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Biology 2e
SENIOR CONTRIBUTING AUTHORS
Mary Ann Clark, Texas Wesleyan University
Jung Choi, Georgia Institute of Technology
Matthew Douglas, Grand Rapids Community College
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_Table of C ontents
Preface.1
The Chemistry of Life
Chapter 1: The Study of Life .9
1.1 The Science of Biology.9
1.2 Themes and Concepts of Biology. 19
Chapter 2: The Chemical Foundation of Life.35
2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks .36
2.2 Water. 49
2.3 Carbon . 56
Chapter 3: Biological Macromolecules. 69
3.1 Synthesis of Biological Macromolecules . 70
3.2 Carbohydrates. 71
3.3 Lipids . 80
3.4 Proteins. 86
3.5 Nucleic Acids. 96
The Cell
Chapter 4: Cell Structure.107
4.1 Studying Cells.107
4.2 Prokaryotic Cells .110
4.3 Eukaryotic Cells.113
4.4 The Endomembrane System and Proteins.121
4.5 The Cytoskeleton.126
4.6 Connections between Cells and Cellular Activities.131
Chapter 5: Structure and Function of Plasma Membranes.143
5.1 Components and Structure.144
5.2 Passive Transport.151
5.3 Active Transport.159
5.4 Bulk Transport.163
Chapter 6: Metabolism.173
6.1 Energy and Metabolism.174
6.2 Potential, Kinetic, Free, and Activation Energy.177
6.3 The Laws of Thermodynamics .182
6.4 ATP: Adenosine Triphosphate.184
6.5 Enzymes.187
Chapter 7: Cellular Respiration.199
7.1 Energy in Living Systems.200
7.2 Glycolysis.204
7.3 Oxidation of Pyruvate and the Citric Acid Cycle.206
7.4 Oxidative Phosphorylation .209
7.5 Metabolism without Oxygen.214
7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways.217
7.7 Regulation of Cellular Respiration.219
Chapter 8: Photosynthesis.227
8.1 Overview of Photosynthesis.227
8.2 The Light-Dependent Reactions of Photosynthesis.232
8.3 Using Light Energy to Make Organic Molecules .239
Chapter 9: Cell Communication.251
9.1 Signaling Molecules and Cellular Receptors.252
9.2 Propagation of the Signal.261
9.3 Response to the Signal.265
9.4 Signaling in Single-Celled Organisms.268
Chapter 10: Cell Reproduction.279
10.1 Cell Division.279
10.2 The Cell Cycle.283
10.3 Control of the Cell Cycle.289
10.4 Cancer and the Cell Cycle.295
10.5 Prokaryotic Cell Division.297
Genetics
Chapter 11: Meiosis and Sexual Reproduction.307
11.1 The Process of Meiosis.308
11.2 Sexual Reproduction.317
Chapter 12: Mendel's Experiments and Heredity.327
12.1 Mendel’s Experiments and the Laws of Probability.328
12.2 Characteristics and Traits.334
12.3 Laws of Inheritance .345
Chapter 13: Modern Understandings of Inheritance .361
13.1 Chromosomal Theory and Genetic Linkage.362
13.2 Chromosomal Basis of Inherited Disorders.366
Chapter 14: DNA Structure and Function .379
14.1 Historical Basis of Modern Understanding.380
14.2 DNA Structure and Sequencing.383
14.3 Basics of DNA Replication.389
14.4 DNA Replication in Prokaryotes.392
14.5 DNA Replication in Eukaryotes .394
14.6 DNA Repair.397
Chapter 15: Genes and Proteins.407
15.1 The Genetic Code.407
15.2 Prokaryotic Transcription.413
15.3 Eukaryotic Transcription.416
15.4 RNA Processing in Eukaryotes .420
15.5 Ribosomes and Protein Synthesis.424
Chapter 16: Gene Expression.435
16.1 Regulation of Gene Expression.436
16.2 Prokaryotic Gene Regulation .438
16.3 Eukaryotic Epigenetic Gene Regulation.442
16.4 Eukaryotic Transcription Gene Regulation.445
16.5 Eukaryotic Post-transcriptional Gene Regulation.447
16.6 Eukaryotic Translational and Post-translational Gene Regulation.450
16.7 Cancer and Gene Regulation.452
Chapter 17: Biotechnology and Genomics.461
17.1 Biotechnology.461
17.2 Mapping Genomes.472
17.3 Whole-Genome Sequencing.476
17.4 Applying Genomics .479
17.5 Genomics and Proteomics.482
Evolutionary Processes
Chapter 18: Evolution and the Origin of Species.491
18.1 Understanding Evolution.492
18.2 Formation of New Species.500
18.3 Reconnection and Speciation Rates.509
Chapter 19: The Evolution of Populations.517
19.1 Population Evolution.518
19.2 Population Genetics.521
19.3 Adaptive Evolution.527
Chapter 20: Phylogenies and the History of Life .537
20.1 Organizing Life on Earth.537
20.2 Determining Evolutionary Relationships.542
20.3 Perspectives on the Phylogenetic Tree.548
Biological Diversity
Chapter 21: Viruses.559
21.1 Viral Evolution, Morphology, and Classification.559
21.2 Virus Infections and Hosts.567
21.3 Prevention and Treatment of Viral Infections .574
21.4 Other Acellular Entities: Prions and Viroids.579
Chapter 22: Prokaryotes: Bacteria and Archaea.589
22.1 Prokaryotic Diversity.590
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
22.2 Structure of Prokaryotes: Bacteria and Archaea.595
22.3 Prokaryotic Metabolism.604
22.4 Bacterial Diseases in Humans.608
22.5 Beneficial Prokaryotes.616
Chapter 23: Protists.629
23.1 Eukaryotic Origins.630
23.2 Characteristics of Protists.637
23.3 Groups of Protists.639
23.4 Ecology of Protists.657
Chapter 24: Fungi.667
24.1 Characteristics of Fungi.668
24.2 Classifications of Fungi .675
24.3 Ecology of Fungi.684
24.4 Fungal Parasites and Pathogens .690
24.5 Importance of Fungi in Human Life .694
Chapter 25: Seedless Plants.701
25.1 Early Plant Life.702
25.2 Green Algae: Precursors of Land Plants.708
25.3 Bryophytes.710
25.4 Seedless Vascular Plants.716
Chapter 26: Seed Plants.731
26.1 Evolution of Seed Plants.732
26.2 Gymnosperms.738
26.3 Angiosperms.743
26.4 The Role of Seed Plants.751
Chapter 27: Introduction to Animal Diversity.763
27.1 Features of the Animal Kingdom.764
27.2 Features Used to Classify Animals .768
27.3 Animal Phylogeny.775
27.4 The Evolutionary History of the Animal Kingdom.778
Chapter 28: Invertebrates.789
28.1 Phylum Porifera.790
28.2 Phylum Cnidaria.794
28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans.802
28.4 Superphylum Lophotrochozoa: Molluscs and Annelids.811
28.5 Superphylum Ecdysozoa: Nematodes and Tardigrades.821
28.6 Superphylum Ecdysozoa: Arthropods.827
28.7 Superphylum Deuterostomia.836
Chapter 29: Vertebrates.849
29.1 Chordates.850
29.2 Fishes.855
29.3 Amphibians .861
29.4 Reptiles .867
29.5 Birds.876
29.6 Mammals.881
29.7 The Evolution of Primates.886
Plant Structure and Function
Chapter 30: Plant Form and Physiology.903
30.1 The Plant Body.904
30.2 Stems .906
30.3 Roots.915
30.4 Leaves.918
30.5 Transport of Water and Solutes in Plants.926
30.6 Plant Sensory Systems and Responses.935
Chapter 31: Soil and Plant Nutrition.951
31.1 Nutritional Requirements of Plants.951
31.2 The Soil .955
31.3 Nutritional Adaptations of Plants.960
Chapter 32: Plant Reproduction .971
32.1 Reproductive Development and Structure . . . .
32.2 Pollination and Fertilization .
32.3 Asexual Reproduction.
Animal Structure and Function
Chapter 33: The Animal Body: Basic Form and Function
33.1 Animal Form and Function.
33.2 Animal Primary Tissues.
33.3 Homeostasis.
Chapter 34: Animal Nutrition and the Digestive System
34.1 Digestive Systems.
34.2 Nutrition and Energy Production.
34.3 Digestive System Processes.
34.4 Digestive System Regulation .
Chapter 35: The Nervous System.
35.1 Neurons and Glial Cells.
35.2 How Neurons Communicate.
35.3 The Central Nervous System .
35.4 The Peripheral Nervous System.
35.5 Nervous System Disorders .
Chapter 36: Sensory Systems .
36.1 Sensory Processes .
36.2 Somatosensation .
36.3 Taste and Smell.
36.4 Hearing and Vestibular Sensation.
36.5 Vision.
Chapter 37: The Endocrine System.
37.1 Types of Hormones .
37.2 How Hormones Work .
37.3 Regulation of Body Processes.
37.4 Regulation of Hormone Production .
37.5 Endocrine Glands.
Chapter 38: The Musculoskeletal System.
38.1 Types of Skeletal Systems.
38.2 Bone.
38.3 Joints and Skeletal Movement.
38.4 Muscle Contraction and Locomotion.
Chapter 39: The Respiratory System.
39.1 Systems of Gas Exchange.
39.2 Gas Exchange across Respiratory Surfaces . .
39.3 Breathing.
39.4 Transport of Gases in Human Bodily Fluids . . .
Chapter 40: The Circulatory System.
40.1 Overview of the Circulatory System.
40.2 Components of the Blood.
40.3 Mammalian Heart and Blood Vessels.
40.4 Blood Flow and Blood Pressure Regulation . . .
Chapter 41: Osmotic Regulation and Excretion.
41.1 Osmoregulation and Osmotic Balance.
41.2 The Kidneys and Osmoregulatory Organs ....
41.3 Excretion Systems.
41.4 Nitrogenous Wastes.
41.5 Hormonal Control of Osmoregulatory Functions .
Chapter 42: The Immune System.
42.1 Innate Immune Response.
42.2 Adaptive Immune Response.
42.3 Antibodies.
42.4 Disruptions in the Immune System .
Chapter 43: Animal Reproduction and Development . .
43.1 Reproduction Methods.
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43.2 Fertilization.1339
43.3 Human Reproductive Anatomy and Gametogenesis .1341
43.4 Hormonal Control of Human Reproduction .1348
43.5 Human Pregnancy and Birth.1353
43.6 Fertilization and Early Embryonic Development.1358
43.7 Organogenesis and Vertebrate Formation.1362
Ecology
Chapter 44: Ecology and the Biosphere.1371
44.1 The Scope of Ecology.1372
44.2 Biogeography .1376
44.3 Terrestrial Biomes.1382
44.4 Aquatic Biomes .1389
44.5 Climate and the Effects of Global Climate Change .1395
Chapter 45: Population and Community Ecology.1407
45.1 Population Demography.1408
45.2 Life Histories and Natural Selection.1413
45.3 Environmental Limits to Population Growth.1417
45.4 Population Dynamics and Regulation.1421
45.5 Human Population Growth.1425
45.6 Community Ecology.1429
45.7 Behavioral Biology: Proximate and Ultimate Causes of Behavior.1441
Chapter 46: Ecosystems .1459
46.1 Ecology of Ecosystems .1459
46.2 Energy Flow through Ecosystems.1468
46.3 Biogeochemical Cycles .1472
Chapter 47: Conservation Biology and Biodiversity .1489
47.1 The Biodiversity Crisis.1490
47.2 The Importance of Biodiversity to Human Life.1500
47.3 Threats to Biodiversity.1504
47.4 Preserving Biodiversity.1511
Appendix A: The Periodic Table of Elements.1521
Appendix B: Geological Time .1523
Appendix C: Measurements and the Metric System.1525
Index.1545
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Preface
1
PREFACE
Welcome to Biology 2e (2nd edition), an OpenStax resource. This textbook was written to increase student
access to high-quality learning materials, maintaining highest standards of academic rigor at little to no cost.
About OpenStax
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Format
You can access this textbook for free in web view or PDF through OpenStax.org, and for a low cost in print.
About Biology 2e
Biology 2e (2nd edition) is designed to cover the scope and sequence requirements of a typical two-semester
biology course for science majors. The text provides comprehensive coverage of foundational research and core
biology concepts through an evolutionary lens. Biology includes rich features that engage students in scientific
inquiry, highlight careers in the biological sciences, and offer everyday applications. The book also includes
various types of practice and homework questions that help students understand — and apply — key concepts.
The 2 nd edition has been revised to incorporate clearer, more current, and more dynamic explanations, while
maintaining the same organization as the first edition. Art and illustrations have been substantially improved, and
the textbook features additional assessments and related resources.
Coverage and scope
Biology was one of the first textbooks published by OpenStax and has been used by hundreds of faculty and
thousands of students since 2012. We mined our adopters’ extensive and helpful feedback to identify the most
2
Preface
significant revision needs while maintaining the organization that many instructors had incorporated into their
courses. Specific surveys, focus groups, and pre-revision reviews, as well as data from our OpenStax Tutor
users, all aided in planning the revision.
The result is a book that thoroughly treats biology’s foundational concepts while adding current and meaningful
coverage in specific areas. Biology 2e retains its manageable scope and contains ample features to draw
learners into the discipline.
Structurally, the textbook remains similar to the first edition, with no chapter reorganization and very targeted
changes at the section level (mostly in biodiversity).
Unit 1: The Chemistry of Life. Our opening unit introduces students to the sciences, including the scientific
method and the fundamental concepts of chemistry and physics that provide a framework within which
learners comprehend biological processes.
Unit 2: The Cell. Students will gain solid understanding of the structures, functions, and processes of the
most basic unit of life: the cell.
Unit 3: Genetics. Our comprehensive genetics unit takes learners from the earliest experiments that
revealed the basis of genetics through the intricacies of DNA to current applications in the emerging studies
of biotechnology and genomics.
Unit 4: Evolutionary Processes. The core concepts of evolution are discussed in this unit with examples
illustrating evolutionary processes. Additionally, the evolutionary basis of biology reappears throughout
the textbook in general discussion and is reinforced through special call-out features highlighting specific
evolution-based topics.
Unit 5: Biological Diversity. The diversity of life is explored with detailed study of various organisms and
discussion of emerging phylogenetic relationships. This unit moves from viruses to living organisms like
bacteria, discusses the organisms formerly grouped as protists, and devotes multiple chapters to plant and
animal life.
Unit 6: Plant Structure and Function. Our plant unit thoroughly covers the fundamental knowledge of
plant life essential to an introductory biology course.
Unit 7: Animal Structure and Function. An introduction to the form and function of the animal body
is followed by chapters on specific body systems and processes. This unit touches on the biology of all
organisms while maintaining an engaging focus on human anatomy and physiology that helps students
connect to the topics.
Unit 8: Ecology. Ecological concepts are broadly covered in this unit, with features highlighting localized,
real-world issues of conservation and biodiversity.
Changes to the Second Edition
OpenStax only undertakes second editions when significant modifications to the text are necessary. In the case
of Biology 2e , user feedback indicated that we needed to focus on a few key areas, which we have done in the
following ways:
Content revisions for clarity, accuracy, and currency. The revision plan varied by chapter based on
need. About twenty chapters were wholly revised with significant updates to conceptual coverage, research-
informed data, and clearer language. In about fifteen other chapters, the revisions focused mostly on
readability and clearer language with fewer conceptual and factual changes.
Additional end-of-chapter questions. The authors added new assessments to nearly every chapter,
including both review and critical thinking questions. The additions total over 350 new items.
Art and illustrations. Under the guidance of the authors and expert scientific illustrators, especially those
well versed in creating accessible art, the OpenStax team made changes to most of the art in Biology. You
will find examples in the section below. The revisions fall into the following categories:
Revisions for accuracy
Redesigns for greater understanding and impact
Recoloring art for overall consistency
Accessibility improvements. As with all OpenStax books, the first edition of Biology was created with a
focus on accessibility. We have emphasized and improved that approach in the second edition.
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Preface
3
To accommodate users of specific assistive technologies, all alternative text was reviewed and revised
for comprehensiveness and clarity.
Many illustrations were revised to improve the color contrast, which is important for some visually
impaired students.
Overall, the OpenStax platform has been continually upgraded to improve accessibility.
A transition guide will be available on OpenStax.org to highlight the specific chapter-level changes to the second
edition.
Pedagogical foundation
The pedagogical choices, chapter arrangements, and learning objective fulfillment were developed and vetted
with the feedback of another one hundred reviewers, who thoroughly read the material and offered detailed
critical commentary.
Evolution Connection features uphold the importance of evolution to all biological study through
discussions like “The Evolution of Metabolic Pathways" and “Algae and Evolutionary Paths to
Photosynthesis.”
Scientific Method Connection call-outs walk students through actual or thought experiments that
elucidate the steps of the scientific process as applied to the topic. Features include “Determining the Time
Spent in Cell Cycle Stages” and “Testing the Hypothesis of Independent Assortment.”
Career Connection features present information on a variety of careers in the biological sciences,
introducing students to the educational requirements and day-to-day work life of a variety of professions,
such as microbiologist, ecologist, neurologist, and forensic scientist.
Everyday Connection features tie biological concepts to emerging issues and discuss science in terms of
everyday life. Topics include “Chesapeake Bay" and “Can Snail Venom Be Used as a Pharmacological Pain
Killer?”
Art and animations that engage
Our art program takes a straightforward approach designed to help students learn the concepts of biology
through simple, effective illustrations, photos, and micrographs. Biology 2e also incorporates links to relevant
animations and interactive exercises that help bring biology to life for students.
Visual Connection features call out core figures in each chapter for student study. Questions about
key figures, including clicker questions that can be used in the classroom, engage students’ critical thinking
and analytical abilities to ensure their genuine understanding.
Link to Learning features direct students to online interactive exercises and animations to add a fuller
context and examples to core content.
Below are a few examples of the revised art for Biology 2e\
4
Preface
Desmosome
Adjacent
plasma
membranes
Plaque
Transmembrane
glycoprotein
(cadherin)
Intermediate
filament
(keratin)
Intercellullar
space
Phagocytosis
Plasma
membrane
Large particle
Vacuole
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Preface
5
membrane cavity
External ear Middle ear Inner ear
Additional resources
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About the authors
Second edition authors and reviewers
Senior Contributing Authors
Mary Ann Clark, Texas Wesleyan University
Jung Choi, Georgia Institute of Technology
Matthew Douglas, Grand Rapids Community College
Reviewers
Kathleen Berlyn, Baltimore City Community College
Bridgett Brinton, Armstrong State University
Jennifer Chase, Northwest Nazarene University
6
Preface
Amy Hoffman, Grayson County College
Olga Kopp, Utah Valley University
Jennifer Larson, Capital University
Jason Locklin, Austin Community College
Hongmei Ma, American University
Melissa Masse, Tulsa Community College
Shannon McDermott, Central Virginia Community College
Bryan Monesson-Olson, University of Massachusetts Amherst
Amber Reece, California State University Fresno
Monique Reed, Texas A&M University
Jeffrey Roberts, American River College
Matthew Smith, North Dakota State University
Dawn Wankowski, Cardinal Stritch University
First edition authors and reviewers
Senior Contributing Authors
Yael Avissar (Cell Biology), Rhode Island College
Jung Choi (Genetics), Georgia Institute of Technology
Jean DeSaix (Evolution), University of North Carolina at Chapel Hill
Vladimir Jurukovski (Animal Physiology), Suffolk County Community College
Robert Wise (Plant Biology), University of Wisconsin, Oshkosh
Connie Rye (General Content Lead), East Mississippi Community College
Contributing Authors and Reviewers
Julie Adams, Aurora University
Summer Allen, Brown University
James Bader, Case Western Reserve University
David Bailey, St. Norbert College
Mark Belk, Brigham Young University
Nancy Boury, Iowa State University
Lisa Bonneau, Metropolitan Community College - Blue River
Graciela Brelles-Marino, California State University Pomona
Mark Browning, Purdue University
Sue Chaplin, University of St. Thomas
George Cline, Jacksonville State University
Deb Cook, Georgia Gwinnett College
Diane Day, Clayton State University
Frank Dirrigl, The University of Texas Pan American
Waneene Dorsey, Grambling State University
Nick Downey, University of Wisconsin La Crosse
Rick Duhrkopf, Baylor University
Kristy Duran, Adams State University
Stan Eisen, Christian Brothers University
Brent Ewers, University of Wyoming
Myriam Feldman, Lake Washington Institute of Technology
Michael Fine, Virginia Commonwealth University
Linda Flora, Delaware County Community College
Thomas Freeland, Walsh University
David Grise, Texas A&M University - Corpus Christi
Andrea Hazard, SUNY Cortland
Michael Hedrick, University of North Texas
Linda Hensel, Mercer University
Mark Kopeny, University of Virginia
Norman Johnson, University of Massachusetts Amherst
Grace Lasker, Lake Washington Institute of Technology; Walden University
Sandy Latourelle, SUNY Plattsburgh
Theo Light, Shippensburg University
Clark Lindgren, Grinnell College
James Malcolm, University of Redlands
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Preface
7
Mark Meade, Jacksonville State University
Richard Merritt, Houston Community College
James Mickle, North Carolina State University
Jasleen Mishra, Houston Community College
Dudley Moon, Albany College of Pharmacy and Health Sciences
Shobhana Natarajan, Brookhaven College
Jonas Okeagu, Fayetteville State University
Diana Oliveras, University of Colorado Boulder
John Peters, College of Charleston
Joel Piperberg, Millersville University
Johanna Porter-Kelley, Winston-Salem State University
Robyn Puffenbarger, Bridgewater College
Dennis Revie, California Lutheran University
Ann Rushing, Baylor University
Sangha Saha, City College of Chicago
Edward Saiff, Ramapo College of New Jersey
Brian Shmaefsky, Lone Star College System
Robert Sizemore, Alcorn State University
Marc Smith, Sinclair Community College
Frederick Spiegel, University of Arkansas
Frederick Sproull, La Roche College
Bob Sullivan, Marist College
Mark Sutherland, Hendrix College
Toure Thompson, Alabama A&M University
Scott Thomson, University of Wisconsin - Parkside
Allison van de Meene, University of Melbourne
Mary White, Southeastern Louisiana University
Steven Wilt, Bellarmine University
James Wise, Hampton University
Renna Wolfe
Virginia Young, Mercer University
Leslie Zeman, University of Washington
Daniel Zurek, Pittsburg State University
Shobhana Natarajan, Alcon Laboratories, inc.
Preface
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Chapter 11 The Study of Life
9
1 1 THE STUDY OF LIFE
Figure 1.1 This NASA image is a composite of several satellite-based views of Earth. To make the whole-Earth image,
NASA scientists combine observations of different parts of the planet, (credit: NASA/GSFC/NOAA/USGS)
Chapter Outline
1.1: The Science of Biology
1.2: Themes and Concepts of Biology
Introduction
Viewed from space, Earth offers no clues about the diversity of life forms that reside there. Scientists believe that
the first forms of life on Earth were microorganisms that existed for billions of years in the ocean before plants
and animals appeared. The mammals, birds, and flowers so familiar to us are all relatively recent, originating 130
to 250 million years ago. The earliest representatives of the genus Homo, to which we belong, have inhabited
this planet for only the last 2.5 million years, and only in the last 300,000 years have humans started looking like
we do today.
1.1 1 The Science of Biology
By the end of this section, you will be able to do the following:
• Identify the shared characteristics of the natural sciences
• Summarize the steps of the scientific method
• Compare inductive reasoning with deductive reasoning
• Describe the goals of basic science and applied science
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Chapter 11 The Study of Life
Figure 1.2 Formerly called blue-green algae, these (a) cyanobacteria, magnified 300x under a light microscope, are
some of Earth’s oldest life forms. These (b) stromatolites along the shores of Lake Thetis in Western Australia are
ancient structures formed by layering cyanobacteria in shallow waters, (credit a: modification of work by NASA; credit
b: modification of work by Ruth Ellison; scale-bar data from Matt Russell)
What is biology? In simple terms, biology is the study of living organisms and their interactions with one another
and their environments. This is a very broad definition because the scope of biology is vast. Biologists may
study anything from the microscopic or submicroscopic view of a cell to ecosystems and the whole living planet
(Figure 1.2). Listening to the daily news, you will quickly realize how many aspects of biology we discuss every
day. For example, recent news topics include Escherichia coli (Figure 1.3) outbreaks in spinach and Salmonella
contamination in peanut butter. Other subjects include efforts toward finding a cure for AIDS, Alzheimer’s
disease, and cancer. On a global scale, many researchers are committed to finding ways to protect the planet,
solve environmental issues, and reduce the effects of climate change. All of these diverse endeavors are related
to different facets of the discipline of biology.
Figure 1.3 Escherichia coli (E. coli) bacteria, in this scanning electron micrograph, are normal residents of our digestive
tracts that aid in absorbing vitamin K and other nutrients. However, virulent strains are sometimes responsible for
disease outbreaks, (credit: Eric Erbe, digital colorization by Christopher Pooley, both of USDA, ARS, EMU)
The Process of Science
Biology is a science, but what exactly is science? What does the study of biology share with other scientific
disciplines? We can define science (from the Latin scientia, meaning “knowledge") as knowledge that covers
general truths or the operation of general laws, especially when acquired and tested by the scientific method. It
becomes clear from this definition that applying scientific method plays a major role in science. The scientific
method is a method of research with defined steps that include experiments and careful observation.
We will examine scientific method steps in detail later, but one of the most important aspects of this method is
the testing of hypotheses by means of repeatable experiments. A hypothesis is a suggested explanation for
an event, which one can test. Although using the scientific method is inherent to science, it is inadequate in
determining what science is. This is because it is relatively easy to apply the scientific method to disciplines
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Chapter 11 The Study of Life
11
such as physics and chemistry, but when it comes to disciplines like archaeology, psychology, and geology, the
scientific method becomes less applicable as repeating experiments becomes more difficult.
These areas of study are still sciences, however. Consider archaeology—even though one cannot perform
repeatable experiments, hypotheses may still be supported. For instance, an archaeologist can hypothesize that
an ancient culture existed based on finding a piece of pottery. He or she could make further hypotheses about
various characteristics of this culture, which could be correct or false through continued support or contradictions
from other findings. A hypothesis may become a verified theory. A theory is a tested and confirmed explanation
for observations or phenomena. Therefore, we may be better off to define science as fields of study that attempt
to comprehend the nature of the universe.
Natural Sciences
What would you expect to see in a museum of natural sciences? Frogs? Plants? Dinosaur skeletons? Exhibits
about how the brain functions? A planetarium? Gems and minerals? Maybe all of the above? Science includes
such diverse fields as astronomy, biology, computer sciences, geology, logic, physics, chemistry, and
mathematics (Figure 1.4). However, scientists consider those fields of science related to the physical world and
its phenomena and processes natural sciences. Thus, a museum of natural sciences might contain any of the
items listed above.
Figure 1.4 The diversity of scientific fields includes astronomy, biology, computer science, geology, logic, physics,
chemistry, mathematics, and many other fields, (credit: ‘‘Image Editor’VFlickr)
There is no complete agreement when it comes to defining what the natural sciences include, however.
For some experts, the natural sciences are astronomy, biology, chemistry, earth science, and physics. Other
scholars choose to divide natural sciences into life sciences, which study living things and include biology, and
physical sciences, which study nonliving matter and include astronomy, geology, physics, and chemistry. Some
disciplines such as biophysics and biochemistry build on both life and physical sciences and are interdisciplinary.
Some refer to natural sciences as “hard science" because they rely on the use of quantitative data. Social
sciences that study society and human behavior are more likely to use qualitative assessments to drive
investigations and findings.
Not surprisingly, the natural science of biology has many branches or subdisciplines. Cell biologists study cell
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Chapter 11 The Study of Life
structure and function, while biologists who study anatomy investigate the structure of an entire organism. Those
biologists studying physiology, however, focus on the internal functioning of an organism. Some areas of biology
focus on only particular types of living things. For example, botanists explore plants, while zoologists specialize
in animals.
Scientific Reasoning
One thing is common to all forms of science: an ultimate goal “to know." Curiosity and inquiry are the driving
forces for the development of science. Scientists seek to understand the world and the way it operates. To do
this, they use two methods of logical thinking: inductive reasoning and deductive reasoning.
Inductive reasoning is a form of logical thinking that uses related observations to arrive at a general conclusion.
This type of reasoning is common in descriptive science. A life scientist such as a biologist makes observations
and records them. These data can be qualitative or quantitative, and one can supplement the raw data with
drawings, pictures, photos, or videos. From many observations, the scientist can infer conclusions (inductions)
based on evidence. Inductive reasoning involves formulating generalizations inferred from careful observation
and analyzing a large amount of data. Brain studies provide an example. In this type of research, scientists
observe many live brains while people are engaged in a specific activity, such as viewing images of food.
The scientist then predicts the part of the brain that “lights up” during this activity to be the part controlling
the response to the selected stimulus, in this case, images of food. Excess absorption of radioactive sugar
derivatives by active areas of the brain causes the various areas to "light up". Scientists use a scanner to observe
the resultant increase in radioactivity. Then, researchers can stimulate that part of the brain to see if similar
responses result.
Deductive reasoning or deduction is the type of logic used in hypothesis-based science. In deductive reason, the
pattern of thinking moves in the opposite direction as compared to inductive reasoning. Deductive reasoning
is a form of logical thinking that uses a general principle or law to forecast specific results. From those general
principles, a scientist can extrapolate and predict the specific results that would be valid as long as the general
principles are valid. Studies in climate change can illustrate this type of reasoning. For example, scientists may
predict that if the climate becomes warmer in a particular region, then the distribution of plants and animals
should change.
Both types of logical thinking are related to the two main pathways of scientific study: descriptive science and
hypothesis-based science. Descriptive (or discovery) science, which is usually inductive, aims to observe,
explore, and discover, while hypothesis-based science, which is usually deductive, begins with a specific
question or problem and a potential answer or solution that one can test. The boundary between these two forms
of study is often blurred, and most scientific endeavors combine both approaches. The fuzzy boundary becomes
apparent when thinking about how easily observation can lead to specific questions. For example, a gentleman
in the 1940s observed that the burr seeds that stuck to his clothes and his dog’s fur had a tiny hook structure. On
closer inspection, he discovered that the burrs’ gripping device was more reliable than a zipper. Fie eventually
experimented to find the best material that acted similar, and produced the hook-and-loop fastener popularly
known today as Velcro. Descriptive science and hypothesis-based science are in continuous dialogue.
The Scientific Method
Biologists study the living world by posing questions about it and seeking science-based responses. Known as
scientific method, this approach is common to other sciences as well. The scientific method was used even in
ancient times, but England’s Sir Francis Bacon (1561-1626) first documented it (Figure 1.5). He set up inductive
methods for scientific inquiry. The scientific method is not used only by biologists; researchers from almost all
fields of study can apply it as a logical, rational problem-solving method.
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Chapter 11 The Study of Life
13
Figure 1.5 Historians credit Sir Francis Bacon (1561-1626) as the first to define the scientific method, (credit: Paul van
Somer)
The scientific process typically starts with an observation (often a problem to solve) that leads to a question. Let’s
think about a simple problem that starts with an observation and apply the scientific method to solve the problem.
One Monday morning, a student arrives at class and quickly discovers that the classroom is too warm. That is an
observation that also describes a problem: the classroom is too warm. The student then asks a question: “Why
is the classroom so warm?”
Proposing a Hypothesis
Recall that a hypothesis is a suggested explanation that one can test. To solve a problem, one can propose
several hypotheses. For example, one hypothesis might be, “The classroom is warm because no one turned on
the air conditioning." However, there could be other responses to the question, and therefore one may propose
other hypotheses. A second hypothesis might be, “The classroom is warm because there is a power failure, and
so the air conditioning doesn’t work.”
Once one has selected a hypothesis, the student can make a prediction. A prediction is similar to a hypothesis
but it typically has the format “If . . . then . . . .” For example, the prediction for the first hypothesis might be, “If
the student turns on the air conditioning, then the classroom will no longer be too warm.”
Testing a Hypothesis
A valid hypothesis must be testable. It should also be falsifiable, meaning that experimental results can disprove
it. Importantly, science does not claim to “prove" anything because scientific understandings are always subject
to modification with further information. This step—openness to disproving ideas—is what distinguishes sciences
from non-sciences. The presence of the supernatural, for instance, is neither testable nor falsifiable. To test
a hypothesis, a researcher will conduct one or more experiments designed to eliminate one or more of the
hypotheses. Each experiment will have one or more variables and one or more controls. A variable is any part
of the experiment that can vary or change during the experiment. The control group contains every feature of
the experimental group except it is not given the manipulation that the researcher hypothesizes. Therefore, if
the experimental group's results differ from the control group, the difference must be due to the hypothesized
manipulation, rather than some outside factor. Look for the variables and controls in the examples that follow. To
test the first hypothesis, the student would find out if the air conditioning is on. If the air conditioning is turned
on but does not work, there should be another reason, and the student should reject this hypothesis. To test
the second hypothesis, the student could check if the lights in the classroom are functional. If so, there is no
power failure and the student should reject this hypothesis. The students should test each hypothesis by carrying
out appropriate experiments. Be aware that rejecting one hypothesis does not determine whether or not one
can accept the other hypotheses. It simply eliminates one hypothesis that is not valid (Figure 1.6). Using the
scientific method, the student rejects the hypotheses that are inconsistent with experimental data.
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Chapter 11 The Study of Life
While this “warm classroom” example is based on observational results, other hypotheses and experiments
might have clearer controls. For instance, a student might attend class on Monday and realize she had difficulty
concentrating on the lecture. One observation to explain this occurrence might be, “When I eat breakfast before
class, I am better able to pay attention.” The student could then design an experiment with a control to test this
hypothesis.
In hypothesis-based science, researchers predict specific results from a general premise. We call this type of
reasoning deductive reasoning: deduction proceeds from the general to the particular. However, the reverse
of the process is also possible: sometimes, scientists reach a general conclusion from a number of specific
observations. We call this type of reasoning inductive reasoning, and it proceeds from the particular to the
general. Researchers often use inductive and deductive reasoning in tandem to advance scientific knowledge
(Figure 1.7).
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Chapter 11 The Study of Life
15
visual
CONNECTION
Figure 1.6 The scientific method consists of a series of well-defined steps. If a hypothesis is not supported by
experimental data, one can propose a new hypothesis.
In the example below, the scientific method is used to solve an everyday problem. Order the scientific
method steps (numbered items) with the process of solving the everyday problem (lettered items). Based
on the results of the experiment, is the hypothesis correct? If it is incorrect, propose some alternative
hypotheses.
1. Observation
2. Question
3. Hypothesis (answer)
4. Prediction
5. Experiment
6. Result
a. There is something wrong with the electrical outlet.
b. If something is wrong with the outlet, my coffeemaker also won’t work when plugged into it.
c. My toaster doesn’t toast my bread.
d. I plug my coffee maker into the outlet.
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Chapter 11 The Study of Life
e. My coffeemaker works.
f. Why doesn’t my toaster work?
visual
CONNECTION
Two Types of Reasoning
Inductive reasoning:
from a number of
observations, a general
conclusion is drawn.
Deductive reasoning:
from a general premise,
specific results are
predicted.
Observations
General premise
• Members of a species
are not all the same.
• Individuals compete for
resources.
• Species are generally
adapted to their
environment.
Individuals most adapted
to their environment are
more likely to survive
and pass their traits on
to the next generation.
T 1
Conclusion
Predicted results
Individuals most adapted
to their environment are
more likely to survive
and pass their traits to
the next generation.
If the average
temperature in an
ecosystem increases
due to climate change,
individuals better
adapted to warmer
temperatures will
outcompete those that
are not.
Figure 1.7 Scientists use two types of reasoning, inductive and deductive reasoning, to advance scientific
knowledge. As is the case in this example, the conclusion from inductive reasoning can often become the premise
for deductive reasoning.
Decide if each of the following is an example of inductive or deductive reasoning.
1. All flying birds and insects have wings. Birds and insects flap their wings as they move through the air.
Therefore, wings enable flight.
2. Insects generally survive mild winters better than harsh ones. Therefore, insect pests will become more
problematic if global temperatures increase.
3. Chromosomes, the carriers of DNA, are distributed evenly between the daughter cells during cell
division. Therefore, each daughter cell will have the same chromosome set as the mother cell.
4. Animals as diverse as humans, insects, and wolves all exhibit social behavior. Therefore, social
behavior must have an evolutionary advantage.
The scientific method may seem too rigid and structured. It is important to keep in mind that, although scientists
often follow this sequence, there is flexibility. Sometimes an experiment leads to conclusions that favor a change
in approach. Often, an experiment brings entirely new scientific questions to the puzzle. Many times, science
does not operate in a linear fashion. Instead, scientists continually draw inferences and make generalizations,
finding patterns as their research proceeds. Scientific reasoning is more complex than the scientific method
alone suggests. Notice, too, that we can apply the scientific method to solving problems that aren’t necessarily
scientific in nature.
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Chapter 11 The Study of Life
17
Two Types of Science: Basic Science and Applied Science
The scientific community has been debating for the last few decades about the value of different types of
science. Is it valuable to pursue science for the sake of simply gaining knowledge, or does scientific knowledge
only have worth if we can apply it to solving a specific problem or to bettering our lives? This question focuses
on the differences between two types of science: basic science and applied science.
Basic science or “pure" science seeks to expand knowledge regardless of the short-term application of that
knowledge. It is not focused on developing a product or a service of immediate public or commercial value. The
immediate goal of basic science is knowledge for knowledge’s sake, although this does not mean that, in the
end, it may not result in a practical application.
In contrast, applied science or “technology," aims to use science to solve real-world problems, making it
possible, for example, to improve a crop yield, find a cure for a particular disease, or save animals threatened
by a natural disaster (Figure 1.8). In applied science, the problem is usually defined for the researcher.
Figure 1.8 After Hurricane Irma struck the Caribbean and Florida in 2017, thousands of baby squirrels like this one
were thrown from their nests. Thanks to applied science, scientists knew how to rehabilitate the squirrel, (credit:
audreyjm529, Flickr)
Some individuals may perceive applied science as “useful” and basic science as “useless." A question these
people might pose to a scientist advocating knowledge acquisition would be, “What for?” However, a careful
look at the history of science reveals that basic knowledge has resulted in many remarkable applications
of great value. Many scientists think that a basic understanding of science is necessary before researchers
develop an application therefore, applied science relies on the results that researchers generate through basic
science. Other scientists think that it is time to move on from basic science in order to find solutions to actual
problems. Both approaches are valid. It is true that there are problems that demand immediate attention;
however, scientists would find few solutions without the help of the wide knowledge foundation that basic science
generates.
One example of how basic and applied science can work together to solve practical problems occurred after the
discovery of DNA structure led to an understanding of the molecular mechanisms governing DNA replication.
DNA strands, unique in every human, are in our cells, where they provide the instructions necessary for
life. When DNA replicates, it produces new copies of itself, shortly before a cell divides. Understanding DNA
replication mechanisms enabled scientists to develop laboratory techniques that researchers now use to identify
genetic diseases, pinpoint individuals who were at a crime scene, and determine paternity. Without basic
science, it is unlikely that applied science would exist.
Another example of the link between basic and applied research is the Human Genome Project, a study in
which researchers analyzed and mapped each human chromosome to determine the precise sequence of
DNA subunits and each gene's exact location. (The gene is the basic unit of heredity. An individual’s complete
collection of genes is his or her genome.) Researchers have studied other less complex organisms as part of this
project in order to gain a better understanding of human chromosomes. The Human Genome Project (Figure
1.9) relied on basic research with simple organisms and, later, with the human genome. An important end goal
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Chapter 11 The Study of Life
eventually became using the data for applied research, seeking cures and early diagnoses for genetically related
diseases.
Figure 1.9 The Human Genome Project was a 13-year collaborative effort among researchers working in several
different science fields. Researchers completed the project, which sequenced the entire human genome, in 2003.
(credit: the U.S. Department of Energy Genome Programs (http://genomics.energy.gov (http:// 0 penstax. 0 rg/l/
genomics gov) )
While scientists usually carefully plan research efforts in both basic science and applied science, note that some
discoveries are made by serendipity, that is, by means of a fortunate accident or a lucky surprise. Scottish
biologist Alexander Fleming discovered penicillin when he accidentally left a petri dish of Staphylococcus
bacteria open. An unwanted mold grew on the dish, killing the bacteria. Fleming's curiosity to investigate the
reason behind the bacterial death, followed by his experiments, led to the discovery of the antibiotic penicillin,
which is produced by the fungus Penicillium. Even in the highly organized world of science, luck—when
combined with an observant, curious mind—can lead to unexpected breakthroughs.
Reporting Scientific Work
Whether scientific research is basic science or applied science, scientists must share their findings in order
for other researchers to expand and build upon their discoveries. Collaboration with other scientists—when
planning, conducting, and analyzing results—are all important for scientific research. For this reason, important
aspects of a scientist’s work are communicating with peers and disseminating results to peers. Scientists can
share results by presenting them at a scientific meeting or conference, but this approach can reach only the
select few who are present, instead, most scientists present their results in peer-reviewed manuscripts that are
published in scientific journals. Peer-reviewed manuscripts are scientific papers that a scientist’s colleagues
or peers review. These colleagues are qualified individuals, often experts in the same research area, who judge
whether or not the scientist’s work is suitable for publication. The process of peer review helps to ensure that
the research in a scientific paper or grant proposal is original, significant, logical, and thorough. Grant proposals,
which are requests for research funding, are also subject to peer review. Scientists publish their work so other
scientists can reproduce their experiments under similar or different conditions to expand on the findings. The
experimental results must be consistent with the findings of other scientists.
A scientific paper is very different from creative writing. Although creativity is required to design experiments,
there are fixed guidelines when it comes to presenting scientific results. First, scientific writing must be brief,
concise, and accurate. A scientific paper needs to be succinct but detailed enough to allow peers to reproduce
the experiments.
The scientific paper consists of several specific sections—introduction, materials and methods, results, and
discussion. This structure is sometimes called the “IMRaD" format. There are usually acknowledgment and
reference sections as well as an abstract (a concise summary) at the beginning of the paper. There might be
additional sections depending on the type of paper and the journal where it will be published. For example, some
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Chapter 11 The Study of Life
19
review papers require an outline.
The introduction starts with brief, but broad, background information about what is known in the field. A good
introduction also gives the rationale of the work. It justifies the work carried out and also briefly mentions the end
of the paper, where the researcher will present the hypothesis or research question driving the research. The
introduction refers to the published scientific work of others and therefore requires citations following the style of
the journal. Using the work or ideas of others without proper citation is plagiarism.
The materials and methods section includes a complete and accurate description of the substances the
researchers use, and the method and techniques they use to gather data. The description should be thorough
enough to allow another researcher to repeat the experiment and obtain similar results, but it does not have to be
verbose. This section will also include information on how the researchers made measurements and the types of
calculations and statistical analyses they used to examine raw data. Although the materials and methods section
gives an accurate description of the experiments, it does not discuss them.
Some journals require a results section followed by a discussion section, but it is more common to combine both.
If the journal does not allow combining both sections, the results section simply narrates the findings without any
further interpretation. The researchers present results with tables or graphs, but they do not present duplicate
information, in the discussion section, the researchers will interpret the results, describe how variables may be
related, and attempt to explain the observations. It is indispensable to conduct an extensive literature search to
put the results in the context of previously published scientific research. Therefore, researchers include proper
citations in this section as well.
Finally, the conclusion section summarizes the importance of the experimental findings. While the scientific
paper almost certainly answers one or more scientific questions that the researchers stated, any good research
should lead to more questions. Therefore, a well-done scientific paper allows the researchers and others to
continue and expand on the findings.
Review articles do not follow the IMRAD format because they do not present original scientific findings, or
primary literature. Instead, they summarize and comment on findings that were published as primary literature
and typically include extensive reference sections.
1.2 | Themes and Concepts of Biology
By the end of this section, you will be able to do the following:
• identify and describe the properties of life
• Describe the levels of organization among living things
• Recognize and interpret a phylogenetic tree
• List examples of different subdisciplines in biology
Biology is the science that studies life, but what exactly is life? This may sound like a silly question with an
obvious response, but it is not always easy to define life. For example, a branch of biology called virology
studies viruses, which exhibit some of the characteristics of living entities but lack others. Although viruses can
attack living organisms, cause diseases, and even reproduce, they do not meet the criteria that biologists use
to define life. Consequently, virologists are not biologists, strictly speaking. Similarly, some biologists study the
early molecular evolution that gave rise to life. Since the events that preceded life are not biological events, these
scientists are also excluded from biology in the strict sense of the term.
From its earliest beginnings, biology has wrestled with three questions: What are the shared properties that
make something “alive"? Once we know something is alive, how do we find meaningful levels of organization in
its structure? Finally, when faced with the remarkable diversity of life, how do we organize the different kinds of
organisms so that we can better understand them? As scientists discover new organisms every day, biologists
continue to seek answers to these and other questions.
Properties of Life
All living organisms share several key characteristics or functions: order, sensitivity or response to the
environment, reproduction, adaptation, growth and development, regulation, homeostasis, energy processing,
and evolution. When viewed together, these nine characteristics serve to define life.
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Chapter 11 The Study of Life
Order
Figure 1.10 A toad represents a highly organized structure consisting of cells, tissues, organs, and organ systems,
(credit: “Ivengo’VWikimedia Commons)
Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple,
single-celled organisms are remarkably complex: inside each cell, atoms comprise molecules. These in turn
comprise cell organelles and other cellular inclusions. In multicellular organisms (Figure 1.10), similar cells form
tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work
together to form organ systems.
Sensitivity or Response to Stimuli
Figure 1.11 The leaves of this sensitive plant (Mimosa pudica) will instantly droop and fold when touched. After a few
minutes, the plant returns to normal, (credit: Alex Lomas)
Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences
and walls, or respond to touch (Figure 1.11). Even tiny bacteria can move toward or away from chemicals
(a process called chemotaxis) or light {phototaxis ). Movement toward a stimulus is a positive response, while
movement away from a stimulus is a negative response.
LINK TQ LEARNING
Watch this video (http:// 0 penstaxc 0 llege. 0 rg/l/m 0 vement_plants) to see how plants respond to a
stimulus—from opening to light, to wrapping a tendril around a branch, to capturing prey.
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Chapter 11 The Study of Life
21
Reproduction
Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares
to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline, gamete,
oocyte, and sperm cells. After fertilization (the fusion of an oocyte and a sperm cell), a new individual develops.
When reproduction occurs, DNA containing genes are passed along to an organism’s offspring. These genes
ensure that the offspring will belong to the same species and will have similar characteristics, such as size and
shape.
Growth and Development
Organisms grow and develop as a result of genes providing specific instructions that will direct cellular growth
and development. This ensures that a species’ young (Figure 1.12) will grow up to exhibit many of the same
characteristics as its parents.
Figure 1.12 Although no two look alike, these kittens have inherited genes from both parents and share many of the
same characteristics, (credit: Rocky Mountain Feline Rescue)
Regulation
Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal
functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions
regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together)
perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients
to every cell, and cooling the body.
Homeostasis
Figure 1.13 Polar bears (Ursus maritimus) and other mammals living in ice-covered regions maintain their body
temperature by generating heat and reducing heat loss through thick fur and a dense layer of fat under their skin,
(credit: “longhorndave'VFlickr)
In order to function properly, cells require appropriate conditions such as proper temperature, pH, and
appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to
the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite
22
Chapter 11 The Study of Life
environmental changes, through homeostasis (literally, “steady state”). For example, an organism needs to
regulate body temperature through the thermoregulation process. Organisms that live in cold climates, such as
the polar bear (Figure 1.13), have body structures that help them withstand low temperatures and conserve
body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates,
organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess
body heat.
Energy Processing
Figure 1.14 The California condor (Gymnogyps californianus) uses chemical energy derived from food to power flight.
California condors are an endangered species. This bird has a wing tag that helps biologists identify the individual,
(credit: Pacific Southwest Region U.S. Fish and Wildlife Service)
All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the
sun and convert it into chemical energy in food. Others use chemical energy in molecules they take in as food
(Figure 1.14).
Levels of Organization of Living Things
Living things are highly organized and structured, following a hierarchy that we can examine on a scale
from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus
surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least
two atoms held together by one or more chemical bonds. Many molecules that are biologically important are
macromolecules, large molecules that are typically formed by polymerization (a polymer is a large molecule
that is made by combining smaller units called monomers, which are simpler than macromolecules). An example
of a macromolecule is deoxyribonucleic acid (DNA) (Figure 1.15), which contains the instructions for the
structure and functioning of all living organisms.
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Chapter 11 The Study of Life
23
Figure 1.15 All molecules, including this DNA molecule, are comprised of atoms, (credit: “brian0918'7Wikimedia
Commons)
LINK TQ LEARNING
Watch this video (http:// 0 penstaxc 0 llege. 0 rg/l/r 0 tating_DNA) that animates the three-dimensional
structure of the DNA molecule in Figure 1.15.
Some cells contain aggregates of macromolecules surrounded by membranes. We call these organelles.
Organelles are small structures that exist within cells. Examples of organelles include mitochondria and
chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while
chloroplasts enable green plants to utilize the energy in sunlight to make sugars. All living things are made
of cells. The cell itself is the smallest fundamental unit of structure and function in living organisms. (This
requirement is why scientists do not consider viruses living: they are not made of cells. To make new viruses,
they have to invade and hijack the reproductive mechanism of a living cell. Only then can they obtain the
materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular.
Scientists classify cells as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that
do not have membrane-bound nuclei. In contrast, the cells of eukaryotes do have membrane-bound organelles
and a membrane-bound nucleus.
In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or
related functions. Organs are collections of tissues grouped together performing a common function. Organs
24
Chapter 11 The Study of Life
are present not only in animals but also in plants. An organ system is a higher level of organization that
consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system
transports blood through the body and to and from the lungs. It includes organs such as the heart and blood
vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled
prokaryotes and single-celled eukaryotes are also organisms, which biologists typically call microorganisms.
Biologists collectively call all the individuals of a species living within a specific area a population. For example,
a forest may include many pine trees, which represent the population of pine trees in this forest. Different
populations may live in the same specific area. For example, the forest with the pine trees includes populations
of flowering plants, insects, and microbial populations. A community is the sum of populations inhabiting a
particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s
community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area
together with the abiotic, nonliving parts of that environment such as nitrogen in the soil or rain water. At the
highest level of organization (Figure 1.16), the biosphere is the collection of all ecosystems, and it represents
the zones of life on Earth. It includes land, water, and even the atmosphere to a certain extent.
visual
CONNECTION
Organelles: The nucleus,
dyed blue in these onion
cefts. is an example of an
organelle
Cells: Human Wood oe*s
_
fm
Tissues: Human skin
tissue
&
Organs and Organ
Systems: Organs, such as
the stomach and intestine
make up the human
digestive system
t
Organisms. Populations,
and Communities: In a
forest each pine tree is an
organism Together, all the
pine trees make up a
ammal species in the forest
compose a community
3E
Ecosystems: The* coastal
ecosystem in the
southeastern United States
includes living orgarwsms
and the environment in
which they bve.
The Biosphere:
Encompasses all the
ecosystems on Earth
Figure 1.16 shows the biological levels of organization of living things. From a single organelle to the entire
biosphere, living organisms are parts of a highly structured hierarchy, (credit “organelles”: modification of work by
Umberto Salvagnin; credit “cells”: modification of work by Bruce Wetzel, Harry Schaefer/ National Cancer Institute;
credit “tissues”: modification of work by Kilbad; Fama Clamosa; Mikael Haggstrom; credit “organs”: modification
of work by Mariana Ruiz Villareal; credit “organisms": modification of work by "Crystal'VFlickr; credit “ecosystems”:
modification of work by US Fish and Wildlife Service Headquarters; credit “biosphere": modification of work by
NASA)
Which of the following statements is false?
a. Tissues exist within organs which exist within organ systems.
b. Communities exist within populations which exist within ecosystems.
c. Organelles exist within cells which exist within tissues.
d. Communities exist within ecosystems which exist in the biosphere.
The Diversity of Life
The fact that biology, as a science, has such a broad scope has to do with the tremendous diversity of life on
earth. The source of this diversity is evolution, the process of gradual change during which new species arise
from older species. Evolutionary biologists study the evolution of living things in everything from the microscopic
world to ecosystems.
A phylogenetic tree (Figure 1.17) can summarize the evolution of various life forms on Earth. It is a diagram
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Chapter 11 The Study of Life
25
showing the evolutionary relationships among biological species based on similarities and differences in genetic
or physical traits or both. Nodes and branches comprise a phylogenetic tree. The internal nodes represent
ancestors and are points in evolution when, based on scientific evidence, researchers believe an ancestor has
diverged to form two new species. The length of each branch is proportional to the time elapsed since the split.
Phylogenetic Tree of Life
= You are here
Bacteria Archaea Eukarya
Green
Figure 1.17 Microbiologist Carl Woese constructed this phylogenetic tree using data that he obtained from sequencing
ribosomal RNA genes. The tree shows the separation of living organisms into three domains: Bacteria, Archaea, and
Eukarya. Bacteria and Archaea are prokaryotes, single-celled organisms lacking intracellular organelles, (credit: Eric
Gaba; NASA Astrobiology Institute)
26
Chapter 11 The Study of Life
V /
e olution CONNECTION
Carl Woese and the Phylogenetic Tree
In the past, biologists grouped living organisms into five kingdoms: animals, plants, fungi, protists, and
bacteria. They based the organizational scheme mainly on physical features, as opposed to physiology,
biochemistry, or molecular biology, all of which modern systematics use. American microbiologist Carl
Woese's pioneering work in the early 1970s has shown, however, that life on Earth has evolved along three
lineages, now called domains—Bacteria, Archaea, and Eukarya. The first two are prokaryotic cells with
microbes that lack membrane-enclosed nuclei and organelles. The third domain contains the eukaryotes
and includes unicellular microorganisms (protists), together with the three remaining kingdoms (fungi,
plants, and animals). Woese defined Archaea as a new domain, and this resulted in a new taxonomic tree
(Figure 1.17). Many organisms belonging to the Archaea domain live under extreme conditions and are
called extremophiles. To construct his tree, Woese used genetic relationships rather than similarities based
on morphology (shape).
Woese constructed his tree from universally distributed comparative gene sequencing that are present
in every organism, and conserved (meaning that these genes have remained essentially unchanged
throughout evolution). Woese’s approach was revolutionary because comparing physical features are
insufficient to differentiate between the prokaryotes that appear fairly similar in spite of their tremendous
biochemical diversity and genetic variability (Figure 1.18). Comparing homologous DNA and RNA
sequences provided Woese with a sensitive device that revealed the extensive variability of prokaryotes,
and which justified separating the prokaryotes into two domains: bacteria and archaea.
(C)
Figure 1.18 These images represent different domains. The (a) bacteria in this micrograph belong to Domain
Bacteria, while the (b) extremophiles (not visible) living in this hot vent belong to Domain Archaea. Both the
(c) sunflower and (d) lion are part of Domain Eukarya. (credit a: modification of work by Drew March; credit b:
modification of work by Steve Jurvetson; credit c: modification of work by Michael Arrighi; credit d: modification of
work by Leszek Leszcynski)
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Chapter 11 The Study of Life
27
Branches of Biological Study
The scope of biology is broad and therefore contains many branches and subdisciplines. Biologists may
pursue one of those subdisciplines and work in a more focused field. For instance, molecular biology and
biochemistry study biological processes at the molecular and chemical level, including interactions among
molecules such as DNA, RNA, and proteins, as well as the way they are regulated. Microbiology, the study of
microorganisms, is the study of the structure and function of single-celled organisms. It is quite a broad branch
itself, and depending on the subject of study, there are also microbial physiologists, ecologists, and geneticists,
among others.
career connection
Forensic Scientist
Forensic science is the application of science to answer questions related to the law. Biologists as well
as chemists and biochemists can be forensic scientists. Forensic scientists provide scientific evidence for
use in courts, and their job involves examining trace materials associated with crimes. Interest in forensic
science has increased in the last few years, possibly because of popular television shows that feature
forensic scientists on the job. Also, developing molecular techniques and establishing DNA databases have
expanded the types of work that forensic scientists can do. Their job activities are primarily related to crimes
against people such as murder, rape, and assault. Their work involves analyzing samples such as hair,
blood, and other body fluids and also processing DNA (Figure 1.19) found in many different environments
and materials. Forensic scientists also analyze other biological evidence left at crime scenes, such as insect
larvae or pollen grains. Students who want to pursue careers in forensic science will most likely have to take
chemistry and biology courses as well as some intensive math courses.
Figure 1.19 This forensic scientist works in a DNA extraction room at the U.S. Army Criminal Investigation
Laboratory at Fort Gillem, GA. (credit: United States Army CID Command Public Affairs)
Another field of biological study, neurobiology, studies the biology of the nervous system, and although it
is a branch of biology, it is also an interdisciplinary field of study known as neuroscience. Because of its
interdisciplinary nature, this subdiscipline studies different nervous system functions using molecular, cellular,
developmental, medical, and computational approaches.
28
Chapter 11 The Study of Life
Figure 1.20 Researchers work on excavating dinosaur fossils at a site in Castellon, Spain, (credit: Mario Modesto)
Paleontology, another branch of biology, uses fossils to study life’s history (Figure 1.20). Zoology and botany
are the study of animals and plants, respectively. Biologists can also specialize as biotechnologists, ecologists,
or physiologists, to name just a few areas. This is just a small sample of the many fields that biologists can
pursue.
Biology is the culmination of the achievements of the natural sciences from their inception to today. Excitingly,
it is the cradle of emerging sciences, such as the biology of brain activity, genetic engineering of custom
organisms, and the biology of evolution that uses the laboratory tools of molecular biology to retrace the earliest
stages of life on Earth. A scan of news headlines—whether reporting on immunizations, a newly discovered
species, sports doping, or a genetically-modified food—demonstrates the way biology is active in and important
to our everyday world.
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Chapter 11 The Study of Life
29
KEY TERMS
abstract opening section of a scientific paper that summarizes the research and conclusions
applied science form of science that aims to solve real-world problems
atom smallest and most fundamental unit of matter
basic science science that seeks to expand knowledge and understanding regardless of the short-term
application of that knowledge
biochemistry study of the chemistry of biological organisms
biology the study of living organisms and their interactions with one another and their environments
biosphere collection of all the ecosystems on Earth
botany study of plants
cell smallest fundamental unit of structure and function in living things
community set of populations inhabiting a particular area
conclusion section of a scientific paper that summarizes the importance of the experimental findings
control part of an experiment that does not change during the experiment
deductive reasoning form of logical thinking that uses a general inclusive statement to forecast specific results
descriptive science (also, discovery science) form of science that aims to observe, explore, and investigate
discussion section of a scientific paper in which the author interprets experimental results, describes how
variables may be related, and attempts to explain the phenomenon in question
ecosystem all the living things in a particular area together with the abiotic, nonliving parts of that environment
eukaryote organism with cells that have nuclei and membrane-bound organelles
evolution process of gradual change during which new species arise from older species and some species
become extinct
falsifiable able to be disproven by experimental results
homeostasis ability of an organism to maintain constant internal conditions
hypothesis suggested explanation for an observation, which one can test
hypothesis-based science form of science that begins with a specific question and potential testable answers
inductive reasoning form of logical thinking that uses related observations to arrive at a general conclusion
introduction opening section of a scientific paper, which provides background information about what was
known in the field prior to the research reported in the paper
life science field of science, such as biology, that studies living things
macromolecule large molecule, typically formed by the joining of smaller molecules
materials and methods section of a scientific paper that includes a complete description of the substances,
methods, and techniques that the researchers used to gather data
microbiology study of the structure and function of microorganisms
30
Chapter 11 The Study of Life
molecular biology study of biological processes and their regulation at the molecular level, including
interactions among molecules such as DNA, RNA, and proteins
molecule chemical structure consisting of at least two atoms held together by one or more chemical bonds
natural science field of science that is related to the physical world and its phenomena and processes
neurobiology study of the biology of the nervous system
organ collection of related tissues grouped together performing a common function
organ system level of organization that consists of functionally related interacting organs
organelle small structures that exist within cells and carry out cellular functions
organism individual living entity
paleontology study of life’s history by means of fossils
peer-reviewed manuscript scientific paper that a scientist’s colleagues review who are experts in the field of
study
phylogenetic tree diagram showing the evolutionary relationships among various biological species based on
similarities and differences in genetic or physical traits or both; in essence, a hypothesis concerning
evolutionary connections
physical science field of science, such as geology, astronomy, physics, and chemistry, that studies nonliving
matter
plagiarism using other people’s work or ideas without proper citation, creating the false impression that those
are the author’s original ideas
population all of the individuals of a species living within a specific area
prokaryote single-celled organism that lacks organelles and does not have nuclei surrounded by a nuclear
membrane
results section of a scientific paper in which the author narrates the experimental findings and presents relevant
figures, pictures, diagrams, graphs, and tables, without any further interpretation
review article paper that summarizes and comments on findings that were published as primary literature
science knowledge that covers general truths or the operation of general laws, especially when acquired and
tested by the scientific method
scientific method method of research with defined steps that include observation, formulation of a hypothesis,
testing, and confirming or falsifying the hypothesis
serendipity fortunate accident or a lucky surprise
theory tested and confirmed explanation for observations or phenomena
tissue group of similar cells carrying out related functions
variable part of an experiment that the experimenter can vary or change
zoology study of animals
CHAPTER SUMMARY
1.1 The Science of Biology
Biology is the science that studies living organisms and their interactions with one another and their
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Chapter 11 The Study of Life
31
environments. Science attempts to describe and understand the nature of the universe in whole or in part by
rational means. Science has many fields. Those fields related to the physical world and its phenomena are
natural sciences.
Science can be basic or applied. The main goal of basic science is to expand knowledge without any
expectation of short-term practical application of that knowledge. The primary goal of applied research,
however, is to solve practical problems.
Science uses two types of logical reasoning. Inductive reasoning uses particular results to produce general
scientific principles. Deductive reasoning is a form of logical thinking that predicts results by applying general
principles. The common thread throughout scientific research is using the scientific method, a step-based
process that consists of making observations, defining a problem, posing hypotheses, testing these
hypotheses, and drawing one or more conclusions. The testing uses proper controls. Scientists present their
results in peer-reviewed scientific papers published in scientific journals. A scientific research paper consists of
several well-defined sections: introduction, materials and methods, results, and, finally, a concluding
discussion. Review papers summarize the conducted research in a particular field over a period of time.
1.2 Themes and Concepts of Biology
Biology is the science of life. All living organisms share several key properties such as order, sensitivity or
response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.
Living things are highly organized parts of a hierarchy that includes atoms, molecules, organelles, cells,
tissues, organs, and organ systems. In turn, biologists group organisms as populations, communities,
ecosystems, and the biosphere. The great diversity of life today evolved from less-diverse ancestral organisms
over billions of years. We can use a phylogenetic tree to show evolutionary relationships among organisms.
Biology is very broad and includes many branches and subdisciplines. Examples include molecular biology,
microbiology, neurobiology, zoology, and botany, among others.
VISUAL CONNECTION QUESTIONS
1. Figure 1.6 In the example below, the scientific
method is used to solve an everyday problem. Order
the scientific method steps (numbered items) with the
process of solving the everyday problem (lettered
items). Based on the results of the experiment, is the
hypothesis correct? If it is incorrect, propose some
alternative hypotheses.
1. Observation
2. Question
3. Hypothesis (answer)
4. Prediction
5. Experiment
6. Result
a. There is something wrong with the electrical
outlet.
b. If something is wrong with the outlet, my
coffeemaker also won’t work when plugged
into it.
c. My toaster doesn’t toast my bread.
d. I plug my coffee maker into the outlet.
e. My coffeemaker works.
f. Why doesn’t my toaster work?
2. Figure 1.7 Decide if each of the following is an
example of inductive or deductive reasoning.
1. All flying birds and insects have wings. Birds
and insects flap their wings as they move
through the air. Therefore, wings enable
flight.
2. insects generally survive mild winters better
than harsh ones. Therefore, insect pests will
become more problematic if global
temperatures increase.
3. Chromosomes, the carriers of DNA,
separate into daughter cells during cell
division. Therefore, each daughter cell will
have the same chromosome set as the
mother cell.
4. Animals as diverse as humans, insects, and
wolves all exhibit social behavior. Therefore,
social behavior must have an evolutionary
advantage.
3. Figure 1.16 Which of the following statements is
false?
a. Tissues exist within organs which exist
within organ systems.
b. Communities exist within populations which
exist within ecosystems.
c. Organelles exist within cells which exist
within tissues.
d. Communities exist within ecosystems which
exist in the biosphere.
32
Chapter 11 The Study of Life
REVIEW QUESTIONS
4. The first forms of life on Earth were_.
a. plants
b. microorganisms
c. birds
d. dinosaurs
5. A suggested and testable explanation for an event
is called a_.
a. hypothesis
b. variable
c. theory
d. control
6. Which of the following sciences is not considered a
natural science?
a. biology
b. astronomy
c. physics
d. computer science
7. The type of logical thinking that uses related
observations to arrive at a general conclusion is
called_.
a. deductive reasoning
b. the scientific method
c. hypothesis-based science
d. inductive reasoning
8. The process of_helps to ensure that a
scientist’s research is original, significant, logical, and
thorough.
a. publication
b. public speaking
c. peer review
d. the scientific method
9. A person notices that her houseplants that are
regularly exposed to music seem to grow more
quickly than those in rooms with no music. As a
result, she determines that plants grow better when
exposed to music. This example most closely
resembles which type of reasoning?
a. inductive reasoning
b. deductive reasoning
c. neither, because no hypothesis was made
d. both inductive and deductive reasoning
CRITICAL THINKING QUESTIONS
16. Although the scientific method is used by most of
the sciences, it can also be applied to everyday
situations. Think about a problem that you may have
at home, at school, or with your car, and apply the
scientific method to solve it.
17. Give an example of how applied science has had
a direct effect on your daily life.
18. Name two topics that are likely to be studied by
biologists, and two areas of scientific study that
10. The smallest unit of biological structure that
meets the functional requirements of “living" is the
a. organ
b. organelle
c. cell
d. macromolecule
11. Viruses are not considered living because they
a. are not made of cells
b. lack cell nuclei
c. do not contain DNA or RNA
d. cannot reproduce
12. The presence of a membrane-enclosed nucleus
is a characteristic of_.
a. prokaryotic cells
b. eukaryotic cells
c. living organisms
d. bacteria
13. A group of individuals of the same species living
in the same area is called a(n)_.
a. family
b. community
c. population
d. ecosystem
14. Which of the following sequences represents the
hierarchy of biological organization from the most
inclusive to the least complex level?
a. organelle, tissue, biosphere, ecosystem,
population
b. organ, organism, tissue, organelle, molecule
c. organism, community, biosphere, molecule,
tissue, organ
d. biosphere, ecosystem, community,
population, organism
15. Where in a phylogenetic tree would you expect to
find the organism that had evolved most recently?
a. at the base
b. within the branches
c. at the nodes
d. at the branch tips
would fall outside the realm of biology.
19. Thinking about the topic of cancer, write a basic
science question and an applied science question
that a researcher interested in this topic might ask.
20. Select two items that biologists agree are
necessary in order to consider an organism “alive.”
For each, give an example of a nonliving object that
otherwise fits the definition of “alive.”
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Chapter 11 The Study of Life
33
21. Consider the levels of organization of the
biological world, and place each of these items in
order from smallest level of organization to most
encompassing: skin cell, elephant, water molecule,
planet Earth, tropical rainforest, hydrogen atom, wolf
pack, liver.
22. You go for a long walk on a hot day. Give an
example of a way in which homeostasis keeps your
body healthy.
23. Using examples, explain how biology can be
studied from a microscopic approach to a global
approach.
34
Chapter 11 The Study of Life
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Chapter 2 | The Chemical Foundation of Life
35
2 | THE CHEMICAL
FOUNDATION OF LIFE
Figure 2.1 Atoms are the building blocks of molecules in the universe—air, soil, water, rocks . . . and also the cells
of all living organisms. In this model of an organic molecule, the atoms of carbon (black), hydrogen (white), nitrogen
(blue), oxygen (red), and sulfur (yellow) are in proportional atomic size. The silver rods indicate chemical bonds, (credit:
modification of work by Christian Guthier)
Chapter Outline
2.1: Atoms, Isotopes, Ions, and Molecules: The Building Blocks
2.2: Water
2.3: Carbon
Introduction
Elements in various combinations comprise all matter, including living things. Some of the most abundant
elements in living organisms include carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus. These form
the nucleic acids, proteins, carbohydrates, and lipids that are the fundamental components of living matter.
Biologists must understand these important building blocks and the unique structures of the atoms that comprise
molecules, allowing for cells, tissues, organ systems, and entire organisms to form.
All biological processes follow the laws of physics and chemistry, so in order to understand how biological
systems work, it is important to understand the underlying physics and chemistry. For example, the flow of blood
within the circulatory system follows the laws of physics that regulate the modes of fluid flow. The breakdown
of the large, complex molecules of food into smaller molecules—and the conversion of these to release energy
to be stored in adenosine triphosphate (ATP)—is a series of chemical reactions that follow chemical laws. The
properties of water and the formation of hydrogen bonds are key to understanding living processes. Recognizing
the properties of acids and bases is important, for example, to our understanding of the digestive process.
Therefore, the fundamentals of physics and chemistry are important for gaining insight into biological processes.
36
Chapter 2 | The Chemical Foundation of Life
2.1 1 Atoms, Isotopes, Ions, and Molecules: The
Building Blocks
By the end of this section, you will be able to do the following:
• Define matter and elements
• Describe the interrelationship between protons, neutrons, and electrons
• Compare the ways in which electrons can be donated or shared between atoms
• Explain the ways in which naturally occurring elements combine to create molecules, cells, tissues,
organ systems, and organisms
At its most fundamental level, life is made up of matter. Matter is any substance that occupies space and
has mass. Elements are unique forms of matter with specific chemical and physical properties that cannot
break down into smaller substances by ordinary chemical reactions. There are 118 elements, but only 98 occur
naturally. The remaining elements are unstable and require scientists to synthesize them in laboratories.
Each element is designated by its chemical symbol, which is a single capital letter or, when the first letter is
already “taken” by another element, a combination of two letters. Some elements follow the English term for the
element, such as C for carbon and Ca for calcium. Other elements’ chemical symbols derive from their Latin
names. For example, the symbol for sodium is Na, referring to natrium, the Latin word for sodium.
The four elements common to all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). In
the nonliving world, elements are found in different proportions, and some elements common to living organisms
are relatively rare on the earth as a whole, as Table 2.1 shows. For example, the atmosphere is rich in nitrogen
and oxygen but contains little carbon and hydrogen, while the earth’s crust, although it contains oxygen and a
small amount of hydrogen, has little nitrogen and carbon. In spite of their differences in abundance, all elements
and the chemical reactions between them obey the same chemical and physical laws regardless of whether they
are a part of the living or nonliving world.
Approximate Percentage of Elements in Living Organisms (Humans) Compared
to the Nonliving World
Element
Life (Humans)
Atmosphere
Earth’s Crust
Oxygen (O)
65%
21%
46%
Carbon (C)
18%
trace
trace
Hydrogen (H)
10%
trace
0.1%
Nitrogen (N)
3%
78%
trace
Table 2.1
The Structure of the Atom
To understand how elements come together, we must first discuss the element's smallest component or building
block, the atom. An atom is the smallest unit of matter that retains all of the element's chemical properties. For
example, one gold atom has all of the properties of gold in that it is a solid metal at room temperature. A gold
coin is simply a very large number of gold atoms molded into the shape of a coin and contains small amounts of
other elements known as impurities. We cannot break down gold atoms into anything smaller while still retaining
the properties of gold.
An atom is composed of two regions: the nucleus, which is in the atom's center and contains protons and
neutrons. The atom's outermost region holds its electrons in orbit around the nucleus, as Figure 2.2 illustrates.
Atoms contain protons, electrons, and neutrons, among other subatomic particles. The only exception is
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Chapter 2 | The Chemical Foundation of Life
37
hydrogen (H), which is made of one proton and one electron with no neutrons.
Nucleus
Figure 2.2 Elements, such as helium, depicted here, are made up of atoms. Atoms are made up of protons and
neutrons located within the nucleus, with electrons in orbitals surrounding the nucleus.
Protons and neutrons have approximately the same mass, about 1.67 x 10~ 24 grams. Scientists arbitrarily define
this amount of mass as one atomic mass unit (amu) or one Dalton, as Table 2.2 shows. Although similar in
mass, protons and neutrons differ in their electric charge. A proton is positively charged; whereas, a neutron
is uncharged. Therefore, the number of neutrons in an atom contributes significantly to its mass, but not to its
po
charge. Electrons are much smaller in mass than protons, weighing only 9.11 x 10 grams, or about 1/1800 of
an atomic mass unit. Hence, they do not contribute much to an element’s overall atomic mass. Therefore, when
considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom’s mass
based on the number of protons and neutrons alone. Although not significant contributors to mass, electrons do
contribute greatly to the atom’s charge, as each electron has a negative charge equal to the proton's positive
charge. In uncharged, neutral atoms, the number of electrons orbiting the nucleus is equal to the number of
protons inside the nucleus. In these atoms, the positive and negative charges cancel each other out, leading to
an atom with no net charge.
Accounting for the sizes of protons, neutrons, and electrons, most of the atom's volume—greater than 99
percent—is empty space. With all this empty space, one might ask why so-called solid objects do not just pass
through one another. The reason they do not is that the electrons that surround all atoms are negatively charged
and negative charges repel each other.
Protons, Neutrons, and Electrons
Charge
Mass (amu)
Location
Proton
+1
1
nucleus
Neutron
0
1
nucleus
Electron
-1
0
orbitals
Table 2.2
Atomic Number and Mass
Atoms of each element contain a characteristic number of protons and electrons. The number of protons
determines an element’s atomic number, which scientists use to distinguish one element from another. The
number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only
in the number of neutrons they possess. Together, the number of protons and neutrons determine an element’s
mass number, as Figure 2.3 illustrates. Note that we disregard the small contribution of mass from electrons in
calculating the mass number. We can use this approximation of mass to easily calculate how many neutrons an
element has by simply subtracting the number of protons from the mass number. Since an element’s isotopes
will have slightly different mass numbers, scientists also determine the atomic mass, which is the calculated
mean of the mass number for its naturally occurring isotopes. Often, the resulting number contains a fraction.
38
Chapter 2 | The Chemical Foundation of Life
For example, the atomic mass of chlorine (Cl) is 35.45 because chlorine is composed of several isotopes, some
(the majority) with atomic mass 35 (17 protons and 18 neutrons) and some with atomic mass 37 (17 protons and
20 neutrons).
visual
CONNECTION
Atomic number
p
V
12
Ml
symbol
13
Mr
Mass number
Figure 2.3 Carbon has an atomic number of six, and two stable isotopes with mass numbers of twelve and
thirteen, respectively. Its relative atomic mass is 12.011
How many neutrons do carbon-12 and carbon-13 have, respectively?
Isotopes
Isotopes are different forms of an element that have the same number of protons but a different number
of neutrons. Some elements—such as carbon, potassium, and uranium—have naturally occurring isotopes.
Carbon-12 contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 (six
protons and six neutrons). Carbon-14 contains six protons, eight neutrons, and six electrons; its atomic mass is
14 (six protons and eight neutrons). These two alternate forms of carbon are isotopes. Some isotopes may emit
neutrons, protons, and electrons, and attain a more stable atomic configuration (lower level of potential energy);
these are radioactive isotopes, or radioisotopes. Radioactive decay (carbon-14 decaying to eventually become
nitrogen-14) describes the energy loss that occurs when an unstable atom’s nucleus releases radiation.
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Chapter 2 | The Chemical Foundation of Life
39
V /
e olution CONNECTION
Carbon Dating
Carbon is normally present in the atmosphere in the form of gaseous compounds like carbon dioxide and
methane. Carbon-14 ( 14 C) is a naturally occurring radioisotope that is created in the atmosphere from
atmospheric 14 N (nitrogen) by the addition of a neutron and the loss of a proton because of cosmic rays.
This is a continuous process, so more 14 C is always being created. As a living organism incorporates 14 C
initially as carbon dioxide fixed in the process of photosynthesis, the relative amount of 14 C in its body is
equal to the concentration of 14 C in the atmosphere. When an organism dies, it is no longer ingesting 14 C,
so the ratio between 14 C and 12 C will decline as 14 C decays gradually to 14 N by a process called beta
decay—electrons or positrons emission. This decay emits energy in a slow process.
After approximately 5,730 years, half of the starting concentration of 14 C will convert back to 14 N. We call
the time it takes for half of the original concentration of an isotope to decay back to its more stable form its
half-life. Because the half-life of 14 C is long, scientists use it to date formerly living objects such as old bones
or wood. Comparing the ratio of the 14 C concentration in an object to the amount of 14 C in the atmosphere,
scientists can determine the amount of the isotope that has not yet decayed. On the basis of this amount,
Figure 2.4 shows that we can calculate the age of the material, such as the pygmy mammoth, with accuracy
if it is not much older than about 50,000 years. Other elements have isotopes with different half lives. For
example, 40 K (potassium-40) has a half-life of 1.25 billion years, and 235 U (Uranium 235) has a half-life of
about 700 million years. Through the use of radiometric dating, scientists can study the age of fossils or
other remains of extinct organisms to understand how organisms have evolved from earlier species.
Figure 2.4 Scientists can determine the age of carbon-containing remains less than about 50,000 years old, such
as this pygmy mammoth, using carbon dating, (credit: Bill Faulkner, NPS)
LINK
T a
LEARNING
To learn more about atoms, isotopes, and how to tell one isotope from another, run the simulation.
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66430/1.3/#eip-
idll65071748010)
40
Chapter 2 | The Chemical Foundation of Life
The Periodic Table
The periodic table organizes and displays different elements. Devised by Russian chemist Dmitri Mendeleev
(1834-1907) in 1869, the table groups elements that, although unique, share certain chemical properties with
other elements. The properties of elements are responsible for their physical state at room temperature: they
may be gases, solids, or liquids. Elements also have specific chemical reactivity, the ability to combine and to
chemically bond with each other.
In the periodic table in Figure 2.5, the elements are organized and displayed according to their atomic number
and are arranged in a series of rows and columns based on shared chemical and physical properties. In addition
to providing the atomic number for each element, the periodic table also displays the element’s atomic mass.
Looking at carbon, for example, its symbol (C) and name appear, as well as its atomic number of six (in the
upper left-hand corner) and its atomic mass of 12.11.
a Group
1
Periodic Table of the Elements
18
1
H
1.008
hydrogen
2
13 14 15 16 17
2
He
4.003
3
Li
6.94
lithium
4
Be
^9.012
3 4 5 6 7 8 9 10 11 12
5
B
10.81
6
c
12.01
7
N
14.01
8
o
16.00
oxygen
9
F
19.00
fluorine
10
Ne
20.18
11
Na
22.99
12
Mg
24.31
magnesium
13 .
Al
26.98
14
Si
28.09
15
P
30.97
phosphorus
16
s
32.06
17
Cl
35.45
18
Ar
39.95
argon
19
K
39.10
potassium
20
Ca
40.08
21
Sc
44.96
scandium
22
Ti
47.87
titanium
23
V
50.94
vanadium
24
Cr
52.00
chromium
25
Mn
54.94
manganese
26
Fe
55.85
27
Co
58.93
28
Ni
58.69
29
Cu
63.55
copper
30
Zn
65.38
31
Ga
69.72
32
Ge
72.63
germanium
33
As
74.92
34
Se
78.97
selenium
35
Br
79.90
bromine
36
Kr
83.80
krypton
37
Rb
85.47
rubidium
38
Sr
87.62
strontium
39
Y
88.91
40
Zr
91.22
41
Nb
92.91
niobium
42
Mo
95.95
molybdenum
43
Tc
[97]
technetium
44
Ru
101.1
ruthenium
45
Rh
102.9
rhodium
46
Pd
106.4
: -i ' ' l i 1 ''
47
Ag
107Tt
silver
48
Cd
112.4
49
In
114.8
50
Sn
118.7
51
Sb
121.8
52
Te
127.6
tellurium
53
1
126.9
54
Xe
131.3
55
Cs
132.9
56
Ba
137.3
barium
57-71
La-
Lu *
72
Hf
178.5
hafnium
73
Ta
180.9
tantalum
74
w
183.8
tungsten
75
Re
186.2
rhenium
76
Os
190.2
osmium
77
lr
192.2
78
Pt
195.1
platinum
79
Au
197.0
goid
80
Hg
200T
mercury
81
TI
204.4
thallium
82
Pb
201.2
83
Bi
209.0
bismuth
84
Po
[209]
polonium
85
At
[210]
astatine
86
Rn
[222]
87
Fr
[223]
francium
88
Ra
[226]
radium
89-103
Ac-
Lr **
104
R f
■utlLJum
105
Db
J270]
dubraum
106
Jl
107
Bh
£70]
bohrlum
108
Hs
[277]
hassium
109
Mt
[276]
meitnenum
110
Ds
a. [281 J
darmstadtum
111
,J28£,
112
Cn
«,KL
113
Nh
£1
114
FI
[289]
flerovium
115
Me
moscovium
116
Lv
.,SL
117
Ts
[294]
tennesslne
118
oganesson
57
La
138.9
lanthanum
58
Ce
140.1
59
Pr
140.9
praseodymium
60
Nd
144.2
neodymium
61
Pm
[145]
promethium
62
Sm
150.4
samarium
63
Eu
152.0
europium
64
Gd
157.3
gadolinium
65
Tb
158.9
66
Dy
162L5
dysprosium
67
Ho
164.9
holmium
68
Er
167,3
erbium
69
Tm
168.9
thulium
70
Yb
173.1
ytterbium
71
Lu
175.0
lutetium
**
89
Ac
[227]
actinium
90
Th
232.0
thorium
91
Pa
231.0
protactinium
92
u
238.0
uranium
93
Np
[237]
neptunium
94
Pu
[244]
plutonium
95
Am
[243]
americium
96
Cm
[247]
97
Bk
[247]
berkelium
98
Cf
JSL
99
Es
[252]
einsteinium
100
Fm
[257]
lermium
101
Md
a,
mendelevium
102
No
[259]
nobellum
103
Lr
[262]
lawrencium
Symbol
Atomic mass
Name
Color Code
H] Metal
Solid
| Metalloid
Liquid
Nonmetal
Gas
Figure 2.5 The periodic table shows each element's atomic mass and atomic number. The atomic number appears
above the symbol for the element and the approximate atomic mass appears below it.
The periodic table groups elements according to chemical properties. Scientists base the differences in chemical
reactivity between the elements on the number and spatial distribution of an atom’s electrons. Atoms that
chemically react and bond to each other form molecules. Molecules are simply two or more atoms chemically
bonded together. Logically, when two atoms chemically bond to form a molecule, their electrons, which form the
outermost region of each atom, come together first as the atoms form a chemical bond.
Electron Shells and the Bohr Model
Note that there is a connection between the number of protons in an element, the atomic number that
distinguishes one element from another, and the number of electrons it has. In all electrically neutral atoms, the
number of electrons is the same as the number of protons. Thus, each element, at least when electrically neutral,
has a characteristic number of electrons equal to its atomic number.
In 1913, Danish scientist Niels Bohr (1885-1962) developed an early model of the atom. The Bohr model shows
the atom as a central nucleus containing protons and neutrons, with the electrons in circular orbitals at specific
distances from the nucleus, as Figure 2.6 illustrates. These orbits form electron shells or energy levels, which
are away of visualizing the number of electrons in the outermost shells. These energy levels are designated by
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Chapter 2 | The Chemical Foundation of Life
41
a number and the symbol “n." For example, In represents the first energy level located closest to the nucleus.
Figure 2.6 In 1913, Niels Bohrs developed the Bohr model in which electrons exist within principal shells. An electron
normally exists in the lowest energy shell available, which is the one closest to the nucleus. Energy from a photon of
light can bump it up to a higher energy shell, but this situation is unstable, and the electron quickly decays back to the
ground state. In the process, it releases a photon of light.
Electrons fill orbitals in a consistent order: they first fill the orbitals closest to the nucleus, then they continue to
fill orbitals of increasing energy further from the nucleus. If there are multiple orbitals of equal energy, they fill
with one electron in each energy level before adding a second electron. The electrons of the outermost energy
level determine the atom's energetic stability and its tendency to form chemical bonds with other atoms to form
molecules.
Under standard conditions, atoms fill the inner shells first, often resulting in a variable number of electrons in
the outermost shell. The innermost shell has a maximum of two electrons but the next two electron shells can
each have a maximum of eight electrons. This is known as the octet rule, which states, with the exception of
the innermost shell, that atoms are more stable energetically when they have eight electrons in their valence
shell, the outermost electron shell. Figure 2.7 shows examples of some neutral atoms and their electron
configurations. Notice that in Figure 2.7, helium has a complete outer electron shell, with two electrons filling
its first and only shell. Similarly, neon has a complete outer 2n shell containing eight electrons. In contrast,
chlorine and sodium have seven and one in their outer shells, respectively, but theoretically they would be more
energetically stable if they followed the octet rule and had eight.
42
Chapter 2 | The Chemical Foundation of Life
visual
CONNECTION
Figure 2.7 Bohr diagrams indicate how many electrons fill each principal shell. Group 18 elements (helium, neon,
and argon) have a full outer, or valence, shell. A full valence shell is the most stable electron configuration.
Elements in other groups have partially filled valence shells and gain or lose electrons to achieve a stable electron
configuration.
An atom may give, take, or share electrons with another atom to achieve a full valence shell, the most stable
electron configuration. Looking at this figure, how many electrons do elements in group 1 need to lose in
order to achieve a stable electron configuration? How many electrons do elements in groups 14 and 17
need to gain to achieve a stable configuration?
Understanding that the periodic table's organization is based on the total number of protons (and electrons)
helps us know how electrons distribute themselves among the shells. The periodic table is arranged in columns
and rows based on the number of electrons and their location. Examine more closely some of the elements in
the table’s far right column in Figure 2.5. The group 18 atoms helium (He), neon (Ne), and argon (Ar) all have
filled outer electron shells, making it unnecessary for them to share electrons with other atoms to attain stability.
They are highly stable as single atoms. Because they are non reactive, scientists coin them inert (or noble
gases). Compare this to the group 1 elements in the left-hand column. These elements, including hydrogen
(H), lithium (Li), and sodium (Na), all have one electron in their outermost shells. That means that they can
achieve a stable configuration and a filled outer shell by donating or sharing one electron with another atom or
a molecule such as water. Hydrogen will donate or share its electron to achieve this configuration, while lithium
and sodium will donate their electron to become stable. As a result of losing a negatively charged electron, they
become positively charged ions. Group 17 elements, including fluorine and chlorine, have seven electrons in
their outmost shells, so they tend to fill this shell with an electron from other atoms or molecules, making them
negatively charged ions. Group 14 elements, of which carbon is the most important to living systems, have four
electrons in their outer shell allowing them to make several covalent bonds (discussed below) with other atoms.
Thus, the periodic table's columns represent the potential shared state of these elements’ outer electron shells
that is responsible for their similar chemical characteristics.
Electron Orbitals
Although useful to explain the reactivity and chemical bonding of certain elements, the Bohr model does not
accurately reflect how electrons spatially distribute themselves around the nucleus. They do not circle the
nucleus like the earth orbits the sun, but we find them in electron orbitals. These relatively complex shapes
result from the fact that electrons behave not just like particles, but also like waves. Mathematical equations from
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Chapter 2 | The Chemical Foundation of Life
43
quantum mechanics, which scientists call wave functions, can predict within a certain level of probability where
an electron might be at any given time. Scientists call the area where an electron is most likely to be found its
orbital.
Recall that the Bohr model depicts an atom’s electron shell configuration. Within each electron shell are
subshells, and each subshell has a specified number of orbitals containing electrons. While it is impossible to
calculate exactly an electron's location, scientists know that it is most probably located within its orbital path.
The letter s, p, d, and f designate the subshells. The s subshell is spherical in shape and has one orbital.
Principal shell In has only a single s orbital, which can hold two electrons. Principal shell 2n has one s and one
p subshell, and can hold a total of eight electrons. The p subshell has three dumbbell-shaped orbitals, as Figure
2.8 illustrates. Subshells d and f have more complex shapes and contain five and seven orbitals, respectively.
We do not show these in the illustration. Principal shell 3n has s, p, and d subshells and can hold 18 electrons.
Principal shell 4n has s, p, d and f orbitals and can hold 32 electrons. Moving away from the nucleus, the number
of electrons and orbitals in the energy levels increases. Progressing from one atom to the next in the periodic
table, we can determine the electron structure by fitting an extra electron into the next available orbital.
Y
2n p subshell
Ins subshell
Figure 2.8 The s subshells are shaped like spheres. Both the In and 2n principal shells have an s orbital, but the size
of the sphere is larger in the 2n orbital. Each sphere is a single orbital. Three dumbbell-shaped orbitals comprise p
subshells. Principal shell 2n has a p subshell, but shell 1 does not.
The closest orbital to the nucleus, the Is orbital, can hold up to two electrons. This orbital is equivalent to the
Bohr model's innermost electron shell. Scientists call it the Is orbital because it is spherical around the nucleus.
The Is orbital is the closest orbital to the nucleus, and it is always filled first, before any other orbital fills.
Hydrogen has one electron; therefore, it occupies only one spot within the Is orbital. We designate this as Is 1 ,
where the superscripted 1 refers to the one electron within the Is orbital. Helium has two electrons; therefore, it
can completely fill the Is orbital with its two electrons. We designate this as Is 2 , referring to the two electrons of
helium in the Is orbital. On the periodic table Figure 2.5, hydrogen and helium are the only two elements in the
first row (period). This is because they only have electrons in their first shell, the Is orbital. Hydrogen and helium
are the only two elements that have the Is and no other electron orbitals in the electrically neutral state.
The second electron shell may contain eight electrons. This shell contains another spherical s orbital and three
“dumbbell” shaped p orbitals, each of which can hold two electrons, as Figure 2.8 shows. After the Is orbital fills,
the second electron shell fills, first filling its 2s orbital and then its three p orbitals. When filling the p orbitals, each
takes a single electron. Once each p orbital has an electron, it may add a second. Lithium (Li) contains three
electrons that occupy the first and second shells. Two electrons fill the Is orbital, and the third electron then fills
the 2s orbital. Its electron configuration is ls 2 2s 1 . Neon (Ne), alternatively, has a total often electrons: two are
in its innermost Is orbital and eight fill its second shell (two each in the 2s and three p orbitals). Thus it is an
44
Chapter 2 | The Chemical Foundation of Life
inert gas and energetically stable as a single atom that will rarely form a chemical bond with other atoms. Larger
elements have additional orbitals, comprising the third electron shell. While the concepts of electron shells
and orbitals are closely related, orbitals provide a more accurate depiction of an atom's electron configuration
because the orbital model specifies the different shapes and special orientations of all the places that electrons
may occupy.
LINK TQ LEARNING
Watch this visual animation to see the spatial arrangement of the p and s orbitals. (This multimedia
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Chemical Reactions and Molecules
All elements are most stable when their outermost shell is filled with electrons according to the octet rule. This
is because it is energetically favorable for atoms to be in that configuration and it makes them stable. However,
since not all elements have enough electrons to fill their outermost shells, atoms form chemical bonds with
other atoms thereby obtaining the electrons they need to attain a stable electron configuration. When two or
more atoms chemically bond with each other, the resultant chemical structure is a molecule. The familiar water
molecule, H 2 O, consists of two hydrogen atoms and one oxygen atom. These bond together to form water, as
Figure 2.9 illustrates. Atoms can form molecules by donating, accepting, or sharing electrons to fill their outer
shells.
Figure 2.9 Two or more atoms may bond with each other to form a molecule. When two hydrogens and an oxygen
share electrons via covalent bonds it forms a water molecule.
Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms
break apart. Scientists call the substances used in the beginning of a chemical reaction reactants (usually on
the left side of a chemical equation), and we call the substances at the end of the reaction products (usually on
the right side of a chemical equation). We typically draw an arrow between the reactants and products to indicate
the chemical reaction's direction. This direction is not always a “one-way street.” To create the water molecule
above, the chemical equation would be:
2H + O —► H 2 0
An example of a simple chemical reaction is breaking down hydrogen peroxide molecules, each of which
consists of two hydrogen atoms bonded to two oxygen atoms (H 2 O 2 ). The reactant hydrogen peroxide breaks
down into water, containing one oxygen atom bound to two hydrogen atoms (H 2 O), and oxygen, which consists
of two bonded oxygen atoms (O 2 ). in the equation below, the reaction includes two hydrogen peroxide molecules
and two water molecules. This is an example of a balanced chemical equation, wherein each element's
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Chapter 2 | The Chemical Foundation of Life
45
number of atoms is the same on each side of the equation. According to the law of conservation of matter, the
number of atoms before and after a chemical reaction should be equal, such that no atoms are, under normal
circumstances, created or destroyed.
2Ht 0 2 (hydrogen peroxide) -> 2H 1 0 (water) + Oo (oxygen)
Even though all of the reactants and products of this reaction are molecules (each atom remains bonded to at
least one other atom), in this reaction only hydrogen peroxide and water are representatives of compounds:
they contain atoms of more than one type of element. Molecular oxygen, alternatively, as Figure 2.10 shows,
consists of two doubly bonded oxygen atoms and is not classified as a compound but as a hononuclear
molecule.
Figure 2.10 A double bond joins the oxygen atoms in an O 2 molecule.
Some chemical reactions, such as the one above, can proceed in one direction until they expend all the
reactants. The equations that describe these reactions contain a unidirectional arrow and are irreversible.
Reversible reactions are those that can go in either direction. In reversible reactions, reactants turn into
products, but when the product's concentration goes beyond a certain threshold (characteristic of the particular
reaction), some of these products convert back into reactants. At this point, product and reactant designations
reverse. This back and forth continues until a certain relative balance between reactants and products occurs—a
state called equilibrium. A chemical equation with a double headed arrow pointing towards both the reactants
and products often denote these reversible reaction situations.
For example, in human blood, excess hydrogen ions (H + ) bind to bicarbonate ions (HCO 3 ") forming an
equilibrium state with carbonic acid (H 2 CO 3 ). If we added carbonic acid to this system, some of it would convert
to bicarbonate and hydrogen ions.
hco 3 _ + h + ^h 2 co 3
However, biological reactions rarely obtain equilibrium because the concentrations of the reactants or products
or both are constantly changing, often with one reaction's product a reactant for another. To return to the example
of excess hydrogen ions in the blood, forming carbonic acid will be the reaction's major direction. However,
the carbonic acid can also leave the body as carbon dioxide gas (via exhalation) instead of converting back to
bicarbonate ion, thus driving the reaction to the right by the law of mass action. These reactions are important
for maintaining homeostasis in our blood.
hco 3 _ +H + ~ h 2 co 3 co 2 +H 2 0
Ions and Ionic Bonds
Some atoms are more stable when they gain or lose an electron (or possibly two) and form ions. This fills their
outermost electron shell and makes them energetically more stable. Because the number of electrons does not
equal the number of protons, each ion has a net charge. Cations are positive ions that form by losing electrons.
Negative ions form by gaining electrons, which we call anions. We designate anions by their elemental name
and change the ending to “-ide”, thus the anion of chlorine is chloride, and the anion of sulfur is sulfide.
Scientists refer to this movement of electrons from one element to another as electron transfer. As Figure 2.11
illustrates, sodium (Na) only has one electron in its outer electron shell. It takes less energy for sodium to donate
that one electron than it does to accept seven more electrons to fill the outer shell. If sodium loses an electron,
it now has 11 protons, 11 neutrons, and only 10 electrons, leaving it with an overall charge of +1. We now refer
to it as a sodium ion. Chlorine (Cl) in its lowest energy state (called the ground state) has seven electrons in its
outer shell. Again, it is more energy-efficient for chlorine to gain one electron than to lose seven. Therefore, it
tends to gain an electron to create an ion with 17 protons, 17 neutrons, and 18 electrons, giving it a net negative
(-1) charge. We now refer to it as a chloride ion. In this example, sodium will donate its one electron to empty its
shell, and chlorine will accept that electron to fill its shell. Both ions now satisfy the octet rule and have complete
outermost shells. Because the number of electrons is no longer equal to the number of protons, each is now an
46
Chapter 2 | The Chemical Foundation of Life
ion and has a +1 (sodium cation) or -1 (chloride anion) charge. Note that these transactions can normally only
take place simultaneously: in order for a sodium atom to lose an electron, it must be in the presence of a suitable
recipient like a chlorine atom.
Figure 2.11 In the formation of an ionic compound, metals lose electrons and nonmetals gain electrons to achieve an
octet.
Ionic bonds form between ions with opposite charges. For instance, positively charged sodium ions and
negatively charged chloride ions bond together to make crystals of sodium chloride, or table salt, creating a
crystalline molecule with zero net charge.
Physiologists refer to certain salts as electrolytes (including sodium, potassium, and calcium), ions necessary
for nerve impulse conduction, muscle contractions, and water balance. Many sports drinks and dietary
supplements provide these ions to replace those lost from the body via sweating during exercise.
Covalent Bonds and Other Bonds and Interactions
Another way to satisfy the octet rule by sharing electrons between atoms to form covalent bonds. These bonds
are stronger and much more common than ionic bonds in the molecules of living organisms. We commonly
find covalent bonds in carbon-based organic molecules, such as our DNA and proteins. We also find covalent
bonds in inorganic molecules like H 2 O, CO 2 , and O 2 . The bonds may share one, two, or three pairs of electrons,
making single, double, and triple bonds, respectively. The more covalent bonds between two atoms, the stronger
their connection. Thus, triple bonds are the strongest.
The strength of different levels of covalent bonding is one of the main reasons living organisms have a difficult
time in acquiring nitrogen for use in constructing their molecules, even though molecular nitrogen, N 2 , is the
most abundant gas in the atmosphere. Molecular nitrogen consists of two nitrogen atoms triple bonded to each
other and, as with all molecules, sharing these three pairs of electrons between the two nitrogen atoms allows
for filling their outer electron shells, making the molecule more stable than the individual nitrogen atoms. This
strong triple bond makes it difficult for living systems to break apart this nitrogen in order to use it as constituents
of proteins and DNA.
Forming water molecules provides an example of covalent bonding. Covalent bonds bind the hydrogen and
oxygen atoms that combine to form water molecules as Figure 2.9 shows. The electron from the hydrogen splits
its time between the hydrogen atoms' incomplete outer shell and the oxygen atoms' incomplete outer shell. To
completely fill the oxygen's outer shell, which has six electrons but which would be more stable with eight, two
electrons (one from each hydrogen atom) are needed: hence, the well-known formula H 2 O. The two elements
share the electrons to fill the outer shell of each, making both elements more stable.
LINK TQ LEARNING
View this short video to see an animation of ionic and covalent bonding. (This multimedia resource will open
in a browser.) (http://cnx.Org/content/m66430/l.3/#eip-idll66283807593)
Polar Covalent Bonds
There are two types of covalent bonds: polar and nonpolar. In a polar covalent bond, Figure 2.12 shows
atoms unequally share the electrons and are attracted more to one nucleus than the other. Because of the
unequal electron distribution between the atoms of different elements, a slightly positive (5+) or slightly negative
(5-) charge develops. This partial charge is an important property of water and accounts for many of its
characteristics.
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Chapter 2 | The Chemical Foundation of Life
47
Water is a polar molecule, with the hydrogen atoms acquiring a partial positive charge and the oxygen a partial
negative charge. This occurs because the oxygen atom's nucleus is more attractive to the hydrogen atoms'
electrons than the hydrogen nucleus is to the oxygen’s electrons. Thus, oxygen has a higher electronegativity
than hydrogen and the shared electrons spend more time near the oxygen nucleus than the hydrogen atoms'
nucleus, giving the oxygen and hydrogen atoms slightly negative and positive charges, respectively. Another
way of stating this is that the probability of finding a shared electron near an oxygen nucleus is more likely than
finding it near a hydrogen nucleus. Either way, the atom’s relative electronegativity contributes to developing
partial charges whenever one element is significantly more electronegative than the other, and the charges that
these polar bonds generate may then be used to form hydrogen bonds based on the attraction of opposite
partial charges. (Hydrogen bonds, which we discuss in detail below, are weak bonds between slightly positively
charged hydrogen atoms to slightly negatively charged atoms in other molecules.) Since macromolecules often
have atoms within them that differ in electronegativity, polar bonds are often present in organic molecules.
Nonpolar Covalent Bonds
Nonpolar covalent bonds form between two atoms of the same element or between different elements that
share electrons equally. For example, molecular oxygen (O 2 ) is nonpolar because the electrons distribute
equally between the two oxygen atoms.
Figure 2.12 also shows another example of a nonpolar covalent bond—methane (CH 4 ). Carbon has four
electrons in its outermost shell and needs four more to fill it. It obtains these four from four hydrogen atoms, each
atom providing one, making a stable outer shell of eight electrons. Carbon and hydrogen do not have the same
electronegativity but are similar; thus, nonpolar bonds form. The hydrogen atoms each need one electron for
their outermost shell, which is filled when it contains two electrons. These elements share the electrons equally
among the carbons and the hydrogen atoms, creating a nonpolar covalent molecule.
Bond type
Molecular shape
Molecular type
Water
Polar covalent
8+^ ^8+
V
8-
Bent
Polar
Methane
Nonpolar covalent
A @
Tetrahedral
Nonpolar
Carbon
dioxide
8 "© = ® 8+
Polar covalent
0 -®-©
Linear
Nonpolar
Figure 2.12 Whether a molecule is polar or nonpolar depends both on bond type and molecular shape. Both water and
carbon dioxide have polar covalent bonds, but carbon dioxide is linear, so the partial charges on the molecule cancel
each other out.
Hydrogen Bonds and Van Der Waals Interactions
Ionic and covalent bonds between elements require energy to break. Ionic bonds are not as strong as covalent,
which determines their behavior in biological systems. However, not all bonds are ionic or covalent bonds.
Weaker bonds can also form between molecules. Two weak bonds that occur frequently are hydrogen bonds
and van der Waals interactions. Without these two types of bonds, life as we know it would not exist. Hydrogen
bonds provide many of the critical, life-sustaining properties of water and also stabilize the structures of proteins
and DNA, the building block of cells.
When polar covalent bonds containing hydrogen form, the hydrogen in that bond has a slightly positive charge
because hydrogen’s electron is pulled more strongly toward the other element and away from the hydrogen.
Because the hydrogen is slightly positive, it will be attracted to neighboring negative charges. When this
happens, a weak interaction occurs between the hydrogen's 5 + from one molecule and another molecule's 5-
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Chapter 2 | The Chemical Foundation of Life
charge on the more electronegative atoms, usually oxygen or nitrogen, or within the same molecule. Scientists
call this interaction a hydrogen bond. This type of bond is common and occurs regularly between water
molecules. Individual hydrogen bonds are weak and easily broken; however, they occur in very large numbers
in water and in organic polymers, creating a major force in combination. Hydrogen bonds are also responsible
for zipping together the DNA double helix.
Like hydrogen bonds, van der Waals interactions are weak attractions or interactions between molecules. Van
der Waals attractions can occur between any two or more molecules and are dependent on slight fluctuations
of the electron densities, which are not always symmetrical around an atom. For these attractions to happen,
the molecules need to be very close to one another. These bonds—along with ionic, covalent, and hydrogen
bonds—contribute to the proteins' three-dimensional structure in our cells that is necessary for their proper
function.
ca eer connection
Pharmaceutical Chemist
Pharmaceutical chemists are responsible for developing new drugs and trying to determine the mode of
action of both old and new drugs. They are involved in every step of the drug development process. We can
find drugs in the natural environment or we can synthesize them in the laboratory. In many cases, chemists
chemically change potential drugs from nature chemically in the laboratory to make them safer and more
effective, and sometimes synthetic versions of drugs substitute for the version we find in nature.
After a drug's initial discovery or synthesis, the chemist then develops the drug, perhaps chemically altering
it, testing it to see if it is toxic, and then designing methods for efficient large-scale production. Then, the
process of approving the drug for human use begins. In the United States, the Food and Drug Administration
(FDA) handles drug approval. This involves a series of large-scale experiments using human subjects to
ensure the drug is not harmful and effectively treats the condition for which it is intended. This process often
takes several years and requires the participation of physicians and scientists, in addition to chemists, to
complete testing and gain approval.
An example of a drug that was originally discovered in a living organism is Paclitaxel (Taxol), an anti-cancer
drug used to treat breast cancer. This drug was discovered in the bark of the pacific yew tree. Another
example is aspirin, originally isolated from willow tree bark. Finding drugs often means testing hundreds of
samples of plants, fungi, and other forms of life to see if they contain any biologically active compounds.
Sometimes, traditional medicine can give modern medicine clues as to where to find an active compound.
For example, mankind has used willow bark to make medicine for thousands of years, dating back to ancient
Egypt. However, it was not until the late 1800s that scientists and pharmaceutical companies purified and
marketed the aspirin molecule, acetylsalicylic acid, for human use.
Occasionally, drugs developed for one use have unforeseen effects that allow usage in other, unrelated
ways. For example, scientists originally developed the drug minoxidil (Rogaine) to treat high blood pressure.
When tested on humans, researchers noticed that individuals taking the drug would grow new hair.
Eventually the pharmaceutical company marketed the drug to men and women with baldness to restore lost
hair.
A pharmaceutical chemist's career may involve detective work, experimentation, and drug development, all
with the goal of making human beings healthier.
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Chapter 2 | The Chemical Foundation of Life
49
2.2 | Water
By the end of this section, you will be able to do the following:
• Describe the properties of water that are critical to maintaining life
• Explain why water is an excellent solvent
• Provide examples of water’s cohesive and adhesive properties
• Discuss the role of acids, bases, and buffers in homeostasis
Why do scientists spend time looking for water on other planets? Why is water so important? It is because water
is essential to life as we know it. Water is one of the more abundant molecules and the one most critical to life
on Earth. Water comprises approximately 60-70 percent of the human body. Without it, life as we know it simply
would not exist.
The polarity of the water molecule and its resulting hydrogen bonding make water a unique substance with
special properties that are intimately tied to the processes of life. Life originally evolved in a watery environment,
and most of an organism’s cellular chemistry and metabolism occur inside the watery contents of the cell’s
cytoplasm. Special properties of water are its high heat capacity and heat of vaporization, its ability to dissolve
polar molecules, its cohesive and adhesive properties, and its dissociation into ions that leads to generating pH.
Understanding these characteristics of water helps to elucidate its importance in maintaining life.
Water’s Polarity
One of water’s important properties is that it is composed of polar molecules: the hydrogen and oxygen within
water molecules (H 2 O) form polar covalent bonds. While there is no net charge to a water molecule, water's
polarity creates a slightly positive charge on hydrogen and a slightly negative charge on oxygen, contributing
to water’s properties of attraction. Water generates charges because oxygen is more electronegative than
hydrogen, making it more likely that a shared electron would be near the oxygen nucleus than the hydrogen
nucleus, thus generating the partial negative charge near the oxygen.
As a result of water’s polarity, each water molecule attracts other water molecules because of the opposite
charges between water molecules, forming hydrogen bonds. Water also attracts or is attracted to other polar
molecules and ions. We call a polar substance that interacts readily with or dissolves in water hydrophilic
(hydro- = “water”; -philic = “loving"). In contrast, nonpolar molecules such as oils and fats do not interact well with
water, as Figure 2.13 shows. A good example of this is vinegar and oil salad dressing (an acidic water solution).
We call such nonpolar compounds hydrophobic (hydro- = “water”; -phobic = “fearing”).
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Chapter 2 | The Chemical Foundation of Life
Figure 2.13 Oil and water do not mix. As this macro image of oil and water shows, oil does not dissolve in water but
forms droplets instead. This is because it is a nonpolar compound, (credit: Gautam Dogra).
Water’s States: Gas, Liquid, and Solid
The formation of hydrogen bonds is an important quality of the liquid water that is crucial to life as we know it. As
water molecules make hydrogen bonds with each other, water takes on some unique chemical characteristics
compared to other liquids and, since living things have a high water content, understanding these chemical
features is key to understanding life. In liquid water, hydrogen bonds constantly form and break as the water
molecules slide past each other. The water molecules' motion (kinetic energy) causes the bonds to break due to
the heat contained in the system. When the heat rises as water boils, the water molecules' higher kinetic energy
causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas (steam
or water vapor). Alternatively, when water temperature reduces and water freezes, the water molecules form a
crystalline structure maintained by hydrogen bonding (there is not enough energy to break the hydrogen bonds)
that makes ice less dense than liquid water, a phenomenon that we do not see when other liquids solidify.
Water’s lower density in its solid form is due to the way hydrogen bonds orient as they freeze: the water
molecules push farther apart compared to liquid water. With most other liquids, solidification when the
temperature drops includes lowering kinetic energy between molecules, allowing them to pack even more tightly
than in liquid form and giving the solid a greater density than the liquid.
The lower density of ice, as Figure 2.14 depicts, an anomaly causes it to float at the surface of liquid water, such
as in an iceberg or ice cubes in a glass of water. In lakes and ponds, ice will form on the water's surface creating
an insulating barrier that protects the animals and plant life in the pond from freezing. Without this insulating
ice layer, plants and animals living in the pond would freeze in the solid block of ice and could not survive. The
expansion of ice relative to liquid water causes the detrimental effect of freezing on living organisms. The ice
crystals that form upon freezing rupture the delicate membranes essential for living cells to function, irreversibly
damaging them. Cells can only survive freezing if another liquid like glycerol temporarily replaces the water in
them.
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51
Figure 2.14 Hydrogen bonding makes ice less dense than liquid water. The (a) lattice structure of ice makes it less
dense than the liquid water’s freely flowing molecules, enabling it to (b) float on water, (credit a: modification of work
by Jane Whitney, image created using Visual Molecular Dynamics (VMD) software ; credit b: modification of work by
Carlos Ponte)
LINK TQ LEARNING
Click here (http://openstaxcollege.Org/l/ice_lattice2) to see a 3-D animation of an ice lattice structure.
(Image credit: Jane Whitney. Image created using Visual Molecular Dynamics VMD software. )
Water’s High Heat Capacity
Water’s high heat capacity is a property that hydrogen bonding among water molecules causes. Water has the
highest specific heat capacity of any liquids. We define specific heat as the amount of heat one gram of a
substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one
calorie. It therefore takes water a long time to heat and a long time to cool. In fact, water's specific heat capacity
is about five times more than that of sand. This explains why the land cools faster than the sea. Due to its
high heat capacity, warm blooded animals use water to more evenly disperse heat in their bodies: it acts in a
similar manner to a car’s cooling system, transporting heat from warm places to cool places, causing the body
to maintain a more even temperature.
Water’s Heat of Vaporization
Water also has a high heat of vaporization, the amount of energy required to change one gram of a liquid
substance to a gas. A considerable amount of heat energy (586 cal) is required to accomplish this change in
water. This process occurs on the water's surface. As liquid water heats up, hydrogen bonding makes it difficult
to separate the liquid water molecules from each other, which is required for it to enter its gaseous phase
(steam). As a result, water acts as a heat sink or heat reservoir and requires much more heat to boil than
does a liquid such as ethanol (grain alcohol), whose hydrogen bonding with other ethanol molecules is weaker
than water’s hydrogen bonding. Eventually, as water reaches its boiling point of 100° Celsius (212° Fahrenheit),
the heat is able to break the hydrogen bonds between the water molecules, and the kinetic energy (motion)
between the water molecules allows them to escape from the liquid as a gas. Even when below its boiling point,
water’s individual molecules acquire enough energy from other water molecules such that some surface water
molecules can escape and vaporize: we call this process evaporation.
The fact that hydrogen bonds need to be broken for water to evaporate means that bonds use a substantial
amount of energy in the process. As the water evaporates, energy is taken up by the process, cooling
the environment where the evaporation is taking place. In many living organisms, including in humans, the
1. W. Humphrey W., A. Dalke, and K. Schulten, “VMD—Visual Molecular Dynamics,” Journal of Molecular Graphics 14 (1996): 33-38.
2. W. Humphrey W., A. Dalke, and K. Schulten, “VMD—Visual Molecular Dynamics,” Journal of Molecular Graphics 14 (1996): 33-38.
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Chapter 2 | The Chemical Foundation of Life
evaporation of sweat, which is 90 percent water, allows the organism to cool so that it can maintain homeostasis
of body temperature.
Water’s Solvent Properties
Since water is a polar molecule with slightly positive and slightly negative charges, ions and polar molecules can
readily dissolve in it. Therefore, we refer to water as a solvent, a substance capable of dissolving other polar
molecules and ionic compounds. The charges associated with these molecules will form hydrogen bonds with
water, surrounding the particle with water molecules. We refer to this as a sphere of hydration, or a hydration
shell, as Figure 2.15 illustrates and serves to keep the particles separated or dispersed in the water.
When we add ionic compounds to water, the individual ions react with the water molecules' polar regions and
their ionic bonds are disrupted in the process of dissociation. Dissociation occurs when atoms or groups of
atoms break off from molecules and form ions. Consider table salt (NaCI, or sodium chloride): when we add NaCI
crystals to water, the NaCI molecules dissociate into Na + and CP ions, and spheres of hydration form around
the ions, as Figure 2.15 illustrates. The partially negative charge of the water molecule’s oxygen surrounds the
positively charged sodium ion. The hydrogen's partially positive charge on the water molecule surrounds the
negatively charged chloride ion.
Figure 2.15 When we mix table salt (NaCI) in water, it forms spheres of hydration around the ions.
Water’s Cohesive and Adhesive Properties
Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows,
the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because
of the property of cohesion. In cohesion, water molecules are attracted to each other (because of hydrogen
bonding), keeping the molecules together at the liquid-gas (water-air) interface, although there is no more room
in the glass.
Cohesion allows for surface tension, the capacity of a substance to withstand rupturing when placed under
tension or stress. This is also why water forms droplets when on a dry surface rather than flattening by gravity.
When we place a small scrap of paper onto a water droplet, the paper floats on top even though paper is denser
(heavier) than the water. Cohesion and surface tension keep the water molecules' hydrogen bonds intact and
support the item floating on the top. It’s even possible to “float” a needle on top of a glass of water if you place it
gently without breaking the surface tension, as Figure 2.16 shows.
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53
Figure 2.16 A needle's weight pulls the surface downward. At the same time, the surface tension pulls it up,
suspending it on the water's surface preventing it from sinking. Notice the indentation in the water around the needle,
(credit: Cory Zanker)
These cohesive forces are related to water’s property of adhesion, or the attraction between water molecules
and other molecules. This attraction is sometimes stronger than water’s cohesive forces, especially when the
water is exposed to charged surfaces such as those on the inside of thin glass tubes known as capillary tubes.
We observe adhesion when water “climbs” up the tube placed in a glass of water: notice that the water appears
to be higher on the tube's sides than in the middle. This is because the water molecules are attracted to the
capillary's charged glass walls more than they are to each other and therefore adhere to it. We call this type of
adhesion capillary action, as Figure 2.17 illustrates.
Figure 2.17 The adhesive forces exerted by the glass' internal surface exceeding the cohesive forces between the
water molecules themselves causes capillary action in a glass tube, (credit: modification of work by Pearson-Scott
Foresman, donated to the Wikimedia Foundation)
Why are cohesive and adhesive forces important for life? Cohesive and adhesive forces are important for
transporting water from the roots to the leaves in plants. These forces create a “pull” on the water column. This
pull results from the tendency of water molecules evaporating on the plant's surface to stay connected to water
molecules below them, and so they are pulled along. Plants use this natural phenomenon to help transport water
from their roots to their leaves. Without these properties of water, plants would be unable to receive the water
and the dissolved minerals they require. In another example, insects such as the water strider, as Figure 2.18
shows, use the water's surface tension to stay afloat on the water's surface layer and even mate there.
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Chapter 2 | The Chemical Foundation of Life
Figure 2.18 Water’s cohesive and adhesive properties allow this water strider (Gems sp.) to stay afloat, (credit: Tim
Vickers)
pH, Buffers, Acids, and Bases
The pH of a solution indicates its acidity or alkalinity.
h 2 o(i) H + (a q) + OH (aq)
You may have used litmus or pH paper, filter paper treated with a natural water-soluble dye for use as a pH
indicator, tests how much acid (acidity) or base (alkalinity) exists in a solution. You might have even used some
to test whether the water in a swimming pool is properly treated. In both cases, the pH test measures hydrogen
ions' concentration in a given solution.
Hydrogen ions spontaneously generate in pure water by the dissociation (ionization) of a small percentage of
water molecules into equal numbers of hydrogen (H + ) ions and hydroxide (OH") ions. While the hydroxide ions
are kept in solution by their hydrogen bonding with other water molecules, the hydrogen ions, consisting of naked
protons, immediately attract to un-ionized water molecules, forming hydronium ions (H 30 + ). Still, by convention,
scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.
The concentration of hydrogen ions dissociating from pure water is 1 x 10" 7 moles H + ions per liter of water.
Moles (mol) are a way to express the amount of a substance (which can be atoms, molecules, ions, etc.).
One mole represents the atomic weight of a substance, expressed in grams, which equals the amount of the
substance containing as many units as there are atoms in 12 grams of 12 C. Mathematically, one mole is equal
qq qq
to 6.02 x io particles of the substance. Therefore, 1 mole of water is equal to 6.02 x io water molecules.
We calculate the pH as the negative of the base 10 logarithm of this concentration. The loglO of 1 x io -7 is -7.0,
and the negative of this number (indicated by the “p” of “pH") yields a pH of 7.0, which is also a neutral pH. The
pH inside of human cells and blood are examples of two body areas where near-neutral pH is maintained.
Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate
positive integers, high concentrations of hydrogen ions yield a low pH number; whereas, low levels of hydrogen
ions result in a high pH. An acid is a substance that increases hydrogen ions' (H + ) concentration in a solution,
usually by having one of its hydrogen atoms dissociate. A base provides either hydroxide ions (OH - ) or other
negatively charged ions that combine with hydrogen ions, reducing their concentration in the solution and
thereby raising the pH. in cases where the base releases hydroxide ions, these ions bind to free hydrogen ions,
generating new water molecules.
The stronger the acid, the more readily it donates H + . For example, hydrochloric acid (HCI) completely
dissociates into hydrogen and chloride ions and is highly acidic; whereas the acids in tomato juice or vinegar
do not completely dissociate and are weak acids. Conversely, strong bases are those substances that readily
donate OH - or take up hydrogen ions. Sodium hydroxide (NaOH) and many household cleaners are highly
alkaline and give up OH - rapidly when we place them in water, thereby raising the pH. An example of a weak
basic solution is seawater, which has a pH near 8.0 This is close enough to a neutral pH that marine organisms
have adapted in order to live and thrive in a saline environment.
The pH scale is, as we previously mentioned, an inverse logarithm and ranges from 0 to 14 (Figure 2.19).
Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline.
Extremes in pH in either direction from 7.0 are usually inhospitable to life. The pH inside cells (6.8) and the pH in
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Chapter 2 | The Chemical Foundation of Life
55
the blood (7.4) are both very close to neutral. However, the environment in the stomach is highly acidic, with a pH
of 1 to 2. As a result, how do stomach cells survive in such an acidic environment? How do they homeostatically
maintain the near neutral pH inside them? The answer is that they cannot do it and are constantly dying. The
stomach constantly produces new cells to replace dead ones, which stomach acids digest. Scientists estimate
that the human body completely replaces the stomach lining every seven to ten days.
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Figure 2.19 The pH scale measures hydrogen ions' (H + )
Edward Stevens)
Bleach
Soapy water
Ammonia solution
Milk of magnesia
Baking soda
Sea water
Distilled water
Urine
Black coffee
Tomato juice
Orange juice
Lemon juice
Gastric acid
concentration in a solution, (credit: modification of work by
LINK TQ LEARNING
Watch this video for a straightforward explanation of pH and its logarithmic scale. (This multimedia
resource will open in a browser..) (http://cnx.Org/content/m66434/l.3/#eip-idll70503069363)
How can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human
drinking orange juice, for example) and survive? Buffers are the key. Buffers readily absorb excess H + or OH - ,
keeping the body's pH carefully maintained in the narrow range required for survival. Maintaining a constant
blood pH is critical to a person’s well-being. The buffer maintaining the pH of human blood involves carbonic
acid (H 2 CO 3 ), bicarbonate ion (HCO 3 - ), and carbon dioxide (CO 2 ). When bicarbonate ions combine with free
hydrogen ions and become carbonic acid, it removes hydrogen ions and moderates pH changes. Similarly, as
Figure 2.20 shows, excess carbonic acid can convert to carbon dioxide gas which we exhale through the lungs.
This prevents too many free hydrogen ions from building up in the blood and dangerously reducing the blood’s
pH. Likewise, if too much OH - enters into the system, carbonic acid will combine with it to create bicarbonate,
lowering the pH. Without this buffer system, the body’s pH would fluctuate enough to put survival in jeopardy.
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Chapter 2 | The Chemical Foundation of Life
H + + HC0 3 ~
H 2 C0 3
h 2 o + co 2
Figure 2.20 This diagram shows the body’s buffering of blood pH levels. The blue arrows show the process of raising
pH as more CO 2 is made. The purple arrows indicate the reverse process: the lowering of pH as more bicarbonate is
created.
Other examples of buffers are antacids that some people use to combat excess stomach acid. Many of these
over-the-counter medications work in the same way as blood buffers, usually with at least one ion capable of
absorbing hydrogen and moderating pH, bringing relief to those who suffer “heartburn” after eating. Water's
unique properties that contribute to this capacity to balance pH—as well as water’s other characteristics—are
essential to sustaining life on Earth.
LINK TQ LEARNING
To learn more about water, visit the U.S. Geological Survey Water Science for Schools
(http:// 0 penstaxc 0 llege. 0 rg/l/all_ab 0 ut_water) All About Water! website.
2.3 | Carbon
By the end of this section, you will be able to do the following:
• Explain why carbon is important for life
• Describe the role of functional groups in biological molecules
Many complex molecules called macromolecules, such as proteins, nucleic acids (RNA and DNA),
carbohydrates, and lipids comprise cells. The macromolecules are a subset of organic molecules (any carbon-
containing liquid, solid, or gas) that are especially important for life. The fundamental component for all of these
macromolecules is carbon. The carbon atom has unique properties that allow it to form covalent bonds to as
many as four different atoms, making this versatile element ideal to serve as the basic structural component, or
“backbone,” of the macromolecules.
Individual carbon atoms have an incomplete outermost electron shell. With an atomic number of 6 (six electrons
and six protons), the first two electrons fill the inner shell, leaving four in the second shell. Therefore, carbon
atoms can form up to four covalent bonds with other atoms to satisfy the octet rule. The methane molecule
provides an example: it has the chemical formula CH 4 . Each of its four hydrogen atoms forms a single covalent
bond with the carbon atom by sharing a pair of electrons. This results in a filled outermost shell.
Hydrocarbons
Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen, such as methane (CH 4 )
described above. We often use hydrocarbons in our daily lives as fuels—like the propane in a gas grill or
the butane in a lighter. The many covalent bonds between the atoms in hydrocarbons store a great amount
of energy, which releases when these molecules burn (oxidize). Methane, an excellent fuel, is the simplest
hydrocarbon molecule, with a central carbon atom bonded to four different hydrogen atoms, as Figure 2.21
illustrates. The shape of its electron orbitals determines the shape of the methane molecule's geometry, where
the atoms reside in three dimensions. The carbons and the four hydrogen atoms form a tetrahedron, with four
triangular faces. For this reason, we describe methane as having tetrahedral geometry.
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Chapter 2 | The Chemical Foundation of Life
57
Figure 2.21 Methane has a tetrahedral geometry, with each of the four hydrogen atoms spaced 109.5° apart.
As the backbone of the large molecules of living things, hydrocarbons may exist as linear carbon chains, carbon
rings, or combinations of both. Furthermore, individual carbon-to-carbon bonds may be single, double, or triple
covalent bonds, and each type of bond affects the molecule's geometry in a specific way. This three-dimensional
shape or conformation of the large molecules of life (macromolecules) is critical to how they function.
Hydrocarbon Chains
Successive bonds between carbon atoms form hydrocarbon chains. These may be branched or unbranched.
Furthermore, a molecule's different geometries of single, double, and triple covalent bonds alter the overall
molecule's geometry as Figure 2.22 illustrates. The hydrocarbons ethane, ethene, and ethyne serve as
examples of how different carbon-to-carbon bonds affect the molecule's geometry. The names of all three
molecules start with the prefix “eth-,” which is the prefix for two carbon hydrocarbons. The suffixes “-ane,” “-ene,”
and “-yne" refer to the presence of single, double, or triple carbon-carbon bonds, respectively. Thus, propane,
propene, and propyne follow the same pattern with three carbon molecules, butane, butene, and butyne for
four carbon molecules, and so on. Double and triple bonds change the molecule's geometry: single bonds allow
rotation along the bond's axis; whereas, double bonds lead to a planar configuration and triple bonds to a linear
one. These geometries have a significant impact on the shape a particular molecule can assume.
Methane (CH 4 )
Ethane (C 2 H 6 )
Ethene (C 2 H 4 )
s
Tetrahedral
(single bond)
Tetrahedral
(single bond)
H
Planar
(double bond)
Figure 2.22 When carbon forms single bonds with other atoms, the shape is tetrahedral. When two carbon atoms form
a double bond, the shape is planar, or flat. Single bonds, like those in ethane, are able to rotate. Double bonds, like
those in ethene cannot rotate, so the atoms on either side are locked in place.
Hydrocarbon Rings
So far, the hydrocarbons we have discussed have been aliphatic hydrocarbons, which consist of linear chains
of carbon atoms. Another type of hydrocarbon, aromatic hydrocarbons, consists of closed rings of carbon
atoms. We find ring structures in hydrocarbons, sometimes with the presence of double bonds, which we
can see by comparing cyclohexane's structure to benzene in Figure 2.23. Examples of biological molecules
that incorporate the benzene ring include some amino acids and cholesterol and its derivatives, including the
hormones estrogen and testosterone. We also find the benzene ring in the herbicide 2,4-D. Benzene is a natural
component of crude oil and has been classified as a carcinogen. Some hydrocarbons have both aliphatic and
aromatic portions. Beta-carotene is an example of such a hydrocarbon.
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Chapter 2 | The Chemical Foundation of Life
Cyclopentane Cyclohexane
Benzene
Pyridine
Figure 2.23 Carbon can form five- and six-membered rings. Single or double bonds may connect the carbons in the
ring, and nitrogen may be substituted for carbon.
Isomers
The three-dimensional placement of atoms and chemical bonds within organic molecules is central to
understanding their chemistry. We call molecules that share the same chemical formula but differ in the
placement (structure) of their atoms and/or chemical bonds isomers. Structural isomers (like butane and
isobutene in Figure 2.24a) differ in the placement of their covalent bonds: both molecules have four carbons
and ten hydrogens (C 4 H 10 ), but the different atom arrangement within the molecules leads to differences in their
chemical properties. For example, butane is suited for use as a fuel for cigarette lighters and torches; whereas,
isobutene is suited for use as a refrigerant and a propellant in spray cans.
Geometric isomers, alternatively have similar placements of their covalent bonds but differ in how these bonds
are made to the surrounding atoms, especially in carbon-to-carbon double bonds. In the simple molecule butene
(C 4 H 8 ), the two methyl groups (CH 3 ) can be on either side of the double covalent bond central to the molecule,
as Figure 2.24b illustrates. When the carbons are bound on the same side of the double bond, this is the
cis configuration. If they are on opposite sides of the double bond, it is a trans configuration. In the trans
configuration, the carbons form a more or less linear structure; whereas, the carbons in the cis configuration
make a bend (change in direction) of the carbon backbone.
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Chapter 2 | The Chemical Foundation of Life
59
visual
CONNECTION
(a) Structural isomers
Butane
Isobutane
H H H H
I I I I
H-C-C-C-C-H
I I I I
H H H H
H H H
I I I
H—C-C—C—H
H-C-H
I
H
(b) Geometric isomers
c/s-2-butene
frans-2-butene
H H
\ /
C = C
/ \
H 3 c ch 3
methyl groups on
same side of double bond
H CH,
\ /
C = C
/ \
3 C H
methyl groups on opposite
sides of double bond
(c) Enantiomers
L-isomer
D-isomer
Figure 2.24 We call molecules that have the same number and type of atoms arranged differently isomers,
(a) Structural isomers have a different covalent arrangement of atoms, (b) Geometric isomers have a different
arrangement of atoms around a double bond, (c) Enantiomers are mirror images of each other.
Which of the following statements is false?
a. Molecules with the formulas CH 3 CH 2 COOH and C 3 H 6 O 2 could be structural isomers.
b. Molecules must have a double bond to be cis-trans isomers.
c. To be enantiomers, a molecule must have at least three different atoms or groups connected to a
central carbon.
d. To be enantiomers, a molecule must have at least four different atoms or groups connected to a central
carbon.
In triglycerides (fats and oils), long carbon chains known as fatty acids may contain double bonds, which can be
in either the cis or trans configuration, as Figure 2.25 illustrates. Fats with at least one double bond between
carbon atoms are unsaturated fats. When some of these bonds are in the cis configuration, the resulting bend in
the chain's carbon backbone means that triglyceride molecules cannot pack tightly, so they remain liquid (oil) at
room temperature. Alternatively, triglycerides with trans double bonds (popularly called trans fats), have relatively
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Chapter 2 | The Chemical Foundation of Life
linear fatty acids that are able to pack tightly together at room temperature and form solid fats. In the human diet,
trans fats are linked to an increased risk of cardiovascular disease, so many food manufacturers have reduced
or eliminated their use in recent years. In contrast to unsaturated fats, we call triglycerides without double bonds
between carbon atoms saturated fats, meaning that they contain all the hydrogen atoms available. Saturated
fats are a solid at room temperature and usually of animal origin.
Eliadic acid
Oleic acid
Figure 2.25 These space-filling models show a c/s (oleic acid) and a trans (eliadic acid) fatty acid. Notice the bend in
the molecule caused by the cis configuration.
Enantiomers
Enantiomers are molecules that share the same chemical structure and chemical bonds but differ in the three-
dimensional placement of atoms so that they are non-superimposable mirror images. Figure 2.26 shows an
amino acid alanine example, where the two structures are nonsuperimposable. In nature, only the L-forms of
amino acids make proteins. Some D forms of amino acids are seen in the cell walls of bacteria, but never in their
proteins. Similarly, the D-form of glucose is the main product of photosynthesis and we rarely see the molecule's
L-form in nature.
D-alanine L-alanine
Figure 2.26 D-alanine and L-alanine are examples of enantiomers or mirror images. Only the L-forms of amino acids
are used to make proteins.
Functional Groups
Functional groups are groups of atoms that occur within molecules and confer specific chemical properties to
those molecules. We find them along the “carbon backbone" of macromolecules. Chains and/or rings of carbon
atoms with the occasional substitution of an element such as nitrogen or oxygen form this carbon backbone.
Molecules with other elements in their carbon backbone are substituted hydrocarbons.
The functional groups in a macromolecule are usually attached to the carbon backbone at one or several
different places along its chain and/or ring structure. Each of the four types of macromolecules—proteins, lipids,
carbohydrates, and nucleic acids—has its own characteristic set of functional groups that contributes greatly to
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Chapter 2 | The Chemical Foundation of Life
61
its differing chemical properties and its function in living organisms.
A functional group can participate in specific chemical reactions. Figure 2.27 shows some of the important
functional groups in biological molecules. They include: hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate,
and sulfhydryl. These groups play an important role in forming molecules like DNA, proteins, carbohydrates, and
lipids. We usually classify functional groups as hydrophobic or hydrophilic depending on their charge or polarity
characteristics. An example of a hydrophobic group is the nonpolar methyl molecule. Among the hydrophilic
functional groups is the carboxyl group in amino acids, some amino acid side chains, and the fatty acids that
form triglycerides and phospholipids. This carboxyl group ionizes to release hydrogen ions (H + ) from the COOH
group resulting in the negatively charged COO" group. This contributes to the hydrophilic nature of whatever
molecule on which it is found. Other functional groups, such as the carbonyl group, have a partially negatively
charged oxygen atom that may form hydrogen bonds with water molecules, again making the molecule more
hydrophilic.
Functional
Group
Structure
Properties
Hydroxyl
O-H
/
R
Polar
Methyl
R — ch 3
Nonpolar
Carbonyl
o
R-C-R'
Polar
Carboxyl
0
C
/ \
R OH
Charged, ionizes to release H + .
Since carboxyl groups can release
H + ions into solution, they are
considered acidic.
Amino
H
/
R -N
\
H
Charged, accepts H + to form NH 3 *.
Since amino groups can remove
H*from solution, they are
considered basic.
Phosphate
O
R P-OH
\/\„
Charged, ionizes to release H + .
Since phosphate groups can
release H + ions into solution,
they are considered acidic.
Sulfhydryl
R-S
\
H
Polar
Figure 2.27 These functional groups are in many different biological molecules. R, also known as R-group, is an
abbreviation for any group in which a carbon or hydrogen atom is attached to the rest of the molecule.
Hydrogen bonds between functional groups (within the same molecule or between different molecules) are
important to the function of many macromolecules and help them to fold properly into and maintain the
appropriate shape for functioning. Hydrogen bonds are also involved in various recognition processes, such as
DNA complementary base pairing and the binding of an enzyme to its substrate, as Figure 2.28 illustrates.
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Chapter 2 | The Chemical Foundation of Life
Hydrogen bonds
Figure 2.28 Hydrogen bonds connect two strands of DNA together to create the double-helix structure.
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63
KEY TERMS
acid molecule that donates hydrogen ions and increases the concentration of hydrogen ions in a solution
adhesion attraction between water molecules and other molecules
aliphatic hydrocarbon hydrocarbon consisting of a linear chain of carbon atoms
anion negative ion that is formed by an atom gaining one or more electrons
aromatic hydrocarbon hydrocarbon consisting of closed rings of carbon atoms
atom the smallest unit of matter that retains all of the chemical properties of an element
atomic mass calculated mean of the mass number for an element’s isotopes
atomic number total number of protons in an atom
balanced chemical equation statement of a chemical reaction with the number of each type of atom equalized
for both the products and reactants
base molecule that donates hydroxide ions or otherwise binds excess hydrogen ions and decreases the
hydrogen ions' concentration in a solution
buffer substance that prevents a change in pH by absorbing or releasing hydrogen or hydroxide ions
calorie amount of heat required to change the temperature of one gram of water by one degree Celsius
capillary action occurs because water molecules are attracted to charges on the inner surfaces of narrow
tubular structures such as glass tubes, drawing the water molecules to the tubes' sides
cation positive ion that is formed by an atom losing one or more electrons
chemical bond interaction between two or more of the same or different atoms that results in forming molecules
chemical reaction process leading to rearranging atoms in molecules
chemical reactivity the ability to combine and to chemically bond with each other
cohesion intermolecular forces between water molecules caused by the polar nature of water; responsible for
surface tension
compound substance composed of molecules consisting of atoms of at least two different elements
covalent bond type of strong bond formed between two atoms of the same or different elements; forms when
electrons are shared between atoms
dissociation release of an ion from a molecule such that the original molecule now consists of an ion and the
charged remains of the original, such as when water dissociates into H + and OH"
electrolyte ion necessary for nerve impulse conduction, muscle contractions, and water balance
electron negatively charged subatomic particle that resides outside of the nucleus in the electron orbital; lacks
functional mass and has a negative charge of-1 unit
electron configuration arrangement of electrons in an atom’s electron shell (for example, ls 2 2s 2 2p 6 )
electron orbital how electrons are spatially distributed surrounding the nucleus; the area where we are most
likely to find an electron
electron transfer movement of electrons from one element to another; important in creating ionic bonds
electronegativity ability of some elements to attract electrons (often of hydrogen atoms), acquiring partial
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Chapter 2 | The Chemical Foundation of Life
negative charges in molecules and creating partial positive charges on the hydrogen atoms
element one of 118 unique substances that cannot break down into smaller substances; each element has
unique properties and a specified number of protons
enantiomers molecules that share overall structure and bonding patterns, but differ in how the atoms are three
dimensionally placed such that they are mirror images of each other
equilibrium steady state of relative reactant and product concentration in reversible chemical reactions in a
closed system
evaporation change from liquid to gaseous state at a body of water's surface, plant leaves, or an organism's
skin
functional group group of atoms that provides or imparts a specific function to a carbon skeleton
geometric isomer isomer with similar bonding patterns differing in the placement of atoms alongside a double
covalent bond
heat of vaporization of water high amount of energy required for liquid water to turn into water vapor
hydrocarbon molecule that consists only of carbon and hydrogen
hydrogen bond weak bond between slightly positively charged hydrogen atoms and slightly negatively charged
atoms in other molecules
hydrophilic describes ions or polar molecules that interact well with other polar molecules such as water
hydrophobic describes uncharged nonpolar molecules that do not interact well with polar molecules such as
water
inert gas (also, noble gas) element with filled outer electron shell that is unreactive with other atoms
ion atom or chemical group that does not contain equal numbers of protons and electrons
ionic bond chemical bond that forms between ions with opposite charges (cations and anions)
irreversible chemical reaction chemical reaction where reactants proceed unidirectionally to form products
isomers molecules that differ from one another even though they share the same chemical formula
isotope one or more forms of an element that have different numbers of neutrons
law of mass action chemical law stating that the rate of a reaction is proportional to the concentration of the
reacting substances
litmus paper (also, pH paper) filter paper treated with a natural water-soluble dye that changes its color as the
pH of the environment changes in order to use it as a pH indicator
mass number total number of protons and neutrons in an atom
matter anything that has mass and occupies space
molecule two or more atoms chemically bonded together
neutron uncharged particle that resides in an atom's nucleus; has a mass of one amu
noble gas see inert gas
nonpolar covalent bond type of covalent bond that forms between atoms when electrons are shared equally
between them
nucleus core of an atom; contains protons and neutrons
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octet rule rule that atoms are most stable when they hold eight electrons in their outermost shells
orbital region surrounding the nucleus; contains electrons
organic molecule any molecule containing carbon (except carbon dioxide)
periodic table organizational chart of elements indicating each element's atomic number and atomic mass;
provides key information about the elements' properties
pH paper see litmus paper
pH scale scale ranging from zero to 14 that is inversely proportional to the hydrogen ions' concentration in a
solution
polar covalent bond type of covalent bond that forms as a result of unequal electron sharing, resulting in
creating slightly positive and negative charged molecule regions
product molecule that is result of chemical reaction
proton positively charged particle that resides in the atom's nucleus; has a mass of one amu and a charge of
+1
radioisotope isotope that emits radiation comprised of subatomic particles to form more stable elements
reactant molecule that takes part in a chemical reaction
reversible chemical reaction chemical reaction that functions bidirectionally, where products may turn into
reactants if their concentration is great enough
solvent substance capable of dissolving another substance
specific heat capacity the amount of heat one gram of a substance must absorb or lose to change its
temperature by one degree Celsius
sphere of hydration when a polar water molecule surrounds charged or polar molecules thus keeping them
dissolved and in solution
structural isomers molecules that share a chemical formula but differ in the placement of their chemical bonds
substituted hydrocarbon hydrocarbon chain or ring containing an atom of another element in place of one of
the backbone carbons
surface tension tension at the surface of a body of liquid that prevents the molecules from separating; created
by the attractive cohesive forces between the liquid's molecules
valence shell outermost shell of an atom
van der Waals interaction very weak interaction between molecules due to temporary charges attracting
atoms that are very close together
CHAPTER SUMMARY
2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks
Matter is anything that occupies space and has mass. It is comprised of elements. All of the 98 elements that
occur naturally have unique qualities that allow them to combine in various ways to create molecules, which in
turn combine to form cells, tissues, organ systems, and organisms. Atoms, which consist of protons, neutrons,
and electrons, are the smallest units of an element that retain all of the properties of that element. Electrons
can transfer, share, or cause charge disparities between atoms to create bonds, including ionic, covalent, and
hydrogen bonds, as well as van der Waals interactions.
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Chapter 2 | The Chemical Foundation of Life
2.2 Water
Water has many properties that are critical to maintaining life. It is a polar molecule, allowing for forming
hydrogen bonds. Hydrogen bonds allow ions and other polar molecules to dissolve in water. Therefore, water is
an excellent solvent. The hydrogen bonds between water molecules cause the water to have a high heat
capacity, meaning it takes considerable added heat to raise its temperature. As the temperature rises, the
hydrogen bonds between water continually break and form anew. This allows for the overall temperature to
remain stable, although energy is added to the system. Water also exhibits a high heat of vaporization, which is
key to how organisms cool themselves by evaporating sweat. Water’s cohesive forces allow for the property of
surface tension; whereas, we see its adhesive properties as water rises inside capillary tubes. The pH value is
a measure of hydrogen ion concentration in a solution and is one of many chemical characteristics that is highly
regulated in living organisms through homeostasis. Acids and bases can change pH values, but buffers tend to
moderate the changes they cause. These properties of water are intimately connected to the biochemical and
physical processes performed by living organisms, and life would be very different if these properties were
altered, if it could exist at all.
2.3 Carbon
The unique properties of carbon make it a central part of biological molecules. Carbon binds to oxygen,
hydrogen, and nitrogen covalently to form the many molecules important for cellular function. Carbon has four
electrons in its outermost shell and can form four bonds. Carbon and hydrogen can form hydrocarbon chains or
rings. Functional groups are groups of atoms that confer specific properties to hydrocarbon (or substituted
hydrocarbon) chains or rings that define their overall chemical characteristics and function.
VISUAL CONNECTION QUESTIONS
1. Figure 2.3 How many neutrons do carbon-12 and
carbon-13 have, respectively?
2. Figure 2.7 An atom may give, take, or share
electrons with another atom to achieve a full valence
shell, the most stable electron configuration. Looking
at this figure, how many electrons do elements in
group 1 need to lose in order to achieve a stable
electron configuration? How many electrons do
elements in groups 14 and 17 need to gain to
achieve a stable configuration?
3. Figure 2.24 Which of the following statements is
REVIEW QUESTIONS
4. If xenon has an atomic number of 54 and a mass
number of 108, how many neutrons does it have?
a. 54
b. 27
c. 100
d. 108
5. Atoms that vary in the number of neutrons found in
their nuclei are called_.
a. ions
b. neutrons
c. neutral atoms
d. isotopes
6 . Potassium has an atomic number of 19. What is its
electron configuration?
false?
a. Molecules with the formulas CH 3 CH 2 COOH
and C 3 H 6 O 2 could be structural isomers.
b. Molecules must have a double bond to be
cis-trans isomers.
c. To be enantiomers, a molecule must have at
least three different atoms or groups
connected to a central carbon.
d. To be enantiomers, a molecule must have at
least four different atoms or groups
connected to a central carbon.
a. shells 1 and 2 are full, and shell 3 has nine
electrons
b. shells 1, 2 and 3 are full and shell 4 has
three electrons
c. shells 1, 2 and 3 are full and shell 4 has one
electron
d. shells 1, 2 and 3 are full and no other
electrons are present
7. Which type of bond represents a weak chemical
bond?
a. hydrogen bond
b. atomic bond
c. covalent bond
d. nonpolar covalent bond
8 . Which of the following statements is not true?
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Chapter 2 | The Chemical Foundation of Life
67
a.
Water is polar.
b.
Water stabilizes temperature.
c.
Water is essential for life.
d.
Water is the most abundant molecule in the
Earth’s atmosphere.
9. When acids are added to a solution, the pH should
a.
decrease
b.
increase
c.
stay the same
d.
cannot tell without testing
10. We call a molecule that binds up excess
hydrogen ions in a solution a(n)
a.
acid
b.
isotope
c.
base
d.
donator
11. Which of the following statements is true?
CRITICAL THINKING QUESTIONS
14. What makes ionic bonds different from covalent
bonds?
15. Why are hydrogen bonds and van der Waals
interactions necessary for cells?
16. Discuss how buffers help prevent drastic swings
in pH.
a.
Acids and bases cannot mix together.
b.
Acids and bases will neutralize each other.
c.
Acids, but not bases, can change the pH of
a solution.
d.
Acids donate hydroxide ions (OH - ); bases
donate hydrogen ions (H + ).
12. Each carbon molecule can bond with as many
as
other atom(s) or molecule(s).
a.
one
b.
two
c.
six
d.
four
13. Which of the following is not a functional group
that can bond with carbon?
a.
sodium
b.
hydroxyl
c.
phosphate
d.
carbonyl
17. Why can some insects walk on water?
18. What property of carbon makes it essential for
organic life?
19. Compare and contrast saturated and unsaturated
triglycerides.
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Chapter 2 | The Chemical Foundation of Life
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Chapter 3 | Biological Macromolecules
69
3 | BIOLOGICAL
MACROMOLECULES
Figure 3.1 Foods such as bread, fruit, and cheese are rich sources of biological macromolecules, (credit: modification
of work by Bengt Nyman)
Chapter Outline
3.1: Synthesis of Biological Macromolecules
3.2: Carbohydrates
3.3: Lipids
3.4: Proteins
3.5: Nucleic Acids
Introduction
Food provides the body with the nutrients it needs to survive. Many of these critical nutrients are biological
macromolecules, or large molecules, necessary for life. Different smaller organic molecule (monomer)
combinations build these macromolecules (polymers). What specific biological macromolecules do living things
require? How do these molecules form? What functions do they serve? We explore these questions in this
chapter.
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Chapter 3 | Biological Macromolecules
3.1 1 Synthesis of Biological Macromolecules
By the end of this section, you will be able to do the following:
• Understand macromolecule synthesis
• Explain dehydration (or condensation) and hydrolysis reactions
As you’ve learned, biological macromolecules are large molecules, necessary for life, that are built from
smaller organic molecules. There are four major biological macromolecule classes (carbohydrates, lipids,
proteins, and nucleic acids). Each is an important cell component and performs a wide array of functions.
Combined, these molecules make up the majority of a cell’s dry mass (recall that water makes up the majority of
its complete mass). Biological macromolecules are organic, meaning they contain carbon. In addition, they may
contain hydrogen, oxygen, nitrogen, and additional minor elements.
Dehydration Synthesis
Most macromolecules are made from single subunits, or building blocks, called monomers. The monomers
combine with each other using covalent bonds to form larger molecules known as polymers. In doing so,
monomers release water molecules as byproducts. This type of reaction is dehydration synthesis, which
means “to put together while losing water.”
Figure 3.2 In the dehydration synthesis reaction above, two glucose molecules link to form the disaccharide maltose.
In the process, it forms a water molecule.
In a dehydration synthesis reaction (Figure 3.2), the hydrogen of one monomer combines with the hydroxyl
group of another monomer, releasing a water molecule. At the same time, the monomers share electrons and
form covalent bonds. As additional monomers join, this chain of repeating monomers forms a polymer. Different
monomer types can combine in many configurations, giving rise to a diverse group of macromolecules. Even
one kind of monomer can combine in a variety of ways to form several different polymers. For example, glucose
monomers are the constituents of starch, glycogen, and cellulose.
Hydrolysis
Polymers break down into monomers during hydrolysis. A chemical reaction occurs when inserting a water
molecule across the bond. Breaking a covalent bond with this water molecule in the compound achieves this
(Figure 3.3). During these reactions, the polymer breaks into two components: one part gains a hydrogen atom
(H+) and the other gains a hydroxyl molecule (OH-) from a split water molecule.
Figure 3.3 In the hydrolysis reaction here, the disaccharide maltose breaks down to form two glucose monomers by
adding a water molecule. Note that this reaction is the reverse of the synthesis reaction in Figure 3.2.
Dehydration and hydrolysis reactions are catalyzed, or “sped up,” by specific enzymes; dehydration reactions
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Chapter 3 | Biological Macromolecules
71
involve the formation of new bonds, requiring energy, while hydrolysis reactions break bonds and release energy.
These reactions are similar for most macromolecules, but each monomer and polymer reaction is specific for
its class. For example, catalytic enzymes in the digestive system hydrolyze or break down the food we ingest
into smaller molecules. This allows cells in our body to easily absorb nutrients in the intestine. A specific
enzyme breaks down each macromolecule. For instance, amylase, sucrase, lactase, or maltase break down
carbohydrates. Enzymes called proteases, such as pepsin and peptidase, and hydrochloric acid break down
proteins. Lipases break down lipids. These broken down macromolecules provide energy for cellular activities.
LINK TQ LEARNING
Visit this site (http:// 0 penstaxc 0 llege. 0 rg/l/hydmlysis) to see visual representations of dehydration
synthesis and hydrolysis.
3.2 | Carbohydrates
By the end of this section, you will be able to do the following:
• Discuss the role of carbohydrates in cells and in the extracellular materials of animals and plants
• Explain carbohydrate classifications
• List common monosaccharides, disaccharides, and polysaccharides
Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we
eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before
important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in
fact, an essential part of our diet. Grains, fruits, and vegetables are all natural carbohydrate sources that provide
energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient
in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.
Molecular Structures
The stoichiometric formula (CH20)n, where n is the number of carbons in the molecule represents
carbohydrates. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules.
This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo") and
the components of water (hence, “hydrate”). Scientists classify carbohydrates into three subtypes:
monosaccharides, disaccharides, and polysaccharides.
Monosaccharides
Monosaccharides (mono- = “one"; sacchar- = “sweet") are simple sugars, the most common of which is
glucose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide
names end with the suffix -ose. If the sugar has an aldehyde group (the functional group with the structure R-
CHO), it is an aldose, and if it has a ketone group (the functional group with the structure RC(=0)R'), it is a
ketose. Depending on the number of carbons in the sugar, they can be trioses (three carbons), pentoses (five
carbons), and/or hexoses (six carbons). Figure 3.4 illustrates monosaccharides.
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Chapter 3 | Biological Macromolecules
MONOSACCHARIDES
Glyceraldehyde
Dihydroxyacetone
H
H -
OH
OH
OH
■ OH
H
Aldose
H
Ketose
Glyceraldehyde
OH
OH
Triose
Figure 3.4 Scientists classify monosaccharides based on the position of their carbonyl group and the number of
carbons in the backbone. Aldoses have a carbonyl group (indicated in green) at the end of the carbon chain, and
ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three-, five-,
and six- carbon backbones, respectively.
The chemical formula for glucose is C 6 H 12 O 6 . In humans, glucose is an important source of energy. During
cellular respiration, energy releases from glucose, and that energy helps make adenosine triphosphate (ATP).
Plants synthesize glucose using carbon dioxide and water, and glucose in turn provides energy requirements
for the plant. Humans and other animals that feed on plants often store excess glucose as catabolized (cell
breakdown of larger molecules) starch.
Galactose (part of lactose, or milk sugar) and fructose (found in sucrose, in fruit) are other common
monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C 6 H 12 O 6 ),
they differ structurally and chemically (and are isomers) because of the different arrangement of functional
groups around the asymmetric carbon. All these monosaccharides have more than one asymmetric carbon
(Figure 3.5).
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Chapter 3 | Biological Macromolecules
73
visual
CONNECTION
Glucose
Galactose
Fructose
Figure 3.5 Glucose, galactose, and fructose are all hexoses. They are structural isomers, meaning they have the
same chemical formula (C 6 H 12 O 6 ) but a different atom arrangement.
What kind of sugars are these, aldose or ketose?
Glucose, galactose, and fructose are isomeric monosaccharides (hexoses), meaning they have the same
chemical formula but have slightly different structures. Glucose and galactose are aldoses, and fructose is a
ketose.
Monosaccharides can exist as a linear chain or as ring-shaped molecules. In aqueous solutions they are usually
in ring forms (Figure 3.6). Glucose in a ring form can have two different hydroxyl group arrangements (OH)
around the anomeric carbon (carbon 1 that becomes asymmetric in the ring formation process). If the hydroxyl
group is below carbon number 1 in the sugar, it is in the alpha (a) position, and if it is above the plane, it is in the
beta (/3) position.
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Chapter 3 | Biological Macromolecules
Conversion between Linear and Ring Forms of Glucose
CH,OH
Glucose
CH,OH
CH 2 OH
Figure 3.6 Five and six carbon monosaccharides exist in equilibrium between linear and ring forms. When the ring
forms, the side chain it closes on locks into an a or (3 position. Fructose and ribose also form rings, although they form
five-membered rings as opposed to the six-membered ring of glucose.
Disaccharides
Disaccharides (di- = “two") form when two monosaccharides undergo a dehydration reaction (or a condensation
reaction or dehydration synthesis). During this process, one monosaccharide's hydroxyl group combines with
another monosaccharide's hydrogen, releasing a water molecule and forming a covalent bond. A covalent bond
forms between a carbohydrate molecule and another molecule (in this case, between two monosaccharides).
Scientists call this a glycosidic bond (Figure 3.7). Glycosidic bonds (or glycosidic linkages) can be an alpha
or beta type. An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring
plane, and a beta bond is formed when the OH group on the carbon-1 is above the ring plane.
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Chapter 3 | Biological Macromolecules
75
6 CH 2 OH
1 ch 2 oh
Fructose
i
Figure 3.7 Sucrose forms when a glucose monomer and a fructose monomer join in a dehydration reaction to form
a glycosidic bond. In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide
are numbered from the terminal carbon closest to the carbonyl group. In sucrose, a glycosidic linkage forms between
carbon 1 in glucose and carbon 2 in fructose.
Common disaccharides include lactose, maltose, and sucrose (Figure 3.8). Lactose is a disaccharide consisting
of the monomers glucose and galactose. It is naturally in milk. Maltose, or malt sugar, is a disaccharide formed
by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table
sugar, which is comprised of glucose and fructose monomers.
76
Chapter 3 | Biological Macromolecules
CH,OH
CH,OH
Sucrose
Figure 3.8 Common disaccharides include maltose (grain sugar), lactose (milk sugar), and sucrose (table sugar).
Polysaccharides
A long chain of monosaccharides linked by glycosidic bonds is a polysaccharide (poly- = “many"). The chain
may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight
may be 100,000 daltons or more depending on the number of joined monomers. Starch, glycogen, cellulose,
and chitin are primary examples of polysaccharides.
Plants store starch in the form of sugars. In plants, an amylose and amylopectic mixture (both glucose polymers)
comprise these sugars. Plants are able to synthesize glucose, and they store the excess glucose, beyond the
their immediate energy needs, as starch in different plant parts, including roots and seeds. The starch in the
seeds provides food for the embryo as it germinates and can also act as a food source for humans and animals.
Enzymes break down the starch that humans consume. For example, an amylase present in saliva catalyzes,
or breaks down this starch into smaller molecules, such as maltose and glucose. The cells can then absorb the
glucose.
Glucose starch comprises monomers that are joined by a 1-4 or a 1-6 glycosidic bonds. The numbers 1-4 and
1-6 refer to the carbon number of the two residues that have joined to form the bond. As Figure 3.9 illustrates,
unbranched glucose monomer chains (only a 1-4 linkages) form the starch; whereas, amylopectin is a branched
polysaccharide (a 1-6 linkages at the branch points).
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Chapter 3 | Biological Macromolecules
77
Figure 3.9 Amylose and amylopectin are two different starch forms. Unbranched glucose monomer chains comprise
amylose by a 1-4 glycosidic linkages. Unbranched glucose monomer chains comprise amylopectin by a 1-4 and a 1-6
glycosidic linkages. Because of the way the subunits are joined, the glucose chains have a helical structure. Glycogen
(not shown) is similar in structure to amylopectin but more highly branched.
Glycogen is the storage form of glucose in humans and other vertebrates and is comprised of monomers of
glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and
muscle cells. Whenever blood glucose levels decrease, glycogen breaks down to release glucose in a process
scientists call glycogenolysis.
Cellulose is the most abundant natural biopolymer. Cellulose mostly comprises a plant's cell wall. This provides
the cell structural support. Wood and paper are mostly cellulosic in nature. Glucose monomers comprise
cellulose that p 1-4 glycosidic bonds link (Figure 3.10).
78
Chapter 3 | Biological Macromolecules
Cellulose fibers
Cellulose structure
Figure 3.10 In cellulose, glucose monomers are linked in unbranched chains by / 3 1-4 glycosidic linkages. Because of
the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear,
fibrous structure.
As Figure 3.10 shows, every other glucose monomer in cellulose is flipped over, and the monomers are packed
tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important
to plant cells. While human digestive enzymes cannot break down the /31-4 linkage, herbivores such as cows,
koalas, and buffalos are able, with the help of the specialized flora in their stomach, to digest plant material that
is rich in cellulose and use it as a food source. In some of these animals, certain species of bacteria and protists
reside in the rumen (part of the herbivore's digestive system) and secrete the enzyme cellulase. The appendix
of grazing animals also contains bacteria that digest cellulose, giving it an important role in ruminants' digestive
systems. Cellulases can break down cellulose into glucose monomers that animals use as an energy source.
Termites are also able to break down cellulose because of the presence of other organisms in their bodies that
secrete cellulases.
Carbohydrates serve various functions in different animals. Arthropods (insects, crustaceans, and others) have
an outer skeleton, the exoskeleton, which protects their internal body parts (as we see in the bee in Figure
3.11). This exoskeleton is made of the biological macromolecule chitin, which is a polysaccharide-containing
nitrogen. It is made of repeating N-acetyl-/3-d-glucosamine units, which are a modified sugar. Chitin is also a
major component of fungal cell walls. Fungi are neither animals nor plants and form a kingdom of their own in
the domain Eukarya.
Figure 3.11 Insects have a hard outer exoskeleton made of chitin, a type of polysaccharide, (credit: Louise Docker)
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Chapter 3 | Biological Macromolecules
79
ca eer connection
Registered Dietitian
Obesity is a worldwide health concern, and many diseases such as diabetes and heart disease are
becoming more prevalent because of obesity. This is one of the reasons why people increasingly seek out
registered dietitians for advice. Registered dietitians help plan nutrition programs for individuals in various
settings. They often work with patients in health care facilities, designing nutrition plans to treat and prevent
diseases. For example, dietitians may teach a patient with diabetes how to manage blood sugar levels by
eating the correct types and amounts of carbohydrates. Dietitians may also work in nursing homes, schools,
and private practices.
To become a registered dietitian, one needs to earn at least a bachelor’s degree in dietetics, nutrition,
food technology, or a related field. In addition, registered dietitians must complete a supervised internship
program and pass a national exam. Those who pursue careers in dietetics take courses in nutrition,
chemistry, biochemistry, biology, microbiology, and human physiology. Dietitians must become experts in
the chemistry and physiology (biological functions) of food (proteins, carbohydrates, and fats).
Benefits of Carbohydrates
Are carbohydrates good for you? Some often tell people who wish to lose weight that carbohydrates are bad
and they should avoid them. Some diets completely forbid carbohydrate consumption, claiming that a low-
carbohydrate diet helps people to lose weight faster. However, carbohydrates have been an important part of
the human diet for thousands of years. Artifacts from ancient civilizations show the presence of wheat, rice, and
corn in our ancestors’ storage areas.
As part of a well balanced diet, we should supplement carbohydrates with proteins, vitamins, and fats. Calorie-
wise, a gram of carbohydrate provides 4.3 Kcal. For comparison, fats provide 9 Kcal/g, a less desirable ratio.
Carbohydrates contain soluble and insoluble elements. The insoluble part, fiber, is mostly cellulose. Fiber
has many uses. It promotes regular bowel movement by adding bulk, and it regulates the blood glucose
consumption rate. Fiber also helps to remove excess cholesterol from the body. Fiber binds to the cholesterol
in the small intestine, then attaches to the cholesterol and prevents the cholesterol particles from entering the
bloodstream. Cholesterol then exits the body via the feces. Fiber-rich diets also have a protective role in reducing
the occurrence of colon cancer. In addition, a meal containing whole grains and vegetables gives a feeling
of fullness. As an immediate source of energy, glucose breaks down during the cellular respiration process,
which produces ATP, the cell's energy currency. Without consuming carbohydrates, we reduce the availability of
“instant energy”. Eliminating carbohydrates from the diet is not the best way to lose weight. A low-calorie diet
that is rich in whole grains, fruits, vegetables, and lean meat, together with plenty of exercise and plenty of water,
is the more sensible way to lose weight.
LINK
T a
LEARNING
For an additional perspective on carbohydrates, explore “Biomolecules: the Carbohydrates" through this
interactive animation (http:// 0 penstaxc 0 llege. 0 rg/l/carb 0 hydrates) .
80
Chapter 3 | Biological Macromolecules
3.3 | Lipids
By the end of this section, you will be able to do the following:
• Describe the four major types of lipids
• Explain the role of fats in storing energy
• Differentiate between saturated and unsaturated fatty acids
• Describe phospholipids and their role in cells
• Define the basic structure of a steroid and some steroid functions
• Explain how cholesterol helps maintain the plasma membrane's fluid nature
Lipids include a diverse group of compounds that are largely nonpolar in nature. This is because they are
hydrocarbons that include mostly nonpolar carbon-carbon or carbon-hydrogen bonds. Non-polar molecules are
hydrophobic (“water fearing"), or insoluble in water. Lipids perform many different functions in a cell. Cells store
energy for long-term use in the form of fats. Lipids also provide insulation from the environment for plants and
animals (Figure 3.12). For example, they help keep aquatic birds and mammals dry when forming a protective
layer over fur or feathers because of their water-repellant hydrophobic nature. Lipids are also the building blocks
of many hormones and are an important constituent of all cellular membranes. Lipids include fats, oils, waxes,
phospholipids, and steroids.
Figure 3.12 Hydrophobic lipids in aquatic mammals' fur, such as this river otter, protect them from the elements, (credit:
Ken Bosma)
Fats and Oils
A fat molecule consists of two main components—glycerol and fatty acids. Glycerol is an organic compound
(alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of
hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid." The number of carbons in the
fatty acid may range from 4 to 36. The most common are those containing 12-18 carbons. In a fat molecule, the
fatty acids attach to each of the glycerol molecule's three carbons with an ester bond through an oxygen atom
(Figure 3.13).
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Chapter 3 | Biological Macromolecules
81
Glycerol
H
I
H — C — OH
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I
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I
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Fatty Acid
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Triacylglycerol
Figure 3.13 Joining three fatty acids to a glycerol backbone in a dehydration reaction forms triacylglycerol. Three water
molecules release in the process.
During this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol
may be similar or dissimilar. We also call fats triacylglycerols or triglycerides because of their chemical
structure. Some fatty acids have common names that specify their origin. For example, palmitic acid, a
saturated fatty acid, is derived from the palm tree. Arachidic acid is derived from Arachis hypogea, the scientific
name for groundnuts or peanuts.
Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between
neighboring carbons in the hydrocarbon chain, the fatty acid is saturated. Saturated fatty acids are saturated
with hydrogen. In other words, the number of hydrogen atoms attached to the carbon skeleton is maximized.
Stearic acid is an example of a saturated fatty acid (Figure 3.14).
82
Chapter 3 | Biological Macromolecules
H
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Figure 3.14 Stearic acid is a common saturated fatty acid.
When the hydrocarbon chain contains a double bond, the fatty acid is unsaturated. Oleic acid is an example of
an unsaturated fatty acid (Figure 3.15).
H
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Figure 3.15 Oleic acid is a common unsaturated fatty acid.
Most unsaturated fats are liquid at room temperature. We call these oils. If there is one double bond in the
molecule, then it is a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is a
polyunsaturated fat (e.g., canola oil).
When a fatty acid has no double bonds, it is a saturated fatty acid because it is not possible to add more
hydrogen to the chain's carbon atoms. A fat may contain similar or different fatty acids attached to glycerol. Long
straight fatty acids with single bonds generally pack tightly and are solid at room temperature. Animal fats with
stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples
of saturated fats. Mammals store fats in specialized cells, or adipocytes, where fat globules occupy most of
the cell’s volume. Plants store fat or oil in many seeds and use them as a source of energy during seedling
development. Unsaturated fats or oils are usually of plant origin and contain c/s unsaturated fatty acids. C/s
and trans indicate the configuration of the molecule around the double bond. If hydrogens are present in the
same plane, it is a cis fat. If the hydrogen atoms are on two different planes, it is a trans fat. The c/s double
bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room
temperature (Figure 3.16). Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats.
Unsaturated fats help to lower blood cholesterol levels; whereas, saturated fats contribute to plaque formation in
the arteries.
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Chapter 3 | Biological Macromolecules
83
Saturated fatty acid
Stearic acid
Unsaturated fatty acids
C/s oleic acid
Trans oleic acid
Figure 3.16 Saturated fatty acids have hydrocarbon chains connected by single bonds only. Unsaturated fatty acids
have one or more double bonds. Each double bond may be in a c/s or trans configuration. In the c/s configuration, both
hydrogens are on the same side of the hydrocarbon chain. In the trans configuration, the hydrogens are on opposite
sides. A c/s double bond causes a kink in the chain.
Trans Fats
The food industry artificially hydrogenates oils to make them semi-solid and of a consistency desirable for many
processed food products. Simply speaking, hydrogen gas is bubbled through oils to solidify them. During this
hydrogenation process, double bonds of the c/s- conformation in the hydrocarbon chain may convert to double
bonds in the trans- conformation.
Margarine, some types of peanut butter, and shortening are examples of artificially hydrogenated trans fats.
Recent studies have shown that an increase in trans fats in the human diet may lead to higher levels of low-
density lipoproteins (LDL), or “bad” cholesterol, which in turn may lead to plaque deposition in the arteries,
resulting in heart disease. Many fast food restaurants have recently banned using trans fats, and food labels are
required to display the trans fat content.
Omega Fatty Acids
Essential fatty acids are those that the human body requires but does not synthesize. Consequently, they have
to be supplemented through ingestion via the diet. Omega-3 fatty acids (like those in Figure 3.17) fall into this
category and are one of only two known for humans (the other is omega-6 fatty acid). These are polyunsaturated
fatty acids and are omega-3 because a double bond connects the third carbon from the hydrocarbon chain's end
to its neighboring carbon.
84
Chapter 3 | Biological Macromolecules
Figure 3.17 Alpha-linolenic acid is an example of an omega-3 fatty acid. It has three c/s double bonds and, as a result,
a curved shape. For clarity, the diagram does not show the carbons. Each singly bonded carbon has two hydrogens
associated with it, which the diagram also does not show.
The farthest carbon away from the carboxyl group is numbered as the omega (co) carbon, and if the double
bond is between the third and fourth carbon from that end, it is an omega-3 fatty acid. Nutritionally important
because the body does not make them, omega-3 fatty acids include alpha-linoleic acid (ALA), eicosapentaenoic
acid (EPA), and docosahexaenoic acid (DHA), all of which are polyunsaturated. Salmon, trout, and tuna are
good sources of omega-3 fatty acids. Research indicates that omega-3 fatty acids reduce the risk of sudden
death from heart attacks, lower triglycerides in the blood, decrease blood pressure, and prevent thrombosis by
inhibiting blood clotting. They also reduce inflammation, and may help lower the risk of some cancers in animals.
Like carbohydrates, fats have received considerable bad publicity. It is true that eating an excess of fried foods
and other “fatty" foods leads to weight gain. However, fats do have important functions. Many vitamins are
fat soluble, and fats serve as a long-term storage form of fatty acids: a source of energy. They also provide
insulation for the body. Therefore, we should consume “healthy” fats in moderate amounts on a regular basis.
Waxes
Wax covers some aquatic birds' feathers and some plants' leaf surfaces. Because of waxes' hydrophobic nature,
they prevent water from sticking on the surface (Figure 3.18). Long fatty acid chains esterified to long-chain
alcohols comprise waxes.
Figure 3.18 Lipids comprise waxy coverings on some leaves, (credit: Roger Griffith)
Phospholipids
Phospholipids are major plasma membrane constituents that comprise cells' outermost layer. Like fats, they
are comprised of fatty acid chains attached to a glycerol or sphingosine backbone. However, instead of three
fatty acids attached as in triglycerides, there are two fatty acids forming diacylglycerol, and a modified phosphate
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Chapter 3 | Biological Macromolecules
85
group occupies the glycerol backbone's third carbon (Figure 3.19). A phosphate group alone attached to a
diaglycerol does not qualify as a phospholipid. It is phosphatidate (diacylglycerol 3-phosphate), the precursor of
phospholipids. An alcohol modifies the phosphate group. Phosphatidylcholine and phosphatidylserine are two
important phospholipids that are in plasma membranes.
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Figure 3.19 A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol
backbone. Adding a charged or polar chemical group may modify the phosphate.
A phospholipid is an amphipathic molecule, meaning it has a hydrophobic and a hydrophilic part. The fatty acid
chains are hydrophobic and cannot interact with water; whereas, the phosphate-containing group is hydrophilic
and interacts with water (Figure 3.20).
Figure 3.20 The phospholipid bilayer is the major component of all cellular membranes. The hydrophilic head groups
of the phospholipids face the aqueous solution. The hydrophobic tails are sequestered in the middle of the bilayer.
The head is the hydrophilic part, and the tail contains the hydrophobic fatty acids. In a membrane, a bilayer of
phospholipids forms the structure's matrix, phospholipids' fatty acid tails face inside, away from water; whereas,
the phosphate group faces the outside, aqueous side (Figure 3.20).
Phospholipids are responsible for the plasma membrane's dynamic nature. If a drop of phospholipids is placed
in water, it spontaneously forms a structure that scientists call a micelle, where the hydrophilic phosphate heads
face the outside and the fatty acids face the structure's interior.
Steroids
Unlike the phospholipids and fats that we discussed earlier, steroids have a fused ring structure. Although
they do not resemble the other lipids, scientists group them with them because they are also hydrophobic and
86
Chapter 3 | Biological Macromolecules
insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, have a short tail
(Figure 3.21). Many steroids also have the -OH functional group, which puts them in the alcohol classification
(sterols).
Cortisol
Figure 3.21 Four fused hydrocarbon rings comprise steroids such as cholesterol and cortisol.
Cholesterol is the most common steroid. The liver synthesizes cholesterol and is the precursor to many steroid
hormones such as testosterone and estradiol, which gonads and endocrine glands secrete. It is also the
precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help emulsifying fats and their
subsequent absorption by cells. Although lay people often speak negatively about cholesterol, it is necessary for
the body's proper functioning. Sterols (cholesterol in animal cells, phytosterol in plants) are components of the
plasma membrane of cells and are found within the phospholipid bilayer.
LINK TO LEARNING
For an additional perspective on lipids, explore the interactive animation
(http:// 0 penstaxc 0 llege. 0 rg/l/lipids) .
‘Biomolecules: The Lipids”
3.4 | Proteins
By the end of this section, you will be able to do the following:
• Describe the functions proteins perform in the cell and in tissues
• Discuss the relationship between amino acids and proteins
• Explain the four levels of protein organization
• Describe the ways in which protein shape and function are linked
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Chapter 3 | Biological Macromolecules
87
Proteins are one of the most abundant organic molecules in living systems and have the most diverse range
of functions of all macromolecules. Proteins may be structural, regulatory, contractile, or protective. They may
serve in transport, storage, or membranes; or they may be toxins or enzymes. Each cell in a living system may
contain thousands of proteins, each with a unique function. Their structures, like their functions, vary greatly.
They are all, however, amino acid polymers arranged in a linear sequence.
Types and Functions of Proteins
Enzymes, which living cells produce, are catalysts in biochemical reactions (like digestion) and are usually
complex or conjugated proteins. Each enzyme is specific for the substrate (a reactant that binds to an enzyme)
upon which it acts. The enzyme may help in breakdown, rearrangement, or synthesis reactions. We call
enzymes that break down their substrates catabolic enzymes. Those that build more complex molecules from
their substrates are anabolic enzymes, and enzymes that affect the rate of reaction are catalytic enzymes. Note
that all enzymes increase the reaction rate and, therefore, are organic catalysts. An example of an enzyme is
salivary amylase, which hydrolyzes its substrate amylose, a component of starch.
Hormones are chemical-signaling molecules, usually small proteins or steroids, secreted by endocrine cells
that act to control or regulate specific physiological processes, including growth, development, metabolism, and
reproduction. For example, insulin is a protein hormone that helps regulate the blood glucose level. Table 3.1
lists the primary types and functions of proteins.
Protein Types and Functions
Type
Examples
Functions
Digestive
Enzymes
Amylase, lipase, pepsin, trypsin
Help in food by catabolizing nutrients into monomeric
units
Transport
Hemoglobin, albumin
Carry substances in the blood or lymph throughout the
body
Structural
Actin, tubulin, keratin
Construct different structures, like the cytoskeleton
Hormones
Insulin, thyroxine
Coordinate different body systems' activity
Defense
Immunoglobulins
Protect the body from foreign pathogens
Contractile
Actin, myosin
Effect muscle contraction
Storage
Legume storage proteins, egg white
(albumin)
Provide nourishment in early embryo development and
the seedling
Table 3.1
Proteins have different shapes and molecular weights. Some proteins are globular in shape; whereas, others
are fibrous in nature. For example, hemoglobin is a globular protein, but collagen, located in our skin, is a
fibrous protein. Protein shape is critical to its function, and many different types of chemical bonds maintain this
shape. Changes in temperature, pH, and exposure to chemicals may lead to permanent changes in the protein's
shape, leading to loss of function, or denaturation. Different arrangements of the same 20 types of amino acids
comprise all proteins. Two rare new amino acids were discovered recently (selenocystein and pirrolysine), and
additional new discoveries may be added to the list.
Amino Acids
Amino acids are the monomers that comprise proteins. Each amino acid has the same fundamental structure,
which consists of a central carbon atom, or the alpha (a) carbon, bonded to an amino group (NH 2 ), a carboxyl
group (COOH), and to a hydrogen atom. Every amino acid also has another atom or group of atoms bonded to
the central atom known as the R group (Figure 3.22).
88
Chapter 3 | Biological Macromolecules
Amino group
Carboxyl group
a carbon
Figure 3.22 Amino acids have a central asymmetric carbon to which an amino group, a carboxyl group, a hydrogen
atom, and a side chain (R group) are attached.
Scientists use the name "amino acid" because these acids contain both amino group and carboxyl-acid-group in
their basic structure. As we mentioned, there are 20 common amino acids present in proteins. Nine of these are
essential amino acids in humans because the human body cannot produce them and we obtain them from our
diet. For each amino acid, the R group (or side chain) is different (Figure 3.23).
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Chapter 3 | Biological Macromolecules
89
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Figure 3.23 There are 20 common amino acids commonly found in proteins, each with a different R group (variant
group) that determines its chemical nature.
Which categories of amino acid would you expect to find on a soluble protein's surface and which would
you expect to find in the interior? What distribution of amino acids would you expect to find in a protein
embedded in a lipid bilayer?
The chemical nature of the side chain determines the amino acid's nature (that is, whether it is acidic, basic,
polar, or nonpolar). For example, the amino acid glycine has a hydrogen atom as the R group. Amino acids such
as valine, methionine, and alanine are nonpolar or hydrophobic in nature, while amino acids such as serine,
threonine, and cysteine are polar and have hydrophilic side chains. The side chains of lysine and arginine are
positively charged, and therefore these amino acids are also basic amino acids. Proline has an R group that
is linked to the amino group, forming a ring-like structure. Proline is an exception to the amino acid's standard
structure since its amino group is not separate from the side chain (Figure 3.23).
A single upper case letter or a three-letter abbreviation represents amino acids. For example, the letter V or
the three-letter symbol val represent valine. Just as some fatty acids are essential to a diet, some amino acids
also are necessary. These essential amino acids in humans include isoleucine, leucine, and cysteine. Essential
amino acids refer to those necessary to build proteins in the body, but not those that the body produces. Which
amino acids are essential varies from organism to organism.
The sequence and the number of amino acids ultimately determine the protein's shape, size, and function. A
covalent bond, or peptide bond, attaches to each amino acid, which a dehydration reaction forms. One amino
acid's carboxyl group and the incoming amino acid's amino group combine, releasing a water molecule. The
resulting bond is the peptide bond (Figure 3.24).
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Chapter 3 | Biological Macromolecules
H
H
H 1
O
H
1 .O
\ 1
*
\
*/
N —C
— c
N
-C —C
H / 1
\
OH
H
/
1 X 0H
H
H
R
0
H
R
H \
1
II
1
1
N
-C -
• c -
- N
-C
-C
h'
1
1
X 0H
H
H
Peptide Bond
Figure 3.24 Peptide bond formation is a dehydration synthesis reaction. The carboxyl group of one amino acid is linked
to the incoming amino acid's amino group. In the process, it releases a water molecule.
The products that such linkages form are peptides. As more amino acids join to this growing chain, the resulting
chain is a polypeptide. Each polypeptide has a free amino group at one end. This end the N terminal, or
the amino terminal, and the other end has a free carboxyl group, also the C or carboxyl terminal. While the
terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically a polymer of
amino acids, whereas the term protein is used for a polypeptide or polypeptides that have combined together,
often have bound non-peptide prosthetic groups, have a distinct shape, and have a unique function. After
protein synthesis (translation), most proteins are modified. These are known as post-translational modifications.
They may undergo cleavage, phosphorylation, or may require adding other chemical groups. Only after these
modifications is the protein completely functional.
LINK
T a
LEARNING
Click through the steps of protein synthesis in this interactive tutorial (http://0penstaxc0llege.0rg/l/
protein_synth).
e olution CONNECTION
The Evolutionary Significance of Cytochrome c
Cytochrome c is an important component of the electron transport chain, a part of cellular respiration, and
it is normally located in the cellular organelle, the mitochondrion. This protein has a heme prosthetic group,
and the heme's central ion alternately reduces and oxidizes during electron transfer. Because this essential
protein’s role in producing cellular energy is crucial, it has changed very little over millions of years. Protein
sequencing has shown that there is a considerable amount of cytochrome c amino acid sequence homology
among different species, in other words, we can assess evolutionary kinship by measuring the similarities
or differences among various species’ DNA or protein sequences.
Scientists have determined that human cytochrome c contains 104 amino acids. For each cytochrome c
molecule from different organisms that scientists have sequenced to date, 37 of these amino acids appear in
the same position in all cytochrome c samples. This indicates that there may have been a common ancestor.
On comparing the human and chimpanzee protein sequences, scientists did not find a sequence difference.
When researchers compared human and rhesus monkey sequences, the single difference was in one amino
acid. In another comparison, human to yeast sequencing shows a difference in the 44th position.
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91
Protein Structure
As we discussed earlier, a protein's shape is critical to its function. For example, an enzyme can bind to a specific
substrate at an active site. If this active site is altered because of local changes or changes in overall protein
structure, the enzyme may be unable to bind to the substrate. To understand how the protein gets its final shape
or conformation, we need to understand the four levels of protein structure: primary, secondary, tertiary, and
quaternary.
Primary Structure
Amino acids' unique sequence in a polypeptide chain is its primary structure. For example, the pancreatic
hormone insulin has two polypeptide chains, A and B, and they are linked together by disulfide bonds. The N
terminal amino acid of the A chain is glycine; whereas, the C terminal amino acid is asparagine (Figure 3.25).
The amino acid sequences in the A and B chains are unique to insulin.
r i
Gty lie Val Glu Gin Cys Cys Ala Ser Val Cys Ser Leu Tyr Gin Leu Glu Asn Tyr Cys Asn
/
Phe Val Asn Gin His Leu Cys Gly Ser His Leu VaJ Glu Ala Leu Tyr Leu Val Cys G jy
B Chain
/TT Yf~ /Z fZ 1
Ala Lys Pro Thr Tyr Pne
Arg
Gly
Figure 3.25 Bovine serum insulin is a protein hormone comprised of two peptide chains, A (21 amino acids long) and
B (30 amino acids long). In each chain, three-letter abbreviations that represent the amino acids' names in the order
they are present indicate primary structure. The amino acid cysteine (cys) has a sulfhydryl (SH) group as a side chain.
Two sulfhydryl groups can react in the presence of oxygen to form a disulfide (S-S) bond. Two disulfide bonds connect
the A and B chains together, and a third helps the A chain fold into the correct shape. Note that all disulfide bonds are
the same length, but we have drawn them different sizes for clarity.
The gene encoding the protein ultimately determines the unique sequence for every protein. A change in
nucleotide sequence of the gene’s coding region may lead to adding a different amino acid to the growing
polypeptide chain, causing a change in protein structure and function. In sickle cell anemia, the hemoglobin /3
chain (a small portion of which we show in Figure 3.26) has a single amino acid substitution, causing a change
in protein structure and function. Specifically, valine in the /3 chain substitutes the amino acid glutamic. What
is most remarkable to consider is that a hemoglobin molecule is comprised of two alpha and two beta chains
that each consist of about 150 amino acids. The molecule, therefore, has about 600 amino acids. The structural
difference between a normal hemoglobin molecule and a sickle cell molecule—which dramatically decreases life
expectancy—is a single amino acid of the 600. What is even more remarkable is that three nucleotides each
encode those 600 amino acids, and a single base change (point mutation), 1 in 1800 bases causes the mutation.
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Chapter 3 | Biological Macromolecules
H H O
II
threonine — proline-N — C — O-lys
CH 2 O
II
threonine — proline-N — C — O-
h 3 c— :h — ch 3
H H O
CH 2 — C -
glutamic acii
II
Amino acid
substitution
Sickle cell
hemoglobin
Normal
hemoglobin
valine
Figure 3.26 The beta chain of hemoglobin is 147 residues in length, yet a single amino acid substitution leads to sickle
cell anemia. In normal hemoglobin, the amino acid at position seven is glutamate. In sickle cell hemoglobin, a valine
replaces glutamate.
Because of this change of one amino acid in the chain, hemoglobin molecules form long fibers that distort the
biconcave, or disc-shaped, red blood cells and causes them to assume a crescent or “sickle" shape, which clogs
blood vessels (Figure 3.27). This can lead to myriad serious health problems such as breathlessness, dizziness,
headaches, and abdominal pain for those affected by this disease.
Figure 3.27 In this blood smear, visualized at 535x magnification using bright field microscopy, sickle cells are crescent
shaped, while normal cells are disc-shaped, (credit: modification of work by Ed Uthman; scale-bar data from Matt
Russell)
Secondary Structure
The local folding of the polypeptide in some regions gives rise to the secondary structure of the protein. The
most common are the or-helix and )3-pleated sheet structures (Figure 3.28). Both structures are held in shape
by hydrogen bonds. The hydrogen bonds form between the oxygen atom in the carbonyl group in one amino
acid and another amino acid that is four amino acids farther along the chain.
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93
Figure 3.28 The a-helix and /3-pleated sheet are secondary structures of proteins that form because of hydrogen
bonding between carbonyl and amino groups in the peptide backbone. Certain amino acids have a propensity to form
an cr-helix, while others have a propensity to form a /3-pleated sheet.
Every helical turn in an alpha helix has 3.6 amino acid residues. The polypeptide's R groups (the variant groups)
protrude out from the a-helix chain. In the /3-pleated sheet, hydrogen bonding between atoms on the polypeptide
chain's backbone form the "pleats". The R groups are attached to the carbons and extend above and below
the pleat's folds. The pleated segments align parallel or antiparallel to each other, and hydrogen bonds form
between the partially positive nitrogen atom in the amino group and the partially negative oxygen atom in the
peptide backbone's carbonyl group. The a-helix and /3-pleated sheet structures are in most globular and fibrous
proteins and they play an important structural role.
Tertiary Structure
The polypeptide's unique three-dimensional structure is its tertiary structure (Figure 3.29). This structure is in
part due to chemical interactions at work on the polypeptide chain. Primarily, the interactions among R groups
create the protein's complex three-dimensional tertiary structure. The nature of the R groups in the amino
acids involved can counteract forming the hydrogen bonds we described for standard secondary structures.
For example, R groups with like charges repel each other and those with unlike charges are attracted to each
other (ionic bonds). When protein folding takes place, the nonpolar amino acids' hydrophobic R groups lie in the
protein's interior; whereas, the hydrophilic R groups lie on the outside. Scientists also call the former interaction
types hydrophobic interactions. Interaction between cysteine side chains forms disulfide linkages in the presence
of oxygen, the only covalent bond that forms during protein folding.
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Chapter 3 | Biological Macromolecules
Polypeptide backbone
Figure 3.29 A variety of chemical interactions determine the proteins' tertiary structure. These include hydrophobic
interactions, ionic bonding, hydrogen bonding, and disulfide linkages.
All of these interactions, weak and strong, determine the protein's final three-dimensional shape. When a protein
loses its three-dimensional shape, it may no longer be functional.
Quaternary Structure
In nature, some proteins form from several polypeptides, or subunits, and the interaction of these subunits forms
the quaternary structure. Weak interactions between the subunits help to stabilize the overall structure. For
example, insulin (a globular protein) has a combination of hydrogen and disulfide bonds that cause it to mostly
clump into a ball shape. Insulin starts out as a single polypeptide and loses some internal sequences in the
presence of post-translational modification after forming the disulfide linkages that hold the remaining chains
together. Silk (a fibrous protein), however, has a jS-pleated sheet structure that is the result of hydrogen bonding
between different chains.
Figure 3.30 illustrates the four levels of protein structure (primary, secondary, tertiary, and quaternary).
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Chapter 3 | Biological Macromolecules
95
Amino acids
Primary Protein structure
sequence of a chain of
animo acids
Secondary Protein structure
hydrogen bonding of the peptide
backbone causes the amino
acids to fold into a repeating
pattern
Tertiary protein structure
three-dimensional folding
pattern of a protein due to side
chain interactions
Figure 3.30 Observe the four levels of protein structure in these illustrations, (credit: modification of work by National
Human Genome Research Institute)
Denaturation and Protein Folding
Each protein has its own unique sequence and shape that chemical interactions hold together. If the protein is
subject to changes in temperature, pH, or exposure to chemicals, the protein structure may change, losing its
shape without losing its primary sequence in what scientists call denaturation. Denaturation is often reversible
because the polypeptide's primary structure is conserved in the process if the denaturing agent is removed,
allowing the protein to resume its function. Sometimes denaturation is irreversible, leading to loss of function.
One example of irreversible protein denaturation is frying an egg. The albumin protein in the liquid egg white
denatures when placed in a hot pan. Not all proteins denature at high temperatures. For instance, bacteria that
survive in hot springs have proteins that function at temperatures close to boiling. The stomach is also very
acidic, has a low pH, and denatures proteins as part of the digestion process; however, the stomach's digestive
enzymes retain their activity under these conditions.
Protein folding is critical to its function. Scientists originally thought that the proteins themselves were
responsible for the folding process. Only recently researchers discovered that often they receive assistance in
the folding process from protein helpers, or chaperones (or chaperonins) that associate with the target protein
during the folding process. They act by preventing polypeptide aggregation that comprise the complete protein
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Chapter 3 | Biological Macromolecules
structure, and they disassociate from the protein once the target protein is folded.
LINK TQ LEARNING
For an additional perspective on proteins, view this animation (http:// 0 penstaxc 0 llege. 0 rg/l/pr 0 teins) called
“Biomolecules: The Proteins.”
3.5 | Nucleic Acids
By the end of this section, you will be able to do the following:
• Describe nucleic acids' structure and define the two types of nucleic acids
• Explain DNA's structure and role
• Explain RNA's structure and roles
Nucleic acids are the most important macromolecules for the continuity of life. They carry the cell's genetic
blueprint and carry instructions for its functioning.
DNA and RNA
The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is
the genetic material in all living organisms, ranging from single-celled bacteria to multicellular mammals. It is in
the nucleus of eukaryotes and in the organelles, chloroplasts, and mitochondria. In prokaryotes, the DNA is not
enclosed in a membranous envelope.
The cell's entire genetic content is its genome, and the study of genomes is genomics. In eukaryotic cells but
not in prokaryotes, DNA forms a complex with histone proteins to form chromatin, the substance of eukaryotic
chromosomes. A chromosome may contain tens of thousands of genes. Many genes contain the information to
make protein products. Other genes code for RNA products. DNA controls all of the cellular activities by turning
the genes “on” or “off.”
The other type of nucleic acid, RNA, is mostly involved in protein synthesis. The DNA molecules never leave
the nucleus but instead use an intermediary to communicate with the rest of the cell. This intermediary is the
messenger RNA (mRNA). Other types of RNA—like rRNA, tRNA, and microRNA—are involved in protein
synthesis and its regulation.
DNA and RNA are comprised of monomers that scientists call nucleotides. The nucleotides combine with each
other to form a polynucleotide, DNA or RNA. Three components comprise each nucleotide: a nitrogenous base,
a pentose (five-carbon) sugar, and a phosphate group (Figure 3.31). Each nitrogenous base in a nucleotide is
attached to a sugar molecule, which is attached to one or more phosphate groups.
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97
Figure 3.31 Three components comprise a nucleotide: a nitrogenous base, a pentose sugar, and one or more
phosphate groups. Carbon residues in the pentose are numbered V through 5' (the prime distinguishes these residues
from those in the base, which are numbered without using a prime notation). The base is attached to the ribose's 1'
position, and the phosphate is attached to the 5' position. When a polynucleotide forms, the incoming nucleotide's 5 r
phosphate attaches to the 3' hydroxyl group at the end of the growing chain. Two types of pentose are in nucleotides,
deoxyribose (found in DNA) and ribose (found in RNA). Deoxyribose is similar in structure to ribose, but it has an H
instead of an OH at the 2' position. We can divide bases into two categories: purines and pyrimidines. Purines have a
double ring structure, and pyrimidines have a single ring.
The nitrogenous bases, important components of nucleotides, are organic molecules and are so named because
they contain carbon and nitrogen. They are bases because they contain an amino group that has the potential
of binding an extra hydrogen, and thus decreasing the hydrogen ion concentration in its environment, making it
more basic. Each nucleotide in DNA contains one of four possible nitrogenous bases: adenine (A), guanine (G)
cytosine (C), and thymine (T).
Scientists classify adenine and guanine as purines. The purine's primary structure is two carbon-nitrogen rings.
Scientists classify cytosine, thymine, and uracil as pyrimidines which have a single carbon-nitrogen ring as
their primary structure (Figure 3.31). Each of these basic carbon-nitrogen rings has different functional groups
attached to it. In molecular biology shorthand, we know the nitrogenous bases by their symbols A, T, G, C, and
U. DNA contains A, T, G, and C; whereas, RNA contains A, U, G, and C.
The pentose sugar in DNA is deoxyribose, and in RNA, the sugar is ribose (Figure 3.31). The difference
between the sugars is the presence of the hydroxyl group on the ribose's second carbon and hydrogen on the
deoxyribose's second carbon. The carbon atoms of the sugar molecule are numbered as 1', 2', 3', 4', and 5' (1'
is read as “one prime"). The phosphate residue attaches to the hydroxyl group of the 5' carbon of one sugar
and the hydroxyl group of the 3' carbon of the sugar of the next nucleotide, which forms a 5-3' phosphodiester
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Chapter 3 | Biological Macromolecules
linkage. A simple dehydration reaction like the other linkages connecting monomers in macromolecules does not
form the phosphodiester linkage. Its formation involves removing two phosphate groups. A polynucleotide may
have thousands of such phosphodiester linkages.
DNA Double-Helix Structure
DNA has a double-helix structure (Figure 3.32). The sugar and phosphate lie on the outside of the helix, forming
the DNA's backbone. The nitrogenous bases are stacked in the interior, like a pair of staircase steps. Hydrogen
bonds bind the pairs to each other. Every base pair in the double helix is separated from the next base pair by
0.34 nm. The helix's two strands run in opposite directions, meaning that the 5' carbon end of one strand will
face the 3' carbon end of its matching strand. (Scientists call this an antiparallel orientation and is important to
DNA replication and in many nucleic acid interactions.)
Figure 3.32 Native DNA is an antiparallel double helix. The phosphate backbone (indicated by the curvy lines) is on
the outside, and the bases are on the inside. Each base from one strand interacts via hydrogen bonding with a base
from the opposing strand, (credit: Jerome Walker/Dennis Myts)
Only certain types of base pairing are allowed. For example, a certain purine can only pair with a certain
pyrimidine. This means A can pair with T, and G can pair with C, as Figure 3.33 shows. This is the base
complementary rule. In other words, the DNA strands are complementary to each other. If the sequence of
one strand is AATTGGCC, the complementary strand would have the sequence TTAACCGG. During DNA
replication, each strand copies itself, resulting in a daughter DNA double helix containing one parental DNA
strand and a newly synthesized strand.
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99
visual
CONNECTION
Figure 3.33 In a double stranded DNA molecule, the two strands run antiparallel to one another so that one strand
runs 5' to 3' and the other 3' to 5'. The phosphate backbone is located on the outside, and the bases are in the
middle. Adenine forms hydrogen bonds (or base pairs) with thymine, and guanine base pairs with cytosine.
A mutation occurs, and adenine replaces cytosine. What impact do you think this will have on the DNA
structure?
RNA
Ribonucleic acid, or RNA, is mainly involved in the process of protein synthesis under the direction of DNA.
RNA is usually single-stranded and is comprised of ribonucleotides that are linked by phosphodiester bonds. A
ribonucleotide in the RNA chain contains ribose (the pentose sugar), one of the four nitrogenous bases (A, U, G,
and C), and the phosphate group.
There are four major types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and
microRNA (miRNA). The first, mRNA, carries the message from DNA, which controls all of the cellular activities
in a cell. If a cell requires synthesizing a certain protein, the gene for this product turns “on” and the messenger
RNA synthesizes in the nucleus. The RNA base sequence is complementary to the DNA's coding sequence from
which it has been copied. However, in RNA, the base T is absent and U is present instead. If the DNA strand
has a sequence AATTGCGC, the sequence of the complementary RNA is UUAACGCG. In the cytoplasm, the
mRNA interacts with ribosomes and other cellular machinery (Figure 3.34).
Figure 3.34 A ribosome has two parts: a large subunit and a small subunit. The mRNA sits in between the two
subunits. A tRNA molecule recognizes a codon on the mRNA, binds to it by complementary base pairing, and adds
the correct amino acid to the growing peptide chain.
The mRNA is read in sets of three bases known as codons. Each codon codes for a single amino acid. In this
way, the mRNA is read and the protein product is made. Ribosomal RNA (rRNA) is a major constituent of
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Chapter 3 | Biological Macromolecules
ribosomes on which the mRNA binds. The rRNA ensures the proper alignment of the mRNA and the Ribosomes.
The ribosome's rRNA also has an enzymatic activity (peptidyl transferase) and catalyzes peptide bond formation
between two aligned amino acids. Transfer RNA (tRNA) is one of the smallest of the four types of RNA, usually
70-90 nucleotides long. It carries the correct amino acid to the protein synthesis site. It is the base pairing
between the tRNA and mRNA that allows for the correct amino acid to insert itself in the polypeptide chain.
MicroRNAs are the smallest RNA molecules and their role involves regulating gene expression by interfering
with the expression of certain mRNA messages. Table 3.2 summarizes DNA and RNA features.
DNA and RNA Features
DNA
RNA
Function
Carries genetic information
Involved in protein synthesis
Location
Remains in the nucleus
Leaves the nucleus
Structure
Double helix
Usually single-stranded
Sugar
Deoxyribose
Ribose
Pyrimidines
Cytosine, thymine
Cytosine, uracil
Purines
Adenine, guanine
Adenine, guanine
Table 3.2
Even though the RNA is single stranded, most RNA types show extensive intramolecular base pairing between
complementary sequences, creating a predictable three-dimensional structure essential for their function.
As you have learned, information flow in an organism takes place from DNA to RNA to protein. DNA dictates
the structure of mRNA in a process scientists call transcription, and RNA dictates the protein's structure in
a process scientists call translation. This is the Central Dogma of Life, which holds true for all organisms;
however, exceptions to the rule occur in connection with viral infections.
LINK TQ LEARNING
To learn more about DNA, explore the Howard Hughes Medical Institute Biolnteractive animations
(http:// 0 penstaxc 0 llege. 0 rg/l/DNA) on the topic of DNA.
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101
KEY TERMS
alpha-helix structure (or-helix) type of secondary protein structure formed by folding the polypeptide into a
helix shape with hydrogen bonds stabilizing the structure
amino acid a protein's monomer; has a central carbon or alpha carbon to which an amino group, a carboxyl
group, a hydrogen, and an R group or side chain is attached; the R group is different for all 20 common
amino acids
beta-pleated sheet (/3-pleated) secondary structure in proteins in which hydrogen bonding forms “pleats”
between atoms on the polypeptide chain's backbone
biological macromolecule large molecule necessary for life that is built from smaller organic molecules
carbohydrate biological macromolecule in which the ratio of carbon to hydrogen and to oxygen is 1:2:1;
carbohydrates serve as energy sources and structural support in cells and form arthropods' cellular
exoskeleton
cellulose polysaccharide that comprises the plants' cell wall; provides structural support to the cell
chaperone (also, chaperonin) protein that helps nascent protein in the folding process
chitin type of carbohydrate that forms the outer skeleton of all arthropods that include crustaceans and insects;
it also forms fungi cell walls
dehydration synthesis (also, condensation) reaction that links monomer molecules, releasing a water
molecule for each bond formed
denaturation loss of shape in a protein as a result of changes in temperature, pH, or chemical exposure
deoxyribonucleic acid (DNA) double-helical molecule that carries the cell's hereditary information
disaccharide two sugar monomers that a glycosidic bond links
enzyme catalyst in a biochemical reaction that is usually a complex or conjugated protein
glycogen storage carbohydrate in animals
glycosidic bond bond formed by a dehydration reaction between two monosaccharides with eliminating a
water molecule
hormone chemical signaling molecule, usually protein or steroid, secreted by endocrine cells that act to control
or regulate specific physiological processes
hydrolysis reaction that causes breakdown of larger molecules into smaller molecules by utilizing water
lipid macromolecule that is nonpolar and insoluble in water
messenger RNA (mRNA) RNA that carries information from DNA to ribosomes during protein synthesis
monomer smallest unit of larger molecules that are polymers
monosaccharide single unit or monomer of carbohydrates
nucleic acid biological macromolecule that carries the cell's genetic blueprint and carries instructions for the
cell's functioning
nucleotide monomer of nucleic acids; contains a pentose sugar, one or more phosphate groups, and a
nitrogenous base
omega fat type of polyunsaturated fat that the body requires; numbering the carbon omega starts from the
methyl end or the end that is farthest from the carboxylic end
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Chapter 3 | Biological Macromolecules
peptide bond bond formed between two amino acids by a dehydration reaction
phosphodiester linkage covalent chemical bond that holds together the polynucleotide chains with a
phosphate group linking neighboring nucleotides' two pentose sugars
phospholipid membranes' major constituent; comprised of two fatty acids and a phosphate-containing group
attached to a glycerol backbone
polymer chain of monomer residues that covalent bonds link; polymerization is the process of polymer
formation from monomers by condensation
polynucleotide long chain of nucleotides
polypeptide long chain of amino acids that peptide bonds link
polysaccharide long chain of monosaccharides; may be branched or unbranched
primary structure linear sequence of amino acids in a protein
protein biological macromolecule comprised of one or more amino acid chains
purine type of nitrogenous base in DNA and RNA; adenine and guanine are purines
pyrimidine type of nitrogenous base in DNA and RNA; cytosine, thymine, and uracil are pyrimidines
quaternary structure association of discrete polypeptide subunits in a protein
ribonucleic acid (RNA) single-stranded, often internally base paired, molecule that is involved in protein
synthesis
ribosomal RNA (rRNA) RNA that ensures the proper alignment of the mRNA and the ribosomes during protein
synthesis and catalyzes forming the peptide linkage
saturated fatty acid long-chain hydrocarbon with single covalent bonds in the carbon chain; the number of
hydrogen atoms attached to the carbon skeleton is maximized
secondary structure regular structure that proteins form by intramolecular hydrogen bonding between the
oxygen atom of one amino acid residue and the hydrogen attached to the nitrogen atom of another amino
acid residue
starch storage carbohydrate in plants
steroid type of lipid comprised of four fused hydrocarbon rings forming a planar structure
tertiary structure a protein's three-dimensional conformation, including interactions between secondary
structural elements; formed from interactions between amino acid side chains
trans fat fat formed artificially by hydrogenating oils, leading to a different arrangement of double bond(s) than
those in naturally occurring lipids
transcription process through which messenger RNA forms on a template of DNA
transfer RNA (tRNA) RNA that carries activated amino acids to the site of protein synthesis on the ribosome
translation process through which RNA directs the protein's formation
triacylglycerol (also, triglyceride) fat molecule; consists of three fatty acids linked to a glycerol molecule
unsaturated fatty acid long-chain hydrocarbon that has one or more double bonds in the hydrocarbon chain
wax lipid comprised of a long-chain fatty acid that is esterified to a long-chain alcohol; serves as a protective
coating on some feathers, aquatic mammal fur, and leaves
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CHAPTER SUMMARY
3.1 Synthesis of Biological Macromolecules
Proteins, carbohydrates, nucleic acids, and lipids are the four major classes of biological
macromolecules—large molecules necessary for life that are built from smaller organic molecules.
Macromolecules are comprised of single units scientists call monomers that are joined by covalent bonds to
form larger polymers. The polymer is more than the sum of its parts: it acquires new characteristics, and leads
to an osmotic pressure that is much lower than that formed by its ingredients. This is an important advantage in
maintaining cellular osmotic conditions. A monomer joins with another monomer with water molecule release,
leading to a covalent bond forming. Scientists call these dehydration or condensation reactions. When
polymers break down into smaller units (monomers), they use a water molecule for each bond broken by these
reactions. Such reactions are hydrolysis reactions. Dehydration and hydrolysis reactions are similar for all
macromolecules, but each monomer and polymer reaction is specific to its class. Dehydration reactions
typically require an investment of energy for new bond formation, while hydrolysis reactions typically release
energy by breaking bonds.
3.2 Carbohydrates
Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural
support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders.
Scientists classify carbohydrates as monosaccharides, disaccharides, and polysaccharides depending on the
number of monomers in the molecule. Monosaccharides are linked by glycosidic bonds that form as a result of
dehydration reactions, forming disaccharides and polysaccharides with eliminating a water molecule for each
bond formed. Glucose, galactose, and fructose are common monosaccharides; whereas, common
disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are
the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be
branched or unbranched. Cellulose is an example of an unbranched polysaccharide; whereas, amylopectin, a
constituent of starch, is a highly branched molecule. Glucose storage, in the form of polymers like starch of
glycogen, makes it slightly less accessible for metabolism; however, this prevents it from leaking out of the cell
or creating a high osmotic pressure that could cause the cell to uptake excessive water.
3.3 Lipids
Lipids are a class of macromolecules that are nonpolar and hydrophobic in nature. Major types include fats and
oils, waxes, phospholipids, and steroids. Fats are a stored form of energy and are also known as
triacylglycerols or triglycerides. Fats are comprised of fatty acids and either glycerol or sphingosine. Fatty acids
may be unsaturated or saturated, depending on the presence or absence of double bonds in the hydrocarbon
chain. If only single bonds are present, they are saturated fatty acids. Unsaturated fatty acids may have one or
more double bonds in the hydrocarbon chain. Phospholipids comprise the membrane's matrix. They have a
glycerol or sphingosine backbone to which two fatty acid chains and a phosphate-containing group are
attached. Steroids are another class of lipids. Their basic structure has four fused carbon rings. Cholesterol is a
type of steroid and is an important constituent of the plasma membrane, where it helps to maintain the
membrane's fluid nature. It is also the precursor of steroid hormones such as testosterone.
3.4 Proteins
Proteins are a class of macromolecules that perform a diverse range of functions for the cell. They help in
metabolism by acting as enzymes, carriers, or hormones, and provide structural support. The building blocks of
proteins (monomers) are amino acids. Each amino acid has a central carbon that bonds to an amino group, a
carboxyl group, a hydrogen atom, and an R group or side chain. There are 20 commonly occurring amino
acids, each of which differs in the R group. A peptide bond links each amino acid to its neighbors. A long amino
acid chain is a polypeptide.
Proteins are organized at four levels: primary, secondary, tertiary, and (optional) quaternary. The primary
structure is the amino acids' unique sequence. The polypeptide's local folding to form structures such as the
cr-helix and ^-pleated sheet constitutes the secondary structure. The overall three-dimensional structure is the
tertiary structure. When two or more polypeptides combine to form the complete protein structure, the
configuration is the protein's quaternary structure. Protein shape and function are intricately linked. Any change
in shape caused by changes in temperature or pH may lead to protein denaturation and a loss in function.
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Chapter 3 | Biological Macromolecules
3.5 Nucleic Acids
Nucleic acids are molecules comprised of nucleotides that direct cellular activities such as cell division and
protein synthesis. Pentose sugar, a nitrogenous base, and a phosphate group comprise each nucleotide. There
are two types of nucleic acids: DNA and RNA. DNA carries the cell's genetic blueprint and passes it on from
parents to offspring (in the form of chromosomes). It has a double-helical structure with the two strands running
in opposite directions, connected by hydrogen bonds, and complementary to each other. RNA is a single-
stranded polymer composed of linked nucleotides made up of a pentose sugar (ribose), a nitrogenous base,
and a phosphate group. RNA is involved in protein synthesis and its regulation. Messenger RNA (mRNA)
copies from the DNA, exports itself from the nucleus to the cytoplasm, and contains information for constructing
proteins. Ribosomal RNA (rRNA) is a part of the ribosomes at the site of protein synthesis; whereas, transfer
RNA (tRNA) carries the amino acid to the site of protein synthesis. The microRNA regulates using mRNA for
protein synthesis.
VISUAL CONNECTION QUESTIONS
1. Figure 3.5 What kind of sugars are these, aldose
or ketose?
2. Figure 3.23 Which categories of amino acid would
you expect to find on the surface of a soluble protein,
and which would you expect to find in the interior?
REVIEW QUESTIONS
4. Dehydration synthesis leads to formation of
a. monomers
b. polymers
c. water and polymers
d. none of the above
5. During the breakdown of polymers, which of the
following reactions takes place?
a. hydrolysis
b. dehydration
c. condensation
d. covalent bond
6. The following chemical reactants produce the ester
ethyl ethanoate (C 4 H 8 O 2 ):
C 2 H 6 O + CH 3 COOH
What type of reaction occurs to make ethyl
ethanoate?
a. condensation
b. hydrolysis
c. combustion
d. acid-base reaction
7. An example of a monosaccharide is_.
a. fructose
b. glucose
c. galactose
d. all of the above
8. Cellulose and starch are examples of:
What distribution of amino acids would you expect to
find in a protein embedded in a lipid bilayer?
3. Figure 3.33 A mutation occurs, and cytosine is
replaced with adenine. What impact do you think this
will have on the DNA structure?
a. monosaccharides
b. disaccharides
c. lipids
d. polysaccharides
9. Plant cell walls contain which of the following in
abundance?
a. starch
b. cellulose
c. glycogen
d. lactose
10. Lactose is a disaccharide formed by the
formation of a_bond between glucose and
a. glycosidic; lactose
b. glycosidic; galactose
c. hydrogen; sucrose
d. hydrogen; fructose
11. Which of the following is not an extracellular
matrix role of carbohydrates?
a. protect an insect’s internal organs from
external trauma
b. prevent plant cells from lysing after the plant
is watered
c. maintain the shape of a fungal spore
d. provide energy for muscle movement
12. Saturated fats have all of the following
characteristics except:
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Chapter 3 | Biological Macromolecules
105
a. they are solid at room temperature
b. they have single bonds within the carbon
chain
c. they are usually obtained from animal
sources
d. they tend to dissolve in water easily
13. Phospholipids are important components of
a. the plasma membrane of cells
b. the ring structure of steroids
c. the waxy covering on leaves
d. the double bond in hydrocarbon chains
14. Cholesterol is an integral part of plasma
membranes. Based on its structure, where is it found
in the membrane?
a. on the extracellular surface
b. embedded with the phospholipid heads
c. within the tail bilayer
d. attached to the intracellular surface
15. The monomers that make up proteins are called
a. nucleotides
b. disaccharides
c. amino acids
d. chaperones
16. The cr-helix and the /3-pleated sheet are part of
which protein structure?
a. primary
b. secondary
c. tertiary
d. quaternary
CRITICAL THINKING QUESTIONS
21. Why are biological macromolecules considered
organic?
22. What role do electrons play in dehydration
synthesis and hydrolysis?
23. Amino acids have the generic structure seen
below, where R represents different carbon-based
H H O
\ I ^
N —C —C
/ I \
side chains. H R OH
Describe how the structure of amino acids allows
them to be linked into long peptide chains to form
proteins.
24. Describe the similarities and differences between
glycogen and starch.
25. Why is it impossible for humans to digest food
that contains cellulose?
26. Draw the ketose and aldose forms of a
17. Mad cow disease is an infectious disease where
one misfolded protein causes all other copies of the
protein to being misfolding. This is an example of a
disease impacting_structure.
a. primary
b. secondary
c. tertiary
d. quaternary
18. A nucleotide of DNA may contain_.
a. ribose, uracil, and a phosphate group
b. deoxyribose, uracil, and a phosphate group
c. deoxyribose, thymine, and a phosphate
group
d. ribose, thymine, and a phosphate group
19. The building blocks of nucleic acids are
a. sugars
b. nitrogenous bases
c. peptides
d. nucleotides
20. How does the double helix structure of DNA
support its role in encoding the genome?
a. The sugar-phosphate backbone provides a
template for DNA replication.
b. tRNA pairing with the template strand
creates proteins encoded by the genome.
c. Complementary base pairing creates a very
stable structure.
d. Complementary base pairing allows for easy
editing of both strands of DNA.
monosaccharide with the chemical formula C 3 H 6 O 3 .
How is the structure of the monosaccharide changed
from one form to the other in the human body?
27. Explain at least three functions that lipids serve in
plants and/or animals.
28. Why have trans fats been banned from some
restaurants? How are they created?
29. Why are fatty acids better than glycogen for
storing large amounts of chemical energy?
30. Part of cortisol’s role in the body involves passing
through the plasma membrane to initiate signaling
inside a cell. Describe how the structures of cortisol
and the plasma membrane allow this to occur.
31. Explain what happens if even one amino acid is
substituted for another in a polypeptide chain.
Provide a specific example.
32. Describe the differences in the four protein
structures.
33. Aquaporins are proteins embedded in the plasma
membrane that allow water molecules to move
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Chapter 3 | Biological Macromolecules
between the extracellular matrix and the intracellular
space. Based on its function and location, describe
the key features of the protein’s shape and the
chemical characteristics of its amino acids.
34. What are the structural differences between RNA
and DNA?
35. What are the four types of RNA and how do they
function?
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Chapter 4 | Cell Structure
107
4 | CELL STRUCTURE
Figure 4.1 (a) Nasal sinus cells (viewed with a light microscope), (b) onion cells (viewed with a light microscope),
and (c) Vibrio tasmaniensis bacterial cells (seen through a scanning electron microscope) are from very different
organisms, yet all share certain basic cell structure characteristics, (credit a: modification of work by Ed Uthman, MD;
credit b: modification of work by Umberto Salvagnin; credit c: modification of work by Anthony D'Onofrio, William H.
Fowle, Eric J. Stewart, and Kim Lewis of the Lewis Lab at Northeastern University; scale-bar data from Matt Russell)
Chapter Outline
4.1: Studying Cells
4.2: Prokaryotic Cells
4.3: Eukaryotic Cells
4.4: The Endomembrane System and Proteins
4.5: The Cytoskeleton
4.6: Connections between Cells and Cellular Activities
Introduction
Close your eyes and picture a brick wall. What is the wall's basic building block? It is a single brick. Like a brick
wall, cells are the building blocks that make up your body.
Your body has many kinds of cells, each specialized for a specific purpose. Just as we use a variety of materials
to build a home, the human body is constructed from many cell types. For example, epithelial cells protect
the body's surface and cover the organs and body cavities within. Bone cells help to support and protect the
body. Immune system cells fight invading bacteria. Additionally, blood and blood cells carry nutrients and oxygen
throughout the body while removing carbon dioxide. Each of these cell types plays a vital role during the body's
growth, development, and day-to-day maintenance. In spite of their enormous variety, however, cells from all
organisms—even ones as diverse as bacteria, onion, and human—share certain fundamental characteristics.
4.1 1 Studying Cells
By the end of this section, you will be able to do the following:
• Describe the role of cells in organisms
• Compare and contrast light microscopy and electron microscopy
• Summarize cell theory
A cell is the smallest unit of a living thing. Whether comprised of one cell (like bacteria) or many cells (like a
human), we call it an organism. Thus, cells are the basic building blocks of all organisms.
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Chapter 4 | Cell Structure
Several cells of one kind that interconnect with each other and perform a shared function form tissues. These
tissues combine to form an organ (your stomach, heart, or brain), and several organs comprise an organ system
(such as the digestive system, circulatory system, or nervous system). Several systems that function together
form an organism (like a human being). Here, we will examine the structure and function of cells.
There are many types of cells, which scientists group into one of two broad categories: prokaryotic and
eukaryotic. For example, we classify both animal and plant cells as eukaryotic cells; whereas, we classify
bacterial cells as prokaryotic. Before discussing the criteria for determining whether a cell is prokaryotic or
eukaryotic, we will first examine how biologists study cells.
Microscopy
Cells vary in size. With few exceptions, we cannot see individual cells with the naked eye, so scientists use
microscopes (micro- = “small”; -scope = “to look at”) to study them. A microscope is an instrument that
magnifies an object. We photograph most cells with a microscope, so we can call these images micrographs.
The optics of a microscope’s lenses change the image orientation that the user sees. A specimen that is right-
side up and facing right on the microscope slide will appear upside-down and facing left when one views through
a microscope, and vice versa. Similarly, if one moves the slide left while looking through the microscope, it will
appear to move right, and if one moves it down, it will seem to move up. This occurs because microscopes use
two sets of lenses to magnify the image. Because of the manner by which light travels through the lenses, this
two lens system produces an inverted image (binocular, or dissecting microscopes, work in a similar manner, but
include an additional magnification system that makes the final image appear to be upright).
Light Microscopes
To give you a sense of cell size, a typical human red blood cell is about eight millionths of a meter or eight
micrometers (abbreviated as eight pm) in diameter. A pin head is about two thousandths of a meter (two mm) in
diameter. That means about 250 red blood cells could fit on a pinhead.
Most student microscopes are light microscopes (Figure 4.2a). Visible light passes and bends through the
lens system to enable the user to see the specimen. Light microscopes are advantageous for viewing living
organisms, but since individual cells are generally transparent, their components are not distinguishable unless
they are colored with special stains. Staining, however, usually kills the cells.
Light microscopes that undergraduates commonly use in the laboratory magnify up to approximately 400 times.
Two parameters that are important in microscopy are magnification and resolving power. Magnification is the
process of enlarging an object in appearance. Resolving power is the microscope's ability to distinguish two
adjacent structures as separate: the higher the resolution, the better the image's clarity and detail. When one
uses oil immersion lenses to study small objects, magnification usually increases to 1,000 times. In order to gain
a better understanding of cellular structure and function, scientists typically use electron microscopes.
(a) (b)
Figure 4.2 (a) Most light microscopes in a college biology lab can magnify cells up to approximately 400 times and
have a resolution of about 200 nanometers, (b) Electron microscopes provide a much higher magnification, 100,000x,
and a have a resolution of 50 picometers. (credit a: modification of work by "GcG'VWikimedia Commons; credit b:
modification of work by Evan Bench)
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Chapter 4 | Cell Structure
109
Electron Microscopes
In contrast to light microscopes, electron microscopes (Figure 4.2b) use a beam of electrons instead of a
beam of light. Not only does this allow for higher magnification and, thus, more detail (Figure 4.3), it also
provides higher resolving power. The method to prepare the specimen for viewing with an electron microscope
kills the specimen. Electrons have short wavelengths (shorter than photons) that move best in a vacuum, so we
cannot view living cells with an electron microscope.
In a scanning electron microscope, a beam of electrons moves back and forth across a cell’s surface, creating
details of cell surface characteristics. In a transmission electron microscope, the electron beam penetrates
the cell and provides details of a cell’s internal structures. As you might imagine, electron microscopes are
significantly more bulky and expensive than light microscopes.
(a) (b)
Figure 4.3 (a) These Salmonella bacteria appear as tiny purple dots when viewed with a light microscope, (b) This
scanning electron microscope micrograph shows Salmonella bacteria (in red) invading human cells (yellow). Even
though subfigure (b) shows a different Salmonella specimen than subfigure (a), you can still observe the comparative
increase in magnification and detail, (credit a: modification of work by CDC/Armed Forces Institute of Pathology,
Charles N. Farmer, Rocky Mountain Laboratories; credit b: modification of work by NIAID, NIH; scale-bar data from
Matt Russell)
LINK TQ LEARNING
For another perspective on cell size, try the HowBig interactive at this site (http:// 0 penstaxc 0 llege. 0 rg/l/
cell_sizes).
Cell Theory
The microscopes we use today are far more complex than those that Dutch shopkeeper Antony van
Leeuwenhoek, used in the 1600s. Skilled in crafting lenses, van Leeuwenhoek observed the movements of
single-celled organisms, which he collectively termed “animalcules.”
In the 1665 publication Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like
structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered
bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled
other scientists to see some components inside cells.
By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and
proposed the unified cell theory, which states that one or more cells comprise all living things, the cell is the
basic unit of life, and new cells arise from existing cells. Rudolf Virchow later made important contributions to
this theory.
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Chapter 4 | Cell Structure
ca eer connection
Cytotechnologist
Have you ever heard of a medical test called a Pap smear (Figure 4.4)? In this test, a doctor takes a small
sample of cells from the patient's uterine cervix and sends it to a medical lab where a cytotechnologist stains
the cells and examines them for any changes that could indicate cervical cancer or a microbial infection.
Cytotechnologists (cyto- = “cell”) are professionals who study cells via microscopic examinations and other
laboratory tests. They are trained to determine which cellular changes are within normal limits and which
are abnormal. Their focus is not limited to cervical cells. They study cellular specimens that come from all
organs. When they notice abnormalities, they consult a pathologist, a medical doctor who interprets and
diagnoses changes that disease in body tissue and fluids cause.
Cytotechnologists play a vital role in saving people’s lives. When doctors discover abnormalities early, a
patient’s treatment can begin sooner, which usually increases the chances of a successful outcome.
10 pm
Figure 4.4 These uterine cervix cells, viewed through a light microscope, are from a Pap smear. Normal cells are
on the left. The cells on the right are infected with human papillomavirus (HPV). Notice that the infected cells are
larger. Also, two of these cells each have two nuclei instead of one, the normal number, (credit: modification of
work by Ed Uthman, MD; scale-bar data from Matt Russell)
4.2 | Prokaryotic Cells
By the end of this section, you will be able to do the following:
• Name examples of prokaryotic and eukaryotic organisms
• Compare and contrast prokaryotic and eukaryotic cells
• Describe the relative sizes of different cells
• Explain why cells must be small
Cells fall into one of two broad categories: prokaryotic and eukaryotic. We classify only the predominantly single-
celled organisms Bacteria and Archaea as prokaryotes (pro- = “before”; -kary- = “nucleus”). Animal cells, plants,
fungi, and protists are all eukaryotes (eu- = “true”).
Components of Prokaryotic Cells
All cells share four common components: 1) a plasma membrane, an outer covering that separates the cell’s
interior from its surrounding environment; 2) cytoplasm, consisting of a jelly-like cytosol within the cell in which
there are other cellular components; 3) DNA, the cell's genetic material; and 4) ribosomes, which synthesize
proteins. However, prokaryotes differ from eukaryotic cells in several ways.
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Chapter 4 | Cell Structure
111
A prokaryote is a simple, mostly single-celled (unicellular) organism that lacks a nucleus, or any other
membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes.
Prokaryotic DNA is in the cell's central part: the nucleoid (Figure 4.5).
(DNA)
Figure 4.5 This figure shows the generalized structure of a prokaryotic cell. All prokaryotes have chromosomal DNA
localized in a nucleoid, ribosomes, a cell membrane, and a cell wall. The other structures shown are present in some,
but not all, bacteria.
Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule (Figure 4.5). The
cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The
capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae.
Flagella are used for locomotion. Pili exchange genetic material during conjugation, the process by which one
bacterium transfers genetic material to another through direct contact. Bacteria use fimbriae to attach to a host
cell.
ca eer connection
Microbiologist
The most effective action anyone can take to prevent the spread of contagious illnesses is to wash his or
her hands. Why? Because microbes (organisms so tiny that they can only be seen with microscopes) are
ubiquitous. They live on doorknobs, money, your hands, and many other surfaces. If someone sneezes
into his hand and touches a doorknob, and afterwards you touch that same doorknob, the microbes from
the sneezer’s mucus are now on your hands. If you touch your hands to your mouth, nose, or eyes, those
microbes can enter your body and could make you sick.
However, not all microbes (also called microorganisms) cause disease; most are actually beneficial. You
have microbes in your gut that make vitamin K. Other microorganisms are used to ferment beer and wine.
Microbiologists are scientists who study microbes. Microbiologists can pursue a number of careers. Not only
do they work in the food industry, they are also employed in the veterinary and medical fields. They can work
in the pharmaceutical sector, serving key roles in research and development by identifying new antibiotic
sources that can treat bacterial infections.
Environmental microbiologists may look for new ways to use specially selected or genetically engineered
microbes to remove pollutants from soil or groundwater, as well as hazardous elements from contaminated
sites. We call using these microbes bioremediation technologies. Microbiologists can also work in the
bioinformatics field, providing specialized knowledge and insight for designing, developing, and specificity
of computer models of, for example, bacterial epidemics.
Cell Size
At 0.1 to 5.0 pm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have
diameters ranging from 10 to 100 pm (Figure 4.6). The prokaryotes' small size allows ions and organic
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Chapter 4 | Cell Structure
molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within
a prokaryotic cell can quickly diffuse. This is not the case in eukaryotic cells, which have developed different
structural adaptations to enhance intracellular transport.
Atom
j
0.1 nm
I
Protein
1
Lipids
Bacteria
Human
egg
Frog
egg
Relative sizes on a logarithmic scale
j_i_i _ i_i_i_i
1 nm 10 nm 100 nm 1 pm 10 pm 100 pm 1 mm
I
Light microscope
t
Chicken
egg
0
Ostrich Adult
e 99 male
10 mm 100 mm 1 m
Naked eye
Electron microscope
Figure 4.6 This figure shows relative sizes of microbes on a logarithmic scale (recall that each unit of increase in a
logarithmic scale represents a 10-fold increase in the quantity measured).
Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is
so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend
to approximate a sphere. You may remember from your high school geometry course that the formula for the
surface area of a sphere is 4nr 2 , while the formula for its volume is 4nr 3 /3. Thus, as the radius of a cell increases,
its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much
more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same
principle would apply if the cell had a cube shape (Figure 4.7). If the cell grows too large, the plasma membrane
will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other
words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide. Another way
is to develop organelles that perform specific tasks. These adaptations lead to developing more sophisticated
cells, which we call eukaryotic cells.
visual
CONNECTION
Figure 4.7 Notice that as a cell increases in size, its surface area-to-volume ratio decreases. When there is
insufficient surface area to support a cell’s increasing volume, a cell will either divide or die. The cell on the left
has a volume of 1 mm 3 and a surface area of 6 mm 2 , with a surface area-to-volume ratio of 6 to 1; whereas, the
cell on the right has a volume of 8 mm 3 and a surface area of 24 mm 2 , with a surface area-to-volume ratio of 3 to
1 .
Prokaryotic cells are much smaller than eukaryotic cells. What advantages might small cell size confer on a
cell? What advantages might large cell size have?
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Chapter 4 | Cell Structure
113
4.3 | Eukaryotic Cells
By the end of this section, you will be able to do the following:
• Describe the structure of eukaryotic cells
• Compare animal cells with plant cells
• State the role of the plasma membrane
• Summarize the functions of the major cell organelles
Have you ever heard the phrase “form follows function?” It’s a philosophy that many industries follow. In
architecture, this means that buildings should be constructed to support the activities that will be carried out
inside them. For example, a skyscraper should include several elevator banks. A hospital should have its
emergency room easily accessible.
Our natural world also utilizes the principle of form following function, especially in cell biology, and this will
become clear as we explore eukaryotic cells ( Figure 4.8). Unlike prokaryotic cells, eukaryotic cells have: 1)
a membrane-bound nucleus; 2) numerous membrane-bound organelles such as the endoplasmic reticulum,
Golgi apparatus, chloroplasts, mitochondria, and others; and 3) several, rod-shaped chromosomes. Because a
membrane surrounds eukaryotic cell’s nucleus, it has a “true nucleus." The word “organelle" means “little organ,”
and, as we already mentioned, organelles have specialized cellular functions, just as your body's organs have
specialized functions.
At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic
cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to
organelles, let’s first examine two important components of the cell: the plasma membrane and the cytoplasm.
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Chapter 4 | Cell Structure
visual
CONNECTION
Nucleus
Nuclear envelope:
membrane enclosing
the nucleus. Protein-lined
pores allow material to
move in and out.
Chromatin: DNAplus
associated proteins.
Nucleolus:
condensed region
where ribosomes
are formed.
Peroxisome:
metabolizes
waste
Endoplasmic
reticulum
Rough: associated
with ribosomes;
makes secretory and
membrane proteins.
Smooth: makes lipids.
Cytoskeleton
Microtubules: form the
mitotic spindle and
maintain cell shape.
Centrosome: microtubule¬
organizing center.
Intermediate filaments:
fibrous proteins that hold
organelles in place.
Microfilaments:
fibrous proteins;
form the cellular
cortex.
Plasma
membrane
Lysosome:
digests food and
waste materials.
Golgi apparatus:
modifies proteins.
Vacuole
Cytoplasm
Mitochondria:
produce energy.
Plasma
membrane
Nucleus contains
chromatin, a
nuclear envelope,
and a nucleolus,
as In an animal cell
Cytoplasm
Ribosomes
Chloroplast site Plastid store
of photosynthesis pigments
Cell wall maintains
cell shape
Central Vacuole
filled with cell sap
that maintains
pressure against
cell wall
Cytoskeleton
microtubules
intermediate
filaments
microfilaments
Golgi
apparatus
Mitochondria
Peroxisome
Plasmodesmata
channels connect
two plant cells
(a)
Endoplasmic Reticulum
smooth rough
(b)
Figure 4.8 These figures show the major organelles and other cell components of (a) a typical animal cell
and (b) a typical eukaryotic plant cell. The plant cell has a cell wall, chloroplasts, plastids, and a central
vacuole—structures not in animal cells. Most cells do not have lysosomes or centrosomes.
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Chapter 4 | Cell Structure
115
If the nucleolus were not able to carry out its function, what other cellular organelles would be affected?
The Plasma Membrane
Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 4.9), a phospholipid bilayer with
embedded proteins that separates the internal contents of the cell from its surrounding environment. A
phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma
membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes
(such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane.
Glycoprotein: protein with
carbohydrate attached
. Glycolipid: lipid with
/ carbohydrate
attached
Phospholipid
bilayer
Integral membrane
protein
Protein channel
Filaments of the cytoskeleton
Figure 4.9 The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.
The plasma membranes of cells that specialize in absorption fold into fingerlike projections that we call microvilli
(singular = microvillus); (Figure 4.10). Such cells typically line the small intestine, the organ that absorbs
nutrients from digested food. This is an excellent example of form following function. People with celiac disease
have an immune response to gluten, which is a protein in wheat, barley, and rye. The immune response
damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping,
and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.
Microvilli
I I
Side of
cell facing
inside of
small
intestine
Plasma membrane Nucleus
Figure 4.10 Microvilli, as they appear on cells lining the small intestine, increase the surface area available for
absorption. These microvilli are only on the area of the plasma membrane that faces the cavity from which substances
will be absorbed, (credit "micrograph": modification of work by Louisa Howard)
The Cytoplasm
The cytoplasm is the cell's entire region between the plasma membrane and the nuclear envelope (a structure
we will discuss shortly). It is comprised of organelles suspended in the gel-like cytosol, the cytoskeleton, and
various chemicals (Figure 4.8). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi¬
solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules
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Chapter 4 | Cell Structure
in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids,
and derivatives of glycerol are also there. Ions of sodium, potassium, calcium, and many other elements also
dissolve in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.
The Nucleus
Typically, the nucleus is the most prominent organelle in a cell (Figure 4.8). The nucleus (plural = nuclei) houses
the cell’s DNA and directs the synthesis of ribosomes and proteins. Let’s look at it in more detail (Figure 4.11).
Figure 4.11 The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The
nucleolus is a condensed chromatin region where ribosome synthesis occurs. We call the nucleus' boundary the
nuclear envelope. It consists of two phospholipid bilayers: an outer and an inner membrane. The nuclear membrane is
continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus.
The Nuclear Envelope
The nuclear envelope is a double-membrane structure that constitutes the nucleus' outermost portion (Figure
4.11). Both the nuclear envelope's inner and outer membranes are phospholipid bilayers.
The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between
the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the
chromatin and the nucleolus.
Chromatin and Chromosomes
To understand chromatin, it is helpful to first explore chromosomes, structures within the nucleus that are made
up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single
circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific
number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46,
while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the
cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins
attach to chromosomes, and they resemble an unwound, jumbled bunch of threads. We call these unwound
protein-chromosome complexes chromatin (Figure 4.12). Chromatin describes the material that makes up the
chromosomes both when condensed and decondensed.
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(a) (b)
Figure 4.12 (a) This image shows various levels of chromatin's organization (DNA and protein), (b) This image shows
paired chromosomes, (credit b: modification of work by NIH; scale-bar data from Matt Russell)
The Nucleolus
We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some
chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus
called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the
ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.
Ribosomes
Ribosomes are the cellular structures responsible for protein synthesis. When we view them through an electron
microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the
cytoplasm. They may be attached to the plasma membrane's cytoplasmic side or the endoplasmic reticulum's
cytoplasmic side and the nuclear envelope's outer membrane (Figure 4.8). Electron microscopy shows us
that ribosomes, which are large protein and RNA complexes, consist of two subunits, large and small (Figure
4.13). Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA transcribes
into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the
sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids
are the building blocks of proteins.
Growing peptide
chain
Amino
acid
Ribosome
large
subunit
tRNA
mRNA
Ribosome
small
subunit
\
Figure 4.13 A large subunit (top) and a small subunit (bottom) comprise ribosomes. During protein synthesis,
ribosomes assemble amino acids into proteins.
Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies,
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pigments, structural components, and surface receptors), there are ribosomes in practically every cell.
Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the
pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes
contain many ribosomes. Thus, we see another example of form following function.
Mitochondria
Scientists often call mitochondria (singular = mitochondrion) the cell's “powerhouses" or “energy factories”
because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying
molecule. ATP represents the cell's short-term stored energy. Cellular respiration is the process of making
ATP using the chemical energy in glucose and other nutrients. In mitochondria, this process uses oxygen and
produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes
from the cellular reactions that produce carbon dioxide as a byproduct.
In keeping with our theme of form following function, it is important to point out that muscle cells have a very high
concentration of mitochondria that produce ATP. Your muscle cells need considerable energy to keep your body
moving. When your cells don’t get enough oxygen, they do not make much ATP. Instead, producing lactic acid
accompanies the small amount of ATP they make in the absence of oxygen.
Mitochondria are oval-shaped, double membrane organelles (Figure 4.14) that have their own ribosomes and
DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae.
We call the area surrounded by the folds the mitochondrial matrix. The cristae and the matrix have different roles
in cellular respiration.
Mitochondrial
matrix
Cristae
Outer membrane
Inner membrane
Figure 4.14 This electron micrograph shows a mitochondrion through an electron microscope. This organelle has
an outer membrane and an inner membrane. The inner membrane contains folds, called cristae, which increase its
surface area. We call the space between the two membranes the intermembrane space, and the space inside the inner
membrane the mitochondrial matrix. ATP synthesis takes place on the inner membrane, (credit: modification of work
by Matthew Britton; scale-bar data from Matt Russell)
Peroxisomes
Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions
that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of
these oxidation reactions release hydrogen peroxide, H 2 O 2 , which would be damaging to cells; however, when
these reactions are confined to peroxisomes, enzymes safely break down the H 2 O 2 into oxygen and water.) For
example, peroxisomes in liver cells detoxify alcohol. Glyoxysomes, which are specialized peroxisomes in plants,
are responsible for converting stored fats into sugars. Plant cells contain many different types of peroxisomes
that play a role in metabolism, pathogene defense, and stress response, to mention a few.
Vesicles and Vacuoles
Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Other than the fact
that vacuoles are somewhat larger than vesicles, there is a very subtle distinction between them. Vesicle
membranes can fuse with either the plasma membrane or other membrane systems within the cell. Additionally,
some agents such as enzymes within plant vacuoles break down macromolecules. The vacuole's membrane
does not fuse with the membranes of other cellular components.
Animal Cells versus Plant Cells
At this point, you know that each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, ribosomes,
mitochondria, peroxisomes, and in some, vacuoles, but there are some striking differences between animal
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and plant cells. While both animal and plant cells have microtubule organizing centers (MTOCs), animal cells
also have centrioles associated with the MTOC: a complex we call the centrosome. Animal cells each have a
centrosome and lysosomes; whereas, most plant cells do not. Plant cells have a cell wall, chloroplasts and other
specialized plastids, and a large central vacuole; whereas, animal cells do not.
The Centrosome
The centrosome is a microtubule-organizing center found near the nuclei of animal cells. It contains a pair of
centrioles, two structures that lie perpendicular to each other (Figure 4.15). Each centriole is a cylinder of nine
triplets of microtubules.
Figure 4.15 The centrosome consists of two centrioles that lie at right angles to each other. Each centriole is a cylinder
comprised of nine triplets of microtubules. Nontubulin proteins (indicated by the green lines) hold the microtubule
triplets together.
The centrosome (the organelle where all microtubules originate) replicates itself before a cell divides, and the
centrioles appear to have some role in pulling the duplicated chromosomes to opposite ends of the dividing cell.
However, the centriole's exact function in cell division isn’t clear, because cells that have had the centrosome
removed can still divide, and plant cells, which lack centrosomes, are capable of cell division.
Lysosomes
Animal cells have another set of organelles that most plant cells do not: lysosomes. The lysosomes are the
cell’s “garbage disposal.” In plant cells, the digestive processes take place in vacuoles. Enzymes within the
lysosomes aid in breaking down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles.
These enzymes are active at a much lower pH than the cytoplasm's. Therefore, the pH within lysosomes is more
acidic than the cytoplasm's pH. Many reactions that take place in the cytoplasm could not occur at a low pH, so
again, the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.
The Cell Wall
If you examine Figure 4.8, the plant cell diagram, you will see a structure external to the plasma membrane.
This is the cell wall, a rigid covering that protects the cell, provides structural support, and gives shape to the
cell. Fungal and some protistan cells also have cell walls. While the prokaryotic cell walls' chief component is
peptidoglycan, the major organic molecule in the plant (and some protists') cell wall is cellulose (Figure 4.16), a
polysaccharide comprised of glucose units. Have you ever noticed that when you bite into a raw vegetable, like
celery, it crunches? That’s because you are tearing the celery cells' rigid cell walls with your teeth.
H 2 C-OH H O H H 2 C-OH H 0 H H 2 C OH
Figure 4.16 Cellulose is a long chain of p-glucose molecules connected by a 1-4 linkage. The dashed lines at each
end of the figure indicate a series of many more glucose units. The size of the page makes it impossible to portray an
entire cellulose molecule.
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Chloroplasts
Like the mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely
different function. Chloroplasts are plant cell organelles that carry out photosynthesis. Photosynthesis is the
series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen. This is a major
difference between plants and animals. Plants (autotrophs) are able to make their own food, like sugars, while
animals (heterotrophs) must ingest their food.
Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a
chloroplast’s inner membrane is a set of interconnected and stacked fluid-filled membrane sacs we call
thylakoids (Figure 4.17). Each thylakoid stack is a granum (plural = grana). We call the fluid enclosed by the
inner membrane that surrounds the grana the stroma.
Outer Intermembrane Inner Stroma
membrane space membrane (aqueous fluid)
Figure 4.17 The chloroplast has an outer membrane, an inner membrane, and membrane structures - thylakoids that
are stacked into grana. We call the space inside the thylakoid membranes the thylakoid space. The light harvesting
reactions take place in the thylakoid membranes, and sugar synthesis takes place in the fluid inside the inner
membrane, which we call the stroma. Chloroplasts also have their own genome, which is contained on a single circular
chromosome.
The chloroplasts contain a green pigment, chlorophyll, which captures the light energy that drives the reactions
of photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria perform
photosynthesis, but their chlorophyll is not relegated to an organelle.
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V /
e olution CONNECTION
Endosymbiosis
We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you
wondered why? Strong evidence points to endosymbiosis as the explanation.
Symbiosis is a relationship in which organisms from two separate species depend on each other for their
survival. Endosymbiosis (endo- = “within”) is a mutually beneficial relationship in which one organism lives
inside the other. Endosymbiotic relationships abound in nature. We have already mentioned that microbes
that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are
unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other
organisms and from drying out, and they receive abundant food from the environment of the large intestine.
Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know
that bacteria have DNA and ribosomes, just like mitochondria and chloroplasts. Scientists believe that host
cells and bacteria formed an endosymbiotic relationship when the host cells ingested both aerobic and
autotrophic bacteria (cyanobacteria) but did not destroy them. Through many millions of years of evolution,
these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming
mitochondria and the autotrophic bacteria becoming chloroplasts.
The Central Vacuole
Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 4.8b, you will
see that plant cells each have a large central vacuole that occupies most of the cell's area. The central vacuole
plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you
ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration
in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and
cytoplasm. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the plant's
cell walls results in the wilted appearance.
The central vacuole also supports the cell's expansion. When the central vacuole holds more water, the cell
becomes larger without having to invest considerable energy in synthesizing new cytoplasm.
4.4 | The Endomembrane System and Proteins
By the end of this section, you will be able to do the following:
• List the components of the endomembrane system
• Recognize the relationship between the endomembrane system and its functions
The endomembrane system (endo = “within”) is a group of membranes and organelles (Figure 4.18) in
eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear
envelope, lysosomes, and vesicles, which we have already mentioned, and the endoplasmic reticulum and Golgi
apparatus, which we will cover shortly. Although not technically within the cell, the plasma membrane is included
in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles.
The endomembrane system does not include either mitochondria or chloroplast membranes.
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Chapter 4 | Cell Structure
visual
Cisternae
trans face
Figure 4.18 Membrane and secretory proteins are synthesized in the rough endoplasmic reticulum (RER). The
RER also sometimes modifies proteins. In this illustration, an attachment of a (purple) carbohydrate modifies a
(green) integral membrane protein in the ER. Vesicles with the integral protein bud from the ER and fuse with the
Golgi apparatus' cis face. As the protein passes along the Golgi’s cisternae, the addition of more carbohydrates
further modifies it. After its synthesis is complete, it exits as an integral membrane protein of the vesicle that buds
from the Golgi's trans face. When the vesicle fuses with the cell membrane, the protein becomes an integral
portion of that cell membrane, (credit: modification of work by Magnus Manske)
If a peripheral membrane protein were synthesized in the lumen (inside) of the ER, would it end up on the
inside or outside of the plasma membrane?
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) (Figure 4.18) is a series of interconnected membranous sacs and tubules
that collectively modifies proteins and synthesizes lipids. However, these two functions take place in separate
areas of the ER: the rough ER and the smooth ER, respectively.
We call the ER tubules' hollow portion the lumen or cisternal space. The ER's membrane, which is a phospholipid
bilayer embedded with proteins, is continuous with the nuclear envelope.
Rough ER
Scientists have named the rough endoplasmic reticulum (RER) as such because the ribosomes attached to
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its cytoplasmic surface give it a studded appearance when viewing it through an electron microscope (Figure
4.19).
Mitochondrion overlaying Rough endoplasmic reticulum
Nuclear envelope
Nuclear pore
Nucleus
Nucleolus
Figure 4.19 This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a
pancreatic cell, (credit: modification of work by Louisa Howard)
Ribosomes transfer their newly synthesized proteins into the RER's lumen where they undergo structural
modifications, such as folding or acquiring side chains. These modified proteins incorporate into cellular
membranes—the ER or the ER's or other organelles' membranes. The proteins can also secrete from the cell
(such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes.
If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations
via transport vesicles that bud from the RER’s membrane (Figure 4.18).
Since the RER is engaged in modifying proteins (such as enzymes, for example) that secrete from the cell, you
would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with liver
cells, for example.
Smooth ER
The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on
its cytoplasmic surface (Figure 4.18). SER functions include synthesis of carbohydrates, lipids, and steroid
hormones; detoxification of medications and poisons; and storing calcium ions.
In muscle cells, a specialized SER, the sarcoplasmic reticulum, is responsible for storing calcium ions that are
needed to trigger the muscle cells' coordinated contractions.
LINK TQ LEARNING
You can watch an excellent animation of the endomembrane system here (http:// 0 penstax. 0 rg/l/
insane_in_the_endomembrane) . At the end of the animation, there is a short self-assessment.
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ca eer connection
Cardiologist
Heart disease is the leading cause of death in the United States. This is primarily due to our sedentary
lifestyle and our high trans-fat diets.
Heart failure is just one of many disabling heart conditions. Heart failure does not mean that the heart has
stopped working. Rather, it means that the heart can’t pump with sufficient force to transport oxygenated
blood to all the vital organs. Left untreated, heart failure can lead to kidney failure and other organ failure.
Cardiac muscle tissue comprises the heart's wall. Heart failure occurs when cardiac muscle cells'
endoplasmic reticula do not function properly. As a result, an insufficient number of calcium ions are
available to trigger a sufficient contractile force.
Cardiologists (cardi- = “heart"; -ologist = “one who studies”) are doctors who specialize in treating heart
diseases, including heart failure. Cardiologists can diagnose heart failure via a physical examination, results
from an electrocardiogram (ECG, a test that measures the heart's electrical activity), a chest X-ray to see
whether the heart is enlarged, and other tests. If the cardiologist diagnoses heart failure, he or she will
typically prescribe appropriate medications and recommend a reduced table salt intake and a supervised
exercise program.
The Golgi Apparatus
We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where
do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles
still need sorting, packaging, and tagging so that they end up in the right place. Sorting, tagging, packaging,
and distributing lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of
flattened membranes (Figure 4.20).
Golgi apparatus
Figure 4.20 The Golgi apparatus in this white blood cell is visible as a stack of semicircular, flattened rings in the lower
portion of the image. You can see several vesicles near the Golgi apparatus, (credit: modification of work by Louisa
Howard)
We call the Golgi apparatus' the cis face. The opposite side is the trans face. The transport vesicles that formed
from the ER travel to the cis face, fuse with it, and empty their contents into the Golgi apparatus' lumen. As the
proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The
most frequent modification is adding short sugar molecule chains. These newly modified proteins and lipids then
tag with phosphate groups or other small molecules in order to travel to their proper destinations.
Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the Golgi's trans
face. While some of these vesicles deposit their contents into other cell parts where they will be used, other
secretory vesicles fuse with the plasma membrane and release their contents outside the cell.
in another example of form following function, cells that engage in a great deal of secretory activity (such as
salivary gland cells that secrete digestive enzymes or immune system cells that secrete antibodies) have an
abundance of Golgi.
In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are
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Chapter 4 | Cell Structure
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incorporated into the cell wall and some of which other cell parts use.
career connection
Geneticist
Many diseases arise from genetic mutations that prevent synthesizing critical proteins. One such disease
is Lowe disease (or oculocerebrorenal syndrome, because it affects the eyes, brain, and kidneys), in Lowe
disease, there is a deficiency in an enzyme localized to the Golgi apparatus. Children with Lowe disease
are born with cataracts, typically develop kidney disease after the first year of life, and may have impaired
mental abilities.
A mutation on the X chromosome causes Lowe disease. The X chromosome is one of the two human sex
chromosomes, as these chromosomes determine a person's sex. Females possess two X chromosomes
while males possess one X and one Y chromosome. In females, the genes on only one of the two X
chromosomes are expressed. Females who carry the Lowe disease gene on one of their X chromosomes
are carriers and do not show symptoms of the disease. However, males only have one X chromosome and
the genes on this chromosome are always expressed. Therefore, males will always have Lowe disease if
their X chromosome carries the Lowe disease gene. Geneticists have identified the mutated gene's location,
as well as many other mutation locations that cause genetic diseases. Through prenatal testing, a woman
can find out if the fetus she is carrying may be afflicted with one of several genetic diseases.
Geneticists analyze prenatal genetic test results and may counsel pregnant women on available options.
They may also conduct genetic research that leads to new drugs or foods, or perform DNA analyses for
forensic investigations.
Lysosomes
in addition to their role as the digestive component and organelle-recycling facility of animal cells, lysosomes
are part of the endomembrane system. Lysosomes also use their hydrolytic enzymes to destroy pathogens
(disease-causing organisms) that might enter the cell. A good example of this occurs in macrophages, a group
of white blood cells which are part of your body’s immune system. In a process that scientists call phagocytosis
or endocytosis, a section of the macrophage's plasma membrane invaginates (folds in) and engulfs a pathogen.
The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and
becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the
pathogen (Figure 4.21).
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Phagocytosis
Food particle Food vacuole
Figure 4.21 A macrophage has engulfed (phagocytized) a potentially pathogenic bacterium and then fuses with
lysosomes within the cell to destroy the pathogen. Other organelles are present in the cell but for simplicity we do not
show them.
4.5 | The Cytoskeleton
By the end of this section, you will be able to do the following:
• Describe the cytoskeleton
• Compare the roles of microfilaments, intermediate filaments, and microtubules
• Compare and contrast cilia and flagella
• Summarize the differences among the components of prokaryotic cells, animal cells, and plant cells
If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be
the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus
a network of protein fibers that help maintain the cell's shape, secure some organelles in specific positions,
allow cytoplasm and vesicles to move within the cell, and enable cells within multicellular organisms to move.
Collectively, scientists call this network of protein fibers the cytoskeleton. There are three types of fibers within
the cytoskeleton: microfilaments, intermediate filaments, and microtubules (Figure 4.22). Here, we will examine
each.
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Figure 4.22 Microfilaments thicken the cortex around the cell's inner edge. Like rubber bands, they resist tension.
There are microtubules in the cell's interior where they maintain their shape by resisting compressive forces. There are
intermediate filaments throughout the cell that hold organelles in place.
Microfilaments
Of the three types of protein fibers in the cytoskeleton, microfilaments are the narrowest. They function in
cellular movement, have a diameter of about 7 nm, and are comprised of two globular protein intertwined
strands, which we call actin (Figure 4.23). For this reason, we also call microfilaments actin filaments.
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Chapter 4 | Cell Structure
Actin filaments
Actin subunit
Figure 4.23 Two intertwined actin strands comprise microfilaments.
ATP powers actin to assemble its filamentous form, which serves as a track for the movement of a motor
protein we call myosin. This enables actin to engage in cellular events requiring motion, such as cell division
in eukaryotic cells and cytoplasmic streaming, which is the cell cytoplasm's circular movement in plant cells.
Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your
muscles contract.
Microfilaments also provide some rigidity and shape to the cell. They can depolymerize (disassemble) and
reform quickly, thus enabling a cell to change its shape and move. White blood cells (your body’s infection¬
fighting cells) make good use of this ability. They can move to an infection site and phagocytize the pathogen.
To see an example of a white blood cell in action, watch a short time-lapse video of the cell capturing two
bacteria. It engulfs one and then moves on to the other. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66454/l.3/#eip-id3083425)
Intermediate Filaments
Several strands of fibrous proteins that are wound together comprise intermediate filaments (Figure 4.24).
Cytoskeleton elements get their name from the fact that their diameter, 8 to 10 nm, is between those of
microfilaments and microtubules.
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Figure 4.24 Intermediate filaments consist of several intertwined strands of fibrous proteins.
Intermediate filaments have no role in cell movement. Their function is purely structural. They bear tension,
thus maintaining the cell's shape, and anchor the nucleus and other organelles in place. Figure 4.22 shows how
intermediate filaments create a supportive scaffolding inside the cell.
The intermediate filaments are the most diverse group of cytoskeletal elements. Several fibrous protein types
are in the intermediate filaments. You are probably most familiar with keratin, the fibrous protein that strengthens
your hair, nails, and the skin's epidermis.
Microtubules
As their name implies, microtubules are small hollow tubes. Polymerized dimers of a-tubulin and p-tubulin, two
globular proteins, comprise the microtubule's walls (Figure 4.25). With a diameter of about 25 nm, microtubules
are cytoskeletons' widest components. They help the cell resist compression, provide a track along which
vesicles move through the cell, and pull replicated chromosomes to opposite ends of a dividing cell. Like
microfilaments, microtubules can disassemble and reform quickly.
13 polymerized dimers
of a-tubulin and p-tubulin
Figure 4.25 Microtubules are hollow. Their walls consist of 13 polymerized dimers of a-tubulin and p-tubulin (right
image). The left image shows the tube's molecular structure.
Microtubules are also the structural elements of flagella, cilia, and centrioles (the latter are the centrosome's two
perpendicular bodies). In animal cells, the centrosome is the microtubule-organizing center. In eukaryotic cells,
flagella and cilia are quite different structurally from their counterparts in prokaryotes, as we discuss below.
Flagella and Cilia
The flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and
enable an entire cell to move (for example, sperm, Euglena, and some prokaryotes). When present, the cell has
just one flagellum or a few flagella. However, when cilia (singular = cilium) are present, many of them extend
along the plasma membrane's entire surface. They are short, hair-like structures that move entire cells (such as
paramecia) or substances along the cell's outer surface (for example, the cilia of cells lining the Fallopian tubes
that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that trap particulate matter
and move it toward your nostrils.)
Despite their differences in length and number, flagella and cilia share a common structural arrangement of
microtubules called a “9 + 2 array.” This is an appropriate name because a single flagellum or cilium is made of
a ring of nine microtubule doublets, surrounding a single microtubule doublet in the center (Figure 4.26).
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Chapter 4 | Cell Structure
Figure 4.26 This transmission electron micrograph of two flagella shows the microtubules' 9 + 2 array: nine microtubule
doublets surround a single microtubule doublet, (credit: modification of work by Dartmouth Electron Microscope
Facility, Dartmouth College; scale-bar data from Matt Russell)
You have now completed a broad survey of prokaryotic and eukaryotic cell components. For a summary of
cellular components in prokaryotic and eukaryotic cells, see Table 4.1.
Components of Prokaryotic and Eukaryotic Cells
Cell
Component
Function
Present in
Prokaryotes?
Present
in
Animal
Cells?
Present
in Plant
Cells?
Plasma
membrane
Separates cell from external environment;
controls passage of organic molecules, ions,
water, oxygen, and wastes into and out of
cell
Yes
Yes
Yes
Cytoplasm
Provides turgor pressure to plant cells as
fluid inside the central vacuole; site of many
metabolic reactions; medium in which
organelles are found
Yes
Yes
Yes
Nucleolus
Darkened area within the nucleus where
ribosomal subunits are synthesized.
No
Yes
Yes
Nucleus
Cell organelle that houses DNA and directs
synthesis of ribosomes and proteins
No
Yes
Yes
Ribosomes
Protein synthesis
Yes
Yes
Yes
Mitochondria
ATP production/cellular respiration
No
Yes
Yes
Peroxisomes
Oxidize and thus break down fatty acids and
amino acids, and detoxify poisons
No
Yes
Yes
Vesicles and
vacuoles
Storage and transport; digestive function in
plant cells
No
Yes
Yes
Centrosome
Unspecified role in cell division in animal
cells; microtubule source in animal cells
No
Yes
No
Lysosomes
Digestion of macromolecules; recycling of
worn-out organelles
No
Yes
Some
Table 4.1
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Chapter 4 | Cell Structure
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Components of Prokaryotic and Eukaryotic Cells
Cell
Component
Function
Present in
Prokaryotes?
Present
in
Animal
Cells?
Present
in Plant
Cells?
Cell wall
Protection, structural support, and
maintenance of cell shape
Yes, primarily
peptidoglycan
No
Yes,
primarily
cellulose
Chloroplasts
Photosynthesis
No
No
Yes
Endoplasmic
reticulum
Modifies proteins and synthesizes lipids
No
Yes
Yes
Golgi apparatus
Modifies, sorts, tags, packages, and
distributes lipids and proteins
No
Yes
Yes
Cytoskeleton
Maintains cell’s shape, secures organelles in
specific positions, allows cytoplasm and
vesicles to move within cell, and enables
unicellular organisms to move independently
Yes
Yes
Yes
Flagella
Cellular locomotion
Some
Some
No, except
for some
plant sperm
cells
Cilia
Cellular locomotion, movement of particles
along plasma membrane's extracellular
surface, and filtration
Some
Some
No
Table 4.1
4.6 | Connections between Cells and Cellular Activities
By the end of this section, you will be able to do the following:
• Describe the extracellular matrix
• List examples of the ways that plant cells and animal cells communicate with adjacent cells
• Summarize the roles of tight junctions, desmosomes, gap junctions, and plasmodesmata
You already know that tissue is a group of similar cells working together. As you might expect, if cells are to work
together, they must communicate with each other, just as you need to communicate with others if you work on a
group project. Let’s take a look at how cells communicate with each other.
Extracellular Matrix of Animal Cells
While cells in most multicellular organisms release materials into the extracellular space, animal cells will be
discussed as an example. The primary components of these materials are proteins, and the most abundant
protein is collagen. Collagen fibers are interwoven with proteoglycans, which are carbohydrate-containing
protein molecules. Collectively, we call these materials the extracellular matrix (Figure 4.27). Not only does
the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to
communicate with each other. How can this happen?
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Chapter 4 | Cell Structure
Proteoglycan complex
Carbohydrates
membrane of the cytoskeleton
Figure 4.27 The extracellular matrix consists of a network of proteins and carbohydrates.
Cells have protein receptors on their plasma membranes' extracellular surfaces. When a molecule within the
matrix binds to the receptor, it changes the receptor's molecular structure. The receptor, in turn, changes
the microfilaments' conformation positioned just inside the plasma membrane. These conformational changes
induce chemical signals inside the cell that reach the nucleus and turn “on” or “off” the transcription of specific
DNA sections, which affects the associated protein production, thus changing the activities within the cell.
Blood clotting provides an example of the extracellular matrix's role in cell communication. When the cells lining
a blood vessel are damaged, they display a protein receptor, which we call tissue factor. When tissue factor
binds with another factor in the extracellular matrix, it causes platelets to adhere to the damaged blood vessel's
wall, stimulates the adjacent smooth muscle cells in the blood vessel to contract (thus constricting the blood
vessel), and initiates a series of steps that stimulate the platelets to produce clotting factors.
Intercellular Junctions
Cells can also communicate with each other via direct contact, or intercellular junctions. There are differences
in the ways that plant and animal and fungal cells communicate. Plasmodesmata are junctions between plant
cells; whereas, animal cell contacts include tight junctions, gap junctions, and desmosomes.
Plasmodesmata
In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another
because the cell wall that surrounds each cell separates them (Figure 4.8). How then, can a plant transfer water
and other soil nutrients from its roots, through its stems, and to its leaves? Such transport uses the vascular
tissues (xylem and phloem) primarily. There also exist structural modifications, which we call plasmodesmata
(singular = plasmodesma). Numerous channels that pass between adjacent plant cells' cell walls connect their
cytoplasm, and enable transport of materials from cell to cell, and thus throughout the plant (Figure 4.28).
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Chapter 4 | Cell Structure
133
Plasmodesma
Cell wall
Cytoplasm
Vacuole
- Pathways through the cytoplasm
Figure 4.28 A plasmodesma is a channel between two adjacent plant cells' cell walls. Plasmodesmata allow materials
to pass from one plant cell's cytoplasm to an adjacent cell's cytoplasm.
Tight Junctions
A tight junction is a watertight seal between two adjacent animal cells (Figure 4.29). Proteins (predominantly
two proteins called claudins and occludins) tightly hold the cells against each other.
Tight junction
Figure 4.29 Tight junctions form watertight connections between adjacent animal cells. Proteins create tight junction
adherence, (credit: modification of work by Mariana Ruiz Villareal)
This tight adherence prevents materials from leaking between the cells; tight junctions are typically found in
epithelial tissues that line internal organs and cavities, and comprise most of the skin. For example, the tight
junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular
space.
Desmosomes
Also only in animal cells are desmosomes, which act like spot welds between adjacent epithelial cells
(Figure 4.30). Cadherins, short proteins in the plasma membrane connect to intermediate filaments to create
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Chapter 4 | Cell Structure
desmosomes. The cadherins connect two adjacent cells and maintain the cells in a sheet-like formation in
organs and tissues that stretch, like the skin, heart, and muscles.
Desmosome
Adjacent
plasma
membranes
Plaque
Transmembrane
glycoprotein
(cadherin)
Intermediate
filament
(keratin)
Intercellullar
space
Figure 4.30 A desmosome forms a very strong spot weld between cells. Linking cadherins and intermediate filaments
create it. (credit: modification of work by Mariana Ruiz Villareal)
Gap Junctions
Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent
cells that allow for transporting ions, nutrients, and other substances that enable cells to communicate (Figure
4.31). Structurally, however, gap junctions and plasmodesmata differ.
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Chapter 4 | Cell Structure
135
Gap junction
Adjacent
plasma
membranes
Gap between
cells
Connexons
(composed of
connexins)
\o <
Figure 4.31 A gap junction is a protein-lined pore that allows water and small molecules to pass between adjacent
animal cells, (credit: modification of work by Mariana Ruiz Villareal)
Gap junctions develop when a set of six proteins (connexins) in the plasma membrane arrange themselves in
an elongated donut-like configuration - a connexon. When the connexon's pores (“doughnut holes") in adjacent
animal cells align, a channel between the two cells forms. Gap junctions are particularly important in cardiac
muscle. The electrical signal for the muscle to contract passes efficiently through gap junctions, allowing the
heart muscle cells to contract in tandem.
LINK TQ LEARNING
To conduct a virtual microscopy lab and review the parts of a cell, work through the steps of this interactive
assignment (http://openstaxcollege. 0 rg/l/microscopy_lab) .
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Chapter 4 | Cell Structure
KEY TERMS
cell theory see unified cell theory
cell wall rigid cell covering comprised of various molecules that protects the cell, provides structural support,
and gives shape to the cell
central vacuole large plant cell organelle that regulates the cell’s storage compartment, holds water, and plays
a significant role in cell growth as the site of macromolecule degradation
centrosome region in animal cells made of two centrioles that serves as an organizing center for microtubules
chlorophyll green pigment that captures the light energy that drives the light reactions of photosynthesis
chloroplast plant cell organelle that carries out photosynthesis
chromatin protein-DNA complex that serves as the chromosomes' building material
chromosome structure within the nucleus that comprises chromatin that contains DNA, the hereditary material
cilium (plural = cilia) short, hair-like structure that extends from the plasma membrane in large numbers and
functions to move an entire cell or move substances along the cell's outer surface
cytoplasm entire region between the plasma membrane and the nuclear envelope, consisting of organelles
suspended in the gel-like cytosol, the cytoskeleton, and various chemicals
cytoskeleton protein fiber network that collectively maintains the cell's shape, secures some organelles in
specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms
to move independently
cytosol the cytoplasm's gel-like material in which cell structures are suspended
desmosome linkages between adjacent epithelial cells that form when cadherins in the plasma membrane
attach to intermediate filaments
electron microscope an instrument that magnifies an object using an electron beam that passes and bends
through a lens system to visualize a specimen
endomembrane system group of organelles and membranes in eukaryotic cells that work together modifying,
packaging, and transporting lipids and proteins
endoplasmic reticulum (ER) series of interconnected membranous structures within eukaryotic cells that
collectively modify proteins and synthesize lipids
eukaryotic cell cell that has a membrane-bound nucleus and several other membrane-bound compartments or
sacs
extracellular matrix material secreted from animal or fungal cells that provides mechanical protection and
anchoring for the cells in the tissue
flagellum (plural = flagella) long, hair-like structure that extends from the plasma membrane and moves the cell
gap junction channel between two adjacent animal cells that allows ions, nutrients, and low molecular weight
substances to pass between cells, enabling the cells to communicate
Golgi apparatus eukaryotic organelle comprised of a series of stacked membranes that sorts, tags, and
packages lipids and proteins for distribution
intermediate filament cytoskeletal component, comprised of several fibrous protein intertwined strands, that
bears tension, supports cell-cell junctions, and anchors cells to extracellular structures
light microscope an instrument that magnifies an object using a beam of visible light that passes and bends
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Chapter 4 | Cell Structure
137
through a lens system to visualize a specimen
lysosome organelle in an animal cell that functions as the cell’s digestive component; it breaks down proteins,
polysaccharides, lipids, nucleic acids, and even worn-out organelles
microfilament the cytoskeleton system's narrowest element; it provides rigidity and shape to the cell and
enables cellular movements
microscope an instrument that magnifies an object
microtubule the cytoskeleton system's widest element; it helps the cell resist compression, provides a track
along which vesicles move through the cell, pulls replicated chromosomes to opposite ends of a dividing
cell, and is the structural element of centrioles, flagella, and cilia
mitochondria (singular = mitochondrion) cellular organelles responsible for carrying out cellular respiration,
resulting in producing ATP, the cell’s main energy-carrying molecule
nuclear envelope double-membrane structure that constitutes the nucleus' outermost portion
nucleoid central part of a prokaryotic cell's central part where the chromosome is located
nucleolus darkly staining body within the nucleus that is responsible for assembling ribosome subunits
nucleoplasm semi-solid fluid inside the nucleus that contains the chromatin and nucleolus
nucleus cell organelle that houses the cell’s DNA and directs ribosome and protein synthesis
organelle compartment or sac within a cell
peroxisome small, round organelle that contains hydrogen peroxide, oxidizes fatty acids and amino acids, and
detoxifies many poisons
plasma membrane phospholipid bilayer with embedded (integral) or attached (peripheral) proteins, and
separates the cell's internal content from its surrounding environment
plasmodesma (plural = plasmodesmata) channel that passes between adjacent plant cells' cell walls, connects
their cytoplasm, and allows transporting of materials from cell to cell
prokaryote unicellular organism that lacks a nucleus or any other membrane-bound organelle
ribosome cellular structure that carries out protein synthesis
rough endoplasmic reticulum (RER) region of the endoplasmic reticulum that is studded with ribosomes and
engages in protein modification and phospholipid synthesis
smooth endoplasmic reticulum (SER) region of the endoplasmic reticulum that has few or no ribosomes on
its cytoplasmic surface and synthesizes carbohydrates, lipids, and steroid hormones; detoxifies certain
chemicals (like pesticides, preservatives, medications, and environmental pollutants), and stores calcium
ions
tight junction protein adherence that creates a firm seal between two adjacent animal cells
unified cell theory a biological concept that states that one or more cells comprise all organisms; the cell is the
basic unit of life; and new cells arise from existing cells
vacuole membrane-bound sac, somewhat larger than a vesicle, which functions in cellular storage and
transport
vesicle small, membrane-bound sac that functions in cellular storage and transport; its membrane is capable of
fusing with the plasma membrane and the membranes of the endoplasmic reticulum and Golgi apparatus
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Chapter 4 | Cell Structure
CHAPTER SUMMARY
4.1 Studying Cells
A cell is the smallest unit of life. Most cells are so tiny that we cannot see them with the naked eye. Therefore,
scientists use microscopes to study cells. Electron microscopes provide higher magnification, higher resolution,
and more detail than light microscopes. The unified cell theory states that one or more cells comprise all
organisms, the cell is the basic unit of life, and new cells arise from existing cells.
4.2 Prokaryotic Cells
Prokaryotes are single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma
membranes, cytoplasm, ribosomes, and DNA that is not membrane-bound. Most have peptidoglycan cell walls
and many have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1 to 5.0 pm.
As a cell increases in size, its surface area-to-volume ratio decreases. If the cell grows too large, the plasma
membrane will not have sufficient surface area to support the rate of diffusion required for the increased
volume.
4.3 Eukaryotic Cells
Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes, but a eukaryotic
cell is typically larger than a prokaryotic cell, has a true nucleus (meaning a membrane surrounds its DNA), and
has other membrane-bound organelles that allow for compartmentalizing functions. The plasma membrane is a
phospholipid bilayer embedded with proteins. The nucleus’s nucleolus is the site of ribosome assembly. We
find ribosomes either in the cytoplasm or attached to the cytoplasmic side of the plasma membrane or
endoplasmic reticulum. They perform protein synthesis. Mitochondria participate in cellular respiration. They
are responsible for the majority of ATP produced in the cell. Peroxisomes hydrolyze fatty acids, amino acids,
and some toxins. Vesicles and vacuoles are storage and transport compartments. In plant cells, vacuoles also
help break down macromolecules.
Animal cells also have a centrosome and lysosomes. The centrosome has two bodies perpendicular to each
other, the centrioles, and has an unknown purpose in cell division. Lysosomes are the digestive organelles of
animal cells.
Plant cells and plant-like cells each have a cell wall, chloroplasts, and a central vacuole. The plant cell wall,
whose primary component is cellulose, protects the cell, provides structural support, and gives the cell shape.
Photosynthesis takes place in chloroplasts. The central vacuole can expand without having to produce more
cytoplasm.
4.4 The Endomembrane System and Proteins
The endomembrane system includes the nuclear envelope, lysosomes, vesicles, the ER, and Golgi apparatus,
as well as the plasma membrane. These cellular components work together to modify, package, tag, and
transport proteins and lipids that form the membranes.
The RER modifies proteins and synthesizes phospholipids in cell membranes. The SER synthesizes
carbohydrates, lipids, and steroid hormones; engages in the detoxification of medications and poisons; and
stores calcium ions. Sorting, tagging, packaging, and distributing lipids and proteins take place in the Golgi
apparatus. Budding RER and Golgi membranes create lysosomes. Lysosomes digest macromolecules, recycle
worn-out organelles, and destroy pathogens.
4.5 The Cytoskeleton
The cytoskeleton has three different protein element types. From narrowest to widest, they are the
microfilaments (actin filaments), intermediate filaments, and microtubules. Biologists often associate
microfilaments with myosin. They provide rigidity and shape to the cell and facilitate cellular movements.
Intermediate filaments bear tension and anchor the nucleus and other organelles in place. Microtubules help
the cell resist compression, serve as tracks for motor proteins that move vesicles through the cell, and pull
replicated chromosomes to opposite ends of a dividing cell. They are also the structural element of centrioles,
flagella, and cilia.
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Chapter 4 | Cell Structure
139
4.6 Connections between Cells and Cellular Activities
Animal cells communicate via their extracellular matrices and are connected to each other via tight junctions,
desmosomes, and gap junctions. Plant cells are connected and communicate with each other via
plasmodesmata.
When protein receptors on the plasma membrane's surface of an animal cell bind to a substance in the
extracellular matrix, a chain of reactions begins that changes activities taking place within the cell.
Plasmodesmata are channels between adjacent plant cells, while gap junctions are channels between adjacent
animal cells. However, their structures are quite different. A tight junction is a watertight seal between two
adjacent cells, while a desmosome acts like a spot weld.
VISUAL CONNECTION QUESTIONS
1. Figure 4.7 Prokaryotic cells are much smaller than
eukaryotic cells. What advantages might small cell
size confer on a cell? What advantages might large
cell size have?
2. Figure 4.8 If the nucleolus were not able to carry
out its function, what other cellular organelles would
REVIEW QUESTIONS
4. When viewing a specimen through a light
microscope, scientists use_to distinguish
the individual components of cells.
a.
a beam of electrons
b.
radioactive isotopes
c.
special stains
d.
high temperatures
5. The
is the basic
a.
organism
b.
cell
c.
tissue
d.
organ
6. Prokaryotes depend on_to obtain some
materials and to get rid of wastes.
a. ribosomes
b. flagella
c. cell division
d. diffusion
7. Bacteria that lack fimbriae are less likely to
a. adhere to cell surfaces
b. swim through bodily fluids
c. synthesize proteins
d. retain the ability to divide
8. Which of the following organisms is a prokaryote?
a. amoeba
b. influenza A virus
c. charophyte algae
d. E. coli
9. Which of the following is surrounded by two
phospholipid bilayers?
be affected?
3. Figure 4.18 If a peripheral membrane protein were
synthesized in the lumen (inside) of the ER, would it
end up on the inside or outside of the plasma
membrane?
a. the ribosomes
b. the vesicles
c. the cytoplasm
d. the nucleoplasm
10. Peroxisomes got their name because hydrogen
peroxide is:
a. used in their detoxification reactions
b. produced during their oxidation reactions
c. incorporated into their membranes
d. a cofactor for the organelles’ enzymes
11. In plant cells, the function of the lysosomes is
carried out by_.
a. vacuoles
b. peroxisomes
c. ribosomes
d. nuclei
12. Which of the following is both in eukaryotic and
prokaryotic cells?
a. nucleus
b. mitochondrion
c. vacuole
d. ribosomes
13. Tay-Sachs disease is a genetic disorder that
results in the destruction of neurons due to a buildup
of sphingolipids in the cells. Which organelle is
malfunctioning in Tay-Sachs?
a. lysosome
b. endoplasmic reticulum
c. peroxisome
d. mitochondria
14. Which of the following is not a component of the
endomembrane system?
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Chapter 4 | Cell Structure
a. mitochondrion
b. Golgi apparatus
c. endoplasmic reticulum
d. lysosome
15. The process by which a cell engulfs a foreign
particle is known as:
a. endosymbiosis
b. phagocytosis
c. hydrolysis
d. membrane synthesis
16. Which of the following is most likely to have the
greatest concentration of smooth endoplasmic
reticulum?
a. a cell that secretes enzymes
b. a cell that destroys pathogens
c. a cell that makes steroid hormones
d. a cell that engages in photosynthesis
17. Which of the following sequences correctly lists in
order the steps involved in the incorporation of a
proteinaceous molecule within a cell?
a. protein synthesis of the protein on the
ribosome; modification in the Golgi
apparatus; packaging in the endoplasmic
reticulum; tagging in the vesicle
b. synthesis of the protein on the lysosome;
tagging in the Golgi; packaging in the
vesicle; distribution in the endoplasmic
reticulum
c. synthesis of the protein on the ribosome;
modification in the endoplasmic reticulum;
tagging in the Golgi; distribution via the
vesicle
d. synthesis of the protein on the lysosome;
packaging in the vesicle; distribution via the
Golgi; tagging in the endoplasmic reticulum
18. Congenital disorders of glycosylation are a
growing class of rare diseases. Which organelle
would be most commonly involved in the glycoprotein
disorder portion of the group?
a. RER
b. ribosomes
c. endosomes
d. Golgi apparatus
CRITICAL THINKING QUESTIONS
25. In your everyday life, you have probably noticed
that certain instruments are ideal for certain
situations. For example, you would use a spoon
rather than a fork to eat soup because a spoon is
shaped for scooping, while soup would slip between
the tines of a fork. The use of ideal instruments also
applies in science. In what situation(s) would the use
of a light microscope be ideal, and why?
26. In what situation(s) would the use of a scanning
electron microscope be ideal, and why?
19. Which of the following have the ability to
disassemble and reform quickly?
a. microfilaments and intermediate filaments
b. microfilaments and microtubules
c. intermediate filaments and microtubules
d. only intermediate filaments
20. Which of the following do not play a role in
intracellular movement?
a. microfilaments and intermediate filaments
b. microfilaments and microtubules
c. intermediate filaments and microtubules
d. only intermediate filaments
21. In humans,_are used to move a cell within
its environment while_are used to move the
environment relative to the cell.
a. cilia, pseudopodia
b. flagella; cilia
c. microtubules; flagella
d. microfilaments; microtubules
22. Which of the following are only in plant cells?
a. gap junctions
b. desmosomes
c. plasmodesmata
d. tight junctions
23. The key components of desmosomes are
cadherins and_.
a. actin
b. microfilaments
c. intermediate filaments
d. microtubules
24. Diseased animal cells may produce molecules
that activate death cascades to kill the cells in a
controlled manner. Why would neighboring healthy
cells also die?
a. The death molecule is passed through
desmosomes.
b. The death molecule is passed through
plasmodesmata.
c. The death molecule disrupts the
extracellular matrix.
d. The death molecule passes through gap
junctions.
27. In what situation(s) would a transmission electron
microscope be ideal, and why?
28. What are the advantages and disadvantages of
each of these types of microscopes?
29. Explain how the formation of an adult human
follows the cell theory.
30. Antibiotics are medicines that are used to fight
bacterial infections. These medicines kill prokaryotic
cells without harming human cells. What part or parts
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Chapter 4 | Cell Structure
141
of the bacterial cell do you think antibiotics target?
Why?
31. Explain why not all microbes are harmful.
32. You already know that ribosomes are abundant in
red blood cells. In what other cells of the body would
you find them in great abundance? Why?
33. What are the structural and functional similarities
and differences between mitochondria and
chloroplasts?
34. Why are plasma membranes arranged as a
bilayer rather than a monolayer?
35. In the context of cell biology, what do we mean by
form follows function? What are at least two
examples of this concept?
36. In your opinion, is the nuclear membrane part of
the endomembrane system? Why or why not?
Defend your answer.
37. What are the similarities and differences between
the structures of centrioles and flagella?
38. How do cilia and flagella differ?
39. Describe how microfilaments and microtubules
are involved in the phagocytosis and destruction of a
pathogen by a macrophage.
40. Compare and contrast the boundaries that plant,
animal, and bacteria cells use to separate
themselves from their surrounding environment.
41. How does the structure of a plasmodesma differ
from that of a gap junction?
42. Explain how the extracellular matrix functions.
43. Pathogenic E. coli have recently been shown to
degrade tight junction proteins during infection. How
would this provide an advantage to the bacteria?
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Chapter 4 | Cell Structure
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Chapter 5 | Structure and Function of Plasma Membranes
143
5 | STRUCTURE AND
FUNCTION OF PLASMA
MEMBRANES
Figure 5.1 Despite its seeming hustle and bustle, Grand Central Station functions with a high level of organization:
People and objects move from one location to another, they cross or are contained within certain boundaries, and they
provide a constant flow as part of larger activity. Analogously, a plasma membrane’s functions involve movement within
the cell and across boundaries' activities, (credit: modification of work by Randy Le’Moine)
Chapter Outline
5.1: Components and Structure
5.2: Passive Transport
5.3: Active Transport
5.4: Bulk Transport
Introduction
The plasma membrane, the cell membrane, has many functions, but the most basic one is to define the cell’s
borders and keep the cell functional. The plasma membrane is selectively permeable. This means that the
membrane allows some materials to freely enter or leave the cell, while other materials cannot move freely, but
require a specialized structure, and occasionally, even energy investment for crossing.
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Chapter 5 | Structure and Function of Plasma Membranes
5.1 1 Components and Structure
By the end of this section, you will be able to do the following:
• Understand the cell membrane fluid mosaic model
• Describe phospholipid, protein, and carbohydrate functions in membranes
• Discuss membrane fluidity
A cell’s plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with
its environment (see Table 5.1 for a summary). Cells exclude some substances, take in others, and excrete still
others, all in controlled quantities. The plasma membrane must be very flexible to allow certain cells, such as
red and white blood cells, to change shape as they pass through narrow capillaries. These are the more obvious
plasma membrane functions. In addition, the plasma membrane's surface carries markers that allow cells to
recognize one another, which is vital for tissue and organ formation during early development, and which later
plays a role in the immune response's “self" versus “non-self" distinction.
Among the most sophisticated plasma membrane functions is the ability for complex, integral proteins, receptors
to transmit signals. These proteins act both as extracellular input receivers and as intracellular processing
activators. These membrane receptors provide extracellular attachment sites for effectors like hormones and
growth factors, and they activate intracellular response cascades when their effectors are bound. Occasionally,
viruses hijack receptors (HIV, human immunodeficiency virus, is one example) that use them to gain entry into
cells, and at times, the genes encoding receptors become mutated, causing the signal transduction process to
malfunction with disastrous consequences.
Fluid Mosaic Model
Scientists identified the plasma membrane in the 1890s, and its chemical components in 1915. The principal
components they identified were lipids and proteins. In 1935, Hugh Davson and James Danielli proposed the
plasma membrane's structure. This was the first model that others in the scientific community widely accepted.
It was based on the plasma membrane's “railroad track” appearance in early electron micrographs. Davson
and Danielli theorized that the plasma membrane's structure resembles a sandwich. They made the analogy of
proteins to bread, and lipids to the filling. In the 1950s, advances in microscopy, notably transmission electron
microscopy (TEM), allowed researchers to see that the plasma membrane's core consisted of a double, rather
than a single, layer. In 1972, S.J. Singer and Garth L. Nicolson proposed a new model that provides microscopic
observations and better explains plasma membrane function.
The explanation, the fluid mosaic model, has evolved somewhat over time, but it still best accounts for
plasma membrane structure and function as we now understand them. The fluid mosaic model describes the
plasma membrane structure as a mosaic of components—including phospholipids, cholesterol, proteins, and
carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in
thickness. For comparison, human red blood cells, visible via light microscopy, are approximately 8 pm wide,
or approximately 1,000 times wider than a plasma membrane. The membrane does look a bit like a sandwich
(Figure 5.2).
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Chapter 5 | Structure and Function of Plasma Membranes
145
Glycoprotein: protein with
carbohydrate attached
>Glycolipid: lipid with
/ carbohydrate
attached
Phospholipid
bilayer
Integral membrane
protein
Protein channel
Cytoskeletal filaments
Figure 5.2 The plasma membrane fluid mosaic model describes the plasma membrane as a fluid combination of
phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins)
extend from the membrane's outward-facing surface.
A plasma membrane's principal components are lipids (phospholipids and cholesterol), proteins, and
carbohydrates attached to some of the lipids and proteins. A phospholipid is a molecule consisting of glycerol,
two fatty acids, and a phosphate-linked head group. Cholesterol, another lipid comprised of four fused carbon
rings, is situated alongside the phospholipids in the membrane's core. The protein, lipid, and carbohydrate
proportions in the plasma membrane vary with cell type, but for a typical human cell, protein accounts for about
50 percent of the composition by mass, lipids (of all types) account for about 40 percent, and carbohydrates
comprise the remaining 10 percent. However, protein and lipid concentration varies with different cell
membranes. For example, myelin, an outgrowth of specialized cells' membrane that insulates the peripheral
nerves' axons, contains only 18 percent protein and 76 percent lipid. The mitochondrial inner membrane contains
76 percent protein and only 24 percent lipid. The plasma membrane of human red blood cells is 30 percent
lipid. Carbohydrates are present only on the plasma membrane's exterior surface and are attached to proteins,
forming glycoproteins, or attached to lipids, forming glycolipids.
Phospholipids
The membrane's main fabric comprises amphiphilic, phospholipid molecules. The hydrophilic or “water-loving”
areas of these molecules (which look like a collection of balls in an artist’s rendition of the model) (Figure 5.2)
are in contact with the aqueous fluid both inside and outside the cell. Hydrophobic, or water-hating molecules,
tend to be non-polar. They interact with other non-polar molecules in chemical reactions, but generally do not
interact with polar molecules. When placed in water, hydrophobic molecules tend to form a ball or cluster. The
phospholipids' hydrophilic regions form hydrogen bonds with water and other polar molecules on both the cell's
exterior and interior. Thus, the membrane surfaces that face the cell's interior and exterior are hydrophilic. In
contrast, the cell membrane's interior is hydrophobic and will not interact with water. Therefore, phospholipids
form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside the cell.
A phospholipid molecule (Figure 5.3) consists of a three-carbon glycerol backbone with two fatty acid molecules
attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. This arrangement
gives the overall molecule a head area (the phosphate-containing group), which has a polar character or
negative charge, and a tail area (the fatty acids), which has no charge. The head can form hydrogen bonds,
but the tail cannot. Scientists call a molecule with a positively or negatively charged area and an uncharged, or
non-polar, area amphiphilic or “dual-loving.”
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(a) Structural formula
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Unsaturated
fatty acid
Fatty
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Hydrophilic
head
Hydrophobic
tails
(c) Phospholipid symbol
Figure 5.3 A hydrophilic head and two hydrophobic tails comprise this phospholipid molecule. The hydrophilic
head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each
containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.
This characteristic is vital to the plasma membrane's structure because, in water, phospholipids arrange
themselves with their hydrophobic tails facing each other and their hydrophilic heads facing out. In this way, they
form a lipid bilayer—a double layered phospholipid barrier that separates the water and other materials on one
side from the water and other materials on the other side. Phosopholipids heated in an aqueous solution usually
spontaneously form small spheres or droplets (micelles or liposomes), with their hydrophilic heads forming the
exterior and their hydrophobic tails on the inside (Figure 5.4).
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Chapter 5 | Structure and Function of Plasma Membranes
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Lipid-bilayer sphere
Single-layer lipid sphere
Lipid-bilayer sheet
Figure 5.4 In an aqueous solution, phospholipids usually arrange themselves with their polar heads facing outward
and their hydrophobic tails facing inward, (credit: modification of work by Mariana Ruiz Villareal)
Proteins
Proteins comprise the plasma membranes' second major component. Integral proteins, or integrins, as their
name suggests, integrate completely into the membrane structure, and their hydrophobic membrane-spanning
regions interact with the phospholipid bilayer's hydrophobic region (Figure 5.2). Single-pass integral membrane
proteins usually have a hydrophobic transmembrane segment that consists of 20-25 amino acids. Some span
only part of the membrane—associating with a single layer—while others stretch from one side to the other,
and are exposed on either side. Up to 12 single protein segments comprise some complex proteins, which are
extensively folded and embedded in the membrane (Figure 5.5). This protein type has a hydrophilic region
or regions, and one or several mildly hydrophobic regions. This arrangement of protein regions orients the
protein alongside the phospholipids, with the protein's hydrophobic region adjacent to the phosopholipids' tails
and the protein's hydrophilic region or regions protruding from the membrane and in contact with the cytosol or
extracellular fluid.
4
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Figure 5.5 Integral membrane proteins may have one or more alpha-helices that span the membrane (examples 1 and
2), or they may have beta-sheets that span the membrane (example 3). (credit: “Foobar’VWikimedia Commons)
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Peripheral proteins are on the membranes' exterior and interior surfaces, attached either to integral proteins
or to phospholipids. Peripheral proteins, along with integral proteins, may serve as enzymes, as structural
attachments for the cytoskeleton's fibers, or as part of the cell’s recognition sites. Scientists sometimes refer to
these as “cell-specific” proteins. The body recognizes its own proteins and attacks foreign proteins associated
with invasive pathogens.
Carbohydrates
Carbohydrates are the third major plasma membrane component. They are always on the cells' exterior surface
and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids) (Figure 5.2). These
carbohydrate chains may consist of 2-60 monosaccharide units and can be either straight or branched. Along
with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize
each other. These sites have unique patterns that allow for cell recognition, much the way that the facial features
unique to each person allow individuals to recognize him or her. This recognition function is very important to
cells, as it allows the immune system to differentiate between body cells (“self”) and foreign cells or tissues
(“non-self"). Similar glycoprotein and glycolipid types are on the surfaces of viruses and may change frequently,
preventing immune cells from recognizing and attacking them.
We collectively refer to these carbohydrates on the cell's exterior surface—the carbohydrate components of both
glycoproteins and glycolipids—as the glycocalyx (meaning “sugar coating"). The glycocalyx is highly hydrophilic
and attracts large amounts of water to the cell's surface. This aids in the cell's interaction with its watery
environment and in the cell’s ability to obtain substances dissolved in the water. As we discussed above, the
glycocalyx is also important for cell identification, self/non-self determination, and embryonic development, and
is used in cell to cell attachments to form tissues.
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V /
How Viruses Infect Specific Organs
Glycoprotein and glycolipid patterns on the cells' surfaces give many viruses an opportunity for infection.
HIV and hepatitis viruses infect only specific organs or cells in the human body. HIV is able to penetrate
the plasma membranes of a subtype of lymphocytes called T-helper cells, as well as some monocytes and
central nervous system cells. The hepatitis virus attacks liver cells.
These viruses are able to invade these cells, because the cells have binding sites on their surfaces that are
specific to and compatible with certain viruses (Figure 5.6). Other recognition sites on the virus’s surface
interact with the human immune system, prompting the body to produce antibodies. Antibodies are made
in response to the antigens or proteins associated with invasive pathogens, or in response to foreign cells,
such as might occur with an organ transplant. These same sites serve as places for antibodies to attach
and either destroy or inhibit the virus' activity. Unfortunately, these recognition sites on HIV change at a
rapid rate because of mutations, making an effective vaccine against the virus very difficult, as the virus
evolves and adapts. A person infected with HIV will quickly develop different populations, or variants, of the
virus that differences in these recognition sites distinguish. This rapid change of surface markers decreases
the effectiveness of the person’s immune system in attacking the virus, because the antibodies will not
recognize the surface patterns' new variations. In the case of HIV, the problem is compounded because the
virus specifically infects and destroys cells involved in the immune response, further incapacitating the host.
Figure 5.6 HIV binds to the CD4 receptor, a glycoprotein on T cell surfaces, (credit: modification of work by NIH,
NIAID)
Membrane Fluidity
The membrane's mosaic characteristic helps to illustrate its nature. The integral proteins and lipids exist in the
membrane as separate but loosely attached molecules. These resemble the separate, multicolored tiles of a
mosaic picture, and they float, moving somewhat with respect to one another. The membrane is not like a
balloon, however, that can expand and contract; rather, it is fairly rigid and can burst if penetrated or if a cell takes
in too much water. However, because of its mosaic nature, a very fine needle can easily penetrate a plasma
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membrane without causing it to burst, and the membrane will flow and self-seal when one extracts the needle.
The membrane's mosaic characteristics explain some but not all of its fluidity. There are two other factors that
help maintain this fluid characteristic. One factor is the nature of the phospholipids themselves. In their saturated
form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms. There are no double bonds
between adjacent carbon atoms. This results in tails that are relatively straight. In contrast, unsaturated fatty
acids do not contain a maximal number of hydrogen atoms, but they do contain some double bonds between
adjacent carbon atoms. A double bond results in a bend in the carbon string of approximately 30 degrees (Figure
5.3).
Thus, if decreasing temperatures compress saturated fatty acids with their straight tails, they press in on each
other, making a dense and fairly rigid membrane. If unsaturated fatty acids are compressed, the “kinks” in
their tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid
molecules. This “elbow room” helps to maintain fluidity in the membrane at temperatures at which membranes
with saturated fatty acid tails in their phospholipids would “freeze” or solidify. The membrane's relative fluidity
is particularly important in a cold environment. A cold environment usually compresses membranes comprised
largely of saturated fatty acids, making them less fluid and more susceptible to rupturing. Many organisms (fish
are one example) are capable of adapting to cold environments by changing the proportion of unsaturated fatty
acids in their membranes in response to lower temperature.
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and mosaic quality.
Animals have an additional membrane constituent that assists in maintaining fluidity. Cholesterol, which lies
alongside the phospholipids in the membrane, tends to dampen temperature effects on the membrane. Thus,
this lipid functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased
temperatures from increasing fluidity too much. Thus, cholesterol extends, in both directions, the temperature
range in which the membrane is appropriately fluid and consequently functional. Cholesterol also serves other
functions, such as organizing clusters of transmembrane proteins into lipid rafts.
Plasma Membrane Components and Functions
Component
Location
Phospholipid
Main membrane fabric
Cholesterol
Attached between phospholipids and between the two
phospholipid layers
Integral proteins (for example, integrins)
Embedded within the phospholipid layer(s); may or may not
penetrate through both layers
Peripheral proteins
On the phospholipid bilayer's inner or outer surface; not
embedded within the phospholipids
Carbohydrates (components of
glycoproteins and glycolipids)
Generally attached to proteins on the outside membrane layer
Table 5.1
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ca eer connection
Immunologist
The variations in peripheral proteins and carbohydrates that affect a cell’s recognition sites are of prime
interest in immunology. In developing vaccines, researchers have been able to conquer many infectious
diseases, such as smallpox, polio, diphtheria, and tetanus.
Immunologists are the physicians and scientists who research and develop vaccines, as well as treat and
study allergies or other immune problems. Some immunologists study and treat autoimmune problems
(diseases in which a person’s immune system attacks his or her own cells or tissues, such as lupus)
and immunodeficiencies, whether acquired (such as acquired immunodeficiency syndrome, or AIDS) or
hereditary (such as severe combined immunodeficiency, or SCID). Immunologists also help treat organ
transplantation patients, who must have their immune systems suppressed so that their bodies will not
reject a transplanted organ. Some immunologists work to understand natural immunity and the effects of a
person’s environment on it. Others work on questions about how the immune system affects diseases such
as cancer. In the past, researchers did not understand the importance of having a healthy immune system
in preventing cancer.
To work as an immunologist, one must have a PhD or MD. In addition, immunologists undertake at least
two to three years of training in an accredited program and must pass the American Board of Allergy
and Immunology exam. Immunologists must possess knowledge of the human body's function as they
relate to issues beyond immunization, and knowledge of pharmacology and medical technology, such as
medications, therapies, test materials, and surgical procedures.
5.2 | Passive Transport
By the end of this section, you will be able to do the following:
• Explain why and how passive transport occurs
• Understand the osmosis and diffusion processes
• Define tonicity and its relevance to passive transport
Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials
from entering and some essential materials from leaving. In other words, plasma membranes are selectively
permeable —they allow some substances to pass through, but not others. If they were to lose this selectivity,
the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts
of specific substances. They must have a way of obtaining these materials from extracellular fluids. This may
happen passively, as certain materials move back and forth, or the cell may have special mechanisms that
facilitate transport. Some materials are so important to a cell that it spends some of its energy, hydrolyzing
adenosine triphosphate (ATP), to obtain these materials. Red blood cells use some of their energy doing just
that. Most cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions
between the cell's interior and exterior, as well as on protein synthesis.
The most direct forms of membrane transport are passive. Passive transport is a naturally occurring
phenomenon and does not require the cell to exert any of its energy to accomplish the movement. In passive
transport, substances move from an area of higher concentration to an area of lower concentration. A physical
space in which there is a single substance concentration range has a concentration gradient.
Selective Permeability
Plasma membranes are asymmetric: the membrane's interior is not identical to its exterior. There is a
considerable difference between the array of phospholipids and proteins between the two leaflets that form a
membrane. On the membrane's interior, some proteins serve to anchor the membrane to cytoskeleton's fibers.
There are peripheral proteins on the membrane's exterior that bind extracellular matrix elements. Carbohydrates,
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attached to lipids or proteins, are also on the plasma membrane's exterior surface. These carbohydrate
complexes help the cell bind required substances in the extracellular fluid. This adds considerably to plasma
membrane's selective nature (Figure 5.7).
>Glycolipid: lipid with
/ carbohydrate
attached
Phospholipid
bilayer
Protein channel
Glycoprotein: protein with
carbohydrate attached
Integral membrane
protein
Cytoskeletal filaments
Figure 5.7 The plasma membrane's exterior surface is not identical to its interior surface.
Recall that plasma membranes are amphiphilic: They have hydrophilic and hydrophobic regions. This
characteristic helps move some materials through the membrane and hinders the movement of others. Non¬
polar and lipid-soluble material with a low molecular weight can easily slip through the membrane's hydrophobic
lipid core. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma
membranes in the digestive tract and other tissues. Fat-soluble drugs and hormones also gain easy entry into
cells and readily transport themselves into the body’s tissues and organs. Oxygen and carbon dioxide molecules
have no charge and pass through membranes by simple diffusion.
Polar substances present problems for the membrane. While some polar molecules connect easily with the
cell's outside, they cannot readily pass through the plasma membrane's lipid core. Additionally, while small ions
could easily slip through the spaces in the membrane's mosaic, their charge prevents them from doing so. ions
such as sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes.
Simple sugars and amino acids also need the help of various transmembrane proteins (channels) to transport
themselves across plasma membranes.
Diffusion
Diffusion is a passive process of transport. A single substance moves from a high concentration to a low
concentration area until the concentration is equal across a space. You are familiar with diffusion of substances
through the air. For example, think about someone opening a bottle of ammonia in a room filled with people.
The ammonia gas is at its highest concentration in the bottle. Its lowest concentration is at the room's edges.
The ammonia vapor will diffuse, or spread away, from the bottle, and gradually, increasingly more people will
smell the ammonia as it spreads. Materials move within the cell’s cytosol by diffusion, and certain materials
move through the plasma membrane by diffusion (Figure 5.8). Diffusion expends no energy. On the contrary,
concentration gradients are a form of potential energy, which dissipates as the gradient is eliminated.
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Chapter 5 | Structure and Function of Plasma Membranes
153
Lipid bilayer
(plasma
membrane)
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Extracellular fluid
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Figure 5.8 Diffusion through a permeable membrane moves a substance from a high concentration area (extracellular
fluid, in this case) down its concentration gradient (into the cytoplasm), (credit: modification of work by Mariana Ruiz
Villareal)
Each separate substance in a medium, such as the extracellular fluid, has its own concentration gradient,
independent of other materials' concentration gradients. In addition, each substance will diffuse according to that
gradient. Within a system, there will be different diffusion rates of various substances in the medium.
Factors That Affect Diffusion
Molecules move constantly in a random manner, at a rate that depends on their mass, their environment, and
the amount of thermal energy they possess, which in turn is a function of temperature. This movement accounts
for molecule diffusion through whatever medium in which they are localized. A substance moves into any space
available to it until it evenly distributes itself throughout. After a substance has diffused completely through a
space, removing its concentration gradient, molecules will still move around in the space, but there will be no net
movement of the number of molecules from one area to another. We call this lack of a concentration gradient in
which the substance has no net movement dynamic equilibrium. While diffusion will go forward in the presence
of a substance's concentration gradient, several factors affect the diffusion rate.
• Extent of the concentration gradient: The greater the difference in concentration, the more rapid the
diffusion. The closer the distribution of the material gets to equilibrium, the slower the diffusion rate.
• Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly.
The reverse is true for lighter molecules.
• Temperature: Higher temperatures increase the energy and therefore the molecules' movement, increasing
the diffusion rate. Lower temperatures decrease the molecules' energy, thus decreasing the diffusion rate.
• Solvent density: As the density of a solvent increases, the diffusion rate decreases. The molecules slow
down because they have a more difficult time passing through the denser medium. If the medium is less
dense, diffusion increases. Because cells primarily use diffusion to move materials within the cytoplasm,
any increase in the cytoplasm’s density will inhibit the movement of the materials. An example of this
is a person experiencing dehydration. As the body’s cells lose water, the diffusion rate decreases in the
cytoplasm, and the cells’ functions deteriorate. Neurons tend to be very sensitive to this effect. Dehydration
frequently leads to unconsciousness and possibly coma because of the decrease in diffusion rate within the
cells.
• Solubility: As we discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes
more easily than polar materials, allowing a faster diffusion rate.
• Surface area and plasma membrane thickness: increased surface area increases the diffusion rate;
whereas, a thicker membrane reduces it.
• Distance travelled: The greater the distance that a substance must travel, the slower the diffusion rate. This
places an upper limitation on cell size. A large, spherical cell will die because nutrients or waste cannot
reach or leave the cell's center, respectively. Therefore, cells must either be small in size, as in the case of
many prokaryotes, or be flattened, as with many single-celled eukaryotes.
A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration
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gradient through a membrane. Sometimes pressure enhances the diffusion rate, causing the substances to filter
more rapidly. This occurs in the kidney, where blood pressure forces large amounts of water and accompanying
dissolved substances, or solutes, out of the blood and into the renal tubules. The diffusion rate in this instance
is almost totally dependent on pressure. One of the effects of high blood pressure is the appearance of protein
in the urine, which abnormally high pressure "squeezes through".
Facilitated transport
In facilitated transport, or facilitated diffusion, materials diffuse across the plasma membrane with the help
of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell
without expending cellular energy. However, these materials are polar molecule ions that the cell membrane's
hydrophobic parts repel. Facilitated transport proteins shield these materials from the membrane's repulsive
force, allowing them to diffuse into the cell.
The transported material first attaches to protein or glycoprotein receptors on the plasma membrane's exterior
surface. This allows removal of material from the extracellular fluid that the cell needs. The substances then
pass to specific integral proteins that facilitate their passage. Some of these integral proteins are collections of
beta-pleated sheets that form a pore or channel through the phospholipid bilayer. Others are carrier proteins
which bind with the substance and aid its diffusion through the membrane.
Channels
The integral proteins involved in facilitated transport are transport proteins, and they function as either
channels for the material or carriers. In both cases, they are transmembrane proteins. Channels are specific
for the transported substance. Channel proteins have hydrophilic domains exposed to the intracellular and
extracellular fluids, in addition, they have a hydrophilic channel through their core that provides a hydrated
opening through the membrane layers (Figure 5.9). Passage through the channel allows polar compounds to
avoid the plasma membrane's nonpolar central layer that would otherwise slow or prevent their entry into the
cell. Aquaporins are channel proteins that allow water to pass through the membrane at a very high rate.
o o o
Cytoplasm
Figure 5.9 Facilitated transport moves substances down their concentration gradients. They may cross the plasma
membrane with the aid of channel proteins, (credit: modification of work by Mariana Ruiz Villareal)
Channel proteins are either open at all times or they are “gated,” which controls the channel's opening. When
a particular ion attaches to the channel protein it may control the opening, or other mechanisms or substances
may be involved. In some tissues, sodium and chloride ions pass freely through open channels; whereas, in
other tissues a gate must open to allow passage. An example of this occurs in the kidney, where there are both
channel forms in different parts of the renal tubules. Cells involved in transmitting electrical impulses, such as
nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes. Opening
and closing these channels changes the relative concentrations on opposing sides of the membrane of these
ions, resulting in facilitating electrical transmission along membranes (in the case of nerve cells) or in muscle
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contraction (in the case of muscle cells).
Carrier Proteins
Another type of protein embedded in the plasma membrane is a carrier protein. This aptly named protein
binds a substance and, thus triggers a change of its own shape, moving the bound molecule from the cell's
outside to its interior (Figure 5.10). Depending on the gradient, the material may move in the opposite direction.
Carrier proteins are typically specific for a single substance. This selectivity adds to the plasma membrane's
overall selectivity. Scientists poorly understand the exact mechanism for the change of shape. Proteins can
change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism. Each
carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane.
This can cause problems in transporting enough material for the cell to function properly. When all of the
proteins are bound to their ligands, they are saturated and the rate of transport is at its maximum. Increasing the
concentration gradient at this point will not result in an increased transport rate.
Ilular fluid
O
Cytoplasm
Figure 5.10 Some substances are able to move down their concentration gradient across the plasma membrane with
the aid of carrier proteins. Carrier proteins change shape as they move molecules across the membrane, (credit:
modification of work by Mariana Ruiz Villareal)
An example of this process occurs in the kidney. In one part, the kidney filters glucose, water, salts, ions, and
amino acids that the body requires. This filtrate, which includes glucose, then reabsorbs in another part of the
kidney. Because there are only a finite number of carrier proteins for glucose, if more glucose is present than
the proteins can handle, the excess is not transported and the body excretes this through urine. In a diabetic
individual, the term is “spilling glucose into the urine.” A different group of carrier proteins, glucose transport
proteins, or GLUTS, are involved in transporting glucose and other hexose sugars through plasma membranes
within the body.
Channel and carrier proteins transport material at different rates. Channel proteins transport much more quickly
than carrier proteins. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second;
whereas, carrier proteins work at a rate of a thousand to a million molecules per second.
Osmosis
Osmosis is the movement of water through a semipermeable membrane according to the water's concentration
gradient across the membrane, which is inversely proportional to the solutes' concentration. While diffusion
transports material across membranes and within cells, osmosis transports only water across a membrane and
the membrane limits the solutes' diffusion in the water. Not surprisingly, the aquaporins that facilitate water
movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney
tubules.
Mechanism
Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration
to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a
semipermeable membrane separating the two sides or halves (Figure 5.11). On both sides of the membrane
the water level is the same, but there are different dissolved substance concentrations, or solute, that cannot
_Plasma
membrane
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cross the membrane (otherwise the solute crossing the membrane would balance concentrations on each side).
If the solution's volume on both sides of the membrane is the same, but the solute's concentrations are different,
then there are different amounts of water, the solvent, on either side of the membrane.
Figure 5.11 In osmosis, water always moves from an area of higher water concentration to one of lower concentration.
In the diagram, the solute cannot pass through the selectively permeable membrane, but the water can.
To illustrate this, imagine two full water glasses. One has a single teaspoon of sugar in it; whereas, the second
one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup
contains more water? Because the large sugar amount in the second cup takes up much more space than the
teaspoon of sugar in the first cup, the first cup has more water in it.
Returning to the beaker example, recall that it has a solute mixture on either side of the membrane. A principle
of diffusion is that the molecules move around and will spread evenly throughout the medium if they can.
However, only the material capable of getting through the membrane will diffuse through it. In this example,
the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this
system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it
is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the water's
concentration gradient goes to zero or until the water's hydrostatic pressure balances the osmotic pressure.
Osmosis proceeds constantly in living systems.
Tonicity
Tonicity describes how an extracellular solution can change a cell's volume by affecting osmosis. A solution's
tonicity often directly correlates with the solution's osmolarity. Osmolarity describes the solution's total solute
concentration. A solution with low osmolarity has a greater number of water molecules relative to the number of
solute particles. A solution with high osmolarity has fewer water molecules with respect to solute particles. In a
situation in which a membrane permeable to water, though not to the solute separates two different osmolarities,
water will move from the membrane's side with lower osmolarity (and more water) to the side with higher
osmolarity (and less water). This effect makes sense if you remember that the solute cannot move across
the membrane, and thus the only component in the system that can move—the water—moves along its own
concentration gradient. An important distinction that concerns living systems is that osmolarity measures the
number of particles (which may be molecules) in a solution. Therefore, a solution that is cloudy with cells may
have a lower osmolarity than a solution that is clear, if the second solution contains more dissolved molecules
than there are cells.
Hypotonic Solutions
Scientists use three terms—hypotonic, isotonic, and hypertonic—to relate the cell's osmolarity to the
extracellular fluid's osmolarity that contains the cells. In a hypotonic situation, the extracellular fluid has lower
osmolarity than the fluid inside the cell, and water enters the cell. (In living systems, the point of reference is
always the cytoplasm, so the prefix hypo- means that the extracellular fluid has a lower solute concentration,
or a lower osmolarity, than the cell cytoplasm.) It also means that the extracellular fluid has a higher water
concentration in the solution than does the cell. In this situation, water will follow its concentration gradient and
enter the cell.
Hypertonic Solutions
As for a hypertonic solution, the prefix hyper- refers to the extracellular fluid having a higher osmolarity than
the cell’s cytoplasm; therefore, the fluid contains less water than the cell does. Because the cell has a relatively
higher water concentration, water will leave the cell.
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157
Isotonic Solutions
In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the cell's osmolarity matches
that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still
move in and out. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic
appearances (Figure 5.12).
visual
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Hypertonic Isotonic Hypotonic
solution solution solution
H2 °^3P tJS? 'p
Figure 5.12 Osmotic pressure changes red blood cells' shape in hypertonic, isotonic, and hypotonic solutions,
(credit: Mariana Ruiz Villareal)
A doctor injects a patient with what the doctor thinks is an isotonic saline solution. The patient dies, and
an autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor
injected was really isotonic?
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dispersion) .
Tonicity in Living Systems
In a hypotonic environment, water enters a cell, and the cell swells. In an isotonic condition, the relative solute
and solvent concentrations are equal on both membrane sides. There is no net water movement; therefore, there
is no change in the cell's size. In a hypertonic solution, water leaves a cell and the cell shrinks. If either the hypo-
or hyper- condition goes to excess, the cell’s functions become compromised, and the cell may be destroyed.
A red blood cell will burst, or lyse, when it swells beyond the plasma membrane’s capability to expand.
Remember, the membrane resembles a mosaic, with discrete spaces between the molecules comprising it. If
the cell swells, and the spaces between the lipids and proteins become too large, the cell will break apart.
In contrast, when excessive water amounts leave a red blood cell, the cell shrinks, or crenates. This has the
effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within
the cell. The cell’s ability to function will be compromised and may also result in the cell's death.
Various living things have ways of controlling the effects of osmosis—a mechanism we call osmoregulation.
Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma
membrane and prevent cell lysis in a hypotonic solution. The plasma membrane can only expand to the
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Chapter 5 | Structure and Function of Plasma Membranes
cell wall's limit, so the cell will not lyse. The cytoplasm in plants is always slightly hypertonic to the cellular
environment, and water will always enter a cell if water is available. This water inflow produces turgor pressure,
which stiffens the plant's cell walls (Figure 5.13). In nonwoody plants, turgor pressure supports the plant.
Conversly, if you do not water the plant, the extracellular fluid will become hypertonic, causing water to leave the
cell. In this condition, the cell does not shrink because the cell wall is not flexible. However, the cell membrane
detaches from the wall and constricts the cytoplasm. We call this plasmolysis. Plants lose turgor pressure in
this condition and wilt (Figure 5.14).
Hypertonic
condition
Isotonic
condition
Hypotonic
condition
Figure 5.13 The turgor pressure within a plant cell depends on the solution's tonicity in which it is bathed, (credit:
modification of work by Mariana Ruiz Villareal)
Figure 5.14 Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting. Watering the
plant (right) will restore the turgor pressure, (credit: Victor M. Vicente Selvas)
Tonicity is a concern for all living things. For example, paramecia and amoebas, which are protists that lack cell
walls, have contractile vacuoles. This vesicle collects excess water from the cell and pumps it out, keeping the
cell from lysing as it takes on water from its environment (Figure 5.15).
Figure 5.15 A paramecium’s contractile vacuole, here visualized using bright field light microscopy at 480x
magnification, continuously pumps water out of the organism's body to keep it from bursting in a hypotonic medium,
(credit: modification of work by NIH; scale-bar data from Matt Russell)
Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with
the water in which they live. Fish, however, must spend approximately five percent of their metabolic energy
maintaining osmotic homeostasis. Freshwater fish live in an environment that is hypotonic to their cells. These
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fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water. Saltwater
fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and
excrete highly concentrated urine.
in vertebrates, the kidneys regulate the water amount in the body. Osmoreceptors are specialized cells in the
brain that monitor solute concentration in the blood. If the solute levels increase beyond a certain range, a
hormone releases that slows water loss through the kidney and dilutes the blood to safer levels. Animals also
have high albumin concentrations, which the liver produces, in their blood. This protein is too large to pass easily
through plasma membranes and is a major factor in controlling the osmotic pressures applied to tissues.
5.3 | Active Transport
By the end of this section, you will be able to do the following:
• Understand how electrochemical gradients affect ions
• Distinguish between primary active transport and secondary active transport
Active transport mechanisms require the cell’s energy, usually in the form of adenosine triphosphate (ATP). If
a substance must move into the cell against its concentration gradient—that is, if the substance's concentration
inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy
to move the substance. Some active transport mechanisms move small-molecular weight materials, such as
ions, through the membrane. Other mechanisms transport much larger molecules.
Electrochemical Gradient
We have discussed simple concentration gradients—a substance's differential concentrations across a space or
a membrane—but in living systems, gradients are more complex. Because ions move into and out of cells and
because cells contain proteins that do not move across the membrane and are mostly negatively charged, there
is also an electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is
electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells
have higher concentrations of potassium (K + ) and lower concentrations of sodium (Na + ) than the extracellular
fluid. Thus in a living cell, the concentration gradient of Na + tends to drive it into the cell, and its electrical gradient
(a positive ion) also drives it inward to the negatively charged interior. However, the situation is more complex for
other elements such as potassium. The electrical gradient of K + , a positive ion, also drives it into the cell, but the
concentration gradient of K + drives K + out of the cell (Figure 5.16). We call the combined concentration gradient
and electrical charge that affects an ion its electrochemical gradient.
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visual
CONNECTION
Figure 5.16 Electrochemical gradients arise from the combined effects of concentration gradients and electrical
gradients. Structures labeled A represent proteins, (credit: “Synaptitude’VWikimedia Commons)
Injecting a potassium solution into a person’s blood is lethal. This is how capital punishment and euthanasia
subjects die. Why do you think a potassium solution injection is lethal?
Moving Against a Gradient
To move substances against a concentration or electrochemical gradient, the cell must use energy. This
energy comes from ATP generated through the cell’s metabolism. Active transport mechanisms, or pumps,
work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active
transport maintains concentrations of ions and other substances that living cells require in the face of these
passive movements. A cell may spend much of its metabolic energy supply maintaining these processes. (A red
blood cell uses most of its metabolic energy to maintain the imbalance between exterior and interior sodium and
potassium levels that the cell requires.) Because active transport mechanisms depend on a cell’s metabolism for
energy, they are sensitive to many metabolic poisons that interfere with the ATP supply.
Two mechanisms exist for transporting small-molecular weight material and small molecules. Primary active
transport moves ions across a membrane and creates a difference in charge across that membrane, which
is directly dependent on ATP. Secondary active transport does not directly require ATP: instead, it is the
movement of material due to the electrochemical gradient established by primary active transport.
Carrier Proteins for Active Transport
An important membrane adaption for active transport is the presence of specific carrier proteins or pumps
to facilitate movement: there are three protein types or transporters (Figure 5.17). A uniporter carries one
specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction. An
antiporter also carries two different ions or molecules, but in different directions. All of these transporters can
also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also
in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active
transport are Na + -K + ATPase, which carries sodium and potassium ions, and H + -K + ATPase, which carries
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hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca 2+
ATPase and H + ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.
Uniporter Symporter Antiporter
A
I
▼
Figure 5.17 A uniporter carries one molecule or ion. A symporter carries two different molecules or ions, both in the
same direction. An antiporter also carries two different molecules or ions, but in different directions, (credit: modification
of work by “Lupask’VWikimedia Commons)
Primary Active Transport
The primary active transport that functions with the active transport of sodium and potassium allows secondary
active transport to occur. The second transport method is still active because it depends on using energy as
does primary transport (Figure 5.18).
Plasma
membrane
Figure 5.18 Primary active transport moves ions across a membrane, creating an electrochemical gradient
(electrogenic transport), (credit: modification of work by Mariana Ruiz Villareal)
One of the most important pumps in animal cells is the sodium-potassium pump (Na + -K + ATPase), which
maintains the electrochemical gradient (and the correct concentrations of Na + and K + ) in living cells. The sodium-
potassium pump moves K + into the cell while moving Na + out at the same time, at a ratio of three Na + for every
two K + ions moved in. The Na + -K + ATPase exists in two forms, depending on its orientation to the cell's interior
or exterior and its affinity for either sodium or potassium ions. The process consists of the following six steps.
1. With the enzyme oriented towards the cell's interior, the carrier has a high affinity for sodium ions. Three
ions bind to the protein.
2. The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it.
3. As a result, the carrier changes shape and reorients itself towards the membrane's exterior. The protein’s
affinity for sodium decreases and the three sodium ions leave the carrier.
4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein.
Subsequently, the low-energy phosphate group detaches from the carrier.
5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself
towards the cell's interior.
6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions moves
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into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.
Several things have happened as a result of this process. At this point, there are more sodium ions outside
the cell than inside and more potassium ions inside than out. For every three sodium ions that move out, two
potassium ions move in. This results in the interior being slightly more negative relative to the exterior. This
difference in charge is important in creating the conditions necessary for the secondary process. The sodium-
potassium pump is, therefore, an electrogenic pump (a pump that creates a charge imbalance), creating an
electrical imbalance across the membrane and contributing to the membrane potential.
LINK
T &
LEARNING
Watch this video (https:// 0 penstax. 0 rg/l/Na_K_ATPase) to see an active transport simulation in a sodium-
potassium ATPase.
Secondary Active Transport (Co-transport)
Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion
concentrations build outside of the plasma membrane because of the primary active transport process, this
creates an electrochemical gradient. If a channel protein exists and is open, the sodium ions will pull through
the membrane. This movement transports other substances that can attach themselves to the transport protein
through the membrane (Figure 5.19). Many amino acids, as well as glucose, enter a cell this way. This
secondary process also stores high-energy hydrogen ions in the mitochondria of plant and animal cells in order
to produce ATP. The potential energy that accumulates in the stored hydrogen ions translates into kinetic energy
as the ions surge through the channel protein ATP synthase, and that energy then converts ADP into ATP.
visual
CONNECTION
Figure 5.19 An electrochemical gradient, which primary active transport creates, can move other substances
against their concentration gradients, a process scientists call co-transport or secondary active transport, (credit:
modification of work by Mariana Ruiz Villareal)
If the pH outside the cell decreases, would you expect the amount of amino acids transported into the cell
to increase or decrease?
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5.4 | Bulk Transport
By the end of this section, you will be able to do the following:
• Describe endocytosis, including phagocytosis, pinocytosis, and receptor-mediated endocytosis
• Understand the process of exocytosis
In addition to moving small ions and molecules through the membrane, cells also need to remove and take in
larger molecules and particles (see Table 5.2 for examples). Some cells are even capable of engulfing entire
unicellular microorganisms. You might have correctly hypothesized that when a cell uptakes and releases large
particles, it requires energy. A large particle, however, cannot pass through the membrane, even with energy
that the cell supplies.
Endocytosis
Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and
even whole cells, into a cell. There are different endocytosis variations, but all share a common characteristic:
the cell's plasma membrane invaginates, forming a pocket around the target particle. The pocket pinches
off, resulting in the particle containing itself in a newly created intracellular vesicle formed from the plasma
membrane.
Phagocytosis
Phagocytosis (the condition of “cell eating”) is the process by which a cell takes in large particles, such as
other cells or relatively large particles. For example, when microorganisms invade the human body, a type of
white blood cell, a neutrophil, will remove the invaders through this process, surrounding and engulfing the
microorganism, which the neutrophil then destroys (Figure 5.20).
Phagocytosis
Plasma
membrane
Vacuole
Figure 5.20 In phagocytosis, the cell membrane surrounds the particle and engulfs it. (credit: modification of work by
Mariana Ruiz Villareal)
In preparation for phagocytosis, a portion of the plasma membrane's inward-facing surface becomes coated with
the protein clathrin, which stabilizes this membrane's section. The membrane's coated portion then extends
from the cell's body and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle
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is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome
for breaking down the material in the newly formed compartment (endosome). When accessible nutrients from
the vesicular contents' degradation have been extracted, the newly formed endosome merges with the plasma
membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part
of the plasma membrane.
Pinocytosis
A variation of endocytosis is pinocytosis. This literally means “cell drinking". Discovered by Warren Lewis in
1929, this American embryologist and cell biologist described a process whereby he assumed that the cell was
purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which
the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis,
and the vesicle does not need to merge with a lysosome (Figure 5.21).
Pinocytosis
Figure 5.21 In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. (credit:
modification of work by Mariana Ruiz Villareal)
A variation of pinocytosis is potocytosis. This process uses a coating protein, caveolin, on the plasma
membrane's cytoplasmic side, which performs a similar function to clathrin. The cavities in the plasma
membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles
or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis brings small
molecules into the cell and transports them through the cell for their release on the other side, a process we call
transcytosis.
Receptor-mediated Endocytosis
A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific
binding affinity for certain substances (Figure 5.22).
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Receptor-mediated
endocytosis
* * a ★
Coated vesicle
Figure 5.22 In receptor-mediated endocytosis, the cell's uptake of substances targets a single type of substance that
binds to the receptor on the cell membrane's external surface, (credit: modification of work by Mariana Ruiz Villareal)
In receptor-mediated endocytosis, as in phagocytosis, clathrin attaches to the plasma membrane's
cytoplasmic side. If a compound's uptake is dependent on receptor-mediated endocytosis and the process is
ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids
and increase in concentration. The failure of receptor-mediated endocytosis causes some human diseases. For
example, receptor mediated endocytosis removes low density lipoprotein or LDL (or "bad" cholesterol) from the
blood. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing
entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells
cannot clear LDL particles.
Although receptor-mediated endocytosis is designed to bring specific substances that are normally in the
extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses,
diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into
cells.
LINK TQ LEARNING
See receptor-mediated endocytosis in action, and click on different parts (http:// 0 penstaxc 0 llege. 0 rg/l/
endocytosis) for a focused animation.
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Exocytosis
The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of
the processes we discussed above in that its purpose is to expel material from the cell into the extracellular
fluid. Waste material is enveloped in a membrane and fuses with the plasma membrane's interior. This fusion
opens the membranous envelope on the cell's exterior, and the waste material expels into the extracellular space
(Figure 5.23). Other examples of cells releasing molecules via exocytosis include extracellular matrix protein
secretion and neurotransmitter secretion into the synaptic cleft by synaptic vesicles.
Exocytosis
Extracellular fluid
Figure 5.23 In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents then release
to the cell's exterior, (credit: modification of work by Mariana Ruiz Villareal)
Methods of Transport, Energy Requirements, and Types of Transported Material
Transport Method
Active/
Passive
Material Transported
Diffusion
Passive
Small-molecular weight material
Osmosis
Passive
Water
Facilitated transport/diffusion
Passive
Sodium, potassium, calcium, glucose
Primary active transport
Active
Sodium, potassium, calcium
Secondary active transport
Active
Amino acids, lactose
Phagocytosis
Active
Large macromolecules, whole cells, or cellular
structures
Pinocytosis and potocytosis
Active
Small molecules (liquids/water)
Table 5.2
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Methods of Transport, Energy Requirements, and Types of Transported Material
Transport Method
Active/
Passive
Material Transported
Receptor-mediated
endocytosis
Active
Large quantities of macromolecules
Table 5.2
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KEY TERMS
active transport method of transporting material that requires energy
amphiphilic molecule possessing a polar or charged area and a nonpolar or uncharged area capable of
interacting with both hydrophilic and hydrophobic environments
antiporter transporter that carries two ions or small molecules in different directions
aquaporin channel protein that allows water through the membrane at a very high rate
carrier protein membrane protein that moves a substance across the plasma membrane by changing its own
shape
caveolin protein that coats the plasma membrane's cytoplasmic side and participates in the liquid uptake
process by potocytosis
channel protein membrane protein that allows a substance to pass through its hollow core across the plasma
membrane
clathrin protein that coats the plasma membrane's inward-facing surface and assists in forming specialized
structures, like coated pits, for phagocytosis
concentration gradient area of high concentration adjacent to an area of low concentration
diffusion passive transport process of low-molecular weight material according to its concentration gradient
electrochemical gradient a combined electrical and chemical force that produces a gradient
electrogenic pump pump that creates a charge imbalance
endocytosis type of active transport that moves substances, including fluids and particles, into a cell
exocytosis process of passing bulk material out of a cell
facilitated transport process by which material moves down a concentration gradient (from high to low
concentration) using integral membrane proteins
fluid mosaic model describes the plasma membrane's structure as a mosaic of components including
phospholipids, cholesterol, proteins, glycoproteins, and glycolipids (sugar chains attached to proteins or
lipids, respectively), resulting in a fluid character (fluidity)
glycolipid combination of carbohydrates and lipids
glycoprotein combination of carbohydrates and proteins
hydrophilic molecule with the ability to bond with water; “water-loving"
hydrophobic molecule that does not have the ability to bond with water; “water-hating”
hypertonic situation in which extracellular fluid has a higher osmolarity than the fluid inside the cell, resulting in
water moving out of the cell
hypotonic situation in which extracellular fluid has a lower osmolarity than the fluid inside the cell, resulting in
water moving into the cell
integral protein protein integrated into the membrane structure that interacts extensively with the membrane
lipids' hydrocarbon chains and often spans the membrane
isotonic situation in which the extracellular fluid has the same osmolarity as the fluid inside the cell, resulting in
no net water movement into or out of the cell
osmolarity total amount of substances dissolved in a specific amount of solution
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osmosis transport of water through a semipermeable membrane according to the water's concentration
gradient across the membrane that results from the presence of solute that cannot pass through the
membrane
passive transport method of transporting material through a membrane that does not require energy
peripheral protein protein at the plasma membrane's surface either on its exterior or interior side
pinocytosis a variation of endocytosis that imports macromolecules that the cell needs from the extracellular
fluid
plasmolysis detaching the cell membrane from the cell wall and constricting the cell membrane when a plant
cell is in a hypertonic solution
potocytosis variation of pinocytosis that uses a different coating protein (caveolin) on the plasma membrane's
cytoplasmic side
primary active transport active transport that moves ions or small molecules across a membrane and may
create a difference in charge across that membrane
pump active transport mechanism that works against electrochemical gradients
receptor-mediated endocytosis variation of endocytosis that involves using specific binding proteins in the
plasma membrane for specific molecules or particles, and clathrin-coated pits that become clathrin-coated
vesicles
secondary active transport movement of material that results from primary active transport to the
electrochemical gradient
selectively permeable membrane characteristic that allows some substances through
solute substance dissolved in a liquid to form a solution
symporter transporter that carries two different ions or small molecules, both in the same direction
tonicity amount of solute in a solution
transport protein membrane protein that facilitates a substance's passage across a membrane by binding it
transporter specific carrier proteins or pumps that facilitate movement
uniporter transporter that carries one specific ion or molecule
CHAPTER SUMMARY
5.1 Components and Structure
Modern scientists refer to the plasma membrane as the fluid mosaic model. A phospholipid bilayer comprises
the plasma membrane, with hydrophobic, fatty acid tails in contact with each other. The membrane's landscape
is studded with proteins, some which span the membrane. Some of these proteins serve to transport materials
into or out of the cell. Carbohydrates are attached to some of the proteins and lipids on the membrane's
outward-facing surface, forming complexes that function to identify the cell to other cells. The membrane's fluid
nature is due to temperature, fatty acid tail configuration (some kinked by double bonds), cholesterol presence
embedded in the membrane, and the mosaic nature of the proteins and protein-carbohydrate combinations,
which are not firmly fixed in place. Plasma membranes enclose and define the cells' borders. Not static, they
are dynamic and constantly in flux.
5.2 Passive Transport
The passive transport forms, diffusion and osmosis, move materials of small molecular weight across
membranes. Substances diffuse from high to lower concentration areas, and this process continues until the
substance evenly distributes itself in a system. In solutions containing more than one substance, each molecule
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type diffuses according to its own concentration gradient, independent of other substances diffusing. Many
factors can affect the diffusion rate, such as concentration gradient, diffusing, particle sizes, and the system's
temperature.
In living systems, the plasma membrane mediates substances diffusing in and out of cells. Some materials
diffuse readily through the membrane, but others are hindered and only can pass through due to specialized
proteins such as channels and transporters. The chemistry of living things occurs in aqueous solutions, and
balancing the concentrations of those solutions is an ongoing problem. In living systems, diffusing some
substances would be slow or difficult without membrane proteins that facilitate transport.
5.3 Active Transport
The combined gradient that affects an ion includes its concentration gradient and its electrical gradient. A
positive ion, for example, might diffuse into a new area, down its concentration gradient, but if it is diffusing into
an area of net positive charge, its electrical gradient hampers its diffusion. When dealing with ions in aqueous
solutions, one must consider electrochemical and concentration gradient combinations, rather than just the
concentration gradient alone. Living cells need certain substances that exist inside the cell in concentrations
greater than they exist in the extracellular space. Moving substances up their electrochemical gradients
requires energy from the cell. Active transport uses energy stored in ATP to fuel this transport. Active transport
of small molecular-sized materials uses integral proteins in the cell membrane to move the materials. These
proteins are analogous to pumps. Some pumps, which carry out primary active transport, couple directly with
ATP to drive their action. In co-transport (or secondary active transport), energy from primary transport can
move another substance into the cell and up its concentration gradient.
5.4 Bulk Transport
Active transport methods require directly using ATP to fuel the transport. In a process scientists call
phagocytosis, other cells can engulf large particles, such as macromolecules, cell parts, or whole cells. In
phagocytosis, a portion of the membrane invaginates and flows around the particle, eventually pinching off and
leaving the particle entirely enclosed by a plasma membrane's envelope. The cell breaks down vesicle
contents, with the particles either used as food or dispatched. Pinocytosis is a similar process on a smaller
scale. The plasma membrane invaginates and pinches off, producing a small envelope of fluid from outside the
cell. Pinocytosis imports substances that the cell needs from the extracellular fluid. The cell expels waste in a
similar but reverse manner. It pushes a membranous vacuole to the plasma membrane, allowing the vacuole to
fuse with the membrane and incorporate itself into the membrane structure, releasing its contents to the
exterior.
VISUAL CONNECTION QUESTIONS
1. Figure 5.12 A doctor injects a patient with what
the doctor thinks is an isotonic saline solution. The
patient dies, and an autopsy reveals that many red
blood cells have been destroyed. Do you think the
solution the doctor injected was really isotonic?
2. Figure 5.16 Injecting a potassium solution into a
REVIEW QUESTIONS
4. Which plasma membrane component can be
either found on its surface or embedded in the
membrane structure?
a. protein
b. cholesterol
c. carbohydrate
d. phospholipid
5. Which characteristic of a phospholipid contributes
to the fluidity of the membrane?
person’s blood is lethal. Capital punishment and
euthanasia utilize this method in their subjects. Why
do you think a potassium solution injection is lethal?
3. Figure 5.19 If the pH outside the cell decreases,
would you expect the amount of amino acids
transported into the cell to increase or decrease?
a. its head
b. cholesterol
c. a saturated fatty acid tail
d. double bonds in the fatty acid tail
6. What is the primary function of carbohydrates
attached to the exterior of cell membranes?
a. identification of the cell
b. flexibility of the membrane
c. strengthening the membrane
d. channels through membrane
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7. A scientist compares the plasma membrane
composition of an animal from the Mediterranean
coast with one from the Mojave Desert. Which
hypothesis is most likely to be correct?
a. The cells from the Mediterranean coast
animal will have more fluid plasma
membranes.
b. The cells from the Mojave Desert animal will
have a higher cholesterol concentration in
the plasma membranes.
c. The cells’ plasma membranes will be
indistinguishable.
d. The cells from the Mediterranean coast
animal will have a higher glycoprotein
content, while the cells from the Mojave
Desert animal will have a higher lipoprotein
content.
8. Water moves via osmosis_.
a. throughout the cytoplasm
b. from an area with a high concentration of
other solutes to a lower one
c. from an area with a high concentration of
water to one of lower concentration
d. from an area with a low concentration of
water to higher concentration
9. The principal force driving movement in diffusion is
the_.
a. temperature
b. particle size
c. concentration gradient
d. membrane surface area
10. What problem is faced by organisms that live in
fresh water?
a. Their bodies tend to take in too much water.
b. They have no way of controlling their
tonicity.
c. Only salt water poses problems for animals
that live in it.
d. Their bodies tend to lose too much water to
their environment.
11. In which situation would passive transport not
use a transport protein for entry into a cell?
a. water flowing into a hypertonic environment
b. glucose being absorbed from the blood
c. an ion flowing into a nerve cell to create an
electrical potential
d. oxygen moving into a cell after oxygen
deprivation
12. Active transport must function continuously
because_.
a. plasma membranes wear out
b. not all membranes are amphiphilic
c. facilitated transport opposes active transport
d. diffusion is constantly moving solutes in
opposite directions
13. How does the sodium-potassium pump make the
interior of the cell negatively charged?
a. by expelling anions
b. by pulling in anions
c. by expelling more cations than are taken in
d. by taking in and expelling an equal number
of cations
14. What is the combination of an electrical gradient
and a concentration gradient called?
a. potential gradient
b. electrical potential
c. concentration potential
d. electrochemical gradient
15. What happens to the membrane of a vesicle after
exocytosis?
a. It leaves the cell.
b. It is disassembled by the cell.
c. It fuses with and becomes part of the
plasma membrane.
d. It is used again in another exocytosis event.
16. Which transport mechanism can bring whole cells
into a cell?
a. pinocytosis
b. phagocytosis
c. facilitated transport
d. primary active transport
17. In what important way does receptor-mediated
endocytosis differ from phagocytosis?
a. It transports only small amounts of fluid.
b. It does not involve the pinching off of
membrane.
c. It brings in only a specifically targeted
substance.
d. It brings substances into the cell, while
phagocytosis removes substances.
18. Many viruses enter host cells through receptor-
mediated endocytosis. What is an advantage of this
entry strategy?
a. The virus directly enters the cytoplasm of
the cell.
b. The virus is protected from recognition by
white blood cells.
c. The virus only enters its target host cell
type.
d. The virus can directly inject its genome into
the cell’s nucleus.
19. Which of the following organelles relies on
exocytosis to complete its function?
a. Golgi apparatus
b. vacuole
c. mitochondria
d. endoplasmic reticulum
20. Imagine a cell can perform exocytosis, but only
minimal endocytosis. What would happen to the cell?
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a. The cell would secrete all its intracellular
proteins.
b. The plasma membrane would increase in
size over time.
c. The cell would stop expressing integral
receptor proteins in its plasma membrane.
d. The cell would lyse.
CRITICAL THINKING QUESTIONS
21. Why is it advantageous for the cell membrane to
be fluid in nature?
22. Why do phospholipids tend to spontaneously
orient themselves into something resembling a
membrane?
23. How can a cell use an extracellular peripheral
protein as the receptor to transmit a signal into the
cell?
24. Discuss why the following affect the rate of
diffusion: molecular size, temperature, solution
density, and the distance that must be traveled.
25. Why does water move through a membrane?
26. Both of the regular intravenous solutions
administered in medicine, normal saline and lactated
Ringer’s solution, are isotonic. Why is this important?
27. Describe two ways that decreasing temperature
would affect the rate of diffusion of molecules across
a cell’s plasma membrane.
28. A cell develops a mutation in its potassium
channels that prevents the ions from leaving the cell.
If the cell’s aquaporins are still active, what will
happen to the cell? Be sure to describe the tonicity
and osmolarity of the cell.
29. Where does the cell get energy for active
transport processes?
30. How does the sodium-potassium pump contribute
to the net negative charge of the interior of the cell?
31. Glucose from digested food enters intestinal
epithelial cells by active transport. Why would
intestinal cells use active transport when most body
cells use facilitated diffusion?
32. The sodium/calcium exchanger (NCX) transports
sodium into and calcium out of cardiac muscle cells.
Describe why this transporter is classified as
secondary active transport.
33. Why is it important that there are different types
of proteins in plasma membranes for the transport of
materials into and out of a cell?
34. Why do ions have a difficult time getting through
plasma membranes despite their small size?
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6 | METABOLISM
Figure 6.1 A hummingbird needs energy to maintain prolonged periods of flight. The bird obtains its energy from taking
in food and transforming the nutrients into energy through a series of biochemical reactions. The flight muscles in birds
are extremely efficient in energy production, (credit: modification of work by Cory Zanker)
Chapter Outline
6.1: Energy and Metabolism
6.2: Potential, Kinetic, Free, and Activation Energy
6.3: The Laws of Thermodynamics
6.4: ATP: Adenosine Triphosphate
6.5: Enzymes
Introduction
Virtually every task performed by living organisms requires energy. Organisms require energy to perform heavy
labor and exercise, but humans also use considerable energy while thinking, and even during sleep. Every
organism's living cells constantly use energy. Organisms import nutrients and other molecules. They metabolize
(break down) and possibly synthesize into new molecules. If necessary, molecules modify, move around the cell
and may distribute themselves to the entire organism. For example, the large proteins that make up muscles are
actively built from smaller molecules. Complex carbohydrates break down into simple sugars that the cell uses
for energy. Just as energy is required to both build and demolish a building, energy is required to synthesize
and break down molecules. Additionally, signaling molecules such as hormones and neurotransmitters transport
between cells. Cells ingest and break down bacteria and viruses. Cells must also export waste and toxins to stay
healthy, and many cells must swim or move surrounding materials via the beating motion of cellular appendages
like cilia and flagella.
The cellular processes that we listed above require a steady supply of energy. From where, and in what form,
does this energy come? How do living cells obtain energy, and how do they use it? This chapter will discuss
different forms of energy and the physical laws that govern energy transfer. This chapter will also describe how
cells use energy and replenish it, and how chemical reactions in the cell perform with great efficiency.
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6.1 1 Energy and Metabolism
By the end of this section, you will be able to do the following:
• Explain metabolic pathways and describe the two major types
• Discuss how chemical reactions play a role in energy transfer
Scientists use the term bioenergetics to discuss the concept of energy flow (Figure 6.2) through living systems,
such as cells. Cellular processes such as building and breaking down complex molecules occur through
stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy; whereas,
others require energy to proceed. Just as living things must continually consume food to replenish what they
have used, cells must continually produce more energy to replenish that which the many energy-requiring
chemical reactions that constantly take place use. All of the chemical reactions that transpire inside cells,
including those that use and release energy, are the cell’s metabolism.
HEAT
Figure 6.2 Most life forms on earth obtain their energy from the sun. Plants use photosynthesis to capture sunlight,
and herbivores eat those plants to obtain energy. Carnivores eat the herbivores, and decomposers digest plant and
animal matter.
Carbohydrate Metabolism
Sugar (chemical reactions) metabolism (a simple carbohydrate) is a classic example of the many cellular
processes that use and produce energy. Living things consume sugar as a major energy source, because sugar
molecules have considerable energy stored within their bonds. The following equation describes the breakdown
of glucose, a simple sugar:
C 6 H 12 0 6 + 60 2 -> 6C0 2 + 6H 2 0 + energy
Consumed carbohydrates have their origins in photosynthesizing organisms like plants (Figure 6.3). During
photosynthesis, plants use the energy of sunlight to convert carbon dioxide gas (CO 2 ) into sugar molecules, like
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glucose (C 6 H 12 O 6 ). Because this process involves synthesizing a larger, energy-storing molecule, it requires an
energy input to proceed. The following equation (notice that it is the reverse of the previous equation) describes
the synthesis of glucose:
6COo + 6H 2 0 + energy -*■ C 6 H 12 0 6 + 60 2
During photosynthesis chemical reactions, energy is in the form of a very high-energy molecule scientists call
ATP, or adenosine triphosphate. This is the primary energy currency of all cells. Just as the dollar is the currency
we use to buy goods, cells use ATP molecules as energy currency to perform immediate work. The sugar
(glucose) is stored as starch or glycogen. Energy-storing polymers like these break down into glucose to supply
ATP molecules.
Solar energy is required to synthesize a glucose molecule during the photosynthesis reactions. In
photosynthesis, light energy from the sun initially transforms into chemical energy that temporally stores itself in
the energy carrier molecules ATP and NADPH (nicotinamide adenine dinucleotide phosphate). Photosynthesis
later uses the stored energy in ATP and NADPH to build one glucose molecule from six molecules of CO 2 . This
process is analogous to eating breakfast in the morning to acquire energy for your body that you can use later in
the day. Under ideal conditions, energy from 18 molecules of ATP is required to synthesize one glucose molecule
during photosynthesis reactions. Glucose molecules can also combine with and convert into other sugar types.
When an organism consumes sugars, glucose molecules eventually make their way into each organism's living
cell. Inside the cell, each sugar molecule breaks down through a complex series of chemical reactions. The goal
of these reactions is to harvest the energy stored inside the sugar molecules. The harvested energy makes high-
energy ATP molecules, which perform work, powering many chemical reactions in the cell. The amount of energy
needed to make one glucose molecule from six carbon dioxide molecules is 18 ATP molecules and 12 NADPH
molecules (each one of which is energetically equivalent to three ATP molecules), or a total of 54 molecule
equivalents required for synthesizing one glucose molecule. This process is a fundamental and efficient way for
cells to generate the molecular energy that they require.
Figure 6.3 Plants, like this oak tree and acorn, use energy from sunlight to make sugar and other organic molecules.
Both plants and animals (like this squirrel) use cellular respiration to derive energy from the organic molecules that
plants originally produced, (credit “acorn”: modification of work by Noel Reynolds; credit “squirrel”: modification of work
by Dawn Huczek)
Metabolic Pathways
The processes of making and breaking down sugar molecules illustrate two types of metabolic pathways.
A metabolic pathway is a series of interconnected biochemical reactions that convert a substrate molecule
or molecules, step-by-step, through a series of metabolic intermediates, eventually yielding a final product
or products. In the case of sugar metabolism, the first metabolic pathway synthesized sugar from smaller
molecules, and the other pathway broke sugar down into smaller molecules. Scientists call these two opposite
processes—the first requiring energy and the second producing energy—anabolic (building) and catabolic
(breaking down) pathways, respectively. Consequently, building (anabolism) and degradation (catabolism)
comprise metabolism.
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V /
e olution CONNECTION
Figure 6.4 This tree shows the evolution of the various branches of life. The vertical dimension is time. Early life
forms, in blue, used anaerobic metabolism to obtain energy from their surroundings.
Evolution of Metabolic Pathways
There is more to the complexity of metabolism than understanding the metabolic pathways alone. Metabolic
complexity varies from organism to organism. Photosynthesis is the primary pathway in which
photosynthetic organisms like plants (planktonic algae perform the majority of global synthesis) harvest
the sun’s energy and convert it into carbohydrates. The by-product of photosynthesis is oxygen, which
some cells require to carry out cellular respiration. During cellular respiration, oxygen aids in the catabolic
breakdown of carbon compounds, like carbohydrates. Among the products are CO 2 and ATP. In addition,
some eukaryotes perform catabolic processes without oxygen (fermentation); that is, they perform or use
anaerobic metabolism.
Organisms probably evolved anaerobic metabolism to survive (living organisms came into existence about
3.8 billion years ago, when the atmosphere lacked oxygen). Despite the differences between organisms
and the complexity of metabolism, researchers have found that all branches of life share some of the
same metabolic pathways, suggesting that all organisms evolved from the same ancient common ancestor
(Figure 6.4). Evidence indicates that over time, the pathways diverged, adding specialized enzymes to
allow organisms to better adapt to their environment, thus increasing their chance to survive. However, the
underlying principle remains that all organisms must harvest energy from their environment and convert it
to ATP to carry out cellular functions.
Anabolic and Catabolic Pathways
Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. Synthesizing
sugar from CO 2 is one example. Other examples are synthesizing large proteins from amino acid building blocks,
and synthesizing new DNA strands from nucleic acid building blocks. These biosynthetic processes are critical
to the cell's life, take place constantly, and demand energy that ATP and other high-energy molecules like NADH
(nicotinamide adenine dinucleotide) and NADPH provide (Figure 6.5).
ATP is an important molecule for cells to have in sufficient supply at all times. The breakdown of sugars illustrates
how a single glucose molecule can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is
a catabolic pathway. Catabolic pathways involve degrading (or breaking down) complex molecules into simpler
ones. Molecular energy stored in complex molecule bonds release in catabolic pathways and harvest in such
a way that it can produce ATP. Other energy-storing molecules, such as fats, also break down through similar
catabolic reactions to release energy and make ATP (Figure 6.5).
It is important to know that metabolic pathway chemical reactions do not take place spontaneously. A protein
called an enzyme facilitates or catalyzes each reaction step. Enzymes are important for catalyzing all types of
biological reactions—those that require energy as well as those that release energy.
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Chapter 6 | Metabolism
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Metabolic pathways
Anabolic: Small molecules assemble into large ones. Energy is required.
O O O Q «*-> —^ OQCO
Catabolic: Large molecules break down into small ones. Energy is released.
+ Energy
Figure 6.5 Anabolic pathways are those that require energy to synthesize larger molecules. Catabolic pathways are
those that generate energy by breaking down larger molecules. Both types of pathways are required for maintaining
the cell's energy balance.
6.2 | Potential, Kinetic, Free, and Activation Energy
By the end of this section, you will be able to do the following:
• Define “energy”
• Explain the difference between kinetic and potential energy
• Discuss the concepts of free energy and activation energy
• Describe endergonic and exergonic reactions
We define energy as the ability to do work. As you’ve learned, energy exists in different forms. For example,
electrical energy, light energy, and heat energy are all different energy types. While these are all familiar energy
types that one can see or feel, there is another energy type that is much less tangible. Scientists associate this
energy with something as simple as an object above the ground. In order to appreciate the way energy flows
into and out of biological systems, it is important to understand more about the different energy types that exist
in the physical world.
Energy Types
When an object is in motion, there is energy. For example, an airplane in flight produces considerable energy.
This is because moving objects are capable of enacting a change, or doing work. Think of a wrecking ball. Even
a slow-moving wrecking ball can do considerable damage to other objects. However, a wrecking ball that is not
in motion is incapable of performing work. Energy with objects in motion is kinetic energy. A speeding bullet,
a walking person, rapid molecule movement in the air (which produces heat), and electromagnetic radiation like
light all have kinetic energy.
What if we lift that same motionless wrecking ball two stories above a car with a crane? If the suspended
wrecking ball is unmoving, can we associate energy with it? The answer is yes. The suspended wrecking ball
has associated energy that is fundamentally different from the kinetic energy of objects in motion. This energy
form results from the potential for the wrecking ball to do work. If we release the ball it would do work. Because
this energy type refers to the potential to do work, we call it potential energy. Objects transfer their energy
between kinetic and potential in the following way: As the wrecking ball hangs motionless, it has 0 kinetic and
100 percent potential energy. Once it releases, its kinetic energy begins to increase because it builds speed due
to gravity. Simultaneously, as it nears the ground, it loses potential energy. Somewhere mid-fall it has 50 percent
kinetic and 50 percent potential energy. Just before it hits the ground, the ball has nearly lost its potential energy
and has near-maximal kinetic energy. Other examples of potential energy include water's energy held behind a
dam (Figure 6.6), or a person about to skydive from an airplane.
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Figure 6.6 Water behind a dam has potential energy. Moving water, such as in a waterfall or a rapidly flowing river, has
kinetic energy, (credit “dam": modification of work by "Pascal"/Flickr; credit “waterfall": modification of work by Frank
Gualtieri)
We associate potential energy only with the matter's location (such as a child sitting on a tree branch), but also
with the matter's structure. A spring on the ground has potential energy if it is compressed; so does a tautly
pulled rubber band. The very existence of living cells relies heavily on structural potential energy. On a chemical
level, the bonds that hold the molecules' atoms together have potential energy. Remember that anabolic cellular
pathways require energy to synthesize complex molecules from simpler ones, and catabolic pathways release
energy when complex molecules break down. That certain chemical bonds' breakdown can release energy
implies that those bonds have potential energy. In fact, there is potential energy stored within the bonds of all the
food molecules we eat, which we eventually harness for use. This is because these bonds can release energy
when broken. Scientists call the potential energy type that exists within chemical bonds that releases when those
bonds break chemical energy (Figure 6.7). Chemical energy is responsible for providing living cells with energy
from food. Breaking the molecular bonds within fuel molecules brings about the energy's release.
H
H
H
H
H
H
H
H
1
H—C
1
-c
1
-c
1
-c
1
-c
1
— c
1
-c
1
-C —H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
1
H
Figure 6.7 The molecules in gasoline contain chemical energy within the chemical bonds. This energy transforms into
kinetic energy that allows a car to race on a racetrack, (credit “car”: modification of work by Russell Trow)
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Chapter 6 | Metabolism
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LINK TQ LEARNING
Visit this site (http:// 0 penstaxc 0 llege. 0 rg/l/simple_pendulum) and select “A simple pendulum” on the menu
(under “Harmonic Motion”) to see the shifting kinetic (K) and potential energy (U) of a pendulum in motion.
Free Energy
After learning that chemical reactions release energy when energy-storing bonds break, an important next
question is how do we quantify and express the chemical reactions with the associated energy? How can we
compare the energy that releases from one reaction to that of another reaction? We use a measurement of free
energy to quantitate these energy transfers. Scientists call this free energy Gibbs free energy (abbreviated with
the letter G) after Josiah Willard Gibbs, the scientist who developed the measurement. Recall that according to
the second law of thermodynamics, all energy transfers involve losing some energy in an unusable form such
as heat, resulting in entropy. Gibbs free energy specifically refers to the energy that takes place with a chemical
reaction that is available after we account for entropy. In other words, Gibbs free energy is usable energy, or
energy that is available to do work.
Every chemical reaction involves a change in free energy, called delta G (AG). We can calculate the change
in free energy for any system that undergoes such a change, such as a chemical reaction. To calculate AG,
subtract the amount of energy lost to entropy (denoted as AS) from the system's total energy change. Scientists
call this total energy change in the system enthalpy and we denote it as AH. The formula for calculating AG is
as follows, where the symbol T refers to absolute temperature in Kelvin (degrees Celsius + 273):
AG = AH - TAS
We express a chemical reaction's standard free energy change as an amount of energy per mole of the reaction
product (either in kilojoules or kilocalories, kJ/mol or kcal/mol; 1 kJ = 0.239 kcal) under standard pH, temperature,
and pressure conditions. We generally calculate standard pH, temperature, and pressure conditions at pH 7.0
in biological systems, 25 degrees Celsius, and 100 kilopascals (1 atm pressure), respectively. Note that cellular
conditions vary considerably from these standard conditions, and so standard calculated AG values for biological
reactions will be different inside the cell.
Endergonic Reactions and Exergonic Reactions
If energy releases during a chemical reaction, then the resulting value from the above equation will be a
negative number. In other words, reactions that release energy have a AG < 0. A negative AG also means
that the reaction's products have less free energy than the reactants, because they gave off some free energy
during the reaction. Scientists call reactions that have a negative AG and consequently release free energy
exergonic reactions. Think: exergonic means energy is exiting the system. We also refer to these reactions as
spontaneous reactions, because they can occur without adding energy into the system. Understanding which
chemical reactions are spontaneous and release free energy is extremely useful for biologists, because these
reactions can be harnessed to perform work inside the cell. We must draw an important distinction between the
term spontaneous and the idea of a chemical reaction that occurs immediately. Contrary to the everyday use
of the term, a spontaneous reaction is not one that suddenly or quickly occurs. Rusting iron is an example of a
spontaneous reaction that occurs slowly, little by little, over time.
If a chemical reaction requires an energy input rather than releasing energy, then the AG for that reaction will be
a positive value. In this case, the products have more free energy than the reactants. Thus, we can think of the
reactions' products as energy-storing molecules. We call these chemical reactions endergonic reactions, and
they are non-spontaneous. An endergonic reaction will not take place on its own without adding free energy.
Let’s revisit the example of the synthesis and breakdown of the food molecule, glucose. Remember that building
complex molecules, such as sugars, from simpler ones is an anabolic process and requires energy. Therefore,
the chemical reactions involved in anabolic processes are endergonic reactions. Alternatively the catabolic
process of breaking sugar down into simpler molecules releases energy in a series of exergonic reactions. Like
the rust example above, the sugar breakdown involves spontaneous reactions, but these reactions do not occur
instantaneously. Figure 6.8 shows some other examples of endergonic and exergonic reactions. Later sections
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Chapter 6 | Metabolism
will provide more information about what else is required to make even spontaneous reactions happen more
efficiently.
visual
CONNECTION
Figure 6.8 This figure shows some examples of endergonic processes (ones that require energy) and exergonic
processes (ones that release energy). These include (a) a compost pile decomposing, (b) a chick developing
from a fertilized egg, (c) sand art destruction, and (d) a ball rolling down a hill, (credit a: modification of work by
Natalie Maynor; credit b: modification of work by USDA; credit c: modification of work by “Athlex’VFlickr; credit d:
modification of work by Harry Malsch)
Look at each of the processes, and decide if it is endergonic or exergonic. In each case, does enthalpy
increase or decrease, and does entropy increase or decrease?
An important concept in studying metabolism and energy is that of chemical equilibrium. Most chemical reactions
are reversible. They can proceed in both directions, releasing energy into their environment in one direction, and
absorbing it from the environment in the other direction (Figure 6.9). The same is true for the chemical reactions
involved in cell metabolism, such as the breaking down and building up of proteins into and from individual
amino acids, respectively. Reactants within a closed system will undergo chemical reactions in both directions
until they reach a state of equilibrium, which is one of the lowest possible free energy and a state of maximal
entropy. To push the reactants and products away from a state of equilibrium requires energy. Either reactants
or products must be added, removed, or changed. If a cell were a closed system, its chemical reactions would
reach equilibrium, and it would die because there would be insufficient free energy left to perform the necessary
work to maintain life. In a living cell, chemical reactions are constantly moving towards equilibrium, but never
reach it. This is because a living cell is an open system. Materials pass in and out, the cell recycles the products
of certain chemical reactions into other reactions, and there is never chemical equilibrium. In this way, living
organisms are in a constant energy-requiring, uphill battle against equilibrium and entropy. This constant energy
supply ultimately comes from sunlight, which produces nutrients in the photosynthesis process.
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Chapter 6 | Metabolism
181
EXERGONIC REACTION: AG < 0
ENDERGONIC REACTION: AG > O
Reaction is spontaneous
Reaction is not spontaneous
>»
o>
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O)
0)
0
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\ Energy is released
c
LU
Energy is added /
0>
LL
reactants
0>
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</>
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-Q
AG < 0 \
</)
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-Q
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Figure 6.9 Exergonic and endergonic reactions result in changes in Gibbs free energy. Exergonic reactions release
energy. Endergonic reactions require energy to proceed.
Activation Energy
There is another important concept that we must consider regarding endergonic and exergonic reactions. Even
exergonic reactions require a small amount of energy input before they can proceed with their energy-releasing
steps. These reactions have a net release of energy, but still require some initial energy. Scientists call this small
amount of energy input necessary for all chemical reactions to occur the activation energy (or free energy of
activation) abbreviated as Ea (Figure 6.10).
Why would an energy-releasing, negative AG reaction actually require some energy to proceed? The reason
lies in the steps that take place during a chemical reaction. During chemical reactions, certain chemical bonds
break and new ones form. For example, when a glucose molecule breaks down, bonds between the molecule's
carbon atoms break. Since these are energy-storing bonds, they release energy when broken. However, to get
them into a state that allows the bonds to break, the molecule must be somewhat contorted. A small energy
input is required to achieve this contorted state. This contorted state is the transition state, and it is a high-
energy, unstable state. For this reason, reactant molecules do not last long in their transition state, but very
quickly proceed to the chemical reaction's next steps. Free energy diagrams illustrate the energy profiles for a
given reaction. Whether the reaction is exergonic or endergonic determines whether the products in the diagram
will exist at a lower or higher energy state than both the reactants and the products. However, regardless of this
measure, the transition state of the reaction exists at a higher energy state than the reactants, and thus, Ea is
always positive.
LINK TQ LEARNING
Watch an animation of the move from free energy to transition state at this (http:// 0 penstaxc 0 llege. 0 rg/l/
energy^reaction) site.
From where does the activation energy that chemical reactants require come? The activation energy's required
source to push reactions forward is typically heat energy from the surroundings. Heat energy (the total bond
energy of reactants or products in a chemical reaction) speeds up the molecule's motion, increasing the
frequency and force with which they collide. It also moves atoms and bonds within the molecule slightly, helping
them reach their transition state. For this reason, heating a system will cause chemical reactants within that
system to react more frequently. Increasing the pressure on a system has the same effect. Once reactants have
absorbed enough heat energy from their surroundings to reach the transition state, the reaction will proceed.
The activation energy of a particular reaction determines the rate at which it will proceed. The higher the
activation energy, the slower the chemical reaction. The example of iron rusting illustrates an inherently slow
reaction. This reaction occurs slowly over time because of its high Ea. Additionally, burning many fuels, which
is strongly exergonic, will take place at a negligible rate unless sufficient heat from a spark overcomes their
activation energy. However, once they begin to burn, the chemical reactions release enough heat to continue the
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Chapter 6 | Metabolism
burning process, supplying the activation energy for surrounding fuel molecules. Like these reactions outside of
cells, the activation energy for most cellular reactions is too high for heat energy to overcome at efficient rates, in
other words, in order for important cellular reactions to occur at appreciable rates (number of reactions per unit
time), their activation energies must be lowered (Figure 6.10). Scientist refer to this as catalysis. This is a very
good thing as far as living cells are concerned. Important macromolecules, such as proteins, DNA, and RNA,
store considerable energy, and their breakdown is exergonic. If cellular temperatures alone provided enough
heat energy for these exergonic reactions to overcome their activation barriers, the cell's essential components
would disintegrate.
visual
Figure 6.10 Activation energy is the energy required for a reaction to proceed, and it is lower if the reaction is
catalyzed. This diagram's horizontal axis describes the sequence of events in time.
If no activation energy were required to break down sucrose (table sugar), would you be able to store it in a
sugar bowl?
EXERGONIC REACTION: AG < O
Reaction is spontaneous
6.3 | The Laws of Thermodynamics
By the end of this section, you will be able to do the following:
• Discuss the concept of entropy
• Explain the first and second laws of thermodynamics
Thermodynamics refers to the study of energy and energy transfer involving physical matter. The matter and its
environment relevant to a particular case of energy transfer are classified as a system, and everything outside
that system is the surroundings. For instance, when heating a pot of water on the stove, the system includes the
stove, the pot, and the water. Energy transfers within the system (between the stove, pot, and water). There are
two types of systems: open and closed. An open system is one in which energy can transfer between the system
and its surroundings. The stovetop system is open because it can lose heat into the air. A closed system is one
that cannot transfer energy to its surroundings.
Biological organisms are open systems. Energy exchanges between them and their surroundings, as they
consume energy-storing molecules and release energy to the environment by doing work. Like all things in the
physical world, energy is subject to the laws of physics. The laws of thermodynamics govern the transfer of
energy in and among all systems in the universe.
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The First Law of Thermodynamics
The first law of thermodynamics deals with the total amount of energy in the universe. It states that this
total amount of energy is constant. In other words, there has always been, and always will be, exactly the
same amount of energy in the universe. Energy exists in many different forms. According to the first law of
thermodynamics, energy may transfer from place to place or transform into different forms, but it cannot be
created or destroyed. The transfers and transformations of energy take place around us all the time. Light bulbs
transform electrical energy into light energy. Gas stoves transform chemical energy from natural gas into heat
energy. Plants perform one of the most biologically useful energy transformations on earth: that of converting
sunlight energy into the chemical energy stored within organic molecules (Figure 6.2). Figure 6.11 examples of
energy transformations.
The challenge for all living organisms is to obtain energy from their surroundings in forms that they can transfer
or transform into usable energy to do work. Living cells have evolved to meet this challenge very well. Chemical
energy stored within organic molecules such as sugars and fats transforms through a series of cellular chemical
reactions into energy within ATP molecules. Energy in ATP molecules is easily accessible to do work. Examples
of the types of work that cells need to do include building complex molecules, transporting materials, powering
the beating motion of cilia or flagella, contracting muscle fibers to create movement, and reproduction.
Figure 6.11 Here are two examples of energy transferring from one system to another and transformed from one form
to another. Humans can convert the chemical energy in food, like this ice cream cone, into kinetic energy (the energy
of movement to ride a bicycle). Plants can convert electromagnetic radiation (light energy) from the sun into chemical
energy, (credit “ice cream”: modification of work by D. Sharon Pruitt; credit “kids on bikes”: modification of work by
Michelle Riggen-Ransom; credit “leaf": modification of work by Cory Zanker)
The Second Law of Thermodynamics
A living cell’s primary tasks of obtaining, transforming, and using energy to do work may seem simple. However,
the second law of thermodynamics explains why these tasks are harder than they appear. None of the energy
transfers that we have discussed, along with all energy transfers and transformations in the universe, is
completely efficient. In every energy transfer, some amount of energy is lost in a form that is unusable. In most
cases, this form is heat energy. Thermodynamically, scientists define heat energy as energy that transfers from
one system to another that is not doing work. For example, when an airplane flies through the air, it loses some of
its energy as heat energy due to friction with the surrounding air. This friction actually heats the air by temporarily
increasing air molecule speed. Likewise, some energy is lost as heat energy during cellular metabolic reactions.
This is good for warm-blooded creatures like us, because heat energy helps to maintain our body temperature.
Strictly speaking, no energy transfer is completely efficient, because some energy is lost in an unusable form.
An important concept in physical systems is that of order and disorder (or randomness). The more energy that a
system loses to its surroundings, the less ordered and more random the system. Scientists refer to the measure
of randomness or disorder within a system as entropy. High entropy means high disorder and low energy
(Figure 6.12). To better understand entropy, think of a student’s bedroom. If no energy or work were put into
it, the room would quickly become messy. It would exist in a very disordered state, one of high entropy. Energy
must be put into the system, in the form of the student doing work and putting everything away, in order to bring
the room back to a state of cleanliness and order. This state is one of low entropy. Similarly, a car or house must
be constantly maintained with work in order to keep it in an ordered state. Left alone, a house's or car's entropy
gradually increases through rust and degradation. Molecules and chemical reactions have varying amounts of
entropy as well. For example, as chemical reactions reach a state of equilibrium, entropy increases, and as
molecules at a high concentration in one place diffuse and spread out, entropy also increases.
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Chapter 6 | Metabolism
scientific methf d CONNECTION
Transfer of Energy and the Resulting Entropy
Set up a simple experiment to understand how energy transfers and how a change in entropy results.
1. Take a block of ice. This is water in solid form, so it has a high structural order. This means that the
molecules cannot move very much and are in a fixed position. The ice's temperature is CPC. As a result,
the system's entropy is low.
2. Allow the ice to melt at room temperature. What is the state of molecules in the liquid water now? How
did the energy transfer take place? Is the system's entropy higher or lower? Why?
3. Heat the water to its boiling point. What happens to the system's entropy when the water is heated?
Think of all physical systems of in this way: Living things are highly ordered, requiring constant energy input to
maintain themselves in a state of low entropy. As living systems take in energy-storing molecules and transform
them through chemical reactions, they lose some amount of usable energy in the process, because no reaction
is completely efficient. They also produce waste and by-products that are not useful energy sources. This
process increases the entropy of the system’s surroundings. Since all energy transfers result in losing some
usable energy, the second law of thermodynamics states that every energy transfer or transformation increases
the universe's entropy. Even though living things are highly ordered and maintain a state of low entropy, the
universe's entropy in total is constantly increasing due to losing usable energy with each energy transfer that
occurs. Essentially, living things are in a continuous uphill battle against this constant increase in universal
entropy.
Increasing
entropy
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Figure 6.12 Entropy is a measure of randomness or disorder in a system. Gases have higher entropy than liquids,
liquids have higher entropy than solids.
and
6.4 | ATP: Adenosine Triphosphate
By the end of this section, you will be able to do the following:
• Explain ATP's role as the cellular energy currency
• Describe how energy releases through ATP hydrolysis
Even exergonic, energy-releasing reactions require a small amount of activation energy in order to proceed.
However, consider endergonic reactions, which require much more energy input, because their products have
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185
more free energy than their reactants. Within the cell, from where does energy to power such reactions come?
The answer lies with an energy-supplying molecule scientists call adenosine triphosphate, or ATP. This is a
small, relatively simple molecule (Figure 6.13), but within some of its bonds, it contains the potential for a quick
burst of energy that can be harnessed to perform cellular work. Think of this molecule as the cells' primary
energy currency in much the same way that money is the currency that people exchange for things they need.
ATP powers the majority of energy-requiring cellular reactions.
Figure 6.13 ATP is the cell's primary energy currency. It has an adenosine backbone with three phosphate groups
attached.
As its name suggests, adenosine triphosphate is comprised of adenosine bound to three phosphate groups
(Figure 6.13). Adenosine is a nucleoside consisting of the nitrogenous base adenine and a five-carbon sugar,
ribose. The three phosphate groups, in order of closest to furthest from the ribose sugar, are alpha, beta, and
gamma. Together, these chemical groups constitute an energy powerhouse. However, not all bonds within this
molecule exist in a particularly high-energy state. Both bonds that link the phosphates are equally high-energy
bonds ( phosphoanhydride bonds) that, when broken, release sufficient energy to power a variety of cellular
reactions and processes. These high-energy bonds are the bonds between the second and third (or beta and
gamma) phosphate groups and between the first and second phosphate groups. These bonds are “high-energy"
because the products of such bond breaking—adenosine diphosphate (ADP) and one inorganic phosphate
group (Pi)—have considerably lower free energy than the reactants: ATP and a water molecule. Because this
reaction takes place using a water molecule, it is a hydrolysis reaction. In other words, ATP hydrolyzes into ADP
in the following reaction:
ATP + H 9 O -* ADP + Pj + free energy
Like most chemical reactions, ATP to ADP hydrolysis is reversible. The reverse reaction regenerates ATP from
ADP + Pi. Cells rely on ATP regeneration just as people rely on regenerating spent money through some sort
of income. Since ATP hydrolysis releases energy, ATP regeneration must require an input of free energy. This
equation expresses ATP formation:
ADP + Pj + free energy -> ATP + H 2 O
Two prominent questions remain with regard to using ATP as an energy source. Exactly how much free energy
releases with ATP hydrolysis, and how does that free energy do cellular work? The calculated AG for the
hydrolysis of one ATP mole into ADP and Pi is -7.3 kcal/mole (-30.5 kJ/mol). Since this calculation is true under
standard conditions, one would expect a different value exists under cellular conditions. In fact, the AG for one
ATP mole's hydrolysis in a living cell is almost double the value at standard conditions: -14 kcal/mol (-57 kJ/
mol).
ATP is a highly unstable molecule. Unless quickly used to perform work, ATP spontaneously dissociates into
ADP + Pi, and the free energy released during this process is lost as heat. The second question we posed
above discusses how ATP hydrolysis energy release performs work inside the cell. This depends on a strategy
scientists call energy coupling. Cells couple the ATP hydrolysis' exergonic reaction allowing them to proceed.
One example of energy coupling using ATP involves a transmembrane ion pump that is extremely important
for cellular function. This sodium-potassium pump (Na + /K + pump) drives sodium out of the cell and potassium
into the cell (Figure 6.14). A large percentage of a cell’s ATP powers this pump, because cellular processes
bring considerable sodium into the cell and potassium out of it. The pump works constantly to stabilize cellular
concentrations of sodium and potassium. In order for the pump to turn one cycle (exporting three Na+ ions
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and importing two K + ions), one ATP molecule must hydrolyze. When ATP hydrolyzes, its gamma phosphate
does not simply float away, but it actually transfers onto the pump protein. Scientists call this process of a
phosphate group binding to a molecule phosphorylation. As with most ATP hydrolysis cases, a phosphate from
ATP transfers onto another molecule. In a phosphorylated state, the Na + /K + pump has more free energy and is
triggered to undergo a conformational change. This change allows it to release Na + to the cell's outside. It then
binds extracellular K + , which, through another conformational change, causes the phosphate to detach from the
pump. This phosphate release triggers the K + to release to the cell's inside. Essentially, the energy released
from the ATP hydrolysis couples with the energy required to power the pump and transport Na + and K + ions.
ATP performs cellular work using this basic form of energy coupling through phosphorylation.
visual
CONNECTION
Figure 6.14 The sodium-potassium pump is an example of energy coupling. The energy derived from exergonic
ATP hydrolysis pumps sodium and potassium ions across the cell membrane.
One ATP molecule's hydrolysis releases 7.3 kcal/mol of energy (AG = -7.3 kcal/mol of energy). If it takes
2.1 kcal/mol of energy to move one Na + across the membrane (AG = +2.1 kcal/mol of energy), how many
sodium ions could one ATP molecule's hydrolysis move?
Often during cellular metabolic reactions, such as nutrient synthesis and breakdown, certain molecules must
alter slightly in their conformation to become substrates for the next step in the reaction series. One example
is during the very first steps of cellular respiration, when a sugar glucose molecule breaks down in the process
of glycolysis. In the first step, ATP is required to phosphorylze glucose, creating a high-energy but unstable
intermediate. This phosphorylation reaction powers a conformational change that allows the phosphorylated
glucose molecule to convert to the phosphorylated sugar fructose. Fructose is a necessary intermediate for
glycolysis to move forward. Here, ATP hydrolysis' exergonic reaction couples with the endergonic reaction of
converting glucose into a phosphorylated intermediate in the pathway. Once again, the energy released by
breaking a phosphate bond within ATP was used for phosphorylyzing another molecule, creating an unstable
intermediate and powering an important conformational change.
LINK
T &
LEARNING
See an interactive animation of the ATP-producing glycolysis process at this site (http://openstaxcollege.org/
l/glycolysis_stgs) .
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6.5 | Enzymes
By the end of this section, you will be able to do the following:
• Describe the role of enzymes in metabolic pathways
• Explain how enzymes function as molecular catalysts
• Discuss enzyme regulation by various factors
A substance that helps a chemical reaction to occur is a catalyst, and the special molecules that catalyze
biochemical reactions are enzymes. Almost all enzymes are proteins, comprised of amino acid chains, and
they perform the critical task of lowering the activation energies of chemical reactions inside the cell. Enzymes
do this by binding to the reactant molecules, and holding them in such a way as to make the chemical bond¬
breaking and bond-forming processes take place more readily. It is important to remember that enzymes do not
change the reaction's AG. In other words, they do not change whether a reaction is exergonic (spontaneous) or
endergonic. This is because they do not change the reactants' or products' free energy. They only reduce the
activation energy required to reach the transition state (Figure 6.15).
Activation Energy
- Reaction without catalyst
- - - - Reaction with catalyst
Figure 6.15 Enzymes lower the reaction's activation energy but do not change the reaction's free energy.
Enzyme Active Site and Substrate Specificity
The chemical reactants to which an enzyme binds are the enzyme’s substrates. There may be one or more
substrates, depending on the particular chemical reaction. In some reactions, a single-reactant substrate breaks
down into multiple products. In others, two substrates may come together to create one larger molecule. Two
reactants might also enter a reaction, both become modified, and leave the reaction as two products. The
location within the enzyme where the substrate binds is the enzyme’s active site. This is where the “action”
happens. Since enzymes are proteins, there is a unique combination of amino acid residues (also side chains,
or R groups) within the active site. Different properties characterize each residue. These can be large or small,
weakly acidic or basic, hydrophilic or hydrophobic, positively or negatively charged, or neutral. The unique
combination of amino acid residues, their positions, sequences, structures, and properties, creates a very
specific chemical environment within the active site. This specific environment is suited to bind, albeit briefly,
to a specific chemical substrate (or substrates). Due to this jigsaw puzzle-like match between an enzyme and
its substrates (which adapts to find the best fit between the transition state and the active site), enzymes are
known for their specificity. The “best fit” results from the shape and the amino acid functional group’s attraction to
the substrate. There is a specifically matched enzyme for each substrate and, thus, for each chemical reaction;
however, there is flexibility as well.
The fact that active sites are so perfectly suited to provide specific environmental conditions also means that they
are subject to local enviromental influences. It is true that increasing the environmental temperature generally
increases reaction rates, enzyme-catalyzed or otherwise. However, increasing or decreasing the temperature
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outside of an optimal range can affect chemical bonds within the active site in such a way that they are less
well suited to bind substrates. High temperatures will eventually cause enzymes, like other biological molecules,
to denature, a process that changes the substance's natural properties. Likewise, the local environment's pH
can also affect enzyme function. Active site amino acid residues have their own acidic or basic properties that
are optimal for catalysis. These residues are sensitive to changes in pH that can impair the way substrate
molecules bind. Enzymes are suited to function best within a certain pH range, and, as with temperature,
extreme environmental pH values (acidic or basic) can cause enzymes to denature.
Induced Fit and Enzyme Function
For many years, scientists thought that enzyme-substrate binding took place in a simple “lock-and-key” fashion.
This model asserted that the enzyme and substrate fit together perfectly in one instantaneous step. However,
current research supports a more refined view scientists call induced fit (Figure 6.16). This model expands
upon the lock-and-key model by describing a more dynamic interaction between enzyme and substrate. As
the enzyme and substrate come together, their interaction causes a mild shift in the enzyme’s structure that
confirms an ideal binding arrangement between the enzyme and the substrate's transition state. This ideal
binding maximizes the enzyme’s ability to catalyze its reaction.
View an induced fit animation at this website (http:// 0 penstaxc 0 llege. 0 rg/l/hex 0 kinase) .
When an enzyme binds its substrate, it forms an enzyme-substrate complex. This complex lowers the reaction's
activation energy and promotes its rapid progression in one of many ways. On a basic level, enzymes promote
chemical reactions that involve more than one substrate by bringing the substrates together in an optimal
orientation. The appropriate region (atoms and bonds) of one molecule is juxtaposed to the other molecule's
appropriate region with which it must react. Another way in which enzymes promote substrate reaction is by
creating an optimal environment within the active site for the reaction to occur. Certain chemical reactions
might proceed best in a slightly acidic or non-polar environment. The chemical properties that emerge from
the particular arrangement of amino acid residues within an active site create the perfect environment for an
enzyme’s specific substrates to react.
You have learned that the activation energy required for many reactions includes the energy involved in
manipulating or slightly contorting chemical bonds so that they can easily break and allow others to reform.
Enzymatic action can aid this process. The enzyme-substrate complex can lower the activation energy by
contorting substrate molecules in such away as to facilitate bond-breaking, helping to reach the transition state.
Finally, enzymes can also lower activation energies by taking part in the chemical reaction itself. The amino acid
residues can provide certain ions or chemical groups that actually form covalent bonds with substrate molecules
as a necessary step of the reaction process. In these cases, it is important to remember that the enzyme will
always return to its original state at the reaction's completion. One of enzymes' hallmark properties is that they
remain ultimately unchanged by the reactions they catalyze. After an enzyme catalyzes a reaction, it releases its
product(s).
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Substrate entering Enzyme/substrate Enzyme/products Products leaving
active site of enzyme complex complex active site of enzyme
Figure 6.16 According to the induced-fit model, both enzyme and substrate undergo dynamic conformational changes
upon binding. The enzyme contorts the substrate into its transition state, thereby increasing the reaction's rate.
Metabolism Control Through Enzyme Regulation
It would seem ideal to have a scenario in which all the encoded enzymes in an organism’s genome existed in
abundant supply and functioned optimally under all cellular conditions, in all cells, at all times. In reality, this is
far from the case. A variety of mechanisms ensure that this does not happen. Cellular needs and conditions vary
from cell to cell, and change within individual cells over time. The required enzymes and energetic demands of
stomach cells are different from those of fat storage cells, skin cells, blood cells, and nerve cells. Furthermore, a
digestive cell works much harder to process and break down nutrients during the time that closely follows a meal
compared with many hours after a meal. As these cellular demands and conditions vary, so do the amounts and
functionality of different enzymes.
Since the rates of biochemical reactions are controlled by activation energy, and enzymes lower and determine
activation energies for chemical reactions, the relative amounts and functioning of the variety of enzymes
within a cell ultimately determine which reactions will proceed and at which rates. This determination is tightly
controlled. In certain cellular environments, environmental factors like pH and temperature partly control enzyme
activity. There are other mechanisms through which cells control enzyme activity and determine the rates at
which various biochemical reactions will occur.
Molecular Regulation of Enzymes
Enzymes can be regulated in ways that either promote or reduce their activity. There are many different kinds
of molecules that inhibit or promote enzyme function, and various mechanisms exist for doing so. For example,
in some cases of enzyme inhibition, an inhibitor molecule is similar enough to a substrate that it can bind to
the active site and simply block the substrate from binding. When this happens, the enzyme is inhibited through
competitive inhibition, because an inhibitor molecule competes with the substrate for active site binding
(Figure 6.17). Alternatively, in noncompetitive inhibition, an inhibitor molecule binds to the enzyme in a location
other than an allosteric site, a binding site away from the active site, and still manages to block substrate binding
to the active site.
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Enzyme Inhibition
Figure 6.17 Competitive and noncompetitive inhibition affect the reaction's rate differently. Competitive inhibitors affect
the initial rate but do not affect the maximal rate; whereas, noncompetitive inhibitors affect the maximal rate.
Some inhibitor molecules bind to enzymes in a location where their binding induces a conformational change
that reduces the enzyme's affinity for its substrate. This type of inhibition is an allosteric inhibition (Figure
6.18). More than one polypeptide comprise most allosterically regulated enzymes, meaning that they have
more than one protein subunit. When an allosteric inhibitor binds to an enzyme, all active sites on the protein
subunits change slightly such that they bind their substrates with less efficiency. There are allosteric activators
as well as inhibitors. Allosteric activators bind to locations on an enzyme away from the active site, inducing a
conformational change that increases the affinity of the enzyme’s active site(s) for its substrate(s).
Figure 6.18 Allosteric inhibitors modify the enzyme's active site so that substrate binding is reduced or prevented. In
contrast, allosteric activators modify the enzyme's active site so that the affinity for the substrate increases.
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everyday CONNECTION
Figure 6.19 Have you ever wondered how pharmaceutical drugs are developed? (credit: Deborah Austin)
Drug Discovery by Looking for Inhibitors of Key Enzymes in
Specific Pathways
Enzymes are key components of metabolic pathways. Understanding how enzymes work and how they can
be regulated is a key principle behind developing many pharmaceutical drugs (Figure 6.19) on the market
today. Biologists working in this field collaborate with other scientists, usually chemists, to design drugs.
Consider statins for example—which is a class of drugs that reduces cholesterol levels. These compounds
are essentially inhibitors of the enzyme HMG-CoA reductase. HMG-CoA reductase is the enzyme that
synthesizes cholesterol from lipids in the body. By inhibiting this enzyme, the drug reduces cholesterol levels
synthesized in the body. Similarly, acetaminophen, popularly marketed under the brand name Tylenol, is an
inhibitor of the enzyme cyclooxygenase. While it is effective in providing relief from fever and inflammation
(pain), scientists still do not completely understand its mechanism of action.
How are drugs developed? One of the first challenges in drug development is identifying the specific
molecule that the drug is intended to target. In the case of statins, HMG-CoA reductase is the drug target.
Researchers identify targets through painstaking research in the laboratory. Identifying the target alone
is not sufficient. Scientists also need to know how the target acts inside the cell and which reactions go
awry in the case of disease. Once researchers identify the target and the pathway, then the actual drug
design process begins. During this stage, chemists and biologists work together to design and synthesize
molecules that can either block or activate a particular reaction. However, this is only the beginning: both if
and when a drug prototype is successful in performing its function, then it must undergo many tests from in
vitro experiments to clinical trials before it can obtain FDA approval to be on the market.
Many enzymes don’t work optimally, or even at all, unless bound to other specific non-protein helper molecules,
either temporarily through ionic or hydrogen bonds or permanently through stronger covalent bonds. Two types
of helper molecules are cofactors and coenzymes. Binding to these molecules promotes optimal conformation
and function for their respective enzymes. Cofactors are inorganic ions such as iron (Fe++) and magnesium
(Mg++). One example of an enzyme that requires a metal ion as a cofactor is the enzyme that builds DNA
molecules, DNA polymerase, which requires a bound zinc ion (Zn++) to function. Coenzymes are organic
helper molecules, with a basic atomic structure comprised of carbon and hydrogen, which are required for
enzyme action. The most common sources of coenzymes are dietary vitamins (Figure 6.20). Some vitamins are
precursors to coenzymes and others act directly as coenzymes. Vitamin C is a coenzyme for multiple enzymes
that take part in building the important connective tissue component, collagen. An important step in breaking
down glucose to yield energy is catalysis by a multi-enzyme complex scientists call pyruvate dehydrogenase.
Pyruvate dehydrogenase is a complex of several enzymes that actually requires one cofactor (a magnesium ion)
and five different organic coenzymes to catalyze its specific chemical reaction. Therefore, enzyme function is, in
part, regulated by an abundance of various cofactors and coenzymes, which the diets of most organisms supply.
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Figure 6.20 Vitamins are important coenzymes or precursors of coenzymes, and are required for enzymes to function
properly. Multivitamin capsules usually contain mixtures of all the vitamins at different percentages.
Enzyme Compartmentalization
In eukaryotic cells, molecules such as enzymes are usually compartmentalized into different organelles. This
allows for yet another level of regulation of enzyme activity. Enzymes required only for certain cellular processes
are sometimes housed separately along with their substrates, allowing for more efficient chemical reactions.
Examples of this sort of enzyme regulation based on location and proximity include the enzymes involved in the
latter stages of cellular respiration, which take place exclusively in the mitochondria, and the enzymes involved
in digesting cellular debris and foreign materials, located within lysosomes.
Feedback Inhibition in Metabolic Pathways
Molecules can regulate enzyme function in many ways. However, a major question remains: What are these
molecules and from where do they come? Some are cofactors and coenzymes, ions, and organic molecules, as
you have learned. What other molecules in the cell provide enzymatic regulation, such as allosteric modulation,
and competitive and noncompetitive inhibition? The answer is that a wide variety of molecules can perform these
roles. Some include pharmaceutical and non-pharmaceutical drugs, toxins, and poisons from the environment.
Perhaps the most relevant sources of enzyme regulatory molecules, with respect to cellular metabolism, are
cellular metabolic reaction products themselves. In a most efficient and elegant way, cells have evolved to use
their own reactions' products for feedback inhibition of enzyme activity. Feedback inhibition involves using a
reaction product to regulate its own further production (Figure 6.21). The cell responds to the abundance of
specific products by slowing down production during anabolic or catabolic reactions. Such reaction products may
inhibit the enzymes that catalyzed their production through the mechanisms that we described above.
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Inhibition of the pathway
Figure 6.21 Metabolic pathways are a series of reactions that multiple enzymes catalyze. Feedback inhibition, where
the pathway's end product inhibits an upstream step, is an important regulatory mechanism in cells.
Producing both amino acids and nucleotides is controlled through feedback inhibition. Additionally, ATP is an
allosteric regulator of some of the enzymes involved in sugar's catabolic breakdown, the process that produces
ATP. In this way, when ATP is abundant, the cell can prevent its further production. Remember that ATP is an
unstable molecule that can spontaneously dissociate into ADP. If too much ATP were present in a cell, much of
it would go to waste. Alternatively, ADP serves as a positive allosteric regulator (an allosteric activator) for some
of the same enzymes that ATP inhibits. Thus, when relative ATP levels are high compared to ATP, the cell is
triggered to produce more ATP through sugar catabolism.
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KEY TERMS
activation energy energy necessary for reactions to occur
active site enzyme's specific region to which the substrate binds
allosteric inhibition inhibition by a binding event at a site different from the active site, which induces a
conformational change and reduces the enzyme's affinity for its substrate
anabolic (also, anabolism) pathways that require an energy input to synthesize complex molecules from simpler
ones
ATP adenosine triphosphate, the cell’s energy currency
bioenergetics study of energy flowing through living systems
catabolic (also, catabolism) pathways in which complex molecules break down into simpler ones
chemical energy potential energy in chemical bonds that releases when those bonds are broken
coenzyme small organic molecule, such as a vitamin or its derivative, which is required to enhance an enzyme's
activity
cofactor inorganic ion, such as iron and magnesium ions, required for optimal enzyme activity regulation
competitive inhibition type of inhibition in which the inhibitor competes with the substrate molecule by binding
to the enzyme's active site
denature process that changes asubtance's natural properties
endergonic describes chemical reactions that require energy input
enthalpy a system's total energy
entropy (S) measure of randomness or disorder within a system
exergonic describes chemical reactions that release free energy
feedback inhibition a product's effect of a reaction sequence to decrease its further production by inhibiting the
first enzyme's activity in the pathway that produces it
free energy Gibbs free energy is the usable energy, or energy that is available to do work
heat energy transferred from one system to another that is not work (energy of the molecules' motion or
particles)
heat energy total bond energy of reactants or products in a chemical reaction
induced fit dynamic fit between the enzyme and its substrate, in which both components modify their structures
to allow for ideal binding
kinetic energy energy type that takes place with objects or particles in motion
metabolism all the chemical reactions that take place inside cells, including anabolism and catabolism
phosphoanhydride bond bond that connects phosphates in an ATP molecule
potential energy energy type that has the potential to do work; stored energy
substrate molecule on which the enzyme acts
thermodynamics study of energy and energy transfer involving physical matter
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transition state high-energy, unstable state (an intermediate form between the substrate and the product)
occurring during a chemical reaction
CHAPTER SUMMARY
6.1 Energy and Metabolism
Cells perform the functions of life through various chemical reactions. A cell’s metabolism refers to the chemical
reactions that take place within it. There are metabolic reactions that involve breaking down complex chemicals
into simpler ones, such as breaking down large macromolecules. Scientists refer to this process as catabolism,
and we associate such reactions an energy release. On the other end of the spectrum, anabolism refers to
metabolic processes that build complex molecules out of simpler ones, such as macromolecule synthesis.
Anabolic processes require energy. Glucose synthesis and glucose breakdown are examples of anabolic and
catabolic pathways, respectively.
6.2 Potential, Kinetic, Free, and Activation Energy
Energy comes in many different forms. Objects in motion do physical work, and kinetic energy is the energy of
objects in motion. Objects that are not in motion may have the potential to do work, and thus, have potential
energy. Molecules also have potential energy because breaking molecular bonds has the potential to release
energy. Living cells depend on harvesting potential energy from molecular bonds to perform work. Free energy
is a measure of energy that is available to do work. A system's free energy changes during energy transfers
such as chemical reactions, and scientists refer to this change as AG.
A reaction's AG can be negative or positive, meaning that the reaction releases energy or consumes energy,
respectively. A reaction with a negative AG that gives off energy is an exergonic reaction. One with a positive
AG that requires energy input is an endergonic reaction. Exergonic reactions are spontaneous because their
products have less energy than their reactants. Endergonic reactions' products have a higher energy state than
the reactants, and so these are nonspontaneous reactions. However, all reactions (including spontaneous -AG
reactions) require an initial energy input in order to reach the transition state, at which they will proceed. This
initial input of energy is the activation energy.
6.3 The Laws of Thermodynamics
In studying energy, scientists use the term “system" to refer to the matter and its environment involved in
energy transfers. Everything outside of the system is the surroundings. Single cells are biological systems. We
can think of systems as having a certain amount of order. It takes energy to make a system more ordered. The
more ordered a system, the lower its entropy. Entropy is a measure of a system's disorder. As a system
becomes more disordered, the lower its energy and the higher its entropy.
The laws of thermodynamics are a series of laws that describe the properties and processes of energy transfer.
The first law states that the total amount of energy in the universe is constant. This means that energy cannot
be created or destroyed, only transferred or transformed. The second law of thermodynamics states that every
energy transfer involves some loss of energy in an unusable form, such as heat energy, resulting in a more
disordered system. In other words, no energy transfer is completely efficient, and all transfers trend toward
disorder.
6.4 ATP: Adenosine Triphosphate
ATP is the primary energy-supplying molecule for living cells. ATP is comprised of a nucleotide, a five-carbon
sugar, and three phosphate groups. The bonds that connect the phosphates (phosphoanhydride bonds) have
high-energy content. The energy released from ATP hydrolysis into ADP + Pi performs cellular work. Cells use
ATP to perform work by coupling ATP hydrolysis' exergonic reaction with endergonic reactions. ATP donates its
phosphate group to another molecule via phosphorylation. The phosphorylated molecule is at a higher-energy
state and is less stable than its unphosphorylated form, and this added energy from phosphate allows the
molecule to undergo its endergonic reaction.
6.5 Enzymes
Enzymes are chemical catalysts that accelerate chemical reactions at physiological temperatures by lowering
their activation energy. Enzymes are usually proteins consisting of one or more polypeptide chains. Enzymes
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have an active site that provides a unique chemical environment, comprised of certain amino acid R groups
(residues). This unique environment is perfectly suited to convert particular chemical reactants for that enzyme,
scientists call substrates, into unstable intermediates that they call transition states. Enzymes and substrates
bind with an induced fit, which means that enzymes undergo slight conformational adjustments upon substrate
contact, leading to full, optimal binding. Enzymes bind to substrates and catalyze reactions in four different
ways: bringing substrates together in an optimal orientation, compromising the bond structures of substrates so
that bonds can break down more easily, providing optimal environmental conditions for a reaction to occur, or
participating directly in their chemical reaction by forming transient covalent bonds with the substrates.
Enzyme action must be regulated so that in a given cell at a given time, the desired reactions catalyze and the
undesired reactions are not. Enzymes are regulated by cellular conditions, such as temperature and pH. They
are also regulated through their location within a cell, sometimes compartmentalized so that they can only
catalyze reactions under certain circumstances. Enzyme inhibition and activation via other molecules are other
important ways that enzymes are regulated. Inhibitors can act competitively, noncompetitively, or allosterically.
Noncompetitive inhibitors are usually allosteric. Activators can also enhance enzyme function allosterically. The
most common method by which cells regulate the enzymes in metabolic pathways is through feedback
inhibition. During feedback inhibition, metabolic pathway products serve as inhibitors (usually allosteric) of one
or more of the enzymes (usually the first committed enzyme of the pathway) involved in the pathway that
produces them.
VISUAL CONNECTION QUESTIONS
1. Figure 6.8 Look at each of the processes shown,
and decide if it is endergonic or exergonic. In each
case, does enthalpy increase or decrease, and does
entropy increase or decrease?
2. Figure 6.10 If no activation energy were required
to break down sucrose (table sugar), would you be
able to store it in a sugar bowl?
REVIEW QUESTIONS
4. Energy is stored long-term in the bonds of_
and used short-term to perform work from a(n)_
molecule.
a. ATP : glucose
b. an anabolic molecule : catabolic molecule
c. glucose : ATP
d. a catabolic molecule : anabolic molecule
5. DNA replication involves unwinding two strands of
parent DNA, copying each strand to synthesize
complementary strands, and releasing the parent and
daughter DNA. Which of the following accurately
describes this process?
a. This is an anabolic process.
b. This is a catabolic process.
c. This is both anabolic and catabolic.
d. This is a metabolic process but is neither
anabolic nor catabolic.
6. Consider a pendulum swinging. Which type(s) of
energy is/are associated with the pendulum in the
following instances: i. the moment at which it
completes one cycle, just before it begins to fall back
towards the other end, ii. the moment that it is in the
middle between the two ends, and iii. just before it
reaches the end of one cycle (just before instant i.).
3. Figure 6.14 The hydrolysis of one ATP molecule
releases 7.3 kcal/mol of energy (AG = -7.3 kcal/mol
of energy). If it takes 2.1 kcal/mol of energy to move
one Na + across the membrane (AG = +2.1 kcal/mol
of energy), how many sodium ions could be moved
by the hydrolysis of one ATP molecule?
a. i. potential and kinetic, ii. potential and
kinetic, iii. kinetic
b. i. potential, ii. potential and kinetic, iii.
potential and kinetic
c. i. potential, ii. kinetic, iii. potential and kinetic
d. i. potential and kinetic, ii. kinetic iii. kinetic
7. Which of the following comparisons or contrasts
between endergonic and exergonic reactions is
false?
a. Endergonic reactions have a positive AG
and exergonic reactions have a negative
AG.
b. Endergonic reactions consume energy and
exergonic reactions release energy.
c. Both endergonic and exergonic reactions
require a small amount of energy to
overcome an activation barrier.
d. Endergonic reactions take place slowly and
exergonic reactions take place quickly.
8. Which of the following is the best way to judge the
relative activation energies between two given
chemical reactions?
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197
a. Compare the AG values between the two
reactions.
b. Compare their reaction rates.
c. Compare their ideal environmental
conditions.
d. Compare the spontaneity between the two
reactions.
9. Which of the following is not an example of an
energy transformation?
a. turning on a light switch
b. solar panels at work
c. formation of static electricity
d. none of the above
10. in each of the three systems, determine the state
of entropy (low or high) when comparing the first and
second: i. the instant that a perfume bottle is sprayed
compared with 30 seconds later, ii. an old 1950s car
compared with a brand new car, and iii. a living cell
compared with a dead cell.
a.
i. low,
ii. high, i
i. low
b.
i. low,
ii. high, i
i. high
c.
i. high
ii. low, i
i. high
d.
i. high
ii. low, i
i. low
11. The energy released by the hydrolysis of ATP
is_
a. primarily stored between the alpha and beta
phosphates
b. equal to -57 kcal/mol
c. harnessed as heat energy by the cell to
perform work
d. providing energy to coupled reactions
12. Which of the following molecules is likely to have
the most potential energy?
CRITICAL THINKING QUESTIONS
16. Does physical exercise involve anabolic and/or
catabolic processes? Give evidence for your answer.
17. Name two different cellular functions that require
energy that parallel human energy-requiring
functions.
18. Explain in your own words the difference between
a spontaneous reaction and one that occurs
instantaneously, and what causes this difference.
19. Describe the position of the transition state on a
vertical energy scale, from low to high, relative to the
position of the reactants and products, for both
endergonic and exergonic reactions.
20. Imagine an elaborate ant farm with tunnels and
passageways through the sand where ants live in a
large community. Now imagine that an earthquake
a. sucrose
b. ATP
c. glucose
d. ADP
13. Which of the following is not true about enzymes:
a. They increase AG of reactions.
b. They are usually made of amino acids.
c. They lower the activation energy of
chemical reactions.
d. Each one is specific to the particular
substrate(s) to which it binds.
14. An allosteric inhibitor does which of the following?
a. Binds to an enzyme away from the active
site and changes the conformation of the
active site, increasing its affinity for
substrate binding.
b. Binds to the active site and blocks it from
binding substrate.
c. Binds to an enzyme away from the active
site and changes the conformation of the
active site, decreasing its affinity for the
substrate.
d. Binds directly to the active site and mimics
the substrate.
15. Which of the following analogies best describes
the induced-fit model of enzyme-substrate binding?
a. a hug between two people
b. a key fitting into a lock
c. a square peg fitting through the square hole
and a round peg fitting through the round
hole of a children’s toy
d. the fitting together of two jigsaw puzzle
pieces
shook the ground and demolished the ant farm. In
which of these two scenarios, before or after the
earthquake, was the ant farm system in a state of
higher or lower entropy?
21. Energy transfers take place constantly in
everyday activities. Think of two scenarios: cooking
on a stove and driving. Explain how the second law
of thermodynamics applies to these two scenarios.
22. Do you think that the Ea for ATP hydrolysis is
relatively low or high? Explain your reasoning.
23. With regard to enzymes, why are vitamins
necessary for good health? Give examples.
24. Explain in your own words how enzyme feedback
inhibition benefits a cell.
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7 | CELLULAR
RESPIRATION
Figure 7.1 This geothermal energy plant transforms thermal energy from deep in the ground into electrical energy,
which can be easily used, (credit: modification of work by the U.S. Department of Defense)
Chapter Outline
7.1: Energy in Living Systems
7.2: Glycolysis
7.3: Oxidation of Pyruvate and the Citric Acid Cycle
7.4: Oxidative Phosphorylation
7.5: Metabolism without Oxygen
7.6: Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
7.7: Regulation of Cellular Respiration
Introduction
The electrical energy plant in Figure 7.1 converts energy from one form to another form that can be more easily
used. This type of generating plant starts with underground thermal energy (heat) and transforms it into electrical
energy that will be transported to homes and factories. Like a generating plant, plants and animals also must
take in energy from the environment and convert it into a form that their cells can use. Mass and its stored
energy enter an organism’s body in one form and are converted into another form that can fuel the organism’s
life functions. In the process of photosynthesis, plants and other photosynthetic producers take in energy in the
form of light (solar energy) and convert it into chemical energy in the form of glucose, which stores this energy
in its chemical bonds. Then, a series of metabolic pathways, collectively called cellular respiration, extracts the
energy from the bonds in glucose and converts it into a form that all living things can use.
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7.1 1 Energy in Living Systems
By the end of this section, you will be able to do the following:
• Discuss the importance of electrons in the transfer of energy in living systems
• Explain how ATP is used by cells as an energy source
Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are
combinations of oxidation and reduction reactions, which occur at the same time. An oxidation reaction strips
an electron from an atom in a compound, and the addition of this electron to another compound is a reduction
reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation
reduction reactions, or redox reactions.
Electrons and Energy
The removal of an electron from a molecule (oxidizing it), results in a decrease in potential energy in the oxidized
compound. The electron (sometimes as part of a hydrogen atom) does not remain unbonded, however, in the
cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound. The
shift of an electron from one compound to another removes some potential energy from the first compound (the
oxidized compound) and increases the potential energy of the second compound (the reduced compound). The
transfer of electrons between molecules is important because most of the energy stored in atoms and used
to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of high-energy
electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in
a single, destructive burst. This chapter focuses on the extraction of energy from food; you will see that as you
track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.
Electron Carriers
In living systems, a small class of compounds functions as electron shuttles: they bind and carry high-energy
electrons between compounds in biochemical pathways. The principal electron carriers we will consider are
derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced
(that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD)
(Figure 7.2) is derived from vitamin B 3 , niacin. NAD + is the oxidized form of the molecule; NADH is the reduced
form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a
hydrogen atom with an extra electron). Note that if a compound has an “H” on it, it is generally reduced (e.g.,
NADH is the reduced form of NAD).
NAD + can accept electrons from an organic molecule according to the general equation:
RH NAD +
Reducing + Oxidizing ->
agent agent
NADH R
Reduced Oxidized
When electrons are added to a compound, it is reduced. A compound that reduces another is called a reducing
agent. In the above equation, RH is a reducing agent, and NAD + is reduced to NADH. When electrons are
removed from a compound, it is oxidized. A compound that oxidizes another is called an oxidizing agent. In the
above equation, NAD + is an oxidizing agent, and RH is oxidized to R.
Similarly, flavin adenine dinucleotide (FAD + ) is derived from vitamin B 2 , also called riboflavin. Its reduced form
is FADH 2 . A second variation of NAD, NADP, contains an extra phosphate group. Both NAD + and FAD + are
extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and
photosynthesis in plants.
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201
Figure 7.2 The oxidized form of the electron carrier (NAD + ) is shown on the left, and the reduced form (NADH) is
shown on the right. The nitrogenous base in NADH has one more hydrogen ion and two more electrons than in NAD + .
ATP in Living Systems
A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of
heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell.
Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release
it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP).
ATP is often called the “energy currency" of the cell, and, like currency, this versatile compound can be used to
fill any energy need of the cell. How? It functions similarly to a rechargeable battery.
When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The
energy is used to do work by the cell, usually when the released phosphate binds to another molecule, thereby
activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the
contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell membranes.
ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and
potassium. In this way, the cell performs work, pumping ions against their electrochemical gradients.
ATP Structure and Function
At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine
molecule bonded to a ribose molecule and to a single phosphate group (Figure 7.3). Ribose is a five-carbon
sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to
this core molecule results in the formation of adenosine diphosphate (ADP); the addition of a third phosphate
group forms adenosine triphosphate (ATP).
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Chapter 7 | Cellular Respiration
Gamma
phosphate
group
O
O
Alpha
phosphate
group
O
O-P—O—P—O—P—O-
I I I
O' 0“ O'
Beta
phosphate
group
OH OH
Ribose
Figure 7.3 ATP (adenosine triphosphate) has three phosphate groups that can be removed by hydrolysis (addition
of H 2 O) to form ADP (adenosine diphosphate) or AMP (adenosine monophosphate). The negative charges on the
phosphate group naturally repel each other, requiring energy to bond them together and releasing energy when these
bonds are broken.
The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and
thus repel one another when they are arranged in series, as they are in ADP and ATP. This repulsion makes the
ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process
called dephosphorylation, releases energy.
Energy from ATP
Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or lysed,
and the resulting hydrogen atom (H + ) and a hydroxyl group (OH'), or hydroxide, are added to the larger
molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (Pi), and the release
of free energy. To carry out life processes, ATP is continuously broken down into ADP, and like a rechargeable
battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group. Water, which
was broken down into its hydrogen atom and hydroxyl group (hydroxide) during ATP hydrolysis, is regenerated
when a third phosphate is added to the ADP molecule, reforming ATP.
Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come from? In
nearly every living thing on Earth, the energy comes from the metabolism of glucose, fructose, or galactose, all
isomers with the chemical formula C 6 H 12 O 6 but different molecular configurations. In this way, ATP is a direct link
between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways
that power living cells.
Phosphorylation
Recall that, in some chemical reactions, enzymes may bind to several substrates that react with each other on
the enzyme, forming an intermediate complex. An intermediate complex is a temporary structure, and it allows
one of the substrates (such as ATP) and reactants to more readily react with each other; in reactions involving
ATP, ATP is one of the substrates and ADP is a product. During an endergonic chemical reaction, ATP forms
an intermediate complex with the substrate and enzyme in the reaction. This intermediate complex allows the
ATP to transfer its third phosphate group, with its energy, to the substrate, a process called phosphorylation.
Phosphorylation refers to the addition of the phosphate (~P). This is illustrated by the following generic
reaction, in which A and B represent two different substrates:
A + enzyme + ATP -> [A — enzyme — ~ P] -> B + enzyme + ADP + phosphate ion
When the intermediate complex breaks apart, the energy is used to modify the substrate and convert it into a
product of the reaction. The ADP molecule and a free phosphate ion are released into the medium and are
available for recycling through cell metabolism.
Substrate Phosphorylation
ATP is generated through two mechanisms during the breakdown of glucose. A few ATP molecules are
generated (that is, regenerated from ADP) as a direct result of the chemical reactions that occur in the catabolic
pathways. A phosphate group is removed from an intermediate reactant in the pathway, and the free energy of
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Chapter 7 | Cellular Respiration
203
the reaction is used to add the third phosphate to an available ADP molecule, producing ATP (Figure 7.4). This
very direct method of phosphorylation is called substrate-level phosphorylation.
ADP
ATP
Figure 7.4 In phosphorylation reactions, the gamma (third) phosphate of ATP is attached to a protein.
Oxidative Phosphorylation
Most of the ATP generated during glucose catabolism, however, is derived from a much more complex process,
chemiosmosis, which takes place in mitochondria (Figure 7.5) within a eukaryotic cell or the plasma membrane
of a prokaryotic cell. Chemiosmosis, a process of ATP production in cellular metabolism, is used to generate
90 percent of the ATP made during glucose catabolism and is also the method used in the light reactions of
photosynthesis to harness the energy of sunlight. The production of ATP using the process of chemiosmosis is
called oxidative phosphorylation because of the involvement of oxygen in the process.
ATP synthase enzymes and the
electron transport chain are
embedded in the inner membrane.
Figure 7.5 In eukaryotes, oxidative phosphorylation takes place in mitochondria. In prokaryotes, this process takes
place in the plasma membrane. (Credit: modification of work by Mariana Ruiz Villareal)
ca eer connection
Mitochondrial Disease Physician
What happens when the critical reactions of cellular respiration do not proceed correctly? This may happen
in mitochondrial diseases, which are genetic disorders of metabolism. Mitochondrial disorders can arise
from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than
is normal in body cells. In type 2 diabetes, for instance, the oxidation efficiency of NADH is reduced,
impacting oxidative phosphorylation but not the other steps of respiration. Symptoms of mitochondrial
diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and
hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases.
Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation
for this profession requires a college education, followed by medical school with a specialization in medical
genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on
to become associated with professional organizations devoted to the study of mitochondrial diseases, such
as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disorders.
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7.2 | Glycolysis
By the end of this section, you will be able to do the following:
• Describe the overall result in terms of molecules produced during the chemical breakdown of glucose by
glycolysis
• Compare the output of glycolysis in terms of ATP molecules and NADH molecules produced
As you have read, nearly all of the energy used by living cells comes to them in the bonds of the sugar glucose.
Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. In fact, nearly
all living organisms carry out glycolysis as part of their metabolism. The process does not use oxygen directly
and therefore is termed anaerobic. Glycolysis takes place in the cytoplasm of both prokaryotic and eukaryotic
cells. Glucose enters heterotrophic cells in two ways. One method is through secondary active transport in which
the transport takes place against the glucose concentration gradient. The other mechanism uses a group of
integral proteins called GLUT proteins, also known as glucose transporter proteins. These transporters assist
in the facilitated diffusion of glucose.
Glycolysis begins with the six-carbon ring-shaped structure of a single glucose molecule and ends with two
molecules of a three-carbon sugar called pyruvate. Glycolysis consists of two distinct phases. The first part of
the glycolysis pathway traps the glucose molecule in the cell and uses energy to modify it so that the six-carbon
sugar molecule can be split evenly into the two three-carbon molecules. The second part of glycolysis extracts
energy from the molecules and stores it in the form of ATP and NADH—remember: this is the reduced form of
NAD.
First Half of Glycolysis (Energy-Requiring Steps)
Step 1. The first step in glycolysis (Figure 7.6) is catalyzed by hexokinase, an enzyme with broad specificity
that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates glucose using ATP as
the source of the phosphate, producing glucose-6-phosphate, a more reactive form of glucose. This reaction
prevents the phosphorylated glucose molecule from continuing to interact with the GLUT proteins, and it can no
longer leave the cell because the negatively charged phosphate will not allow it to cross the hydrophobic interior
of the plasma membrane.
Step 2. In the second step of glycolysis, an isomerase converts glucose-6-phosphate into one of its isomers,
fructose-6-phosphate (this isomer has a phosphate attached at the location of the sixth carbon of the ring). An
isomerase is an enzyme that catalyzes the conversion of a molecule into one of its isomers. (This change from
phosphoglucose to phosphofructose allows the eventual split of the sugar into two three-carbon molecules.)
Step 3. The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme
phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate,
producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a rate-limiting enzyme. It is active
when the concentration of ADP is high; it is less active when ADP levels are low and the concentration of ATP
is high. Thus, if there is “sufficient" ATP in the system, the pathway slows down. This is a type of end product
inhibition, since ATP is the end product of glucose catabolism.
Step 4. The newly added high-energy phosphates further destabilize fructose-1,6-bisphosphate. The fourth step
in glycolysis employs an enzyme, aldolase, to cleave fructose-1,6-bisphosphate into two three-carbon isomers:
dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.
Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer,
glyceraldehyde-3-phosphate. Thus, the pathway will continue with two molecules of a
glyceraldehyde-3-phosphate. At this point in the pathway, there is a net investment of energy from two ATP
molecules in the breakdown of one glucose molecule.
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Chapter 7 | Cellular Respiration
205
h v /
H-C-OH
I
CH-O,
\ M
A OH H j
□ V • /
*C — C'
zz
Glucose
H-C=0
i
H-C-OH
i
HO-C-H
. H-C-OH _
Phosphoglucose
H-C-OHo jsomerase
H-C-O-PsO
H 6'
H
H-C-OH
i
c=o
I
HO-C-H
H-C-OH
zz
Phosphofructo
H-C-OH o' kj nase
H-C-O-PsO
h 6-
y 9'
H-C-0-P=0
• l
c=o o*
I
HO-C-H
H-C-OH
l
H-C-OH O'
• I
H-C-O-PsO
H 6-
H V
H-C“0-P=0
C«0 O'
i
HO-c-H Dihydroxyacetone-
h phosphate
Fructose
bisphosphate
aldolase
Glucose-6-phosphate Fructose-6-phosphate Fructose-1,6-biphosphate
It
H
Triose
phosphate
isomerase
c=o
I
H-C-OH o'
H-C-0 -P=0
H O'
Glyceraldehyde-
3-phosphate
Figure 7.6 The first half of glycolysis uses two ATP molecules in the phosphorylation of glucose, which is then split
into two three-carbon molecules.
Second Half of Glycolysis (Energy-Releasing Steps)
So far, glycolysis has cost the cell two ATP molecules and produced two small, three-carbon sugar molecules.
Both of these molecules will proceed through the second half of the pathway, and sufficient energy will be
extracted to pay back the two ATP molecules used as an initial investment and produce a profit for the cell of
two additional ATP molecules and two even higher-energy NADH molecules.
Step 6 . The sixth step in glycolysis (Figure 7.7) oxidizes the sugar (glyceraldehyde-3-phosphate), extracting
high-energy electrons, which are picked up by the electron carrier NAD + , producing NADH. The sugar is then
phosphorylated by the addition of a second phosphate group, producing 1,3-bisphosphoglycerate. Note that the
second phosphate group does not require another ATP molecule.
H
I
c=o
I
h-c-oho' Glyceraldehyde-
h-c-o-p=o 3-phosphate
h 6-
Glyceraldehyde-
3-phosphate
dehydrogenase
2X
z'
NAD + + P,
NADH + H +
o* ,o-
c
I
c=o
I
H*C-H
H
Pyruvate
ATP ADP o
zz
Pyruvate
kinase
9 ?'
c-o-p=o
■ I
A °‘
H' h
H ? 0
V.
Phosphoenolpyruvate
(PEP)
°V°'
°v /°‘
c
I
H-C-OH o'
ATP ADP
O'
H-C-0-P=0 ^__ wt>Y m _
H-C-OH 6' Phosphoglycerate h-C-0-P=0 Phosphoglycerate
u mutase A i kinase
h H O'
Zi
o-p*o
o
c=o
i
H-C-OH (p‘
H-C-0-P=0
* i
O'
H
2-Phosphoglycerate 3-Phosphoglycerate 1,3-Bisphosphoglycerate
10
Figure 7.7 The second half of glycolysis involves phosphorylation without ATP investment (step 6) and produces two
NADH and four ATP molecules per glucose.
Here again is a potential limiting factor for this pathway. The continuation of the reaction depends upon the
availability of the oxidized form of the electron carrier, NAD + . Thus, NADH must be continuously oxidized back
into NAD + in order to keep this step going. If NAD + is not available, the second half of glycolysis slows down
or stops. If oxygen is available in the system, the NADH will be oxidized readily, though indirectly, and the high-
energy electrons from the hydrogen released in this process will be used to produce ATP. In an environment
without oxygen, an alternate pathway (fermentation) can provide the oxidation of NADH to NAD + .
Step 7. In the seventh step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse reaction),
1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP. (This is an
example of substrate-level phosphorylation.) A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a
carboxyl group, and 3-phosphoglycerate is formed.
Step 8. In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to
the second carbon, producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate). The enzyme catalyzing
this step is a mutase (isomerase).
Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its
structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential
206
Chapter 7 | Cellular Respiration
energy in the remaining phosphate bond and produces phosphoenolpyruvate (PEP).
Step 10. The last step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme in this case is
named for the reverse reaction of pyruvate’s conversion into PEP) and results in the production of a second
ATP molecule by substrate-level phosphorylation and the compound pyruvic acid (or its salt form, pyruvate).
Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can catalyze both
forward and reverse reactions (these may have been described initially by the reverse reaction that takes place
in vitro, under nonphysiological conditions).
LINK TQ LEARNING
Gain a better understanding of the breakdown of glucose by glycolysis by visiting this site
(http:// 0 penstaxc 0 llege. 0 rg/l/glyc 0 lysis) to see the process in action.
Outcomes of Glycolysis
Glycolysis begins with glucose and produces two pyruvate molecules, four new ATP molecules, and two
molecules of NADH. (Note: two ATP molecules are used in the first half of the pathway to prepare the six-carbon
ring for cleavage, so the cell has a net gain of two ATP molecules and two NADH molecules for its use). If the
cell cannot catabolize the pyruvate molecules further, it will harvest only two ATP molecules from one molecule
of glucose. Mature mammalian red blood cells do not have mitochondria and thus are not capable of aerobic
respiration —the process in which organisms convert energy in the presence of oxygen—and glycolysis is their
sole source of ATP. If glycolysis is interrupted, these cells lose their ability to maintain their sodium-potassium
pumps, and eventually, they die.
The last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate,
is not available in sufficient quantities. In this situation, the entire glycolysis pathway will proceed, but only two
ATP molecules will be made in the second half. Thus, pyruvate kinase is a rate-limiting enzyme for glycolysis.
7.3 | Oxidation of Pyruvate and the Citric Acid Cycle
By the end of this section, you will be able to do the following:
• Explain how a circular pathway, such as the citric acid cycle, fundamentally differs from a linear
biochemical pathway, such as glycolysis
• Describe how pyruvate, the product of glycolysis, is prepared for entry into the citric acid cycle
If oxygen is available, aerobic respiration will go forward. In eukaryotic cells, the pyruvate molecules produced
at the end of glycolysis are transported into the mitochondria, which are the sites of cellular respiration. There,
pyruvate is transformed into an acetyl group that will be picked up and activated by a carrier compound called
coenzyme A (CoA). The resulting compound is called acetyl CoA. CoA is derived from vitamin B5, pantothenic
acid. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group
derived from pyruvate to the next stage of the pathway in glucose catabolism.
Breakdown of Pyruvate
In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes. The
conversion is a three-step process (Figure 7.8).
Step 1. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding
medium. This reaction creates a two-carbon hydroxyethyl group bound to the enzyme (pyruvate
dehydrogenase). We should note that this is the first of the six carbons from the original glucose molecule to be
removed. (This step proceeds twice because there are two pyruvate molecules produced at the end of glycolsis
for every molecule of glucose metabolized anaerobically; thus, two of the six carbons will have been removed at
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Chapter 7 | Cellular Respiration
207
the end of both steps.)
Step 2. The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD + , forming
NADH. The high-energy electrons from NADH will be used later to generate ATP.
Step 3. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA.
Oxidation of Pyruvate
O' 1
CoA-SH
3
1
V
S—CoA
0—0
II II
0 0
■W
1
c=o
1
1
NAD* NADH
CH 3
CH 3
+
co 2
Pyruvate
Oxidation
reaction
Acetyl CoA
1
2
3
A carboxyl group
NAD* is reduced
An acetyl group is
is removed from
to NADH.
transferred to
pyruvate, releasing
coenzyme A,
carbon dioxide.
resulting in acetyl
CoA.
Figure 7.8 Upon entering the mitochondrial matrix, a multienzyme complex converts pyruvate into acetyl CoA. In the
process, carbon dioxide is released, and one molecule of NADH is formed.
Note that during the second stage of glucose metabolism, whenever a carbon atom is removed, it is bound to
two oxygen atoms, producing carbon dioxide, one of the major end products of cellular respiration.
Acetyl CoA to CO2
In the presence of oxygen, acetyl CoA delivers its acetyl (2C) group to a four-carbon molecule, oxaloacetate,
to form citrate, a six-carbon molecule with three carboxyl groups; this pathway will harvest the remainder of the
extractable energy from what began as a glucose molecule and release the remaining four CO 2 molecules. This
single pathway is called by different names: the citric acid cycle (for the first intermediate formed—citric acid,
or citrate—when acetate joins to the oxaloacetate), the TCA cycle (because citric acid or citrate and isocitrate
are tricarboxylic acids), and the Krebs cycle, after Hans Krebs, who first identified the steps in the pathway in
the 1930s in pigeon flight muscles.
Citric Acid Cycle
Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria.
Almost all of the enzymes of the citric acid cycle are soluble, with the single exception of the enzyme succinate
dehydrogenase, which is embedded in the inner membrane of the mitochondrion. Unlike glycolysis, the citric
acid cycle is a closed loop: the last part of the pathway regenerates the compound used in the first step. The
eight steps of the cycle are a series of redox, dehydration, hydration, and decarboxylation reactions that produce
two carbon dioxide molecules, one GTP/ATP, and the reduced carriers NADH and FADH 2 (Figure 7.9). This is
considered an aerobic pathway because the NADH and FADH 2 produced must transfer their electrons to the
next pathway in the system, which will use oxygen. If this transfer does not occur, the oxidation steps of the citric
acid cycle also do not occur. Note that the citric acid cycle produces very little ATP directly and does not directly
consume oxygen.
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Figure 7.9 In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule
to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide
molecules for each acetyl group fed into the cycle. In the process, three NAD + molecules are reduced to NADH, one
FAD molecule is reduced to FADH 2 , and one ATP or GTP (depending on the cell type) is produced (by substrate-level
phosphorylation). Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously
in the presence of sufficient reactants, (credit: modification of work by “Yikrazuul’VWikimedia Commons)
Steps in the Citric Acid Cycle
Step 1. Prior to the first step, a transitional phase occurs during which pyruvic acid is converted to acetyl CoA.
Then, the first step of the cycle begins: This condensation step combines the two-carbon acetyl group with a
four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group
(-SH) and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is
highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available. If
ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases.
Step 2. In step two, citrate loses one water molecule and gains another as citrate is converted into its isomer,
isocitrate.
Step 3. In step three, isocitrate is oxidized, producing a five-carbon molecule, a-ketoglutarate, along with a
molecule of CO 2 and two electrons, which reduce NAD + to NADH. This step is also regulated by negative
feedback from ATP and NADH and a positive effect of ADP.
Step 4. Steps three and four are both oxidation and decarboxylation steps, which as we have seen, release
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electrons that reduce NAD + to NADH and release carboxyl groups that form CO 2 molecules. Alpha-ketoglutarate
is the product of step three, and a succinyl group is the product of step four. CoA binds with the succinyl group
to form succinyl CoA. The enzyme that catalyzes step four is regulated by feedback inhibition of ATP, succinyl
CoA, and NADH.
Step 5. In step five, a phosphate group is substituted for coenzyme A, and a high-energy bond is formed. This
energy is used in substrate-level phosphorylation (during the conversion of the succinyl group to succinate) to
form either guanine triphosphate (GTP) or ATP. There are two forms of the enzyme, called isoenzymes, for this
step, depending upon the type of animal tissue in which they are found. One form is found in tissues that use
large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the
enzyme is found in tissues that have a high number of anabolic pathways, such as liver. This form produces
GTP. GTP is energetically equivalent to ATP; however, its use is more restricted. In particular, protein synthesis
primarily uses GTP.
Step 6. Step six is a dehydration process that converts succinate into fumarate. Two hydrogen atoms are
transferred to FAD, reducing it to FADH 2 . (Note: the energy contained in the electrons of these hydrogens is
insufficient to reduce NAD + but adequate to reduce FAD.) Unlike NADH, this carrier remains attached to the
enzyme and transfers the electrons to the electron transport chain directly. This process is made possible by the
localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion.
Step 7. Water is added by hydrolysis to fumarate during step seven, and malate is produced. The last step in
the citric acid cycle regenerates oxaloacetate by oxidizing malate. Another molecule of NADH is then produced
in the process.
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Products of the Citric Acid Cycle
Two carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the six carbons
of one glucose molecule. Two carbon dioxide molecules are released on each turn of the cycle; however, these
do not necessarily contain the most recently added carbon atoms. The two acetyl carbon atoms will eventually be
released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually
incorporated into carbon dioxide. Each turn of the cycle forms three NADH molecules and one FADH 2 molecule.
These carriers will connect with the last portion of aerobic respiration, the electron transport chain, to produce
ATP molecules. One GTP or ATP is also made in each cycle. Several of the intermediate compounds in the
citric acid cycle can be used in synthesizing nonessential amino acids; therefore, the cycle is amphibolic (both
catabolic and anabolic).
7.4 | Oxidative Phosphorylation
By the end of this section, you will be able to do the following:
• Describe how electrons move through the electron transport chain and explain what happens to their
energy levels during this process
• Explain how a proton (H + ) gradient is established and maintained by the electron transport chain
You have just read about two pathways in glucose catabolism—glycolysis and the citric acid cycle—that
generate ATP. Most of the ATP generated during the aerobic catabolism of glucose, however, is not generated
directly from these pathways. Instead, it is derived from a process that begins by moving electrons through
a series of electron carriers that undergo redox reactions. This process causes hydrogen ions to accumulate
within the matrix space. Therefore, a concentration gradient forms in which hydrogen ions diffuse out of the
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matrix space by passing through ATP synthase. The current of hydrogen ions powers the catalytic action of ATP
synthase, which phosphorylates ADP, producing ATP.
Electron Transport Chain
The electron transport chain (Figure 7.10) is the last component of aerobic respiration and is the only part of
glucose metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plant tissues (typically
through stomata), as well as into fungi and bacteria; however, in animals, oxygen enters the body through a
variety of respiratory systems. Electron transport is a series of redox reactions that resembles a relay race or
bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain
where the electrons reduce molecular oxygen and, along with associated protons, produces water. There are
four complexes composed of proteins, labeled i through IV in Figure 7.10, and the aggregation of these four
complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain.
The electron transport chain is present with multiple copies in the inner mitochondrial membrane of eukaryotes
and within the plasma membrane of prokaryotes.
Figure 7.10 The electron transport chain is a series of electron transporters embedded in the inner mitochondrial
membrane that shuttles electrons from NADH and FADH 2 to molecular oxygen. In the process, protons are pumped
from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to form water.
Complex I
First, two electrons are carried to the first complex via NADH. This complex, labeled I, is composed of flavin
mononucleotide (FMN) and an iron-sulfur (Fe-S)-containing protein. FMN, which is derived from vitamin B 2 (also
called riboflavin), is one of several prosthetic groups or cofactors in the electron transport chain. A prosthetic
group is a nonprotein molecule required for the activity of a protein. Prosthetic groups are organic or inorganic,
nonpeptide molecules bound to a protein that facilitate its function. Prosthetic groups include coenzymes, which
are the prosthetic groups of enzymes. The enzyme in complex I is NADH dehydrogenase and is a very large
protein, containing 45 amino acid chains. Complex I can pump four hydrogen ions across the membrane from
the matrix into the intermembrane space, and it is in this way that the hydrogen ion gradient is established and
maintained between the two compartments separated by the inner mitochondrial membrane.
Q and Complex II
Complex II directly receives FADH 2 —which does not pass through complex I. The compound connecting the first
and second complexes to the third is ubiquinone B. The Q molecule is lipid soluble and freely moves through
the hydrophobic core of the membrane. Once it is reduced (QH 2 ), ubiquinone delivers its electrons to the next
complex in the electron transport chain. Q receives the electrons derived from NADH from complex I, and the
electrons derived from FADH 2 from complex II. This enzyme and FADH 2 form a small complex that delivers
electrons directly to the electron transport chain, bypassing the first complex. Since these electrons bypass and
thus do not energize the proton pump in the first complex, fewer ATP molecules are made from the FADH 2
electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons
pumped across the inner mitochondrial membrane.
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Complex III
The third complex is composed of cytochrome b—another Fe-S protein, a Rieske center (2Fe-2S center), and
cytochrome c proteins. This complex is also called cytochrome oxidoreductase. Cytochrome proteins have a
prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not
oxygen. As a result, the iron ion at its core is reduced and oxidized as it passes the electrons, fluctuating between
different oxidation states: Fe ++ (reduced) and Fe +++ (oxidized). The heme molecules in the cytochromes have
slightly different characteristics due to the effects of the different proteins binding to them, giving slightly different
characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to
cytochrome c for transport to the fourth complex of proteins and enzymes. (Cytochrome c receives electrons
from Q; however, whereas Q carries pairs of electrons, cytochrome c can accept only one at a time.)
Complex IV
The fourth complex is composed of cytochrome proteins c, a, and a 3 . This complex contains two heme groups
(one in each of the two cytochromes, a, and a 3 ) and three copper ions (a pair of Cua and one Cub in cytochrome
a 3 ). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is
completely reduced by the gain of two electrons. The reduced oxygen then picks up two hydrogen ions from the
surrounding medium to make water (H 2 O). The removal of the hydrogen ions from the system contributes to the
ion gradient that forms the foundation for the process of chemiosmosis.
Chemiosmosis
in chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen
ions (protons) across the mitochondrial membrane. The uneven distribution of H + ions across the membrane
establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the
hydrogen ions’ positive charge and their aggregation on one side of the membrane.
If the membrane were continuously open to simple diffusion by the hydrogen ions, the ions would tend to
diffuse back across into the matrix, driven by the concentrations producing their electrochemical gradient. Recall
that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion
channels. Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane
by an integral membrane protein called ATP synthase (Figure 7.11). This complex protein acts as a tiny
generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient. The
turning of parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the
potential energy of the hydrogen ion gradient.
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visual
CONNECTION
ATP Synthase
Figure 7.11 ATP synthase is a complex, molecular machine that uses a proton (H + ) gradient to form ATP from
ADP and inorganic phosphate (Pi). (Credit: modification of work by Klaus Hoffmeier)
Dinitrophenol (DNP) is an “uncoupler” that makes the inner mitochondrial membrane “leaky" to protons. It
was used until 1938 as a weight-loss drug. What effect would you expect DNP to have on the change in pH
across the inner mitochondrial membrane? Why do you think this might be an effective weight-loss drug?
Chemiosmosis (Figure 7.12) is used to generate 90 percent of the ATP made during aerobic glucose catabolism;
it is also the method used in the light reactions of photosynthesis to harness the energy of sunlight in the process
of photophosphorylation. Recall that the production of ATP using the process of chemiosmosis in mitochondria
is called oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy
of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the
end of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons
on the oxygen attract hydrogen ions (protons) from the surrounding medium, and water is formed. Thus, oxygen
is the final electron acceptor in the electron transport chain.
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visual
CONNECTION
Figure 7.12 In oxidative phosphorylation, the pH gradient formed by the electron transport chain is used by ATP
synthase to form ATP.
Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning
occurs, would you expect the pH of the intermembrane space to increase or decrease? What effect would
cyanide have on ATP synthesis?
ATP Yield
The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of
hydrogen ions that the electron transport chain complexes can pump through the membrane varies between
species. Another source of variance stems from the shuttle of electrons across the membranes of the
mitochondria. (The NADH generated from glycolysis cannot easily enter mitochondria.) Thus, electrons are
picked up on the inside of mitochondria by either NAD + or FAD + . As you have learned earlier, these FAD +
molecules can transport fewer ions; consequently, fewer ATP molecules are generated when FAD + acts as a
carrier. NAD + is used as the electron transporter in the liver and FAD + acts in the brain.
Another factor that affects the yield of ATP molecules generated from glucose is the fact that intermediate
compounds in these pathways are also used for other purposes. Glucose catabolism connects with the pathways
that build or break down all other biochemical compounds in cells, and the result is somewhat messier than the
ideal situations described thus far. For example, sugars other than glucose are fed into the glycolytic pathway
for energy extraction, in addition, the five-carbon sugars that form nucleic acids are made from intermediates
in glycolysis. Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric
acid cycle. Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways,
and both amino acids and triglycerides are broken down for energy through these pathways. Overall, in living
systems, these pathways of glucose catabolism extract about 34 percent of the energy contained in glucose,
with the remainder being released as heat.
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7.5 | Metabolism without Oxygen
By the end of this section, you will be able to do the following:
• Discuss the fundamental difference between anaerobic cellular respiration and fermentation
• Describe the type of fermentation that readily occurs in animal cells and the conditions that initiate that
fermentation
In aerobic respiration, the final electron acceptor is an oxygen molecule, O 2 . If aerobic respiration occurs, then
ATP will be produced using the energy of high-energy electrons carried by NADH or FADH 2 to the electron
transport chain. If aerobic respiration does not occur, NADH must be reoxidized to NAD + for reuse as an electron
carrier for the glycolytic pathway to continue. How is this done? Some living systems use an organic molecule
as the final electron acceptor. Processes that use an organic molecule to regenerate NAD + from NADH are
collectively referred to as fermentation. In contrast, some living systems use an inorganic molecule as a final
electron acceptor. Both methods are called anaerobic cellular respiration, in which organisms convert energy
for their use in the absence of oxygen.
Anaerobic Cellular Respiration
Certain prokaryotes, including some species in the domains Bacteria and Archaea, use anaerobic respiration.
For example, a group of archaeans called methanogens reduces carbon dioxide to methane to oxidize NADH.
These microorganisms are found in soil and in the digestive tracts of ruminants, such as cows and sheep.
Similarly, sulfate-reducing bacteria, most of which are anaerobic (Figure 7.13), reduce sulfate to hydrogen
sulfide to regenerate NAD + from NADH.
Figure 7.13 The green color seen in these coastal waters is from an eruption of hydrogen sulfide-producing bacteria.
These anaerobic, sulfate-reducing bacteria release hydrogen sulfide gas as they decompose algae in the water,
(credit: modification of work by NASA/Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC, Visible
Earth Catalog of NASA images)
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LINK TQ LEARNING
Visit this site (http://0penstaxc0llege.0rg/l/fermentati0n) to see anaerobic cellular respiration in action.
Lactic Acid Fermentation
The fermentation method used by animals and certain bacteria, such as those in yogurt, is lactic acid
fermentation (Figure 7 . 14 ). This type of fermentation is used routinely in mammalian red blood cells, which do
not have mitochondria, and in skeletal muscle that has an insufficient oxygen supply to allow aerobic respiration
to continue (that is, in muscles used to the point of fatigue), in muscles, lactic acid accumulation must be
removed by the blood circulation, and when the lactic acid loses a hydrogen, the resulting lactate is brought to
the liver for further metabolism. The chemical reactions of lactic acid fermentation are the following:
Pyruvic acid + NADH <-> lactic acid + NAD +
The enzyme used in this reaction is lactate dehydrogenase (LDH). The reaction can proceed in either direction,
but the reaction from left to right is inhibited by acidic conditions. Such lactic acid accumulation was once
believed to cause muscle stiffness, fatigue, and soreness, although more recent research disputes this
hypothesis. Once the lactic acid has been removed from the muscle and circulated to the liver, it can be
reconverted into pyruvic acid and further catabolized for energy.
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visual
Figure 7.14 Lactic acid fermentation is common in muscle cells that have run out of oxygen.
Tremetol, a metabolic poison found in the white snakeroot plant, prevents the metabolism of lactate. When
cows eat this plant, tremetol is concentrated in the milk they produce. Humans who consume the milk
can become seriously ill. Symptoms of this disease, which include vomiting, abdominal pain, and tremors,
become worse after exercise. Why do you think this is the case?
CONNECTION
Lactic Acid Fermentation
Glucose
Net 2 ATP
2 Pyruvate
2 Lactate
2 NAD +
2 NADH
2 NADH
2 NAD +
Alcohol Fermentation
Another familiar fermentation process is alcohol fermentation (Figure 7.15), which produces ethanol. The first
chemical reaction of alcohol fermentation is the following (CO 2 does not participate in the second reaction):
pyruvic acid + H + -h> CO 0 + acetaldehyde + NADH + H + —> ethanol + NAD 4 "
The first reaction is catalyzed by pyruvate decarboxylase, a cytoplasmic enzyme, with a coenzyme of thiamine
pyrophosphate (TPP, derived from vitamin Bi and also called thiamine). A carboxyl group is removed from
pyruvic acid, releasing carbon dioxide as a gas. The loss of carbon dioxide reduces the size of the molecule
by one carbon, producing acetaldehyde. The second reaction is catalyzed by alcohol dehydrogenase to oxidize
NADH to NAD + and reduce acetaldehyde to ethanol. The fermentation of pyruvic acid by yeast produces the
ethanol found in alcoholic beverages. Ethanol tolerance of yeast is variable, ranging from about 5 percent to 21
percent, depending on the yeast strain and environmental conditions.
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Figure 7.15 Fermentation of grape juice into wine produces CO 2 as a byproduct. Fermentation tanks have valves so
that the pressure inside the tanks created by the carbon dioxide produced can be released.
Other Types of Fermentation
Other fermentation methods take place in bacteria. We should note that many prokaryotes are facultatively
anaerobic. This means that they can switch between aerobic respiration and fermentation, depending on the
availability of free oxygen. Certain prokaryotes, such as Clostridia, are obligate anaerobes. Obligate anaerobes
live and grow in the absence of molecular oxygen. Oxygen is a poison to these microorganisms and kills them on
exposure. We should also note that all forms of fermentation, except lactic acid fermentation, produce gas. The
production of particular types of gas is used as an indicator of the fermentation of specific carbohydrates, which
plays a role in the laboratory identification of the bacteria. Various methods of fermentation are used by assorted
organisms to ensure an adequate supply of NAD + for the sixth step in glycolysis. Without these pathways, this
step would not occur, and ATP could not be harvested from the breakdown of glucose.
7.6 | Connections of Carbohydrate, Protein, and Lipid
Metabolic Pathways
By the end of this section, you will be able to do the following:
• Discuss the ways in which carbohydrate metabolic pathways, glycolysis, and the citric acid cycle
interrelate with protein and lipid metabolic pathways
• Explain why metabolic pathways are not considered closed systems
You have learned about the catabolism of glucose, which provides energy to living cells. But living things
consume organic compounds other than glucose for food. How does a turkey sandwich end up as ATP in your
cells? This happens because all of the catabolic pathways for carbohydrates, proteins, and lipids eventually
connect into glycolysis and the citric acid cycle pathways (see Figure 7.17). Metabolic pathways should be
thought of as porous and interconnecting—that is, substances enter from other pathways, and intermediates
leave for other pathways. These pathways are not closed systems! Many of the substrates, intermediates, and
products in a particular pathway are reactants in other pathways.
Connections of Other Sugars to Glucose Metabolism
Glycogen, a polymer of glucose, is an energy storage molecule in animals. When there is adequate ATP
present, excess glucose is stored as glycogen in both liver and muscle cells. The glycogen will be hydrolyzed
into glucose 1-phosphate monomers (G-l-P) if blood sugar levels drop. The presence of glycogen as a source
of glucose allows ATP to be produced for a longer period of time during exercise. Glycogen is broken down into
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glucose-l-phosphate (G-l-P) and converted into glucose-6-phosphate (G-6-P) in both muscle and liver cells,
and this product enters the glycolytic pathway.
Sucrose is a disaccharide with a molecule of glucose and a molecule of fructose bonded together with a
glycosidic linkage. Fructose is one of the three “dietary” monosaccharides, along with glucose and galactose
(part of the milk sugar dissacharide lactose), which are absorbed directly into the bloodstream during digestion.
The catabolism of both fructose and galactose produces the same number of ATP molecules as glucose.
Connections of Proteins to Glucose Metabolism
Proteins are hydrolyzed by a variety of enzymes in cells. Most of the time, the amino acids are recycled into the
synthesis of new proteins. If there are excess amino acids, however, or if the body is in a state of starvation,
some amino acids will be shunted into the pathways of glucose catabolism (Figure 7.16). It is very important to
note that each amino acid must have its amino group removed prior to entry into these pathways. The amino
group is converted into ammonia, in mammals, the liver synthesizes urea from two ammonia molecules and
a carbon dioxide molecule. Thus, urea is the principal waste product in mammals, produced from the nitrogen
originating in amino acids, and it leaves the body in urine. It should be noted that amino acids can be synthesized
from the intermediates and reactants in the cellular respiration cycle.
Figure 7.16 The carbon skeletons of certain amino acids (indicated in boxes) derived from proteins can feed into the
citric acid cycle, (credit: modification of work by Mikael Haggstrom)
Connections of Lipid and Glucose Metabolisms
The lipids connected to the glucose pathway include cholesterol and triglycerides. Cholesterol is a lipid that
contributes to cell membrane flexibility and is a precursor of steroid hormones. The synthesis of cholesterol starts
with acetyl groups and proceeds in only one direction. The process cannot be reversed.
Triglycerides—made from the bonding of glycerol and three fatty acids—are a form of long-term energy
storage in animals. Animals can make most of the fatty acids they need. Triglycerides can be both made
and broken down through parts of the glucose catabolism pathways. Glycerol can be phosphorylated to
glycerol-3-phosphate, which continues through glycolysis. Fatty acids are catabolized in a process called beta-
oxidation, which takes place in the matrix of the mitochondria and converts their fatty acid chains into two-carbon
units of acetyl groups. The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric
acid cycle.
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Figure 7.17 Glycogen from the liver and muscles, as well as other carbohydrates, hydrolyzed into
glucose-l-phosphate, together with fats and proteins, can feed into the catabolic pathways for carbohydrates.
e olution CONNECTION
Pathways of Photosynthesis and Cellular Metabolism
The processes of photosynthesis and cellular metabolism consist of several very complex pathways. It
is generally thought that the first cells arose in an aqueous environment—a “soup” of nutrients—possibly
on the surface of some porous clays, perhaps in warm marine environments. If these cells reproduced
successfully and their numbers climbed steadily, it follows that the cells would begin to deplete the nutrients
from the medium in which they lived as they shifted the nutrients into the components of their own bodies.
This hypothetical situation would have resulted in natural selection favoring those organisms that could exist
by using the nutrients that remained in their environment and by manipulating these nutrients into materials
upon which they could survive. Selection would favor those organisms that could extract maximal value from
the nutrients to which they had access.
An early form of photosynthesis developed that harnessed the sun’s energy using water as a source of
hydrogen atoms, but this pathway did not produce free oxygen (anoxygenic photosynthesis). (Another type
of anoxygenic photosynthesis did not produce free oxygen because it did not use water as the source
of hydrogen ions; instead, it used materials such as hydrogen sulfide and consequently produced sulfur).
It is thought that glycolysis developed at this time and could take advantage of the simple sugars being
produced but that these reactions were unable to fully extract the energy stored in the carbohydrates. The
development of glycolysis probably predated the evolution of photosynthesis, as it was well suited to extract
energy from materials spontaneously accumulating in the “primeval soup.” A later form of photosynthesis
used water as a source of electrons and hydrogen and generated free oxygen. Over time, the atmosphere
became oxygenated, but not before the oxygen released oxidized metals in the ocean and created a “rust”
layer in the sediment, permitting the dating of the rise of the first oxygenic photosynthesizers. Living things
adapted to exploit this new atmosphere that allowed aerobic respiration as we know it to evolve. When
the full process of oxygenic photosynthesis developed and the atmosphere became oxygenated, cells were
finally able to use the oxygen expelled by photosynthesis to extract considerably more energy from the
sugar molecules using the citric acid cycle and oxidative phosphorylation.
7.7 | Regulation of Cellular Respiration
By the end of this section, you will be able to do the following:
• Describe how feedback inhibition would affect the production of an intermediate or product in a pathway
• Identify the mechanism that controls the rate of the transport of electrons through the electron transport
chain
Cellular respiration must be regulated in order to provide balanced amounts of energy in the form of ATP. The
cell also must generate a number of intermediate compounds that are used in the anabolism and catabolism
of macromolecules. Without controls, metabolic reactions would quickly come to a standstill as the forward and
backward reactions reached a state of equilibrium. Resources would be used inappropriately. A cell does not
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need the maximum amount of ATP that it can make all the time: At times, the cell needs to shunt some of the
intermediates to pathways for amino acid, protein, glycogen, lipid, and nucleic acid production. In short, the cell
needs to control its metabolism.
Regulatory Mechanisms
A variety of mechanisms is used to control cellular respiration. Some type of control exists at each stage of
glucose metabolism. Access of glucose to the cell can be regulated using the GLUT (glucose transporter)
proteins that transport glucose (Figure 7.18). Different forms of the GLUT protein control passage of glucose
into the cells of specific tissues.
Figure 7.18 GLUT4 is a glucose transporter that is stored in vesicles. A cascade of events that occurs upon insulin
binding to a receptor in the plasma membrane causes GLUT4-containing vesicles to fuse with the plasma membrane
so that glucose may be transported into the cell.
Some reactions are controlled by having two different enzymes—one each for the two directions of a reversible
reaction. Reactions that are catalyzed by only one enzyme can go to equilibrium, stalling the reaction. In
contrast, if two different enzymes (each specific for a given direction) are necessary for a reversible reaction, the
opportunity to control the rate of the reaction increases, and equilibrium is not reached.
A number of enzymes involved in each of the pathways—in particular, the enzyme catalyzing the first committed
reaction of the pathway—are controlled by attachment of a molecule to an allosteric site on the protein.
The molecules most commonly used in this capacity are the nucleotides ATP, ADP, AMP, NAD + , and NADH.
These regulators—allosteric effectors—may increase or decrease enzyme activity, depending on the prevailing
conditions. The allosteric effector alters the steric structure of the enzyme, usually affecting the configuration of
the active site. This alteration of the protein’s (the enzyme’s) structure either increases or decreases its affinity
for its substrate, with the effect of increasing or decreasing the rate of the reaction. The attachment signals to
the enzyme. This binding can increase or decrease the enzyme’s activity, providing a feedback mechanism. This
feedback type of control is effective as long as the chemical affecting it is attached to the enzyme. Once the
overall concentration of the chemical decreases, it will diffuse away from the protein, and the control is relaxed.
Control of Catabolic Pathways
Enzymes, proteins, electron carriers, and pumps that play roles in glycolysis, the citric acid cycle, and the
electron transport chain tend to catalyze nonreversible reactions. In other words, if the initial reaction takes place,
the pathway is committed to proceeding with the remaining reactions. Whether a particular enzyme activity is
released depends upon the energy needs of the cell (as reflected by the levels of ATP, ADP, and AMP).
Glycolysis
The control of glycolysis begins with the first enzyme in the pathway, hexokinase (Figure 7.19). This enzyme
catalyzes the phosphorylation of glucose, which helps to prepare the compound for cleavage in a later step. The
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221
presence of the negatively charged phosphate in the molecule also prevents the sugar from leaving the cell.
When hexokinase is inhibited, glucose diffuses out of the cell and does not become a substrate for the respiration
pathways in that tissue. The product of the hexokinase reaction is glucose-6-phosphate, which accumulates
when a later enzyme, phosphofructokinase, is inhibited.
*
Regulatory step
Regulatory step
H
H-C-OH
/ CH -°\ H
OH H , c
Yc
Glucose
ATP ADP
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H-C-OH
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Phosphoglucose
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H-C-O-PsO
h 6-
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H-C-OH
i
c=o
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H-C-OHo- kinase
H-C-O-PsO
h 6-
H ?'
H-C-0-P=0
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HO-C-H Fructose
H -C-OH bisphosphate
• aldolase
H-C-OH
H ?'
H-C-0-P=0
C*0 6'
I
ho -c- H Dihydroxyacetone-
h phosphate
Glucose-6-phosphate Fructose-6-phosphate
?■
H-C-0-P=0
h i-
Fructose-1,6-bi phosphate
It
Triose
phosphate
isomerase
Glyceraldehyde-
3-phosphate
dehydrogenase
H
c=o
H-C-OH o'
i i
H-C~0-P=0
• i
H O'
NAD
Glyceraldehyde-
3-phosphate
/O'
c
-c=o
H-C-H
H
Pyruvate
ATP ADP o
XL
Pyruvate
kinase
C
h' n h
O'
o-p=o
4 -
Enolase
V°'
9 9
H00-P=0
H-C-OHO
H
2X
ATP ADP
_ ^_ H-C-OHO' ,yy
Phosphoglycerate h -6-0 -p=0 Pbosphoglycerate
mutase ^ kinase
+ P,
NADH + H +
V
O'
o -p«o
6
c=o
H-C-OH (p'
Phosphoenolpyruvate
(PEP)
10 9
Regulatory step
2-Phosphoglycerate 3-Phosphoglycerate
• •
H-C-0-P=0
H O-
1,3-Bisphosphoglycerate
Figure 7.19 The glycolysis pathway is primarily regulated at the three key enzymatic steps (1, 2, and 7) as indicated.
Note that the first two steps that are regulated occur early in the pathway and involve hydrolysis of ATP.
Phosphofructokinase is the main enzyme controlled in glycolysis. High levels of ATP or citrate or a lower, more
acidic pH decreases the enzyme’s activity. An increase in citrate concentration can occur because of a blockage
in the citric acid cycle. Fermentation, with its production of organic acids such as lactic acid, frequently accounts
for the increased acidity in a cell; however, the products of fermentation do not typically accumulate in cells.
The last step in glycolysis is catalyzed by pyruvate kinase. The pyruvate produced can proceed to be catabolized
or converted into the amino acid alanine. If no more energy is needed and alanine is in adequate supply, the
enzyme is inhibited. The enzyme’s activity is increased when fructose-1,6-bisphosphate levels increase. (Recall
that fructose-1,6-bisphosphate is an intermediate in the first half of glycolysis.) The regulation of pyruvate kinase
involves phosphorylation by a kinase (pyruvate kinase), resulting in a less-active enzyme. Dephosphorylation by
a phosphatase reactivates it. Pyruvate kinase is also regulated by ATP (a negative allosteric effect).
If more energy is needed, more pyruvate will be converted into acetyl CoA through the action of pyruvate
dehydrogenase. If either acetyl groups or NADH accumulates, there is less need for the reaction, and the rate
decreases. Pyruvate dehydrogenase is also regulated by phosphorylation: a kinase phosphorylates it to form an
inactive enzyme, and a phosphatase reactivates it. The kinase and the phosphatase are also regulated.
Citric Acid Cycle
The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two
molecules of NADH (Figure 7.9). These enzymes are isocitrate dehydrogenase and a-ketoglutarate
dehydrogenase. When adequate ATP and NADH levels are available, the rates of these reactions decrease.
When more ATP is needed, as reflected in rising ADP levels, the rate increases. Alpha-ketoglutarate
dehydrogenase will also be affected by the levels of succinyl CoA—a subsequent intermediate in the
cycle—causing a decrease in activity. A decrease in the rate of operation of the pathway at this point is not
necessarily negative, as the increased levels of the a-ketoglutarate not used by the citric acid cycle can be used
by the cell for amino acid (glutamate) synthesis.
Electron Transport Chain
Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron
transport through the pathway is affected by the levels of ADP and ATP. Greater ATP consumption by a cell is
indicated by a buildup of ADP. As ATP usage decreases, the concentration of ADP decreases, and now, ATP
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Chapter 7 | Cellular Respiration
begins to build up in the cell. This change in the relative concentration of ADP to ATP triggers the cell to slow
down the electron transport chain.
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For a summary of feedback controls in cellular respiration, see Table 7.1.
Summary of Feedback Controls in Cellular Respiration
Pathway
Enzyme
affected
Effect on
Elevated levels of effector pathway
activity
glycolysis
hexokinase
glucose-6-phosphate
decrease
phosphofructokinase
low-energy charge (ATP, AMP),
fructose-6-phosphate via
fructose-2,6-bisphosphate
increase
high-energy charge (ATP, AMP), citrate, acidic
PH
decrease
pyruvate kinase
fructose-1,6-bisphosphate
increase
high-energy charge (ATP, AMP), alanine
decrease
pyruvate to acetyl
CoA conversion
pyruvate
dehydrogenase
ADP, pyruvate
increase
acetyl CoA, ATP, NADH
decrease
citric acid cycle
isocitrate
dehydrogenase
ADP
increase
ATP, NADH
decrease
a-ketoglutarate
dehydrogenase
calcium ions, ADP
increase
ATP, NADH, succinyl CoA
decrease
electron transport
chain
ADP
increase
ATP
decrease
Table 7.1
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Chapter 7 | Cellular Respiration
223
KEY TERMS
acetyl CoA combination of an acetyl group derived from pyruvic acid and coenzyme A, which is made from
pantothenic acid (a B-group vitamin)
aerobic respiration process in which organisms convert energy in the presence of oxygen
anaerobic process that does not use oxygen
anaerobic cellular respiration process in which organisms convert energy for their use in the absence of
oxygen
ATP synthase (also FIFO ATP synthase) membrane-embedded protein complex that adds a phosphate to ADP
with energy from protons diffusing through it
chemiosmosis process in which there is a production of adenosine triphosphate (ATP) in cellular metabolism
by the involvement of a proton gradient across a membrane
citric acid cycle (also Krebs cycle) series of enzyme-catalyzed chemical reactions of central importance in all
living cells for extraction of energy from carbohydrates
dephosphorylation removal of a phosphate group from a molecule
fermentation process of regenerating NAD + with either an inorganic or organic compound serving as the final
electron acceptor; occurs in the absence of oxygen
GLUT protein integral membrane protein that transports glucose
glycolysis process of breaking glucose into two three-carbon molecules with the production of ATP and NADH
isomerase enzyme that converts a molecule into its isomer
Krebs cycle (also citric acid cycle) alternate name for the citric acid cycle, named after Hans Krebs, who first
identified the steps in the pathway in the 1930s in pigeon flight muscles; see citric acid cycle
oxidative phosphorylation production of ATP using the process of chemiosmosis in the presence of oxygen
phosphorylation addition of a high-energy phosphate to a compound, usually a metabolic intermediate, a
protein, or ADP
prosthetic group (also prosthetic cofactor) molecule bound to a protein that facilitates the function of the
protein
pyruvate three-carbon sugar that can be decarboxylated and oxidized to make acetyl CoA, which enters the
citric acid cycle under aerobic conditions; the end product of glycolysis
redox reaction chemical reaction that consists of the coupling of an oxidation reaction and a reduction reaction
substrate-level phosphorylation production of ATP from ADP using the excess energy from a chemical
reaction and a phosphate group from a reactant
TCA cycle (also citric acid cycle) alternate name for the citric acid cycle, named after the group name for citric
acid, tricarboxylic acid (TCA); see citric acid cycle
ubiquinone soluble electron transporter in the electron transport chain that connects the first or second complex
to the third
CHAPTER SUMMARY
7.1 Energy in Living Systems
ATP functions as the energy currency for cells. It allows the cell to store energy briefly and transport it within
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Chapter 7 | Cellular Respiration
the cell to support endergonic chemical reactions. The structure of ATP is that of an RNA nucleotide with three
phosphates attached. As ATP is used for energy, a phosphate group or two are detached, and either ADP or
AMP is produced. Energy derived from glucose catabolism is used to convert ADP into ATP. When ATP is used
in a reaction, the third phosphate is temporarily attached to a substrate in a process called phosphorylation.
The two processes of ATP regeneration that are used in conjunction with glucose catabolism are substrate-
level phosphorylation and oxidative phosphorylation through the process of chemiosmosis.
7.2 Glycolysis
Glycolysis is the first pathway within the cytoplasm used in the breakdown of glucose to extract energy. It was
probably one of the earliest metabolic pathways to evolve and is used by nearly all of the organisms on Earth.
Glycolysis consists of two parts: The first part prepares the six-carbon ring of glucose for cleavage into two
three-carbon sugars. ATP is invested in the process during this half to energize the separation. The second half
of glycolysis extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD + . Two
ATP molecules are invested in the first half and four ATP molecules are formed by substrate phosphorylation
during the second half. This produces a net gain of two ATP and two NADH molecules for the cell.
7.3 Oxidation of Pyruvate and the Citric Acid Cycle
in the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of
coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is
delivered to the citric acid cycle for further catabolism. During the conversion of pyruvate into the acetyl group,
a molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide accounts for two
(conversion of two pyruvate molecules) of the six carbons of the original glucose molecule. The electrons are
picked up by NAD + , and the NADH carries the electrons to a later pathway for ATP production. At this point, the
glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential
energy stored within the glucose molecule has been transferred to electron carriers or has been used to
synthesize a few ATPs.
The citric acid cycle is a series of redox and decarboxylation reactions that removes high-energy electrons and
carbon dioxide. The electrons, temporarily stored in molecules of NADH and FADH 2 , are used to generate ATP
in a subsequent pathway. One molecule of either GTP or ATP is produced by substrate-level phosphorylation
on each turn of the cycle. There is no comparison of the cyclic pathway with a linear one.
7.4 Oxidative Phosphorylation
The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron
acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron
transport chain is composed of four large, multiprotein complexes embedded in the inner mitochondrial
membrane and two small diffusible electron carriers shuttling electrons between them. The electrons are
passed through a series of redox reactions, with a small amount of free energy used at three points to transport
hydrogen ions across a membrane. This process contributes to the gradient used in chemiosmosis. The
electrons passing through the electron transport chain gradually lose energy. High-energy electrons donated to
the chain by either NADH or FADH 2 complete the chain, as low-energy electrons reduce oxygen molecules and
form water. The level of free energy of the electrons drops from about 60 kcal/mol in NADH or 45 kcal/mol in
FADH 2 to about 0 kcal/mol in water. The end products of the electron transport chain are water and ATP. A
number of intermediate compounds of the citric acid cycle can be diverted into the anabolism of other
biochemical molecules, such as nonessential amino acids, sugars, and lipids. These same molecules can
serve as energy sources for the glucose pathways.
7.5 Metabolism without Oxygen
If NADH cannot be oxidized through aerobic respiration, another electron acceptor is used. Most organisms will
use some form of fermentation to accomplish the regeneration of NAD + , ensuring the continuation of glycolysis.
The regeneration of NAD + in fermentation is not accompanied by ATP production; therefore, the potential of
NADH to produce ATP using an electron transport chain is not utilized.
7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways
The breakdown and synthesis of carbohydrates, proteins, and lipids connect with the pathways of glucose
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Chapter 7 | Cellular Respiration
225
catabolism. The simple sugars are galactose, fructose, glycogen, and pentose. These are catabolized during
glycolysis. The amino acids from proteins connect with glucose catabolism through pyruvate, acetyl CoA, and
components of the citric acid cycle. Cholesterol synthesis starts with acetyl groups, and the components of
triglycerides come from glycerol-3-phosphate from glycolysis and acetyl groups produced in the mitochondria
from pyruvate.
7.7 Regulation of Cellular Respiration
Cellular respiration is controlled by a variety of means. The entry of glucose into a cell is controlled by the
transport proteins that aid glucose passage through the cell membrane. Most of the control of the respiration
processes is accomplished through the control of specific enzymes in the pathways. This is a type of negative
feedback mechanism, turning the enzymes off. The enzymes respond most often to the levels of the available
nucleosides ATP, ADP, AMP, NAD + , and FAD. Other intermediates of the pathway also affect certain enzymes
in the systems.
VISUAL CONNECTION QUESTIONS
1. Figure 7.11 Dinitrophenol (DNP) is an "uncoupler"
that makes the inner mitochondrial membrane "leaky"
to protons. It was used until 1938 as a weight-loss
drug. What effect would you expect DNP to have on
the change in pH across the inner mitochondrial
membrane? Why do you think this might be an
effective weight-loss drug?
2. Figure 7.12 Cyanide inhibits cytochrome c
oxidase, a component of the electron transport chain.
If cyanide poisoning occurs, would you expect the pH
of the intermembrane space to increase or
REVIEW QUESTIONS
4. The energy currency used by cells is_.
a. ATP
b. ADP
c. AMP
d. adenosine
5. A reducing chemical reaction_.
a. reduces the compound to a simpler form
b. adds an electron to the substrate
c. removes a hydrogen atom from the
substrate
d. is a catabolic reaction
6. During the second half of glycolysis, what occurs?
a. ATP is used up.
b. Fructose is split in two.
c. ATP is made.
d. Glucose becomes fructose.
7. What is removed from pyruvate during its
conversion into an acetyl group?
a. oxygen
b. ATP
c. B vitamin
d. carbon dioxide
8. What do the electrons added to NAD + do?
decrease? What effect would cyanide have on ATP
synthesis?
3. (Figure 7.14) Tremetol, a metabolic poison found
in the white snake root plant, prevents the
metabolism of lactate. When cows eat this plant,
tremetol is concentrated in the milk they produce.
Humans who consume the milk can become
seriously ill. Symptoms of this disease, which include
vomiting, abdominal pain, and tremors, become
worse after exercise. Why do you think this is the
case?
a. They become part of a fermentation
pathway.
b. They go to another pathway for ATP
production.
c. They energize the entry of the acetyl group
into the citric acid cycle.
d. They are converted to NADP.
9. GTP or ATP is produced during the conversion of
a. isocitrate into a-ketoglutarate
b. succinyl CoA into succinate
c. fumarate into malate
d. malate into oxaloacetate
10. How many NADH molecules are produced on
each turn of the citric acid cycle?
a. one
b. two
c. three
d. four
11. What compound receives electrons from NADH?
a. FMN
b. ubiquinone
c. cytochrome ci
d. oxygen
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Chapter 7 | Cellular Respiration
12. Chemiosmosis involves_.
a. the movement of electrons across the cell
membrane
b. the movement of hydrogen atoms across a
mitochondrial membrane
c. the movement of hydrogen ions across a
mitochondrial membrane
d. the movement of glucose through the cell
membrane
13. Which of the following fermentation methods can
occur in animal skeletal muscles?
a. lactic acid fermentation
b. alcohol fermentation
c. mixed acid fermentation
d. propionic fermentation
14. A major connection for sugars in glycolysis is
a. glucose-6-phosphate
b. fructose-1,6-bisphosphate
c. dihydroxyacetone phosphate
d. phosphoenolpyruvate
CRITICAL THINKING QUESTIONS
18. Why is it beneficial for cells to use ATP rather
than energy directly from the bonds of
carbohydrates? What are the greatest drawbacks to
harnessing energy directly from the bonds of several
different compounds?
19. Nearly all organisms on Earth carry out some
form of glycolysis. How does this fact support or not
support the assertion that glycolysis is one of the
oldest metabolic pathways?
20. Because they lose their mitochondria during
development, red blood cells cannot perform aerobic
respiration; however, they do perform glycolysis in
the cytoplasm. Why do all cells need an energy
source, and what would happen if glycolysis were
blocked in a red blood cell?
21. What is the primary difference between a circular
pathway and a linear pathway?
15. Beta-oxidation is_.
a. the breakdown of sugars
b. the assembly of sugars
c. the breakdown of fatty acids
d. the removal of amino groups from amino
acids
16. The effect of high levels of ADP is to_in
cellular respiration.
a. increase the activity of specific enzymes
b. decrease the activity of specific enzymes
c. have no effect on the activity of specific
enzymes
d. slow down the pathway
17. The control of which enzyme exerts the most
control on glycolysis?
a. hexokinase
b. phosphofructokinase
c. glucose-6-phosphatase
d. aldolase
22. How do the roles of ubiquinone and cytochrome c
differ from the roles of the other components of the
electron transport chain?
23. What accounts for the different number of ATP
molecules that are formed through cellular
respiration?
24. What is the primary difference between
fermentation and anaerobic respiration?
25. Would you describe metabolic pathways as
inherently wasteful or inherently economical? Why?
26. How does citrate from the citric acid cycle affect
glycolysis?
27. Why might negative feedback mechanisms be
more common than positive feedback mechanisms in
living cells?
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Chapter 8 | Photosynthesis
227
8 | PHOTOSYNTHESIS
Figure 8.1 This world map shows Earth’s distribution of photosynthetic activity determined by chlorophyll a
concentrations. On land, chlorophyll is evident from terrestrial plants, and within oceanic zones, from chlorophyll
from phytoplankton, (credit: modification of work by SeaWiFS Project, NASA/Goddard Space Flight Center and
ORBIMAGE)
Chapter Outline
8.1: Overview of Photosynthesis
8.2: The Light-Dependent Reactions of Photosynthesis
8.3: Using Light Energy to Make Organic Molecules
Introduction
The metabolic processes in all organisms—from bacteria to humans—require energy. To get this energy, many
organisms access stored energy by eating, that is, by ingesting other organisms. But where does the stored
energy in food originate? All of this energy can be traced back to photosynthesis.
8.1 1 Overview of Photosynthesis
By the end of this section, you will be able to do the following:
• Explain the significance of photosynthesis to other living organisms
• Describe the main structures involved in photosynthesis
• Identify the substrates and products of photosynthesis
Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological
process that can capture energy that originates from sunlight and converts it into chemical compounds
228
Chapter 8 | Photosynthesis
(carbohydrates) that every organism uses to power its metabolism. It is also a source of oxygen necessary for
many living organisms. In brief, the energy of sunlight is “captured” to energize electrons, whose energy is then
stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The
energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and
stored by photosynthesis 350 to 200 million years ago during the Carboniferous Period.
Plants, algae, and a group of bacteria called cyanobacteria are the only organisms capable of performing
photosynthesis (Figure 8.2). Because they use light to manufacture their own food, they are called
photoautotrophs (literally, “self-feeders using light”). Other organisms, such as animals, fungi, and most other
bacteria, are termed heterotrophs (“other feeders”), because they must rely on the sugars produced by
photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars,
not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds. For this reason,
they are referred to as chemoautotrophs.
(a)
(b)
(c)
(d) (e)
Figure 8.2 Photoautotrophs including (a) plants, (b) algae, and (c) cyanobacteria synthesize their organic compounds
via photosynthesis using sunlight as an energy source. Cyanobacteria and planktonic algae can grow over enormous
areas in water, at times completely covering the surface. In a (d) deep sea vent, chemoautotrophs, such as these
(e) thermophilic bacteria, capture energy from inorganic compounds to produce organic compounds. The ecosystem
surrounding the vents has a diverse array of animals, such as tubeworms, crustaceans, and octopuses that derive
energy from the bacteria, (credit a: modification of work by Steve Hillebrand, U.S. Fish and Wildlife Service; credit b:
modification of work by "eutrophication&hypoxia'VFlickr; credit c: modification of work by NASA; credit d: University of
Washington, NOAA; credit e: modification of work by Mark Amend, West Coast and Polar Regions Undersea Research
Center, UAF, NOAA)
The importance of photosynthesis is not just that it can capture sunlight’s energy. After all, a lizard sunning itself
on a cold day can use the sun’s energy to warm up in a process called behavioral thermoregulation. In contrast,
photosynthesis is vital because it evolved as a way to store the energy from solar radiation (the “photo-” part) to
energy in the carbon-carbon bonds of carbohydrate molecules (the “-synthesis” part). Those carbohydrates are
the energy source that heterotrophs use to power the synthesis of ATP via respiration. Therefore, photosynthesis
powers 99 percent of Earth’s ecosystems. When a top predator, such as a wolf, preys on a deer (Figure 8.3),
the wolf is at the end of an energy path that went from nuclear reactions on the surface of the sun, to visible light,
to photosynthesis, to vegetation, to deer, and finally to the wolf.
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Chapter 8 | Photosynthesis
229
Figure 8.3 The energy stored in carbohydrate molecules from photosynthesis passes through the food chain. The
predator that eats these deer receives a portion of the energy that originated in the photosynthetic vegetation that the
deer consumed, (credit: modification of work by Steve VanRiper, U.S. Fish and Wildlife Service)
Main Structures and Summary of Photosynthesis
Photosynthesis is a multi-step process that requires specific wavelengths of visible sunlight, carbon dioxide
(which is low in energy), and water as substrates (Figure 8.4). After the process is complete, it releases oxygen
and produces glyceraldehyde-3-phosphate (GA3P), as well as simple carbohydrate molecules (high in energy)
that can then be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar
molecules contain energy and the energized carbon that all living things need to survive.
Sugars made
Carbon dioxide
Water
Oxygen
Sunlight
PHOTOSYNTHESIS
Figure 8.4 Photosynthesis uses solar energy, carbon dioxide, and water to produce energy-storing carbohydrates.
Oxygen is generated as a waste product of photosynthesis.
The following is the chemical equation for photosynthesis (Figure 8.5):
230
Chapter 8 | Photosynthesis
Photosynthesis Equation
Carbon
dioxide
SUNLIGHT
+
Water
> Sugar
+
Oxygen
/
6C0 2
6H 2 0
C 6 Hi 2 0 6
60 2
Figure 8.5 The basic equation for photosynthesis is deceptively simple. In reality, the process takes place in many
steps involving intermediate reactants and products. Glucose, the primary energy source in cells, is made from two
three-carbon GA3Ps.
Although the equation looks simple, the many steps that take place during photosynthesis are actually quite
complex. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become
familiar with the structures involved.
Basic Photosynthetic Structures
In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells. The process
of photosynthesis occurs in a middle layer called the mesophyll. The gas exchange of carbon dioxide and
oxygen occurs through small, regulated openings called stomata (singular: stoma), which also play roles in the
regulation of gas exchange and water balance. The stomata are typically located on the underside of the leaf,
which helps to minimize water loss due to high temperatures on the upper surface of the leaf. Each stoma is
flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response
to osmotic changes.
in all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. For plants,
chloroplast-containing cells exist mostly in the mesophyll. Chloroplasts have a double membrane envelope
(composed of an outer membrane and an inner membrane), and are ancestrally derived from ancient free-living
cyanobacteria. Within the chloroplast are stacked, disc-shaped structures called thylakoids. Embedded in the
thylakoid membrane is chlorophyll, a pigment (molecule that absorbs light) responsible for the initial interaction
between light and plant material, and numerous proteins that make up the electron transport chain. The thylakoid
membrane encloses an internal space called the thylakoid lumen. As shown in Figure 8.6, a stack of thylakoids
is called a granum, and the liquid-filled space surrounding the granum is called stroma or “bed” (not to be
confused with stoma or “mouth,” an opening on the leaf epidermis).
visual
CONNECTION
Intermembrane
space
Thylakoid
Stroma
(aqueous fluid)
Thylakoid lumen
Outer
membrane
Inner
membrane
Granum
(stack of
thylakoids)
Figure 8.6 Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane.
Stacks of thylakoids called grana form a third membrane layer.
On a hot, dry day, the guard cells of plants close their stomata to conserve water. What impact will this have
on photosynthesis?
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Chapter 8 | Photosynthesis
231
The Two Parts of Photosynthesis
Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent
reactions. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy
is converted into stored chemical energy, in the light-independent reactions, the chemical energy harvested
during the light-dependent reactions drives the assembly of sugar molecules from carbon dioxide. Therefore,
although the light-independent reactions do not use light as a reactant, they require the products of the light-
dependent reactions to function. In addition, however, several enzymes of the light-independent reactions are
activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These
are referred to as energy carriers. The energy carriers that move energy from light-dependent reactions to light-
independent reactions can be thought of as “full” because they are rich in energy. After the energy is released,
the “empty" energy carriers return to the light-dependent reaction to obtain more energy. Figure 8.7 illustrates
the components inside the chloroplast where the light-dependent and light-independent reactions take place.
Figure 8.7 Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle. Light-dependent
reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle,
which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO 2 .
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232
Chapter 8 | Photosynthesis
everyday CONNECTION
Photosynthesis at the Grocery Store
Figure 8.8 Foods that humans consume originate from photosynthesis, (credit: Associagao Brasileira de
Supermercados)
Major grocery stores in the United States are organized into departments, such as dairy, meats, produce,
bread, cereals, and so forth. Each aisle (Figure 8.8) contains hundreds, if not thousands, of different
products for customers to buy and consume.
Although there is a large variety, each item ultimately can be linked back to photosynthesis. Meats and dairy
link, because the animals were fed plant-based foods. The breads, cereals, and pastas come largely from
starchy grains, which are the seeds of photosynthesis-dependent plants. What about desserts and drinks?
All of these products contain sugar—sucrose is a plant product, a disaccharide, a carbohydrate molecule,
which is built directly from photosynthesis. Moreover, many items are less obviously derived from plants:
For instance, paper goods are generally plant products, and many plastics (abundant as products and
packaging) are derived from “algae” (unicellular plant-like organisms, and cyanobacteria). Virtually every
spice and flavoring in the spice aisle was produced by a plant as a leaf, root, bark, flower, fruit, or stem.
Ultimately, photosynthesis connects to every meal and every food a person consumes.
8.2 | The Light-Dependent Reactions of
Photosynthesis
By the end of this section, you will be able to do the following:
• Explain how plants absorb energy from sunlight
• Describe short and long wavelengths of light
• Describe how and where photosynthesis takes place within a plant
How can light energy be used to make food? When a person turns on a lamp, electrical energy becomes light
energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the
case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build basic
carbohydrate molecules (Figure 8.9). However, autotrophs only use a few specific wavelengths of sunlight.
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Figure 8.9 Photoautotrophs can capture visible light energy in specific wavelengths from the sun, converting it into the
chemical energy used to build food molecules, (credit: Gerry Atwell)
What Is Light Energy?
The sun emits an enormous amount of electromagnetic radiation (solar energy in a spectrum from very short
gamma rays to very long radio waves). Humans can see only a tiny fraction of this energy, which we refer to
as “visible light." The manner in which solar energy travels is described as waves. Scientists can determine the
amount of energy of a wave by measuring its wavelength (shorter wavelengths are more powerful than longer
wavelengths)—the distance between consecutive crest points of a wave. Therefore, a single wave is measured
from two consecutive points, such as from crest to crest or from trough to trough (Figure 8.10).
Figure 8.10 The wavelength of a single wave is the distance between two consecutive points of similar position (two
crests or two troughs) along the wave.
Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars.
Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The
electromagnetic spectrum is the range of all possible frequencies of radiation (Figure 8.11). The difference
between wavelengths relates to the amount of energy carried by them.
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Figure 8.11 The sun emits energy in the form of electromagnetic radiation. This radiation exists at different
wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is
characterized by its wavelength.
Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength, the less
energy it carries. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a
piece of moving heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope
move in short, tight waves, a person would need to apply significantly more energy.
The electromagnetic spectrum (Figure 8.11) shows several types of electromagnetic radiation originating from
the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage
cells and DNA, which explains why both X-rays and UV rays can be harmful to living organisms.
Absorption of Light
Light energy initiates the process of photosynthesis when pigments absorb specific wavelengths of visible light.
Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy
levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an
orbital electron to a excited (quantum) state. Energy levels higher than those in blue light will physically tear
the molecules apart, in a process called bleaching. Our retinal pigments can only “see” (absorb) wavelengths
between 700 nm and 400 nm of light, a spectrum that is therefore called visible light. For the same reasons,
plants, pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm; plant physiologists
refer to this range for plants as photosynthetically active radiation.
The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as
a prism or a drop of water, disperse white light to reveal the colors to the human eye. The visible light portion
of the electromagnetic spectrum shows the rainbow of colors, with violet and blue having shorter wavelengths,
and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have
lower energy (Figure 8.12).
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YELLOW
GREEN AAAAAA
blue vAAAAAA/
indigo < /\f\f\j\/\f\y\/\ J ,
violet AAAAAAAAAA
Longer wavelength
Shorter wavelength
Less energy
More energy
Figure 8.12 The colors of visible light do not carry the same amount of energy. Violet has the shortest wavelength and
therefore carries the most energy, whereas red has the longest wavelength and carries the least amount of energy,
(credit: modification of work by NASA)
Understanding Pigments
Different kinds of pigments exist, and each absorbs only specific wavelengths (colors) of visible light. Pigments
reflect or transmit the wavelengths they cannot absorb, making them appear a mixture of the reflected or
transmitted light colors.
Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae;
each class has multiple types of pigment molecules. There are five major chlorophylls: a, b, c and d and a related
molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are found in higher
plant chloroplasts and will be the focus of the following discussion.
With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in
fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange
peel (P-carotene)—are used as advertisements to attract seed dispersers. In photosynthesis, carotenoids
function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a
leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy;
if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the
thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.
Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light: This
is termed the absorption spectrum. The graph in Figure 8.13 shows the absorption spectra for chlorophyll
a, chlorophyll b, and a type of carotenoid pigment called p-carotene (which absorbs blue and green light).
Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of
absorption. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not
green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-
wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.
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(C) (d)
Figure 8.13 (a) Chlorophyll a, (b) chlorophyll b, and (c) /3-carotene are hydrophobic organic pigments found in
the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are
responsible for the green color of leaves. /3-carotene is responsible for the orange color in carrots. Each pigment has
(d) a unique absorbance spectrum.
Many photosynthetic organisms have a mixture of pigments, and by using these pigments, the organism can
absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight.
Some organisms grow underwater where light intensity and quality decrease and change with depth. Other
organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that
comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation
(Figure 8.14).
Figure 8.14 Plants that commonly grow in the shade have adapted to low levels of light by changing the relative
concentrations of their chlorophyll pigments, (credit: Jason Hollinger)
When studying a photosynthetic organism, scientists can determine the types of pigments present by generating
absorption spectra. An instrument called a spectrophotometer can differentiate which wavelengths of light a
substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By
extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify
which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments
include various types of chromatography that separate the pigments by their relative affinities to solid and mobile
phases.
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How Light-Dependent Reactions Work
The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form
of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of
sugar molecules. The light-dependent reactions are depicted in Figure 8.15. Protein complexes and pigment
molecules work together to produce NADPH and ATP. The numbering of the photosystems is derived from the
order in which they were discovered, not in the order of the transfer of electrons.
(a) Photosystem II (P680)
Thylakoid .
membrane
Reaction primary ijqht
Pigment center electron
mnlon iloc \ _
molecules
Light
harvesting''
complex H2 q 1/2 o 2 + 2H +
Thylakoid lumen
(b) Photosystem I (P700)
Light
Figure 8.15 A photosystem consists of 1) a light-harvesting complex and 2) a reaction center. Pigments in the light¬
harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites
an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then
be replaced. In (a) photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste
product. In (b) photosystem I, the electron comes from the chloroplast electron transport chain discussed below.
The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a
photosystem, two types of which are found embedded in the thylakoid membrane: photosystem II (PSII) and
photosystem I (PSI) (Figure 8.16). The two complexes differ on the basis of what they oxidize (that is, the
source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized
electrons).
Both photosystems have the same basic structure; a number of antenna proteins to which the chlorophyll
molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is
serviced by the light-harvesting complex, which passes energy from sunlight to the reaction center; it consists
of multiple antenna proteins that contain a mixture of 300 to 400 chlorophyll a and b molecules as well as other
pigments like carotenoids. The absorption of a single photon or distinct quantity or “packet" of light by any of
the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured
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by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to
chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this
point, only energy has been transferred between molecules, not electrons.
visual
CONNECTION
Stroma
Electron
transport chain
H +
Figure 8.16 In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from
water. The electrons travel through the chloroplast electron transport chain to photosystem I (PSI), which reduces
NADP + to NADPH. The electron transport chain moves protons across the thylakoid membrane into the lumen. At
the same time, splitting of water adds protons to the lumen, and reduction of NADPH removes protons from the
stroma. The net result is a low pH in the thylakoid lumen, and a high pH in the stroma. ATP synthase uses this
electrochemical gradient to make ATP.
What is the initial source of electrons for the chloroplast electron transport chain?
a. water
b. oxygen
c. carbon dioxide
d. NADPH
The reaction center contains a pair of chlorophyll a molecules with a special property. Those two chlorophylls
can undergo oxidation upon excitation; they can actually give up an electron in a process called a photoact. It
is at this step in the reaction center during photosynthesis that light energy is converted into an excited electron.
All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin
cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate. PSII
and PSI are two major components of the photosynthetic electron transport chain, which also includes the
cytochrome complex. The cytochrome complex, an enzyme composed of two protein complexes, transfers the
electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus enabling both the
transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI.
The reaction center of PSII (called P680) delivers its high-energy electrons, one at the time, to the primary
electron acceptor, and through the electron transport chain (Pq to cytochrome complex to plastocyanine) to
PSI. P680’s missing electron is replaced by extracting a low-energy electron from water; thus, water is “split”
during this stage of photosynthesis, and PSII is re-reduced after every photoact. Splitting one H 2 O molecule
releases two electrons, two hydrogen atoms, and one atom of oxygen. However, splitting two molecules is
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239
required to form one molecule of diatomic O 2 gas. About 10 percent of the oxygen is used by mitochondria in the
leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic
organisms to support respiration.
As electrons move through the proteins that reside between PSII and PSI, they lose energy. This energy is used
to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms,
plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP
in a later step. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by
PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center
(called P700). P700 is oxidized and sends a high-energy electron to NADP + to form NADPH. Thus, PSII captures
the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP + into NADPH.
The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the
production of ATP. Other mechanisms exist to fine-tune that ratio to exactly match the chloroplast’s constantly
changing energy needs.
Generating an Energy Carrier: ATP
As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions
inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high
concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in
the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because
they all have the same electrical charge, repelling each other.
To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a
dam. in the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase.
The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to
ADP, which forms a molecule of ATP (Figure 8.16). The flow of hydrogen ions through ATP synthase is called
chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-
permeable structure of the thylakoid.
Visit this site (http:// 0 penstaxc 0 llege. 0 rg/l/light_reacti 0 ns) and click through the animation to view the
process of photosynthesis within a leaf.
8.3 | Using Light Energy to Make Organic Molecules
By the end of this section, you will be able to do the following:
• Describe the Calvin cycle
• Define carbon fixation
• Explain how photosynthesis works in the energy cycle of all living organisms
After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH
molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. The
products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds,
whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can
survive almost indefinitely. The carbohydrate molecules made will have a backbone of carbon atoms. But where
does the carbon come from? it comes from carbon dioxide—the gas that is a waste product of respiration in
microbes, fungi, plants, and animals.
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The Calvin Cycle
In plants, carbon dioxide (CO 2 ) enters the leaves through stomata, where it diffuses over short distances through
intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO 2 diffuses into the stroma
of the chloroplast—the site of light-independent reactions of photosynthesis. These reactions actually have
several names associated with them. Another term, the Calvin cycle, is named for the man who discovered
it, and because these reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name
of another scientist involved in its discovery. The most outdated name is “dark reaction,” because light is not
directly required (Figure 8.17). However, the term dark reaction can be misleading because it implies incorrectly
that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no
longer use it.
Light-independent reactions
Calvin cycle
NADP* + H' NADPH
RuBP
ATP
synthase
NADPH
Thylakoid lumen
NADP*+ H'
Thylakoid membrane
Stroma
Figure 8.17 Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy¬
carrying molecules are made in the stroma where carbon fixation takes place.
The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction,
and regeneration.
Stage 1: Fixation
In the stroma, in addition to CO 2 , two other components are present to initiate the light-independent reactions: an
enzyme called ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBisCO), and three molecules of ribulose
bisphosphate (RuBP), as shown in Figure 8.18. RuBP has five atoms of carbon, flanked by two phosphates.
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visual
CONNECTION
1/2 molecule glucose
Figure 8.18 The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into
an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In
stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon
dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-
carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.
Which of the following statements is true?
a. In photosynthesis, oxygen, carbon dioxide, ATP, and NADPH are reactants. GA3P and water are
products.
b. In photosynthesis, chlorophyll, water, and carbon dioxide are reactants. GA3P and oxygen are
products.
c. In photosynthesis, water, carbon dioxide, ATP, and NADPH are reactants. RuBP and oxygen are
products.
d. In photosynthesis, water and carbon dioxide are reactants. GA3P and oxygen are products.
RuBisCO catalyzes a reaction between CO 2 and RuBP. For each CO 2 molecule that reacts with one RuBP,
two molecules of another compound 3-phospho glyceric acid (3-PGA) form. PGA has three carbons and one
phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules
of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the
reactions (3 C atoms from 3 CO 2 + 15 C atoms from 3RuBP = 18 C atoms in 6 molecules of 3-PGA). This process
is called carbon fixation, because CO 2 is “fixed" from an inorganic form into organic molecules.
Stage 2: Reduction
ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called
glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it involves the gain of electrons by
3-PGA. (Recall that a reduction is the gain of an electron by an atom or molecule.) Six molecules of both ATP
and NADPH are used. For ATP, energy is released with the loss of the terminal phosphate atom, converting
it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP + . Both of these
molecules return to the nearby light-dependent reactions to be reused and re-energized.
Stage 3: Regeneration
Interestingly, at this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the cytoplasm
to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the
chloroplast has three carbon atoms, it takes three “turns" of the Calvin cycle to fix enough net carbon to export
one G3P. But each turn makes two G3Ps, thus three turns make six G3Ps. One is exported while the remaining
five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare
for more CO 2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.
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LINK
T &
LEARNING
This link (http:// 0 penstaxc 0 llege. 0 rg/l/calvin_cycle) leads to an animation of the Calvin cycle. Click stage
1, stage 2, and then stage 3 to see G3P and ATP regenerate to form RuBP.
e olution CONNECTION
Photosynthesis
During the evolution of photosynthesis, a major shift occurred from the bacterial type of photosynthesis
that involves only one photosystem and is typically anoxygenic (does not generate oxygen) into modern
oxygenic (does generate oxygen) photosynthesis, employing two photosystems. This modern oxygenic
photosynthesis is used by many organisms—from giant tropical leaves in the rainforest to tiny
cyanobacterial cells—and the process and components of this photosynthesis remain largely the same.
Photosystems absorb light and use electron transport chains to convert energy into the chemical energy of
ATP and NADH. The subsequent light-independent reactions then assemble carbohydrate molecules with
this energy.
In the harsh dry heat of the desert, plants must conserve every drop of water must be used to survive.
Because stomata must open to allow for the uptake of CO 2 , water escapes from the leaf during active
photosynthesis. Desert plants have evolved processes to conserve water and deal with harsh conditions.
Mechanisms to capture and store CO 2 allows plants to adapt to living with less water. Some plants such as
cacti (Figure 8.19) can prepare materials for photosynthesis during the night by a temporary carbon fixation/
storage process, because opening the stomata at this time conserves water due to cooler temperatures.
During the day cacti use the captured CO 2 for photosynthesis, and keep their stomata closed.
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Chapter 8 | Photosynthesis
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Figure 8.19 The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-
independent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve
water and energy, (credit: Piotr Wojtkowski)
The Energy Cycle
Whether the organism is a bacterium, plant, or animal, all living things access energy by breaking down
carbohydrate and other carbon-rich organic molecules. But if plants make carbohydrate molecules, why would
they need to break them down, especially when it has been shown that the gas organisms release as a “waste
product” (CO 2 ) acts as a substrate for the formation of more food in photosynthesis? Remember, living things
need energy to perform life functions. In addition, an organism can either make its own food or eat another
organism—either way, the food still needs to be broken down. Finally, in the process of breaking down food,
called cellular respiration, heterotrophs release needed energy and produce “waste" in the form of CO 2 gas.
However, in nature, there is no such thing as “waste." Every single atom of matter and energy is conserved,
recycled over and over infinitely. Substances change form or move from one type of molecule to another, but
their constituent atoms never disappear (Figure 8.20).
In reality, CO 2 is no more a form of waste than oxygen is wasteful to photosynthesis. Both are byproducts
of reactions that move on to other reactions. Photosynthesis absorbs light energy to build carbohydrates in
chloroplasts, and aerobic cellular respiration releases energy by using oxygen to metabolize carbohydrates in
the cytoplasm and mitochondria. Both processes use electron transport chains to capture the energy necessary
to drive other reactions. These two powerhouse processes, photosynthesis and cellular respiration, function in
biological, cyclical harmony to allow organisms to access life-sustaining energy that originates millions of miles
away in a burning star humans call the sun.
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Figure 8.20 Photosynthesis consumes carbon dioxide and produces oxygen. Aerobic respiration consumes oxygen
and produces carbon dioxide. These two processes play an important role in the carbon cycle, (credit: modification of
work by Stuart Bassil)
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Chapter 8 | Photosynthesis
245
KEY TERMS
absorption spectrum range of wavelengths of electromagnetic radiation absorbed by a given substance
antenna protein pigment molecule that directly absorbs light and transfers the energy absorbed to other
pigment molecules
Calvin cycle light-independent reactions of photosynthesis that convert carbon dioxide from the atmosphere
into carbohydrates using the energy and reducing power of ATP and NADPH
carbon fixation process of converting inorganic CO 2 gas into organic compounds
carotenoid photosynthetic pigment (yellow-orange-red) that functions to dispose of excess energy
chemoautotroph organism that can build organic molecules using energy derived from inorganic chemicals
instead of sunlight
chlorophyll a form of chlorophyll that absorbs violet-blue and red light and consequently has a bluish-green
color; the only pigment molecule that performs the photochemistry by getting excited and losing an electron
to the electron transport chain
chlorophyll b accessory pigment that absorbs blue and red-orange light and consequently has a yellowish-
green tint
chloroplast organelle in which photosynthesis takes place
cytochrome complex group of reversibly oxidizable and reducible proteins that forms part of the electron
transport chain between photosystem II and photosystem I
electromagnetic spectrum range of all possible frequencies of radiation
electron transport chain group of proteins between PSII and PSI that pass energized electrons and use the
energy released by the electrons to move hydrogen ions against their concentration gradient into the
thylakoid lumen
granum stack of thylakoids located inside a chloroplast
heterotroph organism that consumes organic substances or other organisms for food
light harvesting complex complex that passes energy from sunlight to the reaction center in each
photosystem; it consists of multiple antenna proteins that contain a mixture of 300 to 400 chlorophyll a and
b molecules as well as other pigments like carotenoids
light-dependent reaction first stage of photosynthesis where certain wavelengths of the visible light are
absorbed to form two energy-carrying molecules (ATP and NADPH)
light-independent reaction second stage of photosynthesis, through which carbon dioxide is used to build
carbohydrate molecules using energy from ATP and NADPH
mesophyll middle layer of chlorophyll-rich cells in a leaf
P680 reaction center of photosystem II
P700 reaction center of photosystem I
photoact ejection of an electron from a reaction center using the energy of an absorbed photon
photoautotroph organism capable of producing its own organic compounds from sunlight
photon distinct quantity or “packet" of light energy
photosystem group of proteins, chlorophyll, and other pigments that are used in the light-dependent reactions
of photosynthesis to absorb light energy and convert it into chemical energy
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Chapter 8 | Photosynthesis
photosystem I integral pigment and protein complex in thylakoid membranes that uses light energy to transport
electrons from plastocyanin to NADP + (which becomes reduced to NADPH in the process)
photosystem II integral protein and pigment complex in thylakoid membranes that transports electrons from
water to the electron transport chain; oxygen is a product of PSII
pigment molecule that is capable of absorbing certain wavelengths of light and reflecting others (which
accounts for its color)
primary electron acceptor pigment or other organic molecule in the reaction center that accepts an energized
electron from the reaction center
reaction center complex of chlorophyll molecules and other organic molecules that is assembled around a
special pair of chlorophyll molecules and a primary electron acceptor; capable of undergoing oxidation and
reduction
reduction gain of electron(s) by an atom or molecule
spectrophotometer instrument that can measure transmitted light and compute the absorption
stoma opening that regulates gas exchange and water evaporation between leaves and the environment,
typically situated on the underside of leaves
stroma fluid-filled space surrounding the grana inside a chloroplast where the light-independent reactions of
photosynthesis take place
thylakoid disc-shaped, membrane-bound structure inside a chloroplast where the light-dependent reactions of
photosynthesis take place; stacks of thylakoids are called grana
thylakoid lumen aqueous space bound by a thylakoid membrane where protons accumulate during light-driven
electron transport
wavelength distance between consecutive points of equal position (two crests or two troughs) of a wave in a
graphic representation; inversely proportional to the energy of the radiation
CHAPTER SUMMARY
8.1 Overview of Photosynthesis
The process of photosynthesis transformed life on Earth. By harnessing energy from the sun, the evolution of
photosynthesis allowed living things access to enormous amounts of energy. Because of photosynthesis, living
things gained access to sufficient energy that allowed them to build new structures and achieve the biodiversity
evident today.
Only certain organisms (photoautotrophs), can perform photosynthesis; they require the presence of
chlorophyll, a specialized pigment that absorbs certain wavelengths of the visible spectrum and can capture
energy from sunlight. Photosynthesis uses carbon dioxide and water to assemble carbohydrate molecules and
release oxygen as a byproduct into the atmosphere. Eukaryotic autotrophs, such as plants and algae, have
organelles called chloroplasts in which photosynthesis takes place, and starch accumulates. In prokaryotes,
such as cyanobacteria, the process is less localized and occurs within folded membranes, extensions of the
plasma membrane, and in the cytoplasm.
8.2 The Light-Dependent Reactions of Photosynthesis
The pigments of the first part of photosynthesis, the light-dependent reactions, absorb energy from sunlight. A
photon strikes the antenna pigments of photosystem II to initiate photosynthesis. The energy travels to the
reaction center that contains chlorophyll a and then to the electron transport chain, which pumps hydrogen ions
into the thylakoid interior. This action builds up a high concentration of hydrogen ions. The hydrogen ions flow
through ATP synthase during chemiosmosis to form molecules of ATP, which are used for the formation of
sugar molecules in the second stage of photosynthesis. Photosystem I absorbs a second photon, which results
in the formation of an NADPH molecule, another energy and reducing carrier for the light-independent
reactions.
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247
8.3 Using Light Energy to Make Organic Molecules
Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the
Calvin cycle, take in CO 2 from the atmosphere. An enzyme, RuBisCO, catalyzes a reaction with CO 2 and
another organic compound, RuBP. After three cycles, a three-carbon molecule of G3P leaves the cycle to
become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated
into RuBP, which is then ready to react with more CO 2 . Photosynthesis forms an energy cycle with the process
of cellular respiration. Because plants contain both chloroplasts and mitochondria, they rely upon both
photosynthesis and respiration for their ability to function
interconvert essential metabolites.
VISUAL CONNECTION QUESTIONS
1. Figure 8.6 On a hot, dry day, plants close their
stomata to conserve water. What impact will this
have on photosynthesis?
2. Figure 8.16 What is the source of electrons for the
chloroplast electron transport chain?
a. Water
b. Oxygen
c. Carbon dioxide
d. NADPH
3. Figure 8.18 Which of the following statements is
true?
REVIEW QUESTIONS
4. Which of the following components is not used by
both plants and cyanobacteria to carry out
photosynthesis?
a. chloroplasts
b. chlorophyll
c. carbon dioxide
d. water
5. What two main products result from
photosynthesis?
a. oxygen and carbon dioxide
b. chlorophyll and oxygen
c. sugars/carbohydrates and oxygen
d. sugars/carbohydrates and carbon dioxide
6 . In which compartment of the plant cell do the light-
independent reactions of photosynthesis take place?
a. thylakoid
b. stroma
c. outer membrane
d. mesophyll
7. Which statement about thylakoids in eukaryotes is
not correct?
a. Thylakoids are assembled into stacks.
b. Thylakoids exist as a maze of folded
membranes.
c. The space surrounding thylakoids is called
stroma.
d. Thylakoids contain chlorophyll.
8. Predict the end result if a chloroplast’s light-
in both the light and dark, and to be able to
a. In photosynthesis, oxygen, carbon dioxide,
ATP, and NADPH are reactants. G3P and
water are products.
b. In photosynthesis, chlorophyll, water, and
carbon dioxide are reactants. G3P and
oxygen are products.
c. In photosynthesis, water, carbon dioxide,
ATP, and NADPH are reactants. RuBP and
oxygen are products.
d. In photosynthesis, water and carbon dioxide
are reactants. G3P and oxygen are
products.
independent enzymes developed a mutation that
prevented them from activating in response to light.
a. GA3P accumulation
b. ATP and NADPH accumulation
c. Water accumulation
d. Carbon dioxide depletion
9. How are the NADPH and GA3P molecules made
during photosynthesis similar?
a. They are both end products of
photosynthesis.
b. They are both substrates for
photosynthesis.
c. They are both produced from carbon
dioxide.
d. They both store energy in chemical bonds.
10. Which of the following structures is not a
component of a photosystem?
a. ATP synthase
b. antenna molecule
c. reaction center
d. primary electron acceptor
11. How many photons does it take to fully reduce
one molecule of NADP + to NADPH?
a. 1
b. 2
c. 4
d. 8
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Chapter 8 | Photosynthesis
12. Which complex is not involved in the
establishment of conditions for ATP synthesis?
a. photosystem i
b. ATP synthase
c. photosystem II
d. cytochrome complex
13. From which component of the light-dependent
reactions does NADPH form most directly?
a. photosystem II
b. photosystem I
c. cytochrome complex
d. ATP synthase
14. Three of the same species of plant are each
grown under a different colored light for the same
amount of time. Plant A is grown under blue light,
Plant B is grown under green light, and Plant C is
grown under orange light. Assuming the plants use
only chlorophyll a and chlorophyll b for
photosynthesis, what would be the predicted order of
the plants from most growth to least growth?
a. A, C, B
b. A, B, C
c. C, A, B
d. B, A, C
15. Plants containing only chlorophyll b are exposed
to radiation with the following wavelengths: lOnm (x-
rays), 450nm (blue light), 670nm (red light), and
800nm (infrared light). Which plants harness the
most energy for photosynthesis?
a. X-ray irradiated plants
b. Blue light irradiated plants
c. Red light irradiated plants
d. Infrared irradiated plants
16. Which molecule must enter the Calvin cycle
continually for the light-independent reactions to take
CRITICAL THINKING QUESTIONS
21. What is the overall outcome of the light reactions
in photosynthesis?
22. Why are carnivores, such as lions, dependent on
photosynthesis to survive?
23. Why are energy carriers thought of as either “full”
or “empty"?
24. Describe how the grey wolf population would be
impacted by a volcanic eruption that spewed a dense
ash cloud that blocked sunlight in a section of
Yellowstone National Park.
25. How does the closing of the stomata limit
photosynthesis?
26. Describe the pathway of electron transfer from
photosystem II to photosystem I in light-dependent
reactions.
place?
a. RuBisCO
b. RuBP
c. 3-PGA
d. CO 2
17. Which order of molecular conversions is correct
for the Calvin cycle?
a. RuBP + G3P -* 3-PGA -> sugar
b. RuBisCO ->• CO 2 RuBP G3P
c. RuBP + C0 2 [RuBisCO] 3-PGA -+ G3P
d. C0 2 -* 3-PGA -► RuBP G3P
18. Where in eukaryotic cells does the Calvin cycle
take place?
a. thylakoid membrane
b. thylakoid lumen
c. chloroplast stroma
d. granum
19. Which statement correctly describes carbon
fixation?
a. the conversion of CO 2 into an organic
compound
b. the use of RuBisCO to form 3-PGA
c. the production of carbohydrate molecules
from G3P
d. the formation of RuBP from G3P molecules
e. the use of ATP and NADPH to reduce CO 2
20. If four molecules of carbon dioxide enter the
Calvin cycle (four “turns" of the cycle), how many
G3P molecules are produced and how many are
exported?
a. 4 G3P made, 1 G3P exported
b. 4 G3P made, 2 G3P exported
c. 8 G3P made, 1 G3P exported
d. 8 G3P made, 4 G3P exported
27. What are the roles of ATP and NADPH in
photosynthesis?
28. How and why would the end products of
photosynthesis be changed if a plant had a mutation
that eliminated its photosystem II complex?
29. Why is the third stage of the Calvin cycle called
the regeneration stage?
30. Which part of the light-independent reactions
would be affected if a cell could not produce the
enzyme RuBisCO?
31. Why does it take three turns of the Calvin cycle to
produce G3P, the initial product of photosynthesis?
32. Imagine a sealed terrarium containing a plant and
a beetle. How does each organism provide resources
for the other? Could each organism survive if it was
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Chapter 8 | Photosynthesis
249
the only living thing in the terrarium? Why or why nutrients in a closed, sunny ecosystem consisting of
not? a giraffe and a tree.
33. Compare the flow of energy with the flow of
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9 | CELL
COMMUNICATION
Figure 9.1 Have you ever become separated from a friend while in a crowd? If so, you know the challenge of searching
for someone when surrounded by thousands of other people. If you and your friend have cell phones, your chances
of finding each other are good. Cell phone networks use various methods of encoding to ensure that the signals reach
their intended recipients without interference. Similarly, cells must communicate using specific signals and receptors to
ensure that messages are clear, (credit: modification of work by Vincent and Bella Productions)
Chapter Outline
9.1: Signaling Molecules and Cellular Receptors
9.2: Propagation of the Signal
9.3: Response to the Signal
9.4: Signaling in Single-Celled Organisms
Introduction
Imagine what life would be like if you and the people around you could not communicate. You would not be
able to express your wishes to others, nor could you ask questions about your location. Social organization is
dependent on communication between the individuals that comprise that society; without communication, society
would fall apart.
As with people, it is vital for individual cells to be able to interact with their environment. This is true for both a
one-celled organism growing in a puddle and a large animal living on a savanna. In order to properly respond
to external stimuli, cells have developed complex mechanisms of communication that can receive a message,
transfer the information across the plasma membrane, and then produce changes within the cell in response to
the message.
In multicellular organisms, cells send and receive chemical messages constantly to coordinate the actions of
distant organs, tissues, and cells. The ability to send messages quickly and efficiently enables cells to coordinate
and fine-tune their functions.
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Chapter 9 | Cell Communication
While the necessity for cellular communication in larger organisms seems obvious, even single-celled organisms
communicate with each other. Yeast cells signal each other to aid in finding other yeast cells for reproduction.
Some forms of bacteria coordinate their actions in order to form large complexes called biofilms or to organize
the production of toxins to remove competing organisms. The ability of cells to communicate through chemical
signals originated in single cells and was essential for the evolution of multicellular organisms. The efficient and
relatively error-free function of communication systems is vital for all life as we know it.
9.1 1 Signaling Molecules and Cellular Receptors
By the end of this section, you will be able to do the following:
• Describe four types of signaling mechanisms found in multicellular organisms
• Compare internal receptors with cell-surface receptors
• Recognize the relationship between a ligand’s structure and its mechanism of action
There are two kinds of communication in the world of living cells. Communication between cells is called
intercellular signaling, and communication within a cell is called intracellular signaling. An easy way to
remember the distinction is by understanding the Latin origin of the prefixes: inter- means "between" (for
example, intersecting lines are those that cross each other) and intra- means "inside" (as in intravenous).
Chemical signals are released by signaling cells in the form of small, usually volatile or soluble molecules called
ligands. A ligand is a molecule that binds another specific molecule, in some cases, delivering a signal in the
process. Ligands can thus be thought of as signaling molecules. Ligands interact with proteins in target cells,
which are cells that are affected by chemical signals; these proteins are also called receptors. Ligands and
receptors exist in several varieties; however, a specific ligand will have a specific receptor that typically binds
only that ligand.
Forms of Signaling
There are four categories of chemical signaling found in multicellular organisms: paracrine signaling, endocrine
signaling, autocrine signaling, and direct signaling across gap junctions (Figure 9.2). The main difference
between the different categories of signaling is the distance that the signal travels through the organism to reach
the target cell. We should note here that not all cells are affected by the same signals.
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Figure 9.2 In chemical signaling, a cell may target itself (autocrine signaling), a cell connected by gap junctions,
a nearby cell (paracrine signaling), or a distant cell (endocrine signaling). Paracrine signaling acts on nearby cells,
endocrine signaling uses the circulatory system to transport ligands, and autocrine signaling acts on the signaling cell.
Signaling via gap junctions involves signaling molecules moving directly between adjacent cells.
Paracrine Signaling
Signals that act locally between cells that are close together are called paracrine signals. Paracrine signals
move by diffusion through the extracellular matrix. These types of signals usually elicit quick responses that last
only a short period of time. In order to keep the response localized, paracrine ligand molecules are normally
quickly degraded by enzymes or removed by neighboring cells. Removing the signals will reestablish the
concentration gradient for the signal, allowing them to quickly diffuse through the intracellular space if released
again.
One example of paracrine signaling is the transfer of signals across synapses between nerve cells. A nerve
cell consists of a cell body, several short, branched extensions called dendrites that receive stimuli, and a long
extension called an axon, which transmits signals to other nerve cells or muscle cells. The junction between
nerve cells where signal transmission occurs is called a synapse. A synaptic signal is a chemical signal that
travels between nerve cells. Signals within the nerve cells are propagated by fast-moving electrical impulses.
When these impulses reach the end of the axon, the signal continues on to a dendrite of the next cell by the
release of chemical ligands called neurotransmitters from the presynaptic cell (the cell emitting the signal).
The neurotransmitters are transported across the very small distances (20-40 nanometers) between nerve cells,
which are called chemical synapses (Figure 9.3). The small distance between nerve cells allows the signal to
travel quickly; this enables an immediate response, such as, "Take your hand off the stove!"
When the neurotransmitter binds the receptor on the surface of the postsynaptic cell, the electrochemical
potential of the target cell changes, and the next electrical impulse is launched. The neurotransmitters that are
released into the chemical synapse are degraded quickly or get reabsorbed by the presynaptic cell so that the
recipient nerve cell can recover quickly and be prepared to respond rapidly to the next synaptic signal.
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Chapter 9 ] Cell Communication
Neurotransmitter
Neurotransmitter
attached to
receptor
Figure 9.3 The distance between the presynaptic cell and the postsynaptic cell—called the synaptic gap—is very
small and allows for rapid diffusion of the neurotransmitter. Enzymes in the synapatic gap degrade some types of
neurotransmitters to terminate the signal.
Endocrine Signaling
Signals from distant cells are called endocrine signals, and they originate from endocrine cells. (In the body,
many endocrine cells are located in endocrine glands, such as the thyroid gland, the hypothalamus, and the
pituitary gland.) These types of signals usually produce a slower response but have a longer-lasting effect. The
ligands released in endocrine signaling are called hormones, signaling molecules that are produced in one part
of the body but affect other body regions some distance away.
Hormones travel the large distances between endocrine cells and their target cells via the bloodstream, which
is a relatively slow way to move throughout the body. Because of their form of transport, hormones become
diluted and are present in low concentrations when they act on their target cells. This is different from paracrine
signaling, in which local concentrations of ligands can be very high.
Autocrine Signaling
Autocrine signals are produced by signaling cells that can also bind to the ligand that is released. This means
the signaling cell and the target cell can be the same or a similar cell (the prefix auto- means self, a reminder
that the signaling cell sends a signal to itself). This type of signaling often occurs during the early development
of an organism to ensure that cells develop into the correct tissues and take on the proper function. Autocrine
signaling also regulates pain sensation and inflammatory responses. Further, if a cell is infected with a virus,
the cell can signal itself to undergo programmed cell death, killing the virus in the process. In some cases,
neighboring cells of the same type are also influenced by the released ligand. In embryological development,
this process of stimulating a group of neighboring cells may help to direct the differentiation of identical cells into
the same cell type, thus ensuring the proper developmental outcome.
Direct Signaling Across Gap Junctions
Gap junctions in animals and plasmodesmata in plants are connections between the plasma membranes of
neighboring cells. These fluid-filled channels allow small signaling molecules, called intracellular mediators, to
diffuse between the two cells. Small molecules, such as calcium ions (Ca 2+ ), are able to move between cells, but
large molecules like proteins and DNA cannot fit through the channels. The specificity of the channels ensures
that the cells remain independent but can quickly and easily transmit signals. The transfer of signaling molecules
communicates the current state of the cell that is directly next to the target cell; this allows a group of cells to
coordinate their response to a signal that only one of them may have received. In plants, plasmodesmata are
ubiquitous, making the entire plant into a giant communication network.
Synapse
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Types of Receptors
Receptors are protein molecules in the target cell or on its surface that bind ligand. There are two types of
receptors, internal receptors and cell-surface receptors.
Internal receptors
Internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm of the cell
and respond to hydrophobic ligand molecules that are able to travel across the plasma membrane. Once inside
the cell, many of these molecules bind to proteins that act as regulators of mRNA synthesis (transcription) to
mediate gene expression. Gene expression is the cellular process of transforming the information in a cell's DNA
into a sequence of amino acids, which ultimately forms a protein. When the ligand binds to the internal receptor, a
conformational change is triggered that exposes a DNA-binding site on the protein. The ligand-receptor complex
moves into the nucleus, then binds to specific regulatory regions of the chromosomal DNA and promotes the
initiation of transcription (Figure 9.4). Transcription is the process of copying the information in a cell's DNA into
a special form of RNA called messenger RNA (mRNA); the cell uses information in the mRNA (which moves out
into the cytoplasm and associates with ribosomes) to link specific amino acids in the correct order, producing a
protein. Internal receptors can directly influence gene expression without having to pass the signal on to other
receptors or messengers.
Figure 9.4 Hydrophobic signaling molecules typically diffuse across the plasma membrane and interact with
intracellular receptors in the cytoplasm. Many intracellular receptors are transcription factors that interact with DNA in
the nucleus and regulate gene expression.
Cell-Surface Receptors
Cell-surface receptors, also known as transmembrane receptors, are cell surface, membrane-anchored
(integral) proteins that bind to external ligand molecules. This type of receptor spans the plasma membrane
and performs signal transduction, through which an extracellular signal is converted into an intracellular signal.
Ligands that interact with cell-surface receptors do not have to enter the cell that they affect. Cell-surface
receptors are also called cell-specific proteins or markers because they are specific to individual cell types.
Because cell-surface receptor proteins are fundamental to normal cell functioning, it should come as no surprise
that a malfunction in any one of these proteins could have severe consequences. Errors in the protein structures
of certain receptor molecules have been shown to play a role in hypertension (high blood pressure), asthma,
heart disease, and cancer.
Each cell-surface receptor has three main components: an external ligand-binding domain, a hydrophobic
membrane-spanning region called a transmembrane domain, and an intracellular domain inside the cell. The
ligand-binding domain is also called the extracellular domain. The size and extent of each of these domains
vary widely, depending on the type of receptor.
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Chapter 9 ] Cell Communication
V /
e olution CONNECTION
How Viruses Recognize a Host
Unlike living cells, many viruses do not have a plasma membrane or any of the structures necessary to
sustain metabolic life. Some viruses are simply composed of an inert protein shell enclosing DNA or RNA.
To reproduce, viruses must invade a living cell, which serves as a host, and then take over the hosts cellular
apparatus. But how does a virus recognize its host?
Viruses often bind to cell-surface receptors on the host cell. For example, the virus that causes human
influenza (flu) binds specifically to receptors on membranes of cells of the respiratory system. Chemical
differences in the cell-surface receptors among hosts mean that a virus that infects a specific species (for
example, humans) often cannot infect another species (for example, chickens).
However, viruses have very small amounts of DNA or RNA compared to humans, and, as a result, viral
reproduction can occur rapidly. Viral reproduction invariably produces errors that can lead to changes in
newly produced viruses; these changes mean that the viral proteins that interact with cell-surface receptors
may evolve in such a way that they can bind to receptors in a new host. Such changes happen randomly
and quite often in the reproductive cycle of a virus, but the changes only matter if a virus with new binding
properties comes into contact with a suitable host. In the case of influenza, this situation can occur in
[i]
settings where animals and people are in close contact, such as poultry and swine farms. Once a virus
jumps the former "species barrier" to a new host, it can spread quickly. Scientists watch newly appearing
viruses (called emerging viruses) closely in the hope that such monitoring can reduce the likelihood of global
viral epidemics.
Cell-surface receptors are involved in most of the signaling in multicellular organisms. There are three general
categories of cell-surface receptors: ion channel-linked receptors, G-protein-linked receptors, and enzyme-linked
receptors.
Ion channel-linked receptors bind a ligand and open a channel through the membrane that allows specific
ions to pass through. To form a channel, this type of cell-surface receptor has an extensive membrane-spanning
region. In order to interact with the double layer of phospholipid fatty acid tails that form the center of the plasma
membrane, many of the amino acids in the membrane-spanning region are hydrophobic in nature. Conversely,
the amino acids that line the inside of the channel are hydrophilic to allow for the passage of water or ions.
When a ligand binds to the extracellular region of the channel, there is a conformational change in the protein's
structure that allows ions such as sodium, calcium, magnesium, and hydrogen to pass through (Figure 9.5).
Figure 9.5 Gated ion channels form a pore through the plasma membrane that opens when the signaling molecule
binds. The open pore then allows ions to flow into or out of the cell.
G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein. The activated G-
protein then interacts with either an ion channel or an enzyme in the membrane (Figure 9.6). All G-protein-linked
receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and
G-protein-binding site.
1. A. B. Sigalov, The School of Nature. IV. Learning from Viruses, Self/Nonself 1, no. 4 (2010): 282-298. Y. Cao, X. Koh, L. Dong, X. Du, A.
Wu, X. Ding, H. Deng, Y. Shu, J. Chen, T. Jiang, Rapid Estimation of Binding Activity of Influenza Virus Hemagglutinin to Human and Avian
Receptors, PLoS One 6, no. 4 (2011): el8664.
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Chapter 9 | Cell Communication
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Cell signaling using G-protein-linked receptors occurs as a cyclic series of events. Before the ligand binds,
the inactive G-protein can bind to a newly revealed site on the receptor specific for its binding. Once the G-
protein binds to the receptor, the resulting change in shape activates the G-protein, which releases guanosine
diposphate (GDP) and picks up guanosine 3-phosphate (GTP). The subunits of the G-protein then split into the
a subunit and the py subunit. One or both of these G-protein fragments may be able to activate other proteins as
a result. After awhile, the GTP on the active a subunit of the G-protein is hydrolyzed to GDP and the py subunit
is deactivated. The subunits reassociate to form the inactive G-protein and the cycle begins anew.
When a signaling
molecule binds to
the G-protein-coupled
receptor, the G-protein a
subunit exchanges
Figure 9.6 Heterotrimeric G-proteins have three subunits: a, p, and y. When a signaling molecule binds to a G-protein-
coupled receptor in the plasma membrane, a GDP molecule associated with the a subunit is exchanged for GTP. The
P and y subunits dissociate from the a subunit, and a cellular response is triggered either by the a subunit or the
dissociated Py pair. Hydrolysis of GTP to GDP terminates the signal.
G-protein-linked receptors have been extensively studied and much has been learned about their roles in
maintaining health. Bacteria that are pathogenic to humans can release poisons that interrupt specific G-protein-
linked receptor function, leading to illnesses such as pertussis, botulism, and cholera. In cholera (Figure 9.7),
for example, the water-borne bacterium Vibrio cholerae produces a toxin, choleragen, that binds to cells lining
the small intestine. The toxin then enters these intestinal cells, where it modifies a G-protein that controls the
opening of a chloride channel and causes it to remain continuously active, resulting in large losses of fluids from
the body and potentially fatal dehydration as a result.
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Chapter 9 | Cell Communication
NOTICE.
PREVENTIVES OF
CHOLERA!
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Medical CountrL
BE TEMPERATE IN EATING & DRINKING!
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JAMES KELLY, ft
CAI.KB K WOODRCLL
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Figure 9.7 Transmitted primarily through contaminated drinking water, cholera is a major cause of death in the
developing world and in areas where natural disasters interrupt the availability of clean water. The cholera bacterium,
Vibrio cholerae, creates a toxin that modifies G-protein-mediated cell signaling pathways in the intestines. Modern
sanitation eliminates the threat of cholera outbreaks, such as the one that swept through New York City in 1866. This
poster from that era shows how, at that time, the way that the disease was transmitted was not understood, (credit:
New York City Sanitary Commission)
Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an
enzyme. In some cases, the intracellular domain of the receptor itself is an enzyme. Other enzyme-linked
receptors have a small intracellular domain that interacts directly with an enzyme. The enzyme-linked receptors
normally have large extracellular and intracellular domains, but the membrane-spanning region consists of a
single alpha-helical region of the peptide strand. When a ligand binds to the extracellular domain, a signal is
transferred through the membrane, activating the enzyme. Activation of the enzyme sets off a chain of events
within the cell that eventually leads to a response. One example of this type of enzyme-linked receptor is
the tyrosine kinase receptor (Figure 9.8). A kinase is an enzyme that transfers phosphate groups from ATP
to another protein. The tyrosine kinase receptor transfers phosphate groups to tyrosine molecules (tyrosine
residues). First, signaling molecules bind to the extracellular domain of two nearby tyrosine kinase receptors.
The two neighboring receptors then bond together, or dimerize. Phosphates are then added to tyrosine residues
on the intracellular domain of the receptors (phosphorylation). The phosphorylated residues can then transmit
the signal to the next messenger within the cytoplasm.
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Chapter 9 | Cell Communication
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visual
CONNECTION
. When signaling
molecules bind the
X receptors, the
▼ receptors dimerize.
Signaling
molecule
Tyrosine
residues
Tyrosine residues on
the intracellular domain
are phosphorylated,
.. triggering a downstream
▼ cellular response.
Figure 9.8 A receptor tyrosine kinase is an enzyme-linked receptor with a single helical transmembrane region,
and extracellular and intracellular domains. Binding of a signaling molecule to the extracellular domain causes
the receptor to dimerize. Tyrosine residues on the intracellular domain are then autophosphorylated, triggering a
downstream cellular response. The signal is terminated by a phosphatase that removes the phosphates from the
phosphotyrosine residues.
HER2 is a receptor tyrosine kinase. In 30 percent of human breast cancers, HER2 is permanently activated,
resulting in unregulated cell division. Lapatinib, a drug used to treat breast cancer, inhibits HER2 receptor
tyrosine kinase autophosphorylation (the process by which the receptor adds phosphates onto itself), thus
reducing tumor growth by 50 percent. Besides autophosphorylation, which of the following steps would be
inhibited by Lapatinib?
a. Signaling molecule binding, dimerization, and the downstream cellular response
b. Dimerization, and the downstream cellular response
c. The downstream cellular response
d. Phosphatase activity, dimerization, and the downsteam cellular response
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Chapter 9 | Cell Communication
Signaling Molecules
Produced by signaling cells and the subsequent binding to receptors in target cells, ligands act as chemical
signals that travel to the target cells to coordinate responses. The types of molecules that serve as ligands are
incredibly varied and range from small proteins to small ions like calcium (Ca 2+ ).
Small Hydrophobic Ligands
Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal
receptors. Important members of this class of ligands are the steroid hormones. Steroids are lipids that have
a hydrocarbon skeleton with four fused rings; different steroids have different functional groups attached to the
carbon skeleton. Steroid hormones include the female sex hormone, estradiol, which is a type of estrogen;
the male sex hormone, testosterone; and cholesterol, which is an important structural component of biological
membranes and a precursor of steriod hormones (Figure 9.9). Other hydrophobic hormones include thyroid
hormones and vitamin D. In order to be soluble in blood, hydrophobic ligands must bind to carrier proteins while
they are being transported through the bloodstream.
OH OH
Figure 9.9 Steroid hormones have similar chemical structures to their precursor, cholesterol. Because these molecules
are small and hydrophobic, they can diffuse directly across the plasma membrane into the cell, where they interact
with internal receptors.
Water-Soluble Ligands
Water-soluble ligands are polar and, therefore, cannot pass through the plasma membrane unaided; sometimes,
they are too large to pass through the membrane at all. Instead, most water-soluble ligands bind to the
extracellular domain of cell-surface receptors. This group of ligands is quite diverse and includes small
molecules, peptides, and proteins.
Other Ligands
Nitric oxide (NO) is a gas that also acts as a ligand. It is able to diffuse directly across the plasma membrane, and
one of its roles is to interact with receptors in smooth muscle and induce relaxation of the tissue. NO has a very
short half-life and, therefore, only functions over short distances. Nitroglycerin, a treatment for heart disease,
acts by triggering the release of NO, which causes blood vessels to dilate (expand), thus restoring blood flow to
the heart. NO has become better known recently because the pathway that it affects is targeted by prescription
medications for erectile dysfunction, such as Viagra (erection involves dilated blood vessels).
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9.2 | Propagation of the Signal
By the end of this section, you will be able to do the following:
• Explain how the binding of a ligand initiates signal transduction throughout a cell
• Recognize the role of phosphorylation in the transmission of intracellular signals
• Evaluate the role of second messengers in signal transmission
Once a ligand binds to a receptor, the signal is transmitted through the membrane and into the cytoplasm.
Continuation of a signal in this manner is called signal transduction. Signal transduction only occurs with cell-
surface receptors, which cannot interact with most components of the cell such as DNA. Only internal receptors
are able to interact directly with DNA in the nucleus to initiate protein synthesis.
When a ligand binds to its receptor, conformational changes occur that affect the receptor’s intracellular domain.
Conformational changes of the extracellular domain upon ligand binding can propagate through the membrane
region of the receptor and lead to activation of the intracellular domain or its associated proteins. In some cases,
binding of the ligand causes dimerization of the receptor, which means that two receptors bind to each other
to form a stable complex called a dimer. A dimer is a chemical compound formed when two molecules (often
identical) join together. The binding of the receptors in this manner enables their intracellular domains to come
into close contact and activate each other.
Binding Initiates a Signaling Pathway
After the ligand binds to the cell-surface receptor, the activation of the receptor’s intracellular components sets
off a chain of events that is called a signaling pathway, sometimes called a signaling cascade. In a signaling
pathway, second messengers-enzymes-and activated proteins interact with specific proteins, which are in turn
activated in a chain reaction that eventually leads to a change in the cell’s environment (Figure 9.10), such as
an increase in metabolism or specific gene expression. The events in the cascade occur in a series, much like a
current flows in a river. Interactions that occur before a certain point are defined as upstream events, and events
after that point are called downstream events.
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9 CONNECTION
Upon binding
of epidermal
growth factor
(EGF) to the
EGF receptor
(EGFR), two
proteins
associated with
the receptor
called GRB2
and SOS
activate RAS,
a small
G-protein.
ERK
Stimulates
Translation
Nucleus
Stimulates:
cell proliferation
cell migration and adhesion
angiogenesis (growth of new blood
vessels)
Inhibits:
apoptosis
A protein
kinase called
RAF is
activated by
RAS-GTP. RAF
phosphorylates
MEK, which
in turn
phosphorylates
ERK, a MAP
kinase. The
phosphorylated
ERK enters the
nucleus, where
it triggers
a cellular
response.
Figure 9.10 The epidermal growth factor (EGF) receptor (EGFR) is a receptor tyrosine kinase involved in
the regulation of cell growth, wound healing, and tissue repair. When EGF binds to the EGFR, a cascade of
downstream events causes the cell to grow and divide. If EGFR is activated at inappropriate times, uncontrolled
cell growth (cancer) may occur.
in certain cancers, the GTPase activity of the RAS G-protein is inhibited. This means that the RAS protein
can no longer hydrolyze GTP into GDP. What effect would this have on downstream cellular events?
You can see that signaling pathways can get very complicated very quickly because most cellular proteins can
affect different downstream events, depending on the conditions within the cell. A single pathway can branch
off toward different endpoints based on the interplay between two or more signaling pathways, and the same
ligands are often used to initiate different signals in different cell types. This variation in response is due to
differences in protein expression in different cell types. Another complicating element is signal integration of
the pathways, in which signals from two or more different cell-surface receptors merge to activate the same
response in the cell. This process can ensure that multiple external requirements are met before a cell commits
to a specific response.
The effects of extracellular signals can also be amplified by enzymatic cascades. At the initiation of the signal,
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a single ligand binds to a single receptor. However, activation of a receptor-linked enzyme can activate many
copies of a component of the signaling cascade, which amplifies the signal.
LINK TQ LEARNING
Observe an animation of cell signaling at this site (http:// 0 penstaxc 0 llege. 0 rg/l/cell_signals) .
Methods of Intracellular Signaling
The induction of a signaling pathway depends on the modification of a cellular component by an enzyme. There
are numerous enzymatic modifications that can occur, and they are recognized in turn by the next component
downstream. The following are some of the more common events in intracellular signaling.
Phosphorylation
One of the most common chemical modifications that occurs in signaling pathways is the addition of a phosphate
group (PO 4 3 ) to a molecule such as a protein in a process called phosphorylation. The phosphate can be
added to a nucleotide such as GMP to form GDP or GTP. Phosphates are also often added to serine, threonine,
and tyrosine residues of proteins, where they replace the hydroxyl group of the amino acid (Figure 9.11).
The transfer of the phosphate is catalyzed by an enzyme called a kinase. Various kinases are named for
the substrate they phosphorylate. Phosphorylation of serine and threonine residues often activates enzymes.
Phosphorylation of tyrosine residues can either affect the activity of an enzyme or create a binding site that
interacts with downstream components in the signaling cascade. Phosphorylation may activate or inactivate
enzymes, and the reversal of phosphorylation, dephosphorylation by a phosphatase, will reverse the effect.
OH
- P-OH
OH
= P-OH
Phosphoserine
Phosphothreonine
Phosphotyrosine
Figure 9.11 In protein phosphorylation, a phosphate group (PO 4 " 3 ) is added to residues of the amino acids serine,
threonine, and tyrosine.
Second Messengers
Second messengers are small molecules that propagate a signal after it has been initiated by the binding of the
signaling molecule to the receptor. These molecules help to spread a signal through the cytoplasm by altering
the behavior of certain cellular proteins.
Calcium ion is a widely used second messenger. The free concentration of calcium ions (Ca 2+ ) within a
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cell is very low because ion pumps in the plasma membrane continuously remove it by using adenosine-5'-
triphosphate (ATP). For signaling purposes, Ca 2+ is stored in cytoplasmic vesicles, such as the endoplasmic
reticulum, or accessed from outside the cell. When signaling occurs, ligand-gated calcium ion channels allow
the higher levels of Ca 2+ that are present outside the cell (or in intracellular storage compartments) to flow into
the cytoplasm, which raises the concentration of cytoplasmic Ca 2+ , The response to the increase in Ca 2+ varies
and depends on the cell type involved. For example, in the p-cells of the pancreas, Ca 2+ signaling leads to the
release of insulin, and in muscle cells, an increase in Ca 2+ leads to muscle contractions.
Another second messenger utilized in many different cell types is cyclic AMP (cAMP). Cyclic AMP is
synthesized by the enzyme adenylyl cyclase from ATP (Figure 9.12). The main role of cAMP in cells is to bind to
and activate an enzyme called cAMP-dependent kinase (A-kinase). A-kinase regulates many vital metabolic
pathways: It phosphorylates serine and threonine residues of its target proteins, activating them in the process.
A-kinase is found in many different types of cells, and the target proteins in each kind of cell are different.
Differences give rise to the variation of the responses to cAMP in different cells.
o o
'o—P-O-P—o-
O' O'
O' O'
O'
ATP
Pyrophosphate
cAMP
Figure 9.12 This diagram shows the mechanism for the formation of cyclic AMP (cAMP). cAMP serves as a second
messenger to activate or inactivate proteins within the cell. Termination of the signal occurs when an enzyme called
phosphodiesterase converts cAMP into AMP.
Present in small concentrations in the plasma membrane, inositol phospholipids are lipids that can also be
converted into second messengers. Because these molecules are membrane components, they are located
near membrane-bound receptors and can easily interact with them. Phosphatidylinositol (PI) is the main
phospholipid that plays a role in cellular signaling. Enzymes known as kinases phosphorylate PI to form Pl-
phosphate (PIP) and Pl-bisphosphate (PIP 2 ).
The enzyme phospholipase C cleaves PIP 2 to form diacylglycerol (DAG) and inositol triphosphate (IP 3 )
(Figure 9.13). These products of the cleavage of PIP 2 serve as second messengers. Diacylglycerol (DAG)
remains in the plasma membrane and activates protein kinase C (PKC), which then phosphorylates serine
and threonine residues in its target proteins. IP 3 diffuses into the cytoplasm and binds to ligand-gated calcium
channels in the endoplasmic reticulum to release Ca 2+ that continues the signal cascade.
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PIP 2
Figure 9.13 The enzyme phospholipase C breaks down PIP 2 into IP 3 and DAG, both of which serve as second
messengers.
9.3 | Response to the Signal
By the end of this section, you will be able to do the following:
• Describe how signaling pathways direct protein expression, cellular metabolism, and cell growth
• Identify the function of PKC in signal transduction pathways
• Recognize the role of apoptosis in the development and maintenance of a healthy organism
inside the cell, ligands bind to their internal receptors, allowing them to directly affect the cell’s DNA and protein-
producing machinery. Using signal transduction pathways, receptors in the plasma membrane produce a variety
of effects on the cell. The results of signaling pathways are extremely varied and depend on the type of cell
involved as well as the external and internal conditions. A small sampling of responses is described below.
Gene Expression
Some signal transduction pathways regulate the transcription of RNA. Others regulate the translation of proteins
from mRNA. An example of a protein that regulates translation in the nucleus is the MAP kinase ERK. The
MAPK/ERK pathway (also known as the Ras-Raf-MEK-ERK pathway) is a chain of proteins in the cell that
communicates a signal from a receptor on the surface of the cell to the nuclear DNA. ERK is activated in a
phosphorylation cascade when epidermal growth factor (EGF) binds the EGF receptor (see Figure 9.10). Upon
phosphorylation, ERK enters the nucleus and activates a protein kinase that, in turn, regulates protein translation
(Figure 9.14).
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The MAP kinase ERK
phosphorylates MNK1.
MNK1 in turn
phosphorylates elF-4E,
which is associated
with mRNA. The mRNA
unfolds and protein
sythesis begins.
Figure 9.14 ERK is a MAP kinase that activates translation when it is phosphorylated. ERK phosphorylates MNK1,
which in turn phosphorylates elF-4E, an elongation initiation factor that, with other initiation factors, is associated with
mRNA. When elF-4E becomes phosphorylated, the mRNA unfolds, allowing protein synthesis in the nucleus to begin.
(See Figure 9.10 for the phosphorylation pathway that activates ERK.)
Another mechanism of gene regulation involves PKC, which can interact with is a protein that acts as an inhibitor.
An inhibitor is a molecule that binds to a protein and prevents it from functioning or reduces its function, in this
case, the inhibitor is a protein called Ik-B, which binds to the regulatory protein NF-kB. (The symbol k represents
the Greek letter kappa.) When Ik-B is bound to NF-kB, the complex cannot enter the nucleus of the cell, but
when Ik-B is phosphorylated by PKC, it can no longer bind NF-kB, and NF-kB (a transcription factor) can enter
the nucleus and initiate RNA transcription. In this case, the effect of phosphorylation is to inactivate an inhibitor
and thereby activate the process of transcription.
Increase in Cellular Metabolism
The result of another signaling pathway affects muscle cells. The activation of p-adrenergic receptors in muscle
cells by adrenaline leads to an increase in cyclic AMP (cAMP) inside the cell. Also known as epinephrine,
adrenaline is a hormone (produced by the adrenal gland located on top of the kidney) that readies the
body for short-term emergencies. Cyclic AMP activates PKA (protein kinase A), which in turn phosphorylates
two enzymes. The first enzyme promotes the degradation of glycogen by activating intermediate glycogen
phosphorylase kinase (GPK) that in turn activates glycogen phosphorylase (GP) that catabolizes glycogen into
its constituent glucose monomers. (Recall that your body converts excess glucose to glycogen for short-term
storage. When energy is needed, glycogen is quickly reconverted to glucose.) Phosphorylation of the second
enzyme, glycogen synthase (GS), inhibits its ability to form glycogen from glucose. In this manner, a muscle cell
obtains a ready pool of glucose by activating its formation via glycogen degradation and by inhibiting the use of
glucose to form glycogen, thus preventing a futile cycle of glycogen degradation and synthesis. The glucose is
then available for use by the muscle cell in response to a sudden surge of adrenaline—the “fight or flight” reflex.
Cell Growth
Cell signaling pathways also play a major role in cell division. Cells do not normally divide unless they are
stimulated by signals from other cells. The ligands that promote cell growth are called growth factors. Most
growth factors bind to cell-surface receptors that are linked to tyrosine kinases. These cell-surface receptors
are called receptor tyrosine kinases (RTKs). Activation of RTKs initiates a signaling pathway that includes a G-
1
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protein called RAS, which activates the MAP kinase pathway described earlier. The enzyme MAP kinase then
stimulates the expression of proteins that interact with other cellular components to initiate cell division.
ca eer connection
Cancer Biologist
Cancer biologists study the molecular origins of cancer with the goal of developing new prevention methods
and treatment strategies that will inhibit the growth of tumors without harming the normal cells of the
body. As mentioned earlier, signaling pathways control cell growth. These signaling pathways are controlled
by signaling proteins, which are, in turn, expressed by genes. Mutations in these genes can result in
malfunctioning signaling proteins. This prevents the cell from regulating its cell cycle, triggering unrestricted
cell division and perhaps cancer. The genes that regulate the signaling proteins are one type of oncogene,
which is a gene that has the potential to cause cancer. The gene encoding RAS is an oncogene that
was originally discovered when mutations in the RAS protein were linked to cancer. Further studies have
indicated that 30 percent of cancer cells have a mutation in the RAS gene that leads to uncontrolled growth.
If left unchecked, uncontrolled cell division can lead to tumor formation and metastasis, the growth of cancer
cells in new locations in the body.
Cancer biologists have been able to identify many other oncogenes that contribute to the development of
cancer. For example, HER2 is a cell-surface receptor that is present in excessive amounts in 20 percent
of human breast cancers. Cancer biologists realized that gene duplication led to HER2 overexpression in
25 percent of breast cancer patients and developed a drug called Herceptin (trastuzumab). Herceptin is
a monoclonal antibody that targets HER2 for removal by the immune system. Herceptin therapy helps to
control signaling through HER2. The use of Herceptin in combination with chemotherapy has helped to
increase the overall survival rate of patients with metastatic breast cancer.
More information on cancer biology research can be found at the National Cancer Institute website
(https://www.cancer.gov/research/areas/biology (http:// 0 penstax. 0 rg/l/cancer_research) ).
Cell Death
When a cell is damaged, superfluous, or potentially dangerous to an organism, a cell can initiate a mechanism to
trigger programmed cell death, or apoptosis. Apoptosis allows a cell to die in a controlled manner that prevents
the release of potentially damaging molecules from inside the cell. There are many internal checkpoints that
monitor a cell’s health; if abnormalities are observed, a cell can spontaneously initiate the process of apoptosis.
However, in some cases, such as a viral infection or uncontrolled cell division due to cancer, the cell’s normal
checks and balances fail. External signaling can also initiate apoptosis. For example, most normal animal cells
have receptors that interact with the extracellular matrix, a network of glycoproteins that provides structural
support for cells in an organism. The binding of cellular receptors to the extracellular matrix initiates a signaling
cascade within the cell. However, if the cell moves away from the extracellular matrix, the signaling ceases, and
the cell undergoes apoptosis. This system keeps cells from traveling through the body and proliferating out of
control, as happens with tumor cells that metastasize.
Another example of external signaling that leads to apoptosis occurs in T-cell development. T-cells are immune
cells that bind to foreign macromolecules and particles, and target them for destruction by the immune system.
Normally, T-cells do not target “self” proteins (those of their own organism), a process that can lead to
autoimmune diseases. In order to develop the ability to discriminate between self and non-self, immature T-cells
undergo screening to determine whether they bind to so-called self proteins. If the T-cell receptor binds to self
proteins, the cell initiates apoptosis to remove the potentially dangerous cell.
Apoptosis is also essential for normal embryological development. In vertebrates, for example, early stages of
development include the formation of web-like tissue between individual fingers and toes (Figure 9.15). During
the course of normal development, these unneeded cells must be eliminated, enabling fully separated fingers
and toes to form. A cell signaling mechanism triggers apoptosis, which destroys the cells between the developing
digits.
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Figure 9.15 The histological section of a foot of a 15-day-old mouse embryo, visualized using light microscopy, reveals
areas of tissue between the toes, which apoptosis will eliminate before the mouse reaches its full gestational age at 27
days, (credit: modification of work by Michal Manas)
Termination of the Signal Cascade
The aberrant signaling often seen in tumor cells is proof that the termination of a signal at the appropriate time
can be just as important as the initiation of a signal. One method of stopping a specific signal is to degrade
the ligand or remove it so that it can no longer access its receptor. One reason that hydrophobic hormones
like estrogen and testosterone trigger long-lasting events is because they bind carrier proteins. These proteins
allow the insoluble molecules to be soluble in blood, but they also protect the hormones from degradation by
circulating enzymes.
Inside the cell, many different enzymes reverse the cellular modifications that result from signaling cascades.
For example, phosphatases are enzymes that remove the phosphate group attached to proteins by kinases in
a process called dephosphorylation. Cyclic AMP (cAMP) is degraded into AMP by phosphodiesterase, and the
release of calcium stores is reversed by the Ca 2+ pumps that are located in the external and internal membranes
of the cell.
9.4 | Signaling in Single-Celled Organisms
By the end of this section, you will be able to do the following:
• Describe how single-celled yeasts use cell signaling to communicate with one another
• Relate the role of quorum sensing to the ability of some bacteria to form biofilms
Within-cell signaling allows bacteria to respond to environmental cues, such as nutrient levels. Some single-
celled organisms also release molecules to signal to each other.
Signaling in Yeast
Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those
of cell-surface receptor signals in multicellular organisms. Budding yeasts (Figure 9.16) are able to participate
in a process that is similar to sexual reproduction that entails two haploid cells (cells with one-half the normal
number of chromosomes) combining to form a diploid cell (a cell with two sets of each chromosome, which is
what normal body cells contain). In order to find another haploid yeast cell that is prepared to mate, budding
yeasts secrete a signaling molecule called mating factor. When mating factor binds to cell-surface receptors in
other yeast cells that are nearby, they stop their normal growth cycles and initiate a cell signaling cascade that
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includes protein kinases and GTP-binding proteins that are similar to G-proteins.
J
J
J J _
2 pm
Figure 9.16 Budding Saccharomyces cerevisiae yeast cells can communicate by releasing a signaling molecule
called mating factor. In this micrograph, they are visualized using differential interference contrast microscopy, a light
microscopy technique that enhances the contrast of the sample.
Signaling in Bacteria
Signaling in bacteria enables bacteria to monitor extracellular conditions, ensure that there are sufficient
amounts of nutrients, and ensure that hazardous situations are avoided. There are circumstances, however,
when bacteria communicate with each other.
The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship
with Hawaiian bobtail squid. When the population density of the bacteria reaches a certain level, specific gene
expression is initiated, and the bacteria produce bioluminescent proteins that emit light. Because the number
of cells present in the environment (cell density) is the determining factor for signaling, bacterial signaling was
named quorum sensing. In politics and business, a quorum is the minimum number of members required to be
present to vote on an issue.
Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted
by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small,
hydrophobic molecules, such as acyl-homoserine lactone (AHL), or larger peptide-based molecules; each type
of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors,
which then switch gene expression on or off. When the number of bacteria increases so does the concentration
of the autoinducer, triggering increased expression of certain genes including autoinducers, which results in a
self-amplifying cycle, also known as a positive feedback loop (Figure 9.17). The peptide autoinducers stimulate
more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure
to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different
genes that respond to autoinducers.
Some species of bacteria that use quorum sensing form biofilms, complex colonies of bacteria (often containing
several species) that exchange chemical signals to coordinate the release of toxins that will attack the host.
Bacterial biofilms (Figure 9.18) can sometimes be found on medical equipment; when biofilms invade implants
such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.
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CONNECTION
Figure 9.17 Autoinducers are small molecules or proteins produced by bacteria that regulate gene expression.
Which of the following statements about quorum sensing is false?
a. Autoinducer must bind to receptor to turn on transcription of genes responsible for the production of
more autoinducer.
b. The receptor stays in the bacterial cell, but the autoinducer diffuses out.
c. Autoinducer can only act on a different cell: it cannot act on the cell in which it is made.
d. Autoinducer turns on genes that enable the bacteria to form a biofilm.
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visual
CONNECTION
(a) (b)
Figure 9.18 Cell-cell communication enables these (a) Staphylococcus aureus bacteria to work together to
form a biofilm inside a hospital patient’s catheter, seen here via scanning electron microscopy. S. aureus is the
main cause of hospital-acquired infections, (b) Hawaiian bobtail squid have a symbiotic relationship with the
bioluminescent bacteria Vibrio fischeri. The luminescence makes it difficult to see the squid from below because it
effectively eliminates its shadow. In return for camouflage, the squid provides food for the bacteria. Free-living \/.
fischeri do not produce luciferase, the enzyme responsible for luminescence, but \/. fischeri living in a symbiotic
relationship with the squid do. Quorum sensing determines whether the bacteria should produce the luciferase
enzyme, (credit a: modifications of work by CDC/Janice Carr; credit b: modifications of work by Cliffl066/Flickr)
What advantage might biofilm production confer on the S. aureus inside the catheter?
Research on the details of quorum sensing has led to advances in growing bacteria for industrial purposes.
Recent discoveries suggest that it may be possible to exploit bacterial signaling pathways to control bacterial
growth; this process could replace or supplement antibiotics that are no longer effective in certain situations.
LINK
T a
LEARNING
Watch geneticist Bonnie Bassler discuss her discovery (http:// 0 penstaxc 0 llege. 0 rg/l/bacteria_talk) of
quorum sensing in biofilm bacteria in squid.
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Cellular Communication in Yeasts
The first cellular form of life on our planet likely consisted of single-celled prokaryotic organisms that had
limited interaction with each other. While some external signaling occurs between different species of single-
celled organisms, the majority of signaling within bacteria and yeasts concerns only other members of the
same species. The evolution of cellular communication is an absolute necessity for the development of
multicellular organisms, and this innovation is thought to have required approximately 2 billion years to
appear in early life forms.
Yeasts are single-celled eukaryotes and, therefore, have a nucleus and organelles characteristic of more
complex life forms. Comparisons of the genomes of yeasts, nematode worms, fruit flies, and humans
illustrate the evolution of increasingly complex signaling systems that allow for the efficient inner workings
that keep humans and other complex life forms functioning correctly.
Kinases are a major component of cellular communication, and studies of these enzymes illustrate the
evolutionary connectivity of different species. Yeasts have 130 types of kinases. More complex organisms
such as nematode worms and fruit flies have 454 and 239 kinases, respectively. Of the 130 kinase types
in yeast, 97 belong to the 55 subfamilies of kinases that are found in other eukaryotic organisms. The
only obvious deficiency seen in yeasts is the complete absence of tyrosine kinases. It is hypothesized that
phosphorylation of tyrosine residues is needed to control the more sophisticated functions of development,
differentiation, and cellular communication used in multicellular organisms.
Because yeasts contain many of the same classes of signaling proteins as humans, these organisms
are ideal for studying signaling cascades. Yeasts multiply quickly and are much simpler organisms than
humans or other multicellular animals. Therefore, the signaling cascades are also simpler and easier to
study, although they contain similar counterparts to human signaling. 121
Watch this collection of interview clips with biofilm researchers in “What Are Bacterial Biofilms?” (This
multimedia resource will open in a browser.) (http://cnx.org/content/m66383/ 1.3/#eip-
idll67232076592)
2. G. Manning, G.D. Plowman, T. Hunter, S. Sudarsanam, “Evolution of Protein Kinase Signaling from Yeast to Man," Trends in Biochemical
Sciences 27, no. 10 (2002): 514-520.
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KEY TERMS
apoptosis programmed cell death
autocrine signal signal that is sent and received by the same or similar nearby cells
autoinducer signaling molecule secreted by bacteria to communicate with other bacteria of its kind
cell-surface receptor cell-surface protein that transmits a signal from the exterior of the cell to the interior, even
though the ligand does not enter the cell
chemical synapse small space between axon terminals and dendrites of nerve cells where neurotransmitters
function
cyclic AMP (cAMP) second messenger that is derived from ATP
cyclic AMP-dependent kinase (also, protein kinase A, or PKA) kinase that is activated by binding to cAMP
diacylglycerol (DAG) cleavage product of PIP 2 that is used for signaling within the plasma membrane
dimer chemical compound formed when two molecules join together
dimerization (of receptor proteins) interaction of two receptor proteins to form a functional complex called a
dimer
endocrine cell cell that releases ligands involved in endocrine signaling (hormones)
endocrine signal long-distance signal that is delivered by ligands (hormones) traveling through an organism's
circulatory system from the signaling cell to the target cell
enzyme-linked receptor cell-surface receptor with intracellular domains that are associated with membrane-
bound enzymes
extracellular domain region of a cell-surface receptor that is located on the cell surface
G-protein-linked receptor cell-surface receptor that activates membrane-bound G-proteins to transmit a signal
from the receptor to nearby membrane components
growth factor ligand that binds to cell-surface receptors and stimulates cell growth
inhibitor molecule that binds to a protein (usually an enzyme) and keeps it from functioning
inositol phospholipid lipid present at small concentrations in the plasma membrane that is converted into a
second messenger; it has inositol (a carbohydrate) as its hydrophilic head group
inositol triphosphate (IP 3 ) cleavage product of PIP 2 that is used for signaling within the cell
intercellular signaling communication between a cell
internal receptor (also, intracellular receptor) receptor protein that is located in the cytosol of a cell and binds to
ligands that pass through the plasma membrane
intracellular mediator (also, second messenger) small molecule that transmits signals within a cell
intracellular signaling communication within cells
ion channel-linked receptor cell-surface receptor that forms a plasma membrane channel, which opens when
a ligand binds to the extracellular domain (ligand-gated channels)
kinase enzyme that catalyzes the transfer of a phosphate group from ATP to another molecule
ligand molecule produced by a signaling cell that binds with a specific receptor, delivering a signal in the
process
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mating factor signaling molecule secreted by yeast cells to communicate to nearby yeast cells that they are
available to mate
neurotransmitter chemical ligand that carries a signal from one nerve cell to the next
paracrine signal signal between nearby cells that is delivered by ligands traveling in the liquid medium in the
space between the cells
phosphatase enzyme that removes the phosphate group from a molecule that has been previously
phosphorylated
phosphodiesterase enzyme that degrades cAMP, producing AMP, to terminate signaling
quorum sensing method of cellular communication used by bacteria that informs them of the abundance of
similar (or different) bacteria in the environment
receptor protein in or on a target cell that bind to ligands
second messenger small, non-protein molecule that propagates a signal within the cell after activation of a
receptor causes its release
signal integration interaction of signals from two or more different cell-surface receptors that merge to activate
the same response in the cell
signal transduction propagation of the signal through the cytoplasm (and sometimes also the nucleus) of the
cell
signaling cell cell that releases signal molecules that allow communication with another cell
signaling pathway (also signaling cascade) chain of events that occurs in the cytoplasm of the cell to
propagate the signal from the plasma membrane to produce a response
synaptic signal chemical signal (neurotransmitter) that travels between nerve cells
target cell cell that has a receptor for a signal or ligand from a signaling cell
CHAPTER SUMMARY
9.1 Signaling Molecules and Cellular Receptors
Cells communicate by both inter- and intracellular signaling. Signaling cells secrete ligands that bind to target
cells and initiate a chain of events within the target cell. The four categories of signaling in multicellular
organisms are paracrine signaling, endocrine signaling, autocrine signaling, and direct signaling across gap
junctions. Paracrine signaling takes place over short distances. Endocrine signals are carried long distances
through the bloodstream by hormones, and autocrine signals are received by the same cell that sent the signal
or other nearby cells of the same kind. Gap junctions allow small molecules, including signaling molecules, to
flow between neighboring cells.
Internal receptors are found in the cell cytoplasm. Here, they bind ligand molecules that cross the plasma
membrane; these receptor-ligand complexes move to the nucleus and interact directly with cellular DNA. Cell-
surface receptors transmit a signal from outside the cell to the cytoplasm. Ion channel-linked receptors, when
bound to their ligands, form a pore through the plasma membrane through which certain ions can pass. G-
protein-linked receptors interact with a G-protein on the cytoplasmic side of the plasma membrane, promoting
the exchange of bound GDP for GTP and interacting with other enzymes or ion channels to transmit a signal.
Enzyme-linked receptors transmit a signal from outside the cell to an intracellular domain of a membrane-
bound enzyme. Ligand binding causes activation of the enzyme. Small hydrophobic ligands (like steroids) are
able to penetrate the plasma membrane and bind to internal receptors. Water-soluble hydrophilic ligands are
unable to pass through the membrane; instead, they bind to cell-surface receptors, which transmit the signal to
the inside of the cell.
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9.2 Propagation of the Signal
Ligand binding to the receptor allows for signal transduction through the cell. The chain of events that conveys
the signal through the cell is called a signaling pathway or cascade. Signaling pathways are often very complex
because of the interplay between different proteins. A major component of cell signaling cascades is the
phosphorylation of molecules by enzymes known as kinases. Phosphorylation adds a phosphate group to
serine, threonine, and tyrosine residues in a protein, changing their shapes, and activating or inactivating the
protein. Small molecules like nucleotides can also be phosphorylated. Second messengers are small, non¬
protein molecules that are used to transmit a signal within a cell. Some examples of second messengers are
calcium ions (Ca 2+ ), cyclic AMP (cAMP), diacylglycerol (DAG), and inositol triphosphate (IP 3 ).
9.3 Response to the Signal
The initiation of a signaling pathway is a response to external stimuli. This response can take many different
forms, including protein synthesis, a change in the cell’s metabolism, cell growth, or even cell death. Many
pathways influence the cell by initiating gene expression, and the methods utilized are quite numerous. Some
pathways activate enzymes that interact with DNA transcription factors. Others modify proteins and induce
them to change their location in the cell. Depending on the status of the organism, cells can respond by storing
energy as glycogen or fat, or making it available in the form of glucose. A signal transduction pathway allows
muscle cells to respond to immediate requirements for energy in the form of glucose. Cell growth is almost
always stimulated by external signals called growth factors. Uncontrolled cell growth leads to cancer, and
mutations in the genes encoding protein components of signaling pathways are often found in tumor cells.
Programmed cell death, or apoptosis, is important for removing damaged or unnecessary cells. The use of
cellular signaling to organize the dismantling of a cell ensures that harmful molecules from the cytoplasm are
not released into the spaces between cells, as they are in uncontrolled death, necrosis. Apoptosis also ensures
the efficient recycling of the components of the dead cell. Termination of the cellular signaling cascade is very
important so that the response to a signal is appropriate in both timing and intensity. Degradation of signaling
molecules and dephosphorylation of phosphorylated intermediates of the pathway by phosphatases are two
ways to terminate signals within the cell.
9.4 Signaling in Single-Celled Organisms
Yeasts and multicellular organisms have similar signaling mechanisms. Yeasts use cell-surface receptors and
signaling cascades to communicate information on mating with other yeast cells. The signaling molecule
secreted by yeasts is called mating factor.
Bacterial signaling is called quorum sensing. Bacteria secrete signaling molecules called autoinducers that are
either small, hydrophobic molecules or peptide-based signals. The hydrophobic autoinducers, such as AHL,
bind transcription factors and directly affect gene expression. The peptide-based molecules bind kinases and
initiate signaling cascades in the cells.
VISUAL CONNECTION QUESTIONS
1. Figure 9.8 HER2 is a receptor tyrosine kinase. In
30 percent of human breast cancers, HER2 is
permanently activated, resulting in unregulated cell
division. Lapatinib, a drug used to treat breast
cancer, inhibits HER2 receptor tyrosine kinase
autophosphorylation (the process by which the
receptor adds phosphates onto itself), thus reducing
tumor growth by 50 percent. Besides
autophosphorylation, which of the following steps
would be inhibited by Lapatinib?
a. Signaling molecule binding, dimerization,
and the downstream cellular response.
b. Dimerization, and the downstream cellular
response.
c. The downstream cellular response.
d. Phosphatase activity, dimerization, and the
downsteam cellular response.
2. Figure 9.10 In certain cancers, the GTPase
activity of the RAS G-protein is inhibited. This means
that the RAS protein can no longer hydrolyze GTP
into GDP. What effect would this have on
downstream cellular events?
3. Figure 9.17 Which of the following statements
about quorum sensing is false?
a. Autoinducer must bind to receptor to turn on
transcription of genes responsible for the
production of more autoinducer.
b. The receptor stays in the bacterial cell, but
the autoinducer diffuses out.
c. Autoinducer can only act on a different cell:
it cannot act on the cell in which it is made.
d. Autoinducer turns on genes that enable the
bacteria to form a biofilm.
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4. Figure 9.18 What advantage might biofilm catheter?
production confer on the S. aureus inside the
REVIEW QUESTIONS
5. What property prevents the ligands of cell-surface
receptors from entering the cell?
a. The molecules bind to the extracellular
domain.
b. The molecules are hydrophilic and cannot
penetrate the hydrophobic interior of the
plasma membrane.
c. The molecules are attached to transport
proteins that deliver them through the
bloodstream to target cells.
d. The ligands are able to penetrate the
membrane and directly influence gene
expression upon receptor binding.
6. The secretion of hormones by the pituitary gland is
an example of_.
a. autocrine signaling
b. paracrine signaling
c. endocrine signaling
d. direct signaling across gap junctions
7. Why are ion channels necessary to transport ions
into or out of a cell?
a. Ions are too large to diffuse through the
membrane.
b. Ions are charged particles and cannot
diffuse through the hydrophobic interior of
the membrane.
c. Ions do not need ion channels to move
through the membrane.
d. Ions bind to carrier proteins in the
bloodstream, which must be removed
before transport into the cell.
8. Endocrine signals are transmitted more slowly
than paracrine signals because_.
a. the ligands are transported through the
bloodstream and travel greater distances
b. the target and signaling cells are close
together
c. the ligands are degraded rapidly
d. the ligands don't bind to carrier proteins
during transport
9. A scientist notices that when she adds a small,
water-soluble molecule to a dish of cells, the cells
turn off transcription of a gene. She hypothesizes that
the ligand she added binds to a(n)_receptor.
a. Intracellular
b. Hormone
c. Enzyme-linked
d. Gated ion channel-linked
10. Where do DAG and IP 3 originate?
a. They are formed by phosphorylation of
cAMP.
b. They are ligands expressed by signaling
cells.
c. They are hormones that diffuse through the
plasma membrane to stimulate protein
production.
d. They are the cleavage products of the
inositol phospholipid, PIP 2 .
11. What property enables the residues of the amino
acids serine, threonine, and tyrosine to be
phosphorylated?
a. They are polar.
b. They are non-polar.
c. They contain a hydroxyl group.
d. They occur more frequently in the amino
acid sequence of signaling proteins.
12. Histamine binds to the HI G-protein-linked
receptor to initiate the itchiness and airway
constriction associated with an allergic response. If a
mutation in the associated G-protein’s alpha subunit
prevented the hydrolysis of GTP how would the
allergic response change?
a. More severe allergic response compared to
normal G-protein signaling.
b. Less severe allergic response compared to
normal G-protein signaling.
c. No allergic response.
d. No change compared to normal G-protein
signaling.
13. A scientist observes a mutation in the
transmembrane region of EGFR that eliminates its
ability to be stabilized by binding interactions during
dimerization after ligand binding. Which hypothesis
regarding the effect of this mutation on EGF signaling
is most likely to be correct?
a. EGF signaling cascades would be active for
longer in the cell.
b. EGF signaling cascades would be active for
a shorter period of time in the cell.
c. EGF signaling cascades would not occur.
d. EGF signaling would be unaffected.
14. What is the function of a phosphatase?
a. A phosphatase removes phosphorylated
amino acids from proteins.
b. A phosphatase removes the phosphate
group from phosphorylated amino acid
residues in a protein.
c. A phosphatase phosphorylates serine,
threonine, and tyrosine residues.
d. A phosphatase degrades second
messengers in the cell.
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15. How does NF-kB induce gene expression?
a. A small, hydrophobic ligand binds to NF-kB,
activating it.
b. Phosphorylation of the inhibitor !k-B
dissociates the complex between it and NF-
kB, and allows NF-kB to enter the nucleus
and stimulate transcription.
c. NF-kB is phosphorylated and is then free to
enter the nucleus and bind DNA.
d. NF-kB is a kinase that phosphorylates a
transcription factor that binds DNA and
promotes protein production.
16. Apoptosis can occur in a cell when the cell is
a. damaged
b. no longer needed
c. infected by a virus
d. all of the above
17. What is the effect of an inhibitor binding an
enzyme?
a. The enzyme is degraded.
b. The enzyme is activated.
c. The enzyme is inactivated.
d. The complex is transported out of the cell.
18. How does PKC’s signaling role change in
response to growth factor signaling versus an
immune response?
a. PKC interacts directly with signaling
molecules in both cascades, but only
exhibits kinase activity during growth factor
signaling.
b. PKC interacts directly with signaling
molecules in growth factor cascades, but
interacts with signaling inhibitors during
immune signaling.
c. PKC amplifies growth factor cascades, but
turns off immune cascades.
d. PKC is activated during growth factor
cascades, but is inactivated during immune
response cascades.
CRITICAL THINKING QUESTIONS
23. What is the difference between intracellular
signaling and intercellular signaling?
24. How are the effects of paracrine signaling limited
to an area near the signaling cells?
25. What are the differences between internal
receptors and cell-surface receptors?
26. Cells grown in the laboratory are mixed with a
dye molecule that is unable to pass through the
plasma membrane. If a ligand is added to the cells,
observations show that the dye enters the cells. What
type of receptor did the ligand bind to on the cell
surface?
19. A scientist notices that a cancer cell line fails to
die when he adds an inducer of apoptosis to his
culture of cells. Which hypothesis could explain why
the cells fail to die?
a. The cells have a mutation that prevents the
initiation of apoptosis signaling.
b. The cells have lost expression of the
receptor for the apoptosis-inducing ligand.
c. The cells overexpress a growth factor
pathway that inhibits apoptosis.
d. All of the above.
20. Which type of molecule acts as a signaling
molecule in yeasts?
a. steroid
b. autoinducer
c. mating factor
d. second messenger
21. Quorum sensing is triggered to begin when
a. treatment with antibiotics occurs
b. bacteria release growth hormones
c. bacterial protein expression is switched on
d. a sufficient number of bacteria are present
22. A doctor is researching new ways to treat biofilms
on artificial joints. Which approach would best help
prevent bacterial colonization of the medical
implants?
a. Increase antibiotic dosing
b. Create implants with rougher surfaces
c. Vaccinate patients against all pathogenic
bacteria
d. Inhibit quorum sensing
27. Insulin is a hormone that regulates blood sugar
by binding to its receptor, insulin receptor tyrosine
kinase. How does insulin’s behavior differ from
steroid hormone signaling, and what can you infer
about its structure?
28. The same second messengers are used in many
different cells, but the response to second
messengers is different in each cell. How is this
possible?
29. What would happen if the intracellular domain of
a cell-surface receptor was switched with the domain
from another receptor?
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30. If a cell developed a mutation in its MAP2K1
gene (encodes the MEK protein) that prevented MEK
from being recognized by phosphatases, how would
the EGFR signaling cascade and the cell’s behavior
change?
31. What is a possible result of a mutation in a kinase
that controls a pathway that stimulates cell growth?
32. How does the extracellular matrix control the
growth of cells?
33. A scientist notices that a cancer cell line shows
high levels of phosphorylated ERK in the absence of
EGR What are two possible explanations for the
increase in phosphorylated ERK? Be specific in
which proteins are involved.
34. What characteristics make yeasts a good model
for learning about signaling in humans?
35. Why is signaling in multicellular organisms more
complicated than signaling in single-celled
organisms?
36. Pseudomonas infections are very common in
hospital settings. Why would it be important for
doctors to determine the bacterial load before
treating an infected patient?
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10 I CELL
REPRODUCTION
Figure 10.1 A sea urchin begins life as a single diploid cell (zygote) that (a) divides through cell division to form two
genetically identical daughter cells, visible here through scanning electron microscopy (SEM). After four rounds of cell
division, (b) there are 16 cells, as seen in this SEM image. After many rounds of cell division, the individual develops
into a complex, multicellular organism, as seen in this (c) mature sea urchin, (credit a: modification of work by Evelyn
Spiegel, Louisa Howard; credit b: modification of work by Evelyn Spiegel, Louisa Howard; credit c: modification of work
by Marco Busdraghi; scale-bar data from Matt Russell)
Chapter Outline
10.1: Cell Division
10.2: The Cell Cycle
10.3: Control of the Cell Cycle
10.4: Cancer and the Cell Cycle
10.5: Prokaryotic Cell Division
Introduction
A human, like every sexually reproducing organism, begins life as a fertilized egg (embryo) or zygote. In our
species, billions of cell divisions subsequently must occur in a controlled manner in order to produce a complex,
multicellular human comprising trillions of cells. Thus, the original single-celled zygote is literally the ancestor
of all cells in the body. However, once a human is fully grown, cell reproduction is still necessary to repair and
regenerate tissues, and sometimes to increase our size! In fact, all multicellular organisms use cell division for
growth and the maintenance and repair of cells and tissues. Cell division is closely regulated, and the occasional
failure of this regulation can have life-threatening consequences. Single-celled organisms may also use cell
division as their method of reproduction.
10.1 1 Cell Division
By the end of this section, you will be able to do the following:
• Describe the structure of prokaryotic and eukaryotic genomes
• Distinguish between chromosomes, genes, and traits
• Describe the mechanisms of chromosome compaction
The continuity of life from one cell to another has its foundation in the reproduction of cells by way of the cell
cycle. The cell cycle is an orderly sequence of events that describes the stages of a cell’s life from the division
of a single parent cell to the production of two new genetically identical daughter cells.
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Genomic DNA
Before discussing the steps a cell must undertake to replicate and divide its DNA, a deeper understanding of the
structure and function of a cell’s genetic information is necessary. A cell’s DNA, packaged as a double-stranded
DNA molecule, is called its genome. In prokaryotes, the genome is composed of a single, double-stranded DNA
molecule in the form of a loop or circle (Figure 10.2). The region in the cell containing this genetic material is
called a nucleoid. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for
normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new
genes that the recipient can add to their chromosomal DNA. Antibiotic resistance is one trait that often spreads
through a bacterial colony through plasmid exchange from resistant donors to recipient cells.
(DNA)
Figure 10.2 Prokaryotes, including both Bacteria and Archaea, have a single, circular chromosome located in a central
region called the nucleoid.
In eukaryotes, the genome consists of several double-stranded linear DNA molecules (Figure 10.3). Each
species of eukaryotes has a characteristic number of chromosomes in the nuclei of its cells. Human body
(somatic) cells have 46 chromosomes, while human gametes (sperm or eggs) have 23 chromosomes each.
A typical body cell contains two matched or homologous sets of chromosomes (one set from each biological
parent)—a configuration known as diploid. (Note: The letter n is used to represent a single set of chromosomes;
therefore, a diploid organism is designated 2n.) Human cells that contain one set of chromosomes are called
gametes, or sex cells; these are eggs and sperm, and are designated In, or haploid.
Upon fertilization, each gamete contributes one set of chromosomes, creating a diploid cell containing matched
pairs of chromosomes called homologous (“same knowledge”) chromosomes. Homologous chromosomes are
the same length and have specific nucleotide segments called genes in exactly the same location, or locus.
Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins.
Traits are the variations of those characteristics. For example, hair color is a characteristic with traits that are
blonde, brown, or black, and many colors in between.
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Figure 10.3 There are 23 pairs of homologous chromosomes in a female human somatic cell. The condensed
chromosomes are viewed within the nucleus (top), removed from a cell during mitosis (also called karyokinesis or
nuclear division) and spread out on a slide (right), and artificially arranged according to length (left); an arrangement
like this is called a karyotype. In this image, the chromosomes were exposed to fluorescent stains for differentiation of
the different chromosomes. A method of staining called “chromosome painting" employs fluorescent dyes that highlight
chromosomes in different colors, (credit: National Human Genome Project/NIH)
Each copy of a homologous pair of chromosomes originates from a different parent; therefore, the different
genes (alleles) themselves are not identical, although they code for the same traits such as “hair color." The
variation of individuals within a species is due to the specific combination of the genes inherited from both
parents. Even a slightly altered sequence of nucleotides within a gene can result in an alternative trait. For
example, there are three possible gene sequences on the human chromosome that code for blood type:
sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome
that determines blood type, the blood type (the trait) is determined by the two alleles of the marker gene that are
inherited. It is possible to have two copies of the same gene sequence on both homologous chromosomes, with
one on each (for example, AA, BB, or OO), or two different sequences, such as AB, AO, or BO.
Apparently minor variations of traits, such as blood type, eye color, and handedness, contribute to the natural
variation found within a species, but even though they seem minor, these traits may be connected with the
expression of other traits as of yet unknown. However, if the entire DNA sequence from any pair of human
homologous chromosomes is compared, the difference is much less than one percent. The sex chromosomes,
X and Y, are the single exception to the rule of homologous chromosome uniformity: Other than a small amount
of homology that is necessary to accurately produce gametes, the genes found on the X and Y chromosomes
are different.
Eukaryotic Chromosomal Structure and Compaction
If the DNA from all 46 chromosomes in a human cell nucleus were laid out end-to-end, it would measure
approximately two meters; however, its diameter would be only 2 nm! Considering that the size of a typical
human cell is about 10 pm (100,000 cells lined up to equal one meter), DNA must be tightly packaged to fit in
the cell’s nucleus. At the same time, it must also be readily accessible for the genes to be expressed. For this
reason, the long strands of DNA are condensed into compact chromosomes during certain stages of the cell
cycle. There are a number of ways that chromosomes are compacted.
In the first level of compaction, short stretches of the DNA double helix wrap around a core of eight histone
proteins at regular intervals along the entire length of the chromosome (Figure 10.4). The DNA-histone complex
is called chromatin. The beadlike, histone DNA complex is called a nucleosome, and DNA connecting the
nucleosomes is called linker DNA. A DNA molecule in this form is about seven times shorter than the double
helix without the histones, and the beads are about 10 nm in diameter, in contrast with the 2-nm diameter of a
DNA double helix.
The second level of compaction occurs as the nucleosomes and the linker DNA between them coil into a 30-nm
chromatin fiber. This coiling further condenses the chromosome so that it is now about 50 times shorter than the
extended form.
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in the third level of compaction, a variety of fibrous proteins is used to “pack the chromatin.” These fibrous
proteins also ensure that each chromosome in a non-dividing cell occupies a particular area of the nucleus that
does not overlap with that of any other chromosome (see the top image in Figure 10.3).
Figure 10.4 Double-stranded DNA wraps around histone proteins to form nucleosomes that create the appearance
of “beads on a string.” The nucleosomes are coiled into a 30-nm chromatin fiber. When a cell undergoes mitosis, the
chromosomes condense even further.
DNA replicates in the S phase of interphase, which technically is not a part of mitosis, but must always precede it.
After replication, the chromosomes are composed of two linked sister chromatids. When fully compact, the pairs
of identically packed chromosomes are bound to each other by cohesin proteins. The connection between the
sister chromatids is closest in a region called the centromere. The conjoined sister chromatids, with a diameter
of about 1 pm, are visible under a light microscope. The centromeric region is highly condensed and thus will
appear as a constricted area.
LINK TQ LEARNING
This animation illustrates the different levels of chromosome packing.
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66477/1.3/#eip-
idll65802978636)
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10.2 | The Cell Cycle
By the end of this section, you will be able to do the following:
• Describe the three stages of interphase
• Discuss the behavior of chromosomes during karyokinesis/mitosis
• Explain how the cytoplasmic content is divided during cytokinesis
• Define the quiescent Go phase
The cell cycle is an ordered series of events involving cell growth and cell division that produces two new
daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully
regulated stages of growth, DNA replication, and nuclear and cytoplasmic division that ultimately produces two
identical (clone) cells. The cell cycle has two major phases: interphase and the mitotic phase (Figure 10.5).
During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and
cytoplasmic contents are separated, and the cell cytoplasm is typically partitioned by a third process of the cell
cycle called cytokinesis. We should note, however, that interphase and mitosis (kayrokinesis) may take place
without cytokinesis, in which case cells with multiple nuclei (multinucleate cells) are produced.
Mitotic phase
Intemhase
Formation
of 2 daughter
cells
Interphase
Interphase
Figure 10.5 The cell cycle in multicellular organisms consists of interphase and the mitotic phase. During interphase,
the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase,
the duplicated chromosomes are segregated and distributed into daughter nuclei. Following mitosis, the cytoplasm is
usually divided as well by cytokinesis, resulting in two genetically identical daughter cells.
Interphase
During interphase, the cell undergoes normal growth processes while also preparing for cell division. In order for
a cell to move from interphase into the mitotic phase, many internal and external conditions must be met. The
three stages of interphase are called Gi, S, and G 2 -
Gi Phase (First Gap)
The first stage of interphase is called the Gi phase (first gap) because, from a microscopic point of view, little
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change is visible. However, during the Gi stage, the cell is quite active at the biochemical level. The cell is
accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating
sufficient energy reserves to complete the task of replicating each chromosome in the nucleus.
S Phase (Synthesis of DNA)
Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase,
DNA replication can proceed through the mechanisms that result in the formation of identical pairs of DNA
molecules—sister chromatids—that are firmly attached to the centromeric region. The centrosome is also
duplicated during the S phase. The two centrosomes of homologous chromosomes will give rise to the mitotic
spindle, the apparatus that orchestrates the movement of chromosomes during mitosis. For example, roughly
at the center of each animal cell, the centrosomes are associated with a pair of rod-like objects, the centrioles,
which are positioned at right angles to each other. Centrioles help organize cell division. We should note,
however, that centrioles are not present in the centrosomes of other eukaryotic organisms, such as plants and
most fungi.
G 2 Phase (Second Gap)
In the G 2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome
manipulation and movement. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide
resources for the mitotic phase. There may be additional cell growth during G 2 . The final preparations for the
mitotic phase must be completed before the cell is able to enter the first stage of mitosis.
The Mitotic Phase
The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated,
and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis,
or nuclear division. As we have just seen, the second portion of the mitotic phase (and often viewed as a
process separate from and following mitosis) is called cytokinesis—the physical separation of the cytoplasmic
components into the two daughter cells.
Revisit the stages of mitosis at this site (http:// 0 penstaxc 0 Mege. 0 rg/l/Cell_cycle_mit 0 ) .
Karyokinesis (Mitosis)
Karyokinesis, also known as mitosis, is divided into a series of phases—prophase, prometaphase, metaphase,
anaphase, and telophase—that result in the division of the cell nucleus (Figure 10.6).
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Prophase
Chromosomes condense and become visible
Spindle fibers emerge from the centrosomes
Nuclear envelope breaks down
Nucleolus disappears
Metaphase
Chromosomes continue to condense
Kinetochores appear at the centromeres
Mitotic spindle microtubules
attach to kinetochores
Centrosomes move toward
opposite poles
■ Mitotic spindle is fully developed, centrosomes are
at opposite poles of the cell
1 Chromosomes are lined up at the
metaphase plate
• Each sister chromatid is attached
to a spindle fiber originating from
opposite poles
Anaphase
Cohesin proteins binding the sister chromatids
together break down
Sister chromatids (now called chromosomes) are
pulled toward opposite poles
Non-kmetochore spindle fibers
lengthen, elongating the cell
Chromosomes arrive at opposite poles
and begin to decondense
Nuclear envelope material surrounds
each set of chromosomes
The mitotic spindle breaks
down
Animal cells: a cleavage furrow
separates the daughter cells
Plant cells: a cell plate
separates the daughter cells
Telophase
Cytokinesis
Figure 10.6 Karyokinesis (or mitosis) is divided into five stages—prophase, prometaphase, metaphase, anaphase,
and telophase. We should note that this is a continuous process, and that the divisions between the stages are
not discrete. The pictures at the bottom were taken by fluorescence microscopy (hence, the black background) of
cells artificially stained by fluorescent dyes: blue fluorescence indicates DNA (chromosomes) and green fluorescence
indicates microtubules (spindle apparatus), (credit “mitosis drawings”: modification of work by Mariana Ruiz Villareal;
credit “micrographs”: modification of work by Roy van Heesbeen; credit “cytokinesis micrograph”: Wadsworth Center/
New York State Department of Health; scale-bar data from Matt Russell)
Prophase (the “first phase”): the nuclear envelope starts to dissociate into small vesicles, and the membranous
organelles (such as the Golgi complex [Golgi apparatus] and the endoplasmic reticulum), fragment and disperse
toward the periphery of the cell. The nucleolus disappears (disperses) as well, and the centrosomes begin
to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the
centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil
more tightly with the aid of condensin proteins and now become visible under a light microscope.
Prometaphase (the “first change phase’’): Many processes that began in prophase continue to advance. The
remnants of the nuclear envelope fragment further, and the mitotic spindle continues to develop as more
microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become even
more condensed and discrete. Each sister chromatid develops a protein structure called a kinetochore in
its centromeric region (Figure 10.7). The proteins of the kinetochore attract and bind to the mitotic spindle
microtubules. As the spindle microtubules extend from the centrosomes, some of these microtubules come into
contact with and firmly bind to the kinetochores. Once a mitotic fiber attaches to a chromosome, the chromosome
will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister
chromatids will be attached via their kinetochores to microtubules from opposing poles. Spindle microtubules
that do not engage the chromosomes are called polar microtubules. These microtubules overlap each other
midway between the two poles and contribute to cell elongation. Astral microtubules are located near the poles,
aid in spindle orientation, and are required for the regulation of mitosis.
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Centromeric
region
Mitotic spindle
microtubules
Kinetochore
Sister chromatids
Figure 10.7 During prometaphase, mitotic spindle microtubules from opposite poles attach to each sister chromatid at
the kinetochore. In anaphase, the connection between the sister chromatids breaks down, and the microtubules pull
the chromosomes toward opposite poles.
Metaphase (the “change phase”): All the chromosomes are aligned in a plane called the metaphase plate, or
the equatorial plane, roughly midway between the two poles of the cell. The sister chromatids are still tightly
attached to each other by cohesin proteins. At this time, the chromosomes are maximally condensed.
Anaphase (“upward phase”): The cohesin proteins degrade, and the sister chromatids separate at the
centromere. Each chromatid, now called a single chromosome, is pulled rapidly toward the centrosome to which
its microtubule is attached. The cell becomes visibly elongated (oval shaped) as the polar microtubules slide
against each other at the metaphase plate where they overlap.
Telophase (the “distance phase"): the chromosomes reach the opposite poles and begin to decondense
(unravel), relaxing once again into a stretched-out chromatin configuration. The mitotic spindles are
depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter
cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area.
Cytokinesis
Cytokinesis, or “cell motion," is sometimes viewed as the second main stage of the mitotic phase, during which
cell division is completed via the physical separation of the cytoplasmic components into two daughter cells
However, as we have seen earlier, cytokinesis can also be viewed as a separate phase, which may or may not
take place following mitosis. If cytokinesis does take place, cell division is not complete until the cell components
have been apportioned and completely separated into the two daughter cells. Although the stages of mitosis are
similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such
as plant cells.
in animal cells, cytokinesis typically starts during late anaphase. A contractile ring composed of actin filaments
forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of
the cell inward, forming a fissure. This fissure is called the cleavage furrow. The furrow deepens as the actin
ring contracts, and eventually the membrane is cleaved in two (Figure 10.8).
in plant cells, a new cell wall must form between the daughter cells. During interphase, the Golgi apparatus
accumulates enzymes, structural proteins, and glucose molecules prior to breaking into vesicles and dispersing
throughout the dividing cell. During telophase, these Golgi vesicles are transported on microtubules to form a
phragmoplast (a vesicular structure) at the metaphase plate. There, the vesicles fuse and coalesce from the
center toward the cell walls; this structure is called a cell plate. As more vesicles fuse, the cell plate enlarges
until it merges with the cell walls at the periphery of the cell. Enzymes use the glucose that has accumulated
between the membrane layers to build a new cell wall. The Golgi membranes become parts of the plasma
membrane on either side of the new cell wall (Figure 10.8).
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Animal cell
Figure 10.8 During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate. The ring
contracts, forming a cleavage furrow, which divides the cell in two. In plant cells, Golgi vesicles coalesce at the former
metaphase plate, forming a phragmoplast. A cell plate formed by the fusion of the vesicles of the phragmoplast grows
from the center toward the cell walls, and the membranes of the vesicles fuse to form a plasma membrane that divides
the cell in two.
Go Phase
Not all cells adhere to the classic cell-cycle pattern in which a newly formed daughter cell immediately enters
the preparatory phases of interphase, closely followed by the mitotic phase, and cytokinesis. Cells in Go phase
are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the
cell cycle. Some cells enter Go temporarily due to environmental conditions such as availability of nutrients, or
stimulation by growth factors. The cell will remain in this phase until conditions improve or until an external signal
triggers the onset of Gi. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells,
remain in Go permanently.
visual
CONNECTION
Which of the following is the correct order of events in mitosis?
a. Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic
spindle. The nucleus reforms and the cell divides. Cohesin proteins break down and the sister
chromatids separate.
b. The kinetochore becomes attached to the mitotic spindle. Cohesin proteins break down and the sister
chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus reforms and the
cell divides.
c. The kinetochore becomes attached to the cohesin proteins. Sister chromatids line up at the metaphase
plate. The kinetochore breaks down and the sister chromatids separate. The nucleus reforms and the
cell divides.
d. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase
plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the
cell divides.
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scientific methf d CONNECTION
Determine the Time Spent in Cell-Cycle Stages
Problem: How long does a cell spend in interphase compared to each stage of mitosis?
Background: A prepared microscope slide of whitefish blastula cross-sections will show cells arrested in
various stages of the cell cycle. (Note: It is not visually possible to separate the stages of interphase from
each other, but the mitotic stages are readily identifiable.) If 100 cells are examined, the number of cells in
each identifiable cell-cycle stage will give an estimate of the time it takes for the cell to complete that stage.
Problem Statement: Given the events included in all of interphase and those that take place in each stage
of mitosis, estimate the length of each stage based on a 24-hour cell cycle. Before proceeding, state your
hypothesis.
Test your hypothesis: Test your hypothesis by doing the following:
1. Place a fixed and stained microscope slide of whitefish blastula cross-sections under the scanning
objective of a light microscope.
2. Locate and focus on one of the sections using the low-power objective of your microscope. Notice that
the section is a circle composed of dozens of closely packed individual cells.
3. Switch to the medium-power objective and refocus. With this objective, individual cells are clearly
visible, but the chromosomes will still be very small.
4. Switch to the high-power objective and slowly move the slide left to right, and up and down to view
all the cells in the section (Figure 10.9). As you scan, you will notice that most of the cells are not
undergoing mitosis but are in the interphase period of the cell cycle.
Scan the cells to identify the
mitotic stage of the cells.
(a)
Figure 10.9 Slowly scan whitefish blastula cells with the high-power objective as illustrated in image (a)
to identify their mitotic stage, (b) A microscopic image of the scanned cells is shown, (credit “micrograph”:
modification of work by Linda Flora; scale-bar data from Matt Russell)
5. Practice identifying the various stages of the cell cycle, using the drawings of the stages as a guide
(Figure 10.6).
6. Once you are confident about your identification, begin to record the stage of each cell you encounter
as you scan left to right, and top to bottom across the blastula section.
7. Keep a tally of your observations and stop when you reach 100 cells identified.
8. The larger the sample size (total number of cells counted), the more accurate the results. If possible,
gather and record group data prior to calculating percentages and making estimates.
Record your observations: Make a table similar to Table 10.1 within which to record your observations.
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Results of Cell Stage Identification
Phase or Stage Individual Totals Group Totals Percent
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Totals
100
100
100 percent
Table 10.1
Analyze your data/report your results: To find the length of time whitefish blastula cells spend in each
stage, multiply the percent (recorded as a decimal) by 24 hours. Make a table similar to Table 10.2 to
illustrate your data.
Estimate of Cell Stage Length
Phase or Stage Percent Time in Hours
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Table 10.2
Draw a conclusion: Did your results support your estimated times? Were any of the outcomes unexpected?
If so, discuss those events in that stage that may have contributed to the calculated time.
10.3 | Control of the Cell Cycle
By the end of this section, you will be able to do the following:
• Understand how the cell cycle is controlled by mechanisms that are both internal and external to the cell
• Explain how the three internal “control checkpoints” occur at the end of Gi, at the G 2 /M transition, and
during metaphase
• Describe the molecules that control the cell cycle through positive and negative regulation
The length of the cell cycle is highly variable, even within the cells of a single organism. In humans, the frequency
of cell turnover ranges from a few hours in early embryonic development, to an average of two to five days for
epithelial cells, and to an entire human lifetime spent in Go by specialized cells, such as cortical neurons or
cardiac muscle cells.
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There is also variation in the time that a cell spends in each phase of the cell cycle. When rapidly dividing
mammalian cells are grown in a culture (outside the body under optimal growing conditions), the length of
the cell cycle is about 24 hours. In rapidly dividing human cells with a 24-hour cell cycle, the Gi phase lasts
approximately nine hours, the S phase lasts 10 hours, the G 2 phase lasts about four and one-half hours, and the
M phase lasts approximately one-half hour. By comparison, in fertilized eggs (and early embryos) of fruit flies,
the cell cycle is completed in about eight minutes. This is because the nucleus of the fertilized egg divides many
times by mitosis but does not go through cytokinesis until a multinucleate “zygote” has been produced, with
many nuclei located along the periphery of the cell membrane, thereby shortening the time of the cell division
cycle. The timing of events in the cell cycle of both “invertebrates" and “vertebrates” is controlled by mechanisms
that are both internal and external to the cell.
Regulation of the Cell Cycle by External Events
Both the initiation and inhibition of cell division are triggered by events external to the cell when it is about to
begin the replication process. An event may be as simple as the death of nearby cells or as sweeping as the
release of growth-promoting hormones, such as human growth hormone (HGH or hGH). A lack of HGH can
inhibit cell division, resulting in dwarfism, whereas too much HGH can result in gigantism. Crowding of cells can
also inhibit cell division. In contrast, a factor that can initiate cell division is the size of the cell: As a cell grows, it
becomes physiologically inefficient due to its decreasing surface-to-volume ratio. The solution to this problem is
to divide.
Whatever the source of the message, the cell receives the signal, and a series of events within the cell allows it
to proceed into interphase. Moving forward from this initiation point, every parameter required during each cell
cycle phase must be met or the cycle cannot progress.
Regulation at Internal Checkpoints
It is essential that the daughter cells produced be exact duplicates of the parent cell. Mistakes in the duplication
or distribution of the chromosomes lead to mutations that may be passed forward to every new cell produced
from an abnormal cell. To prevent a compromised cell from continuing to divide, there are internal control
mechanisms that operate at three main cell-cycle checkpoints: A checkpoint is one of several points in the
eukaryotic cell cycle at which the progression of a cell to the next stage in the cycle can be halted until conditions
are favorable. These checkpoints occur near the end of Gi, at the G 2 /M transition, and during metaphase
(Figure 10.10).
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Mitotic phase
Formation
of 2 daughter
cells
Figure 10.10 The cell cycle is controlled at three checkpoints. The integrity of the DNA is assessed at the Gi
checkpoint. Proper chromosome duplication is assessed at the G 2 checkpoint. Attachment of each kinetochore to a
spindle fiber is assessed at the M checkpoint.
The Gi Checkpoint
The Gi checkpoint determines whether all conditions are favorable for cell division to proceed. The Gi
checkpoint, also called the restriction point (in yeast), is a point at which the cell irreversibly commits to the cell
division process. External influences, such as growth factors, play a large role in carrying the cell past the Gi
checkpoint. In addition to adequate reserves and cell size, there is a check for genomic DNA damage at the
Gi checkpoint. A cell that does not meet all the requirements will not be allowed to progress into the S phase.
The cell can halt the cycle and attempt to remedy the problematic condition, or the cell can advance into Go and
await further signals when conditions improve.
The G2 Checkpoint
The G 2 checkpoint bars entry into the mitotic phase if certain conditions are not met. As at the Gi checkpoint,
cell size and protein reserves are assessed. However, the most important role of the G 2 checkpoint is to ensure
that all of the chromosomes have been replicated and that the replicated DNA is not damaged. If the checkpoint
mechanisms detect problems with the DNA, the cell cycle is halted, and the cell attempts to either complete DNA
replication or repair the damaged DNA.
The M Checkpoint
The M checkpoint occurs near the end of the metaphase stage of karyokinesis. The M checkpoint is also known
as the spindle checkpoint, because it determines whether all the sister chromatids are correctly attached to the
spindle microtubules. Because the separation of the sister chromatids during anaphase is an irreversible step,
the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to at least
two spindle fibers arising from opposite poles of the cell.
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LINK
T &
LEARNING
Watch what occurs at the Gi, G 2 , and M checkpoints by visiting this website (http:// 0 penstaxc 0 llege. 0 rg/l/
celLcheckpnts) to see an animation of the cell cycle.
Regulator Molecules of the Cell Cycle
In addition to the internally controlled checkpoints, there are two groups of intracellular molecules that regulate
the cell cycle. These regulatory molecules either promote progress of the cell to the next phase (positive
regulation) or halt the cycle (negative regulation). Regulator molecules may act individually, or they can influence
the activity or production of other regulatory proteins. Therefore, the failure of a single regulator may have almost
no effect on the cell cycle, especially if more than one mechanism controls the same event. However, the effect
of a deficient or non-functioning regulator can be wide-ranging and possibly fatal to the cell if multiple processes
are affected.
Positive Regulation of the Cell Cycle
Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are termed positive regulators.
They are responsible for the progress of the cell through the various checkpoints. The levels of the four cyclin
proteins fluctuate throughout the cell cycle in a predictable pattern (Figure 10.11). Increases in the concentration
of cyclin proteins are triggered by both external and internal signals. After the cell moves to the next stage of the
cell cycle, the cyclins that were active in the previous stage are degraded by cytoplasmic enzymes, as shown in
Figure 10.11 below.
Cyclin Expression Cycle
Figure 10.11 The concentrations of cyclin proteins change throughout the cell cycle. There is a direct correlation
between cyclin accumulation and the three major cell-cycle checkpoints. Also note the sharp decline of cyclin levels
following each checkpoint (the transition between phases of the cell cycle), as cyclin is degraded by cytoplasmic
enzymes, (credit: modification of work by "WikiMiMa'VWikimedia Commons)
Cyclins regulate the cell cycle only when they are tightly bound to Cdks. To be fully active, the Cdk/cyclin complex
must also be phosphorylated in specific locations to activate the complex. Like all kinases, Cdks are enzymes
(kinases) that in turn phosphorylate other proteins. Phosphorylation activates the protein by changing its shape.
The proteins phosphorylated by Cdks are involved in advancing the cell to the next phase. (Figure 10.12).
The levels of Cdk proteins are relatively stable throughout the cell cycle; however, the concentrations of cyclin
fluctuate and determine when Cdk/cyclin complexes form. The different cyclins and Cdks bind at specific points
in the cell cycle and thus regulate different checkpoints.
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293
Cyclin-dependent Kinases
Figure 10.12 Cyclin-dependent kinases (Cdks) are protein kinases that, when fully activated, can phosphorylate and
thus activate other proteins that advance the cell cycle past a checkpoint. To become fully activated, a Cdk must bind
to a cyclin protein and then be phosphorylated by another kinase.
Because the cyclic fluctuations of cyclin levels are largely based on the timing of the cell cycle and not on specific
events, regulation of the cell cycle usually occurs by either the Cdk molecules alone or the Cdk/cyclin complexes.
Without a specific concentration of fully activated cyclin/Cdk complexes, the cell cycle cannot proceed through
the checkpoints.
Although the cyclins are the main regulatory molecules that determine the forward momentum of the cell cycle,
there are several other mechanisms that fine-tune the progress of the cycle with negative, rather than positive,
effects. These mechanisms essentially block the progression of the cell cycle until problematic conditions are
resolved. Molecules that prevent the full activation of Cdks are called Cdk inhibitors. Many of these inhibitor
molecules directly or indirectly monitor a particular cell-cycle event. The block placed on Cdks by inhibitor
molecules will not be removed until the specific event that the inhibitor monitors is completed.
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Negative Regulation of the Cell Cycle
The second group of cell-cycle regulatory molecules are negative regulators, which stop the cell cycle.
Remember that in positive regulation, active molecules cause the cycle to progress.
The best understood negative regulatory molecules are retinoblastoma protein (Rb), p53, and p21.
Retinoblastoma proteins are a group of tumor-suppressor proteins common in many cells. We should note here
that the 53 and 21 designations refer to the functional molecular masses of the proteins (p) in kilodaltons (a
dalton is equal to an atomic mass unit, which is equal to one proton or one neutron or 1 g/mol). Much of what
is known about cell-cycle regulation comes from research conducted with cells that have lost regulatory control.
All three of these regulatory proteins were discovered to be damaged or non-functional in cells that had begun
to replicate uncontrollably (i.e., became cancerous). In each case, the main cause of the unchecked progress
through the cell cycle was a faulty copy of the regulatory protein.
Rb, p53, and p21 act primarily at the Gi checkpoint. p53 is a multi-functional protein that has a major impact on
the commitment of a cell to division because it acts when there is damaged DNA in cells that are undergoing the
preparatory processes during Gi. If damaged DNA is detected, p53 halts the cell cycle and then recruits specific
enzymes to repair the DNA. If the DNA cannot be repaired, p53 can trigger apoptosis, or cell suicide, to prevent
the duplication of damaged chromosomes. As p53 levels rise, the production of p21 is triggered. p21 enforces
the halt in the cycle dictated by p53 by binding to and inhibiting the activity of the Cdk/cyclin complexes. As a cell
is exposed to more stress, higher levels of p53 and p21 accumulate, making it less likely that the cell will move
into the S phase.
Rb, which largely monitors cell size, exerts its regulatory influence on other positive regulator proteins. In the
active, dephosphorylated state, Rb binds to proteins called transcription factors, most commonly, E2F (Figure
10.13). Transcription factors “turn on” specific genes, allowing the production of proteins encoded by that gene.
When Rb is bound to E2F, production of proteins necessary for the Gi/S transition is blocked. As the cell
increases in size, Rb is slowly phosphorylated until it becomes inactivated. Rb releases E2F, which can now turn
on the gene that produces the transition protein, and this particular block is removed. For the cell to move past
each of the checkpoints, all positive regulators must be “turned on,” and all negative regulators must be “turned
off.”
visual
CONNECTION
Rb Regulation of the Cell
Unphosphorylated Rb binds
transcription factor E2F.
E2F cannot bind the DNA, and
transcription is blocked.
Cell growth triggers the
phosphoryation of Rb.
Phosphorylated Rb releases
E2F, which binds the DNA and
turns on gene expression, thus
advancing the cell cycle.
Figure 10.13 Rb halts the cell cycle and releases its hold in response to cell growth.
Rb and other proteins that negatively regulate the cell cycle are sometimes called tumor suppressors. Why
do you think the name tumor suppressor might be appropriate for these proteins?
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Chapter 10 | Cell Reproduction
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10.4 | Cancer and the Cell Cycle
By the end of this section, you will be able to do the following:
• Describe how cancer is caused by uncontrolled cell growth
• Understand how proto-oncogenes are normal cell genes that, when mutated, become oncogenes
• Describe how tumor suppressors function
• Explain how mutant tumor suppressors cause cancer
Cancer comprises many different diseases caused by a common mechanism: uncontrolled cell growth. Despite
the redundancy and overlapping levels of cell-cycle control, errors do occur. One of the critical processes
monitored by the cell-cycle checkpoint surveillance mechanism is the proper replication of DNA during the S
phase. Even when all of the cell-cycle controls are fully functional, a small percentage of replication errors
(mutations) will be passed on to the daughter cells. If changes to the DNA nucleotide sequence occur within a
coding portion of a gene and are not corrected, a gene mutation results. All cancers start when a gene mutation
gives rise to a faulty protein that plays a key role in cell reproduction.
The change in the cell that results from the malformed protein may be minor: perhaps a slight delay in the
binding of Cdk to cyclin or an Rb protein that detaches from its target DNA while still phosphorylated. Even minor
mistakes, however, may allow subsequent mistakes to occur more readily. Over and over, small uncorrected
errors are passed from the parent cell to the daughter cells and amplified as each generation produces more
non-functional proteins from uncorrected DNA damage. Eventually, the pace of the cell cycle speeds up as
the effectiveness of the control and repair mechanisms decreases. Uncontrolled growth of the mutated cells
outpaces the growth of normal cells in the area, and a tumor ("-oma") can result.
Proto-oncogenes
The genes that code for the positive cell-cycle regulators are called proto-oncogenes. Proto-oncogenes are
normal genes that, when mutated in certain ways, become oncogenes— genes that cause a cell to become
cancerous. Consider what might happen to the cell cycle in a cell with a recently acquired oncogene. In most
instances, the alteration of the DNA sequence will result in a less functional (or non-functional) protein. The result
is detrimental to the cell and will likely prevent the cell from completing the cell cycle; however, the organism
is not harmed because the mutation will not be carried forward. If a cell cannot reproduce, the mutation is
not propagated and the damage is minimal. Occasionally, however, a gene mutation causes a change that
increases the activity of a positive regulator. For example, a mutation that allows Cdk to be activated without
being partnered with cyclin could push the cell cycle past a checkpoint before all of the required conditions are
met. If the resulting daughter cells are too damaged to undergo further cell divisions, the mutation would not be
propagated and no harm would come to the organism. However, if the atypical daughter cells are able to undergo
further cell divisions, subsequent generations of cells may accumulate even more mutations, some possibly in
additional genes that regulate the cell cycle.
The Cdk gene in the above example is only one of many genes that are considered proto-oncogenes. In addition
to the cell-cycle regulatory proteins, any protein that influences the cycle can be altered in such a way as to
override cell-cycle checkpoints. An oncogene is any gene that, when altered, leads to an increase in the rate of
cell-cycle progression.
Tumor Suppressor Genes
Like proto-oncogenes, many of the negative cell-cycle regulatory proteins were discovered in cells that had
become cancerous. Tumor suppressor genes are segments of DNA that code for negative regulator proteins,
the type of regulators that, when activated, can prevent the cell from undergoing uncontrolled division. The
collective function of the best-understood tumor suppressor gene proteins, Rb, p53, and p21, is to put up a
roadblock to cell-cycle progression until certain events are completed. A cell that carries a mutated form of a
negative regulator might not be able to halt the cell cycle if there is a problem. Tumor suppressors are similar to
brakes in a vehicle: Malfunctioning brakes can contribute to a car crash!
Mutated p53 genes have been identified in more than 50 percent of all human tumor cells. This discovery is not
surprising in light of the multiple roles that the p53 protein plays at the Gi checkpoint. A cell with a faulty p53 may
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fail to detect errors present in the genomic DNA (Figure 10.14). Even if a partially functional p53 does identify
the mutations, it may no longer be able to signal the necessary DNA repair enzymes. Either way, damaged DNA
will remain uncorrected. At this point, a functional p53 will deem the cell unsalvageable and trigger programmed
cell death (apoptosis). The damaged version of p53 found in cancer cells, however, cannot trigger apoptosis.
visual
CONNECTION
Normal p53
DNA damage
Cell cycle abnormalities
Hypoxia
—
p53
Cell cycle —
Apoptosis
arrest
(programmed
1
cell death)
DNA repair
1
T
Cell cycle
restart
When cellular damage occurs. P53
arrests the cell cycle until the damage
is repaired. If damage cannot be
repaired, apoptosis occurs.
Mutated p53
DNA damage
Cell cycle abnormalities
Hypoxia
1
Mutated p53 does not arrest the cell
cycle. The damaged cell continues to
divide, which may result in cancer.
Figure 10.14 The role of normal p53 is to monitor DNA and the supply of oxygen (hypoxia is a condition of reduced
oxygen supply). If damage is detected, p53 triggers repair mechanisms. If repairs are unsuccessful, p53 signals
apoptosis. A cell with an abnormal p53 protein cannot repair damaged DNA and thus cannot signal apoptosis.
Cells with abnormal p53 can become cancerous, (credit: modification of work by Thierry Soussi)
Human papillomavirus can cause cervical cancer. The virus encodes E6, a protein that binds p53. Based
on this fact and what you know about p53, what effect do you think E6 binding has on p53 activity?
a. E6 activates p53
b. E6 inactivates p53
c. E6 mutates p53
d. E6 binding marks p53 for degradation
The loss of p53 function has other repercussions for the cell cycle. Mutated p53 might lose its ability to trigger
p21 production. Without adequate levels of p21, there is no effective block on Cdk activation. Essentially, without
a fully functional p53, the Gi checkpoint is severely compromised and the cell proceeds directly from Gi to
S regardless of internal and external conditions. At the completion of this shortened cell cycle, two daughter
cells are produced that have inherited the mutated p53 gene. Given the non-optimal conditions under which the
parent cell reproduced, it is likely that the daughter cells will have acquired other mutations in addition to the
faulty tumor-suppressor gene. Cells such as these daughter cells quickly accumulate both oncogenes and non¬
functional tumor-suppressor genes. Again, the result is tumor growth.
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LINK TQ LEARNING
Watch an animation of how cancer results from errors in the cell cycle. (This multimedia resource will open
in a browser.) (http://cnx.Org/content/m66480/l.3/#eip-idll69995709332)
10.5 | Prokaryotic Cell Division
By the end of this section, you will be able to do the following:
• Describe the process of binary fission in prokaryotes
• Explain how FtsZ and tubulin proteins are examples of homology
Prokaryotes, such as bacteria, produce daughter cells by binary fission. For unicellular organisms, cell division
is the only method to produce new individuals, in both prokaryotic and eukaryotic cells, the outcome of cell
reproduction is a pair of daughter cells that are genetically identical to the parent cell. In unicellular organisms,
daughter cells are individuals.
To achieve the outcome of cloned offspring, certain steps are essential. The genomic DNA must be replicated
and then allocated into the daughter cells; the cytoplasmic contents must also be divided to give both new cells
the cellular machinery to sustain life. As we’ve seen with bacterial cells, the genome consists of a single, circular
DNA chromosome; therefore, the process of cell division is simplified. Karyokinesis is unnecessary because
there is no true nucleus and thus no need to direct one copy of the multiple chromosomes into each daughter
cell. This type of cell division is called binary (prokaryotic) fission.
Binary Fission
Due to the relative simplicity of the prokaryotes, the cell division process is a less complicated and much
more rapid process than cell division in eukaryotes. As a review of the general information on cell division
we discussed at the beginning of this chapter, recall that the single, circular DNA chromosome of bacteria
occupies a specific location, the nucleoid region, within the cell (Figure 10.2). Although the DNA of the nucleoid
is associated with proteins that aid in packaging the molecule into a compact size, there are no histone proteins
and thus no nucleosomes in prokaryotes. The packing proteins of bacteria are, however, related to the cohesin
and condensin proteins involved in the chromosome compaction of eukaryotes.
The bacterial chromosome is attached to the plasma membrane at about the midpoint of the cell. The starting
point of replication, the origin, is close to the binding site of the chromosome to the plasma membrane
(Figure 10.15). Replication of the DNA is bidirectional, moving away from the origin on both strands of the
loop simultaneously. As the new double strands are formed, each origin point moves away from the cell
wall attachment toward the opposite ends of the cell. As the cell elongates, the growing membrane aids in
the transport of the chromosomes. After the chromosomes have cleared the midpoint of the elongated cell,
cytoplasmic separation begins. The formation of a ring composed of repeating units of a protein called FtsZ
(short for “filamenting temperature-sensitive mutant Z”) directs the partition between the nucleoids. Formation of
the FtsZ ring triggers the accumulation of other proteins that work together to recruit new membrane and cell
wall materials to the site. A septum is formed between the daughter nucleoids, extending gradually from the
periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.
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Binary Fission in Prokaryotes
Replication of the circular prokaryotic chromosome begins at the origin of replication
and continues in both directions at once.
Origin of replication
Prokaryotes have a single,
circular chromosome
FtsZ protein
The cell begins to elongate. FtsZ proteins migrate toward the midpoint of the cell.
The duplicated chromosomes separate and continue to move away from each other
toward opposite ends of the cell. FtsZ proteins form a ring around the periphery of the
midpoint between the chromosomes.
FtsZ ring
The FtsZ ring directs the formation of a septum that divides the cell. Plasma membrane
and cell wall materials accumulate.
Septum
Septum
After the septum is complete, the cell pinches in two, forming two daughter cells. FtsZ is
dispersed throughout the cytoplasm of the new cells.
Figure 10.15 These images show
“Mcstrother’VWikimedia Commons)
the steps of binary fission in prokaryotes, (credit: modification of work by
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Chapter 10 | Cell Reproduction
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V /
e olution CONNECTION
Mitotic Spindle Apparatus
The precise timing and formation of the mitotic spindle is critical to the success of eukaryotic cell division.
Prokaryotic cells, on the other hand, do not undergo karyokinesis and therefore have no need for a mitotic
spindle. However, the FtsZ protein that plays such a vital role in prokaryotic cytokinesis is structurally
and functionally very similar to tubulin, the building block of the microtubules which make up the mitotic
spindle fibers that are necessary for eukaryotic nuclear division. FtsZ proteins can form filaments, rings,
and other three-dimensional structures that resemble the way tubulin forms microtubules, centrioles, and
various cytoskeletal components. In addition, both FtsZ and tubulin employ the same energy source, GTP
(guanosine triphosphate), to rapidly assemble and disassemble complex structures.
FtsZ and tubulin are considered to be homologous structures derived from common evolutionary origins. In
this example, FtsZ is the ancestor protein to tubulin (an evolutionarily derived protein). While both proteins
are found in extant organisms, tubulin function has evolved and diversified tremendously since evolving
from its FtsZ prokaryotic origin. A survey of mitotic assembly components found in present-day unicellular
eukaryotes reveals crucial intermediary steps to the complex membrane-enclosed genomes of multicellular
eukaryotes (Table 10.3).
Cell Division Apparatus among Various Organisms
Structure of
genetic material
Division of nuclear material
Separation
of
daughter
cells
Prokaryotes
There is no nucleus.
The single, circular
chromosome exists in a
region of cytoplasm
called the nucleoid.
Occurs through binary fission. As the
chromosome is replicated, the two copies move
to opposite ends of the cell by an unknown
mechanism.
FtsZ proteins
assemble into a
ring that
pinches the cell
in two.
Some
protists
Linear chromosomes
exist in the nucleus.
Chromosomes attach to the nuclear envelope,
which remains intact. The mitotic spindle passes
through the envelope and elongates the cell. No
centrioles exist.
Microfilaments
form a
cleavage
furrow that
pinches the cell
in two.
Other
protists
Linear chromosomes
wrapped around
histones exist in the
nucleus.
A mitotic spindle forms from the centrioles and
passes through the nuclear membrane, which
remains intact. Chromosomes attach to the
mitotic spindle, which separates the
chromosomes and elongates the cell.
Microfilaments
form a
cleavage
furrow that
pinches the cell
in two.
Animal cells
Linear chromosomes
exist in the nucleus.
A mitotic spindle forms from the centrosomes.
The nuclear envelope dissolves. Chromosomes
attach to the mitotic spindle, which separates the
chromosomes and elongates the cell.
Microfilaments
form a
cleavage
furrow that
pinches the cell
in two.
Table 10.3
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KEY TERMS
anaphase stage of mitosis during which sister chromatids are separated from each other
binary fission prokaryotic cell division process
cell cycle ordered sequence of events through which a cell passes between one cell division and the next
cell cycle ordered series of events involving cell growth and cell division that produces two new daughter cells
cell plate structure formed during plant cell cytokinesis by Golgi vesicles, forming a temporary structure
(phragmoplast) and fusing at the metaphase plate; ultimately leads to the formation of cell walls that
separate the two daughter cells
cell-cycle checkpoint mechanism that monitors the preparedness of a eukaryotic cell to advance through the
various cell-cycle stages
centriole rod-like structure constructed of microtubules at the center of each animal cell centrosome
centromere region at which sister chromatids are bound together; a constricted area in condensed
chromosomes
chromatid single DNA molecule of two strands of duplicated DNA and associated proteins held together at the
centromere
cleavage furrow constriction formed by an actin ring during cytokinesis in animal cells that leads to cytoplasmic
division
condensin proteins that help sister chromatids coil during prophase
cyclin one of a group of proteins that act in conjunction with cyclin-dependent kinases to help regulate the cell
cycle by phosphorylating key proteins; the concentrations of cyclins fluctuate throughout the cell cycle
cyclin-dependent kinase (Cdk) one of a group of protein kinases that helps to regulate the cell cycle when
bound to cyclin; it functions to phosphorylate other proteins that are either activated or inactivated by
phosphorylation
cytokinesis division of the cytoplasm following mitosis that forms two daughter cells,
diploid cell, nucleus, or organism containing two sets of chromosomes (2n)
FtsZ tubulin-like protein component of the prokaryotic cytoskeleton that is important in prokaryotic cytokinesis
(name origin: Filamenting temperature-sensitive mutant Z)
Go phase distinct from the Gi phase of interphase; a cell in Go is not preparing to divide
Gi phase (also, first gap) first phase of interphase centered on cell growth during mitosis
G 2 phase (also, second gap) third phase of interphase during which the cell undergoes final preparations for
mitosis
gamete haploid reproductive cell or sex cell (sperm, pollen grain, or egg)
gene physical and functional unit of heredity, a sequence of DNA that codes for a protein.
genome total genetic information of a cell or organism
haploid cell, nucleus, or organism containing one set of chromosomes (n)
histone one of several similar, highly conserved, low molecular weight, basic proteins found in the chromatin of
all eukaryotic cells; associates with DNA to form nucleosomes
homologous chromosomes chromosomes of the same morphology with genes in the same location; diploid
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organisms have pairs of homologous chromosomes (homologs), with each homolog derived from a
different parent
interphase period of the cell cycle leading up to mitosis; includes Gi, S, and G 2 phases (the interim period
between two consecutive cell divisions)
karyokinesis mitotic nuclear division
kinetochore protein structure associated with the centromere of each sister chromatid that attracts and binds
spindle microtubules during prometaphase
locus position of a gene on a chromosome
metaphase stage of mitosis during which chromosomes are aligned at the metaphase plate
metaphase plate equatorial plane midway between the two poles of a cell where the chromosomes align during
metaphase
mitosis (also, karyokinesis) period of the cell cycle during which the duplicated chromosomes are separated
into identical nuclei; includes prophase, prometaphase, metaphase, anaphase, and telophase
mitotic phase period of the cell cycle during which duplicated chromosomes are distributed into two nuclei and
cytoplasmic contents are divided; includes karyokinesis (mitosis) and cytokinesis
mitotic spindle apparatus composed of microtubules that orchestrates the movement of chromosomes during
mitosis
nucleosome subunit of chromatin composed of a short length of DNA wrapped around a core of histone
proteins
oncogene mutated version of a normal gene involved in the positive regulation of the cell cycle
origin (also, ORI) region of the prokaryotic chromosome where replication begins (origin of replication)
p21 cell-cycle regulatory protein that inhibits the cell cycle; its levels are controlled by p53
p53 cell-cycle regulatory protein that regulates cell growth and monitors DNA damage; it halts the progression
of the cell cycle in cases of DNA damage and may induce apoptosis
prometaphase stage of mitosis during which the nuclear membrane breaks down and mitotic spindle fibers
attach to kinetochores
prophase stage of mitosis during which chromosomes condense and the mitotic spindle begins to form
proto-oncogene normal gene that when mutated becomes an oncogene
quiescent refers to a cell that is performing normal cell functions and has not initiated preparations for cell
division
retinoblastoma protein (Rb) regulatory molecule that exhibits negative effects on the cell cycle by interacting
with a transcription factor (E2F)
S phase second, or synthesis, stage of interphase during which DNA replication occurs
septum structure formed in a bacterial cell as a precursor to the separation of the cell into two daughter cells
telophase stage of mitosis during which chromosomes arrive at opposite poles, decondense, and are
surrounded by a new nuclear envelope
tumor suppressor gene segment of DNA that codes for regulator proteins that prevent the cell from
undergoing uncontrolled division
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CHAPTER SUMMARY
10.1 Ceil Division
Prokaryotes have a single circular chromosome composed of double-stranded DNA, whereas eukaryotes have
multiple, linear chromosomes composed of chromatin wrapped around histones, all of which are surrounded by
a nuclear membrane. The 46 chromosomes of human somatic cells are composed of 22 pairs of autosomes
(matched pairs) and a pair of sex chromosomes, which may or may not be matched. This is the 2 n or diploid
state. Human gametes have 23 chromosomes, or one complete set of chromosomes; a set of chromosomes is
complete with either one of the sex chromosomes, X or Y. This is the n or haploid state. Genes are segments of
DNA that code for a specific functional molecule (a protein or RNA). An organism’s traits are determined by the
genes inherited from each parent. Duplicated chromosomes are composed of two sister chromatids.
Chromosomes are compacted using a variety of mechanisms during certain stages of the cell cycle. Several
classes of protein are involved in the organization and packing of the chromosomal DNA into a highly
condensed structure. The condensing complex compacts chromosomes, and the resulting condensed structure
is necessary for chromosomal segregation during mitosis.
10.2 The Cell Cycle
The cell cycle is an orderly sequence of events. Cells on the path to cell division proceed through a series of
precisely timed and carefully regulated stages. In eukaryotes, the cell cycle consists of a long preparatory
period, called interphase, during which chromosomes are replicated. Interphase is divided into Gi, S, and G 2
phases. The mitotic phase begins with karyokinesis (mitosis), which consists of five stages: prophase,
prometaphase, metaphase, anaphase, and telophase. The final stage of the cell division process, and
sometimes viewed as the final stage of the mitotic phase, is cytokinesis, during which the cytoplasmic
components of the daughter cells are separated either by an actin ring (animal cells) or by cell plate formation
(plant cells).
10.3 Control of the Cell Cycle
Each step of the cell cycle is monitored by internal controls called checkpoints. There are three major
checkpoints in the cell cycle: one near the end of Gi, a second at the G 2 /M transition, and the third during
metaphase. Positive regulator molecules allow the cell cycle to advance to the next stage of cell division.
Negative regulator molecules monitor cellular conditions and can halt the cycle until specific requirements are
met.
10.4 Cancer and the Cell Cycle
Cancer is the result of unchecked cell division caused by a breakdown of the mechanisms that regulate the cell
cycle. The loss of control begins with a change in the DNA sequence of a gene that codes for one of the
regulatory molecules. Faulty instructions lead to a protein that does not function as it should. Any disruption of
the monitoring system can allow other mistakes to be passed on to the daughter cells. Each successive cell
division will give rise to daughter cells with even more accumulated damage. Eventually, all checkpoints
become nonfunctional, and rapidly reproducing cells crowd out normal cells, resulting in a tumor or leukemia
(blood cancer).
10.5 Prokaryotic Cell Division
in both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and then each copy is allocated
into a daughter cell. In addition, the cytoplasmic contents are divided evenly and distributed to the new cells.
However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single,
circular DNA chromosome but no nucleus. Therefore, mitosis (karyokinesis) is not necessary in bacterial cell
division. Bacterial cytokinesis is directed by a ring composed of a protein called FtsZ. Ingrowth of membrane
and cell wall material from the periphery of the cells results in the formation of a septum that eventually
constructs the separate cell walls of the daughter cells.
VISUAL CONNECTION QUESTIONS
1. Figure 10.6 Which of the following is the correct
order of events in mitosis?
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a. Sister chromatids line up at the metaphase
plate. The kinetochore becomes attached to
the mitotic spindle. The nucleus reforms and
the cell divides. Cohesin proteins break
down and the sister chromatids separate.
b. The kinetochore becomes attached to the
mitotic spindle. Cohesin proteins break
down and the sister chromatids separate.
Sister chromatids line up at the metaphase
plate. The nucleus reforms and the cell
divides.
c. The kinetochore becomes attached to the
cohesin proteins. Sister chromatids line up
at the metaphase plate. The kinetochore
breaks down and the sister chromatids
separate. The nucleus reforms and the cell
divides.
d. The kinetochore becomes attached to the
mitotic spindle. Sister chromatids line up at
the metaphase plate. Cohesin proteins
break down and the sister chromatids
separate. The nucleus reforms and the cell
divides.
2. Figure 10.13 Rb and other proteins that negatively
regulate the cell cycle are sometimes called tumor
suppressors. Why do you think the name tumor
suppressor might be appropriate for these proteins?
3. Figure 10.14 Human papillomavirus can cause
cervical cancer. The virus encodes E6, a protein that
binds p53. Based on this fact and what you know
about p53, what effect do you think E6 binding has
on p53 activity?
a. E6 activates p53
b. E6 inactivates p53
c. E6 mutates p53
d. E6 binding marks p53 for degradation
REVIEW QUESTIONS
4. A diploid cell has_the number of
chromosomes as a haploid cell.
a. one-fourth
b. half
c. twice
d. four times
5. An organism’s traits are determined by the specific
combination of inherited
a.
cells.
b.
genes.
c.
proteins.
d.
chromatids.
6. The first level of DNA organization in a eukaryotic
cell is maintained by which molecule?
a. cohesin
b. condensin
c. chromatin
d. histone
7. Identical copies of chromatin held together by
cohesin at the centromere are called_.
a. histones.
b. nucleosomes.
c. chromatin.
d. sister chromatids.
8. Chromosomes are duplicated during what stage of
the cell cycle?
a. Gi phase
b. S phase
c. prophase
d. prometaphase
9. Which of the following events does not occur
during some stages of interphase?
a. DNA duplication
b. organelle duplication
c. increase in cell size
d. separation of sister chromatids
10. The mitotic spindles arise from which cell
structure?
a. centromere
b. centrosome
c. kinetochore
d. cleavage furrow
11. Attachment of the mitotic spindle fibers to the
kinetochores is a characteristic of which stage of
mitosis?
a. prophase
b. prometaphase
c. metaphase
d. anaphase
12. Unpacking of chromosomes and the formation of
a new nuclear envelope is a characteristic of which
stage of mitosis?
a. prometaphase
b. metaphase
c. anaphase
d. telophase
13. Separation of the sister chromatids is a
characteristic of which stage of mitosis?
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a. prometaphase
b. metaphase
c. anaphase
d. telophase
14. The chromosomes become visible under a light
microscope during which stage of mitosis?
a. prophase
b. prometaphase
c. metaphase
d. anaphase
15. The fusing of Golgi vesicles at the metaphase
plate of dividing plant cells forms what structure?
a. cell plate
b. actin ring
c. cleavage furrow
d. mitotic spindle
16. At which of the cell-cycle checkpoints do external
forces have the greatest influence?
a. Gi checkpoint
b. G 2 checkpoint
c. M checkpoint
d. Go checkpoint
17. What is the main prerequisite for clearance at the
G 2 checkpoint?
a. cell has reached a sufficient size
b. an adequate stockpile of nucleotides
c. accurate and complete DNA replication
d. proper attachment of mitotic spindle fibers to
kinetochores
18. If the M checkpoint is not cleared, what stage of
mitosis will be blocked?
a. prophase
b. prometaphase
c. metaphase
d. anaphase
19. Which protein is a positive regulator that
phosphorylates other proteins when activated?
a. p53
b. retinoblastoma protein (Rb)
c. cyclin
d. cyclin-dependent kinase (Cdk)
20. Many of the negative regulator proteins of the cell
cycle were discovered in what type of cells?
a. gametes
b. cells in Go
c. cancer cells
d. stem cells
CRITICAL THINKING QUESTIONS
28. Compare and contrast a human somatic cell to a
human gamete.
29. What is the relationship between a genome,
21. Which negative regulatory molecule can trigger
cell suicide (apoptosis) if vital cell cycle events do not
occur?
a. p53
b. p21
c. retinoblastoma protein (Rb)
d. cyclin-dependent kinase (Cdk)
22. _are changes to the order of
nucleotides in a segment of DNA that codes for a
protein.
a. Proto-oncogenes
b. Tumor suppressor genes
c. Gene mutations
d. Negative regulators
23. A gene that codes for a positive cell-cycle
regulator is called a(n)_.
a. kinase inhibitor.
b. tumor suppressor gene.
c. proto-oncogene.
d. oncogene.
24. A mutated gene that codes for an altered version
of Cdk that is active in the absence of cyclin is a(n)
a. kinase inhibitor.
b. tumor suppressor gene.
c. proto-oncogene.
d. oncogene.
25. Which molecule is a Cdk inhibitor that is
controlled by p53?
a. cyclin
b. anti-kinase
c. Rb
d. p21
26. Which eukaryotic cell-cycle event is missing in
binary fission?
a. cell growth
b. DNA duplication
c. karyokinesis
d. cytokinesis
27. FtsZ proteins direct the formation of a_
that will eventually form the new cell walls of the
daughter cells.
a. contractile ring
b. cell plate
c. cytoskeleton
d. septum
chromosomes, and genes?
30. Eukaryotic chromosomes are thousands of times
longer than a typical cell. Explain how chromosomes
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can fit inside a eukaryotic nucleus.
31. Briefly describe the events that occur in each
phase of interphase.
32. Chemotherapy drugs such as vincristine (derived
from Madagascar periwinkle plants) and colchicine
(derived from autumn crocus plants) disrupt mitosis
by binding to tubulin (the subunit of microtubules)
and interfering with microtubule assembly and
disassembly. Exactly what mitotic structure is
targeted by these drugs and what effect would that
have on cell division?
33. Describe the similarities and differences between
the cytokinesis mechanisms found in animal cells
versus those in plant cells.
34. List some reasons why a cell that has just
completed cytokinesis might enter the Go phase
instead of the Gi phase.
35. What cell-cycle events will be affected in a cell
that produces mutated (non-functional) cohesin
protein?
36. Describe the general conditions that must be met
at each of the three main cell-cycle checkpoints.
37. Compare and contrast the roles of the positive
cell-cycle regulators negative regulators.
38. What steps are necessary for Cdk to become
fully active?
39. Rb is a negative regulator that blocks the cell
cycle at the Gi checkpoint until the cell achieves a
requisite size. What molecular mechanism does Rb
employ to halt the cell cycle?
40. Outline the steps that lead to a cell becoming
cancerous.
41. Explain the difference between a proto-oncogene
and a tumor-suppressor gene.
42. List the regulatory mechanisms that might be lost
in a cell producing faulty p53.
43. p53 can trigger apoptosis if certain cell-cycle
events fail. How does this regulatory outcome benefit
a multicellular organism?
44. Name the common components of eukaryotic cell
division and binary fission.
45. Describe how the duplicated bacterial
chromosomes are distributed into new daughter cells
without the direction of the mitotic spindle.
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11 1 MEIOSIS AND SEXUAL
REPRODUCTION
(a) (b) (c)
Figure 11.1 Each of us, like the organisms shown above, begins life as a fertilized egg (zygote). After trillions of cell
divisions, each of us develops into a complex, multicellular organism, (credit a: modification of work by Frank Wouters;
credit b: modification of work by Ken Cole, USGS; credit c: modification of work by Martin Pettitt)
Chapter Outline
11.1: The Process of Meiosis
11.2: Sexual Reproduction
Introduction
The ability to reproduce is a basic characteristic of all organisms: Hippopotamuses give birth to hippopotamus
calves; Joshua trees produce seeds from which Joshua tree seedlings emerge; and adult flamingos lay eggs
that hatch into flamingo chicks. However, unlike the organisms shown above, offspring may or may not resemble
their parents. For example, in the case of most insects such as butterflies (with a complete metamorphosis), the
larval forms rarely resemble the adult forms.
Although many unicellular organisms and a few multicellular organisms can produce genetically identical clones
of themselves through asexual reproduction, many single-celled organisms and most multicellular organisms
reproduce regularly using another method —sexual reproduction. This highly evolved method involves the
production by parents of two haploid cells and the fusion of two haploid cells to form a single, genetically
recombined diploid cell—a genetically unique organism. Haploid cells that are part of the sexual reproductive
cycle are produced by a type of cell division called meiosis. Sexual reproduction, involving both meiosis
and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual
reproduction. The vast majority of eukaryotic organisms, both multicellular and unicellular, can or must employ
some form of meiosis and fertilization to reproduce.
In most plants and animals, through thousands of rounds of mitotic cell division, diploid cells (whether produced
by asexual or sexual reproduction) will develop into an adult organism.
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Chapter 111 Meiosis and Sexual Reproduction
11.1 1 The Process of Meiosis
By the end of this section, you will be able to do the following:
• Describe the behavior of chromosomes during meiosis, and the differences between the first and second
meiotic divisions
• Describe the cellular events that take place during meiosis
• Explain the differences between meiosis and mitosis
• Explain the mechanisms within the meiotic process that produce genetic variation among the haploid
gametes
Sexual reproduction requires the union of two specialized cells, called gametes, each of which contains one
set of chromosomes. When gametes unite, they form a zygote, or fertilized egg that contains two sets of
chromosomes. (Note: Cells that contain one set of chromosomes are called haploid; cells containing two
sets of chromosomes are called diploid.) If the reproductive cycle is to continue for any sexually reproducing
species, then the diploid cell must somehow reduce its number of chromosome sets to produce haploid gametes;
otherwise, the number of chromosome sets will double with every future round of fertilization. Therefore, sexual
reproduction requires a nuclear division that reduces the number of chromosome sets by half.
Most animals and plants and many unicellular organisms are diploid and therefore have two sets of
chromosomes. In each somatic cell of the organism (all cells of a multicellular organism except the gametes or
reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes.
Homologous chromosomes are matched pairs containing the same genes in identical locations along their
lengths. Diploid organisms inherit one copy of each homologous chromosome from each parent.
Meiosis is the nuclear division that forms haploid cells from diploid cells, and it employs many of the same
cellular mechanisms as mitosis. However, as you have learned, mitosis produces daughter cells whose nuclei
are genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are
at the same “ploidy level”—diploid in the case of most multicellular most animals. Plants use mitosis to grow as
sporophytes, and to grow and produce eggs and sperm as gametophytes; so they use mitosis for both haploid
and diploid cells (as well as for all other ploidies). In meiosis, the starting nucleus is always diploid and the
daughter nuclei that result are haploid. To achieve this reduction in chromosome number, meiosis consists of
one round of chromosome replication followed by two rounds of nuclear division. Because the events that occur
during each of the division stages are analogous to the events of mitosis, the same stage names are assigned.
However, because there are two rounds of division, the major process and the stages are designated with a “I”
or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and
so on. Likewise, Meiosis II (during which the second round of meiotic division takes place) includes prophase II,
prometaphase II, and so on.
Meiosis I
Meiosis is preceded by an interphase consisting of Gi, S, and G 2 phases, which are nearly identical to the
phases preceding mitosis. The Gi phase (the “first gap phase") is focused on cell growth. During the S
phase—the second phase of interphase—the cell copies or replicates the DNA of the chromosomes. Finally, in
the G 2 phase (the “second gap phase”) the cell undergoes the final preparations for meiosis.
During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies —sister
chromatids that are held together at the centromere by cohesin proteins, which hold the chromatids together
until anaphase II.
Prophase I
Early in prophase I, before the chromosomes can be seen clearly with a microscope, the homologous
chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to
break down, the proteins associated with homologous chromosomes bring the pair closer together. Recall that
in mitosis, homologous chromosomes do not pair together. The synaptonemal complex, a lattice of proteins
between the homologous chromosomes, first forms at specific locations and then spreads outward to cover
the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis. In
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synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other.
The synaptonemal complex supports the exchange of chromosomal segments between homologous nonsister
chromatids—a process called crossing over. Crossing over can be observed visually after the exchange as
chiasmata (singular = chiasma) (Figure 11.2).
in humans, even though the X and Y sex chromosomes are not completely homologous (that is, most of their
genes differ), there is a small region of homology that allows the X and Y chromosomes to pair up during
prophase I. A partial synaptonemal complex develops only between the regions of homology.
Homologous
chromosomes
Centromere
Kinetochore
Synaptonemal complex
Sister chromatids
Figure 11.2 Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are
bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin
proteins at the centromere.
Located at intervals along the synaptonemal complex are large protein assemblies called recombination
nodules. These assemblies mark the points of later chiasmata and mediate the multistep process of
crossover —or genetic recombination—between the nonsister chromatids. Near the recombination nodule, the
double-stranded DNA of each chromatid is cleaved, the cut ends are modified, and a new connection is made
between the nonsister chromatids. As prophase I progresses, the synaptonemal complex begins to break
down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous
chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until
anaphase I. The number of chiasmata varies according to the species and the length of the chromosome.
There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during
meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and
the cohesin connection between homologous pairs is removed. At the end of prophase I, the pairs are held
together only at the chiasmata (Figure 11.3). These pairs are called tetrads because the four sister chromatids
of each pair of homologous chromosomes are now visible.
The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single
crossover event between homologous nonsister chromatids leads to a reciprocal exchange of equivalent DNA
between a maternal chromosome and a paternal chromosome. When a recombinant sister chromatid is moved
into a gamete cell it will carry some DNA from one parent and some DNA from the other parent. The recombinant
chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Crossover
events can occur almost anywhere along the length of the synapsed chromosomes. Different cells undergoing
meiosis will therefore produce different recombinant chromatids, with varying combinations of maternal and
parental genes. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments
of DNA to produce genetically recombined chromosomes.
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Chapter 111 Meiosis and Sexual Reproduction
Homologous Chromatid
chromosomes crossover
Recombinant
chromatids
Non-recombinant
chromosomes
Figure 11.3 Crossover occurs between nonsister chromatids of homologous chromosomes. The result is an exchange
of genetic material between homologous chromosomes.
Prometaphase I
The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at
the centromeres. Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome
to the microtubules of the mitotic spindle. Microtubules grow from microtubule-organizing centers (MTOCs).
In animal cells, MTOCs are centrosomes located at opposite poles of the cell. The microtubules from each
pole move toward the middle of the cell and attach to one of the kinetochores of the two fused homologous
chromosomes. Each member of the homologous pair attaches to a microtubule extending from opposite poles
of the cell so that in the next phase, the microtubules can pull the homologous pair apart. A spindle fiber that
has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is
attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous
chromosomes are still held together at the chiasmata. In addition, the nuclear membrane has broken down
entirely.
Metaphase I
During metaphase I, the homologous chromosomes are arranged at the metaphase plate—roughly in the
midline of the cell, with the kinetochores facing opposite poles. The homologous pairs orient themselves
randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b,
then the chromosomes could line up a-b or b-a. This is important in determining the genes carried by a gamete,
as each will only receive one of the two homologous chromosomes. (Recall that after crossing over takes place,
homologous chromosomes are not identical. They contain slight differences in their genetic information, causing
each gamete to have a unique genetic makeup.)
The randomness in the alignment of recombined chromosomes at the metaphase plate, coupled with the
crossing over events between nonsister chromatids, are responsible for much of the genetic variation in
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the offspring. To clarify this further, remember that the homologous chromosomes of a sexually reproducing
organism are originally inherited as two separate sets, one from each parent. Using humans as an example,
one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of
23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the
original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form
the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form
the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or
paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Thus, any
maternally inherited chromosome may face either pole. Likewise, any paternally inherited chromosome may also
face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.
This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is
the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes
meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of
chromosomes making up a set. There are two possibilities for orientation at the metaphase plate; the possible
number of alignments therefore equals 2 n in a diploid cell, where n is the number of chromosomes per haploid
pq
set. Humans have 23 chromosome pairs, which results in over eight million (2 ) possible genetically-distinct
gametes just from the random alignment of chromosomes at the metaphase plate. This number does not include
the variability that was previously produced by crossing over between the nonsister chromatids. Given these two
mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic
composition (Figure 11.4).
To summarize, meiosis i creates genetically diverse gametes in two ways. First, during prophase I, crossover
events between the nonsister chromatids of each homologous pair of chromosomes generate recombinant
chromatids with new combinations of maternal and paternal genes. Second, the random assortment of tetrads
on the metaphase plate produces unique combinations of maternal and paternal chromosomes that will make
their way into the gametes.
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Figure 11.4 Random, independent assortment during metaphase I can be demonstrated by considering a cell with a
set of two chromosomes (n = 2). In this case, there are two possible arrangements at the equatorial plane in metaphase
I. The total possible number of different gametes is 2 n , where n equals the number of chromosomes in a set. In this
example, there are four possible genetic combinations for the gametes. With n = 23 in human cells, there are over
eight million possible combinations of paternal and maternal chromosomes.
Anaphase I
In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound
together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused
kinetochores pull the homologous chromosomes apart (Figure 11.5).
Telophase I and Cytokinesis
In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase
events may or may not occur, depending on the species. In some organisms, the chromosomes “decondense”
and nuclear envelopes form around the separated sets of chromatids produced during telophase I. In other
organisms, cytokinesis —the physical separation of the cytoplasmic components into two daughter
cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis
separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division).
In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell
plate will ultimately lead to the formation of cell walls that separate the two daughter cells.
Two haploid cells are the result of the first meiotic division of a diploid cell. The cells are haploid because at
each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the
chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set,
even though each chromosome still consists of two sister chromatids. Recall that sister chromatids are merely
duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over).
In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.
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Chapter 111 Meiosis and Sexual Reproduction
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LINK TQ LEARNING
Review the process of meiosis, observing how chromosomes align and migrate, at Meiosis: An Interactive
Animation (http:// 0 penstaxc 0 llege. 0 rg/l/animal_mei 0 sis) .
Meiosis II
In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an
S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of
meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming
four new haploid gametes. The mechanics of meiosis II are similar to mitosis, except that each dividing cell has
only one set of homologous chromosomes, each with two chromatids. Therefore, each cell has half the number
of sister chromatids to separate out as a diploid cell undergoing mitosis. In terms of chromosomal content, cells
at the start of meiosis II are similar to haploid cells in G 2 , preparing to undergo mitosis.
Prophase II
If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they
fragment into vesicles. The MTOCs that were duplicated during interkinesis move away from each other toward
opposite poles, and new spindles are formed.
Prometaphase II
The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms
an individual kinetochore that attaches to microtubules from opposite poles.
Metaphase II
The sister chromatids are maximally condensed and aligned at the equator of the cell.
Anaphase II
The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles.
Nonkinetochore microtubules elongate the cell.
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Chapter 111 Meiosis and Sexual Reproduction
Prometaphase I
Homologous
pairs of
chromosomes
are held
together at the
chiasmata.
Homologous pairs
of chromosomes
Anaphase I
Microtubules attach
to the fused kinetochores
of the sister chromatids.
Sister chromatids
remain attached at
the centromere.
Prometaphase It
Anaphase II
Sister chromatids
are held together
at the centromere.
Sister chromatids are
pulled apart by microtubules
attached to the kinetochore.
Microtubules attach to the
individual kinetochores of
the sister chromatids.
Figure 11.5 The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I,
microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are
arranged at the midline of the cell (the metaphase plate) in metaphase I. In anaphase I, the homologous chromosomes
separate. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids
are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids separate.
Telophase II and Cytokinesis
The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the
chromosomes. If the parent cell was diploid, as is most commonly the case, then cytokinesis now separates
the two cells into four unique haploid cells. The cells produced are genetically unique because of the random
assortment of paternal and maternal homologs and because of the recombination of maternal and paternal
segments of chromosomes (with their sets of genes) that occurs during crossover. The entire process of meiosis
is outlined in Figure 11.6.
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Chapter 111 Meiosis and Sexual Reproduction
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Stage
S phase
Prophase I
Prometaphase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
Prophase It
Prometaphase II
Metaphase II
Anaphase II
Telophase II
and
Cytokinesis
Nuclear
envelope
Centrosomes
(with centriole
pairs)
Chromatin
Microtubule -
attached to
kinetochore
- Centromere
0 ,, i (with
jj X kinetochore)
-)T~ Metaphase
jU
plate
Sister-
chromatids
remain
attached.
-» Homologous
chromosomes
separate.
_ Cleavage
furrow
-Sister
s ~ \ f ^ ■ i
a ^ \ chromat ' ds
separate.
ID (M
Haploid daughter cells
Outcome
Chromosomes are duplicated during interphase. The
resulting sister chromatids are held together at the
centromere. The centrosomes are also duplicated.
Chromosomes condense, and the nuclear envelope
fragments. Homologous chromosomes bind firmly
together along their length, forming a tetrad.
Chiasmata form between non sister chromatids.
Crossing over occurs at the chiasmata. Spindle fibers
emerge from the centrosomes.
Homologous chromosomes are attached to spindle
microtubules at the fused kinetochore shared by
the sister chromatids. Chromosomes continue to
condense, and the nuclear envelope completely
disappears.
Homologous chromosomes randomly assemble at the
metaphase plate, where they have been maneuvered
into place by the microtubules.
Spindle microtubules pull the homologous
chromosomes apart. The sister chromatids are still
attached at the centromere.
Sister chromatids arrive at the poles of the cell and
begin to decondense, A nuclear envelope forms
around each nucleus, and the cytoplasm is divided by
a cleavage furrow. The result is two haploid cells.
Each cell contains one duplicated copy of each
homologous chromosome pair.
Sister chromatids condense. A new spindle begins to
form. The nuclear envelope starts to fragment.
The nuclear envelope disappears, and the spindle
fibers engage the individual kinetochores on the
sister chromatids.
Sister chromatids line up at the metaphase plate.
Sister chromatids are pulled apart by the shortening
of the kinetochore microtubules. Non kinetochore
microtubules lengthen the cell.
Chromosomes arnve at the poles of the cell and
decondense. Nuclear envelopes surround the four
nuclei. Cleavage furrows divide the two cells into
four haploid cells.
Figure 11.6 An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form four
haploid daughter cells.
Comparing Meiosis and Mitosis
Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities,
but also exhibit a number of important and distinct differences that lead to very different outcomes (Figure 11.7).
Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The
nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same
number of sets of chromosomes: one set in the case of haploid cells and two sets in the case of diploid cells.
In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four
new, genetically distinct cells. The four nuclei produced during meiosis are not genetically identical, and they
contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is
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Chapter 111 Meiosis and Sexual Reproduction
diploid.
The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division
than mitosis. In meiosis 1, the homologous chromosome pairs physically meet and are bound together with the
synaptonemal complex. Following this, the chromosomes develop chiasmata and undergo crossover between
nonsister chromatids, in the end, the chromosomes line up along the metaphase plate as tetrads—with
kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog to form a tetrad. All of
these events occur only in meiosis I.
When the chiasmata resolve and the tetrad is broken up with the homologs moving to one pole or another, the
ploidy level—the number of sets of chromosomes in each future nucleus—has been reduced from two to one.
For this reason, meiosis I is referred to as a reductional division. There is no such reduction in ploidy level
during mitosis.
Meiosis II is analogous to a mitotic division. In this case, the duplicated chromosomes (only one set of them)
line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles.
During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid—now referred to
as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were
not for the fact that there had been crossover, the two products of each individual meiosis II division would be
identical (as in mitosis). Instead, they are different because there has always been at least one crossover per
chromosome. Meiosis II is not a reduction division because although there are fewer copies of the genome in
the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.
MEIOSIS
HAPLOID CELLS
Meiosis 1 Meiosis II Cytokinesis
_ 1 _ _ 1 _ x
Interphase Pron
Prophase 1
letaphase 1 Anaphase 1
IK! wg>©
Metaphase 1 Telophase 1
Cytokine
Prophase II Metaphase II Telophase II
Prometaphase II Anaphase II
*/, A - K caV/csYj
sis N ^
Cytokines
MITOSIS
Interphase Prophase
njyS - -, (ra
Cytoki
Metaphase Telophase
_l A ^ P-' \ **■ «- 9 1 1
ise Anaphase
DIPLOI
lesis
D CELLS
-w, 1/
Prometaphc
OUTCOME
PROCESS
DNA
synthesis
Synapsis of
homologous
chromosomes
Crossover
Homologous
chromosomes
line up at
metaphase plate
Sister chromatids
line up at
metaphase plate
Number
and genetic
composition of
daughter cells
MEIOSIS
Occurs in S phase
of interphase
During
prophase 1
During
prophase 1
During
metaphase 1
During
metaphase II
Four haploid
cells at the end
of meiosis II
MITOSIS
Occurs in S phase
of interphase
Does not
occur
in mitosis
Does not
occur
in mitosis
Does not
occur
in mitosis
During
metaphase
Two diploid
cells at the end
of mitosis
Figure 11.7 Meiosis and mitosis are both preceded by one cycle of DNA replication; however, meiosis includes two
nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells
resulting from mitosis are diploid and identical to the parent cell.
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Chapter 111 Meiosis and Sexual Reproduction
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V /
e olution CONNECTION
The Mystery of the Evolution of Meiosis
Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to
remember that they evolved like other simple traits. Meiosis is such an extraordinarily complex series of
cellular events that biologists have had trouble testing hypotheses concerning how it may have evolved.
Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages,
it is important to separate the questions of the evolution of meiosis and the evolution of sex, because early
meiosis may have been advantageous for different reasons than it is now. Thinking outside the box and
imagining what the early benefits from meiosis might have been is one approach to uncovering how it may
have evolved.
Meiosis and mitosis share obvious cellular processes, and it makes sense that meiosis evolved from
mitosis. The difficulty lies in the clear differences between meiosis I and mitosis. Adam Wilkins and Robin
[i]
Holliday summarized the unique events that needed to occur for the evolution of meiosis from mitosis.
These steps are homologous chromosome pairing and synapsis, crossover exchanges, sister chromatids
remaining attached during anaphase, and suppression of DNA replication in interphase. They argue that
the first step is the hardest and most important and that understanding how it evolved would make the
evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of
synapsis.
There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis
exist in single-celled protists. Some appear to be simpler or more “primitive" forms of meiosis. Comparing
the meiotic divisions of different protists may shed light on the evolution of meiosis. Marilee Ramesh and
colleagues compared the genes involved in meiosis in protists to understand when and where meiosis
might have evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests
that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell
us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in
earlier cells.
LINK
T a
LEARNING
Click through the steps of this interactive animation to compare the meiotic process of cell division to that of
mitosis at How Cells Divide (http://0penstaxc0llege.0rg/l/h0w cells_dvide) .
11.2 | Sexual Reproduction
By the end of this section, you will be able to do the following:
• Explain that meiosis and sexual reproduction are highly evolved traits
• Identify variation among offspring as a potential evolutionary advantage of sexual reproduction
• Describe the three different life-cycle types among sexually reproducing multicellular organisms.
1.
Adam S. Wilkins and Robin Holliday, “The Evolution of Meiosis from Mitosis,” Genetics 181 (2009): 3-12.
2. Marilee A. Ramesh, Shehre-Banoo Malik and John M. Logsdon, Jr, “A Phylogenetic Inventory of Meiotic Genes: Evidence for Sex in
Giardia and an Early Eukaryotic Origin of Meiosis," Current Biology 15 (2005):185-91.
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Chapter 111 Meiosis and Sexual Reproduction
Sexual reproduction was likely an early evolutionary innovation after the appearance of eukaryotic cells. It
appears to have been very successful because most eukaryotes are able to reproduce sexually and, in many
animals, it is the only mode of reproduction. And yet, scientists also recognize some real disadvantages to
sexual reproduction. On the surface, creating offspring that are genetic clones of the parent appears to be a
better system. If the parent organism is successfully occupying a habitat, offspring with the same traits should
be similarly successful. There is also the obvious benefit to an organism that can produce offspring whenever
circumstances are favorable by asexual budding, fragmentation, or by producing eggs asexually. These methods
of reproduction do not require another organism of the opposite sex. Indeed, some organisms that lead a solitary
lifestyle have retained the ability to reproduce asexually. In addition, in asexual populations, every individual
is capable of reproduction. In sexual populations, the males are not producing the offspring themselves, so
hypothetically an asexual population could grow twice as fast.
However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why
are meiosis and sexual reproductive strategies so common? These are important (and as yet unanswered)
questions in biology, even though they have been the focus of much research beginning in the latter half of the
20th century. There are several possible explanations, one of which is that the variation that sexual reproduction
creates among offspring is very important to the survival and reproduction of the population. Thus, on average,
a sexually reproducing population will leave more descendants than an otherwise similar asexually reproducing
population. The only source of variation in asexual organisms is mutation. Mutations that take place during the
formation of germ cell lines are also the ultimate source of variation in sexually reproducing organisms. However,
in contrast to mutation during asexual reproduction, the mutations during sexual reproduction can be continually
reshuffled from one generation to the next when different parents combine their unique genomes and the genes
are mixed into different combinations by crossovers during prophase I and random assortment at metaphase I.
V / _
e olution CONNECTION
The Red Queen Hypothesis
Genetic variation is the outcome of sexual reproduction, but why are ongoing variations necessary, even
under seemingly stable environmental conditions? Enter the Red Queen hypothesis, first proposed by Leigh
Van Valen in 1973. The concept was named in reference to the Red Queen's race in Lewis Carroll's book,
Through the Looking-Glass.
All species coevolve (evolve together) with other organisms. For example, predators evolve with their prey,
and parasites evolve with their hosts. Each tiny advantage gained by favorable variation gives a species
a reproductive edge over close competitors, predators, parasites, or even prey. However, survival of any
given genotype or phenotype in a population is dependent on the reproductive fitness of other genotypes
or phenotypes within a given species. The only method that will allow a coevolving species to maintain
its own share of the resources is to also continually improve its fitness (the capacity of the members to
produce more reproductively viable offspring relative to others within a species). As one species gains an
advantage, this increases selection on the other species; they must also develop an advantage or they will
be outcompeted. No single species progresses too far ahead because genetic variation among the progeny
of sexual reproduction provides all species with a mechanism to improve rapidly. Species that cannot keep
up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the
same place.” This is an apt description of coevolution between competing species.
Life Cycles of Sexually Reproducing Organisms
Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends
on the organism’s “reproductive strategy.” The process of meiosis reduces the chromosome number by half.
Fertilization, the joining of two haploid gametes, restores the diploid condition. Some organisms have a
multicellular diploid stage that is most obvious and only produce haploid reproductive cells. Animals, including
humans, have this type of life cycle. Other organisms, such as fungi, have a multicellular haploid stage that is
most obvious. Plants and some algae have alternation of generations, in which they have multicellular diploid
and haploid life stages that are apparent to different degrees depending on the group.
Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the
3. Leigh Van Valen, “A New Evolutionary Law,” Evolutionary Theory 1 (1973): 1-30
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Chapter 111 Meiosis and Sexual Reproduction
319
organism are the gametes. Early in the development of the embryo, specialized diploid cells, called germ cells,
are produced within the gonads (such as the testes and ovaries). Germ cells are capable of mitosis to perpetuate
the germ cell line and meiosis to produce haploid gametes. Once the haploid gametes are formed, they lose
the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two
gametes, usually from different individuals, restoring the diploid state (Figure 11.8).
Figure 11.8 In animals, sexually reproducing adults form haploid gametes from diploid germ cells. Fusion of the
gametes gives rise to a fertilized egg cell, or zygote. The zygote will undergo multiple rounds of mitosis to produce a
multicellular offspring. The germ cells are generated early in the development of the zygote.
Most fungi and algae employ a life-cycle type in which the “body” of the organism—the ecologically important
part of the life cycle—is haploid. The haploid cells that make up the tissues of the dominant multicellular stage
are formed by mitosis. During sexual reproduction, specialized haploid cells from two individuals—designated
the (+) and (-) mating types—join to form a diploid zygote. The zygote immediately undergoes meiosis to
form four haploid cells called spores. Although these spores are haploid like the “parents,” they contain a new
genetic combination from two parents. The spores can remain dormant for various time periods. Eventually,
when conditions are favorable, the spores form multicellular haploid structures through many rounds of mitosis
(Figure 11.9).
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Chapter 111 Meiosis and Sexual Reproduction
visual
CONNECTION
Figure 11.9 Fungi, such as black bread mold (Rhizopus nigricans), have a haploid multicellular stage that
produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The haploid multicellular stage
produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The zygote undergoes meiosis
to produce haploid spores. Each spore gives rise to a multicellular haploid organism by mitosis. Above, different
mating hyphae types (denoted as + and -) join to form a zygospore through nuclear fusion, (credit “zygomycota”
micrograph: modification of work by “Fanaberka’VWikimedia Commons)
If a mutation occurs so that a fungus is no longer able to produce a minus mating type, will it still be able to
reproduce?
The third life-cycle type, employed by some algae and all plants, is a blend of the haploid-dominant and
diploid-dominant extremes. Species with alternation of generations have both haploid and diploid multicellular
organisms as part of their life cycle. The haploid multicellular plants are called gametophytes, because they
produce gametes from specialized cells. Meiosis is not directly involved in the production of gametes in this case,
because the organism that produces the gametes is already haploid. Fertilization between the gametes forms
a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant
called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The
spores will subsequently develop into the gametophytes (Figure 11.10).
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Chapter 111 Meiosis and Sexual Reproduction
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Figure 11.10 Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid
organism. In some plants, such as ferns, both the haploid and diploid plant stages are free-living. The diploid plant
is called a sporophyte because it produces haploid spores by meiosis. The spores develop into multicellular, haploid
plants that are called gametophytes because they produce gametes. The gametes of two individuals will fuse to form
a diploid zygote that becomes the sporophyte. (credit “fern": modification of work by Cory Zanker; credit “sporangia”:
modification of work by "Obsidian Sour'AA/ikimedia Commons; credit “gametophyte and sporophyte": modification of
work by “Vlmastra’VWikimedia Commons)
Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte
and the gametophyte and the relationship between them vary greatly. In plants such as moss, the gametophyte
organism is the free-living plant and the sporophyte is physically dependent on the gametophyte. In other plants,
such as ferns, both the gametophyte and sporophyte plants are free-living; however, the sporophyte is much
larger. In seed plants, such as magnolia trees and daisies, the gametophyte is composed of only a few cells and,
in the case of the female gametophyte, is completely retained within the sporophyte.
Sexual reproduction takes many forms in multicellular organisms. The fact that nearly every multicellular
organism on Earth employs sexual reproduction is strong evidence for the benefits of producing offspring with
unique gene combinations, though there are other possible benefits as well.
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Chapter 111 Meiosis and Sexual Reproduction
KEY TERMS
alternation of generations life-cycle type in which the diploid and haploid stages alternate
chiasmata (singular, chiasma) the structure that forms at the crossover points after genetic material is
exchanged
cohesin proteins that form a complex that seals sister chromatids together at their centromeres until anaphase
II of meiosis
crossover exchange of genetic material between nonsister chromatids resulting in chromosomes that
incorporate genes from both parents of the organism
fertilization union of two haploid cells from two individual organisms
gametophyte a multicellular haploid life-cycle stage that produces gametes
germ cells specialized cell line that produces gametes, such as eggs or sperm
interkinesis (also, interphase II) brief period of rest between meiosis I and meiosis II
life cycle the sequence of events in the development of an organism and the production of cells that produce
offspring
meiosis a nuclear division process that results in four haploid cells
meiosis I first round of meiotic cell division; referred to as reduction division because the ploidy level is reduced
from diploid to haploid
meiosis II second round of meiotic cell division following meiosis I; sister chromatids are separated into
individual chromosomes, and the result is four unique haploid cells
recombination nodules protein assemblies formed on the synaptonemal complex that mark the points of
crossover events and mediate the multistep process of genetic recombination between nonsister
chromatids
reduction division nuclear division that produces daughter nuclei each having one-half as many chromosome
sets as the parental nucleus; meiosis I is a reduction division
somatic cell all the cells of a multicellular organism except the gametes or reproductive cells
spore haploid cell that can produce a haploid multicellular organism or can fuse with another spore to form a
diploid cell
sporophyte a multicellular diploid life-cycle stage that produces haploid spores by meiosis
synapsis formation of a close association between homologous chromosomes during prophase I
synaptonemal complex protein lattice that forms between homologous chromosomes during prophase i,
supporting crossover
tetrad two duplicated homologous chromosomes (four chromatids) bound together by chiasmata during
prophase I
CHAPTER SUMMARY
11.1 The Process of Meiosis
Sexual reproduction requires that organisms produce cells that can fuse during fertilization to produce
offspring. In most animals, meiosis is used to produce haploid eggs and sperm from diploid parent cells so that
the fusion of an egg and sperm produces a diploid zygote. As with mitosis, DNA replication occurs prior to
meiosis during the S-phase of the cell cycle so that each chromosome becomes a pair of sister chromatids. In
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Chapter 111 Meiosis and Sexual Reproduction
323
meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each
with half the number of chromosomes as the parent cell. The first division separates homologs, and the
second—like mitosis—separates chromatids into individual chromosomes. Meiosis generates variation in the
daughter nuclei during crossover in prophase I as well as during the random alignment of tetrads at metaphase
I. The cells that are produced by meiosis are genetically unique.
Meiosis and mitosis share similar processes, but have distinct outcomes. Mitotic divisions are single nuclear
divisions that produce genetically identical daughter nuclei (i.e., each daughter nucleus has the same number
of chromosome sets as the original cell). In contrast, meiotic divisions include two nuclear divisions that
ultimately produce four genetically different daughter nuclei that have only one chromosome set (instead of the
two sets of chromosomes in the parent cell). The main differences between the two nuclear division processes
take place during the first division of meiosis: homologous chromosomes pair, crossover, and exchange
homologous nonsister chromatid segments. The homologous chromosomes separate into different nuclei
during meiosis I, causing a reduction of ploidy level in the first division. The second division of meiosis is similar
to a mitotic division, except that the daughter cells do not contain identical genomes because of crossover and
chromosome recombination in prophase I.
II. 2 Sexual Reproduction
Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by
meiosis provides an important advantage that has made sexual reproduction evolutionarily successful. Meiosis
and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called
gametes, which have half the number of chromosomes as the parent cell. When two haploid gametes fuse, this
restores the diploid condition in the new zygote. Thus, most sexually reproducing organisms alternate between
haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between
meiosis and fertilization vary greatly.
VISUAL CONNECTION QUESTIONS
1. Figure 11.9 If a mutation occurs so that a fungus it still be able to reproduce?
is no longer able to produce a minus mating type, will
REVIEW QUESTIONS
2. Meiosis usually produces_daughter
cells.
a. two haploid
b. two diploid
c. four haploid
d. four diploid
3. What structure is most important in forming the
tetrads?
a. centromere
b. synaptonemal complex
c. chiasma
d. kinetochore
4. At which stage of meiosis are sister chromatids
separated from each other?
a. prophase I
b. prophase II
c. anaphase I
d. anaphase II
5. At metaphase I, homologous chromosomes are
connected only at what structures?
a. chiasmata
b. recombination nodules
c. microtubules
d. kinetochores
6. Which of the following is not true in regard to
crossover?
a. Spindle microtubules guide the transfer of
DNA across the synaptonemal complex.
b. Nonsister chromatids exchange genetic
material.
c. Chiasmata are formed.
d. Recombination nodules mark the crossover
point.
7. What phase of mitotic interphase is missing from
meiotic interkinesis?
a. Go phase
b. Gi phase
c. S phase
d. G 2 phase
8. The part of meiosis that is similar to mitosis is
a. meiosis I
b. anaphase I
c. meiosis II
d. interkinesis
9. If a muscle cell of a typical organism has 32
chromosomes, how many chromosomes will be in a
gamete of that same organism?
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Chapter 111 Meiosis and Sexual Reproduction
a.
8
b.
16
c.
32
d.
64
10. Which statement best describes the genetic
content of the two daughter cells in prophase il of
meiosis?
a. haploid with one copy of each gene
b. haploid with two copies of each gene
c. diploid with two copies of each gene
d. diploid with four copies of each gene
11. The pea plants used in Mendel’s genetic
inheritance studies were diploid, with 14
chromosomes in somatic cells. Assuming no crossing
over events occur, how many unique gametes could
one pea plant produce?
a. 28
b. 128
c. 196
d. 16,384
12. How do telophase I and telophase II differ during
meiosis in animal cells?
a. Cells remain diploid at the end of telophase
I, but are haploid at the end of telophase II.
b. Daughter cells form a cell plate to divide
during telophase I, but divide by cytokinesis
during telophase II.
c. Cells enter interphase after telophase I, but
not after telophase II.
d. Chromosomes can remain condensed at the
end of telophase I, but decondense after
telophase II.
13. What is a likely evolutionary advantage of sexual
reproduction over asexual reproduction?
a. Sexual reproduction involves fewer steps.
b. There is a lower chance of using up the
resources in a given environment.
c. Sexual reproduction results in variation in
the offspring.
d. Sexual reproduction is more cost-effective.
14. Which type of life cycle has both a haploid and
CRITICAL THINKING QUESTIONS
19. Describe the process that results in the formation
of a tetrad.
20. Explain how the random alignment of
homologous chromosomes during metaphase I
contributes to the variation in gametes produced by
meiosis.
21. What is the function of the fused kinetochore
found on sister chromatids in prometaphase I?
22. In a comparison of the stages of meiosis to the
stages of mitosis, which stages are unique to meiosis
and which stages have the same events in both
diploid multicellular stage?
a. asexual life cycles
b. most animal life cycles
c. most fungal life cycles
d. alternation of generations
15. What is the ploidy of the most conspicuous form
of most fungi?
a. diploid
b. haploid
c. alternation of generations
d. asexual
16. A diploid, multicellular life-cycle stage that gives
rise to haploid cells by meiosis is called a_.
a. sporophyte
b. gametophyte
c. spore
d. gamete
17. Hydras and jellyfish both live in a freshwater lake
that is slowly being acidified by the runoff from a
chemical plant built upstream. Which population is
predicted to be better able to cope with the changing
environment?
a. jellyfish
b. hydra
c. The populations will be equally able to cope.
d. Both populations will die.
18. Many farmers are worried about the decreasing
genetic diversity of plants associated with
generations of artificial selection and inbreeding. Why
is limiting random sexual reproduction of food crops
concerning?
a. Mutations during asexual reproduction
decrease plant fitness.
b. Consumers do not trust identical-appearing
produce.
c. Larger portions of the plant populations are
susceptible to the same diseases.
d. Spores are not viable in an agricultural
setting.
meiosis and mitosis?
23. Why would an individual with a mutation that
prevented the formation of recombination nodules be
considered less fit than other members of its
species?
24. Does crossing over occur during prophase II?
From an evolutionary perspective, why is this
advantageous?
25. List and briefly describe the three processes that
lead to variation in offspring with the same parents.
26. Animals and plants both have diploid and haploid
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Chapter 111 Meiosis and Sexual Reproduction
325
cells. How does the animal life cycle differ from the
alternation of generations exhibited by plants?
27. Explain why sexual reproduction is beneficial to a
population but can be detrimental to an individual
offspring.
28. How does the role of meiosis in gamete
production differ between organisms with a diploid-
dominant life cycle and organisms with an alternation
of generations life cycle?
29. How do organisms with haploid-dominant life
cycles ensure continued genetic diversification in
offspring without using a meiotic process to make
gametes?
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Chapter 111 Meiosis and Sexual Reproduction
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Chapter 12 | Mendel's Experiments and Heredity
327
12 | MENDEL'S
EXPERIMENTS AND
HEREDITY
Figure 12.1 Experimenting with thousands of garden peas, Mendel uncovered the fundamentals of genetics, (credit:
modification of work by Jerry Kirkhart)
Chapter Outline
12.1: Mendel’s Experiments and the Laws of Probability
12.2: Characteristics and Traits
12.3: Laws of Inheritance
Introduction
Genetics is the study of heredity. Johann Gregor Mendel set the framework for genetics long before
chromosomes or genes had been identified, at a time when meiosis was not well understood. Mendel selected
a simple biological system and conducted methodical, quantitative analyses using large sample sizes. Because
of Mendel’s work, the fundamental principles of heredity were revealed. We now know that genes, carried
on chromosomes, are the basic functional units of heredity with the capability to be replicated, expressed, or
mutated. Today, the postulates put forth by Mendel form the basis of classical, or Mendelian, genetics. Not all
genes are transmitted from parents to offspring according to Mendelian genetics, but Mendel’s experiments
serve as an excellent starting point for thinking about inheritance.
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Chapter 12 | Mendel's Experiments and Heredity
12.1 1 Mendel’s Experiments and the Laws of
Probability
By the end of this section, you will be able to do the following:
• Describe the scientific reasons for the success of Mendel’s experimental work
• Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles
• Apply the sum and product rules to calculate probabilities
Figure 12.2 Johann Gregor Mendel is considered the father of genetics.
Johann Gregor Mendel (1822-1884) (Figure 12.2) was a lifelong learner, teacher, scientist, and man of
faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech
Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary
and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in
honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient
characteristics used to study a specific biological phenomenon to be applied to other systems). In 1865, Mendel
presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He
demonstrated that traits are transmitted from parents to offspring independently of other traits and in dominant
and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization, in the proceedings
of the Natural History Society of Brunn.
Mendel’s work went virtually unnoticed by the scientific community, which believed, incorrectly, that the process
of inheritance involved a blending of parental traits that produced an intermediate physical appearance in
offspring. The blending theory of inheritance asserted that the original parental traits were lost or absorbed by
the blending in the offspring, but we now know that this is not the case. This hypothetical process appeared to
be correct because of what we know now as continuous variation. Continuous variation results from the action
of many genes to determine a characteristic like human height. Offspring appear to be a “blend” of their parents'
traits.
Instead of continuous characteristics, Mendel worked with traits that were inherited in distinct classes
(specifically, violet versus white flowers); this is referred to as discontinuous variation. Mendel’s choice of
these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring, nor were
1. Johann Gregor Mendel, Versuche uber Pflanzenhybriden Verhandlungen des naturforschenden Vereines in Brunn, Bd. IV fur das Jahr,
1865 Abhandlungen, 3-47. [for English translation see http://www.mendelweb.org/Mendel.plain.html (http:// 0 penstax. 0 rg/l/
mendel_experiments) ]
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Chapter 12 | Mendel's Experiments and Heredity
329
they absorbed, but rather that they kept their distinctness and could be passed on. In 1868, Mendel became
abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized
for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was
rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of
heredity.
Mendel’s Model System
Mendel’s seminal work was accomplished using the garden pea, Pisum sativum, to study inheritance. This
species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals
remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred,
or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By
experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring
that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season,
meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of
garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about
simply by chance.
Mendelian Crosses
Mendel performed hybridizations, which involve mating two true-breeding individuals that have different traits.
In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a
mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. In plants,
pollen carries the male gametes (sperm) to the stigma, a sticky organ that traps pollen and allows the sperm
to move down the pistil to the female gametes (ova) below. To prevent the pea plant that was receiving pollen
from self-fertilizing and confounding his results, Mendel painstakingly removed all of the anthers from the plant’s
flowers before they had a chance to mature.
Plants used in first-generation crosses were called Po, or parental generation one (Figure 12.3). After each
cross, Mendel collected the seeds belonging to the Po plants and grew them the following season. These
offspring were called the Fi, or the first filial (filial = offspring, daughter or son) generation. Once Mendel
examined the characteristics in the Fi generation of plants, he allowed them to self-fertilize naturally. He then
collected and grew the seeds from the Fi plants to produce the F 2 , or second filial, generation. Mendel’s
experiments extended beyond the F 2 generation to the F 3 and F 4 generations, and so on, but it was the ratio
of characteristics in the P 0 -F 1 -F 2 generations that were the most intriguing and became the basis for Mendel’s
postulates.
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Chapter 12 | Mendel's Experiments and Heredity
Figure 12.3 In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for
violet flower color with plants true-breeding for white flower color (the P generation). The resulting hybrids in the Fi
generation all had violet flowers. In the F 2 generation, approximately three quarters of the plants had violet flowers,
and one quarter had white flowers.
Garden Pea Characteristics Revealed the Basics of Heredity
In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each
with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic.
The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and
flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus
violet. To fully examine each characteristic, Mendel generated large numbers of Fi and F 2 plants, reporting
results from 19,959 F 2 plants alone. His findings were consistent.
What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he had plants that
bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed
offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet
flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were
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Chapter 12 | Mendel's Experiments and Heredity
331
physically identical.
Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma
of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel
found that 100 percent of the Fi hybrid generation had violet flowers. Conventional wisdom at that time (the
blending theory) would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal
numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the
offspring. Instead, Mendel’s results demonstrated that the white flower trait in the Fi generation had completely
disappeared.
Importantly, Mendel did not stop his experimentation there. He allowed the Fi plants to self-fertilize and found
that, of F 2 -generation plants, 705 had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet
flowers per one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet
flowers to the stigma of a plant with white flowers and vice versa, he obtained about the same ratio regardless of
which parent, male or female, contributed which trait. This is called a reciprocal cross —a paired cross in which
the respective traits of the male and female in one cross become the respective traits of the female and male in
the other cross. For the other six characteristics Mendel examined, the Fi and F 2 generations behaved in the
same way as they had for flower color. One of the two traits would disappear completely from the Fi generation
only to reappear in the F 2 generation at a ratio of approximately 3:1 (Table 12.1).
The Results of Mendel’s Garden Pea Hybridizations
Characteristic
Contrasting Po
Traits
Fi Offspring
Traits
F 2 Offspring
Traits
F 2 Trait
Ratios
Flower color
Violet vs. white
100 percent violet
705 violet
224 white
3.15:1
Flower position
Axial vs. terminal
100 percent axial
651 axial
207 terminal
3.14:1
Plant height
Tall vs. dwarf
100 percent tall
787 tall
277 dwarf
2.84:1
Seed texture
Round vs. wrinkled
100 percent round
5,474 round
1,850 wrinkled
2.96:1
Seed color
Yellow vs. green
100 percent yellow
6,022 yellow
2,001 green
3.01:1
Pea pod texture
Inflated vs. constricted
100 percent inflated
882 inflated
299 constricted
2.95:1
Pea pod color
Green vs. yellow
100 percent green
428 green
152 yellow
2.82:1
Table 12.1
Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be
divided into expressed and latent traits. He called these, respectively, dominant and recessive traits. Dominant
traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear,
in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid
offspring. An example of a dominant trait is the violet-flower trait. For this same characteristic (flower color),
white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F 2 generation
meant that the traits remained separate (not blended) in the plants of the Fi generation. Mendel also proposed
that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted
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Chapter 12 | Mendel's Experiments and Heredity
one of its two copies to its offspring, where they came together. Moreover, the physical observation of a
dominant trait could mean that the genetic composition of the organism included two dominant versions of the
characteristic or that it included one dominant and one recessive version. Conversely, the observation of a
recessive trait meant that the organism lacked any dominant versions of this characteristic.
So why did Mendel repeatedly obtain 3:1 ratios in his crosses? To understand how Mendel deduced the basic
mechanisms of inheritance that lead to such ratios, we must first review the laws of probability.
Probability Basics
Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by
dividing the number of times the event occurs by the total number of opportunities for the event to occur. It is also
possible to calculate theoretical probabilities by dividing the number of times that an event is expected to occur
by the number of times that it could occur. Empirical probabilities come from observations, like those of Mendel.
Theoretical probabilities, on the other hand, come from knowing how the events are produced and assuming
that the probabilities of individual outcomes are equal. A probability of one for some event indicates that it is
guaranteed to occur, whereas a probability of zero indicates that it is guaranteed not to occur. An example of a
genetic event is a round seed produced by a pea plant.
In one experiment, Mendel demonstrated that the probability of the event “round seed” occurring was one in the
Fi offspring of true-breeding parents, one of which has round seeds and one of which has wrinkled seeds. When
the Fi plants were subsequently self-crossed, the probability of any given F 2 offspring having round seeds was
now three out of four. In other words, in a large population of F 2 offspring chosen at random, 75 percent were
expected to have round seeds, whereas 25 percent were expected to have wrinkled seeds. Using large numbers
of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses.
The Product Rule and Sum Rule
Mendel demonstrated that pea plants transmit characteristics as discrete units from parent to offspring. As
will be discussed, Mendel also determined that different characteristics, like seed color and seed texture,
were transmitted independently of one another and could be considered in separate probability analyses. For
instance, performing a cross between a plant with green, wrinkled seeds and a plant with yellow, round seeds
still produced offspring that had a 3:1 ratio of green:yellow seeds (ignoring seed texture) and a 3:1 ratio of
round:wrinkled seeds (ignoring seed color). The characteristics of color and texture did not influence each other.
The product rule of probability can be applied to this phenomenon of the independent transmission of
characteristics. The product rule states that the probability of two independent events occurring together can be
calculated by multiplying the individual probabilities of each event occurring alone. To demonstrate the product
rule, imagine that you are rolling a six-sided die (D) and flipping a penny (P) at the same time. The die may roll
any number from 1-6 (D#), whereas the penny may turn up heads (Ph) or tails (Pt). The outcome of rolling the
die has no effect on the outcome of flipping the penny and vice versa. There are 12 possible outcomes of this
action (Table 12.2), and each event is expected to occur with equal probability.
Twelve Equally Likely Outcomes of Rolling a Die and Flipping a Penny
Rolling Die
Flipping Penny
Di
Ph
Di
Pt
d 2
Ph
D2
Pt
d 3
Ph
D3
Pt
d 4
Ph
D4
Pt
Table 12.2
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Chapter 12 | Mendel's Experiments and Heredity
333
Twelve Equally Likely Outcomes of Rolling a Die and Flipping a Penny
Rolling Die
Flipping Penny
d 5
Ph
d 5
Pt
d 6
Ph
d 6
Pt
Table 12.2
Of the 12 possible outcomes, the die has a 2/12 (or 1/6) probability of rolling a two, and the penny has a 6/12
(or 1/2) probability of coming up heads. By the product rule, the probability that you will obtain the combined
outcome 2 and heads is: (D 2 ) x (Ph) = (1/6) x (1/2) or 1/12 (Table 12.3). Notice the word “and” in the description
of the probability. The “and” is a signal to apply the product rule. For example, consider how the product rule is
applied to the dihybrid cross: the probability of having both dominant traits in the F 2 progeny is the product of the
probabilities of having the dominant trait for each characteristic, as shown here:
On the other hand, the sum rule of probability is applied when considering two mutually exclusive outcomes
that can come about by more than one pathway. The sum rule states that the probability of the occurrence of
one event or the other event, of two mutually exclusive events, is the sum of their individual probabilities. Notice
the word “or” in the description of the probability. The “or” indicates that you should apply the sum rule. In this
case, let’s imagine you are flipping a penny (P) and a quarter (Q). What is the probability of one coin coming
up heads and one coin coming up tails? This outcome can be achieved by two cases: the penny may be heads
(Ph) and the quarter may be tails (Qt), or the quarter may be heads (Qh) and the penny may be tails (Pj). Either
case fulfills the outcome. By the sum rule, we calculate the probability of obtaining one head and one tail as
[(Ph) x (Qt)] + [(Qh) x (Pt)] = [(1/2) x (1/2)] + [(1/2) x (1/2)] = 1/2 (Table 12.3). You should also notice that we
used the product rule to calculate the probability of Ph and Qt, and also the probability of Pt and Qh, before we
summed them. Again, the sum rule can be applied to show the probability of having just one dominant trait in the
F 2 generation of a dihybrid cross:
_ 3 _ + 1 = 15
16 4 16
The Product Rule and Sum Rule
Product Rule
Sum Rule
For independent events A and B, the probability (P)
of them both occurring (A and B) is (Pa x Pb)
For mutually exclusive events A and B, the probability
(P) that at least one occurs (A or B) is (Pa + Pb)
Table 12.3
To use probability laws in practice, we must work with large sample sizes because small sample sizes are prone
to deviations caused by chance. The large quantities of pea plants that Mendel examined allowed him calculate
the probabilities of the traits appearing in his F 2 generation. As you will learn, this discovery meant that when
parental traits were known, the offspring’s traits could be predicted accurately even before fertilization.
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Chapter 12 | Mendel's Experiments and Heredity
12.2 | Characteristics and Traits
By the end of this section, you will be able to do the following:
• Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems
• Develop a Punnett square to calculate the expected proportions of genotypes and phenotypes in a
monohybrid cross
• Explain the purpose and methods of a test cross
• Identify non-Mendelian inheritance patterns such as incomplete dominance, codominance, recessive
lethals, multiple alleles, and sex linkage
Physical characteristics are expressed through genes carried on chromosomes. The genetic makeup of peas
consists of two similar, or homologous, copies of each chromosome, one from each parent. Each pair of
homologous chromosomes has the same linear order of genes. In other words, peas are diploid organisms in
that they have two copies of each chromosome. The same is true for many other plants and for virtually all
animals. Diploid organisms produce haploid gametes, which contain one copy of each homologous chromosome
that unite at fertilization to create a diploid zygote.
For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that
may or may not encode the same version of that characteristic. Gene variants that arise by mutation and exist at
the same relative locations on homologous chromosomes are called alleles. Mendel examined the inheritance
of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a
natural population.
Phenotypes and Genotypes
Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics.
The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying
genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Mendel’s
hybridization experiments demonstrate the difference between phenotype and genotype. When true-breeding
plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the Fi hybrid
offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent
with yellow pods. Flowever, we know that the allele donated by the parent with green pods was not simply
lost because it reappeared in some of the F 2 offspring. Therefore, the Fi plants must have been genotypically
different from the parent with yellow pods.
The Pi plants that Mendel used in his experiments were each homozygous for the trait he was studying.
Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on
their homologous chromosomes. Mendel’s parental pea plants always bred true because both of the gametes
produced carried the same trait. When Pi plants with contrasting traits were cross-fertilized, all of the offspring
were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles
for the gene being examined.
Dominant and Recessive Alleles
Our discussion of homozygous and heterozygous organisms brings us to why the Fi heterozygous offspring
were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics,
one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele
the expressed unit factor; the recessive allele was referred to as the latent unit factor. We now know that these
so-called unit factors are actually genes on homologous chromosome pairs. For a gene that is expressed in
a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that
is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in
homozygous recessive individuals (Table 12.4).
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Chapter 12 | Mendel's Experiments and Heredity
335
Human Inheritance in Dominant and Recessive Patterns
Dominant Traits
Recessive Traits
Achondroplasia
Albinism
Brachydactyly
Cystic fibrosis
Huntington’s disease
Duchenne muscular dystrophy
Marfan syndrome
Galactosemia
Neurofibromatosis
Phenylketonuria
Widow’s peak
Sickle-cell anemia
Wooly hair
Tay-Sachs disease
Table 12.4
Several conventions exist for referring to genes and alleles. For the purposes of this chapter, we will abbreviate
genes using the first letter of the gene’s corresponding dominant trait. For example, violet is the dominant trait
for a pea plant’s flower color, so the flower-color gene would be abbreviated as V (note that it is customary to
italicize gene designations). Furthermore, we will use uppercase and lowercase letters to represent dominant
and recessive alleles, respectively. Therefore, we would refer to the genotype of a homozygous dominant pea
plant with violet flowers as VV, a homozygous recessive pea plant with white flowers as vv, and a heterozygous
pea plant with violet flowers as Vv.
The Punnett Square Approach for a Monohybrid Cross
When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is
called a monohybrid cross, and the resulting offspring are monohybrids. Mendel performed seven monohybrid
crosses involving contrasting traits for each characteristic. On the basis of his results in Fi and F 2 generations,
Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each
offspring, and every possible combination of unit factors was equally likely.
To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea
seeds. The dominant seed color is yellow; therefore, the parental genotypes were YY for the plants with yellow
seeds and yy for the plants with green seeds, respectively. A Punnett square, devised by the British geneticist
Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic
cross or mating and their expected frequencies. To prepare a Punnett square, all possible combinations of the
parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid, representing
their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes
in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or
fertilized egg, that could result from this mating. Because each possibility is equally likely, genotypic ratios can be
determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic
ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one
type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds (Figure
12.4).
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Chapter 12 | Mendel's Experiments and Heredity
0
YY
Yellow
I
Gametes y
Monohybrid Cross
Green
Each homozygous
parent in the P
generation produces
only one kind of
gamete.
\ ✓
Fi
Gametes
Yy
/ \
y y
The heterozygous F x
offspring produces
two kinds of gamete.
Yellow
YY
Yellow
Yy
Yellow
Yy
b*
Self-pollination of the
F x offspring produces
F 2 offspring with a 3:1
ratio of yellow to green
seeds.
Genotype Phenotype
Phenotypes Genotypes ratio ratio
Yellow
YY
Yy
1
2
3
Green
yy
1
1
Figure 12.4 In the P generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with
plants with the recessive green phenotype. This cross produces Fi heterozygotes with a yellow phenotype. Punnett
square analysis can be used to predict the genotypes of the F 2 generation.
A self-cross of one of the Yy heterozygous offspring can be represented in a 2 x 2 Punnett square because
each parent can donate one of two different alleles. Therefore, the offspring can potentially have one of
four allele combinations: YY, Yy, yY, or yy (Figure 12.4). Notice that there are two ways to obtain the Yy
genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm. Both
of these possibilities must be counted. Recall that Mendel’s pea-plant characteristics behaved in the same
way in reciprocal crosses. Therefore, the two possible heterozygous combinations produce offspring that are
genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different
parents. They are grouped together. Because fertilization is a random event, we expect each combination to be
equally likely and for the offspring to exhibit a ratio of YY.Yy.yy genotypes of 1:2:1 (Figure 12.4). Furthermore,
because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule
of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:l green. Indeed, working with
large sample sizes, Mendel observed approximately this ratio in every F 2 generation resulting from crosses for
individual traits.
Mendel validated these results by performing an F3 cross in which he self-crossed the dominant- and recessive-
expressing F 2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green
seeds, confirming that all green seeds had homozygous genotypes of yy. When he self-crossed the F 2 plants
expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated
at a 3:1 ratio of yellow:green seeds. In this case, the true-breeding plants had homozygous ( YY) genotypes,
whereas the segregating plants corresponded to the heterozygous (Yy) genotype. When these plants self-
fertilized, the outcome was just like the Fi self-fertilizing cross.
The Test Cross Distinguishes the Dominant Phenotype
Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also
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Chapter 12 | Mendel's Experiments and Heredity
337
developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a
homozygote. Called the test cross, this technique is still used by plant and animal breeders. In a test cross,
the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same
characteristic. If the dominant-expressing organism is a homozygote, then all Fi offspring will be heterozygotes
expressing the dominant trait (Figure 12.5). Alternatively, if the dominant expressing organism is a heterozygote,
the Fi offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes (Figure 12.5). The test cross
further validates Mendel’s postulate that pairs of unit factors segregate equally.
visual
CONNECTION
The Test Cross
Gametes from parent
of unknown genotype
Y ?
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c
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0
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0
0
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0
E
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0
Yy
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A test cross resulting in
all dominant offspring
indicates that the parent
is homozygous dominant.
Gametes from parent
of unknown genotype
Y ?
c
Yy
Yy
A test cross resulting
in a 1:1 ratio of yellow
to green offspring
indicates that the
parent is heterozygous.
Figure 12.5 A test cross can be performed to determine whether an organism expressing a dominant trait is a
homozygote or a heterozygote.
In pea plants, round peas (R) are dominant to wrinkled peas (r). You do a test cross between a pea plant with
wrinkled peas (genotype rr) and a plant of unknown genotype that has round peas. You end up with three
plants, all which have round peas. From this data, can you tell if the round pea parent plant is homozygous
dominant or heterozygous? If the round pea parent plant is heterozygous, what is the probability that a
random sample of 3 progeny peas will all be round?
Many human diseases are genetically inherited. A healthy person in a family in which some members suffer
from a recessive genetic disorder may want to know if he or she has the disease-causing gene and what risk
exists of passing the disorder on to his or her offspring. Of course, doing a test cross in humans is unethical
and impractical. Instead, geneticists use pedigree analysis to study the inheritance pattern of human genetic
diseases (Figure 12.6).
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Chapter 12 | Mendel's Experiments and Heredity
visual
CONNECTION
Pedigree Analysis for Alkaptonuria
Hr©
—
a a
A ?
A a
First
generation
Second
generation
Third
generation
Fourth
generation
□ Male O Female Q Unaffected □ Affected
Figure 12.6 Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine,
are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint
damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the
genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often
possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has
the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected
phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal
allele, so their genotype gets the “A?" designation.
What are the genotypes of the individuals labeled 1, 2, and 3?
Alternatives to Dominance and Recessiveness
Mendel’s experiments with pea plants suggested that: (1) two “units" or alleles exist for every gene; (2) alleles
maintain their integrity in each generation (no blending); and (3) in the presence of the dominant allele,
the recessive allele is hidden and makes no contribution to the phenotype. Therefore, recessive alleles can
be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as
“carriers.” Further genetic studies in other plants and animals have shown that much more complexity exists,
but that the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider
some of the extensions of Mendelism. If Mendel had chosen an experimental system that exhibited these genetic
complexities, it’s possible that he would not have understood what his results meant.
Incomplete Dominance
Mendel’s results, that traits are inherited as dominant and recessive pairs, contradicted the view at that time
that offspring exhibited a blend of their parents’ traits. However, the heterozygote phenotype occasionally does
appear to be intermediate between the two parents. For example, in the snapdragon, Antirrhinum majus (Figure
12.7), a cross between a homozygous parent with white flowers ( C W C W ) and a homozygous parent with red
flowers (C C ) will produce offspring with pink flowers (C C ). (Note that different genotypic abbreviations are
used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness.) This
pattern of inheritance is described as incomplete dominance, denoting the expression of two contrasting alleles
such that the individual displays an intermediate phenotype. The allele for red flowers is incompletely dominant
over the allele for white flowers. However, the results of a heterozygote self-cross can still be predicted, just as
R R R 1/1/
with Mendelian dominant and recessive crosses. In this case, the genotypic ratio would be 1 C C 2 C C 1
C w c w , a nd the phenotypic ratio would be 1:2:1 for red:pink:white.
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Figure 12.7 These pink flowers of a heterozygote snapdragon result from incomplete dominance, (credit:
“storebukkebruse'VFlickr)
Codominance
A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are
simultaneously expressed in the heterozygote. An example of codominance is the MN blood groups of humans.
The M and N alleles are expressed in the form of an M or N antigen present on the surface of red blood
cells. Homozygotes [l m L m and L N L N ) express either the M or the N allele, and heterozygotes (L M L N ) express
both alleles equally. In a self-cross between heterozygotes expressing a codominant trait, the three possible
offspring genotypes are phenotypically distinct. However, the 1:2:1 genotypic ratio characteristic of a Mendelian
monohybrid cross still applies.
Multiple Alleles
Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now
know that this is an oversimplification. Although individual humans (and all diploid organisms) can only have two
alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two
alleles are observed. Note that when many alleles exist for the same gene, the convention is to denote the most
common phenotype or genotype among wild animals as the wild type (often abbreviated this is considered
the standard or norm. All other phenotypes or genotypes are considered variants of this standard, meaning that
they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.
An example of multiple alleles is coat color in rabbits (Figure 12.8). Here, four alleles exist for the c gene. The
i _i_ ch) ph
wild-type version, C C , is expressed as brown fur. The chinchilla phenotype, c c , is expressed as black-
tipped white fur. The Himalayan phenotype, c h c h , has black fur on the extremities and white fur elsewhere.
Finally, the albino, or “colorless" phenotype, cc, is expressed as white fur. In cases of multiple alleles, dominance
hierarchies can exist. In this case, the wild-type allele is dominant over all the others, chinchilla is incompletely
dominant over Himalayan and albino, and Himalayan is dominant over albino. This hierarchy, or allelic series,
was revealed by observing the phenotypes of each possible heterozygote offspring.
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Chapter 12 | Mendel's Experiments and Heredity
Allele
C c*> C* c
Genotype
CC C ch c ch c h c h cc
Phenotype
WILD TYPE:
Brown
fur
CHINCHILLA:
Black-tipped
white fur
HIMALAYAN:
White fur
with black
paws, nose,
ears, tail
ALBINO:
White
fur
$
$
9
Figure 12.8 Four different alleles exist for the rabbit coat color (C) gene.
The complete dominance of a wild-type phenotype over all other mutants often occurs as an effect of “dosage”
of a specific gene product, such that the wild-type allele supplies the correct amount of gene product whereas
the mutant alleles cannot. For the allelic series in rabbits, the wild-type allele may supply a given dosage of fur
pigment, whereas the mutants supply a lesser dosage or none at all. Interestingly, the Himalayan phenotype
is the result of an allele that produces a temperature-sensitive gene product that only produces pigment in the
cooler extremities of the rabbit’s body.
Alternatively, one mutant allele can be dominant over all other phenotypes, including the wild type. This may
occur when the mutant allele somehow interferes with the genetic message so that even a heterozygote with
one wild-type allele copy expresses the mutant phenotype. One way in which the mutant allele can interfere is
by enhancing the function of the wild-type gene product or changing its distribution in the body. One example
of this is the Antennapedia mutation in Drosophila (Figure 12.9). In this case, the mutant allele expands the
distribution of the gene product, and as a result, the Antennapedia heterozygote develops legs on its head where
its antennae should be.
Figure 12.9 As seen in comparing the wild-type Drosophila (left) and the Antennapedia mutant (right), the
Antennapedia mutant has legs on its head in place of antennae.
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Chapter 12 | Mendel's Experiments and Heredity
341
V /
e olution CONNECTION
Multiple Alleles Confer Drug Resistance in the Malaria Parasite
Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including
Anopheles gambiae (Figure 12.10a), and is characterized by cyclic high fevers, chills, flu-like symptoms,
and severe anemia. Plasmodium falciparum and P. vivax are the most common causative agents of malaria,
and P. falciparum is the most deadly (Figure 12.10b). When promptly and correctly treated, P. falciparum
malaria has a mortality rate of 0.1 percent. However, in some parts of the world, the parasite has evolved
resistance to commonly used malaria treatments, so the most effective malarial treatments can vary by
geographic region.
(a) (b)
Figure 12.10 The (a) Anopheles gambiae, or African malaria mosquito, acts as a vector in the transmission to
humans of the malaria-causing parasite (b) Plasmodium falciparum, here visualized using false-color transmission
electron microscopy, (credit a: James D. Gathany; credit b: Ute Frevert; false color by Margaret Shear; scale-bar
data from Matt Russell)
In Southeast Asia, Africa, and South America, P. falciparum has developed resistance to the anti-malarial
drugs chloroquine, mefloquine, and sulfadoxine-pyrimethamine. P. falciparum, which is haploid during the
life stage in which it is infectious to humans, has evolved multiple drug-resistant mutant alleles of the dhps
gene. Varying degrees of sulfadoxine resistance are associated with each of these alleles. Being haploid, P.
falciparum needs only one drug-resistant allele to express this trait.
In Southeast Asia, different sulfadoxine-resistant alleles of the dhps gene are localized to different
geographic regions. This is a common evolutionary phenomenon that occurs because drug-resistant
mutants arise in a population and interbreed with other P. falciparum isolates in close proximity. Sulfadoxine-
resistant parasites cause considerable human hardship in regions where this drug is widely used as an
over-the-counter malaria remedy. As is common with pathogens that multiply to large numbers within an
infection cycle, P. falciparum evolves relatively rapidly (over a decade or so) in response to the selective
pressure of commonly used anti-malarial drugs. For this reason, scientists must constantly work to develop
new drugs or drug combinations to combat the worldwide malaria burden.
X-Linked Traits
In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex
chromosomes. The sex chromosomes are one pair of non-homologous chromosomes. Until now, we have only
considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous
pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have
an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome
so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes. When
a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked.
2. Sumiti Vinayak, et al., “Origin and Evolution of Sulfadoxine Resistant Plasmodium falciparum," Public Library of Science Pathogens 6, no.
3 (2010): el000830, doi:10.1371/journal.ppat,1000830.
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Chapter 12 | Mendel's Experiments and Heredity
Eye color in Drosophila was one of the first X-linked traits to be identified. Thomas Hunt Morgan mapped this trait
to the X chromosome in 1910. Like humans, Drosophila males have an XY chromosome pair, and females are
XX. In flies, the wild-type eye color is red (X w ) and it is dominant to white eye color (X w ) (Figure 12.11). Because
of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are
said to be hemizygous, because they have only one allele for any X-linked characteristic. Hemizygosity makes
the descriptions of dominance and recessiveness irrelevant for XY males. Drosophila males lack a second allele
copy on the Y chromosome; that is, their genotype can only be X W Y or X^Y. In contrast, females have two allele
copies of this gene and can be X W X W , X W X W , or X W X W .
Figure 12.11 In Drosophila, several genes determine eye color. The genes for white and vermilion eye colors are
located on the X chromosome. Others are located on the autosomes. Clockwise from top left are brown, cinnabar,
sepia, vermilion, white, and red. Red eye color is wild-type and is dominant to white eye color.
in an X-linked cross, the genotypes of Fi and F 2 offspring depend on whether the recessive trait was expressed
by the male or the female in the Pi generation. With regard to Drosophila eye color, when the Pi male expresses
the white-eye phenotype and the female is homozygous red-eyed, all members of the Fi generation exhibit red
eyes (Figure 12.12). The Fi females are heterozygous (X^X 1 ^), and the males are all X^Y, having received
their X chromosome from the homozygous dominant Pi female and their Y chromosome from the Pi male. A
subsequent cross between the X W X W female and the X^Y male would produce only red-eyed females (with
X W X W or X W X W genotypes) and both red- and white-eyed males (with X^Y or X W Y genotypes). Now, consider
a cross between a homozygous white-eyed female and a male with red eyes. The Fi generation would exhibit
only heterozygous red-eyed females (X^X 1 ^) and only white-eyed males (X W Y). Half of the F 2 females would be
red-eyed (X W X W ) and half would be white-eyed (X W X W ). Similarly, half of the F 2 males would be red-eyed (X W Y)
and half would be white-eyed (X W Y).
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Chapter 12 | Mendel's Experiments and Heredity
343
visual
CONNECTION
Punnett Square Analysis of a Sex-linked Trait
All female offspring
have red eyes.
All male offspring
have white eyes.
Figure 12.12 Punnett square analysis is used to determine the ratio of offspring from a cross between a red-eyed
male fruit fly and a white-eyed female fruit fly.
What ratio of offspring would result from a cross between a white-eyed male and a female that is
heterozygous for red eye color?
Discoveries in fruit fly genetics can be applied to human genetics. When a female parent is homozygous for
a recessive X-linked trait, she will pass the trait on to 100 percent of her offspring. Her male offspring are,
therefore, destined to express the trait, as they will inherit their father's Y chromosome. In humans, the alleles for
certain conditions (some forms of color blindness, hemophilia, and muscular dystrophy) are X-linked. Females
who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects.
These females will pass the disease to half of their sons and will pass carrier status to half of their daughters;
therefore, recessive X-linked traits appear more frequently in males than females.
In some groups of organisms with sex chromosomes, the sex with the non-homologous sex chromosomes is
the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to
appear in the female, in which they are hemizygous.
Human Sex-linked Disorders
Sex-linkage studies in Morgan’s laboratory provided the fundamentals for understanding X-linked recessive
disorders in humans, which include red-green color blindness, and Types A and B hemophilia. Because human
males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately
observed in males. Females must inherit recessive X-linked alleles from both of their parents in order to express
the trait. When they inherit one recessive X-linked mutant allele and one dominant X-linked wild-type allele, they
are carriers of the trait and are typically unaffected. Carrier females can manifest mild forms of the trait due
to the inactivation of the dominant allele located on one of the X chromosomes. However, female carriers can
contribute the trait to their sons, resulting in the son exhibiting the trait, or they can contribute the recessive allele
to their daughters, resulting in the daughters being carriers of the trait (Figure 12.13). Although some Y-linked
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Chapter 12 | Mendel's Experiments and Heredity
recessive disorders exist, typically they are associated with infertility in males and are therefore not transmitted
to subsequent generations.
Figure 12.13 The son of a woman who is a carrier of a recessive X-linked disorder will have a 50 percent chance of
being affected. A daughter will not be affected, but she will have a 50 percent chance of being a carrier like her mother.
LINK TQ LEARNING
Watch this video to learn more about sex-linked traits. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66487/l.4/#eip-idll70504131681)
Lethality
A large proportion of genes in an individual’s genome are essential for survival. Occasionally, a nonfunctional
allele for an essential gene can arise by mutation and be transmitted in a population as long as individuals
with this allele also have a wild-type, functional copy. The wild-type allele functions at a capacity sufficient to
sustain life and is therefore considered to be dominant over the nonfunctional allele. However, consider two
heterozygous parents that have a genotype of wild-type/nonfunctional mutant for a hypothetical essential gene.
In one quarter of their offspring, we would expect to observe individuals that are homozygous recessive for the
nonfunctional allele. Because the gene is essential, these individuals might fail to develop past fertilization, die
in utero, or die later in life, depending on what life stage requires this gene. An inheritance pattern in which an
allele is only lethal in the homozygous form and in which the heterozygote may be normal or have some altered
nonlethal phenotype is referred to as recessive lethal.
For crosses between heterozygous individuals with a recessive lethal allele that causes death before birth when
homozygous, only wild-type homozygotes and heterozygotes would be observed. The genotypic ratio would
therefore be 2:1. In other instances, the recessive lethal allele might also exhibit a dominant (but not lethal)
phenotype in the heterozygote. For instance, the recessive lethal Curly allele in Drosophila affects wing shape
in the heterozygote form but is lethal in the homozygote.
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Chapter 12 | Mendel's Experiments and Heredity
345
A single copy of the wild-type allele is not always sufficient for normal functioning or even survival. The dominant
lethal inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote; this
allele can only be transmitted if the lethality phenotype occurs after reproductive age. Individuals with mutations
that result in dominant lethal alleles fail to survive even in the heterozygote form. Dominant lethal alleles are
very rare because, as you might expect, the allele only lasts one generation and is not transmitted. However,
just as the recessive lethal allele might not immediately manifest the phenotype of death, dominant lethal alleles
also might not be expressed until adulthood. Once the individual reaches reproductive age, the allele may be
unknowingly passed on, resulting in a delayed death in both generations. An example of this in humans is
Huntington’s disease, in which the nervous system gradually wastes away (Figure 12.14). People who are
heterozygous for the dominant Huntington allele ( Hh ) will inevitably develop the fatal disease. However, the
onset of Huntington’s disease may not occur until age 40, at which point the afflicted persons may have already
passed the allele to 50 percent of their offspring.
Figure 12.14 The neuron in the center of this micrograph (yellow) has nuclear inclusions characteristic of Huntington’s
disease (orange area in the center of the neuron). Huntington’s disease occurs when an abnormal dominant allele for
the Huntington gene is present, (credit: Dr. Steven Finkbeiner, Gladstone Institute of Neurological Disease, The Taube-
Koret Center for Huntington's Disease Research, and the University of California San Francisco/Wikimedia)
12.3 | Laws of Inheritance
By the end of this section, you will be able to do the following:
• Explain Mendel’s law of segregation and independent assortment in terms of genetics and the events of
meiosis
• Use the forked-line method and the probability rules to calculate the probability of genotypes and
phenotypes from multiple gene crosses
• Explain the effect of linkage and recombination on gamete genotypes
• Explain the phenotypic outcomes of epistatic effects between genes
Mendel generalized the results of his pea-plant experiments into four postulates, some of which are sometimes
called “laws,” that describe the basis of dominant and recessive inheritance in diploid organisms. As you have
learned, more complex extensions of Mendelism exist that do not exhibit the same F 2 phenotypic ratios (3:1).
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Chapter 12 | Mendel's Experiments and Heredity
Nevertheless, these laws summarize the basics of classical genetics.
Pairs of Unit Factors, or Genes
Mendel proposed first that paired unit factors of heredity were transmitted faithfully from generation to generation
by the dissociation and reassociation of paired factors during gametogenesis and fertilization, respectively. After
he crossed peas with contrasting traits and found that the recessive trait resurfaced in the F 2 generation, Mendel
deduced that hereditary factors must be inherited as discrete units. This finding contradicted the belief at that
time that parental traits were blended in the offspring.
Alleles Can Be Dominant or Recessive
Mendel’s law of dominance states that in a heterozygote, one trait will conceal the presence of another trait
for the same characteristic. Rather than both alleles contributing to a phenotype, the dominant allele will be
expressed exclusively. The recessive allele will remain “latent” but will be transmitted to offspring by the same
manner in which the dominant allele is transmitted. The recessive trait will only be expressed by offspring that
have two copies of this allele (Figure 12.15), and these offspring will breed true when self-crossed.
Since Mendel’s experiments with pea plants, researchers have found that the law of dominance does not always
hold true. Instead, several different patterns of inheritance have been found to exist.
Figure 12.15 The child in the photo expresses albinism, a recessive trait.
Equal Segregation of Alleles
Observing that true-breeding pea plants with contrasting traits gave rise to Fi generations that all expressed
the dominant trait and F 2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel
proposed the law of segregation. This law states that paired unit factors (genes) must segregate equally
into gametes such that offspring have an equal likelihood of inheriting either factor. For the F 2 generation
of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous
dominant, heterozygous, or homozygous recessive. Because heterozygotes could arise from two different
pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes
and homozygous dominant individuals are phenotypically identical, the law supports Mendel’s observed 3:1
phenotypic ratio. The equal segregation of alleles is the reason we can apply the Punnett square to accurately
predict the offspring of parents with known genotypes. The physical basis of Mendel’s law of segregation is the
first division of meiosis, in which the homologous chromosomes with their different versions of each gene are
segregated into daughter nuclei. The role of the meiotic segregation of chromosomes in sexual reproduction was
not understood by the scientific community during Mendel’s lifetime.
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Chapter 12 | Mendel's Experiments and Heredity
347
Independent Assortment
Mendel’s law of independent assortment states that genes do not influence each other with regard to the
sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to
occur. The independent assortment of genes can be illustrated by the dihybrid cross, a cross between two true-
breeding parents that express different traits for two characteristics. Consider the characteristics of seed color
and seed texture for two pea plants, one that has green, wrinkled seeds (yyrr) and another that has yellow,
round seeds (YYRR). Because each parent is homozygous, the law of segregation indicates that the gametes
for the green/wrinkled plant all are yr, and the gametes for the yellow/round plant are all YR. Therefore, the Fi
generation of offspring all are YyRr (Figure 12.16).
visual
CONNECTION
YR
yR
o
>s
rsl
O
B
aj
E Yr
o
E yr
Dihybrid Cross
YYRR
YyRr
I
P Generation
Fj Generation
Phenotype:
gametes from heterozygous parent
YR yR Yr yr
YYRR
YyRR
YYRr
YyRr
YyRR
YyRr
YYRr
YyRr
YYrr
Yyrr
YyRr
Yyrr
F 2 Generation
Phenotype:
9 : 3 :|
Figure 12.16 This dihybrid cross of pea plants involves the genes for seed color and texture.
In pea plants, purple flowers (P) are dominant to white flowers (p) and yellow peas (Y) are dominant to
green peas (y). What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea
plants? How many squares do you need to do a Punnett square analysis of this cross?
For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r
allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into
which an r allele sorted would be equally likely to contain either a Y allele or a y allele. Thus, there are four
equally likely gametes that can be formed when the YyRr heterozygote is self-crossed, as follows: YR, Yr, yR,
and yr. Arranging these gametes along the top and left of a 4 x 4 Punnett square (Figure 12.16) gives us 16
equally likely genotypic combinations. From these genotypes, we infer a phenotypic ratio of 9 round/yellow:3
round/green:3 wrinkled/yellow:l wrinkled/green (Figure 12.16). These are the offspring ratios we would expect,
assuming we performed the crosses with a large enough sample size.
Because of independent assortment and dominance, the 9:3:3:1 dihybrid phenotypic ratio can be collapsed into
two 3:1 ratios, characteristic of any monohybrid cross that follows a dominant and recessive pattern. Ignoring
seed color and considering only seed texture in the above dihybrid cross, we would expect that three quarters
of the F 2 generation offspring would be round, and one quarter would be wrinkled. Similarly, isolating only seed
color, we would assume that three quarters of the F 2 offspring would be yellow and one quarter would be green.
The sorting of alleles for texture and color are independent events, so we can apply the product rule. Therefore,
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Chapter 12 | Mendel's Experiments and Heredity
the proportion of round and yellow F 2 offspring is expected to be (3/4) x ( 3 / 4 ) = 9 / 16 , and the proportion of
wrinkled and green offspring is expected to be (1/4) x ( 1 / 4 ) = 1 / 16 . These proportions are identical to those
obtained using a Punnett square. Round, green and wrinkled, yellow offspring can also be calculated using the
product rule, as each of these genotypes includes one dominant and one recessive phenotype. Therefore, the
proportion of each is calculated as (3/4) x ( 1 / 4 ) = 3 / 16 .
The law of independent assortment also indicates that a cross between yellow, wrinkled ( YYrr ) and green, round
iyyRR) parents would yield the same Fi and F 2 offspring as in the YYRR xyyrr cross.
The physical basis for the law of independent assortment also lies in meiosis I, in which the different homologous
pairs line up in random orientations. Each gamete can contain any combination of paternal and maternal
chromosomes (and therefore the genes on them) because the orientation of tetrads on the metaphase plane is
random.
Forked-Line Method
When more than two genes are being considered, the Punnett-square method becomes unwieldy. For instance,
examining a cross involving four genes would require a 16 x 16 grid containing 256 boxes. It would be extremely
cumbersome to manually enter each genotype. For more complex crosses, the forked-line and probability
methods are preferred.
To prepare a forked-line diagram for a cross between Fi heterozygotes resulting from a cross between AABBCC
and aabbcc parents, we first create rows equal to the number of genes being considered, and then segregate
the alleles in each row on forked lines according to the probabilities for individual monohybrid crosses (Figure
12.17). We then multiply the values along each forked path to obtain the F 2 offspring probabilities. Note that this
process is a diagrammatic version of the product rule. The values along each forked pathway can be multiplied
because each gene assorts independently. For a trihybrid cross, the F 2 phenotypic ratio is 27:9:9:9:3:3:3:1.
Analyzing a Trihybrid Cross
3 yellow
1
1 green
1
3 round
1
1
1 wrinkled
1
1
3 round
1
1
1 wrinkled
1
1
3 tall
1
1 dwarf
1
3 tall
1
1 dwarf
1
3 tall
1
1 dwarf
3 tall
1
1 dwarf
O
O
O
O
O
O
O
O
3 x 3 x 3
= 3x3x1
= 3x1x3=
3xlxl=
1x3x3=
1x3x1
= 1x1x3=
lxlxl=
27 yellow
9 yellow
9 yellow
3 yellow
9 green
3 green
3 green
1 green
round
round
wrinkled
wrinkled
round
round
wrinkled
wrinkled
tall
dwarf
tall
dwarf
tall
dwarf
tall
dwarf
Figure 12.17 The forked-line method can be used to analyze a trihybrid cross. Here, the probability for color in the
F 2 generation occupies the top row (3 yellow:l green). The probability for shape occupies the second row (3 round:
1 wrinkled), and the probability for height occupies the third row (3 tall:l dwarf). The probability for each possible
combination of traits is calculated by multiplying the probability for each individual trait. Thus, the probability of F 2
offspring having yellow, round, and tall traits is 3 x 3 x 3 , or 27.
Probability Method
While the forked-line method is a diagrammatic approach to keeping track of probabilities in a cross, the
probability method gives the proportions of offspring expected to exhibit each phenotype (or genotype) without
the added visual assistance. Both methods make use of the product rule and consider the alleles for each gene
separately. Earlier, we examined the phenotypic proportions for a trihybrid cross using the forked-line method;
now we will use the probability method to examine the genotypic proportions for a cross with even more genes.
For a trihybrid cross, writing out the forked-line method is tedious, albeit not as tedious as using the Punnett-
square method. To fully demonstrate the power of the probability method, however, we can consider specific
genetic calculations. For instance, for a tetrahybrid cross between individuals that are heterozygotes for all four
genes, and in which all four genes are sorting independently and in a dominant and recessive pattern, what
proportion of the offspring will be expected to be homozygous recessive for all four alleles? Rather than writing
out every possible genotype, we can use the probability method. We know that for each gene, the fraction of
homozygous recessive offspring will be 1/4. Therefore, multiplying this fraction for each of the four genes, (1/4)
x (1/4) x ( 1 / 4 ) x (1/4), we determine that 1/256 of the offspring will be quadruply homozygous recessive.
For the same tetrahybrid cross, what is the expected proportion of offspring that have the dominant phenotype
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Chapter 12 | Mendel's Experiments and Heredity
349
at all four loci? We can answer this question using phenotypic proportions, but let’s do it the hard way—using
genotypic proportions. The question asks for the proportion of offspring that are 1) homozygous dominant at A
or heterozygous at A, and 2) homozygous at B or heterozygous at B, and so on. Noting the “or” and “and” in
each circumstance makes clear where to apply the sum and product rules. The probability of a homozygous
dominant at A is 1/4 and the probability of a heterozygote at A is 1/2. The probability of the homozygote or the
heterozygote is 1/4 + 1/2 = 3/4 using the sum rule. The same probability can be obtained in the same way for
each of the other genes, so that the probability of a dominant phenotype at/4 and B and C and D is, using the
product rule, equal to 3/4 x 3/4 x 3/4 x 3/4, or 27/64. If you are ever unsure about how to combine probabilities,
returning to the forked-line method should make it clear.
Rules for Multihybrid Fertilization
Predicting the genotypes and phenotypes of offspring from given crosses is the best way to test your knowledge
of Mendelian genetics. Given a multihybrid cross that obeys independent assortment and follows a dominant
and recessive pattern, several generalized rules exist; you can use these rules to check your results as you
work through genetics calculations (Table 12.5). To apply these rules, first you must determine n, the number
of heterozygous gene pairs (the number of genes segregating two alleles each). For example, a cross between
AaBb and AaBb heterozygotes has an n of 2. In contrast, a cross between AABb and AABb has an n of 1
because A is not heterozygous.
General Rules for Multihybrid Crosses
General Rule
Number of Heterozygous Gene
Pairs
Number of different Fi gametes
2 n
Number of different F 2 genotypes
3 n
Given dominant and recessive inheritance, the number of different F 2
phenotypes
2 n
Table 12.5
Linked Genes Violate the Law of Independent Assortment
Although all of Mendel’s pea characteristics behaved according to the law of independent assortment, we now
know that some allele combinations are not inherited independently of each other. Genes that are located on
separate non-homologous chromosomes will always sort independently. However, each chromosome contains
hundreds or thousands of genes, organized linearly on chromosomes like beads on a string. The segregation
of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each
other on the same chromosome are more likely to be inherited as a pair. However, because of the process of
recombination, or “crossover,” it is possible for two genes on the same chromosome to behave independently, or
as if they are not linked. To understand this, let’s consider the biological basis of gene linkage and recombination.
Homologous chromosomes possess the same genes in the same linear order. The alleles may differ on
homologous chromosome pairs, but the genes to which they correspond do not. In preparation for the first
division of meiosis, homologous chromosomes replicate and synapse. Like genes on the homologs align with
each other. At this stage, segments of homologous chromosomes exchange linear segments of genetic material
(Figure 12.18). This process is called recombination, or crossover, and it is a common genetic process. Because
the genes are aligned during recombination, the gene order is not altered, instead, the result of recombination
is that maternal and paternal alleles are combined onto the same chromosome. Across a given chromosome,
several recombination events may occur, causing extensive shuffling of alleles.
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Chapter 12 | Mendel's Experiments and Heredity
Crossover
Homologous Chromosome
chromosomes crossover
aligned
o
Recombinant
chromosomes
a
b
c
Non-recombinant
chromosomes
Figure 12.18 The process of crossover, or recombination, occurs when two homologous chromosomes align during
meiosis and exchange a segment of genetic material. Here, the alleles for gene C were exchanged. The result is two
recombinant and two non-recombinant chromosomes.
When two genes are located in close proximity on the same chromosome, they are considered linked, and their
alleles tend to be transmitted through meiosis together. To exemplify this, imagine a dihybrid cross involving
flower color and plant height in which the genes are next to each other on the chromosome. If one homologous
chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants
and yellow flowers, then when the gametes are formed, the tall and red alleles will go together into a gamete and
the short and yellow alleles will go into other gametes. These are called the parental genotypes because they
have been inherited intact from the parents of the individual producing gametes. But unlike if the genes were on
different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red
alleles. If you create the Punnett square with these gametes, you will see that the classical Mendelian prediction
of a 9:3:3:1 outcome of a dihybrid cross would not apply. As the distance between two genes increases, the
probability of one or more crossovers between them increases, and the genes behave more like they are
on separate chromosomes. Geneticists have used the proportion of recombinant gametes (the ones not like
the parents) as a measure of how far apart genes are on a chromosome. Using this information, they have
constructed elaborate maps of genes on chromosomes for well-studied organisms, including humans.
Mendel’s seminal publication makes no mention of linkage, and many researchers have questioned whether
he encountered linkage but chose not to publish those crosses out of concern that they would invalidate his
independent assortment postulate. The garden pea has seven chromosomes, and some have suggested that
his choice of seven characteristics was not a coincidence. However, even if the genes he examined were not
located on separate chromosomes, it is possible that he simply did not observe linkage because of the extensive
shuffling effects of recombination.
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scientific methQd CONNECTION
Testing the Hypothesis of Independent Assortment
To better appreciate the amount of labor and ingenuity that went into Mendel’s experiments, proceed
through one of Mendel’s dihybrid crosses.
Question: What will be the offspring of a dihybrid cross?
Background: Consider that pea plants mature in one growing season, and you have access to a large
garden in which you can cultivate thousands of pea plants. There are several true-breeding plants with the
following pairs of traits: tall plants with inflated pods, and dwarf plants with constricted pods. Before the
plants have matured, you remove the pollen-producing organs from the tall/inflated plants in your crosses to
prevent self-fertilization. Upon plant maturation, the plants are manually crossed by transferring pollen from
the dwarf/constricted plants to the stigmata of the tall/inflated plants.
Hypothesis: Both trait pairs will sort independently according to Mendelian laws. When the true-breeding
parents are crossed, all of the Fi offspring are tall and have inflated pods, which indicates that the tall
and inflated traits are dominant over the dwarf and constricted traits, respectively. A self-cross of the Fi
heterozygotes results in 2,000 F 2 progeny.
Test the hypothesis: Because each trait pair sorts independently, the ratios of talkdwarf and
inflatedxonstricted are each expected to be 3:1. The tall/dwarf trait pair is called T/t, and the inflated/
constricted trait pair is designated l/i. Each member of the Fi generation therefore has a genotype of Ttli.
Construct a grid analogous to Figure 12.16, in which you cross two Ttli individuals. Each individual can
donate four combinations of two traits: Tl, Ti, tl, or ti, meaning that there are 16 possibilities of offspring
genotypes. Because the T and / alleles are dominant, any individual having one or two of those alleles
will express the tall or inflated phenotypes, respectively, regardless if they also have a f or / allele. Only
individuals that are tt or /'/' will express the dwarf and constricted alleles, respectively. As shown in Figure
12.19, you predict that you will observe the following offspring proportions: tall/inflated:tall/constricted:dwarf/
inflated:dwarf/constricted in a 9:3:3:1 ratio. Notice from the grid that when considering the tall/dwarf and
inflated/constricted trait pairs in isolation, they are each inherited in 3:1 ratios.
Ttli
Tl
Ti
tl
ti
Tl
TTII
TTIi
Ttli
Ttli
Ti
TTIi
TTii
Ttli
Ttii
tl
Ttli
Ttli
ttli
ttli
ti
Ttli
Ttii
ttli
ttii
Figure 12.19 This figure shows all possible combinations of offspring resulting from a dihybrid cross of pea plants
that are heterozygous for the tall/dwarf and inflated/constricted alleles.
Test the hypothesis: You cross the dwarf and tall plants and then self-cross the offspring. For best results,
this is repeated with hundreds or even thousands of pea plants. What special precautions should be taken
in the crosses and in growing the plants?
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Chapter 12 | Mendel's Experiments and Heredity
Analyze your data: You observe the following plant phenotypes in the F 2 generation: 2706 tall/inflated,
930 tall/constricted, 888 dwarf/inflated, and 300 dwarf/constricted. Reduce these findings to a ratio and
determine if they are consistent with Mendelian laws.
Form a conclusion: Were the results close to the expected 9:3:3:1 phenotypic ratio? Do the results support
the prediction? What might be observed if far fewer plants were used, given that alleles segregate randomly
into gametes? Try to imagine growing that many pea plants, and consider the potential for experimental
error. For instance, what would happen if it was extremely windy one day?
Epistasis
Mendel’s studies in pea plants implied that the sum of an individual’s phenotype was controlled by genes (or as
he called them, unit factors), such that every characteristic was distinctly and completely controlled by a single
gene. In fact, single observable characteristics are almost always under the influence of multiple genes (each
with two or more alleles) acting in unison. For example, at least eight genes contribute to eye color in humans.
Eye color in humans is determined by multiple genes. Use the Eye Color Calculator
(http:// 0 penstaxc 0 llege. 0 rg/l/eye_c 0 l 0 r_calc) to predict the eye color of children from parental eye color.
In some cases, several genes can contribute to aspects of a common phenotype without their gene products
ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially,
with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or
synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype.
Genes may also oppose each other, with one gene modifying the expression of another.
In epistasis, the interaction between genes is antagonistic, such that one gene masks or interferes with the
expression of another. “Epistasis” is a word composed of Greek roots that mean “standing upon.” The alleles
that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking.
Often the biochemical basis of epistasis is a gene pathway in which the expression of one gene is dependent on
the function of a gene that precedes or follows it in the pathway.
An example of epistasis is pigmentation in mice. The wild-type coat color, agouti (AA), is dominant to solid-
colored fur (aa). However, a separate gene (C) is necessary for pigment production. A mouse with a recessive c
allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus A (Figure
12.20). Therefore, the genotypes AAcc, Aacc, and aacc all produce the same albino phenotype. A cross between
heterozygotes for both genes (AaCc xAaCc) would generate offspring with a phenotypic ratio of 9 agouti:3 solid
color:4 albino (Figure 12.20). In this case, the C gene is epistatic to the A gene.
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Chapter 12 | Mendel's Experiments and Heredity
353
Epistasis
AaCc
AaCc
-
X
I
0
0
0
0
AACC
AaCC
AACc
AaCc
0
S'
S' :
-
AaCC
aaCC
AaCc
aaCc
0
-
AACc
AaCc
AAcc
Aacc
0
s ^
AaCc
aaCc
Aacc
aacc
0
* *
Genotypes
Agouti Black Albino
9/16 3/16 4/16
Phenotypic ratio
Figure 12.20 In mice, the mottled agouti coat color (A) is dominant to a solid coloration, such as black or gray. A gene
at a separate locus (C) is responsible for pigment production. The recessive c allele does not produce pigment, and a
mouse with the homozygous recessive cc genotype is albino regardless of the allele present at the A locus. Thus, the
C gene is epistatic to the A gene.
Epistasis can also occur when a dominant allele masks expression at a separate gene. Fruit color in summer
squash is expressed in this way. Homozygous recessive expression of the W gene ( ww ) coupled with
homozygous dominant or heterozygous expression of the Y gene (YY or Yy) generates yellow fruit, and the
wwyy genotype produces green fruit. However, if a dominant copy of the W gene is present in the homozygous
or heterozygous form, the summer squash will produce white fruit regardless of the Y alleles. A cross between
white heterozygotes for both genes (WwYy * WwYy) would produce offspring with a phenotypic ratio of 12
white:3 yellow:l green.
Finally, epistasis can be reciprocal such that either gene, when present in the dominant (or recessive) form,
expresses the same phenotype, in the shepherd’s purse plant (Capsella bursa-pastoris), the characteristic of
seed shape is controlled by two genes in a dominant epistatic relationship. When the genes A and 6 are both
homozygous recessive ( aabb ), the seeds are ovoid. If the dominant allele for either of these genes is present,
the result is triangular seeds. That is, every possible genotype other than aabb results in triangular seeds, and a
cross between heterozygotes for both genes (AaBb x AaBb) would yield offspring with a phenotypic ratio of 15
triangulanl ovoid.
As you work through genetics problems, keep in mind that any single characteristic that results in a phenotypic
ratio that totals 16 is typical of a two-gene interaction. Recall the phenotypic inheritance pattern for Mendel’s
dihybrid cross, which considered two noninteracting genes—9:3:3:1. Similarly, we would expect interacting gene
pairs to also exhibit ratios expressed as 16 parts. Note that we are assuming the interacting genes are not linked;
they are still assorting independently into gametes.
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Chapter 12 | Mendel's Experiments and Heredity
LINK TQ LEARNING
For an excellent review of Mendel’s experiments and to perform your own crosses and identify patterns of
inheritance, visit the Mendel’s Peas (http:// 0 penstaxc 0 llege. 0 rg/l/mendels_peas) web lab.
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Chapter 12 | Mendel's Experiments and Heredity
355
KEY TERMS
allele gene variations that arise by mutation and exist at the same relative locations on homologous
chromosomes
autosomes any of the non-sex chromosomes
blending theory of inheritance hypothetical inheritance pattern in which parental traits are blended together in
the offspring to produce an intermediate physical appearance
codominance in a heterozygote, complete and simultaneous expression of both alleles for the same
characteristic
continuous variation inheritance pattern in which a character shows a range of trait values with small
gradations rather than large gaps between them
dihybrid result of a cross between two true-breeding parents that express different traits for two characteristics
discontinuous variation inheritance pattern in which traits are distinct and are transmitted independently of
one another
dominant trait which confers the same physical appearance whether an individual has two copies of the trait or
one copy of the dominant trait and one copy of the recessive trait
dominant lethal inheritance pattern in which an allele is lethal both in the homozygote and the heterozygote;
this allele can only be transmitted if the lethality phenotype occurs after reproductive age
epistasis antagonistic interaction between genes such that one gene masks or interferes with the expression of
another
Fi first filial generation in a cross; the offspring of the parental generation
F 2 second filial generation produced when Fi individuals are self-crossed or fertilized with each other
genotype underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an
organism
hemizygous presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions
of dominance and recessiveness irrelevant
heterozygous having two different alleles for a given gene on the homologous chromosome
homozygous having two identical alleles for a given gene on the homologous chromosome
hybridization process of mating two individuals that differ with the goal of achieving a certain characteristic in
their offspring
incomplete dominance in a heterozygote, expression of two contrasting alleles such that the individual
displays an intermediate phenotype
law of dominance in a heterozygote, one trait will conceal the presence of another trait for the same
characteristic
law of independent assortment genes do not influence each other with regard to sorting of alleles into
gametes; every possible combination of alleles is equally likely to occur
law of segregation paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an
equal likelihood of inheriting any combination of factors
linkage phenomenon in which alleles that are located in close proximity to each other on the same chromosome
are more likely to be inherited together
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Chapter 12 | Mendel's Experiments and Heredity
model system species or biological system used to study a specific biological phenomenon to be applied to
other different species
monohybrid result of a cross between two true-breeding parents that express different traits for only one
characteristic
Po parental generation in a cross
phenotype observable traits expressed by an organism
product rule probability of two independent events occurring simultaneously can be calculated by multiplying
the individual probabilities of each event occurring alone
Punnett square visual representation of a cross between two individuals in which the gametes of each
individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are
recombined at each box in the grid
recessive trait that appears “latent” or non-expressed when the individual also carries a dominant trait for that
same characteristic; when present as two identical copies, the recessive trait is expressed
recessive lethal inheritance pattern in which an allele is only lethal in the homozygous form; the heterozygote
may be normal or have some altered, nonlethal phenotype
reciprocal cross paired cross in which the respective traits of the male and female in one cross become the
respective traits of the female and male in the other cross
sex-linked any gene on a sex chromosome
sum rule probability of the occurrence of at least one of two mutually exclusive events is the sum of their
individual probabilities
test cross cross between a dominant expressing individual with an unknown genotype and a homozygous
recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or
homozygous for the dominant trait
trait variation in the physical appearance of a heritable characteristic
X-linked gene present on the X, but not the Y chromosome
CHAPTER SUMMARY
12.1 Mendel’s Experiments and the Laws of Probability
Working with garden pea plants, Mendel found that crosses between parents that differed by one trait produced
Fi offspring that all expressed the traits of one parent. Observable traits are referred to as dominant, and non-
expressed traits are described as recessive. When the offspring in Mendel’s experiment were self-crossed, the
F 2 offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait
had been transmitted faithfully from the original Po parent. Reciprocal crosses generated identical Fi and F 2
offspring ratios. By examining sample sizes, Mendel showed that his crosses behaved reproducibly according
to the laws of probability, and that the traits were inherited as independent events.
Two rules in probability can be used to find the expected proportions of offspring of different traits from different
crosses. To find the probability of two or more independent events occurring together, apply the product rule
and multiply the probabilities of the individual events. The use of the word “and” suggests the appropriate
application of the product rule. To find the probability of two or more events occurring in combination, apply the
sum rule and add their individual probabilities together. The use of the word “or” suggests the appropriate
application of the sum rule.
12.2 Characteristics and Traits
When true-breeding or homozygous individuals that differ for a certain trait are crossed, all of the offspring will
be heterozygotes for that trait. If the traits are inherited as dominant and recessive, the Fi offspring will all
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Chapter 12 | Mendel's Experiments and Heredity
357
exhibit the same phenotype as the parent homozygous for the dominant trait. If these heterozygous offspring
are self-crossed, the resulting F 2 offspring will be equally likely to inherit gametes carrying the dominant or
recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous,
and one quarter are homozygous recessive. Because homozygous dominant and heterozygous individuals are
phenotypically identical, the observed traits in the F 2 offspring will exhibit a ratio of three dominant to one
recessive.
Alleles do not always behave in dominant and recessive patterns. Incomplete dominance describes situations
in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes.
Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although
diploid organisms can only have two alleles for any given gene, it is common for more than two alleles of a
gene to exist in a population. In humans, as in many animals and some plants, females have two X
chromosomes and males have one X and one Y chromosome. Genes that are present on the X but not the Y
chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit
two. Finally, some alleles can be lethal. Recessive lethal alleles are only lethal in homozygotes, but dominant
lethal alleles are fatal in heterozygotes as well.
12.3 Laws of Inheritance
Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant
and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either
one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes
independently of one another. That is, alleles are generally not more likely to segregate into a gamete with a
particular allele of another gene. A dihybrid cross demonstrates independent assortment when the genes in
question are on different chromosomes or distant from each other on the same chromosome. For crosses
involving more than two genes, use the forked line or probability methods to predict offspring genotypes and
phenotypes rather than a Punnett square.
Although chromosomes sort independently into gametes during meiosis, Mendel’s law of independent
assortment refers to genes, not chromosomes, and a single chromosome may carry more than 1,000 genes.
When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together.
This results in offspring ratios that violate Mendel's law of independent assortment. However, recombination
serves to exchange genetic material on homologous chromosomes such that maternal and paternal alleles
may be recombined on the same chromosome. This is why alleles on a given chromosome are not always
inherited together. Recombination is a random event occurring anywhere on a chromosome. Therefore, genes
that are far apart on the same chromosome are likely to still assort independently because of recombination
events that occurred in the intervening chromosomal space.
Whether or not they are sorting independently, genes may interact at the level of gene products such that the
expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is
called epistasis.
VISUAL CONNECTION QUESTIONS
1. Figure 12.5 In pea plants, round peas (R) are
dominant to wrinkled peas (r). You do a test cross
between a pea plant with wrinkled peas (genotype rr)
and a plant of unknown genotype that has round
peas. You end up with three plants, all which have
round peas. From this data, can you tell if the round
pea parent plant is homozygous dominant or
heterozygous? If the round pea parent plant is
heterozygous, what is the probability that a random
sample of 3 progeny peas will all be round?
2. Figure 12.6 What are the genotypes of the
individuals labeled 1, 2, and 3?
REVIEW QUESTIONS
5. Mendel performed hybridizations by transferring
3. Figure 12.12 What ratio of offspring would result
from a cross between a white-eyed male and a
female that is heterozygous for red eye color?
4. Figure 12.16 In pea plants, purple flowers (P) are
dominant to white flowers (p) and yellow peas (Y) are
dominant to green peas (y). What are the possible
genotypes and phenotypes for a cross between
PpYY and ppYy pea plants? How many squares do
you need to do a Punnett square analysis of this
cross?
pollen from the_of the male plant to the
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Chapter 12 | Mendel's Experiments and Heredity
female ova.
a. anther
b. pistil
c. stigma
d. seed
6. Which is one of the seven characteristics that
Mendel observed in pea plants?
a. flower size
b. seed texture
c. leaf shape
d. stem color
7. Imagine you are performing a cross involving seed
color in garden pea plants. What Fi offspring would
you expect if you cross true-breeding parents with
green seeds and yellow seeds? Yellow seed color is
dominant over green.
a. 100 percent yellow-green seeds
b. 100 percent yellow seeds
c. 50 percent yellow, 50 percent green seeds
d. 25 percent green, 75 percent yellow seeds
8. Consider a cross to investigate the pea pod texture
trait, involving constricted or inflated pods. Mendel
found that the traits behave according to a dominant/
recessive pattern in which inflated pods were
dominant. If you performed this cross and obtained
650 inflated-pod plants in the F 2 generation,
approximately how many constricted-pod plants
would you expect to have?
a. 600
b. 165
c. 217
d. 468
9. A scientist pollinates a true-breeding pea plant with
violet, terminal flowers with pollen from a true-
breeding pea plant with white, axial flowers. Which of
the following observations would most accurately
describe the F 2 generation?
a. 75% violet flowers; 75% terminal flowers
b. 75% white flowers in a terminal position
c. 75% violet flowers; 75% axial flowers
d. 75% violet flowers in an axial position
10. The observable traits expressed by an organism
are described as its_.
a. phenotype
b. genotype
c. alleles
d. zygote
11. A recessive trait will be observed in individuals
that are_for that trait.
a. heterozygous
b. homozygous or heterozygous
c. homozygous
d. diploid
12. If black and white true-breeding mice are mated
and the result is all gray offspring, what inheritance
pattern would this be indicative of?
a. dominance
b. codominance
c. multiple alleles
d. incomplete dominance
13. The ABO blood groups in humans are expressed
as the l A , l B , and / alleles. The l A allele encodes the
A blood group antigen, l B encodes B, and / encodes
O. Both A and B are dominant to O. If a
heterozygous blood type A parent (l A i) and a
heterozygous blood type B parent (l B i) mate, one
quarter of their offspring will have AB blood type
(l A l B ) in which both antigens are expressed equally.
Therefore, ABO blood groups are an example of:
a. multiple alleles and incomplete dominance
b. codominance and incomplete dominance
c. incomplete dominance only
d. multiple alleles and codominance
14. In a mating between two individuals that are
heterozygous for a recessive lethal allele that is
expressed in utero, what genotypic ratio
(homozygous dominant:heterozygous:homozygous
recessive) would you expect to observe in the
offspring?
a. 1:2:1
b. 3:1:1
c. 1:2:0
d. 0:2:1
15. If the allele encoding polydactyly (six fingers) is
dominant why do most people have five fingers?
a. Genetic elements suppress the polydactyl
gene.
b. Polydactyly is embryonic lethal.
c. The sixth finger is removed at birth.
d. The polydactyl allele is very rare in the
human population.
16. A farmer raises black and white chickens. To his
surprise, when the first generation of eggs hatch all
the chickens are black with white speckles
throughout their feathers. What should the farmer
expect when the eggs laid after interbreeding the
speckled chickens hatch?
a. All the offspring will be speckled.
b. 75% of the offspring will be speckled, and
25% will be black.
c. 50% of the offspring will be speckled, 25%
will be black, and 25% will be white.
d. 50% of the offspring will be black and 50%
of the offspring will be white.
17. Assuming no gene linkage, in a dihybrid cross of
AABB x aabb with AaBb Fi heterozygotes, what is
the ratio of the Fi gametes (AB, aB, Ab, ab) that will
give rise to the F 2 offspring?
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Chapter 12 | Mendel's Experiments and Heredity
359
a.
1:1:1:1
b.
1:3:3:1
c.
1:2:2:1
d.
4:3:2:1
18. The forked line and probability methods make
use of what probability rule?
a. test cross
b. product rule
c. monohybrid rule
d. sum rule
19. How many different offspring genotypes are
expected in a trihybrid cross between parents
heterozygous for all three traits when the traits
behave in a dominant and recessive pattern? How
many phenotypes?
a. 64 genotypes; 16 phenotypes
b. 16 genotypes; 64 phenotypes
c. 8 genotypes; 27 phenotypes
d. 27 genotypes; 8 phenotypes
20. Labrador retriever’s fur color is controlled by two
CRITICAL THINKING QUESTIONS
22. Describe one of the reasons why the garden pea
was an excellent choice of model system for studying
inheritance.
23. How would you perform a reciprocal cross for the
characteristic of stem height in the garden pea?
24. Mendel performs a cross using a true-breeding
pea plant with round, yellow seeds and a true-
breeding pea plant with green, wrinkled seeds. What
is the probability that offspring will have green, round
seeds? Calculate the probability for the Fi and F 2
generations.
25. Calculate the probability of selecting a heart or a
face card from a standard deck of cards. Is this
outcome more or less likely than selecting a heart
suit face card?
26. The gene for flower position in pea plants exists
as axial or terminal alleles. Given that axial is
dominant to terminal, list all of the possible Fi and F 2
genotypes and phenotypes from a cross involving
parents that are homozygous for each trait. Express
genotypes with conventional genetic abbreviations.
27. Use a Punnett square to predict the offspring in a
cross between a dwarf pea plant (homozygous
recessive) and a tall pea plant (heterozygous). What
is the phenotypic ratio of the offspring?
28. Can a human male be a carrier of red-green color
blindness?
alleles, E and B. Any dog with the ee_genotype
develops into a yellow lab, while B_E_ dogs become
black labs and bbE_ dogs become chocolate labs.
This is an example of_.
a. epistasis
b. codominance
c. incomplete dominance
d. linkage
21. Which of the following situations does not follow
the Law of Independent Assortment?
a. A blond man and a brunette woman
produce three offspring over time, all of who
have blond hair.
b. A white cow crossed with a brown bull
produces roan cattle.
c. Mating a hog with a sow produces six
female piglets.
d. Men are more likely to experience
hemophilia than women.
29. Why is it more efficient to perform a test cross
with a homozygous recessive donor than a
homozygous dominant donor? How could the same
information still be found with a homozygous
dominant donor?
30. Use the probability method to calculate the
genotypes and genotypic proportions of a cross
between AABBCc and Aabbcc parents.
31. Explain epistatis in terms of its Greek-language
roots “standing upon.”
32. In Section 12.3, “Laws of Inheritance,” an
example of epistasis was given for the summer
squash. Cross white WwYy heterozygotes to prove
the phenotypic ratio of 12 white:3 yellow:l green that
was given in the text.
33. People with trisomy 21 develop Down’s
syndrome. What law of Mendelian inheritance is
violated in this disease? What is the most likely way
this occurs?
34. A heterozygous pea plant produces violet flowers
and yellow, round seeds. Describe the expected
genotypes of the gametes produced by Mendelian
inheritance. If all three genes are found on the same
arm of one chromosome should a scientist predict
that inheritance patterns will follow Mendelian
genetics?
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Chapter 13 | Modern Understandings of Inheritance
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13 | MODERN
UNDERSTANDINGS OF
INHERITANCE
Figure 13.1 Chromosomes are threadlike nuclear structures consisting of DNA and proteins that serve as the
repositories for genetic information. The chromosomes depicted here were isolated from a fruit fly’s salivary gland,
stained with dye, and visualized under a microscope. Akin to miniature bar codes, chromosomes absorb different dyes
to produce characteristic banding patterns, which allows for their routine identification, (credit: modification of work by
“LPLT’VWikimedia Commons; scale-bar data from Matt Russell)
Chapter Outline
13.1: Chromosomal Theory and Genetic Linkage
13.2: Chromosomal Basis of Inherited Disorders
Introduction
The gene is the physical unit of inheritance, and genes are arranged in a linear order on chromosomes.
Chromosome behavior and interaction during meiosis explain, at a cellular level, inheritance patterns that we
observe in populations. Genetic disorders involving alterations in chromosome number or structure may have
dramatic effects and can prevent a fertilized egg from developing.
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Chapter 13 | Modern Understandings of Inheritance
13.1 1 Chromosomal Theory and Genetic Linkage
By the end of this section, you will be able to do the following:
• Discuss Sutton’s Chromosomal Theory of Inheritance
• Describe genetic linkage
• Explain the process of homologous recombination, or crossing over
• Describe chromosome creation
• Calculate the distances between three genes on a chromosome using a three-point test cross
Long before scientists visualized chromosomes under a microscope, the father of modern genetics, Gregor
Mendel, began studying heredity in 1843. With improved microscopic techniques during the late 1800s, cell
biologists could stain and visualize subcellular structures with dyes and observe their actions during cell division
and meiosis. With each mitotic division, chromosomes replicated, condensed from an amorphous (no constant
shape) nuclear mass into distinct X-shaped bodies (pairs of identical sister chromatids), and migrated to
separate cellular poles.
Chromosomal Theory of Inheritance
The speculation that chromosomes might be the key to understanding heredity led several scientists to examine
Mendel’s publications and reevaluate his model in terms of chromosome behavior during mitosis and meiosis.
In 1902, Theodor Boveri observed that proper sea urchin embryonic development does not occur unless
chromosomes are present. That same year, Walter Sutton observed chromosome separation into daughter cells
during meiosis (Figure 13.2). Together, these observations led to the Chromosomal Theory of Inheritance,
which identified chromosomes as the genetic material responsible for Mendelian inheritance.
w m
Figure 13.2 (a) Walter Sutton and (b) Theodor Boveri developed the Chromosomal Theory of Inheritance, which states
that chromosomes carry the unit of heredity (genes).
The Chromosomal Theory of Inheritance was consistent with Mendel’s laws, which the following observations
supported:
• During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other
chromosome pairs.
• Chromosome sorting from each homologous pair into pre-gametes appears to be random.
• Each parent synthesizes gametes that contain only half their chromosomal complement.
• Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same
number of chromosomes, suggesting equal genetic contributions from each parent.
• The gametic chromosomes combine during fertilization to produce offspring with the same chromosome
number as their parents.
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Chapter 13 | Modern Understandings of Inheritance
363
Despite compelling correlations between chromosome behavior during meiosis and Mendel’s abstract laws,
scientists proposed the Chromosomal Theory of Inheritance long before there was any direct evidence that
chromosomes carried traits. Critics pointed out that individuals had far more independently segregating traits
than they had chromosomes. It was only after several years of carrying out crosses with the fruit fly, Drosophila
melanogaster, that Thomas Hunt Morgan provided experimental evidence to support the Chromosomal Theory
of Inheritance.
Genetic Linkage and Distances
Mendel’s work suggested that traits are inherited independently of each other. Morgan identified a 1:1
correspondence between a segregating trait and the X chromosome, suggesting that random chromosome
segregation was the physical basis of Mendel’s model. This also demonstrated that linked genes disrupt
Mendel’s predicted outcomes. That each chromosome can carry many linked genes explains how individuals
can have many more traits than they have chromosomes. However, researchers in Morgan’s laboratory
suggested that alleles positioned on the same chromosome were not always inherited together. During meiosis,
linked genes somehow became unlinked.
Homologous Recombination
In 1909, Frans Janssen observed chiasmata—the point at which chromatids are in contact with each other and
may exchange segments—prior to the first meiosis division. He suggested that alleles become unlinked and
chromosomes physically exchange segments. As chromosomes condensed and paired with their homologs,
they appeared to interact at distinct points. Janssen suggested that these points corresponded to regions in
which chromosome segments exchanged. We now know that the pairing and interaction between homologous
chromosomes, or synapsis, does more than simply organize the homologs for migration to separate daughter
cells. When synapsed, homologous chromosomes undergo reciprocal physical exchanges at their arms in
homologous recombination, or more simply, “crossing over.”
To better understand the type of experimental results that researchers were obtaining at this time, consider a
heterozygous individual that inherited dominant maternal alleles for two genes on the same chromosome (such
as AB) and two recessive paternal alleles for those same genes (such as ab). If the genes are linked, one would
expect this individual to produce gametes that are either AB or ab with a 1:1 ratio. If the genes are unlinked,
the individual should produce AB, Ab, aB, and ab gametes with equal frequencies, according to the Mendelian
concept of independent assortment. Because they correspond to new allele combinations, the genotypes Ab
and aB are nonparental types that result from homologous recombination during meiosis. Parental types are
progeny that exhibit the same allelic combination as their parents. Morgan and his colleagues, however, found
that when they test crossed such heterozygous individuals to a homozygous recessive parent (AaBb x aabb),
both parental and nonparental cases occurred. For example, 950 offspring might be recovered that were either
AaBb or aabb, but 50 offspring would also result that were either Aabb or aaBb. These results suggested that
linkage occurred most often, but a significant minority of offspring were the products of recombination.
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Chapter 13 | Modern Understandings of Inheritance
visual
CONNECTION
Inheritance Pattern of Linked and Unlinked Genes
Three hypothetical inheritance patterns for a test cross between a heterozygote and a homozygous recessive Individual,
based on gene placement, are shown in A through C. The actual experimental results published by Thomas Hunt Morgan
in 1912 are shown in D.
A. Genes on different chromosomes, independently assorted
Type: parental parental recombinant recombinant
50% of the offspring will be recombinant.
B. Genes on the same chromosome,
no crossover occurs
C. Genes on the same chromosome,
crossover occurs 100% of the time
Ratio of
offspring:
Type: parental parental recombinant recombinant Type:
None ot the offspring will be recombinant.
D. Results from Morgan’s 1912 experiment
GL
parental parental recombinant recombinant
50% of the offspring wiU be recombinant.
Gl
Test cross
GgLI
ggll
GgH
Number of offspring
3 %
-'965
3 %
944
9 %
-206 ~
-IB5-
Ratio of offspring:
Type:
parental parental recombinant recombinant
17% of the offspring are recombinant, indicating that the genes are
on tfte same chromosome and crossover occurs some of the time.
Figure 13.3 This figure shows unlinked and linked gene inheritance patterns. In (a), two genes are located on
different chromosomes so independent assortment occurs during meiosis. The offspring have an equal chance
of being the parental type (inheriting the same combination of traits as the parents) or a nonparental type
(inheriting a different combination of traits than the parents). In (b), two genes are very close together on the
same chromosome so that no crossing over occurs between them. Therefore, the genes are always inherited
together and all the offspring are the parental type. In (c), two genes are far apart on the chromosome such that
crossing over occurs during every meiotic event. The recombination frequency will be the same as if the genes
were on separate chromosomes, (d) The actual recombination frequency of fruit fly wing length and body color
that Thomas Morgan observed in 1912 was 17 percent. A crossover frequency between 0 percent and 50 percent
indicates that the genes are on the same chromosome and crossover sometimes occurs.
in a test cross for two characteristics such as the one here, can the recombinant offspring's predicted
frequency be 60 percent? Why or why not?
Genetic Maps
Janssen did not have the technology to demonstrate crossing over so it remained an abstract idea that
scientists did not widely believe. Scientists thought chiasmata were a variation on synapsis and could not
understand how chromosomes could break and rejoin. Yet, the data were clear that linkage did not always occur.
Ultimately, it took a young undergraduate student and an “all-nighter” to mathematically elucidate the linkage
and recombination problem.
In 1913, Alfred Sturtevant, a student in Morgan’s laboratory, gathered results from researchers in the laboratory,
and took them home one night to mull them over. By the next morning, he had created the first “chromosome
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Chapter 13 | Modern Understandings of Inheritance
365
map,” a linear representation of gene order and relative distance on a chromosome (Figure 13.4).
visual
CONNECTION
Genetic Map Based on Recombination
Frequencies in Drosophila
MUTANT WILD TYPE
Short aristae
0
Long aristae
Black body
48.5
Gray body
Cinnabar eyes
57.5
Red eyes
Vestigial wings
65.5
Normal wings
Brown eyes
104.5
Red eyes
Values in centimorgan (cM) map units; recombination
frequency of 0.01 = 1 cM
Figure 13.4 This genetic map orders Drosophila genes on the basis of recombination frequency.
Which of the following statements is true?
a. Recombination of the body color and red/cinnabar eye alleles will occur more frequently than
recombination of the alleles for wing length and aristae length.
b. Recombination of the body color and aristae length alleles will occur more frequently than
recombination of red/brown eye alleles and the aristae length alleles.
c. Recombination of the gray/black body color and long/short aristae alleles will not occur.
d. Recombination of the red/brown eye and long/short aristae alleles will occur more frequently than
recombination of the alleles for wing length and body color.
As Figure 13.4 shows, by using recombination frequency to predict genetic distance, we can infer the relative
gene order on chromosome 2. The values represent map distances in centimorgans (cM), which correspond to
recombination frequencies (in percent). Therefore, the genes for body color and wing size were 65.5 - 48.5 = 17
cM apart, indicating that the maternal and paternal alleles for these genes recombine in 17 percent of offspring,
on average.
To construct a chromosome map, Sturtevant assumed that genes were ordered serially on threadlike
chromosomes. He also assumed that the incidence of recombination between two homologous chromosomes
could occur with equal likelihood anywhere along the chromosome's length. Operating under these assumptions,
Sturtevant postulated that alleles that were far apart on a chromosome were more likely to dissociate during
meiosis simply because there was a larger region over which recombination could occur. Conversely, alleles
that were close to each other on the chromosome were likely to be inherited together. The average number of
crossovers between two alleles—that is, their recombination frequency —correlated with their genetic distance
from each other, relative to the locations of other genes on that chromosome. Considering the example cross
between AaBb and aabb above, we could calculate the recombination's frequency as 50/1000 = 0.05. That
is, the likelihood of a crossover between genes A/a and B/b was 0.05, or 5 percent. Such a result would
indicate that the genes were definitively linked, but that they were far enough apart for crossovers to occasionally
occur. Sturtevant divided his genetic map into map units, or centimorgans (cM), in which a 0,01 recombination
frequency corresponds to 1 cM.
By representing alleles in a linear map, Sturtevant suggested that genes can range from linking perfectly
(recombination frequency = 0) to unlinking perfectly (recombination frequency = 0.5) when genes are on different
chromosomes or genes separate very far apart on the same chromosome. Perfectly unlinked genes correspond
to the frequencies Mendel predicted to assort independently in a dihybrid cross. A 0.5 recombination frequency
indicates that 50 percent of offspring are recombinants and the other 50 percent are parental types. That is,
every type of allele combination is represented with equal frequency. This representation allowed Sturtevant
to additively calculate distances between several genes on the same chromosome. However, as the genetic
distances approached 0.50, his predictions became less accurate because it was not clear whether the genes
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Chapter 13 | Modern Understandings of Inheritance
were very far apart on the same or on different chromosomes.
in 1931, Barbara McClintock and Harriet Creighton demonstrated the crossover of homologous chromosomes
in corn plants. Weeks later, Curt Stern demonstrated microscopically homologous recombination in Drosophila.
Stern observed several X-linked phenotypes that were associated with a structurally unusual and dissimilar
X chromosome pair in which one X was missing a small terminal segment, and the other X was fused to a
piece of the Y chromosome. By crossing flies, observing their offspring, and then visualizing the offspring’s
chromosomes, Stern demonstrated that every time the offspring allele combination deviated from either of the
parental combinations, there was a corresponding exchange of an X chromosome segment. Using mutant flies
with structurally distinct X chromosomes was the key to observing the products of recombination because DNA
sequencing and other molecular tools were not yet available. We now know that homologous chromosomes
regularly exchange segments in meiosis by reciprocally breaking and rejoining their DNA at precise locations.
LINK TQ LEARNING
Review Sturtevant’s process to create a genetic map on the basis of recombination frequencies here
(http:// 0 penstaxc 0 llege. 0 rg/l/gene_cr 0 ss 0 ver) .
Mendel’s Mapped Traits
Homologous recombination is a common genetic process, yet Mendel never observed it. Had he investigated
both linked and unlinked genes, it would have been much more difficult for him to create a unified model of his
data on the basis of probabilistic calculations. Researchers who have since mapped the seven traits that Mendel
investigated onto a pea plant genome's seven chromosomes have confirmed that all the genes he examined are
either on separate chromosomes or are sufficiently far apart as to be statistically unlinked. Some have suggested
that Mendel was enormously lucky to select only unlinked genes; whereas, others question whether Mendel
discarded any data suggesting linkage. In any case, Mendel consistently observed independent assortment
because he examined genes that were effectively unlinked.
13.2 | Chromosomal Basis of Inherited Disorders
By the end of this section, you will be able to do the following:
• Describe how a karyogram is created
• Explain how nondisjunction leads to disorders in chromosome number
• Compare disorders that aneuploidy causes
• Describe how errors in chromosome structure occur through inversions and translocations
Inherited disorders can arise when chromosomes behave abnormally during meiosis. We can divide
chromosome disorders into two categories: abnormalities in chromosome number and chromosomal structural
rearrangements. Because even small chromosome segments can span many genes, chromosomal disorders
are characteristically dramatic and often fatal.
Chromosome Identification
Chromosome isolation and microscopic observation forms the basis of cytogenetics and is the primary method
by which clinicians detect chromosomal abnormalities in humans. A karyotype is the number and appearance
of chromosomes, and includes their length, banding pattern, and centromere position. To obtain a view of an
individual’s karyotype, cytologists photograph the chromosomes and then cut and paste each chromosome into
a chart, or karyogram. Another name is an ideogram (Figure 13.5).
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Chapter 13 | Modern Understandings of Inheritance
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Figure 13.5 This karyotype is of a female human. Notice that homologous chromosomes are the same size, and have
the same centromere positions and banding patterns. A human male would have an XY chromosome pair instead of
the XX pair, (credit: Andreas Blozer et al)
in a given species, we can identify chromosomes by their number, size, centromere position, and banding
pattern. In a human karyotype, autosomes or “body chromosomes" (all of the non-sex chromosomes) are
generally organized in approximate order of size from largest (chromosome 1) to smallest (chromosome 22).
The X and Y chromosomes are not autosomes. However, chromosome 21 is actually shorter than chromosome
22. Researchers discovered this after naming Down syndrome as trisomy 21, reflecting how this disease results
from possessing one extra chromosome 21 (three total). Not wanting to change the name of this important
disease, scientists retained the numbering of chromosome 21 despite describing it having the shortest set of
chromosomes. We may designate the chromosome “arms” projecting from either end of the centromere as short
or long, depending on their relative lengths. We abbreviate the short arm p (for “petite"); whereas, we abbreviate
the long arm q (because it follows “p” alphabetically). Numbers further subdivide and denote each arm. Using
this naming system, we can describe chromosome locations consistently in the scientific literature.
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Chapter 13 | Modern Understandings of Inheritance
ca eer connection
Geneticists Use Karyograms to Identify Chromosomal Aberrations
Although we refer to Mendel as the “father of modern genetics," he performed his experiments with none
of the tools that the geneticists of today routinely employ. One such powerful cytological technique is
karyotyping, a method in which geneticists can identify traits characterized by chromosomal abnormalities
from a single cell. To observe an individual’s karyotype, a geneticist first collects a person’s cells (like white
blood cells) from a blood sample or other tissue, in the laboratory, he or she stimulates the isolated cells
to begin actively dividing. The geneticist then applies the chemical colchicine to cells to arrest condensed
chromosomes in metaphase. The geneticist then induces swelling in the cells using a hypotonic solution so
the chromosomes spread apart. Finally, the geneticist preserves the sample in a fixative and applies it to a
slide.
The geneticist then stains chromosomes with one of several dyes to better visualize each chromosome
pair's distinct and reproducible banding patterns. Following staining, the geneticist views the chromosomes
using bright-field microscopy. A common stain choice is the Giemsa stain. Giemsa staining results in
approximately 400-800 bands (of tightly coiled DNA and condensed proteins) arranged along all 23
chromosome pairs. An experienced geneticist can identify each band. In addition to the banding patterns,
geneticists further identify chromosomes on the basis of size and centromere location. To obtain the classic
depiction of the karyotype in which homologous chromosome pairs align in numerical order from longest
to shortest, the geneticist obtains a digital image, identifies each chromosome, and manually arranges the
chromosomes into this pattern (Figure 13.5).
At its most basic, the karyogram may reveal genetic abnormalities in which an individual has too many
or too few chromosomes per cell. Examples of this are Down Syndrome, which one identifies by a third
copy of chromosome 21, and Turner Syndrome, which has the presence of only one X chromosome
in women instead of the normal two characterizes. Geneticists can also identify large DNA deletions or
insertions. For instance, geneticists can identify Jacobsen Syndrome—which involves distinctive facial
features as well as heart and bleeding defects—by a deletion on chromosome 11. Finally, the karyotype can
pinpoint translocations, which occur when a segment of genetic material breaks from one chromosome
and reattaches to another chromosome or to a different part of the same chromosome. Translocations are
implicated in certain cancers, including chronic myelogenous leukemia.
During Mendel’s lifetime, inheritance was an abstract concept that one could only infer by performing
crosses and observing the traits that offspring expressed. By observing a karyogram, today’s geneticists
can actually visualize an individual's chromosomal composition to confirm or predict genetic abnormalities
in offspring, even before birth.
Chromosome Number Disorders
Of all of the chromosomal disorders, chromosome number abnormalities are the most obviously identifiable
from a karyogram. Chromosome number disorders include duplicating or losing entire chromosomes, as well
as changes in the number of complete sets of chromosomes. They are caused by nondisjunction, which
occurs when homologous chromosome pairs or sister chromatids fail to separate during meiosis. Misaligned
or incomplete synapsis, or a spindle apparatus dysfunction that facilitates chromosome migration, can cause
nondisjunction. The risk of nondisjunction occurring increases with the parents' age.
Nondisjunction can occur during either meiosis I or II, with differing results (Figure 13.6). If homologous
chromosomes fail to separate during meiosis I, the result is two gametes that lack that particular chromosome
and two gametes with two chromosome copies. If sister chromatids fail to separate during meiosis il, the result
is one gamete that lacks that chromosome, two normal gametes with one chromosome copy, and one gamete
with two chromosome copies.
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Chapter 13 | Modern Understandings of Inheritance
369
visual
CONNECTION
Figure 13.6 Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during
meiosis, resulting in an abnormal chromosome number. Nondisjunction may occur during meiosis I or meiosis II.
Which of the following statements about nondisjunction is true?
a. Nondisjunction only results in gametes with n+1 or n-1 chromosomes.
b. Nondisjunction occurring during meiosis II results in 50 percent normal gametes.
c. Nondisjunction during meiosis I results in 50 percent normal gametes.
d. Nondisjunction always results in four different kinds of gametes.
Aneuploidy
Scientists call an individual with the appropriate number of chromosomes for their species euploid. In humans,
euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes. An individual with an error
in chromosome number is described as aneuploid, a term that includes monosomy (losing one chromosome)
or trisomy (gaining an extraneous chromosome). Monosomic human zygotes missing any one copy of an
autosome invariably fail to develop to birth because they lack essential genes. This underscores the importance
of “gene dosage” in humans. Most autosomal trisomies also fail to develop to birth; however, duplications of
some smaller chromosomes (13, 15, 18, 21, or 22) can result in offspring that survive for several weeks to many
years. Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose. Individuals
with an extra chromosome may synthesize an abundance of the gene products, which that chromosome
encodes. This extra dose (150 percent) of specific genes can lead to a number of functional challenges and
often precludes development. The most common trisomy among viable births is that of chromosome 21, which
corresponds to Down Syndrome. Short stature and stunted digits, facial distinctions that include a broad skull
and large tongue, and significant developmental delays characterize individuals with this inherited disorder. We
can correlate the incidence of Down syndrome with maternal age. Older women are more likely to become
pregnant with fetuses carrying the trisomy 21 genotype (Figure 13.7).
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Chapter 13 | Modern Understandings of Inheritance
Down Syndrome Correlation with Maternal Age
Data source: American Family Physician; Aug 15, 2000
Figure 13.7 The incidence of having a fetus with trisomy 21 increases dramatically with maternal age.
LINK TQ LEARNING
Visualize adding a chromosome that leads to Down syndrome in this video simulation
(http:// 0 penstaxc 0 llege. 0 rg/l/d 0 wn_syndr 0 me) .
Polyploidy
We call an individual with more than the correct number of chromosome sets (two for diploid species) polyploid.
For instance, fertilizing an abnormal diploid egg with a normal haploid sperm would yield a triploid zygote.
Polyploid animals are extremely rare, with only a few examples among the flatworms, crustaceans, amphibians,
fish, and lizards. Polyploid animals are sterile because meiosis cannot proceed normally and instead produces
mostly aneuploid daughter cells that cannot yield viable zygotes. Rarely, polyploid animals can reproduce
asexually by haplodiploidy, in which an unfertilized egg divides mitotically to produce offspring. In contrast,
polyploidy is very common in the plant kingdom, and polyploid plants tend to be larger and more robust than
euploids of their species (Figure 13.8).
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Chapter 13 | Modern Understandings of Inheritance
371
J
I
Figure 13.8 As with many polyploid plants, this triploid orange daylily (Hemerocattis fulva) is particularly large and
robust, and grows flowers with triple the number of petals of its diploid counterparts, (credit: Steve Karg)
Sex Chromosome Nondisjunction in Humans
Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem
counterintuitive that human females and males can function normally, despite carrying different numbers of the
X chromosome. Rather than a gain or loss of autosomes, variations in the number of sex chromosomes occur
with relatively mild effects. In part, this happens because of the molecular process X inactivation. Early in
development, when female mammalian embryos consist of just a few thousand cells (relative to trillions in the
newborn), one X chromosome in each cell inactivates by tightly condensing into a quiescent (dormant) structure,
or a Barr body. The chance that an X chromosome (maternally or paternally derived) inactivates in each cell
is random, but once this occurs, all cells derived from that one will have the same inactive X chromosome or
Barr body. By this process, females compensate for their double genetic dose of X chromosome. In so-called
“tortoiseshell” cats, we observe embryonic X inactivation as color variegation (Figure 13.9). Females that are
heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions
of their body, corresponding to whichever X chromosome inactivates in that region's embryonic cell progenitor.
Figure 13.9 In cats, the gene for coat color is located on the X chromosome. In female cats' embryonic development,
one of the two X chromosomes randomly inactivates in each cell, resulting in a tortoiseshell pattern if the cat has
two different alleles for coat color. Male cats, having only one X chromosome, never exhibit a tortoiseshell coat color,
(credit: Michael Bodega)
An individual carrying an abnormal number of X chromosomes will inactivate all but one X chromosome in each
of her cells. However, even inactivated X chromosomes continue to express a few genes, and X chromosomes
must reactivate for the proper maturation of female ovaries. As a result, X-chromosomal abnormalities typically
occur with mild mental and physical defects, as well as sterility. If the X chromosome is absent altogether, the
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Chapter 13 | Modern Understandings of Inheritance
individual will not develop in utero.
Scientists have identified and characterized several errors in sex chromosome number. Individuals with three
X chromosomes, triplo-X, are phenotypically female but express developmental delays and reduced fertility.
The XXY genotype, corresponding to one type of Klinefelter syndrome, corresponds to phenotypically male
individuals with small testes, enlarged breasts, and reduced body hair. More complex types of Klinefelter
syndrome exist in which the individual has as many as five X chromosomes. In all types, every X chromosome
except one undergoes inactivation to compensate for the excess genetic dosage. We see this as several
Barr bodies in each cell nucleus. Turner syndrome, characterized as an XO genotype (i.e., only a single sex
chromosome), corresponds to a phenotypically female individual with short stature, webbed skin in the neck
region, hearing and cardiac impairments, and sterility.
Duplications and Deletions
In addition to losing or gaining an entire chromosome, a chromosomal segment may duplicate or lose itself.
Duplications and deletions often produce offspring that survive but exhibit physical and mental abnormalities.
Duplicated chromosomal segments may fuse to existing chromosomes or may be free in the nucleus. Cri-du-
chat (from the French for “cry of the cat”) is a syndrome that occurs with nervous system abnormalities and
identifiable physical features that result from a deletion of most 5p (the small arm of chromosome 5) (Figure
13.10). Infants with this genotype emit a characteristic high-pitched cry on which the disorder’s name is based.
Figure 13.10 This figure shows an individual with cri-du-chat syndrome at two, four, nine, and 12 years of age. (credit:
Paola Cerruti Mainardi)
Chromosomal Structural Rearrangements
Cytologists have characterized numerous structural rearrangements in chromosomes, but chromosome
inversions and translocations are the most common. We can identify both during meiosis by the adaptive
pairing of rearranged chromosomes with their former homologs to maintain appropriate gene alignment. If the
genes on two homologs are not oriented correctly, a recombination event could result in losing genes from one
chromosome and gaining genes on the other. This would produce aneuploid gametes.
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Chromosome Inversions
A chromosome inversion is the detachment, 180° rotation, and reinsertion of part of a chromosome. Inversions
may occur in nature as a result of mechanical shear, or from transposable elements' action (special DNA
sequences capable of facilitating rearranging chromosome segments with the help of enzymes that cut and
paste DNA sequences). Unless they disrupt a gene sequence, inversions only change gene orientation and are
likely to have more mild effects than aneuploid errors. However, altered gene orientation can result in functional
changes because regulators of gene expression could move out of position with respect to their targets, causing
aberrant levels of gene products.
An inversion can be pericentric and include the centromere, or paracentric and occur outside the centromere
(Figure 13.11). A pericentric inversion that is asymmetric about the centromere can change the chromosome
arms' relative lengths, making these inversions easily identifiable.
Pericentric and Paracentric Inversions
Normal chromosome
A
B
c 0 D
E
F
Pericentric inversion
A
B
D 0 c
E
F
Paracentric inversion
A
B
c 0 D
F
E
centromere
Figure 13.11 Pericentric inversions include the centromere, and paracentric inversions do not. A pericentric inversion
can change the chromosome arms' relative lengths. A paracentric inversion cannot.
When one homologous chromosome undergoes an inversion but the other does not, the individual is an
inversion heterozygote. To maintain point-for-point synapsis during meiosis, one homolog must form a loop, and
the other homolog must mold around it. Although this topology can ensure that the genes correctly align, it also
forces the homologs to stretch and can occur with imprecise synapsis regions (Figure 13.12).
Figure 13.12 When one chromosome undergoes an inversion but the other does not, one chromosome must form an
inverted loop to retain point-for-point interaction during synapsis. This inversion pairing is essential to maintaining gene
alignment during meiosis and to allow for recombination.
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V /
e olution CONNECTION
The Chromosome 18 Inversion
Not all chromosomes' structural rearrangements produce nonviable, impaired, or infertile individuals. In rare
instances, such a change can result in new species evolving. In fact, a pericentric inversion in chromosome
18 appears to have contributed to human evolution. This inversion is not present in our closest genetic
relatives, the chimpanzees. Humans and chimpanzees differ cytogenetically by pericentric inversions on
several chromosomes and by the fusion of two separate chromosomes in chimpanzees that correspond to
chromosome two in humans.
Scientists believe the pericentric chromosome 18 inversion occurred in early humans following their
divergence from a common ancestor with chimpanzees approximately five million years ago. Researchers
characterizing this inversion have suggested that approximately 19,000 nucleotide bases were duplicated
on 18p, and the duplicated region inverted and reinserted on chromosome 18 of an ancestral human.
A comparison of human and chimpanzee genes in the region of this inversion indicates that two
genes— ROCK1 and USP14 —that are adjacent on chimpanzee chromosome 17 (which corresponds to
human chromosome 18) are more distantly positioned on human chromosome 18. This suggests that one
of the inversion breakpoints occurred between these two genes. Interestingly, humans and chimpanzees
express USP14 at distinct levels in specific cell types, including cortical cells and fibroblasts. Perhaps the
chromosome 18 inversion in an ancestral human repositioned specific genes and reset their expression
levels in a useful way. Because both ROCK1 and USP14 encode cellular enzymes, a change in their
expression could alter cellular function. We do not know how this inversion contributed to hominid evolution,
[i]
but it appears to be a significant factor in the divergence of humans from other primates.
Translocations
A translocation occurs when a chromosome segment dissociates and reattaches to a different, nonhomologous
chromosome. Translocations can be benign or have devastating effects depending on how the positions
of genes are altered with respect to regulatory sequences. Notably, specific translocations have occurred
with several cancers and with schizophrenia. Reciprocal translocations result from exchanging chromosome
seqments between two nonhomoloqous chromosomes such that there is no qenetic information qain or loss
(Figure 13.13).
Figure 13.13 A reciprocal translocation occurs when a DNA segment transfers from one chromosome to another,
nonhomologous chromosome, (credit: modification of work by National Human Genome Research/USA)
1. Violaine Goidts et al., “Segmental duplication associated with the human-specific inversion of chromosome 18: a further example of the
impact of segmental duplications on karyotype and genome evolution in primates,” Human Genetics. 115 (2004):116-122
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Chapter 13 | Modern Understandings of Inheritance
375
KEY TERMS
aneuploid individual with an error in chromosome number; includes chromosome segment deletions and
duplications
autosome any of the non-sex chromosomes
centimorgan (cM) (also, map unit) relative distance that corresponds to a 0,01 recombination frequency
Chromosomal Theory of Inheritance theory proposing that chromosomes are the genes' vehicles and that
their behavior during meiosis is the physical basis of the inheritance patterns that Mendel observed
chromosome inversion detachment, 180° rotation, and chromosome arm reinsertion
euploid individual with the appropriate number of chromosomes for their species
homologous recombination process by which homologous chromosomes undergo reciprocal physical
exchanges at their arms, also crossing over
karyogram a karyotype's photographic image
karyotype an individual's chromosome number and appearance; includes the size, banding patterns, and
centromere position
monosomy otherwise diploid genotype in which one chromosome is missing
nondisjunction failure of synapsed homologs to completely separate and migrate to separate poles during the
meiosis' first cell division
nonparental (recombinant) type progeny resulting from homologous recombination that exhibits a different
allele combination compared with its parents
paracentric inversion that occurs outside the centromere
parental types progeny that exhibits the same allelic combination as its parents
pericentric inversion that involves the centromere
polyploid individual with an incorrect number of chromosome sets
recombination frequency average number of crossovers between two alleles; observed as the number of
nonparental types in a progeny's population
translocation process by which one chromosome segment dissociates and reattaches to a different,
nonhomologous chromosome
trisomy otherwise diploid genotype in which one entire chromosome duplicates
X inactivation condensing X chromosomes into Barr bodies during embryonic development in females to
compensate for the double genetic dose
CHAPTER SUMMARY
13.1 Chromosomal Theory and Genetic Linkage
Sutton and Boveri's Chromosomal Theory of Inheritance states that chromosomes are the vehicles of genetic
heredity. Neither Mendelian genetics nor gene linkage is perfectly accurate, instead, chromosome behavior
involves segregation, independent assortment, and occasionally, linkage. Sturtevant devised a method to
assess recombination frequency and infer linked genes' relative positions and distances on a chromosome on
the basis of the average number of crossovers in the intervening region between the genes. Sturtevant
correctly presumed that genes are arranged in serial order on chromosomes and that recombination between
homologs can occur anywhere on a chromosome with equal likelihood. Whereas linkage causes alleles on the
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Chapter 13 | Modern Understandings of Inheritance
same chromosome to be inherited together, homologous recombination biases alleles toward an independent
inheritance pattern.
13.2 Chromosomal Basis of Inherited Disorders
The number, size, shape, and banding pattern of chromosomes make them easily identifiable in a karyogram
and allows for the assessment of many chromosomal abnormalities. Disorders in chromosome number, or
aneuploidies, are typically lethal to the embryo, although a few trisomic genotypes are viable. Because of X
inactivation, aberrations in sex chromosomes typically have milder phenotypic effects. Aneuploidies also
include instances in which a chromosome's segments duplicate or delete themselves. Inversion or
translocation also may rearrange chromosome structures. Both of these aberrations can result in problematic
phenotypic effects. Because they force chromosomes to assume unnatural topologies during meiosis,
inversions and translocations often occur with reduced fertility because of the likelihood of nondisjunction.
VISUAL CONNECTION QUESTIONS
1. Figure 13.3 In a test cross for two characteristics
such as the one shown here, can the predicted
frequency of recombinant offspring be 60 percent?
Why or why not?
2. Figure 13.4 Which of the following statements is
true?
a. Recombination of the body color and red/
cinnabar eye alleles will occur more
frequently than recombination of the alleles
for wing length and aristae length.
b. Recombination of the body color and aristae
length alleles will occur more frequently than
recombination of red/brown eye alleles and
the aristae length alleles.
c. Recombination of the gray/black body color
and long/short aristae alleles will not occur.
d. Recombination of the red/brown eye and
long/short aristae alleles will occur more
frequently than recombination of the alleles
for wing length and body color.
a. 0
b. 0.25
c. 0.50
d. 0.75
7. Which recombination frequency corresponds to
perfect linkage and violates the law of independent
assortment?
a. 0
b. 0.25
c. 0.50
d. 0.75
8. Which of the following codes describes position 12
on the long arm of chromosome 13?
a. 13pl2
b. 13ql2
c. 12pl3
d. 12ql3
9. In agriculture, polyploid crops (like coffee,
REVIEW QUESTIONS
4. X-linked recessive traits in humans (or in
Drosophila) are observed_.
a. in more males than females
b. in more females than males
c. in males and females equally
d. in different distributions depending on the
trait
5. The first suggestion that chromosomes may
physically exchange segments came from the
microscopic identification of_.
a. synapsis
b. sister chromatids
c. chiasmata
d. alleles
6. Which recombination frequency corresponds to
independent assortment and the absence of linkage?
3. Figure 13.6 Which of the following statements
about nondisjunction is true?
a. Nondisjunction only results in gametes with
n+1 or n-1 chromosomes.
b. Nondisjunction occurring during meiosis II
results in 50 percent normal gametes.
c. Nondisjunction during meiosis I results in 50
percent normal gametes.
d. Nondisjunction always results in four
different kinds of gametes.
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Chapter 13 | Modern Understandings of Inheritance
377
strawberries, or bananas) tend to produce_.
a. more uniformity
b. more variety
c. larger yields
d. smaller yields
10. Assume a pericentric inversion occurred in one of
two homologs prior to meiosis. The other homolog
remains normal. During meiosis, what structure—if
any—would these homologs assume in order to pair
accurately along their lengths?
a. V formation
b. cruciform
c. loop
d. pairing would not be possible
11. The genotype XXY corresponds to
CRITICAL THINKING QUESTIONS
14. Explain how the Chromosomal Theory of
Inheritance helped to advance our understanding of
genetics.
a. Klinefelter syndrome
b. Turner syndrome
c. Triplo-X
d. Jacob syndrome
12. Abnormalities in the number of X chromosomes
tends to have milder phenotypic effects than the
same abnormalities in autosomes because of
a. deletions
b. nonhomologous recombination
c. synapsis
d. X inactivation
13. By definition, a pericentric inversion includes the
a. centromere
b. chiasma
c. telomere
d. synapse
15. Using diagrams, illustrate how nondisjunction can
result in an aneuploid zygote.
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Chapter 13 | Modern Understandings of Inheritance
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Chapter 14 | DNA Structure and Function
379
14 | DNA STRUCTURE
AND FUNCTION
Figure 14.1 Dolly the sheep was the first large mammal to be cloned.
Chapter Outline
14.1: Historical Basis of Modern Understanding
14.2: DNA Structure and Sequencing
14.3: Basics of DNA Replication
14.4: DNA Replication in Prokaryotes
14.5: DNA Replication in Eukaryotes
14.6: DNA Repair
Introduction
The three letters “DNA” have now become synonymous with crime solving and genetic testing. DNA can be
retrieved from hair, blood, or saliva. Each person’s DNA is unique, and it is possible to detect differences
between individuals within a species on the basis of these unique features.
DNA analysis has many practical applications beyond forensics. In humans, DNA testing is applied to numerous
uses: determining paternity, tracing genealogy, identifying pathogens, archeological research, tracing disease
outbreaks, and studying human migration patterns. In the medical field, DNA is used in diagnostics, new vaccine
development, and cancer therapy. It is now possible to determine predisposition to diseases by looking at genes.
Each human cell has 23 pairs of chromosomes: one set of chromosomes is inherited from the mother and the
other set is inherited from the father. There is also a mitochondrial genome, inherited exclusively from the mother,
which can be involved in inherited genetic disorders. On each chromosome, there are thousands of genes that
are responsible for determining the genotype and phenotype of the individual. A gene is defined as a sequence
of DNA that codes for a functional product. The human haploid genome contains 3 billion base pairs and has
between 20,000 and 25,000 functional genes.
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Chapter 14 | DNA Structure and Function
14.1 1 Historical Basis of Modern Understanding
By the end of this section, you will be able to do the following:
• Explain transformation of DNA
• Describe the key experiments that helped identify that DNA is the genetic material
• State and explain Chargaff’s rules
Our current understanding of DNA began with the discovery of nucleic acids followed by the development of
the double-helix model. In the 1860s, Friedrich Miescher (Figure 14.2), a physician by profession, isolated
phosphate-rich chemicals from white blood cells (leukocytes). He named these chemicals (which would
eventually be known as DNA) nuclein because they were isolated from the nuclei of the cells.
Figure 14.2 Friedrich Miescher (1844-1895) discovered nucleic acids.
LINK TQ LEARNING
To see Miescher conduct his experiment that led to his discovery of DNA and associated proteins in the
nucleus, click through this review (http:// 0 penstaxc 0 llege. 0 rg/l/miescherJevene) .
A half century later, in 1928, British bacteriologist Frederick Griffith reported the first demonstration of bacterial
transformation —a process in which external DNA is taken up by a cell, thereby changing its morphology
and physiology. Griffith conducted his experiments with Streptococcus pneumoniae, a bacterium that causes
pneumonia. Griffith worked with two strains of this bacterium called rough (R) and smooth (S). (The two cell
types were called “rough” and “smooth” after the appearance of their colonies grown on a nutrient agar plate.)
The R strain is non-pathogenic (does not cause disease). The S strain is pathogenic (disease-causing), and has
a capsule outside its cell wall. The capsule allows the cell to escape the immune responses of the host mouse.
When Griffith injected the living S strain into mice, they died from pneumonia, in contrast, when Griffith injected
the live R strain into mice, they survived. In another experiment, when he injected mice with the heat-killed S
strain, they also survived. This experiment showed that the capsule alone was not the cause of death, in a
third set of experiments, a mixture of live R strain and heat-killed S strain were injected into mice, and—to his
surprise—the mice died. Upon isolating the live bacteria from the dead mouse, only the S strain of bacteria
was recovered. When this isolated S strain was injected into fresh mice, the mice died. Griffith concluded that
something had passed from the heat-killed S strain into the live R strain and transformed it into the pathogenic
S strain. He called this the transforming principle (Figure 14.3). These experiments are now known as Griffith's
transformation experiments.
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Chapter 14 | DNA Structure and Function
381
Mouse injected with heat-killed virulant S strain Mouse injected with both heat-killed S strain
lives. and live non-virulant R strain dies.
Figure 14.3 Two strains of S. pneumoniae were used in Griffith’s transformation experiments. The R strain is non-
pathogenic, whereas the S strain is pathogenic and causes death. When Griffith injected a mouse with the heat-killed
S strain and a live R strain, the mouse died. The S strain was recovered from the dead mouse. Griffith concluded
that something had passed from the heat-killed S strain to the R strain, transforming the R strain into the S strain in
the process, (credit "living mouse": modification of work by NIH; credit "dead mouse": modification of work by Sarah
Marriage)
Scientists Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944) were interested in exploring this
transforming principle further. They isolated the S strain from the dead mice and isolated the proteins and nucleic
acids (RNA and DNA) as these were possible candidates for the molecule of heredity. They used enzymes that
specifically degraded each component and then used each mixture separately to transform the R strain. They
found that when DNA was degraded, the resulting mixture was no longer able to transform the bacteria, whereas
all of the other combinations were able to transform the bacteria. This led them to conclude that DNA was the
transforming principle.
ca eer connection
Forensic Scientists used DNA analysis evidence for the first time to solve an immigration case. The story
started with a teenage boy returning to London from Ghana to be with his mother, immigration authorities
at the airport were suspicious of him, thinking that he was traveling on a forged passport. After much
persuasion, he was allowed to go live with his mother, but the immigration authorities did not drop the case
against him. All types of evidence, including photographs, were provided to the authorities, but deportation
proceedings were started nevertheless. Around the same time, Dr. Alec Jeffreys of Leicester University in
the United Kingdom had invented a technique known as DNA fingerprinting. The immigration authorities
approached Dr. Jeffreys for help. He took DNA samples from the mother and three of her children, as well
as an unrelated mother, and compared the samples with the boy’s DNA. Because the biological father was
not in the picture, DNA from the three children was compared with the boy’s DNA. He found a match in the
boy’s DNA for both the mother and his three siblings. He concluded that the boy was indeed the mother’s
son.
Forensic scientists analyze many items, including documents, handwriting, firearms, and biological
samples. They analyze the DNA content of hair, semen, saliva, and blood, and compare it with a database
of DNA profiles of known criminals. Analysis includes DNA isolation, sequencing, and sequence analysis.
Forensic scientists are expected to appear at court hearings to present their findings. They are usually
employed in crime labs of city and state government agencies. Geneticists experimenting with DNA
techniques also work for scientific and research organizations, pharmaceutical industries, and college and
university labs. Students wishing to pursue a career as a forensic scientist should have at least a bachelor's
degree in chemistry, biology, or physics, and preferably some experience working in a laboratory.
Although the experiments of Avery, McCarty and McLeod had demonstrated that DNA was the informational
component transferred during transformation, DNA was still considered to be too simple a molecule to carry
biological information. Proteins, with their 20 different amino acids, were regarded as more likely candidates.
The decisive experiment, conducted by Martha Chase and Alfred Hershey in 1952, provided confirmatory
evidence that DNA was indeed the genetic material and not proteins. Chase and Hershey were studying a
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Chapter 14 | DNA Structure and Function
bacteriophage —a virus that infects bacteria. Viruses typically have a simple structure: a protein coat, called
the capsid, and a nucleic acid core that contains the genetic material (either DNA or RNA). The bacteriophage
infects the host bacterial cell by attaching to its surface, and then it injects its nucleic acids inside the
cell. The phage DNA makes multiple copies of itself using the host machinery, and eventually the host cell
bursts, releasing a large number of bacteriophages. Hershey and Chase selected radioactive elements that
would specifically distinguish the protein from the DNA in infected cells. They labeled one batch of phage
with radioactive sulfur, 35 S, to label the protein coat. Another batch of phage were labeled with radioactive
phosphorus, 32 P. Because phosphorous is found in DNA, but not protein, the DNA and not the protein would
be tagged with radioactive phosphorus. Likewise, sulfur is absent from DNA, but present in several amino acids
such as methionine and cysteine.
Each batch of phage was allowed to infect the cells separately. After infection, the phage bacterial suspension
was put in a blender, which caused the phage coat to detach from the host cell. Cells exposed long enough
for infection to occur were then examined to see which of the two radioactive molecules had entered the cell.
The phage and bacterial suspension was spun down in a centrifuge. The heavier bacterial cells settled down
and formed a pellet, whereas the lighter phage particles stayed in the supernatant. In the tube that contained
phage labeled with 35 S, the supernatant contained the radioactively labeled phage, whereas no radioactivity was
detected in the pellet. In the tube that contained the phage labeled with 32 P, the radioactivity was detected in the
pellet that contained the heavier bacterial cells, and no radioactivity was detected in the supernatant. Hershey
and Chase concluded that it was the phage DNA that was injected into the cell and carried information to produce
more phage particles, thus providing evidence that DNA was the genetic material and not proteins (Figure 14.4).
35 S-labeled protein coat
•
One batch of phage
was labeled with
35 S, which is
incorporated into
the protein coat.
Another batch was
labeled with 32 P,
which is
incorporated into
the DNA.
Bacteria were
infected with the
phage.
The cultures were
blended and
centrifuged to
separate the phage
from the bacteria.
•
The bacterial pellet
was cultured.
Bacteria infected with
phage containing
32 P-labeled DNA
produced 32 P-labeled
phage. Bacteria
infected with
35 S-labeled phage
produced unlabeled
phage.
Figure 14.4 In Hershey and Chase's experiments, bacteria were infected with phage radiolabeled with either 35 S,
which labels protein, or 32 P, which labels DNA. Only 32 P entered the bacterial cells, indicating that DNA is the genetic
material.
Around this same time, Austrian biochemist Erwin Chargaff examined the content of DNA in different species
and found that the amounts of adenine, thymine, guanine, and cytosine were not found in equal quantities, and
that relative concentrations of the four nucleotide bases varied from species to species, but not within tissues
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Chapter 14 | DNA Structure and Function
383
of the same individual or between individuals of the same species. He also discovered something unexpected:
That the amount of adenine equaled the amount of thymine, and the amount of cytosine equaled the amount
of guanine (that is, A = T and G = C). Different species had equal amounts of purines (A+G) and pyrimidines
(T + C), but different ratios of A+T to G+C. These observations became known as Chargaff’s rules. Chargaff's
findings proved immensely useful when Watson and Crick were getting ready to propose their DNA double helix
model! You can see after reading the past few pages how science builds upon previous discoveries, sometimes
in a slow and laborious process.
14.2 | DNA Structure and Sequencing
By the end of this section, you will be able to do the following:
• Describe the structure of DNA
• Explain the Sanger method of DNA sequencing
• Discuss the similarities and differences between eukaryotic and prokaryotic DNA
The building blocks of DNA are nucleotides. The important components of the nucleotide are a nitrogenous
(nitrogen-bearing) base, a 5-carbon sugar (pentose), and a phosphate group (Figure 14.5). The nucleotide is
named depending on the nitrogenous base. The nitrogenous base can be a purine such as adenine (A) and
guanine (G), or a pyrimidine such as cytosine (C) and thymine (T).
visual
a CONNECTION
NHo
I
X.
CH
I II
X. XH
H
Cytosine
C
Pyrimidines
O
HN
i
'c'
II
,CH
HN'
I
CH
II
XH
Thymine (in DNA)
T
Uracil (in RNA)
U
N„
NH 2
I
'C^X N
HC" || |
V C ^ CH
H
Adenine
A
HC
N_
^NH
^NH 2
Guanine
G
Figure 14.5 The purines have a double ring structure with a six-membered ring fused to a five-membered ring.
Pyrimidines are smaller in size; they have a single six-membered ring structure.
The images above illustrate the five bases of DNA and RNA. Examine the images and explain why these
are called “nitrogenous bases.” How are the purines different from the pyrimidines? How is one purine
or pyrimidine different from another, e.g., adenine from guanine? How is a nucleoside different from a
nucleotide?
The purines have a double ring structure with a six-membered ring fused to a five-membered ring.
Pyrimidines are smaller in size; they have a single six-membered ring structure.
The sugar is deoxyribose in DNA and ribose in RNA. The carbon atoms of the five-carbon sugar are
numbered 1’, 2’, 3’, 4’, and 5' (1' is read as “one prime”). The phosphate, which makes DNA and RNA acidic,
is connected to the 5' carbon of the sugar by the formation of an ester linkage between phosphoric acid
and the 5'-OH group (an ester is an acid + an alcohol). In DNA nucleotides, the 3' carbon of the sugar
deoxyribose is attached to a hydroxyl (OH) group. In RNA nucleotides, the 2' carbon of the sugar ribose
also contains a hydroxyl group. The base is attached to the l'carbon of the sugar.
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Chapter 14 | DNA Structure and Function
The nucleotides combine with each other to produce phosphodiester bonds. The phosphate residue attached
to the 5' carbon of the sugar of one nucleotide forms a second ester linkage with the hydroxyl group of the 3'
carbon of the sugar of the next nucleotide, thereby forming a 5'-3' phosphodiester bond, in a polynucleotide, one
end of the chain has a free 5' phosphate, and the other end has a free 3'-OH. These are called the 5' and 3' ends
of the chain.
In the 1950s, Francis Crick and James Watson worked together to determine the structure of DNA at the
University of Cambridge, England. Other scientists like Linus Pauling and Maurice Wilkins were also actively
exploring this field. Pauling previously had discovered the secondary structure of proteins using X-ray
crystallography. In Wilkins’ lab, researcher Rosalind Franklin was using X-ray diffraction methods to understand
the structure of DNA. Watson and Crick were able to piece together the puzzle of the DNA molecule on the
basis of Franklin's data because Crick had also studied X-ray diffraction (Figure 14.6). In 1962, James Watson,
Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine. Unfortunately, by then Franklin
had died, and Nobel prizes are not awarded posthumously.
(a) (b)
Figure 14.6 The work of pioneering scientists (a) James Watson, Francis Crick, and Maclyn McCarty led to our present
day understanding of DNA. Scientist Rosalind Franklin discovered (b) the X-ray diffraction pattern of DNA, which
helped to elucidate its double-helix structure, (credit a: modification of work by Marjorie McCarty, Public Library of
Science)
Watson and Crick proposed that DNA is made up of two strands that are twisted around each other to form a
right-handed helix. Base pairing takes place between a purine and pyrimidine on opposite strands, so that A
pairs with T, and G pairs with C (suggested by Chargaff's Rules). Thus, adenine and thymine are complementary
base pairs, and cytosine and guanine are also complementary base pairs. The base pairs are stabilized by
hydrogen bonds: adenine and thymine form two hydrogen bonds and cytosine and guanine form three hydrogen
bonds. The two strands are anti-parallel in nature; that is, the 3' end of one strand faces the 5' end of the other
strand. The sugar and phosphate of the nucleotides form the backbone of the structure, whereas the nitrogenous
bases are stacked inside, like the rungs of a ladder. Each base pair is separated from the next base pair by a
distance of 0.34 nm, and each turn of the helix measures 3.4 nm. Therefore, 10 base pairs are present per turn
of the helix. The diameter of the DNA double-helix is 2 nm, and it is uniform throughout. Only the pairing between
a purine and pyrimidine and the antiparallel orientation of the two DNA strands can explain the uniform diameter.
The twisting of the two strands around each other results in the formation of uniformly spaced major and minor
grooves (Figure 14.7).
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Chapter 14 | DNA Structure and Function
385
Figure 14.7 DNA has (a) a double helix structure and (b) phosphodiester bonds; the dotted lines between Thymine
and Adenine and Guanine and Cytosine represent hydrogen bonds. The (c) major and minor grooves are binding sites
for DNA binding proteins during processes such as transcription (the copying of RNA from DNA) and replication.
DNA Sequencing Techniques
Until the 1990s, the sequencing of DNA (reading the sequence of DNA) was a relatively expensive and long
process. Using radiolabeled nucleotides also compounded the problem through safety concerns. With currently
available technology and automated machines, the process is cheaper, safer, and can be completed in a matter
of hours. Fred Sanger developed the sequencing method used for the human genome sequencing project, which
is widely used today (Figure 14.8).
LINK
T a
LEARNING
Visit this site (http:// 0 penstaxc 0 llege. 0 rg/l/DNA_sequencing) to watch a video explaining the DNA
sequence-reading technique that resulted from Sanger’s work.
The sequencing method is known as the dideoxy chain termination method. The method is based on the use
of chain terminators, the dideoxynucleotides (ddNTPs). The ddNTPSs differ from the deoxynucleotides by the
lack of a free 3' OH group on the five-carbon sugar. If a ddNTP is added to a growing DNA strand, the chain
cannot be extended any further because the free 3' OH group needed to add another nucleotide is not available.
By using a predetermined ratio of deoxyribonucleotides to dideoxynucleotides, it is possible to generate DNA
fragments of different sizes.
Dye-labeled dideoxynucleotides are used to
generate DNA fragments of different lengths
Figure 14.8 In Frederick Sanger's dideoxy chain termination method, dye-labeled dideoxynucleotides are used to
generate DNA fragments that terminate at different points. The DNA is separated by capillary electrophoresis (not
defined) on the basis of size, and from the order of fragments formed, the DNA sequence can be read. The DNA
sequence readout is shown on an electropherogram (not defined) that is generated by a laser scanner.
The DNA sample to be sequenced is denatured (separated into two strands by heating it to high temperatures).
The DNA is divided into four tubes in which a primer, DNA polymerase, and all four nucleoside triphosphates
(A, T, G, and C) are added, in addition, limited quantities of one of the four dideoxynucleoside triphosphates
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(ddCTP, ddATP, ddGTP, and ddTTP) are added to each tube respectively. The tubes are labeled as A, T, G, and
C according to the ddNTP added. For detection purposes, each of the four dideoxynucleotides carries a different
fluorescent label. Chain elongation continues until a fluorescent dideoxy nucleotide is incorporated, after which
no further elongation takes place. After the reaction is over, electrophoresis is performed. Even a difference in
length of a single base can be detected. The sequence is read from a laser scanner that detects the fluorescent
marker of each fragment. For his work on DNA sequencing, Sanger received a Nobel Prize in Chemistry in 1980.
LINK TQ LEARNING
Sanger’s genome sequencing has led to a race to sequence human genomes at rapid speed and low cost,
often referred to as the $1000-in-one-day sequence. Learn more by selecting the Sequencing at Speed
animation here (http:// 0 penstaxc 0 llege. 0 rg/l/DNA and_genomes) .
Gel electrophoresis is a technique used to separate DNA fragments of different sizes. Usually the gel is
made of a chemical called agarose (a polysaccharide polymer extracted from seaweed that is high in galactose
residues). Agarose powder is added to a buffer and heated. After cooling, the gel solution is poured into a casting
tray. Once the gel has solidified, the DNA is loaded on the gel and electric current is applied. The DNA has a
net negative charge and moves from the negative electrode toward the positive electrode. The electric current is
applied for sufficient time to let the DNA separate according to size; the smallest fragments will be farthest from
the well (where the DNA was loaded), and the heavier molecular weight fragments will be closest to the well.
Once the DNA is separated, the gel is stained with a DNA-specific dye for viewing it (Figure 14.9).
Figure 14.9 DNA can be separated on the basis of size using gel electrophoresis, (credit: James Jacob, Tompkins
Cortland Community College)
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V /
Neanderthal Genome: How Are We Related?
The first draft sequence of the Neanderthal genome was recently published by Richard E. Green et al, in
2010. Neanderthals are the closest ancestors of present-day humans. They were known to have lived
in Europe and Western Asia (and now, perhaps, in Northern Africa) before they disappeared from fossil
records approximately 30,000 years ago. Green’s team studied almost 40,000-year-old fossil remains that
were selected from sites across the world. Extremely sophisticated means of sample preparation and DNA
sequencing were employed because of the fragile nature of the bones and heavy microbial contamination.
In their study, the scientists were able to sequence some four billion base pairs. The Neanderthal sequence
was compared with that of present-day humans from across the world. After comparing the sequences, the
researchers found that the Neanderthal genome had 2 to 3 percent greater similarity to people living outside
Africa than to people in Africa. While current theories have suggested that all present-day humans can
be traced to a small ancestral population in Africa, the data from the Neanderthal genome suggest some
interbreeding between Neanderthals and early modern humans.
Green and his colleagues also discovered DNA segments among people in Europe and Asia that are
more similar to Neanderthal sequences than to other contemporary human sequences. Another interesting
observation was that Neanderthals are as closely related to people from Papua New Guinea as to those
from China or France. This is surprising because Neanderthal fossil remains have been located only
in Europe and West Asia. Most likely, genetic exchange took place between Neanderthals and modern
humans as modern humans emerged out of Africa, before the divergence of Europeans, East Asians, and
Papua New Guineans.
Several genes seem to have undergone changes from Neanderthals during the evolution of present-
day humans. These genes are involved in cranial structure, metabolism, skin morphology, and cognitive
development. One of the genes that is of particular interest is RUNX2, which is different in modern day
humans and Neanderthals. This gene is responsible for the prominent frontal bone, bell-shaped rib cage,
and dental differences seen in Neanderthals. It is speculated that an evolutionary change in RUNX2 was
important in the origin of modern-day humans, and this affected the cranium and the upper body.
Watch Svante Paabo’s talk (http:// 0 penstaxc 0 llege. 0 rg/l/neanderthal) explaining the Neanderthal genome
research at the 2011 annual TED (Technology, Entertainment, Design) conference.
DNA Packaging in Cells
Prokaryotes are much simpler than eukaryotes in many of their features (Figure 14.10). Most prokaryotes
contain a single, circular chromosome that is found in an area of the cytoplasm called the nucleoid region.
1. Richard E. Green et al., “A Draft Sequence of the Neandertal Genome,” Science 328 (2010): 710-22.
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Chapter 14 | DNA Structure and Function
CONNECTION
Nucleolus
Chromatir
Nucleus
Nucleoid
(folded
chromosome)
Eukaryote
Prokaryote
Figure 14.10 A eukaryote contains a well-defined nucleus, whereas in prokaryotes, the chromosome lies in the
cytoplasm in an area called the nucleoid.
In eukaryotic cells, DNA and RNA synthesis occur in a separate compartment from protein synthesis.
In prokaryotic cells, both processes occur together. What advantages might there be to separating the
processes? What advantages might there be to having them occur together?
The size of the genome in one of the most well-studied prokaryotes, E.coli, is 4.6 million base pairs
(approximately 1.1 mm, if cut and stretched out). So how does this fit inside a small bacterial cell? The DNA
is twisted by what is known as supercoiling. Supercoiling suggests that DNA is either “under-wound” (less than
one turn of the helix per 10 base pairs) or “over-wound” (more than 1 turn per 10 base pairs) from its normal
relaxed state. Some proteins are known to be involved in the supercoiling; other proteins and enzymes such as
DNA gyrase help in maintaining the supercoiled structure.
Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing
strategy to fit their DNA inside the nucleus (Figure 14.11). At the most basic level, DNA is wrapped around
proteins known as histones to form structures called nucleosomes. The histones are evolutionarily conserved
proteins that are rich in basic amino acids and form an octamer composed of two molecules of each of four
different histones. The DNA (remember, it is negatively charged because of the phosphate groups) is wrapped
tightly around the histone core. This nucleosome is linked to the next one with the help of a linker DNA. This is
also known as the “beads on a string" structure. With the help of a fifth histone, a string of nucleosomes is further
compacted into a 30-nm fiber, which is the diameter of the structure. Metaphase chromosomes are even further
condensed by association with scaffolding proteins. At the metaphase stage, the chromosomes are at their most
compact, approximately 700 nm in width.
In interphase, eukaryotic chromosomes have two distinct regions that can be distinguished by staining. The
tightly packaged region is known as heterochromatin, and the less dense region is known as euchromatin.
Heterochromatin usually contains genes that are not expressed, and is found in the regions of the centromere
and telomeres. The euchromatin usually contains genes that are transcribed, with DNA packaged around
nucleosomes but not further compacted.
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Chapter 14 | DNA Structure and Function
389
Organization of Eukaryotic Chromosomes
DNA double
helix
1 ^
L i
DNA wrapped
around histone
L |
Nucleosomes
coiled into a
chromatin
fiber
1
1 W2W i
Further
condensation
of chromatin
iitt
Duplicated
chromosome
Figure 14.11 These figures illustrate the compaction of the eukaryotic chromosome.
14.3 | Basics of DNA Replication
By the end of this section, you will be able to do the following:
• Explain how the structure of DNA reveals the replication process
• Describe the Meselson and Stahl experiments
The elucidation of the structure of the double helix provided a hint as to how DNA divides and makes copies of
itself. In their 1953 paper, Watson and Crick penned an incredible understatement: "It has not escaped our notice
that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic
material." With specific base pairs, the sequence of one DNA strand can be predicted from its complement.
The double-helix model suggests that the two strands of the double helix separate during replication, and each
strand serves as a template from which the new complementary strand is copied. What was not clear was how
the replication took place. There were three models suggested (Figure 14.12): conservative, semi-conservative,
and dispersive.
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Chapter 14 | DNA Structure and Function
Figure 14.12 The three suggested models of DNA replication. Gray indicates the original DNA strands, and blue
indicates newly synthesized DNA.
In conservative replication, the parental DNA remains together, and the newly formed daughter strands are
together. The semi-conservative method suggests that each of the two parental DNA strands acts as a template
for new DNA to be synthesized; after replication, each double-stranded DNA includes one parental or “old”
strand and one “new” strand. In the dispersive model, both copies of DNA have double-stranded segments of
parental DNA and newly synthesized DNA interspersed.
Meselson and Stahl were interested in understanding how DNA replicates. They grew E. coli for several
generations in a medium containing a “heavy” isotope of nitrogen ( 15 N), which gets incorporated into nitrogenous
bases, and eventually into the DNA (Figure 14.13).
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Chapter 14 | DNA Structure and Function
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0)
D
Figure 14.13 Meselson and Stahl experimented with E. coli grown first in heavy nitrogen ( 15 N) then in 14 N. DNA grown
in 15 N (red band) is heavier than DNA grown in 14 N (orange band), and sediments to a lower level in cesium chloride
solution in an ultracentrifuge. When DNA grown in 15 N is switched to media containing 14 N, after one round of cell
division the DNA sediments halfway between the 15 N and 14 N levels, indicating that it now contains fifty percent 14 N. In
subsequent cell divisions, an increasing amount of DNA contains 14 N only. These data support the semi-conservative
replication model, (credit: modification of work by Mariana Ruiz Villareal)
The E. coli culture was then placed into medium containing 14 N and allowed to grow for several generations.
After each of the first few generations, the cells were harvested and the DNA was isolated, then centrifuged
at high speeds in an ultracentrifuge. During the centrifugation, the DNA was loaded into a gradient (typically a
solution of salt such as cesium chloride or sucrose) and spun at high speeds of 50,000 to 60,000 rpm. Under
these circumstances, the DNA will form a band according to its buoyant density, the density within the gradient
at which it floats. DNA grown in 15 N will form a band at a higher density position (i.e., farther down the centrifuge
tube) than that grown in 14 N. Meselson and Stahl noted that after one generation of growth in 14 N after they
had been shifted from 15 N, the single band observed was intermediate in position in between DNA of cells
grown exclusively in 15 N and 14 N. This suggested either a semi-conservative or dispersive mode of replication.
The DNA harvested from cells grown for two generations in 14 N formed two bands: one DNA band was at the
intermediate position between 15 N and 14 N, and the other corresponded to the band of 14 N DNA. These results
could only be explained if DNA replicates in a semi-conservative manner. And for this reason, therefore, the
other two models were ruled out.
During DNA replication, each of the two strands that make up the double helix serves as a template from which
new strands are copied. The new strands will be complementary to the parental or “old” strands. When two
daughter DNA copies are formed, they have the same sequence and are divided equally into the two daughter
cells.
LINK
T a
LEARNING
Click through this tutorial (http://openstaxcollege.Org/l/DNA_replicatio2) on DNA replication.
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Chapter 14 | DNA Structure and Function
14.4 | DNA Replication in Prokaryotes
By the end of this section, you will be able to do the following:
• Explain the process of DNA replication in prokaryotes
• Discuss the role of different enzymes and proteins in supporting this process
DNA replication has been well studied in prokaryotes primarily because of the small size of the genome
and because of the large variety of mutants that are available. E. coli has 4.6 million base pairs in a single
circular chromosome and all of it gets replicated in approximately 42 minutes, starting from a single site along
the chromosome and proceeding around the circle in both directions. This means that approximately 1000
nucleotides are added per second. Thus, the process is quite rapid and occurs without many mistakes.
DNA replication employs a large number of structural proteins and enzymes, each of which plays a critical role
during the process. One of the key players is the enzyme DNA polymerase, also known as DNA pol, which adds
nucleotides one-by-one to the growing DNA chain that is complementary to the template strand. The addition
of nucleotides requires energy; this energy is obtained from the nucleoside triphosphates ATP, GTP, TTP and
CTP. Like ATP, the other NTPs (nucleoside triphosphates) are high-energy molecules that can serve both as
the source of DNA nucleotides and the source of energy to drive the polymerization. When the bond between
the phosphates is “broken,” the energy released is used to form the phosphodiester bond between the incoming
nucleotide and the growing chain. In prokaryotes, three main types of polymerases are known: DNA pol I, DNA
pol II, and DNA pol III. It is now known that DNA pol III is the enzyme required for DNA synthesis; DNA pol I is
an important accessory enzyme in DNA replication, and along with DNA pol II, is primarily required for repair.
How does the replication machinery know where to begin? It turns out that there are specific nucleotide
sequences called origins of replication where replication begins. In E. coii, which has a single origin of replication
on its one chromosome (as do most prokaryotes), this origin of replication is approximately 245 base pairs long
and is rich in AT sequences. The origin of replication is recognized by certain proteins that bind to this site. An
enzyme called helicase unwinds the DNA by breaking the hydrogen bonds between the nitrogenous base pairs.
ATP hydrolysis is required for this process. As the DNA opens up, Y-shaped structures called replication forks
are formed. Two replication forks are formed at the origin of replication and these get extended bi-directionally as
replication proceeds. Single-strand binding proteins coat the single strands of DNA near the replication fork
to prevent the single-stranded DNA from winding back into a double helix.
DNA polymerase has two important restrictions: it is able to add nucleotides only in the 5' to 3' direction (a
new DNA strand can be only extended in this direction). It also requires a free 3'-OH group to which it can
add nucleotides by forming a phosphodiester bond between the 3'-OH end and the 5' phosphate of the next
nucleotide. This essentially means that it cannot add nucleotides if a free 3'-OH group is not available. Then how
does it add the first nucleotide? The problem is solved with the help of a primer that provides the free 3'-OH
end. Another enzyme, RNA primase, synthesizes an RNA segment that is about five to ten nucleotides long
and complementary to the template DNA. Because this sequence primes the DNA synthesis, it is appropriately
called the primer. DNA polymerase can now extend this RNA primer, adding nucleotides one-by-one that are
complementary to the template strand (Figure 14.14).
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Chapter 14 | DNA Structure and Function
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visual
a CONNECTION
DNA polymerase I DNA polymerase
Lagging
strand
Leading
strand
Primase RNA primer
Topoisomerase
Single-strand
DNA polymerase III binding protein
Figure 14.14 A replication fork is formed when helicase separates the DNA strands at the origin of replication.
The DNA tends to become more highly coiled ahead of the replication fork. Topoisomerase breaks and reforms
DNA’s phosphate backbone ahead of the replication fork, thereby relieving the pressure that results from this
“supercoiling." Single-strand binding proteins bind to the single-stranded DNA to prevent the helix from re-forming.
Primase synthesizes an RNA primer. DNA polymerase III uses this primer to synthesize the daughter DNA strand.
On the leading strand, DNA is synthesized continuously, whereas on the lagging strand, DNA is synthesized in
short stretches called Okazaki fragments. DNA polymerase I replaces the RNA primer with DNA. DNA ligase seals
the gaps between the Okazaki fragments, joining the fragments into a single DNA molecule, (credit: modification
of work by Mariana Ruiz Villareal)
Question: You isolate a cell strain in which the joining of Okazaki fragments is impaired and suspect that
a mutation has occurred in an enzyme found at the replication fork. Which enzyme is most likely to be
mutated?
The replication fork moves at the rate of 1000 nucleotides per second. Topoisomerase prevents the over-winding
of the DNA double helix ahead of the replication fork as the DNA is opening up; it does so by causing temporary
nicks in the DNA helix and then resealing it. Because DNA polymerase can only extend in the 5' to 3' direction,
and because the DNA double helix is antiparallel , there is a slight problem at the replication fork. The two
template DNA strands have opposing orientations: one strand is in the 5' to 3' direction and the other is oriented
in the 3' to 5' direction. Only one new DNA strand, the one that is complementary to the 3' to 5' parental DNA
strand, can be synthesized continuously towards the replication fork. This continuously synthesized strand is
known as the leading strand. The other strand, complementary to the 5' to 3' parental DNA, is extended away
from the replication fork, in small fragments known as Okazaki fragments, each requiring a primer to start the
synthesis. New primer segments are laid down in the direction of the replication fork, but each pointing away
from it. (Okazaki fragments are named after the Japanese scientist who first discovered them. The strand with
the Okazaki fragments is known as the lagging strand.)
The leading strand can be extended from a single primer, whereas the lagging strand needs a new primer for
each of the short Okazaki fragments. The overall direction of the lagging strand will be 3' to 5', and that of the
leading strand 5' to 3'. A protein called the sliding clamp holds the DNA polymerase in place as it continues
to add nucleotides. The sliding clamp is a ring-shaped protein that binds to the DNA and holds the polymerase
in place. As synthesis proceeds, the RNA primers are replaced by DNA. The primers are removed by the
exonuclease activity of DNA pol I, which uses DNA behind the RNA as its own primer and fills in the gaps left by
removal of the RNA nucleotides by the addition of DNA nucleotides. The nicks that remain between the newly
synthesized DNA (that replaced the RNA primer) and the previously synthesized DNA are sealed by the enzyme
DNA ligase, which catalyzes the formation of phosphodiester linkages between the 3'-OH end of one nucleotide
and the 5' phosphate end of the other fragment.
Once the chromosome has been completely replicated, the two DNA copies move into two different cells during
cell division.
The process of DNA replication can be summarized as follows:
1. DNA unwinds at the origin of replication.
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2. Helicase opens up the DNA-forming replication forks; these are extended bidirectionally.
3. Single-strand binding proteins coat the DNA around the replication fork to prevent rewinding of the DNA.
4. Topoisomerase binds at the region ahead of the replication fork to prevent supercoiling.
5. Primase synthesizes RNA primers complementary to the DNA strand.
6. DNA polymerase III starts adding nucleotides to the 3'-OH end of the primer.
7. Elongation of both the lagging and the leading strand continues.
8. RNA primers are removed by exonuclease activity.
9. Gaps are filled by DNA pol I by adding dNTPs.
10. The gap between the two DNA fragments is sealed by DNA ligase, which helps in the formation of
phosphodiester bonds.
Table 14.1 summarizes the enzymes involved in prokaryotic DNA replication and the functions of each.
Prokaryotic DNA Replication: Enzymes and Their Function
Enzyme/protein
Specific Function
DNA pol 1
Removes RNA primer and replaces it with newly synthesized DNA
DNA pol III
Main enzyme that adds nucleotides in the 5'-3' direction
Helicase
Opens the DNA helix by breaking hydrogen bonds between the nitrogenous
bases
Ligase
Seals the gaps between the Okazaki fragments to create one continuous DNA
strand
Primase
Synthesizes RNA primers needed to start replication
Sliding Clamp
Helps to hold the DNA polymerase in place when nucleotides are being added
Topoisomerase
Helps relieve the strain on DNA when unwinding by causing breaks, and then
resealing the DNA
Single-strand binding
proteins (SSB)
Binds to single-stranded DNA to prevent DNA from rewinding back.
Table 14.1
LINK TQ LEARNING
Review the full process of DNA replication here (http:// 0 penstaxc 0 llege. 0 rg/l/replicati 0 n_DNA)
14.5 | DNA Replication in Eukaryotes
By the end of this section, you will be able to do the following:
• Discuss the similarities and differences between DNA replication in eukaryotes and prokaryotes
• State the role of telomerase in DNA replication
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Chapter 14 | DNA Structure and Function
395
Eukaryotic genomes are much more complex and larger in size than prokaryotic genomes. Eukaryotes also
have a number of different linear chromosomes. The human genome has 3 billion base pairs per haploid set of
chromosomes, and 6 billion base pairs are replicated during the S phase of the cell cycle. There are multiple
origins of replication on each eukaryotic chromosome; humans can have up to 100,000 origins of replication
across the genome. The rate of replication is approximately 100 nucleotides per second, much slower than
prokaryotic replication. In yeast, which is a eukaryote, special sequences known as autonomously replicating
sequences (ARS) are found on the chromosomes. These are equivalent to the origin of replication in E. coli.
The number of DNA polymerases in eukaryotes is much more than in prokaryotes: 14 are known, of which five
are known to have major roles during replication and have been well studied. They are known as pol a, pol /3,
pol y, pol 5, and pol e.
The essential steps of replication are the same as in prokaryotes. Before replication can start, the DNA has to be
made available as a template. Eukaryotic DNA is bound to basic proteins known as histones to form structures
called nucleosomes. Histones must be removed and then replaced during the replication process, which helps
to account for the lower replication rate in eukaryotes. The chromatin (the complex between DNA and proteins)
may undergo some chemical modifications, so that the DNA may be able to slide off the proteins or be accessible
to the enzymes of the DNA replication machinery. At the origin of replication, a pre-replication complex is made
with other initiator proteins. Helicase and other proteins are then recruited to start the replication process (Table
14.2).
Difference between Prokaryotic and Eukaryotic Replication
Property
Prokaryotes
Eukaryotes
Origin of replication
Single
Multiple
Rate of replication
1000 nucleotides/s
50 to 100 nucleotides/s
DNA polymerase types
5
14
Telomerase
Not present
Present
RNA primer removal
DNA pol 1
RNase H
Strand elongation
DNA pol III
Pol a, pol 5, pol £
Sliding clamp
Sliding clamp
PCNA
Table 14.2
A helicase using the energy from ATP hydrolysis opens up the DNA helix. Replication forks are formed at each
replication origin as the DNA unwinds. The opening of the double helix causes over-winding, or supercoiling, in
the DNA ahead of the replication fork. These are resolved with the action of topoisomerases. Primers are formed
by the enzyme primase, and using the primer, DNA pol can start synthesis. Three major DNA polymerases are
then involved: a, 5 and £. DNA pol a adds a short (20 to 30 nucleotides) DNA fragment to the RNA primer on both
strands, and then hands off to a second polymerase. While the leading strand is continuously synthesized by the
enzyme pol 5, the lagging strand is synthesized by pol e. A sliding clamp protein known as PCNA (proliferating
cell nuclear antigen) holds the DNA pol in place so that it does not slide off the DNA. As pol 5 runs into the primer
RNA on the lagging strand, it displaces it from the DNA template. The displaced primer RNA is then removed by
RNase H (AKA flap endonuclease) and replaced with DNA nucleotides. The Okazaki fragments in the lagging
strand are joined after the replacement of the RNA primers with DNA. The gaps that remain are sealed by DNA
ligase, which forms the phosphodiester bond.
Telomere replication
Unlike prokaryotic chromosomes, eukaryotic chromosomes are linear. As you’ve learned, the enzyme DNA pol
can add nucleotides only in the 5' to 3' direction. In the leading strand, synthesis continues until the end of the
chromosome is reached. On the lagging strand, DNA is synthesized in short stretches, each of which is initiated
by a separate primer. When the replication fork reaches the end of the linear chromosome, there is no way to
replace the primer on the 5’ end of the lagging strand. The DNA at the ends of the chromosome thus remains
unpaired, and over time these ends, called telomeres, may get progressively shorter as cells continue to divide.
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Chapter 14 | DNA Structure and Function
Telomeres comprise repetitive sequences that code for no particular gene. In humans, a six-base-pair sequence,
TTAGGG, is repeated 100 to 1000 times in the telomere regions. In a way, these telomeres protect the genes
from getting deleted as cells continue to divide. The telomeres are added to the ends of chromosomes by
a separate enzyme, telomerase (Figure 14.16), whose discovery helped in the understanding of how these
repetitive chromosome ends are maintained. The telomerase enzyme contains a catalytic part and a built-in
RNA template. It attaches to the end of the chromosome, and DNA nucleotides complementary to the RNA
template are added on the 3' end of the DNA strand. Once the 3' end of the lagging strand template is sufficiently
elongated, DNA polymerase can add the nucleotides complementary to the ends of the chromosomes. Thus,
the ends of the chromosomes are replicated.
s' i i i i i i i i i i i i i i i T
CCATGCATTGGTTAG
5 , CAAUCCCAAUC
telomerase
Telomerase has an associated RNA that complements
the 3' overhang at the end of the chromosome.
1
CCATGCATTGGTTAGGGTTAG
GGTAC
3' I I I I I 5'
CAAUCCCAAUC
telomerase
The RNA template is used to synthesize die complementary
strand.
1
CCATGCAT T GGTTACGG T TAG
I I I I I > 1 T
GGTAC
3' I—L
CAAUCCCAAUC
telomerase
'telomerase shifts, and the process is repeated.
5' TTTTTT1.I I I I I H . . 3'
CCATGCATTGGTTAGGGTTAGGGTTAG
GGTAC AATCCCAAT
3/ 5' .. ^L-L
Primasc and DNA polymerase synthesize the complementary
strand.
Figure 14.15 The ends of linear chromosomes are maintained by the action of the telomerase enzyme.
Telomerase is typically active in germ cells and adult stem cells. It is not active in adult somatic cells. For their
discovery of telomerase and its action, Elizabeth Blackburn, Carol W. Greider, and Jack W. Szostak (Figure
14.16) received the Nobel Prize for Medicine and Physiology in 2009.
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Chapter 14 | DNA Structure and Function
397
Figure 14.16 Elizabeth Blackburn, 2009 Nobel Laureate, is one of the scientists who discovered how telomerase
works, (credit: US Embassy Sweden)
Telomerase and Aging
Cells that undergo cell division continue to have their telomeres shortened because most somatic cells do not
make telomerase. This essentially means that telomere shortening is associated with aging. With the advent
of modern medicine, preventative health care, and healthier lifestyles, the human life span has increased, and
there is an increasing demand for people to look younger and have a better quality of life as they grow older.
In 2010, scientists found that telomerase can reverse some age-related conditions in mice. This may have
potential in regenerative medicine. Telomerase-deficient mice were used in these studies; these mice have
tissue atrophy, stem cell depletion, organ system failure, and impaired tissue injury responses. Telomerase
reactivation in these mice caused extension of telomeres, reduced DNA damage, reversed neurodegeneration,
and improved the function of the testes, spleen, and intestines. Thus, telomere reactivation may have potential
for treating age-related diseases in humans.
Cancer is characterized by uncontrolled cell division of abnormal cells. The cells accumulate mutations,
proliferate uncontrollably, and can migrate to different parts of the body through a process called metastasis.
Scientists have observed that cancerous cells have considerably shortened telomeres and that telomerase is
active in these cells. Interestingly, only after the telomeres were shortened in the cancer cells did the telomerase
become active. If the action of telomerase in these cells can be inhibited by drugs during cancer therapy, then
the cancerous cells could potentially be stopped from further division.
14.6 | DNA Repair
By the end of this section, you will be able to do the following:
• Discuss the different types of mutations in DNA
• Explain DNA repair mechanisms
DNA replication is a highly accurate process, but mistakes can occasionally occur, such as a DNA polymerase
inserting a wrong base. Uncorrected mistakes may sometimes lead to serious consequences, such as cancer.
Repair mechanisms correct the mistakes. In rare cases, mistakes are not corrected, leading to mutations; in
other cases, repair enzymes are themselves mutated or defective.
Most of the mistakes during DNA replication are promptly corrected by the proofreading ability of DNA
polymerase itself. (Figure 14.17). In proofreading, the DNA pol reads the newly added base before adding
the next one, so a correction can be made. The polymerase checks whether the newly added base has paired
correctly with the base in the template strand. If it is the right base, the next nucleotide is added. If an incorrect
base has been added, the enzyme makes a cut at the phosphodiester bond and releases the wrong nucleotide.
This is performed by the 3' exonuclease action of DNA pol. Once the incorrect nucleotide has been removed, it
2. Jaskelioff et al., “Telomerase reactivation reverses tissue degeneration in aged telomerase-deficient mice,” Nature 469 (2011): 102-7.
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Chapter 14 | DNA Structure and Function
can be replaced by the correct one.
3 '
s i i i i i i i i i i i i i r j
GGTTAGCCGATTCA
CCAATCGGCT AGT AGACGTCATG
3'.■■■■■■. .
DNA polymerase
Figure 14.17 Proofreading by DNA polymerase corrects errors during replication.
Some errors are not corrected during replication, but are instead corrected after replication is completed; this
type of repair is known as mismatch repair (Figure 14.18). Specific repair enzymes recognize the mispaired
nucleotide and excise part of the strand that contains it; the excised region is then resynthesized. If the mismatch
remains uncorrected, it may lead to more permanent damage when the mismatched DNA is replicated. How do
mismatch repair enzymes recognize which of the two bases is the incorrect one? In E. coli, after replication, the
nitrogenous base adenine acquires a methyl group; the parental DNA strand will have methyl groups, whereas
the newly synthesized strand lacks them. Thus, DNA polymerase is able to remove the wrongly incorporated
bases from the newly synthesized, non-methylated strand, in eukaryotes, the mechanism is not very well
understood, but it is believed to involve recognition of unsealed nicks in the new strand, as well as a short¬
term continuing association of some of the replication proteins with the new daughter strand after replication has
completed.
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Chapter 14 | DNA Structure and Function
399
Figure 14.18 In mismatch repair, the incorrectly added base is detected after replication. The mismatch repair proteins
detect this base and remove it from the newly synthesized strand by nuclease action. The gap is now filled with the
correctly paired base.
Another type of repair mechanism, nucleotide excision repair, is similar to mismatch repair, except that it is
used to remove damaged bases rather than mismatched ones. The repair enzymes replace abnormal bases by
making a cut on both the 3' and 5' ends of the damaged base (Figure 14.19). The segment of DNA is removed
and replaced with the correctly paired nucleotides by the action of DNA pol. Once the bases are filled in, the
remaining gap is sealed with a phosphodiester linkage catalyzed by DNA ligase. This repair mechanism is often
employed when UV exposure causes the formation of pyrimidine dimers.
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Chapter 14 | DNA Structure and Function
C A
T -
r
1
C
1 1
T
G T
A
A
G
A C
l
i—m—m—!—r
C A T T C T
G T A A G A C
I I I I I I I
Figure 14.19 Nucleotide excision repairs thymine dimers. When exposed to UV light, thymines lying adjacent to each
other can form thymine dimers. In normal cells, they are excised and replaced.
A well-studied example of mistakes not being corrected is seen in people suffering from xeroderma pigmentosa
(Figure 14.20). Affected individuals have skin that is highly sensitive to UV rays from the sun. When individuals
are exposed to UV light, pyrimidine dimers, especially those of thymine, are formed; people with xeroderma
pigmentosa are not able to repair the damage. These are not repaired because of a defect in the nucleotide
excision repair enzymes, whereas in normal individuals, the thymine dimers are excised and the defect is
corrected. The thymine dimers distort the structure of the DNA double helix, and this may cause problems during
DNA replication. People with xeroderma pigmentosa may have a higher risk of contracting skin cancer than
those who don't have the condition.
Figure 14.20 Xeroderma pigmentosa is a condition in which thymine dimerization from exposure to UV light is not
repaired. Exposure to sunlight results in skin lesions, (credit: James Halpern et al.)
Errors during DNA replication are not the only reason why mutations arise in DNA. Mutations, variations in the
nucleotide sequence of a genome, can also occur because of damage to DNA. Such mutations may be of two
types: induced or spontaneous. Induced mutations are those that result from an exposure to chemicals, UV
rays, x-rays, or some other environmental agent. Spontaneous mutations occur without any exposure to any
environmental agent; they are a result of natural reactions taking place within the body.
Mutations may have a wide range of effects. Point mutations are those mutations that affect a single base pair.
The most common nucleotide mutations are substitutions, in which one base is replaced by another. These
substitutions can be of two types, either transitions or transversions. Transition substitution refers to a purine
or pyrimidine being replaced by a base of the same kind; for example, a purine such as adenine may be replaced
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Chapter 14 | DNA Structure and Function
401
by the purine guanine. Transversion substitution refers to a purine being replaced by a pyrimidine, or vice
versa; for example, cytosine, a pyrimidine, is replaced by adenine, a purine. Some point mutations are not
expressed; these are known as silent mutations. Silent mutations are usually due to a substitution in the third
base of a codon, which often represents the same amino acid as the original codon. Other point mutations
can result in the replacement of one amino acid by another, which may alter the function of the protein. Point
mutations that generate a stop codon can terminate a protein early.
Some mutations can result in an increased number of copies of the same codon. These are called trinucleotide
repeat expansions and result in repeated regions of the same amino acid. Mutations can also be the result of
the addition of a base, known as an insertion, or the removal of a base, also known as deletion. If an insertion or
deletion results in the alteration of the translational reading frame (a frameshift mutation), the resultant protein
is usually nonfunctional. Sometimes a piece of DNA from one chromosome may get translocated to another
chromosome or to another region of the same chromosome; this is also known as translocation. These mutation
types are shown in Figure 14.21.
visual
CONNECTION
Point Mutations
Silent: has no effect on the protein sequence
TTTTTTTTTTTT
AGCGT ACCCT ACi
Ser Val Pro Tyr
I I I I I I I I I I I I
AGCGTTCCCT AC
Ser Val Pro Tyr
Missense: results in an amino acid substitution
i i i i i i i i i i i i
AGCGT ACCCTAC
I l l l l I I l l l I l
AGCGT AACCT AC
Ser Val Pro Tyr
Ser Val Thr Tyr
Nonsense: substitutes a stop codon for an amino acid
I I I I I I I I I I I I
AGCGT ACCCT AC
Ser Val Pro Tyr
I I I I I I I I I I I I
A G C GT A CCC TAG
Ser Val Pro Stop
Frameshift Mutations
Insertions or deletions of nucleotides may result in a
shift in the reading frame or insertion of a stop codon.
I I 1" I T l ,, l I I 'I' I I
AGCGCCCT ACTT
I I I I I I I I I I I I
AGCGT ACCCTAC , . . .
Ser Val Pro Tyr Ser Val Leu Leu
Figure 14.21 Mutations can lead to changes in the protein sequence encoded by the DNA.
A frameshift mutation that results in the insertion of three nucleotides is often less deleterious than a
mutation that results in the insertion of one nucleotide. Why?
Mutations in repair genes have been known to cause cancer. Many mutated repair genes have been implicated
in certain forms of pancreatic cancer, colon cancer, and colorectal cancer. Mutations can affect either somatic
cells or germ cells. If many mutations accumulate in a somatic cell, they may lead to problems such as the
uncontrolled cell division observed in cancer. If a mutation takes place in germ cells, the mutation will be passed
on to the next generation, as in the case of hemophilia and xeroderma pigmentosa.
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Chapter 14 | DNA Structure and Function
KEY TERMS
electrophoresis technique used to separate DNA fragments according to size
helicase during replication, this enzyme helps to open up the DNA helix by breaking the hydrogen bonds
induced mutation mutation that results from exposure to chemicals or environmental agents
lagging strand during replication, the strand that is replicated in short fragments and away from the replication
fork
leading strand strand that is synthesized continuously in the 5'-3' direction, which is synthesized in the direction
of the replication fork
ligase enzyme that catalyzes the formation of a phosphodiester linkage between the 3' OH and 5' phosphate
ends of the DNA
mismatch repair type of repair mechanism in which mismatched bases are removed after replication
mutation variation in the nucleotide sequence of a genome
nucleotide excision repair type of DNA repair mechanism in which the wrong base, along with a few
nucleotides upstream or downstream, are removed
Okazaki fragment DNA fragment that is synthesized in short stretches on the lagging strand
point mutation mutation that affects a single base
primase enzyme that synthesizes the RNA primer; the primer is needed for DNA pol to start synthesis of a new
DNA strand
primer short stretch of nucleotides that is required to initiate replication; in the case of replication, the primer has
RNA nucleotides
proofreading function of DNA pol in which it reads the newly added base before adding the next one
replication fork Y-shaped structure formed during initiation of replication
silent mutation mutation that is not expressed
single-strand binding protein during replication, protein that binds to the single-stranded DNA; this helps in
keeping the two strands of DNA apart so that they may serve as templates
sliding clamp ring-shaped protein that holds the DNA pol on the DNA strand
spontaneous mutation mutation that takes place in the cells as a result of chemical reactions taking place
naturally without exposure to any external agent
telomerase enzyme that contains a catalytic part and an inbuilt RNA template; it functions to maintain telomeres
at chromosome ends
telomere DNA at the end of linear chromosomes
topoisomerase enzyme that causes underwinding or overwinding of DNA when DNA replication is taking place
transformation process in which external DNA is taken up by a cell
transition substitution when a purine is replaced with a purine or a pyrimidine is replaced with another
pyrimidine
transversion substitution when a purine is replaced by a pyrimidine or a pyrimidine is replaced by a purine
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Chapter 14 | DNA Structure and Function
403
CHAPTER SUMMARY
14.1 Historical Basis of Modern Understanding
DNA was first isolated from white blood cells by Friedrich Miescher, who called it nuclein because it was
isolated from nuclei. Frederick Griffith's experiments with strains of Streptococcus pneumoniae provided the
first hint that DNA may be the transforming principle. Avery, MacLeod, and McCarty showed that DNA is
required for the transformation of bacteria. Later experiments by Hershey and Chase using bacteriophage T2
proved that DNA is the genetic material. Chargaff found that the ratio of A = T and C = G, and that the
percentage content of A, T, G, and C is different for different species.
14.2 DNA Structure and Sequencing
The currently accepted model of the double-helix structure of DNA was proposed by Watson and Crick. Some
of the salient features are that the two strands that make up the double helix have complementary base
sequences and anti-parallel orientations. Alternating deoxyribose sugars and phosphates form the backbone of
the structure, and the nitrogenous bases are stacked like rungs inside. The diameter of the double helix, 2 nm,
is uniform throughout. A purine always pairs with a pyrimidine; A pairs with T, and G pairs with C. One turn of
the helix has 10 base pairs. Prokaryotes are much simpler than eukaryotes in many of their features. Most
prokaryotes contain a single, circular chromosome, in general, eukaryotic chromosomes contain a linear DNA
molecule packaged into nucleosomes, and have two distinct regions that can be distinguished by staining,
reflecting different states of packaging and compaction.
14.3 Basics of DNA Replication
During cell division, each daughter cell receives a copy of each molecule of DNA by a process known as DNA
replication. The single chromosome of a prokaryote or each chromosome of a eukaryote consists of a single
continuous double helix. The model for DNA replication suggests that the two strands of the double helix
separate during replication, and each strand serves as a template from which the new complementary strand is
copied. In the conservative model of replication, the parental DNA is conserved, and the daughter DNA is
newly synthesized. The semi-conservative model suggests that each of the two parental DNA strands acts as
template for new DNA to be synthesized; after replication, each double-stranded DNA retains the parental or
“old” strand and one “new” strand. The dispersive model suggested that the two copies of the DNA would have
segments of parental DNA and newly synthesized DNA. The Meselson and Stahl experiment supported the
semi-conservative model of replication, in which an entire replicated chromosome consists of one parental
strand and one newly synthesized strand of DNA.
14.4 DNA Replication in Prokaryotes
Replication in prokaryotes starts from a sequence found on the chromosome called the origin of
replication—the point at which the DNA opens up. Helicase opens up the DNA double helix, resulting in the
formation of the replication fork. Single-strand binding proteins bind to the single-stranded DNA near the
replication fork to keep the fork open. Primase synthesizes an RNA primer to initiate synthesis by DNA
polymerase, which can add nucleotides only to the 3' end of a previously synthesized primer strand. Both new
DNA strands grow according to their respective 5'-3' directions. One strand is synthesized continuously in the
direction of the replication fork; this is called the leading strand. The other strand is synthesized in a direction
away from the replication fork, in short stretches of DNA known as Okazaki fragments. This strand is known as
the lagging strand. Once replication is completed, the RNA primers are replaced by DNA nucleotides and the
DNA is sealed with DNA ligase, which creates phosphodiester bonds between the 3'-OH of one end and the 5'
phosphate of the other strand.
14.5 DNA Replication in Eukaryotes
Replication in eukaryotes starts at multiple origins of replication. The mechanism is quite similar to that in
prokaryotes. A primer is required to initiate synthesis, which is then extended by DNA polymerase as it adds
nucleotides one by one to the growing chain. The leading strand is synthesized continuously, whereas the
lagging strand is synthesized in short stretches called Okazaki fragments. The RNA primers are replaced with
DNA nucleotides; the DNA Okazaki fragments are linked into one continuous strand by DNA ligase. The ends
of the chromosomes pose a problem as the primer RNA at the 5’ ends of the DNA cannot be replaced with
DNA, and the chromosome is progressively shortened. Telomerase, an enzyme with an inbuilt RNA template,
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Chapter 14 | DNA Structure and Function
extends the ends by copying the RNA template and extending one strand of the chromosome. DNA
polymerase can then fill in the complementary DNA strand using the regular replication enzymes. In this way,
the ends of the chromosomes are protected.
14.6 DNA Repair
DNA polymerase can make mistakes while adding nucleotides. It edits the DNA by proofreading every newly
added base. Incorrect bases are removed and replaced by the correct base before proceeding with elongation.
Most mistakes are corrected during replication, although when this does not happen, the mismatch repair
mechanism is employed. Mismatch repair enzymes recognize the wrongly incorporated base and excise it from
the DNA, replacing it with the correct base. In yet another type of repair, nucleotide excision repair, a damaged
base is removed along with a few bases on the 5' and 3' end, and these are replaced by copying the template
with the help of DNA polymerase. The ends of the newly synthesized fragment are attached to the rest of the
DNA using DNA ligase, which creates a phosphodiester bond.
Most mistakes are corrected, and if they are not, they may result in a mutation, defined as a permanent change
in the DNA sequence. Mutations can be of many types, such as substitution, deletion, insertion, and
trinucleotide repeat expansions. Mutations in repair genes may lead to serious consequences such as cancer.
Mutations can be induced or may occur spontaneously.
VISUAL CONNECTION QUESTIONS
1. Figure 14.10 In eukaryotic cells, DNA and RNA
synthesis occur in a separate compartment from
protein synthesis. In prokaryotic cells, both processes
occur together. What advantages might there be to
separating the processes? What advantages might
there be to having them occur together?
2. Figure 14.14 You isolate a cell strain in which the
joining of Okazaki fragments is impaired and suspect
REVIEW QUESTIONS
4. If DNA of a particular species was analyzed and it
was found that it contains 27 percent A, what would
be the percentage of C?
a. 27 percent
b. 30 percent
c. 23 percent
d. 54 percent
5. The experiments by Hershey and Chase helped
confirm that DNA was the hereditary material on the
basis of the finding that:
a. radioactive phage were found in the pellet
b. radioactive cells were found in the
supernatant
c. radioactive sulfur was found inside the cell
d. radioactive phosphorus was found in the cell
6. Bacterial transformation is a major concern in
many medical settings. Why might health care
providers be concerned?
that a mutation has occurred in an enzyme found at
the replication fork. Which enzyme is most likely to
be mutated?
3. Figure 14.21 A frameshift mutation that results in
the insertion of three nucleotides is often less
deleterious than a mutation that results in the
insertion of one nucleotide. Why?
a. Pathogenic bacteria could introduce
disease-causing genes in non-pathogenic
bacteria.
b. Antibiotic resistance genes could be
introduced to new bacteria to create
“superbugs.”
c. Bacteriophages could spread DNA encoding
toxins to new bacteria.
d. All of the above.
7. DNA double helix does not have which of the
following?
a. antiparallel configuration
b. complementary base pairing
c. major and minor grooves
d. uracil
8. In eukaryotes, what is the DNA wrapped around?
a. single-stranded binding proteins
b. sliding clamp
c. polymerase
d. histones
9. Meselson and Stahl's experiments proved that
DNA replicates by which mode?
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Chapter 14 | DNA Structure and Function
405
a.
conservative
a.
DNA ligase
b.
semi-conservative
b.
DNA pol li
c.
dispersive
c.
Primase
d.
none of the above
d.
DNA pol 1
10. If the sequence of the 5'-3' strand is AATGCTAC,
then the complementary sequence has the following
sequence:
a. 3'-AAT GCTAC-5'
b. 3'-CATCGTAA-5'
c. 3'-TTACGATG-5'
d. 3'-GTAGCATT-5'
11. How did Meselson and Stahl support Watson and
Crick’s double-helix model?
a. They demonstrated that each strand serves
as a template for synthesizinq a new strand
of DNA.
b. They showed that the DNA strands break
and recombine without losing genetic
material.
c. They proved that DNA maintains a double¬
helix structure while undergoing semi¬
conservative replication.
d. They demonstrated that conservative
replication maintains the complementary
base pairing of each DNA helix.
12. Which of the following components is not
involved during the formation of the replication fork?
a. single-strand binding proteins
b. helicase
c. origin of replication
d. ligase
13. Which of the following does the enzyme primase
synthesize?
a. DNA primer
b. RNA primer
c. Okazaki fragments
d. phosphodiester linkage
14. In which direction does DNA replication take
place?
a. 5'-3'
b. 3-5'
c. 5'
d. 3'
15. A scientist randomly mutates the DNA of a
bacterium. She then sequences the bacterium’s
daughter cells, and finds that the daughters have
many errors in their replicated DNA. The parent
bacterium likely acquired a mutation in which
enzyme?
16. The ends of the linear chromosomes are
maintained by
a. helicase
b. primase
c. DNA pol
d. telomerase
17. Which of the following is not a true statement
comparing prokaryotic and eukaryotic DNA
replication?
a. Both eukaryotic and prokaryotic DNA
polymerases build off RNA primers made by
primase.
b. Eukaryotic DNA replication requires multiple
replication forks, while prokaryotic
replication uses a single origin to rapidly
replicate the entire genome.
c. DNA replication always occurs in the
nucleus.
d. Eukaryotic DNA replication involves more
polymerases than prokaryotic replication.
18. During proofreading, which of the following
enzymes reads the DNA?
a. primase
b. topoisomerase
c. DNA pol
d. helicase
19. The initial mechanism for repairing nucleotide
errors in DNA is_.
a. mismatch repair
b. DNA polymerase proofreading
c. nucleotide excision repair
d. thymine dimers
20. A scientist creates fruit fly larvae with a mutation
that eliminates the exonuclease function of DNA pol
III. Which prediction about the mutational load in the
adult fruit flies is most likely to be correct?
a. The adults with the DNA pol III mutation will
have significantly more mutations than
average.
b. The adults with the DNA pol III mutation will
have slightly more mutations than average.
c. The adults with the DNA pol III mutation will
have the same number of mutations as
average.
d. The adults with the DNA pol III mutation will
have fewer mutations than average.
CRITICAL THINKING QUESTIONS
21. Explain Griffith's transformation experiments.
What did he conclude from them?
22. Why were radioactive sulfur and phosphorous
used to label bacteriophage in Hershey and Chase's
experiments?
23. When Chargaff was performing his experiments,
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Chapter 14 | DNA Structure and Function
the tetranucleotide hypothesis, which stated that
DNA was composed of GACT nucleotide repeats,
was the most widely accepted view of DNA’s
composition. How did Chargaff disprove this
hypothesis?
24. Provide a brief summary of the Sanger
sequencing method.
25. Describe the structure and complementary base
pairing of DNA.
26. Prokaryotes have a single circular chromosome
while eukaryotes have linear chromosomes. Describe
one advantage and one disadvantage to the
eukaryotic genome packaging compared to the
prokaryotes.
27. How did the scientific community learn that DNA
replication takes place in a semi-conservative
fashion?
28. Imagine the Meselson and Stahl experiments had
supported conservative replication instead of semi¬
conservative replication. What results would you
predict to observe after two rounds of replication? Be
specific regarding percent distributions of DNA
incorporating 15 N and 14 N in the gradient.
29. DNA replication is bidirectional and
discontinuous; explain your understanding of those
concepts.
30. What are Okazaki fragments and how they are
formed?
31. If the rate of replication in a particular prokaryote
is 900 nucleotides per second, how long would it take
1.2 million base pair genomes to make two copies?
32. Explain the events taking place at the replication
fork. If the gene for helicase is mutated, what part of
replication will be affected?
33. What is the role of a primer in DNA replication?
What would happen if you forgot to add a primer in a
tube containing the reaction mix for a DNA
sequencing reaction?
34. Quinolone antibiotics treat bacterial infections by
blocking the activity of topoisomerase. Why does this
treatment work? Explain what occurs at the
molecular level.
35. How do the linear chromosomes in eukaryotes
ensure that its ends are replicated completely?
36. What is the consequence of mutation of a
mismatch repair enzyme? How will this affect the
function of a gene?
37. An adult with a history of tanning has his genome
sequenced. The beginning of a protein-coding region
of his DNA reads ATGGGGATATGGCAT. If the
protein-coding region of a healthy adult reads
ATGGGGATATGAGCAT, identify the site and type of
mutation.
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Chapter 15 | Genes and Proteins
407
15 | GENES AND
PROTEINS
Figure 15.1 Genes, which are carried on (a) chromosomes, are linearly organized instructions for making the RNA
and protein molecules that are necessary for all of the processes of life. The (b) interleukin-2 protein and (c) alpha-2u-
globulin protein are just two examples of the array of different molecular structures that are encoded by genes, (credit
“chromosome: National Human Genome Research Institute; credit “interleukin-2”: Ramin Herati/Created from PDB
1M47 and rendered with Pymol; credit “alpha-2u-globulin": Darren Logan/rendered with AISMIG)
Chapter Outline
15.1: The Genetic Code
15.2: Prokaryotic Transcription
15.3: Eukaryotic Transcription
15.4: RNA Processing in Eukaryotes
15.5: Ribosomes and Protein Synthesis
Introduction
Since the rediscovery of Mendel’s work in 1900, the definition of the gene has progressed from an abstract unit
of heredity to a tangible molecular entity capable of replication, expression, and mutation (Figure 15.1). Genes
are composed of DNA and are linearly arranged on chromosomes. Genes specify the sequences of amino acids,
which are the building blocks of proteins. In turn, proteins are responsible for orchestrating nearly every function
of the cell. Both genes and the proteins they encode are absolutely essential to life as we know it.
15.1 1 The Genetic Code
By the end of this section, you will be able to do the following:
• Explain the “central dogma” of DNA-protein synthesis
• Describe the genetic code and how the nucleotide sequence prescribes the amino acid and the protein
sequence
The cellular process of transcription generates messenger RNA (mRNA), a mobile molecular copy of one or
more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA template on ribosomes converts
nucleotide-based genetic information into a protein product. That is the central dogma of DNA-protein synthesis.
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Chapter 15 | Genes and Proteins
Protein sequences consist of 20 commonly occurring amino acids; therefore, it can be said that the protein
alphabet consists of 20 “letters" (Figure 15.2). Different amino acids have different chemistries (such as acidic
versus basic, or polar and nonpolar) and different structural constraints. Variation in amino acid sequence is
responsible for the enormous variation in protein structure and function.
AMINO ACID
COO"
COO-
coo-
+ 1
+ 1
+ 1
H 3 N —C — H
H3N-C-H
H3N-C-H
w
a
H
CH 3
CH
/ \
3
0
ch 3 ch 3
U)
tr
Glycine
Alanine
Valine
0
coo-
coo-
COO“
«3
.C
+ 1
+ 1
+
H3N-C-H
H3N-C-H
H 3 N —C-H
cd
1
1
1
(C
CH 2
ch 2
H-C-CH3
O
1
1
1
CH
ch 2
ch 2
O
/ \
1
Z
CH3CH3
s
ch 3
ch 3
Leucine
Methionine
Isoleucine
coo-
COO-
coo-
+ 1
+ 1
+ 1
H3N-C-H
H3N-C-H
h 3 n-c-h
ch 2 oh
H-C-OH
ch 2
w
1
1
ch 3
SH
O
CT)
Serine
Threonine
Cysteine
a :
■0
0)
COO”
COO”
COO“
a>
(5
.c
1 / H
cr
+ 1
H3N-C-H
|
♦ 1
H3N-C-H
|
0
/ N
3
h 2 n ch 2
1 1
ch 2
ch 2
«C
O
h 2 c—ch 2
1
c
1
ch 2
/ ^
h 2 n 0
1
c
✓ ^
h 2 n 0
Proline
Asparagine
Glutamine
Figure 15.2 Structures of the 20 amino acids found in proteins are shown. Each amino acid is composed of an amino
group (NH 3 ), a carboxyl group (COO"), and a side chain (blue). The side chain may be nonpolar, polar, or charged,
as well as large or small. It is the variety of amino acid side chains that gives rise to the incredible variation of protein
structure and function.
The Central Dogma: DNA Encodes RNA; RNA Encodes Protein
The flow of genetic information in cells from DNA to mRNA to protein is described by the central dogma (Figure
15.3), which states that genes specify the sequence of mRNAs, which in turn specify the sequence of amino
acids making up all proteins. The decoding of one molecule to another is performed by specific proteins and
RNAs. Because the information stored in DNA is so central to cellular function, it makes intuitive sense that
the cell would make mRNA copies of this information for protein synthesis, while keeping the DNA itself intact
and protected. The copying of DNA to RNA is relatively straightforward, with one nucleotide being added to
the mRNA strand for every nucleotide read in the DNA strand. The translation to protein is a bit more complex
because three mRNA nucleotides correspond to one amino acid in the polypeptide sequence. However, the
translation to protein is still systematic and colinear, such that nucleotides 1 to 3 correspond to amino acid 1,
nucleotides 4 to 6 correspond to amino acid 2, and so on.
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Chapter 15 | Genes and Proteins
409
l l l l l l l l '.^7
T
--r . . .
q[kC cact cat
i i i i i i i 3'
TACCACGTA
*3'
Tacggcgtjagac ^c^m,\^ ^a T /.T,cat
DNA RNA polymerase
RNA processing
Primary RNA transcript
Exon 1 Intron Exon 2 Intron Exon 3
Spliced RNA
Exon 1 Exon 2 Exon 3
AAAAAAA
Translation
— polypeptide chain
Met
vj/xC AAA G °^
AUGUUUCGA
Ribosome
Figure 15.3 Instructions on DNA are transcribed onto messenger RNA. Ribosomes are able to read the genetic
information inscribed on a strand of messenger RNA and use this information to string amino acids together into a
protein.
The Genetic Code Is Degenerate and Universal
Each amino acid is defined by a three-nucleotide sequence called the triplet codon. Given the different numbers
of “letters” in the mRNA and protein “alphabets," scientists theorized that single amino acids must be represented
by combinations of nucleotides. Nucleotide doublets would not be sufficient to specify every amino acid because
there are only 16 possible two-nucleotide combinations (4 2 ). In contrast, there are 64 possible nucleotide triplets
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Chapter 15 | Genes and Proteins
(4 3 ), which is far more than the number of amino acids. Scientists theorized that amino acids were encoded by
nucleotide triplets and that the genetic code was “degenerate.” in other words, a given amino acid could be
encoded by more than one nucleotide triplet. This was later confirmed experimentally: Francis Crick and Sydney
Brenner used the chemical mutagen proflavin to insert one, two, or three nucleotides into the gene of a virus.
When one or two nucleotides were inserted, the normal proteins were not produced. When three nucleotides
were inserted, the protein was synthesized and functional. This demonstrated that the amino acids must be
specified by groups of three nucleotides. These nucleotide triplets are called codons. The insertion of one or two
nucleotides completely changed the triplet reading frame, thereby altering the message for every subsequent
amino acid (Figure 15.5). Though insertion of three nucleotides caused an extra amino acid to be inserted during
translation, the integrity of the rest of the protein was maintained.
Scientists painstakingly solved the genetic code by translating synthetic mRNAs in vitro and sequencing the
proteins they specified (Figure 15.4).
Second letter
CD
ts
CD
c/)
LL
U
C
A
G
u
uuin
uuc.
UUA1
UUG.
■Phe
■Leu
UCU'
ucc
UCA
UCG,
■Ser
UAU1
UAC.
UAA
UAG
-Tyr
stop
Stop
UGU1
UGC J
UGA
UGG
-Cys
Stop
Trp
U
C
A
G
c
CULT
cue
CUA
CUG,
■Leu
ecu'
CCC
CCA
CCG
■Pro
CAU 'I
CAC.
CAA1
CAG.
-His
-Gin
CGU'
CGC
CGA
CGG,
■Arg
U
C
A
G
A
AUU '
AUC
AUA
AUG
■lie
Met
ACU '
ACC
ACA
ACG.
■Thr
AAU1
AACJ
AAA 1
AAG.
■Asn
-Lys
AGU'
AGC -
AGA'
AGG „
-Ser
-Arg
U
C
A
G
G
GUU'
GUC
GUA
GUG
■Val
GCU'
GCC
GCA
GCG
■Ala
GAU1
GAC.
GAA1
GAG.
-Asp
-Glu
GGU'
GGC
GGA
GGG
-Gly
U
C
A
G
Figure 15.4 This figure shows the genetic code for translating each nucleotide triplet in mRNA into an amino acid or a
termination signal in a protein, (credit: modification of work by NIH)
In addition to codons that instruct the addition of a specific amino acid to a polypeptide chain, three of the 64
codons terminate protein synthesis and release the polypeptide from the translation machinery. These triplets
are called nonsense codons, or stop codons. Another codon, AUG, also has a special function. In addition to
specifying the amino acid methionine, it also serves as the start codon to initiate translation. The reading frame
for translation is set by the AUG start codon near the 5' end of the mRNA. Following the start codon, the mRNA
is read in groups of three until a stop codon is encountered.
The arrangement of the coding table reveals the structure of the code. There are sixteen "blocks" of codons,
each specified by the first and second nucleotides of the codons within the block, e.g., the "AC*" block that
corresponds to the amino acid threonine (Thr). Some blocks are divided into a pyrimidine half, in which the codon
ends with U or C, and a purine half, in which the codon ends with A or G. Some amino acids get a whole block
of four codons, like alanine (Ala), threonine (Thr) and proline (Pro). Some get the pyrimidine half of their block,
like histidine (His) and asparagine (Asn). Others get the purine half of their block, like glutamate (Glu) and lysine
(Lys). Note that some amino acids get a block and a half-block for a total of six codons.
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Chapter 15 | Genes and Proteins
411
The specification of a single amino acid by multiple similar codons is called "degeneracy." Degeneracy is
believed to be a cellular mechanism to reduce the negative impact of random mutations. Codons that specify
the same amino acid typically only differ by one nucleotide. In addition, amino acids with chemically similar side
chains are encoded by similar codons. For example, aspartate (Asp) and glutamate (Glu), which occupy the GA*
block, are both negatively charged. This nuance of the genetic code ensures that a single-nucleotide substitution
mutation might specify the same amino acid but have no effect or specify a similar amino acid, preventing the
protein from being rendered completely nonfunctional.
The genetic code is nearly universal. With a few minor exceptions, virtually all species use the same genetic
code for protein synthesis. Conservation of codons means that a purified mRNA encoding the globin protein in
horses could be transferred to a tulip cell, and the tulip would synthesize horse globin. That there is only one
genetic code is powerful evidence that all of life on Earth shares a common origin, especially considering that
there are about 10 possible combinations of 20 amino acids and 64 triplet codons.
LINK TQ LEARNING
Transcribe a gene and translate it to protein using complementary pairing and the genetic code at this site
(http:// 0 penstaxc 0 llege. 0 rg/l/create_pr 0 tein) .
Frameshift Mutations
I 1 1 1 1 1 1 1 1 1 1 1 T" 1 ■ rrr 1 mT H"!~i ^ '
AGCGUACCCUAC AGCGCCCUACUU
Ser
Val
Pro
Tyr
Ser Ala Leu Leu
Figure 15.5 The deletion of two nucleotides shifts the reading frame of an mRNA and changes the entire protein
message, creating a nonfunctional protein or terminating protein synthesis altogether.
412
Chapter 15 | Genes and Proteins
scientific method CONNECTION
Which Has More DNA: A Kiwi or a Strawberry?
Figure 15.6 Do you think that a kiwi or a strawberry has more DNA per fruit? (credit “kiwi": "Kelbv'VFlickr; credit:
“strawberry”: Alisdair McDiarmid)
Question: Would a kiwi and strawberry that are approximately the same size (Figure 15.6) also have
approximately the same amount of DNA?
Background: Genes are carried on chromosomes and are made of DNA. All mammals are diploid, meaning
they have two copies of each chromosome. However, not all plants are diploid. The common strawberry
is octoploid (8n) and the cultivated kiwi is hexaploid (6n). Research the total number of chromosomes in
the cells of each of these fruits and think about how this might correspond to the amount of DNA in these
fruits’ cell nuclei. What other factors might contribute to the total amount of DNA in a single fruit? Read
about the technique of DNA isolation to understand how each step in the isolation protocol helps liberate
and precipitate DNA.
Hypothesis: Hypothesize whether you would be able to detect a difference in DNA quantity from similarly
sized strawberries and kiwis. Which fruit do you think would yield more DNA?
Test your hypothesis: Isolate the DNA from a strawberry and a kiwi that are similarly sized. Perform the
experiment in at least triplicate for each fruit
1. Prepare a bottle of DNA extraction buffer from 900 mL water, 50 ml_ dish detergent, and two teaspoons
of table salt. Mix by inversion (cap it and turn it upside down a few times).
2. Grind a strawberry and a kiwi by hand in a plastic bag, or using a mortar and pestle, or with a metal
bowl and the end of a blunt instrument. Grind for at least two minutes per fruit.
3. Add 10 mL of the DNA extraction buffer to each fruit, and mix well for at least one minute.
4. Remove cellular debris by filtering each fruit mixture through cheesecloth or porous cloth and into a
funnel placed in a test tube or an appropriate container.
5. Pour ice-cold ethanol or isopropanol (rubbing alcohol) into the test tube. You should observe white,
precipitated DNA.
6 . Gather the DNA from each fruit by winding it around separate glass rods.
Record your observations: Because you are not quantitatively measuring DNA volume, you can record for
each trial whether the two fruits produced the same or different amounts of DNA as observed by eye. If one
or the other fruit produced noticeably more DNA, record this as well. Determine whether your observations
are consistent with several pieces of each fruit.
Analyze your data: Did you notice an obvious difference in the amount of DNA produced by each fruit?
Were your results reproducible?
Draw a conclusion: Given what you know about the number of chromosomes in each fruit, can you
conclude that chromosome number necessarily correlates to DNA amount? Can you identify any drawbacks
to this procedure? If you had access to a laboratory, how could you standardize your comparison and make
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Chapter 15 | Genes and Proteins
413
it more quantitative?
15.2 | Prokaryotic Transcription
By the end of this section, you will be able to do the following:
• List the different steps in prokaryotic transcription
• Discuss the role of promoters in prokaryotic transcription
• Describe how and when transcription is terminated
The prokaryotes, which include Bacteria and Archaea, are mostly single-celled organisms that, by definition, lack
membrane-bound nuclei and other organelles. A bacterial chromosome is a closed circle that, unlike eukaryotic
chromosomes, is not organized around histone proteins. The central region of the cell in which prokaryotic DNA
resides is called the nucleoid region. In addition, prokaryotes often have abundant plasmids, which are shorter,
circular DNA molecules that may only contain one or a few genes. Plasmids can be transferred independently
of the bacterial chromosome during cell division and often carry traits such as those involved with antibiotic
resistance.
Transcription in prokaryotes (and in eukaryotes) requires the DNA double helix to partially unwind in the region of
mRN A synthesis. The region of unwinding is called a transcription bubble. Transcription always proceeds from
the same DNA strand for each gene, which is called the template strand. The mRNA product is complementary
to the template strand and is almost identical to the other DNA strand, called the nontemplate strand, or the
coding strand. The only nucleotide difference is that in mRNA, all of the T nucleotides are replaced with U
nucleotides (Figure 15.7). In an RNA double helix, A can bind U via two hydrogen bonds, just as in A-T pairing
in a DNA double helix.
Transcription
5'
O,
C'>c —RNA
Q,
5' , .
ATGCCGCAA 7 ANr x ( ' “ ‘
■ 3'
G
TACCACGTA
3'
TAC^CGTTAGAC^ yc^^^.'^QG ^.ATGTGCAT
r^ £TOCGfGAGT^
RNA polymerase
DNA
Figure 15.7 Messenger RNA is a copy of protein-coding information in the coding strand of DNA, with the substitution
of U in the RNA for T in the coding sequence. However, new RNA nucleotides base pair with the nucleotides of the
template strand. RNA is synthesized in its 5'-3' direction, using the enzyme RNA polymerase. As the template is read,
the DNA unwinds ahead of the polymerase and then rewinds behind it.
The nucleotide pair in the DNA double helix that corresponds to the site from which the first 5' mRNA nucleotide
is transcribed is called the +1 site, or the initiation site. Nucleotides preceding the initiation site are denoted with
a and are designated upstream nucleotides. Conversely, nucleotides following the initiation site are denoted
with “+” numbering and are called downstream nucleotides.
Initiation of Transcription in Prokaryotes
Prokaryotes do not have membrane-enclosed nuclei. Therefore, the processes of transcription, translation, and
414
Chapter 15 | Genes and Proteins
mRNA degradation can all occur simultaneously. The intracellular level of a bacterial protein can quickly be
amplified by multiple transcription and translation events that occur concurrently on the same DNA template.
Prokaryotic genomes are very compact, and prokaryotic transcripts often cover more than one gene or cistron (a
coding sequence for a single protein). Polycistronic mRNAs are then translated to produce more than one kind
of protein.
Our discussion here will exemplify transcription by describing this process in Escherichia coli, a well-studied
eubacterial species. Although some differences exist between transcription in E. coli and transcription in
archaea, an understanding of E. coli transcription can be applied to virtually all bacterial species.
Prokaryotic RNA Polymerase
Prokaryotes use the same RNA polymerase to transcribe all of their genes. In E. coli, the polymerase is
composed of five polypeptide subunits, two of which are identical. Four of these subunits, denoted a, a, /3, and
/3', comprise the polymerase core enzyme. These subunits assemble every time a gene is transcribed, and they
disassemble once transcription is complete. Each subunit has a unique role; the two cr-subunits are necessary to
assemble the polymerase on the DNA; the /3-subunit binds to the ribonucleoside triphosphate that will become
part of the nascent mRNA molecule; and the /3' subunit binds the DNA template strand. The fifth subunit, o, is
involved only in transcription initiation. It confers transcriptional specificity such that the polymerase begins to
synthesize mRNA from an appropriate initiation site. Without o, the core enzyme would transcribe from random
sites and would produce mRNA molecules that specified protein gibberish. The polymerase comprised of all five
subunits is called the holoenzyme.
Prokaryotic Promoters
A promoter is a DNA sequence onto which the transcription machinery, including RNA polymerase, binds
and initiates transcription. In most cases, promoters exist upstream of the genes they regulate. The specific
sequence of a promoter is very important because it determines whether the corresponding gene is transcribed
all the time, some of the time, or infrequently. Although promoters vary among prokaryotic genomes, a few
elements are evolutionarily conserved in many species. At the -10 and -35 regions upstream of the initiation
site, there are two promoter consensus sequences, or regions that are similar across all promoters and
across various bacterial species (Figure 15.8). The -10 sequence, called the -10 region, has the consensus
sequence TATAAT. The -35 sequence has the consensus sequence TTGACA. These consensus sequences
are recognized and bound by o. Once this interaction is made, the subunits of the core enzyme bind to the
site. The A-T-rich -10 region facilitates unwinding of the DNA template, and several phosphodiester bonds are
made. The transcription initiation phase ends with the production of abortive transcripts, which are polymers of
approximately 10 nucleotides that are made and released.
Promoter
TTGACG
TATAAT
-►
-35 Region
-10 Region
+1 Transcription
a Factor
start site
RNA Polymerase
Figure 15.8 The o subunit of prokaryotic RNA polymerase recognizes consensus sequences found in the promoter
region upstream of the transcription start site. The o subunit dissociates from the polymerase after transcription has
been initiated.
LINK TQ LEARNING
View this MolecularMovies animation (http:// 0 penstaxc 0 llege. 0 rg/l/transcripti 0 n) to see the first part of
transcription and the base sequence repetition of the TATA box.
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Chapter 15 | Genes and Proteins
415
Elongation and Termination in Prokaryotes
The transcription elongation phase begins with the release of the o subunit from the polymerase. The
dissociation of a allows the core enzyme to proceed along the DNA template, synthesizing mRNA in the 5' to 3'
direction at a rate of approximately 40 nucleotides per second. As elongation proceeds, the DNA is continuously
unwound ahead of the core enzyme and rewound behind it. The base pairing between DNA and RNA is not
stable enough to maintain the stability of the mRNA synthesis components. Instead, the RNA polymerase acts
as a stable linker between the DNA template and the nascent RNA strands to ensure that elongation is not
interrupted prematurely.
Prokaryotic Termination Signals
Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA
template and liberate the newly made mRNA. Depending on the gene being transcribed, there are two kinds
of termination signals. One is protein-based and the other is RNA-based. Rho-dependent termination is
controlled by the rho protein, which tracks along behind the polymerase on the growing mRNA chain. Near the
end of the gene, the polymerase encounters a run of G nucleotides on the DNA template and it stalls. As a result,
the rho protein collides with the polymerase. The interaction with rho releases the mRNA from the transcription
bubble.
Rho-independent termination is controlled by specific sequences in the DNA template strand. As the
polymerase nears the end of the gene being transcribed, it encounters a region rich in C-G nucleotides. The
mRNA folds back on itself, and the complementary C-G nucleotides bind together. The result is a stable hairpin
that causes the polymerase to stall as soon as it begins to transcribe a region rich in A-T nucleotides. The
complementary U-A region of the mRNA transcript forms only a weak interaction with the template DNA. This,
coupled with the stalled polymerase, induces enough instability for the core enzyme to break away and liberate
the new mRNA transcript.
Upon termination, the process of transcription is complete. By the time termination occurs, the prokaryotic
transcript would already have been used to begin synthesis of numerous copies of the encoded protein
because these processes can occur concurrently. The unification of transcription, translation, and even mRNA
degradation is possible because all of these processes occur in the same 5' to 3' direction, and because there
is no membranous compartmentalization in the prokaryotic cell (Figure 15.9). In contrast, the presence of a
nucleus in eukaryotic cells precludes simultaneous transcription and translation.
Figure 15.9 Multiple polymerases can transcribe a single bacterial gene while numerous ribosomes concurrently
translate the mRNA transcripts into polypeptides. In this way, a specific protein can rapidly reach a high concentration
in the bacterial cell.
LINK TQ LEARNING
Visit this BioStudio animation (http://openstaxcollege.Org/l/transcription2) to see the process of
prokaryotic transcription.
416
Chapter 15 | Genes and Proteins
15.3 | Eukaryotic Transcription
By the end of this section, you will be able to do the following:
• List the steps in eukaryotic transcription
• Discuss the role of RNA polymerases in transcription
• Compare and contrast the three RNA polymerases
• Explain the significance of transcription factors
Prokaryotes and eukaryotes perform fundamentally the same process of transcription, with a few key
differences. The most important difference between prokaryote and eukaryote transcription is due to the latter’s
membrane-bound nucleus and organelles. With the genes bound in a nucleus, the eukaryotic cell must be
able to transport its mRNA to the cytoplasm and must protect its mRNA from degrading before it is translated.
Eukaryotes also employ three different polymerases that each transcribe a different subset of genes. Eukaryotic
mRNAs are usually monogenic , meaning that they specify a single protein.
Initiation of Transcription in Eukaryotes
Unlike the prokaryotic polymerase that can bind to a DNA template on its own, eukaryotes require several other
proteins, called transcription factors, to first bind to the promoter region and then to help recruit the appropriate
polymerase.
The Three Eukaryotic RNA Polymerases
The features of eukaryotic mRNA synthesis are markedly more complex than those of prokaryotes, instead of a
single polymerase comprising five subunits, the eukaryotes have three polymerases that are each made up of
10 subunits or more. Each eukaryotic polymerase also requires a distinct set of transcription factors to bring it to
the DNA template.
RNA polymerase I is located in the nucleolus, a specialized nuclear substructure in which ribosomal RNA (rRNA)
is transcribed, processed, and assembled into ribosomes (Table 15.1). The rRNA molecules are considered
structural RNAs because they have a cellular role but are not translated into protein. The rRNAs are components
of the ribosome and are essential to the process of translation. RNA polymerase I synthesizes all of the rRNAs
from the tandemly duplicated set of 18S, 5.8S, and 28S ribosomal genes. (Note that the “S” designation applies
to “Svedberg” units, a nonadditive value that characterizes the speed at which a particle sediments during
centrifugation.)
Locations, Products, and Sensitivities of the Three Eukaryotic RNA Polymerases
RNA
Polymerase
Cellular
Compartment
Product of Transcription
a-Amanitin
Sensitivity
1
Nucleolus
All rRNAs except 5S rRNA
Insensitive
II
Nucleus
All protein-coding nuclear pre-
mRNAs
Extremely sensitive
III
Nucleus
5S rRNA, tRNAs, and small nuclear
RNAs
Moderately sensitive
Table 15.1
RNA polymerase II is located in the nucleus and synthesizes all protein-coding nuclear pre-mRNAs. Eukaryotic
pre-mRNAs undergo extensive processing after transcription but before translation. For clarity, this module’s
discussion of transcription and translation in eukaryotes will use the term “mRNAs” to describe only the mature,
processed molecules that are ready to be translated. RNA polymerase II is responsible for transcribing the
overwhelming majority of eukaryotic genes.
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Chapter 15 | Genes and Proteins
417
RNA polymerase III is also located in the nucleus. This polymerase transcribes a variety of structural RNAs
that includes the 5S pre-rRNA, transfer pre-RNAs (pre-tRNAs), and small nuclear pre- RNAs. The tRNAs
have a critical role in translation; they serve as the “adaptor molecules” between the mRNA template and the
growing polypeptide chain. Small nuclear RNAs have a variety of functions, including “splicing” pre-mRNAs and
regulating transcription factors.
A scientist characterizing a new gene can determine which polymerase transcribes it by testing whether the gene
is expressed in the presence of a-amanitin, an oligopeptide toxin produced by the fly agaric toadstool mushroom
and other species of Amanita. Interestingly, the a-amanitin affects the three polymerases very differently (Table
15.1). RNA polymerase I is completely insensitive to a-amanitin, meaning that the polymerase can transcribe
DNA in vitro in the presence of this poison. RNA polymerase III is moderately sensitive to the toxin. In contrast,
RNA polymerase II is extremely sensitive to a-amanitin. The toxin prevents the enzyme from progressing down
the DNA, and thus inhibits transcription. Knowing the transcribing polymerase can provide clues as to the
general function of the gene being studied. Because RNA polymerase II transcribes the vast majority of genes,
we will focus on this polymerase in our subsequent discussions about eukaryotic transcription factors and
promoters.
RNA Polymerase II Promoters and Transcription Factors
Eukaryotic promoters are much larger and more intricate than prokaryotic promoters. However, both have a
sequence similar to the -10 sequence of prokaryotes. In eukaryotes, this sequence is called the TATA box, and
has the consensus sequence TATAAA on the coding strand. It is located at -25 to -35 bases relative to the
initiation (+1) site (Figure 15.10). This sequence is not identical to the E. coli -10 box, but it conserves the
A-T rich element. The thermostability of A-T bonds is low and this helps the DNA template to locally unwind in
preparation for transcription.
Instead of the simple a factor that helps bind the prokaryotic RNA polymerase to its promoter, eukaryotes
assemble a complex of transcription factors required to recruit RNA polymerase II to a protein coding gene.
Transcription factors that bind to the promoter are called basal transcription factors. These basal factors are all
called TF1I (for Transcription Factor/polymerase II) plus an additional letter (A-J). The core complex is TFIID,
which includes a TATA-binding protein (TBP). The other transcription factors systematically fall into place on the
DNA template, with each one further stabilizing the pre-initiation complex and contributing to the recruitment of
RNA polymerase II.
418
Chapter 15 | Genes and Proteins
visual
CONNECTION
Promoter
JL
1
TATAAA
_.-30
+1
Transcription
start site
TFIID
TBP
TFIID
TBP
TFIIB
X^
I-►
+1
Transcription
start site
|-►
+1
Transcription
start site
TFIIE TFIIF
____ TFIIB
TFIIA TFIID TFIIH >
TBP
+ 1
RNA Polymerase II Transcription
start site
Figure 15.10 A generalized promoter of a gene transcribed by RNA polymerase II is shown. Transcription factors
recognize the promoter. RNA polymerase II then binds and forms the transcription initiation complex. A scientist
splices a eukaryotic promoter in front of a bacterial gene and inserts the gene in a bacterial chromosome. Would
you expect the bacteria to transcribe the gene?
Some eukaryotic promoters also have a conserved CAAT box (GGCCAATCT) at approximately -80. Further
upstream of the TATA box, eukaryotic promoters may also contain one or more GC-rich boxes (GGCG) or
octamer boxes (ATTTGCAT). These elements bind cellular factors that increase the efficiency of transcription
initiation and are often identified in more “active" genes that are constantly being expressed by the cell.
Basal transcription factors are crucial in the formation of a preinitiation complex on the DNA template that
subsequently recruits RNA polymerase II for transcription initiation. The complexity of eukaryotic transcription
does not end with the polymerases and promoters. An army of other transcription factors, which bind to upstream
enhancers and silencers, also help to regulate the frequency with which pre-mRNA is synthesized from a gene.
Enhancers and silencers affect the efficiency of transcription but are not necessary for transcription to proceed.
Promoter Structures for RNA Polymerases I and III
The processes of bringing RNA polymerases I and III to the DNA template involve slightly less complex
collections of transcription factors, but the general theme is the same.
The conserved promoter elements for genes transcribed by polymerases I and III differ from those transcribed
by RNA polymerase II. RNA polymerase I transcribes genes that have two GC-rich promoter sequences in the
-45 to +20 region. These sequences alone are sufficient for transcription initiation to occur, but promoters with
additional sequences in the region from -180 to -105 upstream of the initiation site will further enhance initiation.
Genes that are transcribed by RNA polymerase III have upstream promoters or promoters that occur within the
genes themselves.
Eukaryotic transcription is a tightly regulated process that requires a variety of proteins to interact with each
other and with the DNA strand. Although the process of transcription in eukaryotes involves a greater metabolic
investment than in prokaryotes, it ensures that the cell transcribes precisely the pre-mRNAs that it needs for
protein synthesis.
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Chapter 15 | Genes and Proteins
419
V /
e olution CONNECTION
The Evolution of Promoters
The evolution of genes may be a familiar concept. Mutations can occur in genes during DNA replication, and
the result may or may not be beneficial to the cell. By altering an enzyme, structural protein, or some other
factor, the process of mutation can transform functions or physical features. However, eukaryotic promoters
and other gene regulatory sequences may evolve as well. For instance, consider a gene that, over many
generations, becomes more valuable to the cell. Maybe the gene encodes a structural protein that the cell
needs to synthesize in abundance for a certain function. If this is the case, it would be beneficial to the cell
for that gene’s promoter to recruit transcription factors more efficiently and increase gene expression.
Scientists examining the evolution of promoter sequences have reported varying results. In part, this is
because it is difficult to infer exactly where a eukaryotic promoter begins and ends. Some promoters occur
within genes; others are located very far upstream, or even downstream, of the genes they are regulating.
However, when researchers limited their examination to human core promoter sequences that were defined
experimentally as sequences that bind the preinitiation complex, they found that promoters evolve even
faster than protein-coding genes.
It is still unclear how promoter evolution might correspond to the evolution of humans or other complex
organisms. However, the evolution of a promoter to effectively make more or less of a given gene product
is an intriguing alternative to the evolution of the genes themselves.
Eukaryotic Elongation and Termination
Following the formation of the preinitiation complex, the polymerase is released from the other transcription
factors, and elongation is allowed to proceed as it does in prokaryotes with the polymerase synthesizing
pre-mRNA in the 5' to 3' direction. As discussed previously, RNA polymerase II transcribes the major share
of eukaryotic genes, so in this section we will focus on how this polymerase accomplishes elongation and
termination.
Although the enzymatic process of elongation is essentially the same in eukaryotes and prokaryotes, the DNA
template is considerably more complex. When eukaryotic cells are not dividing, their genes exist as a diffuse
mass of DNA and proteins called chromatin. The DNA is tightly packaged around charged histone proteins at
repeated intervals. These DNA-histone complexes, collectively called nucleosomes, are regularly spaced and
include 146 nucleotides of DNA wound around eight histones like thread around a spool.
For polynucleotide synthesis to occur, the transcription machinery needs to move histones out of the way
every time it encounters a nucleosome. This is accomplished by a special protein complex called FACT, which
stands for “facilitates chromatin transcription This complex pulls histones away from the DNA template as the
polymerase moves along it. Once the pre-mRNA is synthesized, the FACT complex replaces the histones to
recreate the nucleosomes.
The termination of transcription is different for the different polymerases. Unlike in prokaryotes, elongation by
RNA polymerase II in eukaryotes takes place 1,000 to 2,000 nucleotides beyond the end of the gene being
transcribed. This pre-mRNA tail is subsequently removed by cleavage during mRNA processing. On the other
hand, RNA polymerases I and III require termination signals. Genes transcribed by RNA polymerase I contain a
specific 18-nucleotide sequence that is recognized by a termination protein. The process of termination in RNA
polymerase III involves an mRNA hairpin similar to rho-independent termination of transcription in prokaryotes.
1. H Liang et al., “Fast evolution of core promoters in primate genomes,” Molecular Biology and Evolution 25 (2008): 1239-44.
420
Chapter 15 | Genes and Proteins
15.4 | RNA Processing in Eukaryotes
By the end of this section, you will be able to do the following:
• Describe the different steps in RNA processing
• Understand the significance of exons, introns, and splicing for mRNAs
• Explain how tRNAs and rRNAs are processed
After transcription, eukaryotic pre-mRNAs must undergo several processing steps before they can be translated.
Eukaryotic (and prokaryotic) tRNAs and rRNAs also undergo processing before they can function as
components in the protein-synthesis machinery.
mRNA Processing
The eukaryotic pre-mRNA undergoes extensive processing before it is ready to be translated. Eukaryotic
protein-coding sequences are not continuous, as they are in prokaryotes. The coding sequences (exons) are
interrupted by noncoding introns, which must be removed to make a translatable mRNA. The additional steps
involved in eukaryotic mRNA maturation also create a molecule with a much longer half-life than a prokaryotic
mRNA. Eukaryotic mRNAs last for several hours, whereas the typical E. coli mRNA lasts no more than five
seconds.
Pre-mRNAs are first coated in RNA-stabilizing proteins; these protect the pre-mRNA from degradation while it
is processed and exported out of the nucleus. The three most important steps of pre-mRNA processing are the
addition of stabilizing and signaling factors at the 5' and 3' ends of the molecule, and the removal of the introns
(Figure 15.11). In rare cases, the mRNA transcript can be “edited" after it is transcribed.
Primary RNA transcript
Exon 1
Intron
Exon 2
Intron Exon 3
I
RNA processing
Spliced RNA
Exon 1 Exon 2 Exon 3
5’ cap
/
AAAAAAA
Poly-A tail
5' untranslated 3' untranslated
region region
Figure 15.11 Eukaryotic mRNA contains introns that must be spliced out. A 5' cap and 3' poly-A tail are also added.
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Chapter 15 | Genes and Proteins
421
V /
RNA Editing in Trypanosomes
The trypanosomes are a group of protozoa that include the pathogen Trypanosoma brucei, which causes
nagana in cattle and sleeping sickness in humans throughout great areas of Africa (Figure 15.12). The
trypanosome is carried by biting flies in the genus Glossina (commonly called tsetse flies). Trypanosomes,
and virtually all other eukaryotes, have organelles called mitochondria that supply the cell with chemical
energy. Mitochondria are organelles that express their own DNA and are believed to be the remnants
of a symbiotic relationship between a eukaryote and an engulfed prokaryote. The mitochondrial DNA of
trypanosomes exhibit an interesting exception to the central dogma: their pre-mRNAs do not have the
correct information to specify a functional protein. Usually, this is because the mRNA is missing several U
nucleotides. The cell performs an additional RNA processing step called RNA editing to remedy this.
I
10 pm
Figure 15.12 Trypanosoma brucei is the causative agent of sleeping sickness in humans. The mRNAs of this
pathogen must be modified by the addition of nucleotides before protein synthesis can occur, (credit: modification
of work by Torsten Ochsenreiter)
Other genes in the mitochondrial genome encode 40- to 80-nucleotide guide RNAs. One or more of
these molecules interacts by complementary base pairing with some of the nucleotides in the pre-mRNA
transcript. However, the guide RNA has more A nucleotides than the pre-mRNA has U nucleotides with
which to bind. In these regions, the guide RNA loops out. The 3' ends of guide RNAs have a long poly-U tail,
and these U bases are inserted in regions of the pre-mRNA transcript at which the guide RNAs are looped.
This process is entirely mediated by RNA molecules. That is, guide RNAs—rather than proteins—serve as
the catalysts in RNA editing.
RNA editing is not just a phenomenon of trypanosomes. In the mitochondria of some plants, almost all
pre-mRNAs are edited. RNA editing has also been identified in mammals such as rats, rabbits, and even
humans. What could be the evolutionary reason for this additional step in pre-mRNA processing? One
possibility is that the mitochondria, being remnants of ancient prokaryotes, have an equally ancient RNA-
based method for regulating gene expression. In support of this hypothesis, edits made to pre-mRNAs differ
depending on cellular conditions. Although speculative, the process of RNA editing may be a holdover from
a primordial time when RNA molecules, instead of proteins, were responsible for catalyzing reactions.
5' Capping
While the pre-mRNA is still being synthesized, a 7-methylguanosine cap is added to the 5' end of the growing
transcript by a phosphate linkage. This functional group protects the nascent mRNA from degradation. In
addition, factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.
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3' Poly-A Tail
Once elongation is complete, the pre-mRNA is cleaved by an endonuclease between an AAUAAA consensus
sequence and a GU-rich sequence, leaving the AAUAAA sequence on the pre-mRNA. An enzyme called poly-A
polymerase then adds a string of approximately 200 A residues, called the poly-A tail. This modification further
protects the pre-mRNA from degradation and is also the binding site for a protein necessary for exporting the
processed mRNA to the cytoplasm.
Pre-mRNA Splicing
Eukaryotic genes are composed of exons, which correspond to protein-coding sequences (ex-on signifies that
they are expressed), and /'nfervening sequences called introns (/'nf-ron denotes their /nfervening role), which
may be involved in gene regulation but are removed from the pre-mRNA during processing. Intron sequences in
mRNA do not encode functional proteins.
The discovery of introns came as a surprise to researchers in the 1970s who expected that pre-mRNAs would
specify protein sequences without further processing, as they had observed in prokaryotes. The genes of higher
eukaryotes very often contain one or more introns. These regions may correspond to regulatory sequences;
however, the biological significance of having many introns or having very long introns in a gene is unclear. It is
possible that introns slow down gene expression because it takes longer to transcribe pre-mRNAs with lots of
introns. Alternatively, introns may be nonfunctional sequence remnants left over from the fusion of ancient genes
throughout the course of evolution. This is supported by the fact that separate exons often encode separate
protein subunits or domains. For the most part, the sequences of introns can be mutated without ultimately
affecting the protein product.
All of a pre-mRNAs introns must be completely and precisely removed before protein synthesis. If the process
errs by even a single nucleotide, the reading frame of the rejoined exons would shift, and the resulting protein
would be dysfunctional. The process of removing introns and reconnecting exons is called splicing (Figure
15.13). Introns are removed and degraded while the pre-mRNA is still in the nucleus. Splicing occurs by a
sequence-specific mechanism that ensures introns will be removed and exons rejoined with the accuracy and
precision of a single nucleotide. Although the intron itself is noncoding, the beginning and end of each intron is
marked with specific nucleotides: GU at the 5' end and AG at the 3' end of the intron. The splicing of pre-mRNAs
is conducted by complexes of proteins and RNA molecules called spliceosomes.
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Chapter 15 | Genes and Proteins
423
CONNECTION
snRNPs
Intron
Exon 1 GU
AG Exon 2
Spliceosome
Exon 1 Exon 2
Intron
Figure 15.13 Pre-mRNA splicing involves the precise removal of introns from the primary RNA transcript. The
splicing process is catalyzed by protein complexes called spliceosomes that are composed of proteins and RNA
molecules called small nuclear RNAs (snRNAs). Spliceosomes recognize sequences at the 5' and 3' end of the
intron.
Errors in splicing are implicated in cancers and other human diseases. What kinds of mutations might lead
to splicing errors? Think of different possible outcomes if splicing errors occur.
Note that more than 70 individual introns can be present, and each has to undergo the process of splicing—in
addition to 5' capping and the addition of a poly-A tail—just to generate a single, translatable mRNA molecule.
See how introns are removed during RNA splicing at this website (http:// 0 penstaxc 0 llege. 0 rg/l/
RNA_splicing) .
Processing of tRNAs and rRNAs
The tRNAs and rRNAs are structural molecules that have roles in protein synthesis; however, these RNAs
are not themselves translated. Pre-rRNAs are transcribed, processed, and assembled into ribosomes in the
nucleolus. Pre-tRNAs are transcribed and processed in the nucleus and then released into the cytoplasm where
they are linked to free amino acids for protein synthesis.
Most of the tRNAs and rRNAs in eukaryotes and prokaryotes are first transcribed as a long precursor molecule
that spans multiple rRNAs or tRNAs. Enzymes then cleave the precursors into subunits corresponding to each
structural RNA. Some of the bases of pre-rRNAs are methylated ; that is, a -CH3 methyl functional group is
added for stability. Pre-tRNA molecules also undergo methylation. As with pre-mRNAs, subunit excision occurs
in eukaryotic pre-RNAs destined to become tRNAs or rRNAs.
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Mature rRNAs make up approximately 50 percent of each ribosome. Some of a ribosome’s RNA molecules are
purely structural, whereas others have catalytic or binding activities. Mature tRNAs take on a three-dimensional
structure through local regions of base pairing stabilized by intramolecular hydrogen bonding. The tRNA folds
to position the amino acid binding site at one end and the anticodon at the other end (Figure 15.14). The
anticodon is a three-nucleotide sequence in a tRNA that interacts with an mRNA codon through complementary
base pairing.
Figure 15.14 This is a space-filling model of a tRNA molecule that adds the amino acid phenylalanine to a growing
polypeptide chain. The anticodon AAG binds the Codon UUC on the mRNA. The amino acid phenylalanine is attached
to the other end of the tRNA.
15.5 | Ribosomes and Protein Synthesis
By the end of this section, you will be able to do the following:
• Describe the different steps in protein synthesis
• Discuss the role of ribosomes in protein synthesis
The synthesis of proteins consumes more of a cell’s energy than any other metabolic process. In turn, proteins
account for more mass than any other component of living organisms (with the exception of water), and proteins
perform virtually every function of a cell. The process of translation, or protein synthesis, involves the decoding of
an mRNA message into a polypeptide product. Amino acids are covalently strung together by interlinking peptide
bonds in lengths ranging from approximately 50 to more than 1000 amino acid residues. Each individual amino
acid has an amino group (NH 2 ) and a carboxyl (COOH) group. Polypeptides are formed when the amino group
of one amino acid forms an amide (i.e., peptide) bond with the carboxyl group of another amino acid (Figure
15.15). This reaction is catalyzed by ribosomes and generates one water molecule.
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Chapter 15 | Genes and Proteins
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R'
\ I
N —C —C
/
I
H
OH H
R'
\ I *
N —C —C
/ i \
I
H
O
OH
H
H
R’ O H R'
> I II I I
N-C—C-N—C-C
H
I
H
//
\
OH
Peptide Bond
Figure 15.15 A peptide bond links the carboxyl end of one amino acid with the amino end of another, producing one
water molecule during the process. For simplicity in this image, only the functional groups involved in the peptide bond
are shown. The R and R' designations refer to the rest of each amino acid structure.
The Protein Synthesis Machinery
In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation.
The composition of each component may vary across species; for example, ribosomes may consist of different
numbers of rRNAs and polypeptides depending on the organism. However, the general structures and functions
of the protein synthesis machinery are comparable from bacteria to human cells. Translation requires the input
of an mRNA template, ribosomes, tRNAs, and various enzymatic factors. (Note: A ribosome can be thought of
as an enzyme whose amino acid binding sites are specified by mRNA.)
LINK TQ LEARNING
Click through the steps of this PBS interactive (http:// 0 penstaxc 0 llege. 0 rg/l/pr 0 kary_pmtein) to see
protein synthesis in action.
Ribosomes
Even before an mRNA is translated, a cell must invest energy to build each of its ribosomes. In E. coli, there
are between 10,000 and 70,000 ribosomes present in each cell at any given time. A ribosome is a complex
macromolecule composed of structural and catalytic rRNAs, and many distinct polypeptides. In eukaryotes, the
nucleolus is completely specialized for the synthesis and assembly of rRNAs.
Ribosomes exist in the cytoplasm of prokaryotes and in the cytoplasm and rough endoplasmic reticulum of
eukaryotes. Mitochondria and chloroplasts also have their own ribosomes in the matrix and stroma, which look
more similar to prokaryotic ribosomes (and have similar drug sensitivities) than the ribosomes just outside their
outer membranes in the cytoplasm. Ribosomes dissociate into large and small subunits when they are not
synthesizing proteins and reassociate during the initiation of translation. In E. coli, the small subunit is described
as 30S, and the large subunit is 50S, for a total of 70S (recall that Svedberg units are not additive). Mammalian
ribosomes have a small 40S subunit and a large 60S subunit, for a total of 80S. The small subunit is responsible
for binding the mRNA template, whereas the large subunit sequentially binds tRNAs. Each mRNA molecule is
simultaneously translated by many ribosomes, all synthesizing protein in the same direction: reading the mRNA
from 5' to 3' and synthesizing the polypeptide from the N terminus to the C terminus. The complete mRNA/poly-
ribosome structure is called a polysome.
tRNAs
The tRNAs are structural RNA molecules that were transcribed from genes by RNA polymerase III. Depending
on the species, 40 to 60 types of tRNAs exist in the cytoplasm. Transfer RNAs serve as adaptor molecules.
Each tRNA carries a specific amino acid and recognizes one or more of the mRNA codons that define the order
of amino acids in a protein. Aminoacyl-tRNAs bind to the ribosome and add the corresponding amino acid to
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the polypeptide chain. Therefore, tRNAs are the molecules that actually “translate” the language of RNA into the
language of proteins.
Of the 64 possible mRNA codons—or triplet combinations of A, U, G, and C—three specify the termination of
protein synthesis and 61 specify the addition of amino acids to the polypeptide chain. Of these 61, one codon
(AUG) also encodes the initiation of translation. Each tRNA anticodon can base pair with one or more of the
mRNA codons for its amino acid. For instance, if the sequence CUA occurred on an mRNA template in the
proper reading frame, it would bind a leucine tRNA expressing the complementary sequence, GAU. The ability
of some tRNAs to match more than one codon is what gives the genetic code its blocky structure.
As the adaptor molecules of translation, it is surprising that tRNAs can fit so much specificity into such a small
package. Consider that tRNAs need to interact with three factors: 1) they must be recognized by the correct
aminoacyl synthetase (see below); 2) they must be recognized by ribosomes; and 3) they must bind to the
correct sequence in mRNA.
Aminoacyl tRNA Synthetases
The process of pre-tRNA synthesis by RNA polymerase IN only creates the RNA portion of the adaptor molecule.
The corresponding amino acid must be added later, once the tRNA is processed and exported to the cytoplasm.
Through the process of tRNA “charging,” each tRNA molecule is linked to its correct amino acid by one of a
group of enzymes called aminoacyl tRNA synthetases. At least one type of aminoacyl tRNA synthetase exists
for each of the 20 amino acids; the exact number of aminoacyl tRNA synthetases varies by species. These
enzymes first bind and hydrolyze ATP to catalyze a high-energy bond between an amino acid and adenosine
monophosphate (AMP); a pyrophosphate molecule is expelled in this reaction. The activated amino acid is then
transferred to the tRNA, and AMP is released. The term "charging" is appropriate, since the high-energy bond
that attaches an amino acid to its tRNA is later used to drive the formation of the peptide bond. Each tRNA is
named for its amino acid.
The Mechanism of Protein Synthesis
As with mRNA synthesis, protein synthesis can be divided into three phases: initiation, elongation, and
termination. The process of translation is similar in prokaryotes and eukaryotes. Here we’ll explore how
translation occurs in E. coii, a representative prokaryote, and specify any differences between prokaryotic and
eukaryotic translation.
Initiation of Translation
Protein synthesis begins with the formation of an initiation complex, in E. coii, this complex involves the small
30S ribosome, the mRNA template, three initiation factors (IFs; IF-1, IF-2, and IF-3), and a special initiator
tRNA, called tRNA Metf .
In E. coii mRNA, a sequence upstream of the first AUG codon, called the Shine-Dalgarno sequence (AGGAGG),
interacts with the rRNA molecules that compose the ribosome. This interaction anchors the 30S ribosomal
subunit at the correct location on the mRNA template. Guanosine triphosphate (GTP), which is a purine
nucleotide triphosphate, acts as an energy source during translation—both at the start of elongation and during
the ribosome’s translocation. Binding of the mRNA to the 30S ribosome also requires IF-lll.
The initiator tRNA then interacts with the start codon AUG (or rarely, GUG). This tRNA carries the amino
acid methionine, which is formylated after its attachment to the tRNA. The formylation creates a "faux" peptide
bond between the formyl carboxyl group and the amino group of the methionine. Binding of the fMet-tRNA Metf
is mediated by the initiation factor IF-2. The fMet begins every polypeptide chain synthesized by E. coii, but
it is usually removed after translation is complete. When an in-frame AUG is encountered during translation
elongation, a non-formylated methionine is inserted by a regular Met-tRNA Met . After the formation of the initiation
complex, the 30S ribosomal subunit is joined by the 50S subunit to form the translation complex. In eukaryotes,
a similar initiation complex forms, comprising mRNA, the 40S small ribosomal subunit, eukaryotic IFs, and
nucleoside triphosphates (GTP and ATP). The methionine on the charged initiator tRNA, called Met-tRNAi, is not
formylated. However, Met-tRNAi is distinct from other Met-tRNAs in that it can bind IFs.
Instead of depositing at the Shine-Dalgarno sequence, the eukaryotic initiation complex recognizes the
7-methylguanosine cap at the 5' end of the mRNA. A cap-binding protein (CBP) and several other IFs assist the
movement of the ribosome to the 5' cap. Once at the cap, the initiation complex tracks along the mRNA in the 5'
to 3' direction, searching for the AUG start codon. Many eukaryotic mRNAs are translated from the first AUG, but
this is not always the case. According to Kozak’s rules, the nucleotides around the AUG indicate whether it is
the correct start codon. Kozak’s rules state that the following consensus sequence must appear around the AUG
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Chapter 15 | Genes and Proteins
427
of vertebrate genes: 5'-gccRccAUGG-3'. The R (for purine) indicates a site that can be either A or G, but cannot
be C or U. Essentially, the closer the sequence is to this consensus, the higher the efficiency of translation.
Once the appropriate AUG is identified, the other proteins and CBP dissociate, and the 60S subunit binds
to the complex of Met-tRNAi, mRNA, and the 40S subunit. This step completes the initiation of translation in
eukaryotes.
Translation, Elongation, and Termination
in prokaryotes and eukaryotes, the basics of elongation are the same, so we will review elongation from the
perspective of E. coii. When the translation complex is formed, the tRNA binding region of the ribosome consists
of three compartments. The A (aminoacyl) site binds incoming charged aminoacyl tRNAs. The P (peptidyl) site
binds charged tRNAs carrying amino acids that have formed peptide bonds with the growing polypeptide chain
but have not yet dissociated from their corresponding tRNA. The E (exit) site releases dissociated tRNAs so that
they can be recharged with free amino acids. The initiating methionyl-tRNA, however, occupies the P site at the
beginning of the elongation phase of translation in both prokaryotes and eukaryotes.
During translation elongation, the mRNA template provides tRNA binding specificity. As the ribosome moves
along the mRNA, each mRNA codon comes into register, and specific binding with the corresponding charged
tRNA anticodon is ensured. If mRNA were not present in the elongation complex, the ribosome would bind
tRNAs nonspecifically and randomly (?).
Elongation proceeds with charged tRNAs sequentially entering and leaving the ribosome as each new amino
acid is added to the polypeptide chain. Movement of a tRNA from A to P to E site is induced by conformational
changes that advance the ribosome by three bases in the 3' direction. The energy for each step along the
ribosome is donated by elongation factors that hydrolyze GTP GTP energy is required both for the binding of
a new aminoacyl-tRNA to the A site and for its translocation to the P site after formation of the peptide bond.
Peptide bonds form between the amino group of the amino acid attached to the A-site tRNA and the carboxyl
group of the amino acid attached to the P-site tRNA. The formation of each peptide bond is catalyzed by peptidyl
transferase, an RNA-based enzyme that is integrated into the 50S ribosomal subunit. The energy for each
peptide bond formation is derived from the high-energy bond linking each amino acid to its tRNA. After peptide
bond formation, the A-site tRNA that now holds the growing peptide chain moves to the P site, and the P-site
tRNA that is now empty moves to the E site and is expelled from the ribosome (Figure 15.16). Amazingly, the
E. coii translation apparatus takes only 0.05 seconds to add each amino acid, meaning that a 200-amino-acid
protein can be translated in just 10 seconds.
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visual
CONNECTION
mRNA
Large ribosomal
subunit
-tRNA
UAC
AUGUUCCGA
| 3'
0
Met
< r'
p A
UAC
AUGUUCCGA
Phe
Small ribosomal
subunit
E
U A c
P
AAG
A
G C(y
AUGUUCCGA
Figure 15.16 Translation begins when an initiator tRNA anticodon recognizes a start codon on mRNA bound to
a small ribosomal subunit. The large ribosomal subunit joins the small subunit, and a second tRNA is recruited.
As the mRNA moves relative to the ribosome, successive tRNAs move through the ribosome and the polypeptide
chain is formed. Entry of a release factor into the A site terminates translation and the components dissociate.
Many antibiotics inhibit bacterial protein synthesis. For example, tetracycline blocks the A site on the
bacterial ribosome, and chloramphenicol blocks peptidyl transfer. What specific effect would you expect
each of these antibiotics to have on protein synthesis?
Tetracycline would directly affect:
a. tRNA binding to the ribosome
b. ribosome assembly
c. growth of the protein chain
Chloramphenicol would directly affect:
a. tRNA binding to the ribosome
b. ribosome assembly
c. growth of the protein chain
Termination of translation occurs when a nonsense codon (UAA, UAG, or UGA) is encountered. Upon aligning
with the A site, these nonsense codons are recognized by protein release factors that resemble tRNAs. The
releasing factors in both prokaryotes and eukaryotes instruct peptidyl transferase to add a water molecule to
the carboxyl end of the P-site amino acid. This reaction forces the P-site amino acid to detach from its tRNA,
and the newly made protein is released. The small and large ribosomal subunits dissociate from the mRNA and
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Chapter 15 | Genes and Proteins
429
from each other; they are recruited almost immediately into another translation initiation complex. After many
ribosomes have completed translation, the mRNA is degraded so the nucleotides can be reused in another
transcription reaction.
Protein Folding, Modification, and Targeting
During and after translation, individual amino acids may be chemically modified, signal sequences appended,
and the new protein “folded” into a distinct three-dimensional structure as a result of intramolecular interactions.
A signal sequence is a short sequence at the amino end of a protein that directs it to a specific cellular
compartment. These sequences can be thought of as the protein’s “train ticket” to its ultimate destination, and
are recognized by signal-recognition proteins that act as conductors. For instance, a specific signal sequence
terminus will direct a protein to the mitochondria or chloroplasts (in plants). Once the protein reaches its cellular
destination, the signal sequence is usually clipped off.
Many proteins fold spontaneously, but some proteins require helper molecules, called chaperones, to prevent
them from aggregating during the complicated process of folding. Even if a protein is properly specified by
its corresponding mRNA, it could take on a completely dysfunctional shape if abnormal temperature or pH
conditions prevent it from folding correctly.
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KEY TERMS
7-methylguanosine cap modification added to the 5' end of pre-mRNAs to protect mRNA from degradation and
assist translation
aminoacyl tRNA synthetase enzyme that “charges” tRNA molecules by catalyzing a bond between the tRNA
and a corresponding amino acid
anticodon three-nucleotide sequence in a tRNA molecule that corresponds to an mRNA codon
CAAT box (GGCCAATCT) essential eukaryotic promoter sequence involved in binding transcription factors
central dogma states that genes specify the sequence of mRNAs, which in turn specify the sequence of
proteins
codon three consecutive nucleotides in mRNA that specify the insertion of an amino acid or the release of a
polypeptide chain during translation
colinear in terms of RNA and protein, three “units” of RNA (nucleotides) specify one “unit” of protein (amino
acid) in a consecutive fashion
consensus DNA sequence that is used by many species to perform the same or similar functions
core enzyme prokaryotic RNA polymerase consisting of a, a, /3, and /3' but missing o\ this complex performs
elongation
degeneracy (of the genetic code) describes that a given amino acid can be encoded by more than one
nucleotide triplet; the code is degenerate, but not ambiguous
downstream nucleotides following the initiation site in the direction of mRNA transcription; in general,
sequences that are toward the 3' end relative to a site on the mRNA
exon sequence present in protein-coding mRNA after completion of pre-mRNA splicing
FACT complex that “facilitates chromatin transcription” by disassembling nucleosomes ahead of a transcribing
RNA polymerase II and reassembling them after the polymerase passes by
GC-rich box (GGCG) nonessential eukaryotic promoter sequence that binds cellular factors to increase the
efficiency of transcription; may be present several times in a promoter
hairpin structure of RNA when it folds back on itself and forms intramolecular hydrogen bonds between
complementary nucleotides
holoenzyme prokaryotic RNA polymerase consisting of a, a, /3, /3', and cr; this complex is responsible for
transcription initiation
initiation site nucleotide from which mRNA synthesis proceeds in the 5' to 3' direction; denoted with a “+1”
initiator tRNA j n prokaryotes, called tRNA ^ et ; in eukaryotes, called tRNAi; a tRNA that interacts with a start
codon, binds directly to the ribosome P site, and links to a special methionine to begin a polypeptide chain
intron non-protein-coding intervening sequences that are spliced from mRNA during processing
Kozak’s rules determines the correct initiation AUG in a eukaryotic mRNA; the following consensus sequence
must appear around the AUG: 5’-GCC(purine)CCALJGG-3’; the bolded bases are most important
nonsense codon one of the three mRNA codons that specifies termination of translation
nontemplate strand strand of DNA that is not used to transcribe mRNA; this strand is identical to the mRNA
except that T nucleotides in the DNA are replaced by U nucleotides in the mRNA
Octamer box (ATTTGCAT) nonessential eukaryotic promoter sequence that binds cellular factors to increase
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Chapter 15 | Genes and Proteins
431
the efficiency of transcription; may be present several times in a promoter
peptidyl transferase RNA-based enzyme that is integrated into the 50S ribosomal subunit and catalyzes the
formation of peptide bonds
plasmid extrachromosomal, covalently closed, circular DNA molecule that may only contain one or a few genes;
common in prokaryotes
poly-A tail modification added to the 3' end of pre-mRNAs to protect mRNA from degradation and assist mRNA
export from the nucleus
polysome mRNA molecule simultaneously being translated by many ribosomes all going in the same direction
preinitiation complex cluster of transcription factors and other proteins that recruit RNA polymerase II for
transcription of a DNA template
promoter DNA sequence to which RNA polymerase and associated factors bind and initiate transcription
reading frame sequence of triplet codons in mRNA that specify a particular protein; a ribosome shift of one or
two nucleotides in either direction completely abolishes synthesis of that protein
rho-dependent termination in prokaryotes, termination of transcription by an interaction between RNA
polymerase and the rho protein at a run of G nucleotides on the DNA template
rho-independent termination sequence-dependent termination of prokaryotic mRNA synthesis; caused by
hairpin formation in the mRNA that stalls the polymerase
RNA editing direct alteration of one or more nucleotides in an mRNA that has already been synthesized
Shine-Dalgarno sequence (AGGAGG); initiates prokaryotic translation by interacting with rRNA molecules
comprising the 30S ribosome
signal sequence short tail of amino acids that directs a protein to a specific cellular compartment
small nuclear RNA molecules synthesized by RNA polymerase III that have a variety of functions, including
splicing pre-mRNAs and regulating transcription factors
splicing process of removing introns and reconnecting exons in a pre-mRNA
start codon AUG (or rarely, GUG) on an mRNA from which translation begins; always specifies methionine
TATA box conserved promoter sequence in eukaryotes and prokaryotes that helps to establish the initiation site
for transcription
template strand strand of DNA that specifies the complementary mRNA molecule
transcription bubble region of locally unwound DNA that allows for transcription of mRNA
upstream nucleotides preceding the initiation site; in general, sequences toward the 5' end relative to a site on
the mRNA
CHAPTER SUMMARY
15.1 The Genetic Code
The genetic code refers to the DNA alphabet (A, T, C, G), the RNA alphabet (A, U, C, G), and the polypeptide
alphabet (20 amino acids). The central dogma describes the flow of genetic information in the cell from genes
to mRNA to proteins. Genes are used to make mRNA by the process of transcription; mRNA is used to
synthesize proteins by the process of translation. The genetic code is degenerate because 64 triplet codons in
mRNA specify only 20 amino acids and three nonsense codons. Most amino acids have several similar
codons. Almost every species on the planet uses the same genetic code.
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15.2 Prokaryotic Transcription
in prokaryotes, mRNA synthesis is initiated at a promoter sequence on the DNA template comprising two
consensus sequences that recruit RNA polymerase. The prokaryotic polymerase consists of a core enzyme of
four protein subunits and a a protein that assists only with initiation. Elongation synthesizes mRNA in the 5' to
3' direction at a rate of 40 nucleotides per second. Termination liberates the mRNA and occurs either by rho
protein interaction or by the formation of an mRNA hairpin.
15.3 Eukaryotic Transcription
Transcription in eukaryotes involves one of three types of polymerases, depending on the gene being
transcribed. RNA polymerase II transcribes all of the protein-coding genes, whereas RNA polymerase I
transcribes the tandemly duplicated rRNA genes, and RNA polymerase III transcribes various small RNAs, like
the 5S rRNA, tRNA, and small nuclear RNA genes. The initiation of transcription in eukaryotes involves the
binding of several transcription factors to complex promoter sequences that are usually located upstream of the
gene being copied. The mRNA is synthesized in the 5' to 3' direction, and the FACT complex moves and
reassembles nucleosomes as the polymerase passes by. Whereas RNA polymerases I and III terminate
transcription by protein- or RNA hairpin-dependent methods, RNA polymerase II transcribes for 1,000 or more
nucleotides beyond the gene template and cleaves the excess during pre-mRNA processing.
15.4 RNA Processing in Eukaryotes
Eukaryotic pre-mRNAs are modified with a 5' methylguanosine cap and a poly-A tail. These structures protect
the mature mRNA from degradation and help export it from the nucleus. Pre-mRNAs also undergo splicing, in
which introns are removed and exons are reconnected with single-nucleotide accuracy. Only finished mRNAs
that have undergone 5' capping, 3' polyadenylation, and intron splicing are exported from the nucleus to the
cytoplasm. Pre-rRNAs and pre-tRNAs may be processed by intramolecular cleavage, splicing, methylation, and
chemical conversion of nucleotides. Rarely, RNA editing is also performed to insert missing bases after an
mRNA has been synthesized.
15.5 Ribosomes and Protein Synthesis
The players in translation include the mRNA template, ribosomes, tRNAs, and various enzymatic factors. The
small ribosomal subunit binds to the mRNA template either at the Shine-Dalgarno sequence (prokaryotes) or
the 5' cap (eukaryotes). Translation begins at the initiating AUG on the mRNA, specifying methionine. The
formation of peptide bonds occurs between sequential amino acids matched to the mRNA template by their
tRNAs according to the genetic code. Charged tRNAs enter the ribosomal A site, and their amino acid bonds
with the amino acid at the P site. The entire mRNA is translated in three-nucleotide “steps” of the ribosome.
When a nonsense codon is encountered, a release factor binds and dissociates the components and frees the
new protein. Folding of the protein occurs during and after translation.
VISUAL CONNECTION QUESTIONS
1. Figure 15.11 A scientist splices a eukaryotic
promoter in front of a bacterial gene and inserts the
gene in a bacterial chromosome. Would you expect
the bacteria to transcribe the gene?
2. Figure 15.13 Errors in splicing are implicated in
cancers and other human diseases. What kinds of
mutations might lead to splicing errors? Think of
different possible outcomes if splicing errors occur.
3. Figure 15.16 Many antibiotics inhibit bacterial
protein synthesis. For example, tetracycline blocks
the A site on the bacterial ribosome, and
chloramphenicol blocks peptidyl transfer. What
specific effect would you expect each of these
antibiotics to have on protein synthesis?
Tetracycline would directly affect:
a. tRNA binding to the ribosome
b. ribosome assembly
c. growth of the protein chain
Chloramphenicol would directly affect
a. tRNA binding to the ribosome
b. ribosome assembly
c. growth of the protein chain
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Chapter 15 | Genes and Proteins
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REVIEW QUESTIONS
4. The AUC and AUA codons in mRNA both specify
isoleucine. What feature of the qenetic code explains
this?
a. complementarity
b. nonsense codons
c. universality
d. degeneracy
5. How many nucleotides are in 12 mRNA codons?
a. 12
b. 24
c. 36
d. 48
6. Which event contradicts the central dogma of
molecular biology?
a. Poly-A polymerase enzymes process mRNA
in the nucleus.
b. Endonuclease enzymes splice out and
repair damaged DNA.
c. Scientists use reverse transcriptase
enzymes to make DNA from RNA.
d. Codons specifying amino acids are
degenerate and universal.
7. Which subunit of the E. coli polymerase confers
specificity to transcription?
a. a
b. p
c. p
d. o
8. The -10 and -35 regions of prokaryotic promoters
are called consensus sequences because_.
a. they are identical in all bacterial species
b. they are similar in all bacterial species
c. they exist in all organisms
d. they have the same function in all
organisms
9. Three different bacteria species have the following
consensus sequences upstream of a conserved
gene.
Species A
Species B
Species C
-10
TAATAAT
TTTAAT
TATATT
-35
TTGACA
TTGGCC
TTGAAA
Table 15.2
Order the bacteria from most to least efficient
initiation of gene transcription.
a. A > B > C
b. B > C > A
c. C > B > A
d. A > C > B
10. Which feature of promoters can be found in both
prokaryotes and eukaryotes?
a. GC box
b. TATA box
c. octamer box
d. -10 and -35 sequences
11. What transcripts will be most affected by low
levels of a-amanitin?
a. 18S and 28S rRNAs
b. pre-mRNAs
c. 5S rRNAs and tRNAs
d. other small nuclear RNAs
12. How do enhancers and promoters differ?
a. Enhancers bind transcription factors to
silence gene expression, while promoters
activate transcription.
b. Enhancers increase the efficiency of gene
expression, but are not essential for
transcription. Promoter recognition is
essential to transcription initiation.
c. Promoters bind transcription factors to
increase the efficiency of transcription.
Enhancers bind RNA polymerases to initiate
transcription.
d. There is no difference. Both are
transcription factor-binding sequences in
DNA.
13. Which pre-mRNA processing step is important for
initiating translation?
a. poly-A tail
b. RNA editing
c. splicing
d. 7-methylguanosine cap
14. What processing step enhances the stability of
pre-tRNAs and pre-rRNAs?
a. methylation
b. nucleotide modification
c. cleavage
d. splicing
15. A scientist identifies a pre-mRNA with the
What is the predicted size of the corresponding
mature mRNA in base pairs (bp), excluding the 5’
cap and 3’ poly-A tail?
following structure.
100bp
50bp
75bp
90bp
120bp
Exon
Intron
Exon
Intron
Exon
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Chapter 15 | Genes and Proteins
a.
220bp
b.
295bp
c.
140bp
d.
435bp
16. The RNA components of ribosomes are
synthesized in the_.
a. cytoplasm
b. nucleus
c. nucleolus
d. endoplasmic reticulum
17. In any given species, there are at least how many
types of aminoacyl tRNA synthetases?
a. 20
b. 40
c. 100
d. 200
CRITICAL THINKING QUESTIONS
19. Imagine if there were 200 commonly occurring
amino acids instead of 20. Given what you know
about the genetic code, what would be the shortest
possible codon length? Explain.
20. Discuss how degeneracy of the genetic code
makes cells more robust to mutations.
21. A scientist sequencing mRNA identifies the
following strand:
CUAUGUGUCGUAACAGCCGAUGACCCG
What is the sequence of the amino acid chain this
mRNA makes when it is translated?
22. If mRNA is complementary to the DNA template
strand and the DNA template strand is
complementary to the DNA nontemplate strand, then
why are base sequences of mRNA and the DNA
nontemplate strand not identical? Could they ever
be?
23. In your own words, describe the difference
between rho-dependent and rho-independent
termination of transcription in prokaryotes.
24. A fragment of bacterial DNA reads:
3’
-TACCTATAAT CT CAATT GATAGAAGCACT CTAC-
5’
Assuming that this fragment is the template strand,
what is the sequence of mRNA that would be
18. A scientist introduces a mutation that makes the
60S ribosomal subunit nonfunctional in a human cell
line. What would be the predicted effect on
translation?
a. Translation stalls after the initiation AUG
codon is identified.
b. The ribosome cannot catalyze the formation
of peptide bonds between the tRNAs in the
A and P sites.
c. The ribosome cannot interact with mRNAs.
d. tRNAs cannot exit the E site of the
ribosome.
transcribed? (Hint: Be sure to identify the initiation
site.)
25. A scientist observes that a cell has an RNA
polymerase deficiency that prevents it from making
proteins. Describe three additional observations that
would together support the conclusion that a defect in
RNA polymerase I activity, and not problems with the
other polymerases, causes the defect.
26. Chronic lymphocytic leukemia patients often
harbor nonsense mutations in their spliceosome
machinery. Describe how this mutation of the
spliceosome would change the final location and
sequence of a pre-mRNA.
27. Transcribe and translate the following DNA
sequence (nontemplate strand): 5'-
ATGGCCGGTTATTAAGCA-3'
28. Explain how single nucleotide changes can have
vastly different effects on protein function.
29. A normal mRNA that reads 5’ -
UGCCAUGGUAAUAACACAUGAGGCCUGAAC- 3’
has an insertion mutation that changes the sequence
to 5’
-UGCCAUGGUUAAUAACACAUGAGGCCUGAAC-
3'. Translate the original mRNA and the mutated
mRNA, and explain how insertion mutations can
have dramatic effects on proteins. (Hint: Be sure to
find the initiation site.)
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Chapter 16 | Gene Expression
435
16 | GENE EXPRESSION
Figure 16.1 The genetic content of each somatic cell in an organism is the same, but not all genes are expressed in
every cell. The control of which genes are expressed dictates whether a cell is, for example, (a) an eye cell or (b) a liver
cell. It is the differential gene expression patterns that arise in different cells that give rise to (c) a complete organism.
Chapter Outline
16.1: Regulation of Gene Expression
16.2: Prokaryotic Gene Regulation
16.3: Eukaryotic Epigenetic Gene Regulation
16.4: Eukaryotic Transcription Gene Regulation
16.5: Eukaryotic Post-transcriptional Gene Regulation
16.6: Eukaryotic Translational and Post-translational Gene Regulation
16.7: Cancer and Gene Regulation
Introduction
Each somatic cell in the body generally contains the same DNA. A few exceptions include red blood cells, which
contain no DNA in their mature state, and some immune system cells that rearrange their DNA while producing
antibodies. In general, however, the genes that determine whether you have green eyes, brown hair, and how
fast you metabolize food are the same in the cells in your eyes and your liver, even though these organs function
quite differently. If each cell has the same DNA, how is it that cells or organs are different? Why do cells in the
eye differ so dramatically from cells in the liver?
Whereas each cell shares the same genome and DNA sequence, each cell does not turn on, or express, the
same set of genes. Each cell type needs a different set of proteins to perform its function. Therefore, only a
small subset of proteins is expressed in a cell. For the proteins to be expressed, the DNA must be transcribed
into RNA and the RNA must be translated into protein. In a given cell type, not all genes encoded in the DNA
are transcribed into RNA or translated into protein because specific cells in our body have specific functions.
Specialized proteins that make up the eye (iris, lens, and cornea) are only expressed in the eye, whereas the
specialized proteins in the heart (pacemaker cells, heart muscle, and valves) are only expressed in the heart.
At any given time, only a subset of all of the genes encoded by our DNA are expressed and translated into
proteins. The expression of specific genes is a highly regulated process with many levels and stages of control.
This complexity ensures the proper expression in the proper cell at the proper time.
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Chapter 16 | Gene Expression
16.1 1 Regulation of Gene Expression
By the end of this section, you will be able to do the following:
• Discuss why every cell does not express all of its genes all of the time
• Describe how prokaryotic gene regulation occurs at the transcriptional level
• Discuss how eukaryotic gene regulation occurs at the epigenetic, transcriptional, post-transcriptional,
translational, and post-translational levels
For a cell to function properly, necessary proteins must be synthesized at the proper time and place. All cells
control or regulate the synthesis of proteins from information encoded in their DNA. The process of turning on
a gene to produce RNA and protein is called gene expression. Whether in a simple unicellular organism or a
complex multi-cellular organism, each cell controls when and how its genes are expressed. For this to occur,
there must be internal chemical mechanisms that control when a gene is expressed to make RNA and protein,
how much of the protein is made, and when it is time to stop making that protein because it is no longer needed.
The regulation of gene expression conserves energy and space. It would require a significant amount of energy
for an organism to express every gene at all times, so it is more energy efficient to turn on the genes only when
they are required. In addition, only expressing a subset of genes in each cell saves space because DNA must be
unwound from its tightly coiled structure to transcribe and translate the DNA. Cells would have to be enormous
if every protein were expressed in every cell all the time.
The control of gene expression is extremely complex. Malfunctions in this process are detrimental to the cell and
can lead to the development of many diseases, including cancer.
Prokaryotic versus Eukaryotic Gene Expression
To understand how gene expression is regulated, we must first understand how a gene codes for a functional
protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different manners.
Prokaryotic organisms are single-celled organisms that lack a cell nucleus, and their DNA therefore floats
freely in the cell cytoplasm. To synthesize a protein, the processes of transcription and translation occur almost
simultaneously. When the resulting protein is no longer needed, transcription stops. As a result, the primary
method to control what type of protein and how much of each protein is expressed in a prokaryotic cell is
the regulation of DNA transcription. All of the subsequent steps occur automatically. When more protein is
required, more transcription occurs. Therefore, in prokaryotic cells, the control of gene expression is mostly at
the transcriptional level.
Eukaryotic cells, in contrast, have intracellular organelles that add to their complexity. In eukaryotic cells, the
DNA is contained inside the cell’s nucleus and there it is transcribed into RNA. The newly synthesized RNA
is then transported out of the nucleus into the cytoplasm, where ribosomes translate the RNA into protein.
The processes of transcription and translation are physically separated by the nuclear membrane; transcription
occurs only within the nucleus, and translation occurs only outside the nucleus in the cytoplasm. The regulation
of gene expression can occur at all stages of the process (Figure 16.2). Regulation may occur when the DNA
is uncoiled and loosened from nucleosomes to bind transcription factors ( epigenetic level), when the RNA
is transcribed ( transcriptional level), when the RNA is processed and exported to the cytoplasm after it is
transcribed ( post-transcriptional level), when the RNA is translated into protein ( translational level), or after
the protein has been made ( post-translational level).
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Chapter 16 | Gene Expression
437
Figure 16.2 Regulation in prokaryotes and eukaryotes. Prokaryotic transcription and translation occur simultaneously
in the cytoplasm, and regulation occurs at the transcriptional level. Eukaryotic gene expression is regulated during
transcription and RNA processing, which take place in the nucleus, and during protein translation, which takes place
in the cytoplasm. Further regulation may occur through post-translational modifications of proteins.
The differences in the regulation of gene expression between prokaryotes and eukaryotes are summarized in
Table 16.1. The regulation of gene expression is discussed in detail in subsequent modules.
Differences in the Regulation of Gene Expression of Prokaryotic and Eukaryotic
Organisms
Prokaryotic organisms
Eukaryotic organisms
Lack a membrane-bound nucleus
Contain nucleus
DNA is found in the cytoplasm
DNA is confined to the nuclear compartment
RNA transcription and protein
formation occur almost
simultaneously
RNA transcription occurs prior to protein formation, and it takes place in
the nucleus. Translation of RNA to protein occurs in the cytoplasm.
Gene expression is regulated
primarily at the transcriptional
level
Gene expression is regulated at many levels (epigenetic, transcriptional,
nuclear shuttling, post-transcriptional, translational, and post-translational)
Table 16.1
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Chapter 16 | Gene Expression
V /
e olution CONNECTION
Evolution of Gene Regulation
Prokaryotic cells can only regulate gene expression by controlling the amount of transcription. As eukaryotic
cells evolved, the complexity of the control of gene expression increased. For example, with the evolution
of eukaryotic cells came compartmentalization of important cellular components and cellular processes. A
nuclear region that contains the DNA was formed. Transcription and translation were physically separated
into two different cellular compartments. It therefore became possible to control gene expression by
regulating transcription in the nucleus, and also by controlling the RNA levels and protein translation present
outside the nucleus.
Most gene regulation is done to conserve cell resources. However, other regulatory processes may be
defensive. Cellular processes such as developed to protect the cell from viral or parasitic infections. If the
cell could quickly shut off gene expression for a short period of time, it would be able to survive an infection
when other organisms could not. Therefore, the organism evolved a new process that helped it survive, and
it was able to pass this new development to offspring.
16.2 | Prokaryotic Gene Regulation
By the end of this section, you will be able to do the following:
• Describe the steps involved in prokaryotic gene regulation
• Explain the roles of activators, inducers, and repressors in gene regulation
The DNA of prokaryotes is organized into a circular chromosome, supercoiled within the nucleoid region of the
cell cytoplasm. Proteins that are needed for a specific function, or that are involved in the same biochemical
pathway, are encoded together in blocks called operons. For example, all of the genes needed to use lactose
as an energy source are coded next to each other in the lactose (or lac) operon, and transcribed into a single
mRNA.
In prokaryotic cells, there are three types of regulatory molecules that can affect the expression of operons:
repressors, activators, and inducers. Repressors and activators are proteins produced in the cell. Both
repressors and activators regulate gene expression by binding to specific DNA sites adjacent to the genes they
control. In general, activators bind to the promoter site, while repressors bind to operator regions. Repressors
prevent transcription of a gene in response to an external stimulus, whereas activators increase the
transcription of a gene in response to an external stimulus. Inducers are small molecules that may be produced
by the cell or that are in the cell’s environment. Inducers either activate or repress transcription depending on
the needs of the cell and the availability of substrate.
The trp Operon: A Repressible Operon
Bacteria such as Escherichia coli need amino acids to survive, and are able to synthesize many of them.
Tryptophan is one such amino acid that E. coli can either ingest from the environment or synthesize using
enzymes that are encoded by five genes. These five genes are next to each other in what is called the
tryptophan (trp) operon (Figure 16.3). The genes are transcribed into a single mRNA, which is then translated
to produce all five enzymes. If tryptophan is present in the environment, then E. coli does not need to synthesize
it and the trp operon is switched off. However, when tryptophan availability is low, the switch controlling
the operon is turned on, the mRNA is transcribed, the enzyme proteins are translated, and tryptophan is
synthesized.
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Chapter 16 | Gene Expression
439
When tryptophan is present, the trp repressor binds
the operator, and RNA synthesis is blocked.
Promoter
Operator
trpE
trpD
trpC
trpB
trpA
RNA Polymerase^- Repressor
-Tryptophan
In the absence of tryptophan, the repressor dissociates
from the operator, and RNA synthesis proceeds.
Promoter
Operator
trpE
trpD
trpC
trpB
trpA
RNA Polymerase
ir
Repressor
Figure 16.3 The tryptophan operon. The five genes that are needed to synthesize tryptophan in E. coli are located
next to each other in the trp operon. When tryptophan is plentiful, two tryptophan molecules bind the repressor protein
at the operator sequence. This physically blocks the RNA polymerase from transcribing the tryptophan genes. When
tryptophan is absent, the repressor protein does not bind to the operator and the genes are transcribed.
The trp operon includes three important regions: the coding region, the trp operator and the trp promoter. The
coding region includes the genes for the five tryptophan biosynthesis enzymes. Just before the coding region is
the transcriptional start site. The promoter sequence, to which RNA polymerase binds to initiate transcription,
is before or “upstream” of the transcriptional start site. Between the promoter and the transcriptional start site is
the operator region.
The trp operator contains the DNA code to which the trp repressor protein can bind. However, the repressor
alone cannot bind to the operator. When tryptophan is present in the cell, two tryptophan molecules bind to
the trp repressor, which changes the shape of the repressor protein to a form that can bind to the trp operator.
Binding of the tryptophan-repressor complex at the operator physically prevents the RNA polymerase from
binding to the promoter and transcribing the downstream genes.
When tryptophan is not present in the cell, the repressor by itself does not bind to the operator, the polymerase
can transcribe the enzyme genes, and tryptophan is synthesized. Because the repressor protein actively binds
to the operator to keep the genes turned off, the trp operon is said to be negatively regulated and the proteins
that bind to the operator to silence trp expression are negative regulators.
LINK TQ LEARNING
Watch this video to learn more about the trp operon. (This multimedia resource will open in a browser.)
(http://cnx.Org/content/m66504/l.3/#eip-idll69842033659)
Catabolite Activator Protein (CAP): A Transcriptional Activator
Just as the trp operon is negatively regulated by tryptophan molecules, there are proteins that bind to the
promoter sequences that act as positive regulators to turn genes on and activate them. For example, when
glucose is scarce, E. coli bacteria can turn to other sugar sources for fuel. To do this, new genes to process
these alternate sugars must be transcribed. When glucose levels drop, cyclic AMP (cAMP) begins to accumulate
in the cell. The cAMP molecule is a signaling molecule that is involved in glucose and energy metabolism in
E. coli. Accumulating cAMP binds to the positive regulator catabolite activator protein (CAP), a protein that
440
Chapter 16 | Gene Expression
binds to the promoters of operons which control the processing of alternative sugars. When cAMP binds to CAP,
the complex then binds to the promoter region of the genes that are needed to use the alternate sugar sources
(Figure 16.4). In these operons, a CAP-binding site is located upstream of the RNA-polymerase-binding site
in the promoter. CAP binding stabilizes the binding of RNA polymerase to the promoter region and increases
transcription of the associated protein-coding genes.
c«.
In the absence of cAMP, CAP does
not bind the promoter. Transcription
occurs at a low rate.
Promoter
Operator
lacZ
lacY
lacA
RNA Polymerase
-►
In the presence of cAMP, CAP binds
the promoter and increases RNA
polymerase activity.
cAMP + promoter
CAP
Operator
lacZ
lacY
lacA
RNA Polymerase
Figure 16.4 Transcriptional activation by the CAP protein. When glucose levels fall, E. coli may use other sugars for
fuel but must transcribe new genes to do so. As glucose supplies become limited, cAMP levels increase. This cAMP
binds to the CAP protein, a positive regulator that binds to a promoter region upstream of the genes required to use
other sugar sources.
The lac Operon: An Inducible Operon
The third type of gene regulation in prokaryotic cells occurs through inducible operons, which have proteins that
bind to activate or repress transcription depending on the local environment and the needs of the cell. The lac
operon is a typical inducible operon. As mentioned previously, E. coli is able to use other sugars as energy
sources when glucose concentrations are low. One such sugar source is lactose. The lac operon encodes the
genes necessary to acquire and process the lactose from the local environment. The Z gene of the lac operon
encodes beta-galactosidase, which breaks lactose down to glucose and galactose.
However, for the lac operon to be activated, two conditions must be met. First, the level of glucose must be very
low or non-existent. Second, lactose must be present. Only when glucose is absent and lactose is present will
the lac operon be transcribed (Figure 16.5). In the absence of glucose, the binding of the CAP protein makes
transcription of the lac operon more effective. When lactose is present, it binds to the lac repressor and changes
its shape so that it cannot bind to the lac operator to prevent transcription. This combination of conditions makes
sense for the cell, because it would be energetically wasteful to synthesize the enzymes to process lactose if
glucose was plentiful or lactose was not available.
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Chapter 16 | Gene Expression
441
visual
Figure 16.5 Regulation of the lac operon. Transcription of the lac operon is carefully regulated so that its
expression only occurs when glucose is limited and lactose is present to serve as an alternative fuel source.
Question: in E. coli, the trp operon is on by default, while the lac operon is off. Why do you think this is the
case?
CONNECTION
In the absence of lactose, the lac repressor
binds the operator, and transcription is
blocked.
Promoter
Operator
lacZ
lacY
lacA
RNA Polymerase^^- Repressor
In the presence of lactose, the lac repressor
is released from the operator, and
transcription proceeds at a slow rate.
Promoter
Operator
lacZ
lacY
lacA
RNA Polymerase
Repressor
■ Lactose
cAMP-CAP complex stimulates RNA
Polymerase activity and increases RNA
synthesis.
CAMP + p romoter
CAP -
Operator
lacZ
lacY
lacA
RNA Polymerase Q
T>
However, even in the presence of
cAMP-CAP complex, RNA synthesis is
blocked when repressor is bound to
the operator.
CAMP+ promoter
CAP
Operator
lacZ
lacY
lacA
RNA Polymerase^- Repressor
If glucose is present, then CAP fails to bind to the promoter sequence to activate transcription. If lactose is
absent, then the repressor binds to the operator to prevent transcription. If either of these conditions is met,
then transcription remains off. Only when glucose is absent and lactose is present is the lac operon transcribed
(Table 16.2).
Signals that Induce or Repress Transcription of the lac Operon
Glucose
CAP binds
Lactose
Repressor binds
Transcription
+
-
-
+
No
Table 16.2
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Chapter 16 | Gene Expression
Signals that Induce or Repress Transcription of the lac Operon
Glucose
CAP binds
Lactose
Repressor binds
Transcription
+
-
+
-
Some
-
+
-
+
No
-
+
+
-
Yes
Table 16.2
LINK TQ LEARNING
Watch an animated tutorial about the workings of lac operon here. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66504/l.3/#eip-idll65239273914)
16.3 | Eukaryotic Epigenetic Gene Regulation
By the end of this section, you will be able to do the following:
• Explain how chromatin remodeling controls transcriptional access
• Describe how access to DNA is controlled by histone modification
• Describe how DNA methylation is related to epigenetic gene changes
Eukaryotic gene expression is more complex than prokaryotic gene expression because the processes of
transcription and translation are physically separated. Unlike prokaryotic cells, eukaryotic cells can regulate gene
expression at many different levels. Epigenetic changes are inheritable changes in gene expression that do not
result from changes in the DNA sequence. Eukaryotic gene expression begins with control of access to the
DNA. Transcriptional access to the DNA can be controlled in two general ways: chromatin remodeling and DNA
methylation. Chromatin remodeling changes the way that DNA is associated with chromosomal histones. DNA
methylation is associated with developmental changes and gene silencing.
Epigenetic Control: Regulating Access to Genes within the
Chromosome
The human genome encodes over 20,000 genes, with hundreds to thousands of genes on each of the 23 human
chromosomes. The DNA in the nucleus is precisely wound, folded, and compacted into chromosomes so that it
will fit into the nucleus. It is also organized so that specific segments can be accessed as needed by a specific
cell type.
The first level of organization, or packing, is the winding of DNA strands around histone proteins. Histones
package and order DNA into structural units called nucleosome complexes, which can control the access of
proteins to the DNA regions (Figure 16.6a). Under the electron microscope, this winding of DNA around histone
proteins to form nucleosomes looks like small beads on a string (Figure 16.6b).
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Chapter 16 | Gene Expression
443
(a) (b)
Figure 16.6 DNA is folded around histone proteins to create (a) nucleosome complexes. These nucleosomes control
the access of proteins to the underlying DNA. When viewed through an electron microscope (b), the nucleosomes look
like beads on a string, (credit “micrograph": modification of work by Chris Woodcock)
These beads (histone proteins) can move along the string (DNA) to expose different sections of the molecule. If
DNA encoding a specific gene is to be transcribed into RNA, the nucleosomes surrounding that region of DNA
can slide down the DNA to open that specific chromosomal region and allow for the transcriptional machinery
(RNA polymerase) to initiate transcription (Figure 16.7).
visual
CONNECTION
Methylation of DNA and
histones causes nucleosomes
to pack tightly together.
Transcription factors cannot
bind the DNA, and genes are
not expressed.
Histone acetylation results
in loose packing of nucleo¬
somes. Transcription factors
can bind the DNA and genes
are expressed.
Figure 16.7 Nucleosomes can slide along DNA. When nucleosomes are spaced closely together (top),
transcription factors cannot bind and gene expression is turned off. When the nucleosomes are spaced far apart
(bottom), the DNA is exposed. Transcription factors can bind, allowing gene expression to occur. Modifications to
the histones and DNA affect nucleosome spacing.
In females, one of the two X chromosomes is inactivated during embryonic development because of
epigenetic changes to the chromatin. What impact do you think these changes would have on nucleosome
packing?
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Chapter 16 | Gene Expression
How closely the histone proteins associate with the DNA is regulated by signals found on both the histone
proteins and on the DNA. These signals are functional groups added to histone proteins or to DNA and
determine whether a chromosomal region should be open or closed (Figure 16.8 depicts modifications to
histone proteins and DNA). These tags are not permanent, but may be added or removed as needed. Some
chemical groups (phosphate, methyl, or acetyl groups) are attached to specific amino acids in histone "tails" at
the N-terminus of the protein. These groups do not alter the DNA base sequence, but they do alter how tightly
wound the DNA is around the histone proteins. DNA is a negatively charged molecule and unmodified histones
are positively charged; therefore, changes in the charge of the histone will change how tightly wound the DNA
molecule will be. By adding chemical modifications like acetyl groups, the charge becomes less positive, and
the binding of DNA to the histones is relaxed. Altering the location of nucleosomes and the tightness of histone
binding opens some regions of chromatin to transcription and closes others.
The DNA molecule itself can also be modified by methylation. DNA methylation occurs within very specific
regions called CpG islands. These are stretches with a high frequency of cytosine and guanine dinucleotide DNA
pairs (CG) found in the promoter regions of genes. The cytosine member of the CG pair can be methylated (a
methyl group is added). Methylated genes are usually silenced, although methylation may have other regulatory
effects. In some cases, genes that are silenced during the development of the gametes of one parent are
transmitted in their silenced condition to the offspring. Such genes are said to be imprinted. Parental diet or
other environmental conditions may also affect the methylation patterns of genes, which in turn modifies gene
expression. Changes in chromatin organization interact with DNA methylation. DNA methyltransferases appear
to be attracted to chromatin regions with specific histone modifications. Highly methylated ( hypermethylated )
DNA regions with deacetylated histones are tightly coiled and transcriptionally inactive.
CHROMATIN
} METHYL GROUP
HISTONE TAIL
EPIGENETIC CHANGES TO THE
CHROMATIN MAY RESULT FROM
• Development (in utero, childhood)
• Environmental chemicals
• Drugs/Pharmaceuticals
• Aging
• Diet
EPIGENETIC CHANGES
MAY RESULT IN
• Cancer
• Autoimmune disease
• Mental disorders
• Diabetes
A Acetyl group
CHROMOSOME
HISTONE TAIL
GENE
DNA accessible, gene active
Histones are proteins around
which DNA winds for compaction
and gene regulation.
HISTONE'
DNA inaccessible, gene inactive
DNA methylation and chemical
modification of histone tails alter the
spacing of nucleosomes and change
gene expression.
Figure 16.8 Histone proteins and DNA nucleotides can be modified chemically. Modifications affect nucleosome
spacing and gene expression, (credit: modification of work by NIH)
Epigenetic changes are not permanent, although they often persist through multiple rounds of cell division and
may even cross generational lines. Chromatin remodeling alters the chromosomal structure (open or closed) as
needed. If a gene is to be transcribed, the histone proteins and DNA in the chromosomal region encoding that
gene are modified in a way that opens the promoter region to allow RNA polymerase and other proteins, called
transcription factors, to bind and initiate transcription. If a gene is to remain turned off, or silenced, the histone
proteins and DNA have different modifications that signal a closed chromosomal configuration. In this closed
configuration, the RNA polymerase and transcription factors do not have access to the DNA and transcription
cannot occur (Figure 16.8).
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Chapter 16 | Gene Expression
445
LINK TQ LEARNING
View this video that describes how epigenetic regulation controls gene expression. (This multimedia
resource will open in a browser.) (http://cnx.Org/content/m66505/l.3/#eip-idll69842033590)
16.4 | Eukaryotic Transcription Gene Regulation
By the end of this section, you will be able to do the following:
• Discuss the role of transcription factors in gene regulation
• Explain how enhancers and repressors regulate gene expression
Like prokaryotic cells, the transcription of genes in eukaryotes requires the action of an RNA polymerase to bind
to a DNA sequence upstream of a gene in order to initiate transcription. However, unlike prokaryotic cells, the
eukaryotic RNA polymerase requires other proteins, or transcription factors, to facilitate transcription initiation.
RNA polymerase by itself cannot initiate transcription in eukaryotic cells. There are two types of transcription
factors that regulate eukaryotic transcription: General (or basal) transcription factors bind to the core promoter
region to assist with the binding of RNA polymerase. Specific transcription factors bind to various regions outside
of the core promoter region and interact with the proteins at the core promoter to enhance or repress the activity
of the polymerase.
LINK TQ LEARNING
View the process of transcription—the making of RNA from a DNA template. (This multimedia
resource will open in a browser.) (http://cnx.Org/content/m66506/l.3/#eip-idll68020166468)
The Promoter and the Transcription Machinery
Genes are organized to make the control of gene expression easier. The promoter region is immediately
upstream of the coding sequence. This region can be short (only a few nucleotides in length) or quite long
(hundreds of nucleotides long). The longer the promoter, the more available space for proteins to bind. This
also adds more control to the transcription process. The length of the promoter is gene-specific and can
differ dramatically between genes. Consequently, the level of control of gene expression can also differ quite
dramatically between genes. The purpose of the promoter is to bind transcription factors that control the
initiation of transcription.
Within the core promoter region, 25 to 35 bases upstream of the transcriptional start site, resides the TATA
box. The TATA box has the consensus sequence of 5’-TATAAA-3’. The TATA box is the binding site for a
protein complex called TFIID, which contains aTATA-binding protein. Binding of TFIID recruits other transcription
factors, including TFIIB, TFIIE, TFIIF, and TFIIH. Some of these transcription factors help to bind the RNA
polymerase to the promoter, and others help to activate the transcription initiation complex.
in addition to the TATA box, other binding sites are found in some promoters. Some biologists prefer to restrict
the range of the eukaryotic promoter to the core promoter, or polymerase binding site, and refer to these
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additional sites as promoter-proximal elements, because they are usually found within a few hundred base pairs
upstream of the transcriptional start site. Examples of these elements are the CAAT box, with the consensus
sequence 5’-CCAAT-3’ and the GC box, with the consensus sequence 5’-GGGCGG-3’. Specific transcription
factors can bind to these promoter-proximal elements to regulate gene transcription. A given gene may have its
own combination of these specific transcription-factor binding sites. There are hundreds of transcription factors
in a cell, each of which binds specifically to a particular DNA sequence motif. When transcription factors bind to
the promoter just upstream of the encoded gene, it is referred to as a cis-acting element, because it is on the
same chromosome just next to the gene. Transcription factors respond to environmental stimuli that cause the
proteins to find their binding sites and initiate transcription of the gene that is needed.
Enhancers and Transcription
In some eukaryotic genes, there are additional regions that help increase or enhance transcription. These
regions, called enhancers, are not necessarily close to the genes they enhance. They can be located upstream
of a gene, within the coding region of the gene, downstream of a gene, or may be thousands of nucleotides
away.
Enhancer regions are binding sequences, or sites, for specific transcription factors. When a protein transcription
factor binds to its enhancer sequence, the shape of the protein changes, allowing it to interact with proteins at
the promotor site. However, since the enhancer region may be distant from the promoter, the DNA must bend to
allow the proteins at the two sites to come into contact. DNA bending proteins help to bend the DNA and bring
the enhancer and promoter regions together (Figure 16.9). This shape change allows for the interaction of the
specific activator proteins bound to the enhancers with the general transcription factors bound to the promoter
region and the RNA polymerase.
DNA bending protein Enhancer
Promoter Gene A
RNA polymerase
Gene B
Figure 16.9 Interaction between proteins at the promoter and enhancer sites. An enhancer is a DNA sequence that
promotes transcription. Each enhancer is made up of short DNA sequences called distal control elements. Activators
bound to the distal control elements interact with mediator proteins and transcription factors. Two different genes may
have the same promoter but different distal control elements, enabling differential gene expression.
Turning Genes Off: Transcriptional Repressors
Like prokaryotic cells, eukaryotic cells also have mechanisms to prevent transcription. Transcriptional repressors
can bind to promoter or enhancer regions and block transcription. Like the transcriptional activators, repressors
respond to external stimuli to prevent the binding of activating transcription factors.
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16.5 | Eukaryotic Post-transcriptional Gene Regulation
By the end of this section, you will be able to do the following:
• Understand RNA splicing and explain its role in regulating gene expression
• Describe the importance of RNA stability in gene regulation
RNA is transcribed, but must be processed into a mature form before translation can begin. This processing
that takes place after an RNA molecule has been transcribed, but before it is translated into a protein, is
called post-transcriptional modification. As with the epigenetic and transcriptional stages of processing, this post-
transcriptional step can also be regulated to control gene expression in the cell. If the RNA is not processed,
shuttled, or translated, then no protein will be synthesized.
RNA Splicing, the First Stage of Post-transcriptional Control
In eukaryotic cells, the RNA transcript often contains regions, called introns, that are removed prior to translation.
The regions of RNA that code for protein are called exons. (Figure 16.10). After an RNA molecule has been
transcribed, but prior to its departure from the nucleus to be translated, the RNA is processed and the introns
are removed by splicing. Splicing is done by spliceosomes, ribonucleoprotein complexes that can recognize the
two ends of the intron, cut the transcript at those two points, and bring the exons together for ligation.
pre-mRNA
spliced mRNA
Figure 16.10 Pre-mRNA can be alternatively spliced to create different proteins.
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Alternative RNA Splicing
In the 1970s, genes were first observed that exhibited alternative RNA splicing. Alternative RNA splicing is a
mechanism that allows different protein products to be produced from one gene when different combinations
of exons are combined to form the mRNA (Figure 16.11). This alternative splicing can be haphazard, but
more often it is controlled and acts as a mechanism of gene regulation, with the frequency of different
splicing alternatives controlled by the cell as a way to control the production of different protein products in
different cells or at different stages of development. Alternative splicing is now understood to be a common
mechanism of gene regulation in eukaryotes; according to one estimate, 70 percent of genes in humans are
expressed as multiple proteins through alternative splicing. Although there are multiple ways to alternatively
splice RNA transcripts, the original 5'-3' order of the exons is always conserved. That is, a transcript with
exons 1 2 3 4 5 6 7 might be spliced 1 2 4 5 6 7 or 1 2 3 6 7, but never 1 2 5 4 3 6 7.
Exon skipping
Mutually exclusive exons
Alternative 5’ donor sites
Alternative 3' acceptor sites
Intron retention
Figure 16.11 There are five basic modes of alternative splicing.
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e olution CONNECTION
How could alternative splicing evolve? Introns have a beginning- and ending-recognition sequence; it is
easy to imagine the failure of the splicing mechanism to identify the end of an intron and instead find the
end of the next intron, thus removing two introns and the intervening exon. In fact, there are mechanisms in
place to prevent such intron skipping, but mutations are likely to lead to their failure. Such “mistakes" would
more than likely produce a nonfunctional protein. Indeed, the cause of many genetic diseases is abnormal
splicing rather than mutations in a coding sequence. However, alternative splicing could possibly create a
protein variant without the loss of the original protein, opening up possibilities for adaptation of the new
variant to new functions. Gene duplication has played an important role in the evolution of new functions in
a similar way by providing genes that may evolve without eliminating the original, functional protein.
Question: In the corn snake Pantherophis guttatus, there are several different color variants, including
amelanistic snakes whose skin patterns display only red and yellow pigments. The cause of amelanism in
these snakes was recently identified as the insertion of a transposable element into an intron in the OCA2
(oculocutaneous albinism) gene. How might the insertion of extra genetic material into an intron lead to a
nonfunctional protein?
LINK
T a
LEARNING
Visualize how mRNA splicing happens by watching the process in action in this video. (This multimedia
resource will open in a browser.) (http://cnx.Org/content/m66507/l.5/#eip-idll71119155972)
Control of RNA Stability
Before the mRNA leaves the nucleus, it is given two protective "caps" that prevent the ends of the strand
from degrading during its journey. 5' and 3' exonucleases can degrade unprotected RNAs. The 5' cap, which
is placed on the 5' end of the mRNA, is usually composed of a methylated guanosine triphosphate molecule
(GTP). The GTP is placed "backward" on the 5' end of the mRNA, so that the 5' carbons of the GTP and the
terminal nucleotide are linked through three phosphates. The poly-A tail, which is attached to the 3' end, is
usually composed of a long chain of adenine nucleotides. These changes protect the two ends of the RNA from
exonuclease attack.
Once the RNA is transported to the cytoplasm, the length of time that the RNA resides there can be controlled.
Each RNA molecule has a defined lifespan and decays at a specific rate. This rate of decay can influence
how much protein is in the cell. If the decay rate is increased, the RNA will not exist in the cytoplasm as long,
shortening the time available for translation of the mRNA to occur. Conversely, if the rate of decay is decreased,
the mRNA molecule will reside in the cytoplasm longer and more protein can be translated. This rate of decay is
referred to as the RNA stability. If the RNA is stable, it will be detected for longer periods of time in the cytoplasm.
Binding of proteins to the RNA can also influence its stability. Proteins called RNA-binding proteins, or RBPs,
can bind to the regions of the mRNA just upstream or downstream of the protein-coding region. These regions in
the RNA that are not translated into protein are called the untranslated regions, or UTRs. They are not introns
(those have been removed in the nucleus). Rather, these are regions that regulate mRNA localization, stability,
and protein translation. The region just before the protein-coding region is called the 5' UTR, whereas the region
after the coding region is called the 3' UTR (Figure 16.12). The binding of RBPs to these regions can increase
or decrease the stability of an RNA molecule, depending on the specific RBP that binds.
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RNA-binding proteins
5' cap poly-A tail
Figure 16.12 RNA-binding proteins. The protein-coding region of this processed mRNA is flanked by 5' and 3'
untranslated regions (UTRs). The presence of RNA-binding proteins at the 5' or 3' UTR influences the stability of the
RNA molecule.
RNA Stability and microRNAs
In addition to RBPs that bind to and control (increase or decrease) RNA stability, other elements called
microRNAs can bind to the RNA molecule. These microRNAs, or miRNAs, are short RNA molecules that
are only 21 to 24 nucleotides in length. The miRNAs are made in the nucleus as longer pre-miRNAs. These
pre-miRNAs are chopped into mature miRNAs by a protein called Dicer. Like transcription factors and RBPs,
mature miRNAs recognize a specific sequence and bind to the RNA; however, miRNAs also associate with a
ribonucleoprotein complex called the RNA-induced silencing complex (RISC). The RNA component of the
RISC base-pairs with complementary sequences on an mRNA and either impede translation of the message or
lead to the degradation of the mRNA.
16.6 | Eukaryotic Translational and Post-translational
Gene Regulation
By the end of this section, you will be able to do the following:
• Understand the process of translation and discuss its key factors
• Describe how the initiation complex controls translation
• Explain the different ways in which the post-translational control of gene expression takes place
After RNA has been transported to the cytoplasm, it is translated into protein. Control of this process is largely
dependent on the RNA molecule. As previously discussed, the stability of the RNA will have a large impact on
its translation into a protein. As the stability changes, the amount of time that it is available for translation also
changes.
The Initiation Complex and Translation Rate
Like transcription, translation is controlled by proteins that bind and initiate the process. In translation, the
complex that assembles to start the process is referred to as the translation initiation complex. In eukaryotes,
translation is initiated by binding the initiating met-tRNAi to the 40S ribosome. This tRNA is brought to the 40S
ribosome by a protein initiation factor, eukaryotic initiation factor-2 (elF-2). The elF-2 protein binds to the
high-energy molecule guanosine triphosphate (GTP). The tRNA-elF2-GTP complex then binds to the 40S
ribosome. A second complex forms on the mRNA. Several different initiation factors recognize the 5' cap of the
mRNA and proteins bound to the poly-A tail of the same mRNA, forming the mRNA into a loop. The cap-binding
protein elF4F brings the mRNA complex together with the 40S ribosome complex. The ribosome then scans
along the mRNA until it finds a start codon AUG. When the anticodon of the initiator tRNA and the start codon
are aligned, the GTP is hydrolyzed, the initiation factors are released, and the large 60S ribosomal subunit
binds to form the translation complex. The binding of elF-2 to the RNA is controlled by phosphorylation. If elF-2 is
phosphorylated, it undergoes a conformational change and cannot bind to GTP. Therefore, the initiation complex
cannot form properly and translation is impeded (Figure 16.13). When elF-2 remains unphosphorylated, the
initiation complex can form normally and translation can proceed.
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CONNECTION
When elF-2 is
phosphorylated,
translation is
blocked.
Ribosome
small (40S) subunit
No
Translation
Ribosome
small (40S) subunit
el
elF-
When elF-2 is not
phosphorylated,
translation
occurs.
Translation
occurs
Figure 16.13 Gene expression can be controlled by factors that bind the translation initiation complex.
An increase in phosphorylation levels of elF-2 has been observed in patients with neurodegenerative
diseases such as Alzheimer’s, Parkinson’s, and Huntington’s. What impact do you think this might have on
protein synthesis?
Chemical Modifications, Protein Activity, and Longevity
Proteins can be chemically modified with the addition of groups including methyl, phosphate, acetyl, and
ubiquitin groups. The addition or removal of these groups from proteins regulates their activity or the length of
time they exist in the cell. Sometimes these modifications can regulate where a protein is found in the cell—for
example, in the nucleus, in the cytoplasm, or attached to the plasma membrane.
Chemical modifications occur in response to external stimuli such as stress, the lack of nutrients, heat, or
ultraviolet light exposure. These changes can alter epigenetic accessibility, transcription, mRNA stability, or
translation—all resulting in changes in expression of various genes. This is an efficient way for the cell to rapidly
change the levels of specific proteins in response to the environment. Because proteins are involved in every
stage of gene regulation, the phosphorylation of a protein (depending on the protein that is modified) can alter
accessibility to the chromosome, can alter translation (by altering transcription factor binding or function), can
change nuclear shuttling (by influencing modifications to the nuclear pore complex), can alter RNA stability (by
binding or not binding to the RNA to regulate its stability), can modify translation (increase or decrease), or can
change post-translational modifications (add or remove phosphates or other chemical modifications).
The addition of an ubiquitin group to a protein marks that protein for degradation. Ubiquitin acts like a flag
indicating that the protein lifespan is complete. These proteins are moved to the proteasome, an organelle that
functions to remove proteins, to be degraded (Figure 16.14). One way to control gene expression, therefore, is
to alter the longevity of the protein.
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Amino acids
Figure 16.14 Proteins with ubiquitin tags are marked for degradation within the proteasome.
16.7 | Cancer and Gene Regulation
By the end of this section, you will be able to do the following:
• Describe how changes to gene expression can cause cancer
• Explain how changes to gene expression at different levels can disrupt the cell cycle
• Discuss how understanding regulation of gene expression can lead to better drug design
Cancer is not a single disease but includes many different diseases. In cancer cells, mutations modify cell-cycle
control and cells don’t stop growing as they normally would. Mutations can also alter the growth rate or the
progression of the cell through the cell cycle. One example of a gene modification that alters the growth rate is
increased phosphorylation of cyclin B, a protein that controls the progression of a cell through the cell cycle and
serves as a cell-cycle checkpoint protein.
For cells to move through each phase of the cell cycle, the cell must pass through checkpoints. This ensures
that the cell has properly completed the step and has not encountered any mutation that will alter its function.
Many proteins, including cyclin B, control these checkpoints. The phosphorylation of cyclin B, a post-translational
event, alters its function. As a result, cells can progress through the cell cycle unimpeded, even if mutations
exist in the cell and its growth should be terminated. This post-translational change of cyclin B prevents it from
controlling the cell cycle and contributes to the development of cancer.
Cancer: Disease of Altered Gene Expression
Cancer can be described as a disease of altered gene expression. There are many proteins that are turned
on or off (gene activation or gene silencing) that dramatically alter the overall activity of the cell. A gene that is
not normally expressed in that cell can be switched on and expressed at high levels. This can be the result of
gene mutation or changes in gene regulation (epigenetic, transcription, post-transcription, translation, or post¬
translation).
Changes in epigenetic regulation, transcription, RNA stability, protein translation, and post-translational control
can be detected in cancer. While these changes don’t occur simultaneously in one cancer, changes at each of
these levels can be detected when observing cancer at different sites in different individuals. Therefore, changes
in histone acetylation (epigenetic modification that leads to gene silencing), activation of transcription factors
by phosphorylation, increased RNA stability, increased translational control, and protein modification can all be
detected at some point in various cancer cells. Scientists are working to understand the common changes that
give rise to certain types of cancer or how a modification might be exploited to destroy a tumor cell.
Tumor Suppressor Genes, Oncogenes, and Cancer
In normal cells, some genes function to prevent excess, inappropriate cell growth. These are tumor-suppressor
genes, which are active in normal cells to prevent uncontrolled cell growth. There are many tumor-suppressor
genes in cells. The most studied tumor-suppressor gene is p53, which is mutated in over 50 percent of all cancer
types. The p53 protein itself functions as a transcription factor. It can bind to sites in the promoters of genes to
initiate transcription. Therefore, the mutation of p53 in cancer will dramatically alter the transcriptional activity of
its target genes.
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LINK TQ LEARNING
Watch this animation (http://openstaxcollege.Org/l/p53_cancer) to learn more about the use of p53 in
fighting cancer.
Proto-oncogenes are positive cell-cycle regulators. When mutated, proto-oncogenes can become oncogenes
and cause cancer. Overexpression of the oncogene can lead to uncontrolled cell growth. This is because
oncogenes can alter transcriptional activity, stability, or protein translation of another gene that directly or
indirectly controls cell growth. An example of an oncogene involved in cancer is a protein called myc. Myc
is a transcription factor that is aberrantly activated in Burkett’s Lymphoma, a cancer of the lymph system.
Overexpression of myc transforms normal B cells into cancerous cells that continue to grow uncontrollably.
High B-cell numbers can result in tumors that can interfere with normal bodily function. Patients with Burkett’s
lymphoma can develop tumors on their jaw or in their mouth that interfere with the ability to eat.
Cancer and Epigenetic Alterations
Silencing genes through epigenetic mechanisms is also very common in cancer cells. There are characteristic
modifications to histone proteins and DNA that are associated with silenced genes. In cancer cells, the DNA
in the promoter region of silenced genes is methylated on cytosine DNA residues in CpG islands. Histone
proteins that surround that region lack the acetylation modification that is present when the genes are expressed
in normal cells. This combination of DNA methylation and histone deacetylation (epigenetic modifications that
lead to gene silencing) is commonly found in cancer. When these modifications occur, the gene present in
that chromosomal region is silenced. Increasingly, scientists understand how epigenetic changes are altered in
cancer. Because these changes are temporary and can be reversed—for example, by preventing the action of
the histone deacetylase protein that removes acetyl groups, or by DNA methyl transferase enzymes that add
methyl groups to cytosines in DNA—it is possible to design new drugs and new therapies to take advantage
of the reversible nature of these processes. Indeed, many researchers are testing how a silenced gene can be
switched back on in a cancer cell to help re-establish normal growth patterns.
Genes involved in the development of many other illnesses, ranging from allergies to inflammation to autism,
are thought to be regulated by epigenetic mechanisms. As our knowledge of how genes are controlled deepens,
new ways to treat diseases like cancer will emerge.
Cancer and Transcriptional Control
Alterations in cells that give rise to cancer can affect the transcriptional control of gene expression. Mutations
that activate transcription factors, such as increased phosphorylation, can increase the binding of a transcription
factor to its binding site in a promoter. This could lead to increased transcriptional activation of that gene that
results in modified cell growth. Alternatively, a mutation in the DNA of a promoter or enhancer region can
increase the binding ability of a transcription factor. This could also lead to the increased transcription and
aberrant gene expression that is seen in cancer cells.
Researchers have been investigating how to control the transcriptional activation of gene expression in cancer.
Identifying how a transcription factor binds, or a pathway that activates where a gene can be turned off, has led to
new drugs and new ways to treat cancer. In breast cancer, for example, many proteins are overexpressed. This
can lead to increased phosphorylation of key transcription factors that increase transcription. One such example
is the overexpression of the epidermal growth-factor receptor (EGFR) in a subset of breast cancers. The EGFR
pathway activates many protein kinases that, in turn, activate many transcription factors which control genes
involved in cell growth. New drugs that prevent the activation of EGFR have been developed and are used to
treat these cancers.
Cancer and Post-transcriptional Control
Changes in the post-transcriptional control of a gene can also result in cancer. Recently, several groups of
researchers have shown that specific cancers have altered expression of miRNAs. Because miRNAs bind to the
3' UTR of RNA molecules to degrade them, overexpression of these miRNAs could be detrimental to normal
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cellular activity. Too many miRNAs could dramatically decrease the RNA population, leading to a decrease in
protein expression. Several studies have demonstrated a change in the miRNA population in specific cancer
types. It appears that the subset of miRNAs expressed in breast cancer cells is quite different from the subset
expressed in lung cancer cells or even from normal breast cells. This suggests that alterations in miRNA activity
can contribute to the growth of breast cancer cells. These types of studies also suggest that if some miRNAs
are specifically expressed only in cancer cells, they could be potential drug targets. It would, therefore, be
conceivable that new drugs that turn off miRNA expression in cancer could be an effective method to treat
cancer.
Cancer and Translational/Post-translational Control
There are many examples of how translational or post-translational modifications of proteins arise in cancer.
Modifications are found in cancer cells from the increased translation of a protein to changes in protein
phosphorylation to alternative splice variants of a protein. An example of how the expression of an alternative
form of a protein can have dramatically different outcomes is seen in colon cancer cells. The c-Flip protein, a
protein involved in mediating the cell-death pathway, comes in two forms: long (c-FLIPL) and short (c-FLIPS).
Both forms appear to be involved in initiating controlled cell-death mechanisms in normal cells. However, in
colon cancer cells, expression of the long form results in increased cell growth instead of cell death. Clearly, the
expression of the wrong protein dramatically alters cell function and contributes to the development of cancer.
New Drugs to Combat Cancer: Targeted Therapies
Scientists are using what is known about the regulation of gene expression in disease states, including cancer, to
develop new ways to treat and prevent disease development. Many scientists are designing drugs on the basis
of the gene expression patterns within individual tumors. This idea, that therapy and medicines can be tailored
to an individual, has given rise to the field of personalized medicine. With an increased understanding of gene
regulation and gene function, medicines can be designed to specifically target diseased cells without harming
healthy cells. Some new medicines, called targeted therapies, have exploited the overexpression of a specific
protein or the mutation of a gene to develop a new medication to treat disease. One such example is the use
of anti-EGF receptor medications to treat the subset of breast cancer tumors that have very high levels of the
EGF protein. Undoubtedly, more targeted therapies will be developed as scientists learn more about how gene
expression changes can cause cancer.
ca eer connection
Clinical Trial Coordinator
A clinical trial coordinator is the person managing the proceedings of the clinical trial. This job includes
coordinating patient schedules and appointments, maintaining detailed notes, building the database to track
patients (especially for long-term follow-up studies), ensuring proper documentation has been acquired and
accepted, and working with the nurses and doctors to facilitate the trial and publication of the results. A
clinical trial coordinator may have a science background, like a nursing degree, or other certification. People
who have worked in science labs or in clinical offices are also qualified to become a clinical trial coordinator.
These jobs are generally in hospitals; however, some clinics and doctor’s offices also conduct clinical trials
and may hire a coordinator.
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KEY TERMS
3' UTR 3' untranslated region; region just downstream of the protein-coding region in an RNA molecule that is
not translated
5' cap a methylated guanosine triphosphate (GTP) molecule that is attached to the 5' end of a messenger RNA
to protect the end from degradation
5' UTR 5' untranslated region; region just upstream of the protein-coding region in an RNA molecule that is not
translated
activator protein that binds to prokaryotic operators to increase transcription
catabolite activator protein (CAP) protein that complexes with cAMP to bind to the promoter sequences of
operons which control sugar processing when glucose is not available
c/s-acting element transcription factor binding sites within the promoter that regulate the transcription of a gene
adjacent to it
Dicer enzyme that chops the pre-miRNA into the mature form of the miRNA
DNA methylation epigenetic modification that leads to gene silencing; a process involving adding a methyl
group to the DNA molecule
enhancer segment of DNA that is upstream, downstream, perhaps thousands of nucleotides away, or on
another chromosome that influence the transcription of a specific gene
epigenetic heritable changes that do not involve changes in the DNA sequence
eukaryotic initiation factor-2 (elF-2) protein that binds first to an mRNA to initiate translation
gene expression processes that control the turning on or turning off of a gene
guanine diphosphate (GDP) molecule that is left after the energy is used to start translation
guanine triphosphate (GTP) energy-providing molecule that binds to elF-2 and is needed for translation
histone acetylation epigenetic modification that leads to gene silencing; a process involving adding or
removing an acetyl functional group
inducible operon operon that can be activated or repressed depending on cellular needs and the surrounding
environment
initiation complex protein complex containing elF-2 that starts translation
lac operon operon in prokaryotic cells that encodes genes required for processing and intake of lactose
large 60S ribosomal subunit second, larger ribosomal subunit that binds to the RNA to translate it into protein
microRNA (miRNA) small RNA molecules (approximately 21 nucleotides in length) that bind to RNA molecules
to degrade them
myc oncogene that causes cancer in many cancer cells
negative regulator protein that prevents transcription
operator region of DNA outside of the promoter region that binds activators or repressors that control gene
expression in prokaryotic cells
operon collection of genes involved in a pathway that are transcribed together as a single mRNA in prokaryotic
cells
poly-A tail a series of adenine nucleotides that are attached to the 3' end of an mRNA to protect the end from
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Chapter 16 | Gene Expression
degradation
positive regulator protein that increases transcription
post-transcriptional control of gene expression after the RNA molecule has been created but before it is
translated into protein
post-translational control of gene expression after a protein has been created
proteasome organelle that degrades proteins
repressor protein that binds to the operator of prokaryotic genes to prevent transcription
RISC protein complex that binds along with the miRNA to the RNA to degrade it
RNA stability how long an RNA molecule will remain intact in the cytoplasm
RNA-binding protein (RBP) protein that binds to the 3' or 5' UTR to increase or decrease the RNA stability
small 40S ribosomal subunit ribosomal subunit that binds to the RNA to translate it into protein
frans-acting element transcription factor binding site found outside the promoter or on another chromosome
that influences the transcription of a particular gene
transcription factor protein that binds to the DNA at the promoter or enhancer region and that influences
transcription of a gene
transcription factor binding site sequence of DNA to which a transcription factor binds
transcriptional start site site at which transcription begins
trp operon series of genes necessary to synthesize tryptophan in prokaryotic cells
tryptophan amino acid that can be synthesized by prokaryotic cells when necessary
untranslated region segment of the RNA molecule that is not translated into protein. These regions lie before
(upstream or 5') and after (downstream or 3') the protein-coding region
CHAPTER SUMMARY
16.1 Regulation of Gene Expression
While all somatic cells within an organism contain the same DNA, not all cells within that organism express the
same proteins. Prokaryotic organisms express most of their genes most of the time. However, some genes are
expressed only when they are needed. Eukaryotic organisms, on the other hand, express only a subset of their
genes in any given cell. To express a protein, the DNA is first transcribed into RNA, which is then translated
into proteins, which are then targeted to specific cellular locations. In prokaryotic cells, transcription and
translation occur almost simultaneously. In eukaryotic cells, transcription occurs in the nucleus and is separate
from the translation that occurs in the cytoplasm. Gene expression in prokaryotes is mostly regulated at the
transcriptional level (some epigenetic and post-translational regulation is also present), whereas in eukaryotic
cells, gene expression is regulated at the epigenetic, transcriptional, post-transcriptional, translational, and
post-translational levels.
16.2 Prokaryotic Gene Regulation
The regulation of gene expression in prokaryotic cells occurs at the transcriptional level. There are two majors
kinds of proteins that control prokaryotic transcription: repressors and activators. Repressors bind to an
operator region to block the action of RNA polymerase. Activators bind to the promoter to enhance the binding
of RNA polymerase. Inducer molecules can increase transcription either by inactivating repressors or by
activating activator proteins. In the trp operon, the trp repressor is itself activated by binding to tryptophan.
Therefore, if tryptophan is not needed, the repressor is bound to the operator and transcription remains off. The
lac operon is activated by the CAP (catabolite activator protein), which binds to the promoter to stabilize RNA
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polymerase binding. CAP is itself activated by cAMP, whose concentration rises as the concentration of
glucose falls. However, the lac operon also requires the presence of lactose for transcription to occur. Lactose
inactivates the lac repressor, and prevents the repressor protein from binding to the lac operator. With the
repressor inactivated, transcription may proceed. Therefore glucose must be absent and lactose must be
present for effective transcription of the lac operon.
16.3 Eukaryotic Epigenetic Gene Regulation
In eukaryotic cells, the first stage of gene-expression control occurs at the epigenetic level. Epigenetic
mechanisms control access to the chromosomal region to allow genes to be turned on or off. Chromatin
remodeling controls how DNA is packed into the nucleus by regulating how tightly the DNA is wound around
histone proteins. The DNA itself may be methylated to selectively silence genes. The addition or removal of
chemical modifications (or flags) to histone proteins or DNA signals the cell to open or close a chromosomal
region. Therefore, eukaryotic cells can control whether a gene is expressed by controlling accessibility to the
binding of RNA polymerase and its transcription factors.
16.4 Eukaryotic Transcription Gene Regulation
To start transcription, general transcription factors, such as TFIID, TFIIB, and others, must first bind to the TATA
box and recruit RNA polymerase to that location. Additional transcription factors may also bind to other
regulatory elements at the promoter to increase or prevent transcription, in addition to promoter sequences,
enhancer regions help augment transcription. Enhancers can be upstream, downstream, within a gene itself, or
on other chromosomes. Specific transcription factors bound to enhancer regions may either increase or prevent
transcription.
16.5 Eukaryotic Post-transcriptional Gene Regulation
Post-transcriptional control can occur at any stage after transcription, including RNA splicing and RNA stability.
Once RNA is transcribed, it must be processed to create a mature RNA that is ready to be translated. This
involves the removal of introns that do not code for protein. Spliceosomes bind to the signals that mark the
exon/intron border to remove the introns and ligate the exons together. Once this occurs, the RNA is mature
and can be translated. Alternative splicing can produce more than one mRNA from a given transcript. Different
splicing variants may be produced under different conditions.
RNA is created and spliced in the nucleus, but needs to be transported to the cytoplasm to be translated. RNA
is transported to the cytoplasm through the nuclear pore complex. Once the RNA is in the cytoplasm, the length
of time it resides there before being degraded, called RNA stability, can also be altered to control the overall
amount of protein that is synthesized. The RNA stability can be increased, leading to longer residency time in
the cytoplasm, or decreased, leading to shortened time and less protein synthesis. RNA stability is controlled
by RNA-binding proteins (RPBs) and microRNAs (miRNAs). These RPBs and miRNAs bind to the 5' UTR or
the 3' UTR of the RNA to increase or decrease RNA stability. MicroRNAs associated with RISC complexes may
repress translation or lead to mRNA breakdown.
16.6 Eukaryotic Translational and Post-translational Gene Regulation
Changing the status of the RNA or the protein itself can affect the amount of protein, the function of the protein,
or how long it is found in the cell. To translate the protein, a protein initiator complex must assemble on the
RNA. Modifications (such as phosphorylation) of proteins in this complex can prevent proper translation from
occurring. Once a protein has been synthesized, it can be modified (phosphorylated, acetylated, methylated, or
ubiquitinated). These post-translational modifications can greatly impact the stability, degradation, or function of
the protein.
16.7 Cancer and Gene Regulation
Cancer can be described as a disease of altered gene expression. Changes at every level of eukaryotic gene
expression can be detected in some form of cancer at some point in time. In order to understand how changes
to gene expression can cause cancer, it is critical to understand how each stage of gene regulation works in
normal cells. By understanding the mechanisms of control in normal, non-diseased cells, it will be easier for
scientists to understand what goes wrong in disease states including complex ones like cancer.
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Chapter 16 | Gene Expression
VISUAL CONNECTION QUESTIONS
1. Figure 16.5 In E. coli, the trp operon is on by
default, while the lac operon is off. Why do you think
that this is the case?
2. Figure 16.7 in females, one of the two X
chromosomes is inactivated during embryonic
development because of epigenetic changes to the
chromatin. What impact do you think these changes
REVIEW QUESTIONS
4. Control of gene expression in eukaryotic cells
occurs at which level(s)?
a. only the transcriptional level
b. epigenetic and transcriptional levels
c. epigenetic, transcriptional, and translational
levels
d. epigenetic, transcriptional, post-
transcriptional, translational, and post-
translational levels
5. Post-translational control refers to:
a. regulation of gene expression after
transcription
b. regulation of gene expression after
translation
c. control of epigenetic activation
d. period between transcription and translation
6. How does the regulation of gene expression
support continued evolution of more complex
organisms?
a. Cells can become specialized within a
multicellular organism.
b. Organisms can conserve energy and
resources.
c. Cells grow larger to accommodate protein
production.
d. Both A and B.
7. If glucose is absent, but so is lactose, the lac
operon will be_.
a. activated
b. repressed
c. activated, but only partially
d. mutated
8. Prokaryotic cells lack a nucleus. Therefore, the
genes in prokaryotic cells are:
a. all expressed, all of the time
b. transcribed and translated almost
simultaneously
c. transcriptionally controlled because
translation begins before transcription ends
d. b and c are both true
9. The ara operon is an inducible operon that controls
the production of the sugar arabinose. When
arabinose is present in a bacterium it binds to the
protein AraC, and the complex binds to the initiator
would have on nucleosome packing?
3. Figure 16.13 An increase in phosphorylation
levels of elF-2 has been observed in patients with
neurodegenerative diseases such as Alzheimer’s,
Parkinson’s, and Huntington’s. What impact do you
think this might have on protein synthesis?
site to promote transcription. In this scenario, AraC is
a(n)_.
a. activator
b. inducer
c. repressor
d. operator
10. What are epigenetic modifications?
a. the addition of reversible changes to histone
proteins and DNA
b. the removal of nucleosomes from the DNA
c. the addition of more nucleosomes to the
DNA
d. mutation of the DNA sequence
11. Which of the following are true of epigenetic
changes?
a. allow DNA to be transcribed
b. move histones to open or close a
chromosomal region
c. are temporary
d. all of the above
12. The binding of_is required for
transcription to start.
a. a protein
b. DNA polymerase
c. RNA polymerase
d. a transcription factor
13. What will result from the binding of a transcription
factor to an enhancer region?
a. decreased transcription of an adjacent gene
b. increased transcription of a distant gene
c. alteration of the translation of an adjacent
gene
d. initiation of the recruitment of RNA
polymerase
14. A scientist compares the promoter regions of two
genes. Gene A’s core promoter plus proximal
promoter elements encompasses 70bp. Gene B’s
core promoter plus proximal promoter elements
encompasses 250bp. Which of the scientist’s
hypotheses is most likely to be correct?
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Chapter 16 | Gene Expression
459
a. More transcripts will be made from Gene B
b. Transcription of Gene A involves fewer
transcription factors.
c. Enhancers control Gene B’s transcription.
d. Transcription of Gene A is more controlled
than transcription of Gene B.
15. Which of the following are involved in post-
transcriptional control?
a. control of RNA splicing
b. control of RNA shuttling
c. control of RNA stability
d. all of the above
16. Binding of an RNA binding protein will
the stability of the RNA molecule.
a. increase
b. decrease
c. neither increase nor decrease
d. either increase or decrease
17. An unprocessed pre-mRNA has the following
structure.
20bp
50bp HObp 70bp I 130bp
Exon Intron Exon ^ Exon
Intron
30bp
75bp ^
Intron ,
Exon
Which of the following is not a possible size (in bp) of
the mature mRNA?
a. 205bp
b. 180bp
c. 150bp
d. lOObp
18. Alternative splicing has been estimated to occur
in more than 95% of multi-exon genes. Which of the
following is not an evolutionary advantage of
alternative splicing?
a. Alternative splicing increases diversity
without increasing genome size.
b. Different gene isoforms can be expressed in
different tissues.
c. Alternative splicing creates shorter mRNA
transcripts.
d. Different gene isoforms can be expressed
during different stages of development.
CRITICAL THINKING QUESTIONS
23. Name two differences between prokaryotic and
eukaryotic cells and how these differences benefit
multicellular organisms.
24. Describe how controlling gene expression will
alter the overall protein levels in the cell.
25. Describe how transcription in prokaryotic cells
can be altered by external stimulation such as excess
lactose in the environment.
26. What is the difference between a repressible and
19. Post-translational modifications of proteins can
affect which of the following?
a. protein function
b. transcriptional regulation
c. chromatin modification
d. all of the above
20. A scientist mutates elF-2 to eliminate its GTP
hydrolysis capability. How would this mutated form of
elF-2 alter translation?
a. Initiation factors would not be able to bind to
mRNA.
b. The large ribosomal subunit would not be
able to interact with mRNA transcripts.
c. tRNAi-Met would not scan mRNA transcripts
for the start codon.
d. elF-2 would not be able to interact with the
small ribosomal subunit.
21. Cancer causing genes are called_.
a. transformation genes
b. tumor suppressor genes
c. oncogenes
d. mutated genes
22. Targeted therapies are used in patients with a set
gene expression pattern. A targeted therapy that
prevents the activation of the estrogen receptor in
breast cancer would be beneficial to which type of
patient?
a. patients who express the EGFR receptor in
normal cells
b. patients with a mutation that inactivates the
estrogen receptor
c. patients with lots of the estrogen receptor
expressed in their tumor
d. patients that have no estrogen receptor
expressed in their tumor
an inducible operon?
27. In cancer cells, alteration to epigenetic
modifications turns off genes that are normally
expressed. Hypothetically, how could you reverse
this process to turn these genes back on?
28. A scientific study demonstrated that rat mothering
behavior impacts the stress response in their pups.
Rats that were born and grew up with attentive
mothers showed low activation of stress-response
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Chapter 16 | Gene Expression
genes later in life, while rats with inattentive mothers
had high activation of stress-response genes in the
same situation. An additional study that swapped the
pups at birth (i.e., rats born to inattentive mothers
grew up with attentive mothers and vice versa)
showed the same positive effect of attentive
mothering. How do genetics and/or epigenetics
explain the results of this study?
29. Some autoimmune diseases show a positive
correlation with dramatically decreased expression of
histone deacetylase 9 (HDAC9, an enzyme that
removes acetyl groups from histones). Why would
the decreased expression of HDAC9 cause immune
cells to produce inflammatory genes at inappropriate
times?
30. A mutation within the promoter region can alter
transcription of a gene. Describe how this can
happen.
31. What could happen if a cell had too much of an
activating transcription factor present?
32. A scientist identifies a potential transcription
regulation site 300bp downstream of a gene and
hypothesizes that it is a repressor. What experiment
(with results) could he perform to support this
hypothesis?
33. Describe how RBPs can prevent miRNAs from
degrading an RNA molecule.
34. How can external stimuli alter post-transcriptional
control of gene expression?
35. Protein modification can alter gene expression in
many ways. Describe how phosphorylation of
proteins can alter gene expression.
36. Alternative forms of a protein can be beneficial or
harmful to a cell. What do you think would happen if
too much of an alternative protein bound to the 3'
UTR of an RNA and caused it to degrade?
37. Changes in epigenetic modifications alter the
accessibility and transcription of DNA. Describe how
environmental stimuli, such as ultraviolet light
exposure, could modify gene expression.
38. A scientist discovers a virus encoding a Protein X
that degrades a subunit of the elF4F complex.
Knowing that this virus transcribes its own mRNAs in
the cytoplasm of human cells, why would Protein X
be an effective virulence factor?
39. New drugs are being developed that decrease
DNA methylation and prevent the removal of acetyl
groups from histone proteins. Explain how these
drugs could affect gene expression to help kill tumor
cells.
40. How can understanding the gene expression
pattern in a cancer cell tell you something about that
specific form of cancer?
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Chapter 17 | Biotechnology and Genomics
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17 | BIOTECHNOLOGY
AND GENOMICS
Figure 17.1 Genomics compares the DNA of different organisms, enabling scientists to create maps with which to
navigate different organisms' DNA. (credit "map": modification of photo by NASA)
Chapter Outline
17.1: Biotechnology
17.2: Mapping Genomes
17.3: Whole-Genome Sequencing
17.4: Applying Genomics
17.5: Genomics and Proteomics
Introduction
The study of nucleic acids began with the discovery of DNA, progressed to the study of genes and small
fragments, and has now exploded to the field of genomics. Genomics is the study of entire genomes, including
the complete set of genes, their nucleotide sequence and organization, and their interactions within a species
and with other species. DNA sequencing technology has contributed to advances in genomics. Just as
information technology has led to Google maps that enable people to obtain detailed information about locations
around the globe, researchers use genomic information to create similar DNA maps of different organisms.
These findings have helped anthropologists to better understand human migration and have aided the medical
field through mapping human genetic diseases. Genomic information can contribute to scientific understanding
in various ways and knowledge in the field is quickly growing.
17.1 1 Biotechnology
By the end of this section, you will be able to do the following:
• Describe gel electrophoresis
• Explain molecular and reproductive cloning
• Describe biotechnology uses in medicine and agriculture
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Chapter 17 | Biotechnology and Genomics
Biotechnology is the use of biological agents for technological advancement. Biotechnology was used for
breeding livestock and crops long before people understood the scientific basis of these techniques. Since the
discovery of the structure of DNA in 1953, the biotechnology field has grown rapidly through both academic
research and private companies. The primary applications of this technology are in medicine (vaccine and
antibiotic production) and agriculture (crop genetic modification in order to increase yields). Biotechnology also
has many industrial applications, such as fermentation, treating oil spills, and producing biofuels (Figure 17.2).
Figure 17.2 Fungi, bacteria, and other organisms that have antimicrobial properties produce antibiotics. The first
antibiotic discovered was penicillin. Pharmaceutical companies now commercially produce and test antibiotics for their
potential to inhibit bacterial growth, (credit "advertisement": modification of work by NIH; credit "test plate": modification
of work by Don Stalons/CDC; scale-bar data from Matt Russell)
Basic Techniques to Manipulate Genetic Material (DNA and RNA)
To understand the basic techniques used to work with nucleic acids, remember that nucleic acids are
macromolecules made of nucleotides (a sugar, a phosphate, and a nitrogenous base) linked by phosphodiester
bonds. The phosphate groups on these molecules each have a net negative charge. An entire set of DNA
molecules in the nucleus is called the genome. DNA has two complementary strands linked by hydrogen bonds
between the paired bases. Exposure to high temperatures (DNA denaturation) can separate the two strands
and cooling can reanneal them. The DNA polymerase enzyme can replicate the DNA. Unlike DNA, which is
located in the eukaryotic cells' nucleus, RNA molecules leave the nucleus. The most common type of RNA that
researchers analyze is the messenger RNA (mRNA) because it represents the protein-coding genes that are
actively expressed. However, RNA molecules present some other challenges to analysis, as they are often less
stable than DNA.
DNA and RNA Extraction
To study or manipulate nucleic acids, one must first isolate or extract the DNA or RNA from the cells.
Researchers use various techniques to extract different types of DNA (Figure 17.3). Most nucleic acid extraction
techniques involve steps to break open the cell and use enzymatic reactions to destroy all macromolecules that
are not desired (such as unwanted molecule degradation and separation from the DNA sample). A lysis buffer
(a solution which is mostly a detergent) breaks cells. Note that lysis means "to split". These enzymes break
apart lipid molecules in the cell membranes and nuclear membranes. Enzymes such as proteases that break
down proteins inactivate macromolecules, and ribonucleases (RNAses) that break down RNA. Using alcohol
precipitates the DNA. Human genomic DNA is usually visible as a gelatinous, white mass. One can store the
DNA samples frozen at -80°C for several years.
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Chapter 17 | Biotechnology and Genomics
463
DNA Extraction
Cells are lysed
using a detergent
that disrupts the
plasma membrane.
Cell contents
are treated with
protease to
destroy protein,
and RNAase to
destroy RNA.
Cell debris is
pelleted in a
centrifuge. The
supernatant (liquid)
containing the DNA
is transferred to a
clean tube.
The DNA is
precipitated
with ethanol.
It forms viscous
strands that can
be spooled on
a glass rod.
Figure 17.3 This diagram shows the basic method of DNA extraction.
Scientists perform RNA analysis to study gene expression patterns in cells. RNA is naturally very unstable
because RNAses are commonly present in nature and very difficult to inactivate. Similar to DNA, RNA extraction
involves using various buffers and enzymes to inactivate macromolecules and preserve the RNA.
Gel Electrophoresis
Because nucleic acids are negatively charged ions at neutral or basic pH in an aqueous environment, an electric
field can mobilize them. Gel electrophoresis is a technique that scientists use to separate molecules on the
basis of size, using this charge. One can separate the nucleic acids as whole chromosomes or fragments.
The nucleic acids load into a slot near the semisolid, porous gel matrix's negative electrode, and pulled toward
the positive electrode at the gel's opposite end. Smaller molecules move through the gel's pores faster than
larger molecules. This difference in the migration rate separates the fragments on the basis of size. There
are molecular weight standard samples that researchers can run alongside the molecules to provide a size
comparison. We can observe nucleic acids in a gel matrix using various fluorescent or colored dyes. Distinct
nucleic acid fragments appear as bands at specific distances from the gel's top (the negative electrode end) on
the basis of their size (Figure 17.4). A mixture of genomic DNA fragments of varying sizes appear as a long
smear; whereas, uncut genomic DNA is usually too large to run through the gel and forms a single large band at
the gel's top.
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Chapter 17 | Biotechnology and Genomics
(a) (b)
Figure 17.4 a) Shown are DNA fragments from seven samples run on a gel, stained with a fluorescent dye, and viewed
under UV light; and b) a researcher from International Rice Research Institute, reviewing DNA profiles using UV light,
(credit: a: James Jacob, Tompkins Cortland Community College b: International Rice Research Institute)
Nucleic Acid Fragment Amplification by Polymerase Chain Reaction
Although genomic DNA is visible to the naked eye when it is extracted in bulk, DNA analysis often requires
focusing on one or more specific genome regions. Polymerase chain reaction (PCR) is a technique that
scientists use to amplify specific DNA regions for further analysis (Figure 17.5). Researchers use PCR for many
purposes in laboratories, such as cloning gene fragments to analyze genetic diseases, identifying contaminant
foreign DNA in a sample, and amplifying DNA for sequencing. More practical applications include determining
paternity and detecting genetic diseases.
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Chapter 17 | Biotechnology and Genomics
465
Polymerase Chain Reaction (PCR)
The PCR cycle consists of three steps—denaturation,
annealing, and DNA synthesis—that occur at high, low,
and intermediate temperatures, respectively. The cycle
is repeated again and again, resulting in a doubling of
DNA molecules each time. After several cycles, the vast
majority of strands produced are the same length as the
distance between the two primers.
Figure 17.5 Scientists use polymerase chain reaction, or PCR, to amplify a specific DNA sequence. Primers—short
pieces of DNA complementary to each end of the target sequence combine with genomic DNA, Taq polymerase, and
deoxynucleotides. Taq polymerase is a DNA polymerase isolated from the thermostable bacterium Thermus aquaticus
that is able to withstand the high temperatures that scientists use in PCR. Thermus aquaticus grows in the Lower
Geyser Basin of Yellowstone National Park. Reverse transcriptase PCR (RT-PCR) is similar to PCR, but cDNA is made
from an RNA template before PCR begins.
DNA fragments can also be amplified from an RNA template in a process called reverse transcriptase PCR
(RT-PCR). The first step is to recreate the original DNA template strand (called cDNA) by applying DNA
nucleotides to the mRNA. This process is called reverse transcription. This requires the presence of an enzyme
called reverse transcriptase. After the cDNA is made, regular PCR can be used to amplify it.
LINK TQ LEARNING
Deepen your understanding of the polymerase chain reaction by clicking through this interactive exercise
(http://0penstaxc0llege.0rg/l/PCR) .
Hybridization, Southern Blotting, and Northern Blotting
Scientists can probe nucleic acid samples, such as fragmented genomic DNA and RNA extracts, for the
presence of certain sequences. Scientists design and label short DNA fragments, or probes with radioactive or
fluorescent dyes to aid detection. Gel electrophoresis separates the nucleic acid fragments according to their
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Chapter 17 | Biotechnology and Genomics
size. Scientists then transfer the fragments in the gel onto a nylon membrane in a procedure we call blotting
(Figure 17.6). Scientists can then probe the nucleic acid fragments that are bound to the membrane's surface
with specific radioactively or fluorescently labeled probe sequences. When scientists transfer DNA to a nylon
membrane, they refer to the technique as Southern blotting. When they transfer the RNA to a nylon membrane,
they call it Northern blotting. Scientists use Southern blots to detect the presence of certain DNA sequences in
a given genome, and Northern blots to detect gene expression.
Southern Blotting
Electrophoresis is used to
separate DNA fragments by
size. There can be so many
fragments that they appear
as a smear on the gel.
Nylon
The DNA is transferred
from the agarose gel to
a nylon membrane.
Filter paper
The membrane is bathed in a solution
containing a probe, a short piece of
DNA complementary to the sequence
of interest. The probe is labeled or
tagged with a fluorescent dye so that
the location of DNA fragments to
which it hybridizes can be visualized.
Figure 17.6 Scientists use Southern blotting to find a particular sequence in a DNA sample. Scientists separate DNA
fragments on a gel, transfer them to a nylon membrane, and incubate them with a DNA probe complementary to the
sequence of interest. Northern blotting is similar to Southern blotting, but scientists run RNA on the gel instead of DNA.
In Western blotting, scientists run proteins on a gel and detect them using antibodies.
Molecular Cloning
In general, the word “cloning" means the creation of a perfect replica; however, in biology, the re-creation of
a whole organism is referred to as “reproductive cloning.” Long before attempts were made to clone an entire
organism, researchers learned how to reproduce desired regions or fragments of the genome, a process that is
referred to as molecular cloning.
Cloning small genome fragments allows researchers to manipulate and study specific genes (and their protein
products), or noncoding regions in isolation. A plasmid, or vector, is a small circular DNA molecule that replicates
independently of the chromosomal DNA. In cloning, scientists can use the plasmid molecules to provide a
"folder" in which to insert a desired DNA fragment. Plasmids are usually introduced into a bacterial host for
proliferation. In the bacterial context, scientists call the DNA fragment from the human genome (or the genome
of another studied organism) foreign DNA, or a transgene, to differentiate it from the bacterium's DNA, or the
host DNA.
Plasmids occur naturally in bacterial populations (such as Escherichia coii) and have genes that can contribute
favorable traits to the organism, such as antibiotic resistance (the ability to be unaffected by antibiotics).
Scientists have repurposed and engineered plasmids as vectors for molecular cloning and the large-scale
production of important reagents, such as insulin and human growth hormone. An important feature of plasmid
vectors is the ease with which scientists can introduce a foreign DNA fragment via the multiple cloning
site (MCS). The MCS is a short DNA sequence containing multiple sites that different commonly available
restriction endonucleases can cut. Restriction endonucleases recognize specific DNA sequences and cut
them in a predictable manner. They are naturally produced by bacteria as a defense mechanism against
foreign DNA. Many restriction endonucleases make staggered cuts in the two DNA strands, such that the
cut ends have a 2- or 4-base single-stranded overhang. Because these overhangs are capable of annealing
with complementary overhangs, we call them “sticky ends.” Adding the enzyme DNA ligase permanently joins
the DNA fragments via phosphodiester bonds. In this way, scientists can splice any DNA fragment generated
by restriction endonuclease cleavage between the plasmid DNA's two ends that has been cut with the same
restriction endonuclease (Figure 17.7).
Recombinant DNA Molecules
Plasmids with foreign DNA inserted into them are called recombinant DNA molecules because they are
created artificially and do not occur in nature. They are also called chimeric molecules because the origin of
different molecule parts of the molecules can be traced back to different species of biological organisms or
even to chemical synthesis. We call proteins that are expressed from recombinant DNA molecules recombinant
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Chapter 17 | Biotechnology and Genomics
467
proteins. Not all recombinant plasmids are capable of expressing genes. The recombinant DNA may need to
move into a different vector (or host) that is better designed for gene expression. Scientists may also engineer
plasmids to express proteins only when certain environmental factors stimulate them, so they can control the
recombinant proteins' expression.
visual
CONNECTION
Molecular Cloning
Foreign DNA
Plasmid gGATC /
<^ C G C
Restriction site
g g ta g g - LacZ gene
, Ampicillin
resistance
gene
G *TC C
■Sticky-
ends
-V
V
WA?
&
▼ ^AGfe.
%
ip
Oo°
Bacteria (may take
up plasmid with or
without the insert,
or may not take up
plasmid at all).
Bacterial genome is
missing the lacZ gene
Blue colonies
have plasmids
without insert.
White colonies
have plasmids
with the foreign
insert.
The foreign DNA and plasmid are cut with the same
restriction enzyme, which recognizes a particular
sequence of DNA called a restriction site. The restriction
site occurs only once in the plasmid, and is located within
the lacZ gene, a gene necessary for metabolizing
lactose.
The restriction enzyme creates sticky ends that allow the
foreign DNA and cloning vector to anneal. An enzyme
called ligase glues the annealed fragments together.
The ligated cloning vector is transformed into a bacterial
host strain that is ampicillin sensitive and is missing the
lacZ gene from its genome.
Bacteria are grown on media containing ampicillin and
X-gal, a chemical that is metabolized by the same
pathway as lactose. The ampicillin kills bacteria without
plasmid. Plasmids lacking the foreign insert have an
intact lacZ gene and are able to metabolize X-gal,
releasing a dye that turns the colony blue. Plasmids with
an insert have a disrupted lacZ gene and produce white
colonies.
Figure 17.7 This diagram shows the steps involved in molecular cloning.
You are working in a molecular biology lab and, unbeknownst to you, your lab partner left the foreign
genomic DNA that you are planning to clone on the lab bench overnight instead of storing it in the freezer.
As a result, it was degraded by nucleases, but still used in the experiment. The plasmid, on the other hand,
is fine. What results would you expect from your molecular cloning experiment?
a. There will be no colonies on the bacterial plate.
b. There will be blue colonies only.
c. There will be blue and white colonies.
d. The will be white colonies only.
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Chapter 17 | Biotechnology and Genomics
LINK TQ LEARNING
View an animation of recombination in cloning (http:// 0 penstaxc 0 llege. 0 rg/l/rec 0 mbinati 0 n) from the
DNA Learning Center.
Cellular Cloning
Unicellular organisms, such as bacteria and yeast, naturally produce clones of themselves when they replicate
asexually by binary fission; this is known as cellular cloning. The nuclear DNA duplicates by the process of
mitosis, which creates an exact replica of the genetic material.
Reproductive Cloning
Reproductive cloning is a method scientists use to clone or identically copy an entire multicellular organism.
Most multicellular organisms undergo reproduction by sexual means, which involves genetic hybridization of
two individuals (parents), making it impossible to generate an identical copy or a clone of either parent. Recent
advances in biotechnology have made it possible to artificially induce mammal asexual reproduction in the
laboratory.
Parthenogenesis, or “virgin birth," occurs when an embryo grows and develops without egg fertilization. This is a
form of asexual reproduction. An example of parthenogenesis occurs in species in which the female lays an egg
and if the egg is fertilized, it is a diploid egg and the individual develops into a female. If the egg is not fertilized,
it remains a haploid egg and develops into a male. The unfertilized egg is a parthenogenic, or virgin egg. Some
insects and reptiles lay parthenogenic eggs that can develop into adults.
Sexual reproduction requires two cells. When the haploid egg and sperm cells fuse, a diploid zygote results.
The zygote nucleus contains the genetic information to produce a new individual. However, early embryonic
development requires the cytoplasmic material contained in the egg cell. This idea forms the basis for
reproductive cloning. Therefore, if we replace the egg cell's haploid nucleus with a diploid nucleus from the cell
of any individual of the same species (a donor), it will become a zygote that is genetically identical to the donor.
Somatic cell nuclear transfer is the technique of transferring a diploid nucleus into an enucleated egg. Scientists
can use it for either therapeutic cloning or reproductive cloning.
The first cloned animal was Dolly, a sheep born in 1996. The reproductive cloning success rate at the time was
very low. Dolly lived for seven years and died of respiratory complications (Figure 17.8). There is speculation
that because the cell DNA belongs to an older individual, DNA's age may affect a cloned individual's life
expectancy. Since Dolly, scientists have cloned successfully several animals such as horses, bulls, and goats,
although these animals often exhibit facial, limb, and cardiac abnormalities. There have been attempts at
producing cloned human embryos as sources of embryonic stem cells for therapeutic purposes. Therapeutic
cloning produces stem cells in the attempt to remedy detrimental diseases or defects (unlike reproductive
cloning, which aims to reproduce an organism). Still, some have met therapeutic cloning efforts with resistance
because of bioethical considerations.
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Chapter 17 | Biotechnology and Genomics
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visual
CONNECTION
Scottish Blackface Finn-Dorset
(Cytoplasmic donor) (Nuclear donor)
Enucleation Mammary cells
Blastocyst
Dolly
Figure 17.8 Dolly the sheep was the first mammal to be cloned. To create Dolly, they removed the nucleus from a
donor egg cell. They then introduced the nucleus from a second sheep into the cell, which divided to the blastocyst
stage before they implanted it in a surrogate mother, (credit: modification of work by "Squidonius'VWikimedia
Commons)
Do you think Dolly was a Finn-Dorset or a Scottish Blackface sheep?
Genetic Engineering
Genetic engineering is the alteration of an organism’s genotype using recombinant DNA technology to modify
an organism’s DNA to achieve desirable traits. The addition of foreign DNA in the form of recombinant DNA
vectors generated by molecular cloning is the most common method of genetic engineering. The organism that
receives the recombinant DNA is a genetically modified organism (GMO). If the foreign DNA comes from a
different species, the host organism is transgenic. Scientists have genetically modified bacteria, plants, and
animals since the early 1970s for academic, medical, agricultural, and industrial purposes, in the US, GMOs
such as Roundup-ready soybeans and borer-resistant corn are part of many common processed foods.
Gene Targeting
Although classical methods of studying gene function began with a given phenotype and determined the genetic
basis of that phenotype, modern techniques allow researchers to start at the DNA sequence level and ask: "What
does this gene or DNA element do?" This technique, reverse genetics, has resulted in reversing the classic
genetic methodology. This method would be similar to damaging a body part to determine its function. An insect
that loses a wing cannot fly, which means that the wing's function is flight. The classical genetic method would
compare insects that cannot fly with insects that can fly, and observe that the non-flying insects have lost wings.
Similarly, mutating or deleting genes provides researchers with clues about gene function. We collectively call
the methods they use to disable gene function gene targeting. Gene targeting is the use of recombinant DNA
vectors to alter a particular gene's expression, either by introducing mutations in a gene, or by eliminating a
certain gene's expression by deleting a part or all of the gene sequence from the organism's genome.
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Biotechnology in Medicine and Agriculture
It is easy to see how biotechnology can be used for medicinal purposes. Knowledge of the genetic makeup of
our species, the genetic basis of heritable diseases, and the invention of technology to manipulate and fix mutant
genes provides methods to treat the disease. Biotechnology in agriculture can enhance resistance to disease,
pest, and environmental stress, and improve both crop yield and quality.
Genetic Diagnosis and Gene Therapy
Scientists call the process of testing for suspected genetic defects before administering treatment genetic
diagnosis by genetic testing. Depending on the inheritance patterns of a disease-causing gene, family
members are advised to undergo genetic testing. For example, doctors usually advise women diagnosed with
breast cancer to have a biopsy so that the medical team can determine the genetic basis of cancer development.
Doctors base treatment plans on genetic test findings that determine the type of cancer. If inherited gene
mutations cause the cancer, doctors also advise other female relatives to undergo genetic testing and periodic
screening for breast cancer. Doctors also offer genetic testing for fetuses (or embryos with in vitro fertilization) to
determine the presence or absence of disease-causing genes in families with specific debilitating diseases.
Gene therapy is a genetic engineering technique used to cure disease. In its simplest form, it involves the
introduction of a good gene at a random location in the genome to aid the cure of a disease that is caused by a
mutated gene. The good gene is usually introduced into diseased cells as part of a vector transmitted by a virus
that can infect the host cell and deliver the foreign DNA (Figure 17.9). More advanced forms of gene therapy try
to correct the mutation at the original site in the genome, such as is the case with treatment of severe combined
immunodeficiency (SCID).
Vector binds to
cell membrane
Viral New Viral
DNA Gene DNA
I I-1—I
Modified DNA infected
into vector
iiilu vet-im
Gene therapy using
an adenovirus vector
Vector i« packaged
in veside
Vesicle breaks
down releasing
vector
Figure 17.9 Gene therapy using an adenovirus vector can be used to cure certain genetic diseases in which a person
has a defective gene, (credit: NIH)
Production of Vaccines, Antibiotics, and Hormones
Traditional vaccination strategies use weakened or inactive forms of microorganisms to mount the initial immune
response. Modern techniques use the genes of microorganisms cloned into vectors to mass produce the desired
antigen. Doctors then introduce the antigen into the body to stimulate the primary immune response and trigger
immune memory. The medical field has used genes cloned from the influenza virus to combat the constantly
changing strains of this virus.
Antibiotics are a biotechnological product. Microorganisms, such as fungi, naturally produce them to attain an
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advantage over bacterial populations. Cultivating and manipulating fungal cells produces antibodies.
Scientists used recombinant DNA technology to produce large-scale quantities of human insulin in E. coli as
early as 1978. Previously, it was only possible to treat diabetes with pig insulin, which caused allergic reactions
in humans because of differences in the gene product. In addition, doctors use human growth hormone (HGH)
to treat growth disorders in children. Researchers cloned the HGH gene from a cDNA library and inserted it into
E. coli cells by cloning it into a bacterial vector.
Transgenic Animals
Although several recombinant proteins in medicine are successfully produced in bacteria, some proteins require
a eukaryotic animal host for proper processing. For this reason, the desired genes are cloned and expressed
in animals, such as sheep, goats, chickens, and mice. We call animals that have been modified to express
recombinant DNA transgenic animals. Several human proteins are expressed in transgenic sheep and goat milk,
and some are expressed in chicken eggs. Scientists have used mice extensively for expressing and studying
recombinant gene and mutation effects.
Transgenic Plants
Manipulating the DNA of plants (i.e., creating GMOs) has helped to create desirable traits, such as disease
resistance, herbicide and pesticide resistance, better nutritional value, and better shelf-life (Figure 17.10). Plants
are the most important source of food for the human population. Farmers developed ways to select for plant
varieties with desirable traits long before modern-day biotechnology practices were established.
Figure 17.10 Corn, a major agricultural crop used to create products for a variety of industries, is often modified
through plant biotechnology, (credit: Keith Weller, USDA)
We call plants that have received recombinant DNA from other species transgenic plants. Because they are
not natural, government agencies closely monitor transgenic plants and other GMOs to ensure that they are fit
for human consumption and do not endanger other plant and animal life. Because foreign genes can spread to
other species in the environment, extensive testing is required to ensure ecological stability. Staples like corn,
potatoes, and tomatoes were the first crop plants that scientists genetically engineered.
Transformation of Plants Using Agrobacterium tumefaciens
Gene transfer occurs naturally between species in microbial populations. Many viruses that cause human
diseases, such as cancer, act by incorporating their DNA into the human genome. In plants, tumors caused by
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the bacterium Agrobacterium tumefaciens occur by DNA transfer from the bacterium to the plant. Although the
tumors do not kill the plants, they stunt the plants and they become more susceptible to harsh environmental
conditions. A. tumefaciens affects many plants such as walnuts, grapes, nut trees, and beets. Artificially
introducing DNA into plant cells is more challenging than in animal cells because of the thick plant cell wall.
Researchers used the natural transfer of DNA from Agrobacterium to a plant host to introduce DNA fragments of
their choice into plant hosts. In nature, the disease-causing A. tumefaciens have a set of plasmids, Ti plasmids
(tumor-inducing plasmids), that contain genes to produce tumors in plants. DNA from the Ti plasmid integrates
into the infected plant cell’s genome. Researchers manipulate the Ti plasmids to remove the tumor-causing
genes and insert the desired DNA fragment for transfer into the plant genome. The Ti plasmids carry antibiotic
resistance genes to aid selection and researchers can propagate them in E. coli cells as well.
The Organic Insecticide Bacillus thuringiensis
Bacillus thuringiensis (Bt) is a bacterium that produces protein crystals during sporulation that are toxic to many
insect species that affect plants. Insects need to ingest Bt toxin in order to activate the toxin. Insects that have
eaten Bt toxin stop feeding on the plants within a few hours. After the toxin activates in the insects' intestines,
they die within a couple of days. Modern biotechnology has allowed plants to encode their own crystal Bt
toxin that acts against insects. Scientists have cloned the crystal toxin genes from Bt and introduced them into
plants. Bt toxin is safe for the environment, nontoxic to humans and other mammals, and organic farmers have
approved it as a natural insecticide.
Flavr Savr Tomato
The first GM crop on the market was the Flavr Savr Tomato in 1994. Scientists used antisense RNA technology
to slow the softening and rotting process caused by fungal infections, which led to increased shelf life of the
GM tomatoes. Additional genetic modification improved the tomato's flavor. The Flavr Savr tomato did not
successfully stay in the market because of problems maintaining and shipping the crop.
17.2 | Mapping Genomes
By the end of this section, you will be able to do the following:
• Define genomics
• Describe genetic and physical maps
• Describe genomic mapping methods
Genomics is the study of entire genomes, including the complete set of genes, their nucleotide sequence and
organization, and their interactions within a species and with other species. Genome mapping is the process
of finding the locations of genes on each chromosome. The maps that genome mapping create are comparable
to the maps that we use to navigate streets. A genetic map is an illustration that lists genes and their location
on a chromosome. Genetic maps provide the big picture (similar to an interstate highway map) and use genetic
markers (similar to landmarks). A genetic marker is a gene or sequence on a chromosome that co-segregates
(shows genetic linkage) with a specific trait. Early geneticists called this linkage analysis. Physical maps present
the intimate details of smaller chromosome regions (similar to a detailed road map). A physical map is a
representation of the physical distance, in nucleotides, between genes or genetic markers. Both genetic linkage
maps and physical maps are required to build a genome’s complete picture. Having a complete genome map of
the genome makes it easier for researchers to study individual genes. Human genome maps help researchers
in their efforts to identify human disease-causing genes related to illnesses like cancer, heart disease, and cystic
fibrosis. We can use genome mapping in a variety of other applications, such as using live microbes to clean
up pollutants or even prevent pollution. Research involving plant genome mapping may lead to producing higher
crop yields or developing plants that better adapt to climate change.
Genetic Maps
The study of genetic maps begins with linkage analysis, a procedure that analyzes the recombination frequency
between genes to determine if they are linked or show independent assortment. Scientists used the term
linkage before the discovery of DNA. Early geneticists relied on observing phenotypic changes to understand
an organism’s genotype. Shortly after Gregor Mendel (the father of modern genetics) proposed that traits were
determined by what we now call genes, other researchers observed that different traits were often inherited
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together, and thereby deduced that the genes were physically linked by their location on the same chromosome.
Gene mapping relative to each other based on linkage analysis led to developing the first genetic maps.
Observations that certain traits were always linked and certain others were not linked came from studying the
offspring of crosses between parents with different traits. For example, in garden pea experiments, researchers
discovered, that the flower’s color and plant pollen’s shape were linked traits, and therefore the genes encoding
these traits were in close proximity on the same chromosome. We call exchanging DNA between homologous
chromosome pairs genetic recombination, which occurs by crossing over DNA between homologous DNA
strands, such as nonsister chromatids. Linkage analysis involves studying the recombination frequency between
any two genes. The greater the distance between two genes, the higher the chance that a recombination event
will occur between them, and the higher the recombination frequency between them. Figure 17.11 shows two
possibilities for recombination between two nonsister chromatids during meiosis. If the recombination frequency
between two genes is less than 50 percent, they are linked.
Crossover region resulting
in A-B recombination
Crossover region resulting
in B-C recombination
Figure 17.11 Crossover may occur at different locations on the chromosome. Recombination between genes A and 6
is more frequent than recombination between genes 6 and C because genes A and B are farther apart. Therefore, a
crossover is more likely to occur between them.
The generation of genetic maps requires markers, just as a road map requires landmarks (such as rivers
and mountains). Scientists based early genetic maps on using known genes as markers. Scientists now use
more sophisticated markers, including those based on non-coding DNA, to compare individuals’ genomes in a
population. Although individuals of a given species are genetically similar, they are not identical. Every individual
has a unique set of traits. These minor differences in the genome between individuals in a population are useful
for genetic mapping purposes. In general, a good genetic marker is a region on the chromosome that shows
variability or polymorphism (multiple forms) in the population.
Some genetic markers that scientists use in generating genetic maps are restriction fragment length
polymorphisms (RFLP), variable number of tandem repeats (VNTRs), microsatellite polymorphisms, and
the single nucleotide polymorphisms (SNPs). We can detect RFLPs (sometimes pronounced “rif-lips”) when
the DNA of an individual is cut with a restriction endonuclease that recognizes specific sequences in the DNA
to generate a series of DNA fragments, which we can then analyze using gel electrophoresis. Every individual’s
DNA will give rise to a unique pattern of bands when cut with a particular set of restriction endonucleases.
Scientists sometimes refer to this as an individual’s DNA ‘‘fingerprint.” Certain chromosome regions that are
subject to polymorphism will lead to generating the unique banding pattern. VNTRs are repeated sets of
nucleotides present in DNA’s non-coding regions. Non-coding, or “junk," DNA has no known biological function;
however, research shows that much of this DNA is actually transcribed. While its function is uncertain, it is
certainly active, and it may be involved in regulating coding genes. The number of repeats may vary in a
population’s individual organisms. Microsatellite polymorphisms are similar to VNTRs, but the repeat unit is very
small. SNPs are variations in a single nucleotide.
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Because genetic maps rely completely on the natural process of recombination, natural increases or decreases
in the recombination level given genome area affects mapping. Some parts of the genome are recombination
hotspots; whereas, others do not show a propensity for recombination. For this reason, it is important to look at
mapping information developed by multiple methods.
Physical Maps
A physical map provides detail of the actual physical distance between genetic markers, as well as the number
of nucleotides. There are three methods scientists use to create a physical map: cytogenetic mapping, radiation
hybrid mapping, and sequence mapping. Cytogenetic mapping uses information from microscopic analysis
of stained chromosome sections (Figure 17.12). It is possible to determine the approximate distance between
genetic markers using cytogenetic mapping, but not the exact distance (number of base pairs). Radiation
hybrid mapping uses radiation, such as x-rays, to break the DNA into fragments. We can adjust the radiation
amount to create smaller or larger fragments. This technique overcomes the limitation of genetic mapping,
and we can adjust the radiation so that increased or decreased recombination frequency does not affect it.
Sequence mapping resulted from DNA sequencing technology that allowed for creating detailed physical maps
with distances measured in terms of the number of base pairs. Creating genomic libraries and complementary
DNA (cDNA) libraries (collections of cloned sequences or all DNA from a genome) has sped the physical
mapping process. A genetic site that scientists use to generate a physical map with sequencing technology (a
sequence-tagged site, or STS) is a unique sequence in the genome with a known exact chromosomal location.
An expressed sequence tag (EST) and a single sequence length polymorphism (SSLP) are common STSs. An
EST is a short STS that we can identify with cDNA libraries, while we obtain SSLPs from known genetic markers,
which provide a link between genetic and physical maps.
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Figure 17.12 A cytogenetic map shows the appearance of a chromosome after scientists stain and exam it under a
microscope, (credit: National Human Genome Research Institute)
Genetic and Physical Maps Integration
Genetic maps provide the outline and physical maps provide the details. It is easy to understand why both
genome mapping technique types are important to show the big picture. Scientists use information from each
technique in combination to study the genome. Scientists are using genomic mapping with different model
organisms for research. Genome mapping is still an ongoing process, and as researchers develop more
advanced techniques, they expect more breakthroughs. Genome mapping is similar to completing a complicated
puzzle using every piece of available data. Mapping information generated in laboratories all over the world
goes into central databases, such as GenBank at the National Center for Biotechnology Information (NCBI).
Researchers are making efforts for the information to be more easily accessible to other researchers and the
general public. Just as we use global positioning systems instead of paper maps to navigate through roadways,
NCBI has created a genome viewer tool to simplify the data-mining process.
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How to Use a Genome Map Viewer
Problem statement: Do the human, macaque, and mouse genomes contain common DNA sequences?
Develop a hypothesis.
To test the hypothesis, click this link (http://www.ncbi.nlm.nih.gov/mapview/) .
in Search box on the left panel, type any gene name or phenotypic characteristic, such as iris pigmentation
(eye color). Select the species you want to study, and then press Enter. The genome map viewer will
indicate which chromosome encodes the gene in your search. Click each hit in the genome viewer for more
detailed information. This type of search is the most basic use of the genome viewer. You can also use it to
compare sequences between species, as well as many other complicated tasks.
Is the hypothesis correct? Why or why not?
Online Mendelian Inheritance in Man (OMIM) is a searchable online catalog of human genes and genetic
disorders. This website shows genome mapping information, and also details the history and research of
each trait and disorder. Click this link (http://openstaxcollege. 0 rg/l/OMIM) to search for traits (such as
handedness) and genetic disorders (such as diabetes).
17.3 | Whole-Genome Sequencing
By the end of this section, you will be able to do the following:
• Describe three types of sequencing
• Define whole-genome sequencing
Although there have been significant advances in the medical sciences in recent years, doctors are still
confounded by some diseases, and they are using whole-genome sequencing to discover the root of the
problem. Whole-genome sequencing is a process that determines an entire genome’s DNA sequence. Whole-
genome sequencing is a brute-force approach to problem solving when there is a genetic basis at the core of a
disease. Several laboratories now provide services to sequence, analyze, and interpret entire genomes.
For example, whole-exome sequencing is a lower-cost alternative to whole genome sequencing. In exome
sequencing, the doctor sequences only the DNA’s coding, exon-producing regions. In 2010, doctors used
whole-exome sequencing to save a young boy whose intestines had multiple mysterious abscesses. The child
had several colon operations with no relief. Finally, they performed whole-exome sequencing, which revealed
a defect in a pathway that controls apoptosis (programmed cell death). The doctors used a bone-marrow
transplant to overcome this genetic disorder, leading to a cure for the boy. He was the first person to receive
successful treatment based on a whole-exome sequencing diagnosis. Today, human genome sequencing is
more readily available and results are available within two days for about $1000.
Strategies Used in Sequencing Projects
The basic sequencing technique used in all modern day sequencing projects is the chain termination method
(also known as the dideoxy method), which Fred Sanger developed in the 1970s. The chain termination method
involves DNA replication of a single-stranded template by using a primer and a regular deoxynucleotide
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(dNTP), which is a monomer, or a single DNA unit. The primer and dNTP mix with a small proportion of
fluorescently labeled dideoxynucleotides (ddNTPs). The ddNTPs are monomers that are missing a hydroxyl
group (-OH) at the site at which another nucleotide usually attaches to form a chain (Figure 17.13). Scientists
label each ddNTP with a different color of fluorophore. Every time a ddNTP incorporates in the growing
complementary strand, it terminates the DNA replication process, which results in multiple short strands of
replicated DNA that each terminate at a different point during replication. When gel electrophoresis processes
the reaction mixture after separating into single strands, the multiple newly replicated DNA strands form a ladder
because of the differing sizes. Because the ddNTPs are fluorescently labeled, each band on the gel reflects the
DNA strand’s size and the ddNTP that terminated the reaction. The different colors of the fluorophore-labeled
ddNTPs help identify the ddNTP incorporated at that position. Reading the gel on the basis of each band’s color
on the ladder produces the template strand’s sequence (Figure 17.14).
Dideoxynucleotide (ddNTP)
©-©-©-och 2 ^.(X Base
OH H
Deoxynucleotide (dNTP)
Figure 17.13 A dideoxynucleotide is similar in structure to a deoxynucleotide, but is missing the 3' hydroxyl group
(indicated by the box). When a dideoxynucleotide is incorporated into a DNA strand, DNA synthesis stops.
ddCTP
ddATP
►
ddGTP
ddTTP
►
►
Dye-labeled dideoxynucleotides are used to
generate DNA fragments of different lengths.
GAT A A AT CT GGTCTT ATTTCC
Figure 17.14 This figure illustrates Frederick Sanger's dideoxy chain termination method. Using dideoxynucleotides,
the DNA fragment can terminate at different points. The DNA separates on the basis of size, and we can read these
bands based on the fragments’ size.
Early Strategies: Shotgun Sequencing and Pair-Wise End Sequencing
In shotgun sequencing method, several DNA fragment copies cut randomly into many smaller pieces
(somewhat like what happens to a round shot cartridge when fired from a shotgun). All of the segments
sequence using the chain-sequencing method. Then, with sequence computer assistance, scientists can
analyze the fragments to see where their sequences overlap. By matching overlapping sequences at each
fragment’s end, scientists can reform the entire DNA sequence. A larger sequence that is assembled from
overlapping shorter sequences is called a contig. As an analogy, consider that someone has four copies of
a landscape photograph that you have never seen before and know nothing about how it should appear. The
person then rips up each photograph with their hands, so that different size pieces are present from each copy.
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The person then mixes all of the pieces together and asks you to reconstruct the photograph. In one of the
smaller pieces you see a mountain. In a larger piece, you see that the same mountain is behind a lake. A third
fragment shows only the lake, but it reveals that there is a cabin on the shore of the lake. Therefore, from looking
at the overlapping information in these three fragments, you know that the picture contains a mountain behind
a lake that has a cabin on its shore. This is the principle behind reconstructing entire DNA sequences using
shotgun sequencing.
Originally, shotgun sequencing only analyzed one end of each fragment for overlaps. This was sufficient for
sequencing small genomes. However, the desire to sequence larger genomes, such as that of a human, led
to developing double-barrel shotgun sequencing, or pairwise-end sequencing. In pairwise-end sequencing,
scientists analyze each fragment’s end for overlap. Pairwise-end sequencing is, therefore, more cumbersome
than shotgun sequencing, but it is easier to reconstruct the sequence because there is more available
information.
Next-generation Sequencing
Since 2005, automated sequencing techniques used by laboratories are under the umbrella of next-generation
sequencing, which is a group of automated techniques used for rapid DNA sequencing. These automated low-
cost sequencers can generate sequences of hundreds of thousands or millions of short fragments (25 to 500
base pairs) in the span of one day. These sequencers use sophisticated software to get through the cumbersome
process of putting all the fragments in order.
\ / _
e olution CONNECTION
Comparing Sequences
A sequence alignment is an arrangement of proteins, DNA, or RNA. Scientists use it to identify similar
regions between cell types or species, which may indicate function or structure conservation. We can use
sequence alignments to construct phylogenetic trees. The following website uses a software program called
BLAST (basic local alignment search tool) (http://blast.ncbi.nlm.nih.gov/Blast.cgi) .
Under “Basic Blast,” click “Nucleotide Blast.” Input the following sequence into the large "query sequence"
box: ATTGCTTCGATTGCA. Below the box, locate the "Species" field and type "human" or "Homo sapiens".
Then click “BLAST” to compare the inputted sequence against the human genome’s known sequences.
The result is that this sequence occurs in over a hundred places in the human genome. Scroll down below
the graphic with the horizontal bars and you will see a short description of each of the matching hits. Pick
one of the hits near the top of the list and click on "Graphics". This will bring you to a page that shows the
sequence’s location within the entire human genome. You can move the slider that looks like a green flag
back and forth to view the sequences immediately around the selected gene. You can then return to your
selected sequence by clicking the "ATG" button.
Use of Whole-Genome Sequences of Model Organisms
British biochemist and Nobel Prize winner Fred Sanger used a bacterial virus, the bacteriophage fxl74 (5368
base pairs), to completely sequence the first genome. Other scientists later sequenced several other organelle
and viral genomes. American biotechnologist, biochemist, geneticist, and businessman Craig Venter sequenced
the bacterium Haemophilus influenzae in the 1980s. Approximately 74 different laboratories collaborated on
sequencing the genome of the yeast Saccharomyces cerevisiae, which began in 1989 and was completed in
1996, because it was 60 times bigger than any other genome sequencing. By 1997, the genome sequences
of two important model organisms were available: the bacterium Escherichia coli K12 and the yeast
Saccharomyces cerevisiae. We now know the genomes of other model organisms, such as the mouse Mus
musculus, the fruit fly Drosophila melanogaster, the nematode Caenorhabditis. elegans, and humans Homo
sapiens. Researchers perform extensive basic research in model organisms because they can apply the
information to genetically similar organisms. A model organism is a species that researchers use as a model
to understand the biological processes in other species that the model organism represents. Having entire
genomes sequenced helps with the research efforts in these model organisms. The process of attaching
biological information to gene sequences is genome annotation. Annotating gene sequences helps with basic
experiments in molecular biology, such as designing PCR primers and RNA targets.
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LINK TQ LEARNING
Click through each genome sequencing step at this site (http:// 0 penstaxc 0 llege. 0 rg/l/DNA_sequence)
Genome Sequence Uses
DNA microarrays are methods that scientists use to detect gene expression by analyzing different DNA
fragments that are fixed to a glass slide or a silicon chip to identify active genes and sequences. We can discover
almost one million genotypic abnormalities using microarrays; whereas, whole-genome sequencing can provide
information about all six billion base pairs in the human genome. Although studying genome sequencing medical
applications is interesting, this discipline dwells on abnormal gene function. Knowing about the entire genome
will allow researchers to discover future onset diseases and other genetic disorders early. This will allow for
more informed decisions about lifestyle, medication, and having children. Genomics is still in its infancy, although
someday it may become routine to use whole-genome sequencing to screen every newborn to detect genetic
abnormalities.
in addition to disease and medicine, genomics can contribute to developing novel enzymes that convert biomass
to biofuel, which results in higher crop and fuel production, and lower consumer cost. This knowledge should
allow better methods of control over the microbes that industry uses to produce biofuels. Genomics could
also improve monitoring methods that measure the impact of pollutants on ecosystems and help clean up
environmental contaminants. Genomics has aided in developing agrochemicals and pharmaceuticals that could
benefit medical science and agriculture.
It sounds great to have all the knowledge we can get from whole-genome sequencing; however, humans have a
responsibility to use this knowledge wisely. Otherwise, it could be easy to misuse the power of such knowledge,
leading to discrimination based on a person's genetics, human genetic engineering, and other ethical concerns.
This information could also lead to legal issues regarding health and privacy.
17.4 | Applying Genomics
By the end of this section, you will be able to do the following:
• Explain pharmacogenomics
• Define polygenic
introducing DNA sequencing and whole genome sequencing projects, particularly the Human Genome project,
has expanded the applicability of DNA sequence information. Many fields, such as metagenomics,
pharmacogenomics, and mitochondrial genomics are using genomics. Understanding and finding cures for
diseases is the most common application of genomics.
Predicting Disease Risk at the Individual Level
Predicting disease risk involves screening currently healthy individuals by genome analysis at the individual
level. Health care professionals can recommend intervention with lifestyle changes and drugs before disease
onset. However, this approach is most applicable when the problem resides within a single gene defect. Such
defects only account for approximately 5 percent of diseases in developed countries. Most of the common
diseases, such as heart disease, are multi-factored or polygenic, which is a phenotypic characteristic that
involves two or more genes, and also involve environmental factors such as diet. In April 2010, scientists
at Stanford University published the genome analysis of a healthy individual (Stephen Quake, a scientist at
Stanford University, who had his genome sequenced. The analysis predicted his propensity to acquire various
diseases. The medical team performed a risk assessment to analyze Quake’s percentage of risk for 55 different
medical conditions. The team found a rare genetic mutation, which showed him to be at risk for sudden heart
attack. The results also predicted that Quake had a 23 percent risk of developing prostate cancer and a 1.4
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percent risk of developing Alzheimer’s. The scientists used databases and several publications to analyze the
genomic data. Even though genomic sequencing is becoming more affordable and analytical tools are becoming
more reliable, researchers still must address ethical issues surrounding genomic analysis at a population level.
visual
CONNECTION
PCA3
Step 1:
PC A3 mRNA
anneals to
complementary
DNA primers
that are attached
to beads.
Step 2:
The mRNA is
amplified using
reverse-
transcriptase
PCR.
Step 3:
The mRNA is
detected using
a chemilumi¬
nescent probe.
Figure 17.15 PCA3 is a gene that is expressed in prostate epithelial cells and overexpressed in cancerous cells.
A high PCA3 concentration in urine is indicative of prostate cancer. The PCA3 test is a better indicator of cancer
than the more well known PSA test, which measures the level of PSA (prostate-specific antigen) in the blood.
In 2011, the United States Preventative Services Task Force recommended against using the PSA test to
screen healthy men for prostate cancer. Their recommendation is based on evidence that screening does
not reduce the risk of death from prostate cancer. Prostate cancer often develops very slowly and does not
cause problems, while the cancer treatment can have severe side effects. The PCA3 test is more accurate,
but screening may still result in men who would not have been harmed by the cancer itself suffering side
effects from treatment. What do you think? Should all healthy men receive prostate cancer screenings using
the PCA3 or PSA test? Should people in general receive screenings to find out if they have a genetic risk
for cancer or other diseases?
Pharmacogenomics and Toxicogenomics
Pharmacogenomics, or toxicogenomics, involves evaluating drug effectiveness and safety on the basis of
information from an individual's genomic sequence. We can study genomic responses to drugs using
experimental animals (such as laboratory rats or mice) or live cells in the laboratory before embarking on studies
with humans. Studying changes in gene expression could provide information about the transcription profile
in the drug's presence, which we can use as an early indicator of the potential for toxic effects. For example,
genes involved in cellular growth and controlled cell death, when disturbed, could lead to cancerous cell growth.
Genome-wide studies can also help to find new genes involved in drug toxicity. Medical professionals can use
personal genome sequence information to prescribe medications that will be most effective and least toxic on
the basis of the individual patient’s genotype. The gene signatures may not be completely accurate, but medical
professionals can test them further before pathologic symptoms arise.
Microbial Genomics: Metagenomics
Traditionally, scholars have taught microbiology with the view that it is best to study microorganisms under
pure culture conditions. This involves isolating a single cell type and culturing it in the laboratory. Because
microorganisms can go through several generations in a matter of hours, their gene expression profiles adapt to
the new laboratory environment very quickly. In addition, the vast majority of bacterial species resist culturing in
isolation. Most microorganisms do not live as isolated entities, but in microbial communities or biofilms. For all of
these reasons, pure culture is not always the best way to study microorganisms. Metagenomics is the study of
the collective genomes of multiple species that grow and interact in an environmental niche. Metagenomics can
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be used to identify new species more rapidly and to analyze the effect of pollutants on the environment (Figure
17.16).
Each color
represents
DNAfrom a
different
species.
All the genomic DNAfrom a
particular environment is
cut into fragments and
ligated into a cloning vector.
The fragments are
sequenced, and regions
of overlap are used to
determine the genomic
sequences.
\
Figure 17.16 Metagenomics involves isolating DNAfrom multiple species within an environmental niche.
Microbial Genomics: Creation of New Biofuels
Knowledge of the genomics of microorganisms is being used to find better ways to harness biofuels from
algae and cyanobacteria. The primary sources of fuel today are coal, oil, wood, and other plant products, such
as ethanol. Although plants are renewable resources, there is still a need to find more alternative renewable
sources of energy to meet our population’s energy demands. The microbial world is one of the largest resources
for genes that encode new enzymes and produce new organic compounds, and it remains largely untapped.
Microorganisms are used to create products, such as enzymes that are used in research, antibiotics, and other
antimicrobial mechanisms. Microbial genomics is helping to develop diagnostic tools, improved vaccines, new
disease treatments, and advanced environmental cleanup techniques.
Mitochondrial Genomics
Mitochondria are intracellular organelles that contain their own DNA. Mitochondrial DNA mutates at a rapid
rate and scientists often use it to study evolutionary relationships. Another feature that makes studying the
mitochondrial genome interesting is that the mitochondrial DNA in most multicellular organisms passes from
the mother during the fertilization process. For this reason, scientists often use mitochondrial genomics to trace
genealogy.
Experts have used information and clues from DNA samples at crime scenes as evidence in court cases, and
they have used genetic markers in forensic analysis. Genomic analysis has also become useful in this field. The
first publication showcasing the first use of genomics in forensics came out in 2001. It was a collaborative attempt
between academic research institutions and the FBI to solve the mysterious cases of anthrax communicated
via the US Postal Service. Using microbial genomics, researchers determined that the culprit used a specific
anthrax strain in all the mailings.
Genomics in Agriculture
Genomics can reduce the trials and failures involved in scientific research to a certain extent, which could
improve agricultural crop yield quality and quantity. Linking traits to genes or gene signatures helps improve crop
breeding to generate hybrids with the most desirable qualities. Scientists use genomic data to identify desirable
traits, and then transfer those traits to a different organism. Researchers are discovering how genomics can
improve agricultural production's quality and quantity. For example, scientists could use desirable traits to create
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a useful product or enhance an existing product, such as making a drought-sensitive crop more tolerant of the
dry season.
17.5 | Genomics and Proteomics
By the end of this section, you will be able to do the following:
• Explain systems biology
• Describe a proteome
• Define protein signature
Proteins are the final products of genes, which help perform the function that the gene encodes. Amino acids
comprise proteins and play important roles in the cell. All enzymes (except ribozymes) are proteins that act
as catalysts to affect the rate of reactions. Proteins are also regulatory molecules, and some are hormones.
Transport proteins, such as hemoglobin, help transport oxygen to various organs. Antibodies that defend against
foreign particles are also proteins. In the diseased state, protein function can be impaired because of changes
at the genetic level or because of direct impact on a specific protein.
A proteome is the entire set of proteins that a cell type produces. We can study proteoms using the knowledge
of genomes because genes code for mRNAs, and the mRNAs encode proteins. Although mRNA analysis is a
step in the right direction, not all mRNAs are translated into proteins. Proteomics is the study of proteomes'
function. Proteomics complements genomics and is useful when scientists want to test their hypotheses that they
based on genes. Even though all multicellular organisms' cells have the same set of genes, the set of proteins
produced in different tissues is different and dependent on gene expression. Thus, the genome is constant, but
the proteome varies and is dynamic within an organism. In addition, RNAs can be alternately spliced (cut and
pasted to create novel combinations and novel proteins) and many proteins modify themselves after translation
by processes such as proteolytic cleavage, phosphorylation, glycosylation, and ubiquitination. There are also
protein-protein interactions, which complicate studying proteomes. Although the genome provides a blueprint,
the final architecture depends on several factors that can change the progression of events that generate the
proteome.
Metabolomics is related to genomics and proteomics. Metabolomics involves studying small molecule
metabolites in an organism. The metabolome is the complete set of metabolites that are related to an
organism's genetic makeup. Metabolomics offers an opportunity to compare genetic makeup and physical
characteristics, as well as genetic makeup and environmental factors. The goal of metabolome research is to
identify, quantify, and catalogue all the metabolites in living organisms' tissues and fluids.
Basic Techniques in Protein Analysis
The ultimate goal of proteomics is to identify or compare the proteins expressed from a given genome under
specific conditions, study the interactions between the proteins, and use the information to predict cell behavior
or develop drug targets. Just as scientists analyze the genome using the basic DNA sequencing technique,
proteomics requires techniques for protein analysis. The basic technique for protein analysis, analogous to DNA
sequencing, is mass spectrometry. Mass spectrometry identifies and determines a molecule's characteristics.
Advances in spectrometry have allowed researchers to analyze very small protein samples. X-ray
crystallography, for example, enables scientists to determine a protein crystal's three-dimensional structure
at atomic resolution. Another protein imaging technique, nuclear magnetic resonance (NMR), uses atoms'
magnetic properties to determine the protein's three-dimensional structure in aqueous solution. Scientists have
also used protein microarrays to study protein interactions. Large-scale adaptations of the basic two-hybrid
screen (Figure 17.17) have provided the basis for protein microarrays. Scientists use computer software to
analyze the vast amount of data for proteomic analysis.
Genomic- and proteomic-scale analyses are part of systems biology, which is the study of whole biological
systems (genomes and proteomes) based on interactions within the system. The European Bioinformatics
Institute and the Human Proteome Organization (HUPO) are developing and establishing effective tools to sort
through the enormous pile of systems biology data. Because proteins are the direct products of genes and
reflect activity at the genomic level, it is natural to use proteomes to compare the protein profiles of different cells
to identify proteins and genes involved in disease processes. Most pharmaceutical drug trials target proteins.
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Researchers use information that they obtain from proteomics to identify novel drugs and to understand their
mechanisms of action.
Bait
Promoter
If the bait protein interacts with the prey protein, the transcription
factor's activator domain binds to the binding domain, and
transcription occurs.
BD
Promoter
Reporter gene
If the prey doesn’t catch the bait, no transcription occurs.
Figure 17.17 Scientists use two-hybrid screening to determine whether two proteins interact. In this method, a
transcription factor splits into a DNA-binding domain (BD) and an activator domain (AD). The binding domain is able to
bind the promoter in the activator domain's absence, but it does not turn on transcription. The bait protein attaches to
the BD, and the prey protein attaches to the AD. Transcription occurs only if the prey “catches" the bait.
Scientists are challenged when implementing proteomic analysis because it is difficult to detect small protein
quantities. Although mass spectrometry is good for detecting small protein amounts, variations in protein
expression in diseased states can be difficult to discern. Proteins are naturally unstable molecules, which makes
proteomic analysis much more difficult than genomic analysis.
Cancer Proteomics
Researchers are studying patients' genomes and proteomes to understand the genetic basis of diseases. The
most prominent disease researchers are studying with proteomic approaches is cancer. These approaches
improve screening and early cancer detection. Researchers are able to identify proteins whose expression
indicates the disease process. An individual protein is a biomarker; whereas, a set of proteins with altered
expression levels is a protein signature. For a biomarker or protein signature to be useful as a candidate for
early cancer screening and detection, they must secrete in body fluids, such as sweat, blood, or urine, such
that health professionals can perform large-scale screenings in a noninvasive fashion. The current problem with
using biomarkers for early cancer detection is the high rate of false-negative results. A false negative is an
incorrect test result that should have been positive. In other words, many cancer cases go undetected, which
makes biomarkers unreliable. Some examples of protein biomarkers in cancer detection are CA-125 for ovarian
cancer and PSA for prostate cancer. Protein signatures may be more reliable than biomarkers to detect cancer
cells. Researchers are also using proteomics to develop individualized treatment plans, which involves predicting
whether or not an individual will respond to specific drugs and the side effects that the individual may experience.
Researchers also use proteomics to predict the possibility of disease recurrence.
The National Cancer Institute has developed programs to improve cancer detection and treatment. The Clinical
Proteomic Technologies for Cancer and the Early Detection Research Network are efforts to identify protein
signatures specific to different cancer types. The Biomedical Proteomics Program identifies protein signatures
and designs effective therapies for cancer patients.
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KEY TERMS
antibiotic resistance ability of an organism to be unaffected by an antibiotic's actions
biomarker individual protein that is uniquely produced in a diseased state
biotechnology use of biological agents for technological advancement
cDNA library collection of cloned cDNA sequences
cellular cloning production of identical cell populations by binary fission
chain termination method method of DNA sequencing using labeled dideoxynucleotides to terminate DNA
replication; it is also called the dideoxy method or the Sanger method
clone exact replica
contig larger sequence of DNA assembled from overlapping shorter sequences
cytogenetic mapping technique that uses a microscope to create a map from stained chromosomes
deoxynucleotide individual DNA monomer (single unit)
dideoxynucleotide individual DNA monomer that is missing a hydroxyl group (-OH)
DNA microarray method to detect gene expression by analyzing many DNA fragments that are fixed to a glass
slide or a silicon chip to identify active genes and identify sequences
expressed sequence tag (EST) short STS that is identified with cDNA
false negative incorrect test result that should have been positive
foreign DNA DNA that belongs to a different species or DNA that is artificially synthesized
gel electrophoresis technique used to separate molecules on the basis of size using electric charge
gene targeting method for altering the sequence of a specific gene by introducing the modified version on a
vector
gene therapy technique used to cure inheritable diseases by replacing mutant genes with good genes
genetic diagnosis diagnosis of the potential for disease development by analyzing disease-causing genes
genetic engineering alteration of the genetic makeup of an organism
genetic map outline of genes and their location on a chromosome
genetic marker gene or sequence on a chromosome with a known location that is associated with a specific
trait
genetic recombination DNA exchange between homologous chromosome pairs
genetic testing process of testing for the presence of disease-causing genes
genetically modified organism (GMO) organism whose genome has been artificially changed
genome annotation process of attaching biological information to gene sequences
genome mapping process of finding the location of genes on each chromosome
genomic library collection of cloned DNA which represents all of the sequences and fragments from a genome
genomics study of entire genomes including the complete set of genes, their nucleotide sequence and
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organization, and their interactions within a species and with other species
host DNA DNA that is present in the genome of the organism of interest
linkage analysis procedure that analyzes recombining genes to determine if they are linked
lysis buffer solution to break the cell membrane and release cell contents
metabolome complete set of metabolites which are related to an organism's genetic makeup
metabolomics study of small molecule metabolites in an organism
metagenomics study of multiple species' collective genomes that grow and interact in an environmental niche
microsatellite polymorphism variation between individuals in the sequence and number of microsatellite DNA
repeats
model organism species that researchers study and use as a model to understand the biological processes in
other species represented by the model organism
molecular cloning cloning of DNA fragments
multiple cloning site (MCS) site that multiple restriction endonucleases can recognize
next-generation sequencing group of automated techniques for rapid DNA sequencing
Northern blotting transfer of RNA from a gel to a nylon membrane
pharmacogenomics study of drug interactions with the genome or proteome; also called toxicogenomics
physical map representation of the physical distance between genes or genetic markers
polygenic phenotypic characteristic caused by two or more genes
polymerase chain reaction (PCR) technique to amplify DNA
probe small DNA fragment to determine if the complementary sequence is present in a DNA sample
protease enzyme that breaks down proteins
protein signature set of uniquely expressed proteins in the diseased state
proteome entire set of proteins that cell type produces
proteomics study of proteomes' function
pure culture growth of a single cell type in the laboratory
radiation hybrid mapping information obtained by fragmenting the chromosome with x-rays
recombinant DNA combining DNA fragments that molecular cloning generates that do not exist in nature; also
a chimeric molecule
recombinant protein a gene's protein product derived by molecular cloning
reproductive cloning entire organism cloning
restriction endonuclease enzyme that can recognize and cleave specific DNA sequences
restriction fragment length polymorphism (RFLP) variation between individuals in the length of DNA
fragments, which restriction endonucleases generate
reverse genetics method of determining the gene's function by starting with the gene itself instead of starting
with the gene product
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reverse transcriptase PCR (RT-PCR) PCR technique that involves converting RNA to DNA by reverse
transcriptase
ribonuclease enzyme that breaks down RNA
sequence mapping mapping information obtained after DNA sequencing
shotgun sequencing method used to sequence multiple DNA fragments to generate the sequence of a large
piece of DNA
single nucleotide polymorphism (SNP) variation between individuals in a single nucleotide
Southern blotting DNA transfer from a gel to a nylon membrane
systems biology study of whole biological systems (genomes and proteomes) based on interactions within the
system
Ti plasmid plasmid system derived from Agrobacterium tumifaciens that scientists have used to introduce
foreign DNA into plant cells
transgenic organism that receives DNA from a different species
variable number of tandem repeats (VNTRs) variation in the number of tandem repeats between individuals
in the population
whole-genome sequencing process that determines an entire genome's DNA sequence
CHAPTER SUMMARY
17.1 Biotechnology
Nucleic acids can be isolated from cells for the purposes of further analysis by breaking open the cells and
enzymatically destroying all other major macromolecules. Fragmented or whole chromosomes can separate on
the basis of size by gel electrophoresis. PCR can amplify short DNA or RNA stretches. Researchers can use
Southern and Northern blotting to detect the presence of specific short sequences in a DNA or RNA sample.
The term “cloning” may refer to cloning small DNA fragments (molecular cloning), cloning cell populations
(cellular cloning), or cloning entire organisms (reproductive cloning). Medical professionals perform genetic
testing to identify disease-causing genes, and use gene therapy to cure an inheritable disease.
Transgenic organisms possess DNA from a different species, usually generated by molecular cloning
techniques. Vaccines, antibiotics, and hormones are examples of products obtained by recombinant DNA
technology. Scientists usually create transgenic plants to improve crop plant characteristics.
17.2 Mapping Genomes
Genome mapping is similar to solving a big, complicated puzzle with pieces of information coming from
laboratories all over the world. Genetic maps provide an outline for locating genes within a genome, and they
estimate the distance between genes and genetic markers on the basis of recombination frequencies during
meiosis. Physical maps provide detailed information about the physical distance between the genes. The most
detailed information is available through sequence mapping. Researchers combine information from all
mapping and sequencing sources to study an entire genome.
17.3 Whole-Genome Sequencing
Whole-genome sequencing is the latest available resource to treat genetic diseases. Some doctors are using
whole-genome sequencing to save lives. Genomics has many industrial applications including biofuel
development, agriculture, pharmaceuticals, and pollution control. The basic principle of all modern-day
sequencing strategies involves the chain termination method of sequencing.
Although the human genome sequences provide key insights to medical professionals, researchers use whole-
genome sequences of model organisms to better understand the species' genome. Automation and the
decreased cost of whole-genome sequencing may lead to personalized medicine in the future.
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17.4 Applying Genomics
Imagination is the only barrier to the applicability of genomics. Researchers are applying genomics to most
fields of biology. They use it for personalized medicine, prediction of disease risks at an individual level,
studying drug interactions before conducting clinical trials, and studying microorganisms in the environment as
opposed to the laboratory. They are also applying it to developments such as generating new biofuels,
genealogical assessment using mitochondria, advances in forensic science, and improvements in agriculture.
17.5 Genomics and Proteomics
Proteomics is the study of the entire set of proteins expressed by a given type of cell under certain
environmental conditions. In a multicellular organism, different cell types will have different proteomes, and
these will vary with environmental changes. Unlike a genome, a proteome is dynamic and in constant flux,
which makes it both more complicated and more useful than the knowledge of genomes alone.
Proteomics approaches rely on protein analysis. Researchers are constantly upgrading these techniques.
Researchers have used proteomics to study different cancer types. Medical professionals are using different
biomarkers and protein signatures to analyze each cancer type. The future goal is to have a personalized
treatment plan for each individual.
VISUAL CONNECTION QUESTIONS
1. Figure 17.7 You are working in a molecular
biology lab and, unbeknownst to you, your lab
partner left the foreign genomic DNA that you are
planning to clone on the lab bench overnight instead
of storing it in the freezer. As a result, it was
degraded by nucleases, but still used in the
experiment. The plasmid, on the other hand, is fine.
What results would you expect from your molecular
cloning experiment?
a. There will be no colonies on the bacterial
plate.
b. There will be blue colonies only.
c. There will be blue and white colonies.
d. The will be white colonies only.
2. Figure 17.8 Do you think Dolly was a Finn-Dorset
or a Scottish Blackface sheep?
REVIEW QUESTIONS
4. GMOs are created by_.
a. generating genomic DNA fragments with
restriction endonucleases
b. introducing recombinant DNA into an
organism by any means
c. overexpressing proteins in E. coli
d. all of the above
5. Gene therapy can be used to introduce foreign
DNA into cells_.
a. for molecular cloning
b. by PCR
c. of tissues to cure inheritable disease
d. all of the above
6. Insulin produced by molecular cloning:
3. Figure 17.15 In 2011, the United States
Preventative Services Task Force recommended
against using the PSA test to screen healthy men for
prostate cancer. Their recommendation is based on
evidence that screening does not reduce the risk of
death from prostate cancer. Prostate cancer often
develops very slowly and does not cause problems,
while the cancer treatment can have severe side
effects. The PCA3 test is considered to be more
accurate, but screening may still result in men who
would not have been harmed by the cancer itself
suffering side effects from treatment. What do you
think? Should all healthy men be screened for
prostate cancer using the PCA3 or PSA test? Should
people in general be screened to find out if they have
a genetic risk for cancer or other diseases?
a. is of pig origin
b. is a recombinant protein
c. is made by the human pancreas
d. is recombinant DNA
7. Bt toxin is considered to be_.
a. a gene for modifying insect DNA
b. an organic insecticide produced by bacteria
c. useful for humans to fight against insects
d. a recombinant protein
8. The Flavr Savr Tomato:
a. is a variety of vine-ripened tomato in the
supermarket
b. was created to have better flavor and shelf-
life
c. does not undergo soft rot
d. all of the above
9. ESTs are
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a. generated after a cDNA library is made
b. unique sequences in the genome
c. useful for mapping using sequence
information
d. all of the above
10. Linkage analysis_.
a. is used to create a physical map
b. is based on the natural recombination
process
c. requires radiation hybrid mapping
d. involves breaking and rejoining of DNA
artificially
11. Genetic recombination occurs by which process?
a. independent assortment
b. crossing over
c. chromosome segregation
d. sister chromatids
12. Individual genetic maps in a given species are:
a. genetically similar
b. genetically identical
c. genetically dissimilar
d. not useful in species analysis
13. Information obtained by microscopic analysis of
stained chromosomes is used in:
a. radiation hybrid mapping
b. sequence mapping
c. RFLP mapping
d. cytogenetic mapping
14. The chain termination method of sequencing:
a. uses labeled ddNTPs
b. uses only dideoxynucleotides
c. uses only deoxynucleotides
d. uses labeled dNTPs
15. Whole-genome sequencing can be used for
advances in:
a. the medical field
b. agriculture
c. biofuels
d. all of the above
16. Sequencing an individual person’s genome
CRITICAL THINKING QUESTIONS
22. Describe the process of Southern blotting.
23. A researcher wants to study cancer cells from a
patient with breast cancer. Is cloning the cancer cells
an option?
24. How would a scientist introduce a gene for
herbicide resistance into a plant?
25. If you had a chance to get your genome
sequenced, what are some questions you might be
a. is currently possible
b. could lead to legal issues regarding
discrimination and privacy
c. could help make informed choices about
medical treatment
d. all of the above
17. What is the most challenging issue facing
genome sequencing?
a. the inability to develop fast and accurate
sequencing techniques
b. the ethics of using information from
genomes at the individual level
c. the availability and stability of DNA
d. all of the above
18. Genomics can be used in agriculture to:
a. generate new hybrid strains
b. improve disease resistance
c. improve yield
d. all of the above
19. Genomics can be used on a personal level to:
a. decrease transplant rejection
b. predict genetic diseases that a person may
have inherited
c. determine the risks of genetic diseases for
an individual’s children
d. all of the above
20. What is a biomarker?
a. the color coding of different genes
b. a protein that is uniquely produced in a
diseased state
c. a molecule in the genome or proteome
d. a marker that is genetically inherited
21. A protein signature is:
a. the path followed by a protein after it is
synthesized in the nucleus
b. the path followed by a protein in the
cytoplasm
c. a protein expressed on the cell surface
d. a unique set of proteins present in a
diseased state
able to have answered about yourself?
26. Why is so much effort being poured into genome
mapping applications?
27. How could a genetic map of the human genome
help find a cure for cancer?
28. Explain why metagenomics is probably the most
revolutionary application of genomics.
29. How can genomics be used to predict disease
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risk and treatment options? detection and treatment?
30. How has proteomics been used in cancer 31. What is personalized medicine?
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18 | EVOLUTION AND THE
ORIGIN OF SPECIES
Figure 18.1 All organisms are products of evolution adapted to their environment, (a) Saguaro (Carnegiea gigantea)
can soak up 750 liters of water in a single rain storm, enabling these cacti to survive the dry conditions of the Sonora
desert in Mexico and the Southwestern United States, (b) The Andean semiaquatic lizard (Potamites montanicola)
discovered in Peru in 2010 lives between 1,570 to 2,100 meters in elevation, and, unlike most lizards, is nocturnal
and swims. Scientists still do not know how these cold-blood animals are able to move in the cold (10 to 15°C)
temperatures of the Andean night, (credit a: modification of work by Gentry George, U.S. Fish and Wildlife Service;
credit b: modification of work by German Chavez and Diego Vasquez, ZooKeys)
Chapter Outline
18.1: Understanding Evolution
18.2: Formation of New Species
18.3: Reconnection and Speciation Rates
Introduction
All living organisms, from bacteria to baboons to blueberries, evolved at some point from a different species.
Although it may seem that living things today stay much the same, that is not the case—evolution is an ongoing
process.
The theory of evolution is the unifying theory of biology, meaning it is the framework within which biologists ask
questions about the living world. Its power is that it provides direction for predictions about living things that are
borne out in ongoing experiments. The Ukrainian-born American geneticist Theodosius Dobzhansky famously
wrote that “nothing makes sense in biology except in the light of evolution.” He meant that the tenet that all life
has evolved and diversified from a common ancestor is the foundation from which we approach all questions in
biology.
1. Theodosius Dobzhansky. “Biology, Molecular and Organismic." American Zoologist 4, no. 4 (1964): 449.
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18.1 1 Understanding Evolution
By the end of this section, you will be able to do the following:
• Describe how scientists developed the present-day theory of evolution
• Define adaptation
• Explain convergent and divergent evolution
• Describe homologous and vestigial structures
• Discuss misconceptions about the theory of evolution
Evolution by natural selection describes a mechanism for how species change over time. Scientists,
philosophers, researchers, and others had made suggestions and debated this topic well before Darwin began
to explore this idea. Classical Greek philosopher Plato emphasized in his writings that species were static
and unchanging, yet there were also ancient Greeks who expressed evolutionary ideas. In the eighteenth
century, naturalist Georges-Louis Leclerc Comte de Buffon reintroduced ideas about the evolution of animals
and observed that various geographic regions have different plant and animal populations, even when the
environments are similar. Some at this time also accepted that there were extinct species.
Also during the eighteenth century, James Hutton, a Scottish geologist and naturalist, proposed that geological
change occurred gradually by accumulating small changes from processes operating like they are today over
long periods of time. This contrasted with the predominant view that the planet's geology was a consequence
of catastrophic events occurring during a relatively brief past. Nineteenth century geologist Charles Lyell
popularized Hutton's view. A friend to Darwin. Lyell’s ideas were influential on Darwin’s thinking: Lyell’s notion
of the greater age of Earth gave more time for gradual change in species, and the process of change provided
an analogy for this change. In the early nineteenth century, Jean-Baptiste Lamarck published a book that
detailed a mechanism for evolutionary change. We now refer to this mechanism as an inheritance of acquired
characteristics by which the environment causes modifications in an individual, or offspring could use or disuse
of a structure during its lifetime, and thus bring about change in a species. While many discredited this
mechanism for evolutionary change, Lamarck’s ideas were an important influence on evolutionary thought.
Charles Darwin and Natural Selection
In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently
conceived and described the actual mechanism for evolution. Importantly, each naturalist spent time exploring
the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S.
Beagle, including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil
to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to
1862. Darwin’s journey, like Wallace’s later journeys to the Malay Archipelago, included stops at several island
chains, the last being the Galapagos Islands west of Ecuador. On these islands, Darwin observed species of
organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground
finches inhabiting the Galapagos Islands comprised several species with a unique beak shape (Figure 18.2).
The species on the islands had a graded series of beak sizes and shapes with very small differences between
the most similar. He observed that these finches closely resembled another finch species on the South American
mainland. Darwin imagined that the island species might be species modified from one of the original mainland
species. Upon further study, he realized that each finch's varied beaks helped the birds acquire a specific type of
food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches
had spear-like beaks for stabbing their prey.
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1 o
\
1. Geospiza magnirostris 2. Geospiza fortis
3. Geospiza parvula 4. Certhidea olivacea
Finches from Galapagos Archipelago
Figure 18.2 Darwin observed that beak shape varies among finch species. He postulated that ancestral species' beaks
had adapted over time to equip the finches to acquire different food sources.
Wallace and Darwin both observed similar patterns in other organisms and they independently developed the
same explanation for how and why such changes could take place. Darwin called this mechanism natural
selection. Natural selection, or “survival of the fittest,” is the more prolific reproduction of individuals with
favorable traits that survive environmental change because of those traits. This leads to evolutionary change.
For example, Darwin observed a population of giant tortoises in the Galapagos Archipelago to have longer necks
than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could
reach more leaves and access more food than those with short necks. In times of drought when fewer leaves
would be available, those that could reach more leaves had a better chance to eat and survive than those that
couldn’t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively
successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be
present in the population.
Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First,
most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including
Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more
offspring are produced than are able to survive, so resources for survival and reproduction are limited. The
capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus,
there is competition for those resources in each generation. Both Darwin and Wallace’s understanding of this
principle came from reading economist Thomas Malthus' essay that explained this principle in relation to human
populations. Third, offspring vary among each other in regard to their characteristics and those variations are
inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best
compete for limited resources will survive and have more offspring than those individuals with variations that are
less able to compete. Because characteristics are inherited, these traits will be better represented in the next
generation. This will lead to change in populations over generations in a process that Darwin called descent with
modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment.
It is the only mechanism known for adaptive evolution.
In 1858, Darwin and Wallace (Figure 18.3) presented papers at the Linnean Society in London that discussed
the idea of natural selection. The following year Darwin’s book, On the Origin of Species, was published. His
book outlined in considerable detail his arguments for evolution by natural selection.
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(a) (b)
Figure 18.3 Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that they
presented together at the Linnean Society in 1858.
It is difficult and time-consuming to document and present examples of evolution by natural selection. The
Galapagos finches are an excellent example. Peter and Rosemary Grant and their colleagues have studied
Galapagos finch populations every year since 1976 and have provided important evidence of natural selection.
The Grants found changes from one generation to the next in beak shape distribution with the medium ground
finch on the Galapagos island of Daphne Major. The birds have inherited a variation in their bill shape with some
having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal
because of an El Nino, there was a lack of large hard seeds of which the large-billed birds ate; however, there
was an abundance of the small soft seeds which the small-billed birds ate. Therefore, the small-billed birds
were able to survive and reproduce. In the years following this El Nino, the Grants measured beak sizes in the
population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with
smaller bills had more offspring and the bill evolved into a much smaller size. As conditions improved in 1987
and larger seeds became more available, the trend toward smaller average bill size ceased.
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Field Biologist
Many people hike, explore caves, scuba dive, or climb mountains for recreation. People often participate
in these activities hoping to see wildlife. Experiencing the outdoors can be incredibly enjoyable and
invigorating. What if your job entailed working in the wilderness? Field biologists by definition work outdoors
in the “field.” The term field in this case refers to any location outdoors, even under water. A field biologist
typically focuses research on a certain species, group of organisms, or a single habitat (Figure 18.4).
Figure 18.4 A field biologist tranquilizes a polar bear for study, (credit: Karen Rhode)
One objective of many field biologists includes discovering new, unrecorded species. Not only do such
findings expand our understanding of the natural world, but they also lead to important innovations in fields
such as medicine and agriculture. Plant and microbial species, in particular, can reveal new medicinal and
nutritive knowledge. Other organisms can play key roles in ecosystems or if rare require protection. When
discovered, researchers can use these important species as evidence for environmental regulations and
laws.
Processes and Patterns of Evolution
Natural selection can only take place if there is variation, or differences, among individuals in a population.
Importantly, these differences must have some genetic basis; otherwise, the selection will not lead to change in
the next generation. This is critical because nongenetic reasons can cause variation among individuals such as
an individual's height because of better nutrition rather than different genes.
Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction. Mutation,
a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population. The genetic
changes that mutation causes can have one of three outcomes on the phenotype. A mutation affects the
organism's phenotype in a way that gives it reduced fitness—lower likelihood of survival or fewer offspring. A
mutation may produce a phenotype with a beneficial effect on fitness. Many mutations will also have no effect
on the phenotype's fitness. We call these neutral mutations. Mutations may also have a whole range of effect
sizes on the organism's fitness that expresses them in their phenotype, from a small effect to a great effect.
Sexual reproduction also leads to genetic diversity: when two parents reproduce, unique combinations of alleles
assemble to produce the unique genotypes and thus phenotypes in each offspring.
We call a heritable trait that helps an organism's survival and reproduction in its present environment an
adaptation. Scientists describe groups of organisms adapting to their environment when a genetic variation
occurs over time that increases or maintains the population's “fit" to its environment. A platypus's webbed feet
are an adaptation for swimming. A snow leopard's thick fur is an adaptation for living in the cold. A cheetah's fast
speed is an adaptation for catching prey.
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Whether or not a trait is favorable depends on the current environmental conditions. The same traits are not
always selected because environmental conditions can change. For example, consider a plant species that grew
in a moist climate and did not need to conserve water. Large leaves were selected because they allowed the
plant to obtain more energy from the sun. Large leaves require more water to maintain than small leaves, and the
moist environment provided favorable conditions to support large leaves. After thousands of years, the climate
changed, and the area no longer had excess water. The direction of natural selection shifted so that plants
with small leaves were selected because those populations were able to conserve water to survive the new
environmental conditions.
The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives
rise to groups of organisms that become tremendously different from each other. We call two species that evolve
in diverse directions from a common point divergent evolution. We can see such divergent evolution in the
forms of the reproductive organs of flowering plants which share the same basic anatomies; however, they can
look very different as a result of selection in different physical environments and adaptation to different kinds of
pollinators (Figure 18.5).
(a) (b)
Figure 18.5 Flowering plants evolved from a common ancestor. Notice that the (a) dense blazing star (Liatrus spicata)
and the (b) purple coneflower (Echinacea purpurea) vary in appearance, yet both share a similar basic morphology,
(credit a: modification of work by Drew Avery; credit b: modification of work by Cory Zanker)
in other cases, similar phenotypes evolve independently in distantly related species. For example, flight has
evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations
to flight. However, bat and insect wings have evolved from very different original structures. We call this
phenomenon convergent evolution, where similar traits evolve independently in species that do not share a
common ancestry. The two species came to the same function, flying, but did so separately from each other.
These physical changes occur over enormous time spans and help explain how evolution occurs. Natural
selection acts on individual organisms, which can then shape an entire species. Although natural selection may
work in a single generation on an individual, it can take thousands or even millions of years for an entire species'
genotype to evolve. It is over these large time spans that life on earth has changed and continues to change.
Evidence of Evolution
The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems,
biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the
Origin of Species, to identifying patterns in nature that were consistent with evolution, and since Darwin, our
understanding has become clearer and broader.
Fossils
Fossils provide solid evidence that organisms from the past are not the same as those today, and fossils show
a progression of evolution. Scientists determine the age of fossils and categorize them from all over the world to
determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past
and shows the evolution of form over millions of years (Figure 18.6). For example, scientists have recovered
highly detailed records showing the evolution of humans and horses (Figure 18.6). The whale flipper shares
a similar morphology to bird and mammal appendages (Figure 18.7) indicating that these species share a
common ancestor.
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(a)
(b)
Figure 18.6 In this (a) display, fossil hominids are arranged from oldest (bottom) to newest (top). As hominids evolved,
the skull's shape changed. An artist’s rendition of (b) extinct species of the genus Equus reveals that these ancient
species resembled the modern horse (Equus ferus) but varied in size.
Anatomy and Embryology
Another type of evidence for evolution is the presence of structures in organisms that share the same basic form.
For example, the bones in human, dog, bird, and whale appendages all share the same overall construction
(Figure 18.7) resulting from their origin in a common ancestor's appendages. Over time, evolution led to
changes in the bones' shapes and sizes different species, but they have maintained the same overall layout.
Scientists call these synonymous parts homologous structures.
Bird
Whale
Human
Dog
Figure 18.7 The similar construction of these appendages indicates that these organisms share a common ancestor.
Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a
past common ancestor. We call these unused structures without function vestigial structures. Other examples
of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales.
Visit this interactive site (http:// 0 penstaxc 0 llege. 0 rg/l/b 0 ne_structures) to guess which bone structures
are homologous and which are analogous, and see examples of evolutionary adaptations to illustrate these
concepts.
Another evidence of evolution is the convergence of form in organisms that share similar environments. For
example, species of unrelated animals, such as the arctic fox and ptarmigan, living in the arctic region have
been selected for seasonal white phenotypes during winter to blend with the snow and ice (Figure 18.8). These
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similarities occur not because of common ancestry, but because of similar selection pressures—the benefits of
predators not seeing them.
(a) (b)
Figure 18.8 The white winter coat of the (a) arctic fox and the (b) ptarmigan’s plumage are adaptations to their
environments, (credit a: modification of work by Keith Morehouse)
Embryology, the study of the anatomy of an organism's development to its adult form, also provides evidence
of relatedness between now widely divergent groups of organisms. Mutational tweaking in the embryo can have
such magnified consequences in the adult that tends to conserve embryo formation. As a result, structures that
are absent in some groups often appear in their embryonic forms and disappear when they reach the adult or
juvenile form. For example, all vertebrate embryos, including humans, exhibit gill slits and tails at some point in
their early development. These disappear in the adults of terrestrial groups but adult forms of aquatic groups
such as fish and some amphibians maintain them. Great ape embryos, including humans, have a tail structure
during their development that they lose when they are born.
Biogeography
The geographic distribution of organisms on the planet follows patterns that we can explain best by evolution
in conjunction with tectonic plate movement over geological time. Broad groups that evolved before the
supercontinent Pangaea broke up (about 200 million years ago) are distributed worldwide. Groups that evolved
since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern
continents that formed from the supercontinent Laurasia and of the southern continents that formed from the
supercontinent Gondwana. The presence of members of the plant family Proteaceae in Australia, southern
Africa, and South America was most predominant prior to the southern supercontinent Gondwana breaking up.
Marsupial diversification in Australia and the absence of other mammals reflect Australia’s long isolation.
Australia has an abundance of endemic species—species found nowhere else—which is typical of islands
whose isolation by expanses of water prevents species to migrate. Over time, these species diverge
evolutionarily into new species that look very different from their ancestors that may exist on the mainland.
Australia's marsupials, the Galapagos' finches, and many species on the Hawaiian Islands are all unique to their
one point of origin, yet they display distant relationships to ancestral species on mainlands.
Molecular Biology
Like anatomical structures, the molecular structures of life reflect descent with modification. DNA's universality
reflects evidence of a common ancestor for all of life. Fundamental divisions in life between the genetic code,
DNA replication, and expression are reflected in major structural differences in otherwise conservative structures
such as ribosome components and membrane structures. In general, the relatedness of groups of organisms is
reflected in the similarity of their DNA sequences—exactly the pattern that we would expect from descent and
diversification from a common ancestor.
DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the
evolution of new functions for proteins commonly occurs after gene duplication events that allow freely modifying
one copy by mutation, selection, or drift (changes in a population’s gene pool resulting from chance), while the
second copy continues to produce a functional protein.
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Misconceptions of Evolution
Although the theory of evolution generated some controversy when Darwin first proposed it, biologists almost
universally accepted it, particularly younger biologists, within 20 years after publication of On the Origin of
Species. Nevertheless, the theory of evolution is a difficult concept and misconceptions about how it works
abound.
This site (http:// 0 penstaxc 0 llege. 0 rg/l/misc 0 ncepti 0 ns) addresses some of the main misconceptions
associated with the theory of evolution.
Evolution Is Just a Theory
Critics of the theory of evolution dismiss its importance by purposefully confounding the everyday usage of
the word “theory" with the way scientists use the word. In science, we understand a “theory" to be a body of
thoroughly tested and verified explanations for a set of observations of the natural world. Scientists have a theory
of the atom, a theory of gravity, and the theory of relativity, each which describes understood facts about the
world, in the same way, the theory of evolution describes facts about the living world. As such, a theory in science
has survived significant efforts to discredit it by scientists. In contrast, a “theory" in common vernacular is a word
meaning a guess or suggested explanation. This meaning is more akin to the scientific concept of “hypothesis.”
When critics of evolution say it is “just a theory,” they are implying that there is little evidence supporting it and
that it is still in the process of rigorous testing. This is a mischaracterization.
Individuals Evolve
Evolution is the change in a population's genetic composition over time, specifically over generations, resulting
from differential reproduction of individuals with certain alleles. Individuals do change over their lifetime,
obviously, but this is development and involves changes programmed by the set of genes the individual acquired
at birth in coordination with the individual’s environment. When thinking about the evolution of a characteristic, it
is probably best to think about the change of the average value of the characteristic in the population over time.
For example, when natural selection leads to bill-size change in medium ground finches in the Galapagos, this
does not mean that individual bills on the finches are changing. If one measures the average bill size among
all individuals in the population at one time and then measures them in the population several years later, this
average value will be different as a result of evolution. Although some individuals may survive from the first time
to the second, they will still have the same bill size; however, there will be many new individuals who contribute
to the shift in average bill size.
Evolution Explains the Origin of Life
It is a common misunderstanding that evolution includes an explanation of life’s origins. Conversely, some of the
theory’s critics believe that it cannot explain the origin of life. The theory does not try to explain the origin of life.
The theory of evolution explains how populations change over time and how life diversifies the origin of species.
It does not shed light on the beginnings of life including the origins of the first cells, which define life. Importantly,
biologists believe that the presence of life on Earth precludes the possibility that the events that led to life on
Earth can repeat themselves because the intermediate stages would immediately become food for existing living
things.
However, once a mechanism of inheritance was in place in the form of a molecule like DNA either within a cell
or pre-cell, these entities would be subject to the principle of natural selection. More effective reproducers would
increase in frequency at the expense of inefficient reproducers. While evolution does not explain the origin of life,
it may have something to say about some of the processes operating once pre-living entities acquired certain
properties.
Organisms Evolve on Purpose
Statements such as “organisms evolve in response to a change in an environment” are quite common, but
such statements can lead to two types of misunderstandings. First, do not interpret the statement to mean that
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individual organisms evolve. The statement is shorthand for “a population evolves in response to a changing
environment.” However, a second misunderstanding may arise by interpreting the statement to mean that
the evolution is somehow intentional. A changed environment results in some individuals in the population,
those with particular phenotypes, benefiting and therefore producing proportionately more offspring than other
phenotypes. This results in change in the population if the characteristics are genetically determined.
It is also important to understand that the variation that natural selection works on is already in a population
and does not arise in response to an environmental change. For example, applying antibiotics to a population of
bacteria will, over time, select a population of bacteria that are resistant to antibiotics. The resistance, which a
gene causes, did not arise by mutation because of applying the antibiotic. The gene for resistance was already
present in the bacteria's gene pool, likely at a low frequency. The antibiotic, which kills the bacterial cells without
the resistance gene, strongly selects individuals that are resistant, since these would be the only ones that
survived and divided. Experiments have demonstrated that mutations for antibiotic resistance do not arise as a
result of antibiotic.
In a larger sense, evolution is not goal directed. Species do not become “better" over time. They simply track
their changing environment with adaptations that maximize their reproduction in a particular environment at a
particular time. Evolution has no goal of making faster, bigger, more complex, or even smarter species, despite
the commonness of this kind of language in popular discourse. What characteristics evolve in a species are a
function of the variation present and the environment, both of which are constantly changing in a nondirectional
way. A trait that fits in one environment at one time may well be fatal at some point in the future. This holds
equally well for insect and human species.
18.2 | Formation of New Species
By the end of this section, you will be able to do the following:
• Define species and describe how scientists identify species as different
• Describe genetic variables that lead to speciation
• Identify prezygotic and postzygotic reproductive barriers
• Explain allopatric and sympatric speciation
• Describe adaptive radiation
Although all life on earth shares various genetic similarities, only certain organisms combine genetic information
by sexual reproduction and have offspring that can then successfully reproduce. Scientists call such organisms
members of the same biological species.
Species and the Ability to Reproduce
A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to
this definition, one species is distinguished from another when, in nature, it is not possible for matings between
individuals from each species to produce fertile offspring.
Members of the same species share both external and internal characteristics, which develop from their
DNA. The closer relationship two organisms share, the more DNA they have in common, just like people
and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin or
grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and therefore
share characteristics and behaviors that lead to successful reproduction.
Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though
domestic dogs (Canis lupus familiaris ) display phenotypic differences, such as size, build, and coat, most dogs
can interbreed and produce viable puppies that can mature and sexually reproduce (Figure 18.9).
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(a) (b) (c)
Figure 18.9 The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo.
(credit a: modification of work by Sally Eller, Tom Reese; credit b: modification of work by Jeremy McWilliams; credit c:
modification of work by Kathleen Conklin)
in other cases, individuals may appear similar although they are not members of the same species. For example,
even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer ) are both birds
and eagles, each belongs to a separate species group (Figure 18.10). If humans were to artificially intervene
and fertilize a bald eagle's egg with an African fish eagle's sperm and a chick did hatch, that offspring, called
a hybrid (a cross between two species), would probably be infertile—unable to successfully reproduce after it
reached maturity. Different species may have different genes that are active in development; therefore, it may
not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization
may take place, the two species still remain separate.
(a) (b)
Figure 18.10 The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of
different species, (credit a: modification of work by Nigel Wedge; credit b: modification of work by U.S. Fish and Wildlife
Service)
Populations of species share a gene pool: a collection of all the gene variants in the species. Again, the
basis to any changes in a group or population of organisms must be genetic for this is the only way to share
and pass on traits. When variations occur within a species, they can only pass to the next generation along
two main pathways: asexual reproduction or sexual reproduction. The change will pass on asexually simply if
the reproducing cell possesses the changed trait. For the changed trait to pass on by sexual reproduction, a
gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing
organisms can experience several genetic changes in their body cells, but if these changes do not occur in
a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve.
Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In
short, organisms must be able to reproduce with each other to pass new traits to offspring.
Speciation
The biological definition of species, which works for sexually reproducing organisms, is a group of actual or
potential interbreeding individuals. There are exceptions to this rule. Many species are similar enough that hybrid
offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. The
presence in nature of hybrids between similar species suggests that they may have descended from a single
interbreeding species, and the speciation process may not yet be completed.
Given the extraordinary diversity of life on the planet there must be mechanisms for speciation: the formation
of two species from one original species. Darwin envisioned this process as a branching event and diagrammed
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the process in the only illustration in On the Origin of Species (Figure 18.11a). Compare this illustration to the
diagram of elephant evolution (Figure 18.11), which shows that as one species changes over time, it branches
to form more than one new species, repeatedly, as long as the population survives or until the organism becomes
extinct.
(a) (b)
Figure 18.11 The only illustration in Darwin's On the Origin of Species is (a) a diagram showing speciation events
leading to biological diversity. The diagram shows similarities to phylogenetic charts that today illustrate the
relationships of species, (b) Modern elephants evolved from the Palaeomastodon, a species that lived in Egypt 35-50
million years ago.
For speciation to occur, two new populations must form from one original population and they must evolve in
such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists
have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation
(alio- = "other"; -patric = "homeland") involves geographic separation of populations from a parent species
and subsequent evolution. Sympatric speciation (sym- = "same"; -patric = "homeland") involves speciation
occurring within a parent species remaining in one location.
Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There
is no reason why more than two species might not form at one time except that it is less likely and we can
conceptualize multiple events as single splits occurring close in time.
Allopatric Speciation
A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the
movement of alleles across a species' range, is relatively free because individuals can move and then mate with
individuals in their new location. Thus, an allele's frequency at one end of a distribution will be similar to the
allele's frequency at the other end. When populations become geographically discontinuous, it prevents alleles'
free-flow. When that separation lasts for a period of time, the two populations are able to evolve along different
trajectories. Thus, their allele frequencies at numerous genetic loci gradually become increasingly different as
new alleles independently arise by mutation in each population. Typically, environmental conditions, such as
climate, resources, predators, and competitors for the two populations will differ causing natural selection to
favor divergent adaptations in each group.
Isolation of populations leading to allopatric speciation can occur in a variety of ways: a river forming a new
branch, erosion creating a new valley, a group of organisms traveling to a new location without the ability to
return, or seeds floating over the ocean to an island. The nature of the geographic separation necessary to
isolate populations depends entirely on the organism's biology and its potential for dispersal. If two flying insect
populations took up residence in separate nearby valleys, chances are, individuals from each population would
fly back and forth continuing gene flow. However, if a new lake divided two rodent populations continued gene
flow would be unlikely; therefore, speciation would be more likely.
Biologists group allopatric processes into two categories: dispersal and vicariance. Dispersal is when a few
members of a species move to a new geographical area, and vicariance is when a natural situation arises to
physically divide organisms.
Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west
coast of the United States, two separate spotted owl subspecies exist. The northern spotted owl has genetic and
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phenotypic differences from its close relative: the Mexican spotted owl, which lives in the south (Figure 18.12).
Mexican Spotted Owl
Figure 18.12 The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with
different climates and ecosystems. The owl is an example of allopatric speciation. (credit "northern spotted owl":
modification of work by John and Karen Hollingsworth; credit "Mexican spotted owl": modification of work by Bill Radke)
Additionally, scientists have found that the further the distance between two groups that once were the same
species, the more likely it is that speciation will occur. This seems logical because as the distance increases, the
various environmental factors would likely have less in common than locations in close proximity. Consider the
two owls: in the north, the climate is cooler than in the south. The types of organisms in each ecosystem differ,
as do their behaviors and habits. Also, the hunting habits and prey choices of the southern owls vary from the
northern owls. These variances can lead to evolved differences in the owls, and speciation likely will occur.
Adaptive Radiation
In some cases, a population of one species disperses throughout an area, and each finds a distinct niche
or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events
originating from a single species. We call this adaptive radiation because many adaptations evolve from a
single point of origin; thus, causing the species to radiate into several new ones. Island archipelagos like the
Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island
which leads to geographical isolation for many organisms. The Hawaiian honeycreeper illustrates one example
of adaptive radiation. From a single species, the founder species, numerous species have evolved, including the
six in Figure 18.13.
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Figure 18.13 The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others
evolved, each with its own distinctive characteristics.
Notice the differences in the species’ beaks in Figure 18.13. Evolution in response to natural selection based
on specific food sources in each new habitat led to evolution of a different beak suited to the specific food
source. The seed-eating bird has a thicker, stronger beak which is suited to break hard nuts. The nectar-eating
birds have long beaks to dip into flowers to reach the nectar. The insect-eating birds have beaks like swords,
appropriate for stabbing and impaling insects. Darwin’s finches are another example of adaptive radiation in an
archipelago.
LINK TQ LEARNING
Click through this interactive site (http:// 0 penstaxc 0 llege. 0 rg/l/bird_ev 0 luti 0 n) to see how island birds
evolved in evolutionary increments from 5 million years ago to today.
Sympatric Speciation
Can divergence occur if no physical barriers are in place to separate individuals who continue to live and
reproduce in the same habitat? The answer is yes. We call the process of speciation within the same space
sympatric. The prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric”
meaning “other homeland.” Scientists have proposed and studied many mechanisms.
One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell
division event chromosomes replicate, pair up, and then separate so that each new cell has the same number
of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few
individual chromosomes in a condition that we call aneuploidy (Figure 18.14).
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visual
CONNECTION
Figure 18.14 Aneuploidy results when the gametes have too many or too few chromosomes due to nondisjunction
during meiosis. In this example, the resulting offspring will have 2n+l or 2n-l chromosomes.
Which is most likely to survive, offspring with 2n+l chromosomes or offspring with 2n-l chromosomes?
Polyploidy is a condition in which a cell or organism has an extra set, or sets, of chromosomes. Scientists have
identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploidy
state. Reproductive isolation is the inability to interbreed. In some cases, a polyploid individual will have two or
more complete sets of chromosomes from its own species in a condition that we call autopolyploidy (Figure
18.15). The prefix “auto-” means “self,” so the term means multiple chromosomes from one’s own species.
Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of
separating.
Figure 18.15 Autopolyploidy results when mitosis is not followed by cytokinesis.
For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2 n = 6, when
they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new
gametes will be incompatible with the normal gametes that this plant species produces. However, they could
either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number.
In this way, sympatric speciation can occur quickly by forming offspring with 4n that we call a tetraploid. These
individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral
species.
The other form of polyploidy occurs when individuals of two different species reproduce to form a viable
offspring that we call an allopolyploid. The prefix “alio-” means “other” (recall from allopatric): therefore, an
allopolyploid occurs when gametes from two different species combine. Figure 18.16 illustrates one possible
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way an allopolyploid can form. Notice how it takes two generations, or two reproductive acts, before the viable
fertile hybrid results.
Figure 18.16 Alloploidy results when two species mate to produce viable offspring. In this example, a normal gamete
from one species fuses with a polyploidy gamete from another. Two matings are necessary to produce viable offspring.
The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs
occasionally in animals, it takes place most commonly in plants. (Animals with any of the types of chromosomal
aberrations that we describe here are unlikely to survive and produce normal offspring.) Scientists have
discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With
such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as
an adaptation than as an error.
Reproductive Isolation
Given enough time, the genetic and phenotypic divergence between populations will affect characters that
influence reproduction: if individuals of the two populations were brought together, mating would be less likely,
but if mating occurred, offspring would be nonviable or infertile. Many types of diverging characters may affect
the reproductive isolation, the ability to interbreed, of the two populations.
Reproductive isolation can take place in a variety of ways. Scientists organize them into two groups: prezygotic
barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of an organism's
development that reproduces sexually. Therefore, a prezygotic barrier is a mechanism that blocks reproduction
from taking place. This includes barriers that prevent fertilization when organisms attempt reproduction. A
postzygotic barrier occurs after zygote formation. This includes organisms that don’t survive the embryonic
stage and those that are born sterile.
Some types of prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times
of the year, often just annually. Differences in breeding schedules, which we call temporal isolation, can act as
a form of reproductive isolation. For example, two frog species inhabit the same area, but one reproduces from
January to March; whereas, the other reproduces from March to May (Figure 18.17).
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(a) (b)
Figure 18.17 These two related frog species exhibit temporal reproductive isolation, (a) Rana aurora breeds earlier in
the year than (b) Rana boylii. (credit a: modification of work by Mark R. Jennings, USFWS; credit b: modification of
work by Alessandro Catenazzi)
In some cases, populations of a species move or are moved to a new habitat and take up residence in a
place that no longer overlaps with the same species' other populations. We call this situation habitat isolation.
Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically
independent. For example, a cricket population that was divided after a flood could no longer interact with each
other. Over time, natural selection forces, mutation, and genetic drift will likely result in the two groups diverging
(Figure 18.18).
(a) Cryllus pennsylvanicus prefers (b) Cryllus firmus prefers loamy soil,
sandy soil.
Figure 18.18 Speciation can occur when two populations occupy different habitats. The habitats need not be far apart.
The cricket (a) Gryllus pennsylvanicus prefers sandy soil, and the cricket (b) Gryllus firmus prefers loamy soil. The two
species can live in close proximity, but because of their different soil preferences, they became genetically isolated.
Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction. For
example, male fireflies use specific light patterns to attract females. Various firefly species display their lights
differently. If a male of one species tried to attract the female of another, she would not recognize the light pattern
and would not mate with the male.
Other prezygotic barriers work when differences in their gamete cells (eggs and sperm) prevent fertilization from
taking place. We call this a gametic barrier. Similarly, in some cases closely related organisms try to mate, but
their reproductive structures simply do not fit together. For example, damselfly males of different species have
differently shaped reproductive organs. If one species tries to mate with the female of another, their body parts
simply do not fit together. (Figure 18.19).
<5T im,
Figure 18.19 The shape of the male reproductive organ varies among male damselfly species, and is only compatible
with the female of that species. Reproductive organ incompatibility keeps the species reproductively isolated.
In plants, certain structures aimed to attract one type of pollinator simultaneously prevent a different pollinator
from accessing the pollen. The tunnel through which an animal must access nectar can vary widely in length and
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diameter, which prevents the plant from cross-pollinating with a different species (Figure 18.20).
(a) Honeybee drinking nectar (b) Ruby-throated hummingbird drinking nectar from a
from a foxglove flower trumpet creeper flower
Figure 18.20 Some flowers have evolved to attract certain pollinators. The (a) wide foxglove flower is adapted for
pollination by bees, while the (b) long, tube-shaped trumpet creeper flower is adapted for pollination by hummingbirds.
When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction. Hybrid
individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic
stages. We call this hybrid inviability because the hybrid organisms simply are not viable. In another
postzygotic situation, reproduction leads to hybrid birth and growth that is sterile. Therefore, the organisms are
unable to reproduce offspring of their own. We call this hybrid sterility.
Habitat Influence on Speciation
Sympatric speciation may also take place in ways other than polyploidy. For example, consider a fish species
that lives in a lake. As the population grows, competition for food increases. Under pressure to find food, suppose
that a group of these fish had the genetic flexibility to discover and feed off another resource that other fish did
not use. What if this new food source was located at a different depth of the lake? Over time, those feeding
on the second food source would interact more with each other than the other fish; therefore, they would breed
together as well. Offspring of these fish would likely behave as their parents: feeding and living in the same area
and keeping separate from the original population. If this group of fish continued to remain separate from the
first population, eventually sympatric speciation might occur as more genetic differences accumulated between
them.
This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is
Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of
sympatric speciation events in these fish, which have not only happened in great number, but also over a short
period of time. Figure 18.21 shows this type of speciation among a cichlid fish population in Nicaragua. In this
locale, two types of cichlids live in the same geographic location but have come to have different morphologies
that allow them to eat various food sources.
Thin-lipped cichlid Thick-lipped cichlid
Figure 18.21 Cichlid fish from Lake Apoyeque, Nicaragua, show evidence of sympatric speciation. Lake Apoyeque,
a crater lake, is 1800 years old, but genetic evidence indicates that a single population of cichlid fish populated the
lake only 100 years ago. Nevertheless, two populations with distinct morphologies and diets now exist in the lake, and
scientists believe these populations may be in an early stage of speciation.
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18.3 | Reconnection and Speciation Rates
By the end of this section, you will be able to do the following:
• Describe pathways of species evolution in hybrid zones
• Explain the two major theories on rates of speciation
Speciation occurs over a span of evolutionary time, so when a new species arises, there is a transition period
during which the closely related species continue to interact.
Reconnection
After speciation, two species may recombine or even continue interacting indefinitely. Individual organisms will
mate with any nearby individual with whom they are capable of breeding. We call an area where two closely
related species continue to interact and reproduce, forming hybrids a hybrid zone. Over time, the hybrid zone
may change depending on the fitness of the hybrids and the reproductive barriers (Figure 18.22). If the hybrids
are less fit than the parents, speciation reinforcement occurs, and the species continue to diverge until they can
no longer mate and produce viable offspring. If reproductive barriers weaken, fusion occurs and the two species
become one. Barriers remain the same if hybrids are fit and reproductive: stability may occur and hybridization
continues.
visual
CONNECTION
Changes in the Hybrid Zone over Time
Reinforcement:
Hybrids are less fit
than either purebred
species. The species
continue to diverge
until hybridization
can no longer occur.
Fusion:
Reproductive barriers
weaken until the two
species become one.
Stability:
Fit hybrids continue
to be produced.
Figure 18.22 After speciation has occurred, the two separate but closely related species may continue to
produce offspring in an area called the hybrid zone. Reinforcement, fusion, or stability may result, depending on
reproductive barriers and the relative fitness of the hybrids.
If two species eat a different diet but one of the food sources is eliminated and both species are forced to
eat the same foods, what change in the hybrid zone is most likely to occur?
Hybrids can be either less fit than the parents, more fit, or about the same. Usually hybrids tend to be less fit;
therefore, such reproduction diminishes over time, nudging the two species to diverge further in a process we
call reinforcement. Scientists use this term because the hybrids' low success reinforces the original speciation.
If the hybrids are as fit or more fit than the parents, the two species may fuse back into one species (Figure
18.23). Scientists have also observed that sometimes two species will remain separate but also continue to
interact to produce some individuals. Scientists classify this as stability because no real net change is taking
place.
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Varying Rates of Speciation
Scientists around the world study speciation, documenting observations both of living organisms and those
found in the fossil record. As their ideas take shape and as research reveals new details about how life evolves,
they develop models to help explain speciation rates. In terms of how quickly speciation occurs, we can observe
two current patterns: gradual speciation model and punctuated equilibrium model.
In the gradual speciation model, species diverge gradually over time in small steps. In the punctuated
equilibrium model, a new species undergoes changes quickly from the parent species, and then remains
largely unchanged for long periods of time afterward (Figure 18.23). We call this early change model punctuated
equilibrium, because it begins with a punctuated or periodic change and then remains in balance afterward.
While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism.
visual
CONNECTION
Gradual Speciation
Founder species
Figure 18.23 In (a) gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In
(b) punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time.
Which of the following statements is false?
a. Punctuated equilibrium is most likely to occur in a small population that experiences a rapid change in
its environment.
b. Punctuated equilibrium is most likely to occur in a large population that lives in a stable climate.
c. Gradual speciation is most likely to occur in species that live in a stable climate.
d. Gradual speciation and punctuated equilibrium both result in the divergence of species.
The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions,
selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form
for many thousands of years. Layers of their fossils would appear similar for a long time. When a change in the
environment takes place—such as a drop in the water level—a small number of organisms are separated from
the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces
new environmental conditions. Because its gene pool quickly became so small, any variation that surfaces and
that aids in surviving the new conditions becomes the predominant form.
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LINK TQ LEARNING
Visit this website (http:// 0 penstaxc 0 llege. 0 rg/l/snails) to continue the speciation story of the snails.
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KEY TERMS
adaptation heritable trait or behavior in an organism that aids in its survival and reproduction in its present
environment
adaptive radiation speciation when one species radiates to form several other species
allopatric speciation speciation that occurs via geographic separation
allopolyploid polyploidy formed between two related, but separate species
aneuploidy condition of a cell having an extra chromosome or missing a chromosome for its species
autopolyploid polyploidy formed within a single species
behavioral isolation type of reproductive isolation that occurs when a specific behavior or lack of one prevents
reproduction from taking place
convergent evolution process by which groups of organisms independently evolve to similar forms
dispersal allopatric speciation that occurs when a few members of a species move to a new geographical area
divergent evolution process by which groups of organisms evolve in diverse directions from a common point
gametic barrier prezygotic barrier occurring when closely related individuals of different species mate, but
differences in their gamete cells (eggs and sperm) prevent fertilization from taking place
gradual speciation model model that shows how species diverge gradually over time in small steps
habitat isolation reproductive isolation resulting when species' populations move or are moved to a new
habitat, taking up residence in a place that no longer overlaps with the same species' other populations
homologous structures parallel structures in diverse organisms that have a common ancestor
hybrid offspring of two closely related individuals, not of the same species
hybrid zone area where two closely related species continue to interact and reproduce, forming hybrids
natural selection reproduction of individuals with favorable genetic traits that survive environmental change
because of those traits, leading to evolutionary change
postzygotic barrier reproductive isolation mechanism that occurs after zygote formation
prezygotic barrier reproductive isolation mechanism that occurs before zygote formation
punctuated equilibrium model for rapid speciation that can occur when an event causes a small portion of a
population to be cut off from the rest of the population
reinforcement continued speciation divergence between two related species due to low fitness of hybrids
between them
reproductive isolation situation that occurs when a species is reproductively independent from other species;
behavior, location, or reproductive barriers may cause this to happen
speciation formation of a new species
species group of populations that interbreed and produce fertile offspring
sympatric speciation speciation that occurs in the same geographic space
temporal isolation differences in breeding schedules that can act as a form of prezygotic barrier leading to
reproductive isolation
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variation genetic differences among individuals in a population
vestigial structure physical structure present in an organism but that has no apparent function and appears to
be from a functional structure in a distant ancestor
vicariance allopatric speciation that occurs when something in the environment separates organisms of the
same species into separate groups
CHAPTER SUMMARY
18.1 Understanding Evolution
Evolution is the process of adaptation through mutation which allows more desirable characteristics to pass to
the next generation. Over time, organisms evolve more characteristics that are beneficial to their survival. For
living organisms to adapt and change to environmental pressures, genetic variation must be present. With
genetic variation, individuals have differences in form and function that allow some to survive certain conditions
better than others. These organisms pass their favorable traits to their offspring. Eventually, environments
change, and what was once a desirable, advantageous trait may become an undesirable trait and organisms
may further evolve. Evolution may be convergent with similar traits evolving in multiple species or divergent
with diverse traits evolving in multiple species that came from a common ancestor. We can observe evidence of
evolution by means of DNA code and the fossil record, and also by the existence of homologous and vestigial
structures.
18.2 Formation of New Species
Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through
mechanisms that occur within a shared habitat (sympatric speciation). Both pathways isolate a population
reproductively in some form. Mechanisms of reproductive isolation act as barriers between closely related
species, enabling them to diverge and exist as genetically independent species. Prezygotic barriers block
reproduction prior to formation of a zygote; whereas, postzygotic barriers block reproduction after fertilization
occurs. For a new species to develop, something must cause a breach in the reproductive barriers. Sympatric
speciation can occur through errors in meiosis that form gametes with extra chromosomes (polyploidy).
Autopolyploidy occurs within a single species; whereas, allopolyploidy occurs between closely related species.
18.3 Reconnection and Speciation Rates
Speciation is not a precise division: overlap between closely related species can occur in areas called hybrid
zones. Organisms reproduce with other similar organisms. The fitness of these hybrid offspring can affect the
two species' evolutionary path. Scientists propose two models for the rate of speciation: one model illustrates
how a species can change slowly over time. The other model demonstrates how change can occur quickly from
a parent generation to a new species. Both models continue to follow natural selection patterns.
VISUAL CONNECTION QUESTIONS
1. Figure 18.14 Which is most likely to survive,
offspring with 2n+l chromosomes or offspring with
2n-l chromosomes?
2. Figure 18.22 If two species eat a different diet but
one of the food sources is eliminated and both
species are forced to eat the same foods, what
change in the hybrid zone is most likely to occur?
3. Figure 18.23 Which of the following statements is
false?
a. Punctuated equilibrium is most likely to
occur in a small population that experiences
a rapid change in its environment.
b. Punctuated equilibrium is most likely to
occur in a large population that lives in a
stable climate.
c. Gradual speciation is most likely to occur in
species that live in a stable climate.
d. Gradual speciation and punctuated
equilibrium both result in the evolution of
new species.
REVIEW QUESTIONS
4. Which scientific concept did Charles Darwin and
Alfred Wallace independently discover?
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Chapter 18 | Evolution and the Origin of Species
a. mutation
b. natural selection
c. overbreeding
d. sexual reproduction
5. Which of the following situations will lead to natural
selection?
a. The seeds of two plants land near each
other and one grows larger than the other.
b. Two types of fish eat the same kind of food,
and one is better able to gather food than
the other.
c. Male lions compete for the right to mate with
females, with only one possible winner.
d. all of the above
6. Which description is an example of a phenotype?
a. A certain duck has a blue beak.
b. A mutation occurred to a flower.
c. Most cheetahs live solitary lives.
d. both a and c
7. Which situation is most likely an example of
convergent evolution?
a. Squid and humans have eyes similar in
structure.
b. Worms and snakes both move without legs.
c. Some bats and birds have wings that allow
them to fly.
d. all of the above
8. Which situation would most likely lead to allopatric
speciation?
a. Flood causes the formation of a new lake.
b. A storm causes several large trees to fall
down.
c. A mutation causes a new trait to develop.
d. An injury causes an organism to seek out a
new food source.
9. What is the main difference between dispersal and
vicariance?
a. One leads to allopatric speciation, whereas
the other leads to sympatric speciation.
b. One involves the movement of the
organism, and the other involves a change
in the environment.
c. One depends on a genetic mutation
occurring, and the other does not.
d. One involves closely related organisms, and
the other involves only individuals of the
same species.
10. Which variable increases the likelihood of
allopatric speciation taking place more quickly?
a. lower rate of mutation
b. longer distance between divided groups
c. increased instances of hybrid formation
d. equivalent numbers of individuals in each
population
11. What is the main difference between
autopolyploid and allopolyploid?
a. the number of chromosomes
b. the functionality of the chromosomes
c. the source of the extra chromosomes
d. the number of mutations in the extra
chromosomes
12. Which reproductive combination produces
hybrids?
a. when individuals of the same species in
different geographical areas reproduce
b. when any two individuals sharing the same
habitat reproduce
c. when members of closely related species
reproduce
d. when offspring of the same parents
reproduce
13. Which condition is the basis for a species to be
reproductively isolated from other members?
a. It does not share its habitat with related
species.
b. It does not exist out of a single habitat.
c. It does not exchange genetic information
with other species.
d. It does not undergo evolutionary changes
for a significant period of time.
14. Which situation is not an example of a prezygotic
barrier?
a. Two species of turtles breed at different
times of the year.
b. Two species of flowers attract different
pollinators.
c. Two species of birds display different mating
dances.
d. Two species of insects produce infertile
offspring.
15. Which term is used to describe the continued
divergence of species based on the low fitness of
hybrid offspring?
a. reinforcement
b. fusion
c. stability
d. punctuated equilibrium
16. Which components of speciation would be least
likely to be a part of punctuated equilibrium?
a. a division of populations
b. a change in environmental conditions
c. ongoing gene flow among all individuals
d. a large number of mutations taking place at
once
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CRITICAL THINKING QUESTIONS
17. If a person scatters a handful of garden pea plant
seeds in one area, how would natural selection work
in this situation?
18. Why do scientists consider vestigial structures
evidence for evolution?
19. How does the scientific meaning of “theory” differ
from the common vernacular meaning?
20. Explain why the statement that a monkey is more
evolved than a mouse is incorrect.
21. Why do island chains provide ideal conditions for
adaptive radiation to occur?
22. Two species of fish had recently undergone
sympatric speciation. The males of each species had
a different coloring through which the females could
identify and choose a partner from her own species.
After some time, pollution made the lake so cloudy
that it was hard for females to distinguish colors.
What might take place in this situation?
23. Why can polyploidy individuals lead to speciation
fairly quickly?
24. What do both rate of speciation models have in
common?
25. Describe a situation where hybrid reproduction
would cause two species to fuse into one.
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Chapter 19 | The Evolution of Populations
517
19 | THE EVOLUTION OF
POPULATIONS
Figure 19.1 Living things may be single-celled or complex, multicellular organisms. They may be plants, animals, fungi,
bacteria, or archaea. This diversity results from evolution, (credit "wolf": modification of work by Gary Kramer; credit
"coral": modification of work by William Harrigan, NOAA; credit "river": modification of work by Vojtech Dostal; credit
"fish" modification of work by Christian Mehlfuhrer; credit "mushroom": modification of work by Cory Zanker; credit
"tree": modification of work by Joseph Kranak; credit "bee": modification of work by Cory Zanker)
Chapter Outline
19.1: Population Evolution
19.2: Population Genetics
19.3: Adaptive Evolution
Introduction
All life on Earth is related. Evolutionary theory states that humans, beetles, plants, and bacteria all share
a common ancestor, but that millions of years of evolution have shaped each of these organisms into the
forms we see today. Scientists consider evolution a key concept to understanding life. It is one of the most
dominant evolutionary forces. Natural selection acts to promote traits and behaviors that increase an organism’s
chances of survival and reproduction, while eliminating those traits and behaviors that are detrimental to the
organism. However, natural selection can only, as its name implies, select—it cannot create. We can attribute
novel traits and behaviors to another evolutionary force—mutation. Mutation and other sources of variation
among individuals, as well as the evolutionary forces that act upon them, alter populations and species. This
combination of processes has led to the world of life we see today.
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19.1 1 Population Evolution
By the end of this section, you will be able to do the following:
• Define population genetics and describe how scientists use population genetics in studying population
evolution
• Define the Hardy-Weinberg principle and discuss its importance
People did not understand the mechanisms of inheritance, or genetics, at the time Charles Darwin and Alfred
Russel Wallace were developing their idea of natural selection. This lack of knowledge was a stumbling block
to understanding many aspects of evolution. The predominant (and incorrect) genetic theory of the time,
blending inheritance, made it difficult to understand how natural selection might operate. Darwin and Wallace
were unaware of the Austrian monk Gregor Mendel's 1866 publication "Experiments in Plant Hybridization",
which came out not long after Darwin's book, On the Origin of Species. Scholars rediscovered Mendel’s
work in the early twentieth century at which time geneticists were rapidly coming to an understanding of the
basics of inheritance. Initially, the newly discovered particulate nature of genes made it difficult for biologists
to understand how gradual evolution could occur. However, over the next few decades scientists integrated
genetics and evolution in what became known as the modern synthesis —the coherent understanding of the
relationship between natural selection and genetics that took shape by the 1940s. Generally, this concept is
generally accepted today. In short, the modern synthesis describes how evolutionary processes, such as natural
selection, can affect a population’s genetic makeup, and, in turn, how this can result in the gradual evolution
of populations and species. The theory also connects population change over time (microevolution), with the
processes that gave rise to new species and higher taxonomic groups with widely divergent characters, called
(macroevolution).
everyday CONNECTION
Evolution and Flu Vaccines
Every fall, the media starts reporting on flu vaccinations and potential outbreaks. Scientists, health experts,
and institutions determine recommendations for different parts of the population, predict optimal production
and inoculation schedules, create vaccines, and set up clinics to provide inoculations. You may think of the
annual flu shot as media hype, an important health protection, or just a briefly uncomfortable prick in your
arm. However, do you think of it in terms of evolution?
The media hype of annual flu shots is scientifically grounded in our understanding of evolution. Each year,
scientists across the globe strive to predict the flu strains that they anticipate as most widespread and
harmful in the coming year. They base this knowledge on how flu strains have evolved over time and over
the past few flu seasons. Scientists then work to create the most effective vaccine to combat those selected
strains. Pharmaceutical companies produce hundreds of millions of doses in a short period in order to
provide vaccinations to key populations at the optimal time.
Because viruses, like the flu, evolve very quickly (especially in evolutionary time), this poses quite a
challenge. Viruses mutate and replicate at a fast rate, so the vaccine developed to protect against last year’s
flu strain may not provide the protection one needs against the coming year’s strain. Evolution of these
viruses means continued adaptions to ensure survival, including adaptations to survive previous vaccines.
Population Genetics
Recall that a gene for a particular character may have several alleles, or variants, that code for different traits
associated with that character. For example, in the ABO blood type system in humans, three alleles determine
the particular blood-type protein on the surface of red blood cells. Each individual in a population of diploid
organisms can only carry two alleles for a particular gene, but more than two may be present in the individuals
that comprise the population. Mendel followed alleles as they were inherited from parent to offspring. In the early
twentieth century, biologists in the area of population genetics began to study how selective forces change a
population through changes in allele and genotypic frequencies.
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519
The allele frequency (or gene frequency) is the rate at which a specific allele appears within a population. Until
now we have discussed evolution as a change in the characteristics of a population of organisms, but behind that
phenotypic change is genetic change. In population genetics, scientists define the term evolution as a change in
the allele's frequency in a population. Using the ABO blood type system as an example, the frequency of one of
the alleles, / A , is the number of copies of that allele divided by all the copies of the ABO gene in the population.
[1] A R 0
For example, a study in Jordan found a frequency of / to be 26.1 percent. The / and / alleles comprise
13.4 percent and 60.5 percent of the alleles respectively, and all of the frequencies added up to 100 percent. A
change in this frequency over time would constitute evolution in the population.
The allele frequency within a given population can change depending on environmental factors; therefore,
certain alleles become more widespread than others during the natural selection process. Natural selection
can alter the population’s genetic makeup. An example is if a given allele confers a phenotype that allows an
individual to better survive or have more offspring. Because many of those offspring will also carry the beneficial
allele, and often the corresponding phenotype, they will have more offspring of their own that also carry the allele,
thus, perpetuating the cycle. Overtime, the allele will spread throughout the population. Some alleles will quickly
become fixed in this way, meaning that every individual of the population will carry the allele, while detrimental
mutations may be swiftly eliminated if derived from a dominant allele from the gene pool. The gene pool is the
sum of all the alleles in a population.
Sometimes, allele frequencies within a population change randomly with no advantage to the population over
existing allele frequencies. We call this phenomenon genetic drift. Natural selection and genetic drift usually
occur simultaneously in populations and are not isolated events. It is hard to determine which process dominates
because it is often nearly impossible to determine the cause of change in allele frequencies at each occurrence.
We call an event that initiates an allele frequency change in an isolated part of the population, which is not typical
of the original population, the founder effect. Natural selection, random drift, and founder effects can lead to
significant changes in a population's genome.
Hardy-Weinberg Principle of Equilibrium
In the early twentieth century, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg
stated the principle of equilibrium to describe the population's genetic makeup. The theory, which later became
known as the Hardy-Weinberg principle of equilibrium, states that a population’s allele and genotype frequencies
are inherently stable— unless some kind of evolutionary force is acting upon the population, neither the
allele nor the genotypic frequencies would change. The Hardy-Weinberg principle assumes conditions with no
mutations, migration, emigration, or selective pressure for or against genotype, plus an infinite population. While
no population can satisfy those conditions, the principle offers a useful model against which to compare real
population changes.
Working under this theory, population geneticists represent different alleles as different variables in their
mathematical models. The variable p, for example, often represents the frequency of a particular allele, say Y for
the trait of yellow in Mendel’s peas, while the variable q represents the frequency of y alleles that confer the color
green. If these are the only two possible alleles for a given locus in the population, p + q = 1. In other words, all
the p alleles and all the q alleles comprise all of the alleles for that locus in the population.
However, what ultimately interests most biologists is not the frequencies of different alleles, but the frequencies
of the resulting genotypes, known as the population’s genetic structure, from which scientists can surmise
phenotype distribution. If we observe the phenotype, we can know only the homozygous recessive allele's
genotype. The calculations provide an estimate of the remaining genotypes. Since each individual carries two
alleles per gene, if we know the allele frequencies (p and q), predicting the genotypes' frequencies is a simple
mathematical calculation to determine the probability of obtaining these genotypes if we draw two alleles at
random from the gene pool. In the above scenario, an individual pea plant could be pp (YY), and thus produce
yellow peas; pq (Yy), also yellow; or qq (yy), and thus produce green peas (Figure 19.2). In other words,
the frequency of pp individuals is simply p 1 2 ; the frequency of pq individuals is 2pq; and the frequency of qq
individuals is q 2 . Again, if p and q are the only two possible alleles for a given trait in the population, these
genotypes frequencies will sum to one: p 2 + 2pq + q 2 = 1.
1. Sahar S. Hanania, Dhia S. Hassawi, and Nidal M. Irshaid, “Allele Frequency and Molecular Genotypes of ABO Blood Group System in a
Jordanian Population," Journal of Medical Sciences 7 (2007): 51-58, doi:10.3923/jms.2007.51.58.
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Figure 19.2 When populations are in the Hardy-Weinberg equilibrium, the allelic frequency is stable from
generation to generation and we can determine the allele distribution from the Hardy-Weinberg equation. If the
allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what
evolutionary forces are at play.
In plants, violet flower color (V) is dominant over white (v). If p = 0.8 and q = 0.2 in a population of 500
plants, how many individuals would you expect to be homozygous dominant (VV), heterozygous (Vv), and
homozygous recessive (vv)? How many plants would you expect to have violet flowers, and how many
would have white flowers?
In theory, if a population is at equilibrium—that is, there are no evolutionary forces acting upon it—generation
after generation would have the same gene pool and genetic structure, and these equations would all hold
true all of the time. Of course, even Hardy and Weinberg recognized that no natural population is immune
to evolution. Populations in nature are constantly changing in genetic makeup due to drift, mutation, possibly
migration, and selection. As a result, the only way to determine the exact distribution of phenotypes in a
population is to go out and count them. However, the Hardy-Weinberg principle gives scientists a mathematical
baseline of a non-evolving population to which they can compare evolving populations and thereby infer what
evolutionary forces might be at play. If the frequencies of alleles or genotypes deviate from the value expected
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from the Hardy-Weinberg equation, then the population is evolving.
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Use this online calculator (http:// 0 penstaxc 0 llege. 0 rg/l/hardy-weinberg) to determine a population's
genetic structure.
19.2 | Population Genetics
By the end of this section, you will be able to do the following:
• Describe the different types of variation in a population
• Explain why only natural selection can act upon heritable variation
• Describe genetic drift and the bottleneck effect
• Explain how each evolutionary force can influence a population's allele frequencies
A population's individuals often display different phenotypes, or express different alleles of a particular gene,
which scientists refer to as polymorphisms. We call populations with two or more variations of particular
characteristics polymorphic. A number of factors, including the population’s genetic structure and the
environment (Figure 19.3) influence population variation, the distribution of phenotypes among individuals.
Understanding phenotypic variation sources in a population is important for determining how a population will
evolve in response to different evolutionary pressures.
Figure 19.3 The distribution of phenotypes in this litter of kittens illustrates population variation, (credit: Pieter Lanser)
Genetic Variance
Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s
genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors
may be selected, while deleterious alleles may not. Acquired traits, for the most part, are not heritable. For
example, if an athlete works out in the gym every day, building up muscle strength, the athlete’s offspring will not
necessarily grow up to be a body builder. If there is a genetic basis for the ability to run fast, on the other hand,
a parent may pass this to a child.
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Before Darwinian evolution became the prevailing theory of the field, French naturalist Jean-Baptiste Lamarck
theorized that organisms could inherit acquired traits. While the majority of scientists have not supported
this hypothesis, some have recently begun to realize that Lamarck was not completely wrong. Visit this site
(http:// 0 penstaxc 0 llege. 0 rg/l/epigenetic) to learn more.
Heritability is the fraction of phenotype variation that we can attribute to genetic differences, or genetic variance,
among individuals in a population. The greater the heritability of a population’s phenotypic variation, the more
susceptible it is to the evolutionary forces that act on heritable variation.
We call the diversity of alleles and genotypes within a population genetic variance. When scientists are involved
in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a
population’s genetic variance to preserve as much of the phenotypic diversity as possible. This also helps reduce
associated risks of inbreeding, the mating of closely related individuals, which can have the undesirable effect
of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease.
For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only
manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy
population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent
of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will
not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population,
and as a result, the allele maintains itself at low levels in the gene pool. However, if a family of carriers begins
to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually
producing diseased offspring, a phenomenon that scientists call inbreeding depression.
Changes in allele frequencies that we identify in a population can shed light on how it is evolving. In addition
to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation,
nonrandom mating, and environmental variances.
Genetic Drift
The theory of natural selection stems from the observation that some individuals in a population are more likely
to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next
generation. A big, powerful male gorilla, for example, is much more likely than a smaller, weaker one to become
the population’s silverback, the pack’s leader who mates far more than the other males of the group. The pack
leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like
their father. Over time, the genes for bigger size will increase in frequency in the population, and the population
will, as a result, grow larger on average. That is, this would occur if this particular selection pressure, or driving
selective force, were the only one acting on the population. In other examples, better camouflage or a stronger
resistance to drought might pose a selection pressure.
Another way a population’s allele and genotype frequencies can change is genetic drift (Figure 19.4), which
is simply the effect of chance. By chance, some individuals will have more offspring than others—not due to an
advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right
place at the right time (when the receptive female walked by) or because the other one happened to be in the
wrong place at the wrong time (when a fox was hunting).
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Figure 19.4 Genetic drift in a population can lead to eliminating an allele from a population by chance. In this
example, rabbits with the brown coat color allele (6) are dominant over rabbits with the white coat color allele ( b ).
In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values
of .5. Only half of the individuals reproduce, resulting in a second generation with p and q values of .7 and
.3, respectively. Only two individuals in the second generation reproduce, and by chance these individuals are
homozygous dominant for brown coat color. As a result, in the third generation the recessive b allele is lost.
Do you think genetic drift would happen more quickly on an island or on the mainland?
Small populations are more susceptible to the forces of genetic drift. Large populations, alternatively, are
buffered against the effects of chance. If one individual of a population of 10 individuals happens to die at a
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young age before it leaves any offspring to the next generation, all of its genes—1/10 of the population’s gene
pool—will be suddenly lost. In a population of 100, that’s only 1 percent of the overall gene pool; therefore, it is
much less impactful on the population’s genetic structure.
Go to this site (http:// 0 penstaxc 0 llege. 0 rg/l/genetic_drift) to watch an animation of random sampling and
genetic drift in action.
Natural events, such as an earthquake disaster that kills—at random—a large portion of the population, can
magnify genetic drift. Known as the bottleneck effect, it results in suddenly wiping out a large portion of
the genome (Figure 19.5). At once, the survivors' genetic structure becomes the entire population's genetic
structure, which may be very different from the pre-disaster population.
\
Figure 19.5 A chance event or catastrophe can reduce the genetic variability within a population.
Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the
population leaves to start a new population in a new location or if a physical barrier divides a population. In this
situation, those individuals are an unlikely representation of the entire population, which results in the founder
effect. The founder effect occurs when the genetic structure changes to match that of the new population’s
founding fathers and mothers. Researchers believe that the founder effect was a key factor in the genetic history
of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in
Afrikaners but rare in most other populations. This is probably because a higher-than-normal proportion of the
founding colonists carried these mutations. As a result, the population expresses unusually high incidences of
Huntington’s disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause blood marrow and
congenital abnormalities—even cancer.
2. A. J. Tipping et al., “Molecular and Genealogical Evidence for a Founder Effect in Fanconi Anemia Families of the Afrikaner Population of
South Africa,” PNAS 98, no. 10 (2001): 5734-5739, doi: 10.1073/pnas.091402398.
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Watch this short video to learn more about the founder and bottleneck effects. (This multimedia
resource will open in a browser.) (http://cnx.Org/content/m66524/l.3/#eip-idll64438765350)
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Testing the Bottleneck Effect
Question: How do natural disasters affect a population's genetic structure?
Background: When an earthquake or hurricane suddenly wipes out much of a population, the surviving
individuals are usually a random sampling of the original group. As a result, the population's genetic makeup
can change dramatically. We call this phenomenon the bottleneck effect.
Hypothesis: Repeated natural disasters will yield different population genetic structures; therefore, each
time one runs this experiment the results will vary.
Test the hypothesis: Count out the original population using different colored beads. For example, red,
blue, and yellow beads might represent red, blue, and yellow individuals. After recording the number of
each individual in the original population, place them all in a bottle with a narrow neck that will only allow a
few beads out at a time. Then, pour 1/3 of the bottle’s contents into a bowl. This represents the surviving
individuals after a natural disaster kills a majority of the population. Count the number of the different colored
beads in the bowl, and record it. Then, place all of the beads back in the bottle and repeat the experiment
four more times.
Analyze the data: Compare the five populations that resulted from the experiment. Do the populations all
contain the same number of different colored beads, or do they vary? Remember, these populations all
came from the same exact parent population.
Form a conclusion: Most likely, the five resulting populations will differ quite dramatically. This is because
natural disasters are not selective—they kill and spare individuals at random. Now think about how this
might affect a real population. What happens when a hurricane hits the Mississippi Gulf Coast? How do the
seabirds that live on the beach fare?
Gene Flow
Another important evolutionary force is gene flow: the flow of alleles in and out of a population due to the
migration of individuals or gametes (Figure 19.6). While some populations are fairly stable, others experience
more flux. Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate other
populations of the same species some distance away. Even a population that may initially appear to be stable,
such as a pride of lions, can experience its fair share of immigration and emigration as developing males leave
their mothers to seek out a new pride with genetically unrelated females. This variable flow of individuals in and
out of the group not only changes the population's gene structure, but it can also introduce new genetic variation
to populations in different geological locations and habitats.
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Figure 19.6 Gene flow can occur when an individual travels from one geographic location to another.
Mutation
Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species
evolve because of mutations accumulating over time. The appearance of new mutations is the most common
way to introduce novel genotypic and phenotypic variance. Some mutations are unfavorable or harmful and are
quickly eliminated from the population by natural selection. Others are beneficial and will spread through the
population. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism
survive to sexual maturity and reproduce. Some mutations do not do anything and can linger, unaffected by
natural selection, in the genome. Some can have a dramatic effect on a gene and the resulting phenotype.
Nonrandom Mating
If individuals nonrandomly mate with their peers, the result can be a changing population. There are many
reasons nonrandom mating occurs. One reason is simple mate choice. For example, female peahens may
prefer peacocks with bigger, brighter tails. Natural selection picks traits that lead to more mating selections for an
individual. One common form of mate choice, called assortative mating, is an individual’s preference to mate
with partners who are phenotypically similar to themselves.
Another cause of nonrandom mating is physical location. This is especially true in large populations spread over
vast geographic distances where not all individuals will have equal access to one another. Some might be miles
apart through woods or over rough terrain, while others might live immediately nearby.
Environmental Variance
Genes are not the only players involved in determining population variation. Other factors, such as the
environment (Figure 19.7) also influence phenotypes. A beachgoer is likely to have darker skin than a city
dweller, for example, due to regular exposure to the sun, an environmental factor. For some species, the
environment determines some major characteristics, such as gender. For example, some turtles and other
reptiles have temperature-dependent sex determination (TSD). TSD means that individuals develop into males
if their eggs are incubated within a certain temperature range, or females at a different temperature range.
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Figure 19.7 The temperature at which the eggs are incubated determine the American alligator's (Alligator
mississippiensis) sex. Eggs incubated at 30°C produce females, and eggs incubated at 33°C produce males, (credit:
Steve Hillebrand, USFWS)
Geographic separation between populations can lead to differences in the phenotypic variation between those
populations. We see such geographical variation between most populations and it can be significant. We
can observe one type of geographic variation, a cline, as given species' populations vary gradually across an
ecological gradient. Species of warm-blooded animals, for example, tend to have larger bodies in the cooler
climates closer to the earth’s poles, allowing them to better conserve heat. This is a latitudinal cline. Alternatively,
flowering plants tend to bloom at different times depending on where they are along a mountain slope. This is an
altitudinal cline.
If there is gene flow between the populations, the individuals will likely show gradual differences in phenotype
along the cline. Restricted gene flow, alternatively can lead to abrupt differences, even speciation.
19.3 | Adaptive Evolution
By the end of this section, you will be able to do the following:
• Explain the different ways natural selection can shape populations
• Describe how these different forces can lead to different outcomes in terms of the population variation
Natural selection only acts on the population’s heritable traits: selecting for beneficial alleles and thus increasing
their frequency in the population, while selecting against deleterious alleles and thereby decreasing their
frequency. Scientists call this process adaptive evolution. Natural selection does not act on individual alleles,
but on entire organisms. An individual may carry a very beneficial genotype with a resulting phenotype that,
for example, increases the ability to reproduce (fecundity), but if that same individual also carries an allele that
results in a fatal childhood disease, that fecundity phenotype will not pass to the next generation because the
individual will not live to reach reproductive age. Natural selection acts at the individual's level. It selects for
individuals with greater contributions to the gene pool of the next generation. Scientists call this an organism’s
evolutionary (Darwinian) fitness.
Fitness is often quantifiable and is measured by scientists in the field. However, it is not an individual's absolute
fitness that counts, but rather how it compares to the other organisms in the population. Scientists call this
concept relative fitness, which allows researchers to determine which individuals are contributing additional
offspring to the next generation, and thus, how the population might evolve.
There are several ways selection can affect population variation: stabilizing selection, directional selection,
diversifying selection, frequency-dependent selection, and sexual selection. As natural selection influences
the allele frequencies in a population, individuals can either become more or less genetically similar and the
phenotypes can become more similar or more disparate.
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Stabilizing Selection
If natural selection favors an average phenotype, selecting against extreme variation, the population will undergo
stabilizing selection (Figure 19.8). In a mouse population that live in the woods, for example, natural selection
is likely to favor mice that best blend in with the forest floor and are less likely for predators to spot. Assuming
the ground is a fairly consistent shade of brown, those mice whose fur is most closely matched to that color
will be most likely to survive and reproduce, passing on their genes for their brown coat. Mice that carry alleles
that make them a bit lighter or a bit darker will stand out against the ground and be more likely to fall victim to
predation. As a result of this selection, the population’s genetic variance will decrease.
Directional Selection
When the environment changes, populations will often undergo directional selection (Figure 19.8), which
selects for phenotypes at one end of the spectrum of existing variation. A classic example of this type of selection
is the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial
Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored
trees and lichens in their environment. However, as soot began spewing from factories, the trees darkened, and
the light-colored moths became easier for predatory birds to spot. Over time, the frequency of the moth's melanic
form increased because they had a higher survival rate in habitats affected by air pollution because their darker
coloration blended with the sooty trees. Similarly, the hypothetical mouse population may evolve to take on a
different coloration if something were to cause the forest floor where they live to change color. The result of this
type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.
In science, we sometimes believe some things are true, and then new information becomes available
that changes our understanding. The peppered moth story is an example: some scientists recently have
questioned the facts behind the selection toward darker moths. Read this article (http://openstaxcollege.org/
l/peppered_moths) to learn more.
Diversifying Selection
Sometimes two or more distinct phenotypes can each have their advantages for natural selection, while the
intermediate phenotypes are, on average, less fit. Scientists call this diversifying selection (Figure 19.8)
We see this in many animal populations that have multiple male forms. Large, dominant alpha males use
brute force to obtain mates, while small males can sneak in for furtive copulations with the females in an
alpha male’s territory. In this case, both the alpha males and the “sneaking" males will be selected for, but
medium-sized males, who can’t overtake the alpha males and are too big to sneak copulations, are selected
against. Diversifying selection can also occur when environmental changes favor individuals on either end of
the phenotypic spectrum. Imagine a mouse population living at the beach where there is light-colored sand
interspersed with patches of tall grass. In this scenario, light-colored mice that blend in with the sand would be
favored, as well as dark-colored mice that can hide in the grass. Medium-colored mice, alternatively would not
blend in with either the grass or the sand, and thus predators would most likely eat them. The result of this type
of selection is increased genetic variance as the population becomes more diverse.
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(a) Stabilizing selection
Robins typically lay four eggs,
an example of stabilizing
selection. Larger clutches may
result in malnourished chicks,
while smaller clutches may
result in no viable offspring.
(b) Directional selection
Light-colored peppered moths
are better camouflaged against a
pristine environment; likewise,
dark-colored peppered moths
are better camouflaged against a
sooty environment. Thus, as the
Industrial Revolution progressed
in nineteenth-century England,
the color of the moth population
shifted from light to dark, an
example of directional selection.
(c) Diversifying selection
In a hyphothetical population,
gray and Himalayan (gray and
white) rabbits are better able to
blend with a rocky environment
than white rabbits, resulting in
diversifying selection.
Figure 19.8 Different types of natural selection can impact the distribution of phenotypes within a population. In
(a) stabilizing selection, an average phenotype is favored. In (b) directional selection, a change in the environment
shifts the spectrum of observed phenotypes. In (c) diversifying selection, two or more extreme phenotypes are
selected for, while the average phenotype is selected against.
In recent years, factories have become cleaner, and release less soot into the environment. What impact do
you think this has had on the distribution of moth color in the population?
Frequency-Dependent Selection
Another type of selection, frequency-dependent selection, favors phenotypes that are either common (positive
frequency-dependent selection) or rare (negative frequency-dependent selection). We can observe an
interesting example of this type of selection in a unique group of Pacific Northwest lizards. Male common side-
blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different
reproductive strategy: orange males are the strongest and can fight other males for access to their females.
Blue males are medium-sized and form strong pair bonds with their mates. Yellow males (Figure 19.9) are
the smallest, and look a bit like females, which allows them to sneak copulations. Like a game of rock-paper-
scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. That is,
the big, strong orange males can fight off the blue males to mate with the blue’s pair-bonded females, the blue
males are successful at guarding their mates against yellow sneaker males, and the yellow males can sneak
copulations from the potential mates of the large, polygynous orange males.
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Figure 19.9 A yellow-throated side-blotched lizard is smaller than either the blue-throated or orange-throated males
and appears a bit like the females of the species, allowing it to sneak copulations, (credit: “tinyfroglet'VFlickr)
In this scenario, natural selection favors orange males when blue males dominate the population. Blue males
will thrive when the population is mostly yellow males, and yellow males will be selected for when orange
males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these
phenotypes—in one generation, orange might predominate, and then yellow males will begin to rise in frequency.
Once yellow males comprise a majority of the population, blue males will be selected. Finally, when blue males
become common, orange males once again will be favored.
Negative frequency-dependent selection serves to increase the population’s genetic variance by selecting
for rare phenotypes; whereas, positive frequency-dependent selection usually decreases genetic variance by
selecting for common phenotypes.
Sexual Selection
Males and females of certain species are often quite different from one another in ways beyond the reproductive
organs. Males are often larger, for example, and display many elaborate colors and adornments, like the
peacock’s tail, while females tend to be smaller and duller in decoration. We call such differences sexual
dimorphisms (Figure 19.10), which arise in many populations, particularly animal populations, where there
is more variance in the male's reproductive success than that of the females. That is, some males—often the
bigger, stronger, or more decorated males—obtain the vast majority of the total matings, while others receive
none. This can occur because the males are better at fighting off other males, or because females will choose to
mate with the bigger or more decorated males. In either case, this variation in reproductive success generates a
strong selection pressure among males to obtain those matings, resulting in the evolution of bigger body size and
elaborate ornaments to attract the females’ attention. Females, however, tend to achieve a handful of selected
matings; therefore, they are more likely to select more desirable males.
Sexual dimorphism varies widely among species, and some species are even sex-role reversed. In such cases,
females tend to have a greater variance in their reproductive success than males and are correspondingly
selected for the bigger body size and elaborate traits usually characteristic of males.
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(a) (b) (c)
Figure 19.10 Sexual dimorphism in (a) peacocks and peahens, (b) Argiope appensa spiders (the female spider is the
large one), and in (c) wood ducks, (credit “spiders": modification of work by “Sanba38"/Wikimedia Commons; credit
“duck": modification of work by Kevin Cole)
We call the selection pressures on males and females to obtain matings sexual selection. It can result in
developing secondary sexual characteristics that do not benefit the individual’s likelihood of survival but help
to maximize its reproductive success. Sexual selection can be so strong that it selects traits that are actually
detrimental to the individual’s survival. Think, once again, about the peacock’s tail. While it is beautiful and the
male with the largest, most colorful tail is more likely to win the female, it is not the most practical appendage. In
addition to greater visibility to predators, it makes the males slower in their attempted escapes. There is some
evidence that this risk is why females like the big tails in the first place. The speculation is that large tails carry
risk, and only the best males survive that risk: the bigger the tail, the more fit the male. We call this the handicap
principle.
The good genes hypothesis states that males develop these impressive ornaments to show off their efficient
metabolism or their ability to fight disease. Females then choose males with the most impressive traits because
it signals their genetic superiority, which they will then pass on to their offspring. Although one may argue that
females should not be picky because it will likely reduce their number of offspring, if better males father more fit
offspring, it may be beneficial. Fewer, healthier offspring may increase the chances of survival more than many,
weaker offspring.
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In 1915, biologist Ronald Fisher proposed another model of sexual selection: the Fisherian runaway model
(http:// 0 penstaxc 0 llege. 0 rg/l/sexual_select) , which suggests that selection of certain traits is a result of
sexual preference.
In both the handicap principle and the good genes hypothesis, the trait is an honest signal of the males’ quality,
thus giving females a way to find the fittest mates— males that will pass the best genes to their offspring.
No Perfect Organism
Natural selection is a driving force in evolution and can generate populations that are better adapted to
survive and successfully reproduce in their environments. However, natural selection cannot produce the perfect
organism. Natural selection can only select on existing variation in the population. It does not create anything
from scratch. Thus, it is limited by a population’s existing genetic variance and whatever new alleles arise
through mutation and gene flow.
Natural selection is also limited because it works at the individual, not allele level, and some alleles are
linked due to their physical proximity in the genome, making them more likely to pass on together (linkage
disequilibrium). Any given individual may carry some beneficial and some unfavorable alleles. It is the alleles'
net effect, or the organism’s fitness, upon which natural selection can act. As a result, good alleles can be lost
if individuals who carry them also have several overwhelmingly bad alleles. Likewise, bad alleles can be kept if
individuals who have enough good alleles to result in an overall fitness benefit carry them.
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Furthermore, natural selection can be constrained by the relationships between different polymorphisms. One
morph may confer a higher fitness than another, but may not increase in frequency because going from the less
beneficial to the more beneficial trait would require going through a less beneficial phenotype. Think back to the
mice that live at the beach. Some are light-colored and blend in with the sand, while others are dark and blend in
with the patches of grass. The dark-colored mice may be, overall, more fit than the light-colored mice, and at first
glance, one might expect the light-colored mice to be selected for a darker coloration. However, remember that
the intermediate phenotype, a medium-colored coat, is very bad for the mice—they cannot blend in with either
the sand or the grass and predators are more likely to eat them. As a result, the light-colored mice would not be
selected for a dark coloration because those individuals who began moving in that direction (began selection for
a darker coat) would be less fit than those that stayed light.
Finally, it is important to understand that not all evolution is adaptive. While natural selection selects the fittest
individuals and often results in a more fit population overall, other forces of evolution, including genetic drift and
gene flow, often do the opposite: introducing deleterious alleles to the population’s gene pool. Evolution has no
purpose—it is not changing a population into a preconceived ideal, it is simply the sum of the various forces that
we have described in this chapter and how they influence the population's genetic and phenotypic variance.
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KEY TERMS
adaptive evolution increase in frequency of beneficial alleles and decrease in deleterious alleles due to
selection
allele frequency (also, gene frequency) rate at which a specific allele appears within a population
assortative mating when individuals tend to mate with those who are phenotypically similar to themselves
bottleneck effect magnification of genetic drift as a result of natural events or catastrophes
cline gradual geographic variation across an ecological gradient
directional selection selection that favors phenotypes at one end of the spectrum of existing variation
diversifying selection selection that favors two or more distinct phenotypes
evolutionary fitness (also, Darwinian fitness) individual’s ability to survive and reproduce
founder effect event that initiates an allele frequency change in part of the population, which is not typical of the
original population
frequency-dependent selection selection that favors phenotypes that are either common (positive frequency-
dependent selection) or rare (negative frequency-dependent selection)
gene flow flow of alleles in and out of a population due to the individual or gamete migration
gene pool all the alleles that the individuals in the population carry
genetic drift effect of chance on a population’s gene pool
genetic structure distribution of the different possible genotypes in a population
genetic variance diversity of alleles and genotypes in a population
geographical variation differences in the phenotypic variation between populations that are separated
geographically
good genes hypothesis theory of sexual selection that argues individuals develop impressive ornaments to
show off their efficient metabolism or ability to fight disease
handicap principle theory of sexual selection that argues only the fittest individuals can afford costly traits
heritability fraction of population variation that can be attributed to its genetic variance
honest signal trait that gives a truthful impression of an individual’s fitness
inbreeding mating of closely related individuals
inbreeding depression increase in abnormalities and disease in inbreeding populations
macroevolution broader scale evolutionary changes that scientists see over paleontological time
microevolution changes in a population’s genetic structure
modern synthesis overarching evolutionary paradigm that took shape by the 1940s and scientists generally
accept today
nonrandom mating changes in a population’s gene pool due to mate choice or other forces that cause
individuals to mate with certain phenotypes more than others
population genetics study of how selective forces change the allele frequencies in a population over time
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Chapter 19 | The Evolution of Populations
population variation distribution of phenotypes in a population
relative fitness individual’s ability to survive and reproduce relative to the rest of the population
selective pressure environmental factor that causes one phenotype to be better than another
sexual dimorphism phenotypic difference between a population's males and females
stabilizing selection selection that favors average phenotypes
CHAPTER SUMMARY
19.1 Population Evolution
The modern synthesis of evolutionary theory grew out of the cohesion of Darwin’s, Wallace’s, and Mendel’s
thoughts on evolution and heredity, along with the more modern study of population genetics. It describes the
evolution of populations and species, from small-scale changes among individuals to large-scale changes over
paleontological time periods. To understand how organisms evolve, scientists can track populations’ allele
frequencies over time. If they differ from generation to generation, scientists can conclude that the population is
not in Hardy-Weinberg equilibrium, and is thus evolving.
19.2 Population Genetics
Both genetic and environmental factors can cause phenotypic variation in a population. Different alleles can
confer different phenotypes, and different environments can also cause individuals to look or act differently.
Only those differences encoded in an individual’s genes, however, can pass to its offspring and, thus, be a
target of natural selection. Natural selection works by selecting for alleles that confer beneficial traits or
behaviors, while selecting against those for deleterious qualities. Genetic drift stems from the chance
occurrence that some individuals in the gene line have more offspring than others. When individuals leave or
join the population, allele frequencies can change as a result of gene flow. Mutations to an individual’s DNA
may introduce new variation into a population. Allele frequencies can also alter when individuals do not
randomly mate with others in the group.
19.3 Adaptive Evolution
Because natural selection acts to increase the frequency of beneficial alleles and traits while decreasing the
frequency of deleterious qualities, it is adaptive evolution. Natural selection acts at the individual level, selecting
for those that have a higher overall fitness compared to the rest of the population. If the fit phenotypes are
those that are similar, natural selection will result in stabilizing selection, and an overall decrease in the
population’s variation. Directional selection works to shift a population’s variance toward a new, fit phenotype,
as environmental conditions change. In contrast, diversifying selection results in increased genetic variance by
selecting for two or more distinct phenotypes.
Other types of selection include frequency-dependent selection, in which individuals with either common
(positive frequency-dependent selection) or rare (negative frequency-dependent selection) are selected.
Finally, sexual selection results from one sex having more variance in the reproductive success than the other.
As a result, males and females experience different selective pressures, which can often lead to the evolution
of phenotypic differences, or sexual dimorphisms, between the two.
VISUAL CONNECTION QUESTIONS
1. Figure 19.2 In plants, violet flower color (V) is
dominant over white (v). If p = .8 and q = 0.2 in a
population of 500 plants, how many individuals would
you expect to be homozygous dominant (VV),
heterozygous (Vv), and homozygous recessive (vv)?
How many plants would you expect to have violet
flowers, and how many would have white flowers?
2. Figure 19.4 Do you think genetic drift would
happen more quickly on an island or on the
mainland?
3. Figure 19.8 In recent years, factories have
become cleaner, and less soot is released into the
environment. What impact do you think this has had
on the distribution of moth color in the population?
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Chapter 19 | The Evolution of Populations
535
REVIEW QUESTIONS
4. What is the difference between micro- and
macroevolution?
a. Microevolution describes the evolution of
small organisms, such as insects, while
macroevolution describes the evolution of
large organisms, like people and elephants.
b. Microevolution describes the evolution of
microscopic entities, such as molecules and
proteins, while macroevolution describes the
evolution of whole organisms.
c. Microevolution describes the evolution of
organisms in populations, while
macroevolution describes the evolution of
species over long periods of time.
d. Microevolution describes the evolution of
organisms over their lifetimes, while
macroevolution describes the evolution of
organisms over multiple generations.
5. Population genetics is the study of:
a. how selective forces change the allele
frequencies in a population over time
b. the genetic basis of population-wide traits
c. whether traits have a genetic basis
d. the degree of inbreeding in a population
6. Which of the following populations is not in Hardy-
Weinberg equilibrium?
a. a population with 12 homozygous recessive
individuals (yy), 8 homozygous dominant
individuals (YY), and 4 heterozygous
individuals (Yy)
b. a population in which the allele frequencies
do not change over time
c. p 2 + 2pq + q 2 = 1
d. a population undergoing natural selection
7. One of the original Amish colonies rose from a
ship of colonists that came from Europe. The ship’s
captain, who had polydactyly, a rare dominant trait,
was one of the original colonists. Today, we see a
much higher frequency of polydactyly in the Amish
population. This is an example of:
a. natural selection
b. genetic drift
c. founder effect
d. b and c
8. When male lions reach sexual maturity, they leave
their group in search of a new pride. This can alter
the allele frequencies of the population through which
of the following mechanisms?
a. natural selection
b. genetic drift
c. gene flow
d. random mating
9. Which of the following evolutionary forces can
introduce new genetic variation into a population?
a. natural selection and genetic drift
b. mutation and gene flow
c. natural selection and nonrandom mating
d. mutation and genetic drift
10. What is assortative mating?
a. when individuals mate with those who are
similar to themselves
b. when individuals mate with those who are
dissimilar to themselves
c. when individuals mate with those who are
the most fit in the population
d. when individuals mate with those who are
least fit in the population
11. When closely related individuals mate with each
other, or inbreed, the offspring are often not as fit as
the offspring of two unrelated individuals. Why?
a. Close relatives are genetically incompatible.
b. The DNA of close relatives reacts negatively
in the offspring.
c. Inbreeding can bring together rare,
deleterious mutations that lead to harmful
phenotypes.
d. Inbreeding causes normally silent alleles to
be expressed.
12. What is a cline?
a. the slope of a mountain where a population
lives
b. the degree to which a mutation helps an
individual survive
c. the number of individuals in the population
d. gradual geographic variation across an
ecological gradient
13. Which type of selection results in greater genetic
variance in a population?
a. stabilizing selection
b. directional selection
c. diversifying selection
d. positive frequency-dependent selection
14. When males and females of a population look or
act differently, it is referred to as_.
a. sexual dimorphism
b. sexual selection
c. diversifying selection
d. a cline
15. The good genes hypothesis is a theory that
explains what?
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Chapter 19 | The Evolution of Populations
a. why more fit individuals are more likely to
have more offspring
b. why alleles that confer beneficial traits or
behaviors are selected for by natural
selection
c. why some deleterious mutations are
maintained in the population
d. why individuals of one sex develop
impressive ornamental traits
CRITICAL THINKING QUESTIONS
16. Solve for the genetic structure of a population
with 12 homozygous recessive individuals (yy), 8
homozygous dominant individuals (YY), and 4
heterozygous individuals (Yy).
17. Explain the Hardy-Weinberg principle of
equilibrium theory.
18. Imagine you are trying to test whether a
population of flowers is undergoing evolution. You
suspect there is selection pressure on the color of the
flower: bees seem to cluster around the red flowers
more often than the blue flowers. In a separate
experiment, you discover blue flower color is
dominant to red flower color. In a field, you count 600
blue flowers and 200 red flowers. What would you
expect the genetic structure of the flowers to be?
19. Describe a situation in which a population would
undergo the bottleneck effect and explain what
impact that would have on the population’s gene
pool.
20. Describe natural selection and give an example
of natural selection at work in a population.
21. Explain what a cline is and provide examples.
22. Give an example of a trait that may have evolved
as a result of the handicap principle and explain your
reasoning.
23. List the ways in which evolution can affect
population variation and describe how they influence
allele frequencies.
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Chapter 20 | Phylogenies and the History of Life
537
20 | PHYLOGENIES AND
THE HISTORY OF LIFE
Figure 20.1 A bee's life is very different from a flower’s, but the two organisms are related. Both are members of the
domain Eukarya and have cells containing many similar organelles, genes, and proteins, (credit: modification of work
by John Beetham)
Chapter Outline
20.1: Organizing Life on Earth
20.2: Determining Evolutionary Relationships
20.3: Perspectives on the Phylogenetic Tree
Introduction
This bee and Echinacea flower (Figure 20.1) could not look more different, yet they are related, as are all living
organisms on Earth. By following pathways of similarities and changes—both visible and genetic—scientists
seek to map the evolutionary past of how life developed from single-celled organisms to the tremendous
collection of creatures that have germinated, crawled, floated, swum, flown, and walked on this planet.
20.1 1 Organizing Life on Earth
By the end of this section, you will be able to do the following:
• Discuss the need for a comprehensive classification system
• List the different levels of the taxonomic classification system
• Describe how systematics and taxonomy relate to phylogeny
• Discuss a phylogenetic tree's components and purpose
In scientific terms, phylogeny is the evolutionary history and relationship of an organism or group of organisms.
A phylogeny describes the organisim's relationships, such as from which organisms it may have evolved, or to
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Chapter 20 | Phytogenies and the History of Life
which species it is most closely related. Phylogenetic relationships provide information on shared ancestry but
not necessarily on how organisms are similar or different.
Phylogenetic Trees
Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among
organisms. A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or
groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since
one cannot go back to confirm the proposed relationships. In other words, we can construct a “tree of life” to
illustrate when different organisms evolved and to show the relationships among different organisms (Figure
20 . 2 ).
Unlike a taxonomic classification diagram, we can read a phylogenetic tree like a map of evolutionary history.
Many phylogenetic trees have a single lineage at the base representing a common ancestor. Scientists call such
trees rooted, which means there is a single ancestral lineage (typically drawn from the bottom or left) to which
all organisms represented in the diagram relate. Notice in the rooted phylogenetic tree that the three domains—
Bacteria, Archaea, and Eukarya—diverge from a single point and branch off. The small branch that plants and
animals (including humans) occupy in this diagram shows how recent and miniscule these groups are compared
with other organisms. Unrooted trees do not show a common ancestor but do show relationships among species.
Spirochetes
Proteobacteria
Cyanobacteria \
Planctomyces
fila
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mentous ci imp
acteria Entamoebae 'r
Methanosarcina \ .
Methanobacterium \ \
Methanococcus \ Halophiles\\/<
T. celeer \ \ / X.
Thermoproteus
Pryodictium \1 y S' 'yO
Animals
V/ Fungi
Plants
^ Ciliates
^"Flagellates
Animals Fun0i
Slime molds \ J
Plants v
Algae
Protozoa^//
Gram-positives
/ Chlamydiae
// /Green nonsulfur bacteria
/ Actinobacteria
sC / Planctomycetes
/ \\_ Spirochaetes
cytophaga
T richomonads
s' s ‘ Microsporidia
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Fusobacteria
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Aquifex -
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algae)
Ns Thermophilic
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' [ . Acidobacteria
I -Bacteria -Archea - Eukarya Proteobacteria
(a) Rooted phylogenetic tree (b) Unrooted phylogenetic tree
Figure 20.2 Both of these phylogenetic trees show the relationship of the three domains of life—Bacteria, Archaea,
and Eukarya—but the (a) rooted tree attempts to identify when various species diverged from a common ancestor
while the (b) unrooted tree does not. (credit a: modification of work by Eric Gaba)
In a rooted tree, the branching indicates evolutionary relationships (Figure 20.3). The point where a split occurs,
a branch point, represents where a single lineage evolved into a distinct new one. We call a lineage that evolved
early from the root that remains unbranched a basal taxon. We call two lineages stemming from the same
branch point sister taxa. A branch with more than two lineages is a polytomy and serves to illustrate where
scientists have not definitively determined all of the relationships. Note that although sister taxa and polytomy do
share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in
two taxa may have split at a specific branch point, but neither taxon gave rise to the other.
Figure 20.3 A phylogenetic tree's root indicates that an ancestral lineage gave rise to all organisms on the tree. A
branch point indicates where two lineages diverged. A lineage that evolved early and remains unbranched is a basal
taxon. When two lineages stem from the same branch point, they are sister taxa. A branch with more than two lineages
is a polytomy.
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Chapter 20 | Phylogenies and the History of Life
539
The diagrams above can serve as a pathway to understanding evolutionary history. We can trace the pathway
from the origin of life to any individual species by navigating through the evolutionary branches between the two
points. Also, by starting with a single species and tracing back towards the "trunk" of the tree, one can discover
species' ancestors, as well as where lineages share a common ancestry. In addition, we can use the tree to
study entire groups of organisms.
Another point to mention on phylogenetic tree structure is that rotation at branch points does not change
the information. For example, if a branch point rotated and the taxon order changed, this would not alter the
information because each taxon's evolution from the branch point was independent of the other.
Many disciplines within the study of biology contribute to understanding how past and present life evolved over
time; these disciplines together contribute to building, updating, and maintaining the “tree of life." Systematics is
the field that scientists use to organize and classify organisms based on evolutionary relationships. Researchers
may use data from fossils, from studying the body part structures, or molecules that an organism uses, and
DNA analysis. By combining data from many sources, scientists can construct an organism's phylogeny Since
phylogenetic trees are hypotheses, they will continue to change as researchers discover new types of life and
learn new information.
Limitations of Phylogenetic Trees
It may be easy to assume that more closely related organisms look more alike, and while this is often the case,
it is not always true. If two closely related lineages evolved under significantly varied surroundings, it is possible
for the two groups to appear more different than other groups that are not as closely related. For example, the
phylogenetic tree in Figure 20.4 shows that lizards and rabbits both have amniotic eggs; whereas, frogs do not.
Yet lizards and frogs appear more similar than lizards and rabbits.
Egg with amnion?
Legs?
Flinged jaw?
Vertebral
column?
YES
NO
YES
NO
YES
NO
Hair?
YES
YES *
NO
NO
-► Rabbit
■+• Lizard
■+■ Frog
Fish
Lamprey
Lancelet
Figure 20.4 An organism that lacked a vertebral column roots this ladder-like phylogenetic tree of vertebrates. At each
branch point, scientists place organisms with different characters in different groups based on shared characteristics.
Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for length
of time, only the evolutionary order. In other words, a branch's length does not typically mean more time passed,
nor does a short branch mean less time passed— unless specified on the diagram. For example, in Figure 20.4,
the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the
tree does show is the order in which things took place. Again using Figure 20.4, the tree shows that the oldest
trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a
part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops.
Thus, for the organisms in Figure 20.4, just because a vertebral column evolved does not mean that invertebrate
evolution ceased. It only means that a new branch formed. Also, groups that are not closely related, but evolve
under similar conditions, may appear more phenotypically similar to each other than to a close relative.
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Chapter 20 | Phylogenies and the History of Life
LINK TQ LEARNING
Head to this website (http:// 0 penstaxc 0 llege. 0 rg/l/tree_ 0 f_life) to see interactive exercises that allow you
to explore the evolutionary relationships among species.
Classification Levels
Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct
internationally shared classification systems with each organism placed into increasingly more inclusive
groupings. Think about a grocery store's organization. One large space is divided into departments, such as
produce, dairy, and meats. Then each department further divides into aisles, then each aisle into categories
and brands, and then finally a single product. We call this organization from larger to smaller, more specific
categories a hierarchical system.
The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a
Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups
become more specific, until one branch ends as a single species. For example, after the common beginning of
all life, scientists divide organisms into three large categories called domains: Bacteria, Archaea, and Eukarya.
Within each domain is a second category called a kingdom. After kingdoms, the subsequent categories of
increasing specificity are: phylum, class, order, family, genus, and species (Figure 20.5).
Figure 20.5 The taxonomic classification system uses a hierarchical model to organize living organisms into
increasingly specific categories. The common dog, Canis lupus familiaris, is a subspecies of Canis lupus, which also
includes the wolf and dingo, (credit “dog”: modification of work by Janneke Vreugdenhil)
The kingdom Animalia stems from the Eukarya domain. Figure 20.5 above shows the classification for the
common dog. Therefore, the full name of an organism technically has eight terms. For the dog it is: Eukarya,
Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, and lupus. Notice that each name is capitalized
except for species, and the genus and species names are italicized. Scientists generally refer to an organism
only by its genus and species, which is its two-word scientific name, or binomial nomenclature. Therefore, the
scientific name of the dog is Canis lupus. The name at each level is also a taxon. In other words, dogs are in
order Carnivora. Carnivora is the name of the taxon at the order level; Canidae is the taxon at the family level,
and so forth. Organisms also have a common name that people typically use, in this case, dog. Note that the
dog is additionally a subspecies: the “familiaris” in Canis lupus familiaris. Subspecies are members of the same
species that are capable of mating and reproducing viable offspring, but they are separate subspecies due to
geographic or behavioral isolation or other factors.
Figure 20.6 shows how the levels move toward specificity with other organisms. Notice how the dog shares a
domain with the widest diversity of organisms, including plants and butterflies. At each sublevel, the organisms
become more similar because they are more closely related. Historically, scientists classified organisms using
characteristics, but as DNA technology developed, they have determined more precise phylogenies.
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Chapter 20 | Phylogenies and the History of Life
541
visual
CONNECTION
Subspecies:
Canis lupus familiaris
Species: Canis lupus
Genus: Canis
Family: Canidae
Order: Carnivora
Class: Mammalia
Phylum: Chordata
Kingdom: Animalia
Domain: Eukarya
'/'MU
Dog
RTf
WoH Dog
' 1
Jackal Wolf Dog
< 1
Hi ' i
Fox Jackal Wolf Dog
’d'~
Cat Fox Jackal Wolf Dog
m k •»>
Rabbit Cat Fox Jackal Wolf Dog
r I
Fish Rabbit Cat Fox Jackal Wolf Dog
is
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j ; * T. -.
Insect Fish Rabbit Cat Fox Jackal Wolf Dog
Plant Insect Fish Rabbit Cat Fox Jackal Wolf Dog
Figure 20.6 At each sublevel in the taxonomic classification system, organisms become more similar. Dogs
and wolves are the same species because they can breed and produce viable offspring, but they are different
enough to be classified as different subspecies, (credit “plant”: modification of work by "berduchwal"/Flickr;
credit “insect”: modification of work by Jon Sullivan; credit “fish": modification of work by Christian Mehlfuhrer;
credit “rabbit”: modification of work by Aidan Wojtas; credit “cat”: modification of work by Jonathan Lidbeck;
credit “fox”: modification of work by Kevin Bacher, NPS; credit “jackal": modification of work by Thomas A.
Hermann, NBII, USGS; credit “wolf”: modification of work by Robert Dewar; credit “dog": modification of work by
"digital_image_fan"/Flickr)
At what levels are cats and dogs part of the same group?
LINK
T a
LEARNING
Visit this website (http:// 0 penstaxc 0 llege. 0 rg/l/classifyJife) to classify three organisms—bear, orchid, and
sea cucumber—from kingdom to species. To launch the game, under Classifying Life, click the picture of the
bear or the Launch Interactive button.
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Chapter 20 | Phytogenies and the History of Life
Recent genetic analysis and other advancements have found that some earlier phylogenetic classifications do
not align with the evolutionary past; therefore, researchers must make changes and updates as new discoveries
occur. Recall that phylogenetic trees are hypotheses and are modified as data becomes available. In addition,
classification historically has focused on grouping organisms mainly by shared characteristics and does not
necessarily illustrate how the various groups relate to each other from an evolutionary perspective. For example,
despite the fact that a hippopotamus resembles a pig more than a whale, the hippopotamus may be the whale's
closest living relative.
20.2 | Determining Evolutionary Relationships
By the end of this section, you will be able to do the following:
• Compare homologous and analogous traits
• Discuss the purpose of cladistics
• Describe maximum parsimony
Scientists must collect accurate information that allows them to make evolutionary connections among
organisms. Similar to detective work, scientists must use evidence to uncover the facts. In the case of phylogeny,
evolutionary investigations focus on two types of evidence: morphologic (form and function) and genetic.
Two Options for Similarities
In general, organisms that share similar physical features and genomes are more closely related than those that
do not. We refer to such features that overlap both morphologically (in form) and genetically as homologous
structures. They stem from developmental similarities that are based on evolution. For example, the bones in
bat and bird wings have homologous structures (Figure 20.7).
Homologous Structures
(a) Bird wing (b) Bat wing
Figure 20.7 Bat and bird wings are homologous structures, indicating that bats and birds share a common evolutionary
past, (credit a: modification of work by Steve Hillebrand, USFWS; credit b: modification of work by U.S. DOI BLM)
Notice it is not simply a single bone, but rather a grouping of several bones arranged in a similar way. The more
complex the feature, the more likely any kind of overlap is due to a common evolutionary past. Imagine two
people from different countries both inventing a car with all the same parts and in exactly the same arrangement
without any previous or shared knowledge. That outcome would be highly improbable. However, if two people
both invented a hammer, we can reasonably conclude that both could have the original idea without the help
of the other. The same relationship between complexity and shared evolutionary history is true for homologous
structures in organisms.
Misleading Appearances
Some organisms may be very closely related, even though a minor genetic change caused a major
morphological difference to make them look quite different. Similarly, unrelated organisms may be distantly
related, but appear very much alike. This usually happens because both organisms were in common adaptations
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Chapter 20 | Phylogenies and the History of Life
543
that evolved within similar environmental conditions. When similar characteristics occur because of
environmental constraints and not due to a close evolutionary relationship, it is an analogy or homoplasy. For
example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin is completely
different. These are analogous structures (Figure 20.8).
Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin.
Analogous organs have a similar function. For example, the bones in a whale's front flipper are homologous to
the bones in the human arm. These structures are not analogous. A butterfly or bird's wings are analogous but
not homologous. Some structures are both analogous and homologous: bird and bat wings are both homologous
and analogous. Scientists must determine which type of similarity a feature exhibits to decipher the organisms'
phylogeny.
(a) Bat wing (b) Bird wing
(c) Insect wing
Figure 20.8 The (c) wing of a honeybee is similar in shape to a (b) bird wing and (a) bat wing, and it serves the same
function. However, the honeybee wing is not composed of bones and has a distinctly different structure and embryonic
origin. These wing types (insect versus bat and bird) illustrate an analogy—similar structures that do not share an
evolutionary history, (credit a: modification of work by U.S. DOI BLM; credit b: modification of work by Steve Hillebrand,
USFWS; credit c: modification of work by Jon Sullivan)
LINK
T a
LEARNING
This website (http:// 0 penstaxc 0 llege. 0 rg/l/relati 0 nships) has several examples to show how appearances
can be misleading in understanding organisms' phylogenetic relationships.
Molecular Comparisons
The advancement of DNA technology has given rise to molecular systematics, which is use of molecular data
in taxonomy and biological geography (biogeography). New computer programs not only confirm many earlier
classified organisms, but also uncover previously made errors. As with physical characteristics, even the DNA
sequence can be tricky to read in some cases. For some situations, two very closely related organisms can
appear unrelated if a mutation occurred that caused a shift in the genetic code. Inserting or deleting a mutation
would move each nucleotide base over one place, causing two similar codes to appear unrelated.
Sometimes two segments of DNA code in distantly related organisms randomly share a high percentage of
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Chapter 20 | Phylogenies and the History of Life
bases in the same locations, causing these organisms to appear closely related when they are not. For both of
these situations, computer technologies help identify the actual relationships, and, ultimately, the coupled use of
both morphologic and molecular information is more effective in determining phylogeny.
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Chapter 20 | Phytogenies and the History of Life
545
V /
e olution CONNECTION
Why Does Phylogeny Matter?
Evolutionary biologists could list many reasons why understanding phylogeny is important to everyday life
in human society. For botanists, phylogeny acts as a guide to discovering new plants that can be used to
benefit people. Think of all the ways humans use plants—food, medicine, and clothing are a few examples.
If a plant contains a compound that is effective in treating cancer, scientists might want to examine all of the
compounds for other useful drugs.
A research team in China identified a DNA segment that they thought to be common to some medicinal
plants in the family Fabaceae (the legume family). They worked to identify which species had this segment
(Figure 20.9). After testing plant species in this family, the team found a DNA marker (a known location
on a chromosome that enabled them to identify the species) present. Then, using the DNA to uncover
phylogenetic relationships, the team could identify whether a newly discovered plant was in this family and
assess its potential medicinal properties.
xxiv.
»nrw* «««»». “»»»UUMt-t
D&lberg-ia Sissoo, Roxb
Figure 20.9 Dalbergia sissoo (D. sissoo) is in the Fabaceae, or legume family. Scientists found that D. sissoo
shares a DNA marker with species within the Fabaceae family that have antifungal properties. Subsequently,
researchers found that D. sissoo had fungicidal activity, supporting the idea that DNA markers are useful to screen
plants with potential medicinal properties.
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Chapter 20 | Phytogenies and the History of Life
Building Phylogenetic Trees
How do scientists construct phylogenetic trees? After they sort the homologous and analogous traits, scientists
often organize the homologous traits using cladistics. This system sorts organisms into clades: groups of
organisms that descended from a single ancestor. For example, in Figure 20.10, all the organisms in the
orange region evolved from a single ancestor that had amniotic eggs. Consequently, these organisms also have
amniotic eggs and make a single clade, or a monophyletic group. Clades must include all descendants from a
branch point.
visual
CONNECTION
Lancelet Lamprey Fish Lizard Rabbit
Human
Figure 20.10 Lizards, rabbits, and humans all descend from a common ancestor that had an amniotic egg. Thus,
lizards, rabbits, and humans all belong to the clade Amniota. Vertebrata is a larger clade that also includes fish
and lamprey.
Which animals in this figure belong to a clade that includes animals with hair? Which evolved first, hair or
the amniotic egg?
Clades can vary in size depending on which branch point one references. The important factor is that all
organisms in the clade or monophyletic group stem from a single point on the tree. You can remember
this because monophyletic breaks down into “mono,” meaning one, and “phyletic,” meaning evolutionary
relationship. Figure 20.11 shows various clade examples. Notice how each clade comes from a single point;
whereas, the non-clade groups show branches that do not share a single point.
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547
CONNECTION
Clades
Slime
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Entamoebae molds Animals
Trichomonads
Microsporidia
Diplomonads
Fungi
Plants
Ciliates
Flagellates
Trichomonads
Microsporidia
Diplomonads
Fungi
Plants
Ciliates
Flagellates
Not Clades
Slime
Entamoebae molds Animals
Slime
Entamoebae molds Animals
Trichomonads
Microsporidia
Diplomonads
Flagellates
Fungi
Plants
Ciliates
Trichomonads
Microsporidia
Diplomonads
Fungi
Plants
Ciliates
Flagellates
Figure 20.11 All the organisms within a clade stem from a single point on the tree. A clade may contain multiple
groups, as in the case of animals, fungi and plants, or a single group, as in the case of flagellates. Groups that
diverge at a different branch point, or that do not include all groups in a single branch point, are not clades.
What is the largest clade in this diagram?
Shared Characteristics
Organisms evolve from common ancestors and then diversify. Scientists use the phrase “descent with
modification” because even though related organisms have many of the same characteristics and genetic codes,
changes occur. This pattern repeats as one goes through the phylogenetic tree of life:
1. A change in an organism's genetic makeup leads to a new trait which becomes prevalent in the group.
2. Many organisms descend from this point and have this trait.
3. New variations continue to arise: some are adaptive and persist, leading to new traits.
4. With new traits, a new branch point is determined (go back to step 1 and repeat).
If a characteristic is found in the ancestor of a group, it is considered a shared ancestral character because
all of the organisms in the taxon or clade have that trait. The vertebrate in Figure 20.10 is a shared ancestral
character. Now consider the amniotic egg characteristic in the same figure. Only some of the organisms in
Figure 20.10 have this trait, and to those that do, it is called a shared derived character because this trait
derived at some point but does not include all of the ancestors in the tree.
The tricky aspect to shared ancestral and shared derived characters is that these terms are relative. We can
consider the same trait one or the other depending on the particular diagram that we use. Returning to Figure
20.10, note that the amniotic egg is a shared ancestral character for the Amniota clade, while having hair is
a shared derived character for some organisms in this group. These terms help scientists distinguish between
clades in building phylogenetic trees.
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Chapter 20 | Phytogenies and the History of Life
Choosing the Right Relationships
Imagine being the person responsible for organizing all department store items properly—an overwhelming task.
Organizing the evolutionary relationships of all life on Earth proves much more difficult: scientists must span
enormous blocks of time and work with information from long-extinct organisms. Trying to decipher the proper
connections, especially given the presence of homologies and analogies, makes the task of building an accurate
tree of life extraordinarily difficult. Add to that advancing DNA technology, which now provides large quantities
of genetic sequences for researchers to use and analzye. Taxonomy is a subjective discipline: many organisms
have more than one connection to each other, so each taxonomist will decide the order of connections.
To aid in the tremendous task of describing phylogenies accurately, scientists often use the concept of maximum
parsimony, which means that events occurred in the simplest, most obvious way. For example, if a group of
people entered a forest preserve to hike, based on the principle of maximum parsimony, one could predict that
most would hike on established trails rather than forge new ones.
For scientists deciphering evolutionary pathways, the same idea is used: the pathway of evolution probably
includes the fewest major events that coincide with the evidence at hand. Starting with all of the homologous
traits in a group of organisms, scientists look for the most obvious and simple order of evolutionary events that
led to the occurrence of those traits.
LINK TQ LEARNING
Head to this website (http:// 0 penstaxc 0 llege. 0 rg/l/using_parsim 0 ny) to learn how researchers use
maximum parsimony to create phylogenetic trees.
These tools and concepts are only a few strategies scientists use to tackle the task of revealing the
evolutionary history of life on Earth. Recently, newer technologies have uncovered surprising discoveries with
unexpected relationships, such as the fact that people seem to be more closely related to fungi than fungi are
to plants. Sound unbelievable? As the information about DNA sequences grows, scientists will become closer
to mapping the evolutionary history of all life on Earth.
20.3 | Perspectives on the Phylogenetic Tree
By the end of this section, you will be able to do the following:
• Describe horizontal gene transfer
• illustrate how prokaryotes and eukaryotes transfer genes horizontally
• Identify the web and ring models of phylogenetic relationships and describe how they differ from the
original phylogenetic tree concept
Phylogenetic modeling concepts are constantly changing. It is one of the most dynamic fields of study in all
biology. Over the last several decades, new research has challenged scientists’ ideas about how organisms are
related. The scientific community has proposed new models of these relationships.
Many phylogenetic trees are models of the evolutionary relationship among species. Phylogenetic trees
originated with Charles Darwin, who sketched the first phylogenetic tree in 1837 (Figure 20.12a). This served
as a prototype for subsequent studies for more than a century. The phylogenetic tree concept with a single trunk
representing a common ancestor, with the branches representing the divergence of species from this ancestor,
fits well with the structure of many common trees, such as the oak (Figure 20.12b). However, evidence from
modern DNA sequence analysis and newly developed computer algorithms has caused skepticism about the
standard tree model's validity in the scientific community.
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Chapter 20 | Phylogenies and the History of Life
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nt ~'
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(a) (b)
Figure 20.12 The (a) concept of the “tree of life" dates to an 1837 Charles Darwin sketch. Like an (b) oak tree, the “tree
of life” has a single trunk and many branches, (credit b: modification of work by "Amada44"/Wikimedia Commons)
Limitations to the Classic Model
Classical thinking about prokaryotic evolution, included in the classic tree model, is that species evolve clonally.
That is, they produce offspring themselves with only random mutations causing the descent into the variety
of modern-day and extinct species known to science. This view is somewhat complicated in eukaryotes that
reproduce sexually, but the laws of Mendelian genetics explain the variation in offspring, again, to be a result of
a mutation within the species. Scientists did not consider the concept of genes transferring between unrelated
species as a possibility until relatively recently. Horizontal gene transfer (HGT), or lateral gene transfer, is the
transfer of genes between unrelated species. HGT is an ever-present phenomenon, with many evolutionists
postulating a major role for this process in evolution, thus complicating the simple tree model. Genes pass
between species which are only distantly related using standard phylogeny, thus adding a layer of complexity to
understanding phylogenetic relationships.
The various ways that HGT occurs in prokaryotes is important to understanding phylogenies. Although at
present some do not view HGT as important to eukaryotic evolution, HGT does occur in this domain as well.
Finally, as an example of the ultimate gene transfer, some scientists have proposed genome fusion theories
between symbiotic or endosymbiotic organisms to explain an event of great importance—the evolution of the
first eukaryotic cell, without which humans could not have come into existence.
Horizontal Gene Transfer
Horizontal gene transfer (HGT) is the introduction of genetic material from one species to another species by
mechanisms other than the vertical transmission from parent(s) to offspring. These transfers allow even distantly
related species to share genes, influencing their phenotypes. Scientists believe that HGT is more prevalent in
prokaryotes, but that this process transfers only about 2% of the prokaryotic genome. Some researchers believe
such estimates are premature: we must view the actual importance of HGT to evolutionary processes as a work
in progress. As scientists investigate this phenomenon more thoroughly, they may reveal more HGT transfer.
Many scientists believe that HGT and mutation are (especially in prokaryotes) a significant source of genetic
variation, which is the raw material in the natural selection process. These transfers may occur between any two
species that share an intimate relationship (Table 20.1).
Prokaryotic and Eukaryotic HGT Mechanisms Summary
Mechanism
Mode of Transmission
Example
Prokaryotes
transformation
DNA uptake
many prokaryotes
transduction
bacteriophage (virus)
bacteria
Table 20.1
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Chapter 20 | Phylogenies and the History of Life
Prokaryotic and Eukaryotic HGT Mechanisms Summary
Mechanism Mode of Transmission Example
conjugation
pilus
many prokaryotes
gene transfer agents
phage-like particles
purple non-sulfur bacteria
Eukaryotes
from food organisms
unknown
aphid
jumping genes
transposons
rice and millet plants
epiphytes/parasites
unknown
yew tree fungi
from viral infections
Table 20.1
HGT in Prokaryotes
HGT mechanisms are quite common in the Bacteria and Archaea domains, thus significantly changing the
way scientists view their evolution. The majority of evolutionary models, such as in the Endosymbiont Theory,
propose that eukaryotes descended from multiple prokaryotes, which makes HGT all the more important
to understanding the phylogenetic relationships of all extant and extinct species. The Endosymbiont Theory
purports that the eukaryotes' mitochondria and the green plants' chloroplasts and flagellates originated as free-
living prokaryotes that invaded primitive eukaryotic cells and become established as permanent symbionts in the
cytoplasm.
Microbiology students are well aware that genes transfer among common bacteria. These gene transfers
between species are the major mechanism whereby bacteria acquire resistance to antibiotics. Classically,
scientists believe that three different mechanisms drive such transfers.
1. Transformation: bacteria takes up naked DNA
2. Transduction: a virus transfers the genes
3. Conjugation: a hollow tube, or pilus transfers genes between organisms
More recently, scientists have discovered a fourth gene transfer mechanism between prokaryotes. Small, virus¬
like particles, or gene transfer agents (GTAs) transfer random genomic segments from one prokaryote species
to another. GTAs are responsible for genetic changes, sometimes at a very high frequency compared to other
evolutionary processes. Scientists characterized the first GTA in 1974 using purple, non-sulfur bacteria. These
GTAs, which are most likely bacteriophages that lost the ability to reproduce on their own, carry random DNA
pieces from one organism to another. Controlled studies using marine bacteria have demonstrated GTAs' ability
to act with high frequency. Scientists have estimated gene transfer events in marine prokaryotes, either by GTAs
or by viruses, to be as high as 10 13 per year in the Mediterranean Sea alone. GTAs and viruses are efficient
HGT vehicles with a major impact on prokaryotic evolution.
As a consequence of this modern DNA analysis, the idea that eukaryotes evolved directly from Archaea has
fallen out of favor. While eukaryotes share many features that are absent in bacteria, such as the TATA box
(located in many genes' promoter region), the discovery that some eukaryotic genes were more homologous with
bacterial DNA than Archaea DNA made this idea less tenable. Furthermore, scientists have proposed genome
fusion from Archaea and Bacteria by endosymbiosis as the ultimate event in eukaryotic evolution.
HGT in Eukaryotes
Although it is easy to see how prokaryotes exchange genetic material by HGT, scientists initially thought that
this process was absent in eukaryotes. After all, prokaryotes are but single cells exposed directly to their
environment; whereas, the multicellular organisms' sex cells are usually sequestered in protected parts of the
body. It follows from this idea that the gene transfers between multicellular eukaryotes should be more difficult.
Scientists believe this process is rarer in eukaryotes and has a much smaller evolutionary impact than in
prokaryotes. In spite of this, HGT between distantly related organisms is evident in several eukaryotic species,
and it is possible that scientists will discover more examples in the future.
In plants, researchers have observed gene transfer in species that cannot cross-pollinate by normal means.
Transposons or “jumping genes” have shown a transfer between rice and millet plant species. Furthermore,
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551
fungal species feeding on yew trees, from which the anti-cancer drug TAXOL® is derived from the bark, have
acquired the ability to make taxol themselves, a clear example of gene transfer.
In animals, a particularly interesting example of HGT occurs within the aphid species (Figure 20.13). Aphids
are insects that vary in color based on carotenoid content. Carotenoids are pigments that a variety of plants,
fungi, and microbes produce, and they serve a variety of functions in animals, who obtain these chemicals from
their food. Humans require carotenoids to synthesize vitamin A, and we obtain them by eating orange fruits and
vegetables: carrots, apricots, mangoes, and sweet potatoes. Alternatively, aphids have acquired the ability to
make the carotenoids on their own. According to DNA analysis, this ability is due to fungal genes transferring
into the insect by HGT, presumably as the insect consumed fungi for food. A carotenoid enzyme, or desaturase,
is responsible for the red coloration in certain aphids, and when mutation activates this gene, the aphids revert
to their more common green color (Figure 20.13).
Figure 20.13 (a) Red aphids get their color from red carotenoid pigment. Genes necessary to make this pigment are
present in certain fungi, and scientists speculate that aphids acquired these genes through HGT after consuming fungi
for food. If mutation inactivates the genes for making carotenoids, the aphids revert back to (b) their green color. Red
coloration makes the aphids considerably more conspicuous to predators, but evidence suggests that red aphids are
more resistant to insecticides than green ones. Thus, red aphids may be more fit to survive in some environments than
green ones, (credit a: modification of work by Benny Mazur; credit b: modification of work by Mick Talbot)
Genome Fusion and Eukaryote Evolution
Scientists believe the ultimate in HGT occurs through genome fusion between different prokaryote species
when two symbiotic organisms become endosymbiotic. This occurs when one species is taken inside another
species' cytoplasm, which ultimately results in a genome consisting of genes from both the endosymbiont
and the host. This mechanism is an aspect of the Endosymbiont Theory, which most biologists accept as
the mechanism whereby eukaryotic cells obtained their mitochondria and chloroplasts. However, the role of
endosymbiosis in developing the nucleus is more controversial. Scientists believe that nuclear and mitochondrial
DNA have different (separate) evolutionary origins, with the mitochondrial DNA derived from the bacteria's
circular genomes ancient prokaryotic cells engulfed. We can regard mitochondrial DNA as the smallest
chromosome. Interestingly enough, mitochondrial DNA is inherited only from the mother. The mitochondrial DNA
degrades in sperm when the sperm degrades in the fertilized egg or in other instances when the mitochondria
located in the sperm's flagellum fails to enter the egg.
Within the past decade, James Lake of the UCLA/NASA Astrobiology Institute proposed that the genome fusion
process is responsible for the evolution of the first eukaryotic cells (Figure 20.14a). Using DNA analysis and
a new mathematical algorithm, conditioned reconstruction (CR), his laboratory proposed that eukaryotic cells
developed from an endosymbiotic gene fusion between two species, one an Archaea and the other a Bacteria.
As mentioned, some eukaryotic genes resemble those of Archaea; whereas, others resemble those from
Bacteria. An endosymbiotic fusion event, such as Lake has proposed, would clearly explain this observation.
Alternatively, this work is new and the CR algorithm is relatively unsubstantiated, which causes many scientists
to resist this hypothesis.
Lake's more recent work (Figure 20.14b) proposes that gram-negative bacteria, which are unique within their
domain in that they contain two lipid bilayer membranes, resulted from an endosymbiotic fusion of archaeal and
bacterial species. The double membrane would be a direct result of the endosymbiosis, with the endosymbiont
picking up the second membrane from the host as it was internalized. Scientists have also used this mechanism
to explain the double membranes in mitochondria and chloroplasts. Lake’s work is not without skepticism, and
the biological science community still debates his ideas. In addition to Lake’s hypothesis, there are several other
competing theories as to the origin of eukaryotes. How did the eukaryotic nucleus evolve? One theory is that the
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prokaryotic cells produced an additional membrane that surrounded the bacterial chromosome. Some bacteria
have the DNA enclosed by two membranes; however, there is no evidence of a nucleolus or nuclear pores.
Other proteobacteria also have membrane-bound chromosomes. If the eukaryotic nucleus evolved this way, we
would expect one of the two types of prokaryotes to be more closely related to eukaryotes.
(a) Genome fusion by endosymbiosis
Operational genes
(from ancestral
bacteria)
Informational genes
(from ancestral
archaebacteria)
(b) Endosymbiotic formation of Gram-negative bacteria
Archaea Gram-positive Gram-negative
bacteria bacteria
oo
Figure 20.14 The scientific community now widely accepts the theory that mitochondria and chloroplasts are
endosymbiotic in origin. More controversial is the proposal that (a) the eukaryotic nucleus resulted from fusing archaeal
and bacterial genomes, and that (b) Gram-negative bacteria, which have two membranes, resulted from fusing
Archaea and Gram-positive bacteria, each of which has a single membrane.
The nucleus-first hypothesis proposes that the nucleus evolved in prokaryotes first (Figure 20.15a), followed by
a later fusion of the new eukaryote with bacteria that became mitochondria. The mitochondria-first hypothesis
proposes that mitochondria were first established in a prokaryotic host (Figure 20.15b), which subsequently
acquired a nucleus, by fusion or other mechanisms, to become the first eukaryotic cell. Most interestingly, the
eukaryote-first hypothesis proposes that prokaryotes actually evolved from eukaryotes by losing genes and
complexity (Figure 20.15c). All of these hypotheses are testable. Only time and more experimentation will
determine which hypothesis data best supports.
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Models for Evolution of the Three Domains
(a) Nucleus-first hypothesis
(c) Eukaryote-first hypothesis
Figure 20.15 Three alternate hypotheses of eukaryotic and prokaryotic evolution are (a) the nucleus-first hypothesis,
(b) the mitochondrion-first hypothesis, and (c) the eukaryote-first hypothesis.
Web and Network Models
Recognizing the importance of HGT, especially in prokaryote evolution, has caused some to propose
abandoning the classic “tree of life” model. In 1999, W. Ford Doolittle proposed a phylogenetic model that
resembles a web or a network more than a tree. The hypothesis is that eukaryotes evolved not from a single
prokaryotic ancestor, but from a pool of many species that were sharing genes by HGT mechanisms. As
Figure 20.16a shows, some individual prokaryotes were responsible for transferring the bacteria that caused
mitochondrial development to the new eukaryotes; whereas, other species transferred the bacteria that gave
rise to chloroplasts. Scientists often call this model the “ web of life.” In an effort to save the tree analogy, some
have proposed using the Ficus tree (Figure 20.16b) with its multiple trunks as a phylogenetic way to represent
a diminished evolutionary role for HGT.
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Chapter 20 | Phylogenies and the History of Life
Hyperthermophilic
bacteria
cur IKo rarchaeota
□ □n
□ □□
Ancestral community of primitive cells
(a) (b)
Figure 20.16 In W. Ford Doolittle's (a) phylogenetic model, the “tree of life" arose from a community of ancestral cells,
has multiple trunks, and has connections between branches where horizontal gene transfer has occurred. Visually, this
concept is better represented by (b) the multi-trunked Ficus than by an oak's single trunk similar to Darwin's tree in
Figure 20.12. (credit b: modification of work by "psyberartist'VFlickr)
Ring of Life Models
Others have proposed abandoning any tree-like model of phylogeny in favor of a ring structure, the so-called “
ring of life” (Figure 20.17). This is a phylogenetic model where all three domains of life evolved from a pool
of primitive prokaryotes. Lake, again using the conditioned reconstruction algorithm, proposes a ring-like model
in which species of all three domains—Archaea, Bacteria, and Eukarya—evolved from a single pool of gene¬
swapping prokaryotes. His laboratory proposes that this structure is the best fit for data from extensive DNA
analyses performed in his laboratory, and that the ring model is the only one that adequately takes HGT and
genomic fusion into account. However, other phylogeneticists remain highly skeptical of this model.
Figure 20.17 According to the “ring of life" phylogenetic model, the three domains of life evolved from a pool of primitive
prokaryotes.
In summary, we must modify Darwin's “tree of life” model to include HGT. Does this mean abandoning the
tree model completely? Even Lake argues that scientists should attempt to modify the tree model to allow it to
accurately fit his data, and only the inability to do so will sway people toward his ring proposal.
This doesn’t mean a tree, web, or a ring will correlate completely to an accurate description of phylogenetic
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Chapter 20 | Phylogenies and the History of Life
555
relationships of life. A consequence of the new thinking about phylogenetic models is the idea that Darwin’s
original phylogenetic tree concept is too simple, but made sense based on what scientists knew at the time.
However, the search for a more useful model moves on: each model serves as hypotheses to test with
the possibility of developing new models. This is how science advances. Researchers use these models as
visualizations to help construct hypothetical evolutionary relationships and understand the massive amount of
data that requires analysis.
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Chapter 20 | Phytogenies and the History of Life
KEY TERMS
analogy (also, homoplasy) characteristic that is similar between organisms by convergent evolution, not due to
the same evolutionary path
basal taxon branch on a phylogenetic tree that has not diverged significantly from the root ancestor
binomial nomenclature system of two-part scientific names for an organism, which includes genus and
species names
branch point node on a phylogenetic tree where a single lineage splits into distinct new ones
cladistics system to organize homologous traits to describe phylogenies
class division of phylum in the taxonomic classification system
eukaryote-first hypothesis proposal that prokaryotes evolved from eukaryotes
family division of order in the taxonomic classification system
gene transfer agent (GTA) bacteriophage-like particle that transfers random genomic segments from one
species of prokaryote to another
genome fusion fusion of two prokaryotic genomes, presumably by endosymbiosis
genus division of family in the taxonomic classification system; the first part of the binomial scientific name
horizontal gene transfer (HGT) (also, lateral gene transfer) transfer of genes between unrelated species
kingdom domain division in the taxonomic classification system
maximum parsimony applying the simplest, most obvious way with the least number of steps
mitochondria-first hypothesis proposal that prokaryotes acquired a mitochondrion first, followed by nuclear
development
molecular systematics technique using molecular evidence to identify phylogenetic relationships
monophyletic group (also, clade) organisms that share a single ancestor
nucleus-first hypothesis proposal that prokaryotes acquired a nucleus first, and then the mitochondrion
order class division in the taxonomic classification system
phylogenetic tree diagram that reflects the evolutionary relationships among organisms or groups of
organisms
phylogeny evolutionary history and relationship of an organism or group of organisms
phylum (plural: phyla) kingdom division in the taxonomic classification system
polytomy branch on a phylogenetic tree with more than two groups or taxa
ring of life phylogenetic model where all three domains of life evolved from a pool of primitive prokaryotes
rooted single ancestral lineage on a phylogenetic tree to which all organisms represented in the diagram relate
shared ancestral character describes a characteristic on a phylogenetic tree that all organisms on the tree
share
shared derived character describes a characteristic on a phylogenetic tree that only a certain clade of
organisms share
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sister taxa two lineages that diverged from the same branch point
systematics field of organizing and classifying organisms based on evolutionary relationships
taxon (plural: taxa) single level in the taxonomic classification system
taxonomy science of classifying organisms
web of life phylogenetic model that attempts to incorporate the effects of horizontal gene transfer on evolution
CHAPTER SUMMARY
20.1 Organizing Life on Earth
Scientists continually gain new information that helps understand the evolutionary history of life on Earth. Each
group of organisms went through its own evolutionary journey, or its phylogeny. Each organism shares
relatedness with others, and based on morphologic and genetic evidence, scientists attempt to map the
evolutionary pathways of all life on Earth. Historically, scientists organized organisms into a taxonomic
classification system. However, today many scientists build phylogenetic trees to illustrate evolutionary
relationships.
20.2 Determining Evolutionary Relationships
To build phylogenetic trees, scientists must collect accurate information that allows them to make evolutionary
connections between organisms. Using morphologic and molecular data, scientists work to identify homologous
characteristics and genes. Similarities between organisms can stem either from shared evolutionary history
(homologies) or from separate evolutionary paths (analogies). Scientists can use newer technologies to help
distinguish homologies from analogies. After identifying homologous information, scientists use cladistics to
organize these events as a means to determine an evolutionary timeline. They then apply the concept of
maximum parsimony, which states that the order of events probably occurred in the most obvious and simple
way with the least amount of steps. For evolutionary events, this would be the path with the least number of
major divergences that correlate with the evidence.
20.3 Perspectives on the Phylogenetic Tree
The phylogenetic tree, which Darwin first used, is the classic “tree of life" model describing phylogenetic
relationships among species, and the most common model that scientists use today. New ideas about HGT
and genome fusion have caused some to suggest revising the model to resemble webs or rings.
VISUAL CONNECTION QUESTIONS
1. Figure 20.6 At what levels are cats and dogs
considered part of the same group?
2. Figure 20.10 Which animals in this figure belong
to a clade that includes animals with hair? Which
REVIEW QUESTIONS
4. What is used to determine phylogeny?
a. mutations
b. DNA
c. evolutionary history
d. organisms on earth
5. What do scientists in the field of systematics
accomplish?
a. discover new fossil sites
b. organize and classify organisms
c. name new species
d. communicate among field biologists
evolved first, hair or the amniotic egg?
3. Figure 20.11 What is the largest clade in this
diagram?
6. Which statement about the taxonomic
classification system is correct?
a. There are more domains than kingdoms.
b. Kingdoms are the top category of
classification.
c. Classes are divisions of orders.
d. Subspecies are the most specific category
of classification.
7. On a phylogenetic tree, which term refers to
lineages that diverged from the same place?
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Chapter 20 | Phylogenies and the History of Life
a. sister taxa
b. basal taxa
c. rooted taxa
d. dichotomous taxa
8. Which statement about analogies is correct?
a. They occur only as errors.
b. They are synonymous with homologous
traits.
c. They are derived by similar environmental
constraints.
d. They are a form of mutation.
9. What do scientists use to apply cladistics?
a. homologous traits
b. homoplasies
c. analogous traits
d. monophyletic groups
10. What is true about organisms that are a part of
the same clade?
a. They all share the same basic
characteristics.
b. They evolved from a shared ancestor.
c. They usually fall into the same classification
taxa.
d. They have identical phylogenies.
11. Why do scientists apply the concept of maximum
parsimony?
CRITICAL THINKING QUESTIONS
16. How does a phylogenetic tree relate to the
passing of time?
17. Some organisms that appear very closely related
on a phylogenetic tree may not actually be closely
related. Why is this?
18. List the different levels of the taxonomic
classification system.
19. Dolphins and fish have similar body shapes. Is
this feature more likely a homologous or analogous
trait?
a. to decipher accurate phylogenies
b. to eliminate analogous traits
c. to identify mutations in DNA codes
d. to locate homoplasies
12. The transfer of genes by a mechanism not
involving asexual reproduction is called:
a. meiosis
b. web of life
c. horizontal gene transfer
d. gene fusion
13. Particles that transfer genetic material from one
species to another, especially in marine prokaryotes:
a. horizontal gene transfer
b. lateral gene transfer
c. genome fusion device
d. gene transfer agents
14. What does the trunk of the classic phylogenetic
tree represent?
a. single common ancestor
b. pool of ancestral organisms
c. new species
d. old species
15. Which phylogenetic model proposes that all three
domains of life evolved from a pool of primitive
prokaryotes?
a. tree of life
b. web of life
c. ring of life
d. network model
20. Why is it so important for scientists to distinguish
between homologous and analogous characteristics
before building phylogenetic trees?
21. Describe maximum parsimony.
22. Compare three different ways that eukaryotic
cells may have evolved.
23. Describe how aphids acquired the ability to
change color.
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Chapter 211 Viruses
559
21 1 VIRUSES
Figure 21.1 The tobacco mosaic virus, seen here by transmission electron microscopy (left), was the first virus to be
discovered. The virus causes disease in tobacco and other plants, such as the orchid (right), (credit a: USDA ARS;
credit b: modification of work by USDA Forest Service, Department of Plant Pathology Archive North Carolina State
University; scale-bar data from Matt Russell)
Chapter Outline
21.1: Viral Evolution, Morphology, and Classification
21.2: Virus Infections and Hosts
21.3: Prevention and Treatment of Viral Infections
21.4: Other Acellular Entities: Prions and Viroids
Introduction
Viruses are noncellular parasitic entities that cannot be classified within any kingdom. They can infect organisms
as diverse as bacteria, plants, and animals. In fact, viruses exist in a sort of netherworld between a living
organism and a nonliving entity. Living things grow, metabolize, and reproduce. In contrast, viruses are not
cellular, do not have a metabolism or grow, and cannot divide by cell division. Viruses can copy, or replicate
themselves; however, they are entirely dependent on resources derived from their host cells to produce progeny
viruses—which are assembled in their mature form. No one knows exactly when or how viruses evolved or
from what ancestral source because viruses have not left a fossil record. Some virologists contend that modern
viruses are a mosaic of bits and pieces of nucleic acids picked up from various sources along their respective
evolutionary paths.
21.1 1 Viral Evolution, Morphology, and Classification
By the end of this section, you will be able to do the following:
• Describe how viruses were first discovered and how they are detected
• Discuss three hypotheses about how viruses evolved
• Describe the general structure of a virus
• Recognize the basic shapes of viruses
• Understand past and emerging classification systems for viruses
• Describe the basis for the Baltimore classification system
Viruses are diverse entities: They vary in structure, methods of replication, and the hosts they infect. Nearly all
forms of life—from prokaryotic bacteria and archaeans, to eukaryotes such as plants, animals, and fungi—have
viruses that infect them. While most biological diversity can be understood through evolutionary history (such as
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how species have adapted to changing environmental conditions and how different species are related to one
another through common descent), much about virus origins and evolution remains unknown.
Discovery and Detection
Viruses were first discovered after the development of a porcelain filter—the Chamberland-Pasteur filter—that
could remove all bacteria visible in the microscope from any liquid sample. In 1886, Adolph Meyer demonstrated
that a disease of tobacco plants— tobacco mosaic disease —could be transferred from a diseased plant to a
healthy one via liquid plant extracts. In 1892, Dmitri Ivanowski showed that this disease could be transmitted in
this way even after the Chamberland-Pasteur filter had removed all viable bacteria from the extract. Still, it was
many years before it was proved that these “filterable” infectious agents were not simply very small bacteria but
were a new type of very small, disease-causing particle.
Most virions, or single virus particles, are very small, about 20 to 250 nanometers in diameter. However, some
recently discovered viruses from amoebae range up to 1000 nm in diameter. With the exception of large virions,
like the poxvirus and other large DNA viruses, viruses cannot be seen with a light microscope. It was not until the
development of the electron microscope in the late 1930s that scientists got their first good view of the structure
of the tobacco mosaic virus (TMV) (Figure 21.1), discussed above, and other viruses (Figure 21.2). The surface
structure of virions can be observed by both scanning and transmission electron microscopy, whereas the
internal structures of the virus can only be observed in images from a transmission electron microscope. The
use of electron microscopy and other technologies has allowed for the discovery of many viruses of all types of
living organisms.
Jf - ' - ■ •!/-• .*
jmr ' • ■
-v ■
50 nm
(a) (b)
Figure 21.2 Most virus particles are visible only by electron microscopy. In these transmission electron micrographs,
(a) a virus is as dwarfed by the bacterial cell it infects, as (b) these E. coli cells are dwarfed by cultured colon cells,
(credit a: modification of work by U.S. Dept, of Energy, Office of Science, LBL, PBD; credit b: modification of work by
J.P. Nataro and S. Sears, unpub. data, CDC; scale-bar data from Matt Russell)
Evolution of Viruses
Although biologists have a significant amount of knowledge about how present-day viruses mutate and adapt,
much less is known about how viruses originated in the first place. When exploring the evolutionary history
of most organisms, scientists can look at fossil records and similar historic evidence. However, viruses do not
fossilize, as far as we know, so researchers must extrapolate from investigations of how today’s viruses evolve
and by using biochemical and genetic information to create speculative virus histories.
Most scholars agree that viruses don’t have a single common ancestor, nor is there a single reasonable
hypothesis about virus origins. There are current evolutionary scenarios that may explain the origin of viruses.
One such hypothesis, the “devolution” or the regressive hypothesis, suggests that viruses evolved from free-
living cells, or from intracellular prokaryotic parasites. However, many components of how this process might
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561
have occurred remain a mystery. A second hypothesis, the escapist or the progressive hypothesis, suggests
that viruses originated from RNA and DNA molecules that escaped from a host cell. A third hypothesis, the
seif-replicating hypothesis, suggests that viruses may have originated from self-replicating entities similar to
transposons or other mobile genetic elements. In all cases, viruses are probably continuing to evolve along with
the cells on which they rely on as hosts.
As technology advances, scientists may develop and refine additional hypotheses to explain the origins of
viruses. The emerging field called virus molecular systematics attempts to do just that through comparisons
of sequenced genetic material. These researchers hope one day to better understand the origin of viruses—a
discovery that could lead to advances in the treatments for the ailments they produce.
Viral Morphology
Viruses are noncellular, meaning they are biological entities that do not have a cellular structure. They therefore
lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane. A virion
consists of a nucleic acid core, an outer protein coating or capsid, and sometimes an outer envelope made of
protein and phospholipid membranes derived from the host cell. Viruses may also contain additional proteins,
such as enzymes, within the capsid or attached to the viral genome. The most obvious difference between
members of different viral families is the variation in their morphology, which is quite diverse. An interesting
feature of viral complexity is that the complexity of the host does not necessarily correlate with the complexity
of the virion. In fact, some of the most complex virion structures are found in the bacteriophages —viruses that
infect the simplest living organisms, bacteria.
Morphology
Viruses come in many shapes and sizes, but these features are consistent for each viral family. As we have
seen, all virions have a nucleic acid genome covered by a protective capsid. The proteins of the capsid are
encoded in the viral genome, and are called capsomeres. Some viral capsids are simple helices or polyhedral
“spheres,” whereas others are quite complex in structure (Figure 21.3).
Human rhinovirus HRV14
Variola virus
Helical
Icosahedral
Complex
Figure 21.3 Viral capsids can be (a) helical, (b) polyhedral, or (c) have a complex shape, (credit a “micrograph”:
modification of work by USDA ARS; credit b “micrograph”: modification of work by U.S. Department of Energy)
In general, the capsids of viruses are classified into four groups: helical, icosahedral, enveloped, and head-and-
tail. Helical capsids are long and cylindrical. Many plant viruses are helical, including TMV. Icosahedral viruses
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have shapes that are roughly spherical, such as those of poliovirus or herpesviruses. Enveloped viruses have
membranes derived from the host cell that surrounds the capsids. Animal viruses, such as HIV, are frequently
enveloped. Head-and-tail viruses infect bacteria and have a head that is similar to icosahedral viruses and a tail
shaped like helical viruses.
Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral
receptors. For these viruses, attachment is required for later penetration of the cell membrane; only after
penetration takes place can the virus complete its replication inside the cell. The receptors that viruses use are
molecules that are normally found on cell surfaces and have their own physiological functions. It appears that
viruses have simply evolved to make use of these molecules for their own replication. For example, HIV uses
the CD4 molecule on T lymphocytes as one of its receptors (Figure 21.4). CD4 is a type of molecule called a
cell adhesion molecule, which functions to keep different types of immune cells in close proximity to each other
during the generation of a T lymphocyte immune response.
Figure 21.4 A virus and its host receptor protein. The HIV virus binds the CD4 receptor on the surface of human
cells. CD4 receptors help white blood cells to communicate with other cells of the immune system when producing an
immune response, (credit: modification of work by NIAID, NIH)
One of the most complex virions known, the T4 bacteriophage (which infects the Escherichia coll) bacterium,
has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA.
Adenovirus, a non-enveloped animal virus that causes respiratory illnesses in humans, uses glycoprotein spikes
protruding from its capsomeres to attach to host cells. Non-enveloped viruses also include those that cause polio
(poliovirus), plantar warts (papillomavirus), and hepatitis A (hepatitis A virus).
Enveloped virions, such as the influenza virus, consist of nucleic acid (RNA in the case of influenza) and capsid
proteins surrounded by a phospholipid bilayer envelope that contains virus-encoded proteins. Glycoproteins
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embedded in the viral envelope are used to attach to host cells. Other envelope proteins are the matrix proteins
that stabilize the envelope and often play a role in the assembly of progeny virions. Chicken pox, HIV, and
mumps are other examples of diseases caused by viruses with envelopes. Because of the fragility of the
envelope, non-enveloped viruses are more resistant to changes in temperature, pH, and some disinfectants than
enveloped viruses.
Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease
the virus may cause or what species it might infect, but they are still useful means to begin viral classification
(Figure 21.5).
visual
CONNECTION
Bacteriophage T4
Head-
DNA
Tail fibers
Adenovirus
Glycoproteins / Capsomere
DNA
-Capsid
Influenza Virus
Hemagglutinin
Nucleoprotein
Capsid-
Neuraminidase
Figure 21.5 Complex Viruses. Viruses can be either complex or relatively simple in shape. This figure shows three
relatively complex virions: the bacteriophage T4, with its DNA-containing head group and tail fibers that attach
to host cells; adenovirus, which uses spikes from its capsid to bind to host cells; and the influenza virus, which
uses glycoproteins embedded in its envelope to bind to host cells. The influenza virus also has matrix proteins,
internal to the envelope, which help stabilize the virion’s shape, (credit “bacteriophage, adenovirus”: modification
of work by NCBI, NIH; credit "influenza virus": modification of work by Dan Higgins, Centers for Disease Control
and Prevention)
Which of the following statements about virus structure is true?
a. All viruses are encased in a viral membrane.
b. The capsomere is made up of small protein subunits called capsids.
c. DNA is the genetic material in all viruses.
d. Glycoproteins help the virus attach to the host cell.
Types of Nucleic Acid
Unlike nearly all living organisms that use DNA as their genetic material, viruses may use either DNA or RNA.
The virus core contains the genome—the total genetic content of the virus. Viral genomes tend to be small,
containing only those genes that encode proteins which the virus cannot get from the host cell. This genetic
material may be single- or double-stranded. It may also be linear or circular. While most viruses contain a single
nucleic acid, others have genomes divided into several segments. The RNA genome of the influenza virus is
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Chapter 211 Viruses
segmented, which contributes to its variability and continuous evolution, and explains why it is difficult to develop
a vaccine against it.
in DNA viruses, the viral DNA directs the host cell’s replication proteins to synthesize new copies of the viral
genome and to transcribe and translate that genome into viral proteins. Human diseases caused by DNA viruses
include chickenpox, hepatitis B, and adenoviruses. Sexually transmitted DNA viruses include the herpes virus
and the human papilloma virus (HPV), which has been associated with cervical cancer and genital warts.
RNA viruses contain only RNA as their genetic material. To replicate their genomes in the host cell, the RNA
viruses must encode their own enzymes that can replicate RNA into RNA or, in the retroviruses, into DNA. These
RNA polymerase enzymes are more likely to make copying errors than DNA polymerases, and therefore often
make mistakes during transcription. For this reason, mutations in RNA viruses occur more frequently than in
DNA viruses. This causes them to change and adapt more rapidly to their host. Human diseases caused by RNA
viruses include influenza, hepatitis C, measles, and rabies. The HIV virus, which is sexually transmitted, is an
RNA retrovirus.
The Challenge of Virus Classification
Because most viruses probably evolved from different ancestors, the systematic methods that scientists have
used to classify prokaryotic and eukaryotic cells are not very useful. If viruses represent “remnants” of different
organisms, then even genomic or protein analysis is not useful. Why?, Because viruses have no common
genomic sequence that they all share. For example, the 16S rRNA sequence so useful for constructing
prokaryote phylogenies is of no use for a creature with no ribosomes! Biologists have used several classification
systems in the past. Viruses were initially grouped by shared morphology. Later, groups of viruses were classified
by the type of nucleic acid they contained, DNA or RNA, and whether their nucleic acid was single- or double-
stranded. However, these earlier classification methods grouped viruses differently, because they were based
on different sets of characters of the virus. The most commonly used classification method today is called the
Baltimore classification scheme, and is based on how messenger RNA (mRNA) is generated in each particular
type of virus.
Past Systems of Classification
Viruses contain only a few elements by which they can be classified: the viral genome, the type of capsid, and
the envelope structure for the enveloped viruses. All of these elements have been used in the past for viral
classification (Table 21.1 and Figure 21.6). Viral genomes may vary in the type of genetic material (DNA or
RNA) and its organization (single- or double-stranded, linear or circular, and segmented or non-segmented). In
some viruses, additional proteins needed for replication are associated directly with the genome or contained
within the viral capsid.
Virus Classification by Genome Structure
Genome Structure Examples
RNA
Rabies virus, retroviruses
DNA
Herpesviruses, smallpox virus
Single-stranded
Rabies virus, retroviruses
Double-stranded
Herpesviruses, smallpox virus
Linear
Circular
Rabies virus, retroviruses, herpesviruses,
smallpox virus
Papillomaviruses, many bacteriophages
Non-segmented: genome consists of a single segment of
genetic material
Segmented: genome is divided into multiple segments
Parainfluenza viruses
Influenza viruses
Table 21.1
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(a) Rabies virus (b) Variola virus
Figure 21.6 Viruses can be classified according to their core genetic material and capsid design, (a) Rabies virus
has a single-stranded RNA (ssRNA) core and an enveloped helical capsid, whereas (b) variola virus, the causative
agent of smallpox, has a double-stranded DNA (dsDNA) core and a complex capsid. Rabies transmission occurs
when saliva from an infected mammal enters a wound. The virus travels through neurons in the peripheral nervous
system to the central nervous system, where it impairs brain function, and then travels to other tissues. The virus can
infect any mammal, and most die within weeks of infection. Smallpox is a human virus transmitted by inhalation of the
variola virus, localized in the skin, mouth, and throat, which causes a characteristic rash. Before its eradication in 1979,
infection resulted in a 30 to 35 percent mortality rate, (credit “rabies diagram": modification of work by CDC; “rabies
micrograph”: modification of work by Dr. Fred Murphy, CDC; credit “small pox micrograph": modification of work by Dr.
Fred Murphy, Sylvia Whitfield, CDC; credit “smallpox photo”: modification of work by CDC; scale-bar data from Matt
Russell)
Viruses can also be classified by the design of their capsids (Table 21.2 and Figure 21.7). Capsids are classified
as naked icosahedral, enveloped icosahedral, enveloped helical, naked helical, and complex. The type of
genetic material (DNA or RNA) and its structure (single- or double-stranded, linear or circular, and segmented or
non-segmented) are used to classify the virus core structures (Table 21.2).
Virus Classification by Capsid Structure
Capsid Classification
Examples
Naked icosahedral
Hepatitis A virus, polioviruses
Enveloped icosahedral
Epstein-Barr virus, herpes simplex virus, rubella
virus, yellow fever virus, HIV-1
Enveloped helical
Influenza viruses, mumps virus, measles virus,
rabies virus
Naked helical
Tobacco mosaic virus
Complex with many proteins; some have combinations of
icosahedral and helical capsid structures
Herpesviruses, smallpox virus, hepatitis B virus,
T4 bacteriophage
Table 21.2
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Chapter 211 Viruses
(d) (e)
Figure 21.7 Transmission electron micrographs of various viruses show their capsid structures. The capsid of the (a)
polio virus is naked icosahedral; (b) the Epstein-Barr virus capsid is enveloped icosahedral; (c) the mumps virus capsid
is an enveloped helix; (d) the tobacco mosaic virus capsid is naked helical; and (e) the herpesvirus capsid is complex,
(credit a: modification of work by Dr. Fred Murphy, Sylvia Whitfield; credit b: modification of work by Liza Gross; credit
c: modification of work by Dr. F. A. Murphy, CDC; credit d: modification of work by USDA ARS; credit e: modification of
work by Linda Stannard, Department of Medical Microbiology, University of Cape Town, South Africa, NASA; scale-bar
data from Matt Russell)
Baltimore Classification
The most commonly and currently used system of virus classification was first developed by Nobel Prize-winning
biologist David Baltimore in the early 1970s. In addition to the differences in morphology and genetics mentioned
above, the Baltimore classification scheme groups viruses according to how the mRNA is produced during the
replicative cycle of the virus.
Group I viruses contain double-stranded DNA (dsDNA) as their genome. Their mRNA is produced by
transcription in much the same way as with cellular DNA, using the enzymes of the host cell.
Group II viruses have single-stranded DNA (ssDNA) as their genome. They convert their single-stranded
genomes into a dsDNA intermediate before transcription to mRNA can occur.
Group III viruses use dsRNA as their genome. The strands separate, and one of them is used as a template for
the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus.
Group IV viruses have ssRNA as their genome with a positive polarity, which means that the genomic RNA can
serve directly as mRNA. Intermediates of dsRNA, called replicative intermediates, are made in the process of
copying the genomic RNA. Multiple, full-length RNA strands of negative polarity (complementary to the positive-
stranded genomic RNA) are formed from these intermediates, which may then serve as templates for the
production of RNA with positive polarity, including both full-length genomic RNA and shorter viral mRNAs.
Group V viruses contain ssRNA genomes with a negative polarity, meaning that their sequence is
complementary to the mRNA. As with Group IV viruses, dsRNA intermediates are used to make copies of the
genome and produce mRNA. In this case, the negative-stranded genome can be converted directly to mRNA.
Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-
stranded genome.
Group VI viruses have diploid (two copies) ssRNA genomes that must be converted, using the enzyme reverse
transcriptase, to dsDNA; the dsDNA is then transported to the nucleus of the host cell and inserted into the
host genome. Then, mRNA can be produced by transcription of the viral DNA that was integrated into the host
genome.
Group VII viruses have partial dsDNA genomes and make ssRNA intermediates that act as mRNA, but are also
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converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication.
The characteristics of each group in the Baltimore classification are summarized in Table 21.3 with examples of
each group.
Baltimore Classification
Group
Characteristics
Mode of mRNA Production
Example
1
Double-stranded
DNA
mRNA is transcribed directly from the DNA template
Herpes simplex
(herpesvirus)
II
Single-stranded
DNA
DNA is converted to double-stranded form before RNA is
transcribed
Canine
parvovirus
(parvovirus)
Double-stranded
RNA
Childhood
III
mRNA is transcribed from the RNA genome
gastroenteritis
(rotavirus)
IV
Single stranded
RNA (+)
Genome functions as mRNA
Common cold
(picornavirus)
V
Single stranded
RNA (-)
mRNA is transcribed from the RNA genome
Rabies
(rhabdovirus)
VI
Single stranded
RNA viruses with
reverse
transcriptase
Reverse transcriptase makes DNA from the RNA
genome; DNA is then incorporated in the host genome;
mRNA is transcribed from the incorporated DNA
Human
immunodeficiency
virus (HIV)
VII
Double stranded
DNA viruses with
reverse
transcriptase
The viral genome is double-stranded DNA, but viral DNA
is replicated through an RNA intermediate; the RNA may
serve directly as mRNA or as a template to make mRNA
Hepatitis B virus
(hepadnavirus)
Table 21.3
21.2 | Virus Infections and Hosts
By the end of this section, you will be able to do the following:
• List the steps of replication and explain what occurs at each step
• Describe the lytic and lysogenic cycles of virus replication
• Explain the transmission of plant and animal viruses
• Discuss some of the diseases caused by plant and animal viruses
• Discuss the economic impact of plant and animal viruses
Viruses are obligate, intracellular parasites. A virus must first recognize and attach to a specific living cell prior to
entering it. After penetration, the invading virus must copy its genome and manufacture its own proteins. Finally,
the progeny virions must escape the host cell so that they can infect other cells. Viruses can infect only certain
species of hosts and only certain cells within that host. Specific host cells that a virus must occupy and use
to replicate are called permissive. In most cases, the molecular basis for this specificity is due to a particular
surface molecule known as the viral receptor on the host cell surface. A specific viral receptor is required for
the virus to attach. In addition, differences in metabolism and host-cell immune responses (based on differential
gene expression) are a likely factor in determining which cells a virus may target for replication.
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Steps of Virus Infections
A virus must use its host-cell processes to replicate. The viral replication cycle can produce dramatic biochemical
and structural changes in the host cell, which may cause cell damage. These changes, called cytopathic
effects, can change cell functions or even destroy the cell. Some infected cells, such as those infected by the
common cold virus known as rhinovirus, die through lysis (bursting) or apoptosis (programmed cell death or
“cell suicide”), releasing all progeny virions at once. The symptoms of viral diseases result both from such cell
damage caused by the virus and from the immune response to the virus, which attempts to control and eliminate
the virus from the body.
Many animal viruses, such as HIV (human immunodeficiency virus), leave the infected cells of the immune
system by a process known as budding, where virions leave the cell individually. During the budding process,
the cell does not undergo lysis and is not immediately killed. However, the damage to the cells that the virus
infects may make it impossible for the cells to function normally, even though the cells remain alive for a period
of time. Most productive viral infections follow similar steps in the virus replication cycle: attachment, penetration,
uncoating, replication, assembly, and release (Figure 21.8).
Attachment
A virus attaches to a specific receptor site on the host cell membrane through attachment proteins in the capsid
or via glycoproteins embedded in the viral envelope. The specificity of this interaction determines the host—and
the cells within the host—that can be infected by a particular virus. This can be illustrated by thinking of several
keys and several locks, where each key will fit only one specific lock.
LINK TQ LEARNING
This video (http:// 0 penstaxc 0 llege. 0 rg/l/influenza) explains how influenza attacks the body.
Entry
Viruses may enter a host cell either with or without the viral capsid. The nucleic acid of bacteriophages enters
the host cell “naked,” leaving the capsid outside the cell. Plant and animal viruses can enter through endocytosis
(as you may recall, the cell membrane surrounds and engulfs the entire virus). Some enveloped viruses enter
the cell when the viral envelope fuses directly with the cell membrane. Once inside the cell, the viral capsid
degrades, and then the viral nucleic acid is released and becomes available for replication and transcription.
Replication and Assembly
The replication mechanism depends on the viral genome. DNA viruses usually use host-cell proteins and
enzymes to replicate the viral DNA and to transcribe viral mRNA, which is then used to direct viral protein
synthesis. RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA and mRNA.
The viral mRNA directs the host cell to synthesize viral enzymes and capsid proteins, and assemble new virions.
Of course, there are exceptions to this pattern. If a host cell does not provide the enzymes necessary for viral
replication, viral genes supply the information to direct synthesis of the missing proteins. Retroviruses, such as
HIV (group VI of the Baltimore classification scheme), have an RNA genome that must be reverse transcribed
into DNA, which then is incorporated into the host cell genome. To convert RNA into DNA, retroviruses must
contain genes that encode the virus-specific enzyme reverse transcriptase that transcribes an RNA template
to DNA. Reverse transcription never occurs in uninfected host cells—the enzyme reverse transcriptase is only
derived from the expression of viral genes within the infected host cells. The fact that HIV produces some of its
own enzymes not found in the host has allowed researchers to develop drugs that inhibit these enzymes without
affecting the host’s metabolism.
This approach has led to the development of a variety of drugs used to treat HIV and has been effective at
reducing the number of infectious virions (copies of viral RNA) in the blood to non-detectable levels in many
HIV-infected individuals.
Egress
The last stage of viral replication is the release of the new virions produced in the host organism, where they are
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able to infect adjacent cells and repeat the replication cycle. As you’ve learned, some viruses are released when
the host cell dies, and other viruses can leave infected cells by budding through the membrane without directly
killing the cell.
visual
CONNECTION
where it is replicated by the
viral RNA polymerase.
The cell, which is not killed in the
process, continues to make new virus.
Figure 21.8 The influenza reproductive cycle. In influenza virus infection, glycoproteins on the capsid attach to a
host epithelial cell. Following this, the virus is engulfed. RNA and proteins are then made and assembled into new
virions.
Influenza virus is packaged in a viral envelope that fuses with the plasma membrane. This way, the virus
can exit the host cell without killing it. What advantage does the virus gain by keeping the host cell alive?
LINK
T a
LEARNING
Watch a video (https://www.khanacademy.org/science/biology/her/tree-of-lifeMviruses) on viruses,
identifying structures, modes of transmission, replication, and more.
Different Hosts and Their Viruses
As you’ve learned, viruses often infect very specific hosts, as well as specific cells within the host. This feature
of a virus makes it specific to one or a few species of life on Earth. On the other hand, so many different types of
viruses exist on Earth that nearly every living organism has its own set of viruses trying to infect its cells. Even
prokaryotes, the smallest and simplest of cells, may be attacked by specific types of viruses. In the following
section, we will look at some of the features of viral infection of prokaryotic cells. As we have learned, viruses
that infect bacteria are called bacteriophages (Figure 21.9). Archaea have their own similar viruses.
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Bacteriophages
Figure 21.9 Bacteriophages attached to a host cell (transmission electron micrograph). In bacteriophage with tails, like
the one shown here, the tails serve as a passageway for transmission of the phage genome, (credit: modification of
work by Dr. Graham Beards; scale-bar data from Matt Russell)
Most bacteriophages are dsDNA viruses, which use host enzymes for DNA replication and RNA transcription.
Phage particles must bind to specific surface receptors and actively insert the genome into the host cell. (The
complex tail structures seen in many bacteriophages are actively involved in getting the viral genome across
the prokaryotic cell wall.) When infection of a cell by a bacteriophage results in the production of new virions,
the infection is said to be productive. If the virions are released by bursting the cell, the virus replicates by
means of a lytic cycle (Figure 21.10). An example of a lytic bacteriophage is T4, which infects Escherichia
coli found in the human intestinal tract. Sometimes, however, a virus can remain within the cell without being
released. For example, when a temperate bacteriophage infects a bacterial cell, it replicates by means of a
lysogenic cycle (Figure 21.10), and the viral genome is incorporated into the genome of the host cell. When
the phage DNA is incorporated into the host-cell genome, it is called a prophage. An example of a lysogenic
bacteriophage is the A (lambda) virus, which also infects the E. coli bacterium. Viruses that infect plant or animal
cells may sometimes undergo infections where they are not producing virions for long periods. An example is
the animal herpesviruses, including herpes simplex viruses, the cause of oral and genital herpes in humans. In a
process called latency, these viruses can exist in nervous tissue for long periods of time without producing new
virions, only to leave latency periodically and cause lesions in the skin where the virus replicates. Even though
there are similarities between lysogeny and latency, the term lysogenic cycle is usually reserved to describe
bacteriophages. Latency will be described in more detail in the next section.
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CONNECTION
Lytic cycle
Host bacterial cell
The phage infects a cell. The phage ONA circularizes
Phage DNA replicates and The cell lyses, releasing
phage proteins are made. phage.
New phage particles are
assembled. .
remaining separate from
the host DNA.
Lysogenic cycle
t
The phage infects a cell. The phage DNA becomes
The cell divides, and
prophage DNA is passed
on to daughter cells.
Under stressful conditions,
the phage DNA is excised
from the bacterial
chromosome and enters
the lytic cycle.
incorporated into the host
genome.
Figure 21.10 A temperate bacteriophage has both lytic and lysogenic cycles. In the lytic cycle, the phage
replicates and lyses the host cell. In the lysogenic cycle, phage DNA is incorporated into the host genome,
where it is passed on to subsequent generations. Environmental stressors such as starvation or exposure to toxic
chemicals may cause the prophage to excise and enter the lytic cycle.
Which of the following statements is false?
a. In the lytic cycle, new phages are produced and released into the environment.
b. In the lysogenic cycle, phage DNA is incorporated into the host genome.
c. An environmental stressor can cause the phage to initiate the lysogenic cycle.
d. Cell lysis only occurs in the lytic cycle.
Plant Viruses
Most plant viruses, like the tobacco mosaic virus, have single-stranded (+) RNA genomes. However, there
are also plant viruses in most other virus categories. Unlike bacteriophages, plant viruses do not have active
mechanisms for delivering the viral genome across the protective cell wall. For a plant virus to enter a new host
plant, some type of mechanical damage must occur. This damage is often caused by weather, insects, animals,
fire, or human activities like farming or landscaping. Movement from cell to cell within a plant can be facilitated by
viral modification of plasmodesmata (cytoplasmic threads that pass from one plant cell to the next). Additionally,
plant offspring may inherit viral diseases from parent plants. Plant viruses can be transmitted by a variety of
vectors, through contact with an infected plant’s sap, by living organisms such as insects and nematodes, and
through pollen. The transfer of a virus from one plant to another is known as horizontal transmission, whereas
the inheritance of a virus from a parent is called vertical transmission.
Symptoms of viral diseases vary according to the virus and its host (Table 21.4). One common symptom is
hyperplasia, the abnormal proliferation of cells that causes the appearance of plant tumors known as galls.
Other viruses induce hypoplasia, or decreased cell growth, in the leaves of plants, causing thin, yellow areas to
appear. Still other viruses affect the plant by directly killing plant cells, a process known as cell necrosis. Other
symptoms of plant viruses include malformed leaves; black streaks on the stems of the plants; altered growth of
stems, leaves, or fruits; and ring spots, which are circular or linear areas of discoloration found in a leaf.
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Some Common Symptoms of Plant Viral Diseases
Symptom
Appears as
Hyperplasia
Galls (tumors)
Hypoplasia
Thinned, yellow splotches on leaves
Cell necrosis
Dead, blackened stems, leaves, or fruit
Abnormal growth patterns
Malformed stems, leaves, or fruit
Discoloration
Yellow, red, or black lines, or rings in stems, leaves, or fruit
Table 21.4
Plant viruses can seriously disrupt crop growth and development, significantly affecting our food supply. They
are responsible for poor crop quality and quantity globally, and can bring about huge economic losses annually.
Others viruses may damage plants used in landscaping. Some viruses that infect agricultural food plants include
the name of the plant they infect, such as tomato spotted wilt virus, bean common mosaic virus, and cucumber
mosaic virus. In plants used for landscaping, two of the most common viruses are peony ring spot and rose
mosaic virus. There are far too many plant viruses to discuss each in detail, but symptoms of bean common
mosaic virus result in lowered bean production and stunted, unproductive plants. In the ornamental rose, the
rose mosaic disease causes wavy yellow lines and colored splotches on the leaves of the plant.
Animal Viruses
Animal viruses, unlike the viruses of plants and bacteria, do not have to penetrate a cell wall to gain access to
the host cell. The virus may even induce the host cell to cooperate in the infection process. Non-enveloped or
“naked” animal viruses may enter cells in two different ways. As a protein in the viral capsid binds to its receptor
on the host cell, the virus may be taken inside the cell via a vesicle during the normal cell process of receptor-
mediated endocytosis. An alternative method of cell penetration used by non-enveloped viruses is for capsid
proteins to undergo shape changes after binding to the receptor, creating channels in the host cell membrane.
The viral genome is then “injected” into the host cell through these channels in a manner analogous to that used
by many bacteriophages.
Enveloped viruses also have two ways of entering cells after binding to their receptors: receptor-mediated
endocytosis, or fusion. Many enveloped viruses enter the cell by receptor-mediated endocytosis in a fashion
similar to that seen in some non-enveloped viruses. On the other hand, fusion only occurs with enveloped
virions. These viruses, which include HIV among others, use special fusion proteins in their envelopes to cause
the envelope to fuse with the plasma membrane of the cell, thus releasing the genome and capsid of the virus
into the cell cytoplasm.
After making their proteins and copying their genomes, animal viruses complete the assembly of new virions
and exit the cell. As we have already discussed using the example the influenza virus, enveloped animal viruses
may bud from the cell membrane as they assemble themselves, taking a piece of the cell’s plasma membrane in
the process. On the other hand, non-enveloped viral progeny, such as rhinoviruses, accumulate in infected cells
until there is a signal for lysis or apoptosis, and all virions are released together.
As you will learn in the next module, animal viruses are associated with a variety of human diseases. Some
of them follow the classic pattern of acute disease, where symptoms get increasingly worse for a short period
followed by the elimination of the virus from the body by the immune system and eventual recovery from the
infection. Examples of acute viral diseases are the common cold and influenza. Other viruses cause long-term
chronic infections, such as the virus causing hepatitis C, whereas others, like herpes simplex virus, only cause
intermittent symptoms. Still other viruses, such as human herpesviruses 6 and 7, which in some cases can
cause the minor childhood disease roseola, often successfully cause productive infections without causing any
symptoms at all in the host, and thus we say these patients have an asymptomatic infection.
In hepatitis C infections, the virus grows and reproduces in liver cells, causing low levels of liver damage. The
damage is so low that infected individuals are often unaware that they are infected, and many infections are
detected only by routine blood work on patients with risk factors such as intravenous drug use. On the other
hand, since many of the symptoms of viral diseases are caused by immune responses, a lack of symptoms
is an indication of a weak immune response to the virus. This allows the virus to escape elimination by the
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immune system and persist in individuals for years, all the while producing low levels of progeny virions in what
is known as a chronic viral disease. Chronic infection of the liver by this virus leads to a much greater chance of
developing liver cancer, sometimes as much as 30 years after the initial infection.
As already discussed, herpes simplex virus can remain in a state of latency in nervous tissue for months, even
years. As the virus “hides” in the tissue and makes few if any viral proteins, there is nothing for the immune
response to act against, and immunity to the virus slowly declines. Under certain conditions, including various
types of physical and psychological stress, the latent herpes simplex virus may be reactivated and undergo a
lytic replication cycle in the skin, causing the lesions associated with the disease. Once virions are produced
in the skin and viral proteins are synthesized, the immune response is again stimulated and resolves the skin
lesions in a few days or weeks by destroying viruses in the skin. As a result of this type of replicative cycle,
appearances of cold sores and genital herpes outbreaks only occur intermittently, even though the viruses
remain in the nervous tissue for life. Latent infections are common with other herpesviruses as well, including
the varicella-zoster virus that causes chickenpox. After having a chickenpox infection in childhood, the varicella-
zoster virus can remain latent for many years and reactivate in adults to cause the painful condition known as
“shingles" (Figure 21.11).
Figure 21.11 A latent virus infection, (a) Varicella-zoster, the virus that causes chickenpox, has an enveloped
icosahedral capsid visible in this transmission electron micrograph. Its double-stranded DNA genome becomes
incorporated in the host DNA and can reactivate after latency in the form of (b) shingles, often exhibiting a rash, (credit
a: modification of work by Dr. Erskine Palmer, B. G. Martin, CDC; credit b: modification of work by “rosmary'VFlickr;
scale-bar data from Matt Russell)
Some animal-infecting viruses, including the hepatitis C virus discussed above, are known as oncogenic
viruses: They have the ability to cause cancer. These viruses interfere with the normal regulation of the host
cell cycle either by introducing genes that stimulate unregulated cell growth (oncogenes) or by interfering with
the expression of genes that inhibit cell growth. Oncogenic viruses can be either DNA or RNA viruses. Cancers
known to be associated with viral infections include cervical cancer, caused by human papillomavirus (HPV)
(Figure 21.12), liver cancer caused by hepatitis B virus, T-cell leukemia, and several types of lymphoma.
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Figure 21.12 HPV, or human papillomavirus, has a naked icosahedral capsid visible in this transmission electron
micrograph and a double-stranded DNA genome that is incorporated into the host DNA. The virus, which is sexually
transmitted, is oncogenic and can lead to cervical cancer, (credit: modification of work by NCI, NIH; scale-bar data from
Matt Russell)
Visit the interactive animations (http:// 0 penstaxc 0 llege. 0 rg/l/animaLviruses) showing the various stages
of the replicative cycles of animal viruses and click on the flash animation links.
21.3 | Prevention and Treatment of Viral Infections
By the end of this section, you will be able to do the following:
• Identify major viral illnesses that affect humans
• Compare vaccinations and anti-viral drugs as medical approaches to viruses
Viruses cause a variety of diseases in animals, including humans, ranging from the common cold to potentially
fatal illnesses like meningitis (Figure 21.13). These diseases can be treated by antiviral drugs or by vaccines;
however, some viruses, such as HIV, are capable both of avoiding the immune response and of mutating within
the host organism to become resistant to antiviral drugs.
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Overview of Viral Infections
Common
- Rhinoviruses
- Parainfluenza virus
- Respiratory syncytial
virus
Encephalitis/
meningitis
- JC virus
- Measles
- LCM virus
- Arbovirus
- Rabies
Pharyngitis
- Adenovirus ’ Her P es sim P lex ‘VP e 1
■ Epstein-Barr virus
- Cytomegalovirus
Cardiovascular
- Coxsackie B virus
Hepatitis
- Hepatitis virus
types A, B, C, D, and E
Skin infections
- Varicella-zoster virus
- Human herpesvirus 6
- Smallpox
- Molluscum contagiosum
- Human papillomavirus
- Parvovirus B19
- Rubella
- Measles
- Coxsackie A virus
Sexually transmitted
diseases
- Herpes simplex type 2
- Human papillomavirus
- HIV
infections
- Herpes simplex virus
- Adenovirus
- Cytomegalovirus
Parotitis /Pneumonia
- Mumps/ - Influenza virus
virus / types A and B
- Parainfluenza
virus
- Respiratory
syncytial virus
- Adenovirus
- SARS coronavirus
Myelitis
- Poliovirus
- HTLV-I
Gastroenteritis
- Adenovirus
- Rotavirus
- Norovirus
- Astrovirus
- Coronavirus
Pancreatitis
- Coxsackie B virus
Figure 21.13 A sampling of human viruses. Viruses can cause dozens of ailments in humans, ranging from mild
illnesses to serious diseases, (credit: modification of work by Mikael Haggstrom)
Vaccines for Prevention
The primary method of controlling viral disease is by vaccination, which is intended to prevent outbreaks by
building immunity to a virus or virus family (Figure 21.14). Vaccines may be prepared using live viruses, killed
viruses, or molecular subunits of the virus. Note that the killed viral vaccines and subunit viruses are both
incapable of causing disease, nor is there any valid evidence that vaccinations contribute to autism.
Live viral vaccines are designed in the laboratory to cause few symptoms in recipients while giving them
protective immunity against future infections. Polio was one disease that represented a milestone in the use
of vaccines. Mass immunization campaigns in the 1950s (killed vaccine) and 1960s (live vaccine) significantly
reduced the incidence of the disease, which caused muscle paralysis in children and generated a great amount
of fear in the general population when regional epidemics occurred. The success of the polio vaccine paved the
way for the routine dispensation of childhood vaccines against measles, mumps, rubella, chickenpox, and other
diseases.
The issue with using live vaccines (which are usually more effective than killed vaccines), is the low but
significant danger that these viruses will revert to their disease-causing form by back mutations. Live vaccines
are usually made by attenuating (weakening) the “wild-type” (disease-causing) virus by growing it in the
laboratory in tissues or at temperatures different from what the virus is accustomed to in the host. Adaptations
to these new cells or temperatures induce mutations in the genomes of the virus, allowing it to grow better in the
laboratory while inhibiting its ability to cause disease when reintroduced into conditions found in the host. These
attenuated viruses thus still cause infection, but they do not grow very well, allowing the immune response to
develop in time to prevent major disease. Back mutations occur when the vaccine undergoes mutations in the
host such that it readapts to the host and can again cause disease, which can then be spread to other humans in
an epidemic. This type of scenario happened as recently as 2007 in Nigeria where mutations in a polio vaccine
led to an epidemic of polio in that country.
Some vaccines are in continuous development because certain viruses, such as influenza and HIV, have a
high mutation rate compared to that of other viruses and normal host cells. With influenza, mutations in the
surface molecules of the virus help the organism evade the protective immunity that may have been obtained
in a previous influenza season, making it necessary for individuals to get vaccinated every year. Other viruses,
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Chapter 211 Viruses
such as those that cause the childhood diseases measles, mumps, and rubella, mutate so infrequently that the
same vaccine is used year after year.
Figure 21.14 Vaccinations are designed to boost immunity to a virus to prevent infection, (credit: USACE Europe
District)
Watch this NOVA video (http://openstaxcollege.Org/l/1918_flu) to learn how microbiologists are attempting
to replicate the deadly 1918 Spanish influenza virus so they can understand more about virology.
Vaccines and Antiviral Drugs for Treatment
in some cases, vaccines can be used to treat an active viral infection. The concept behind this is that by giving
the vaccine, immunity is boosted without adding more disease-causing virus. In the case of rabies , a fatal
neurological disease transmitted via the saliva of rabies virus-infected animals, the progression of the disease
from the time of the animal bite to the time it enters the central nervous system may be two weeks or longer.
This is enough time to vaccinate individuals who suspect that they have been bitten by a rabid animal, and their
boosted immune response is sufficient to prevent the virus from entering nervous tissue. Thus, the potentially
fatal neurological consequences of the disease are averted, and the individual only has to recover from the
infected bite. This approach is also being used for the treatment of Ebola, one of the fastest and most deadly
viruses on Earth. Transmitted by bats and great apes, this disease can cause death in 70 to 90 percent of
infected humans within two weeks. Using newly developed vaccines that boost the immune response in this
way, there is hope that affected individuals will be better able to control the virus, potentially saving a greater
percentage of infected persons from a rapid and very painful death.
Another way of treating viral infections is the use of antiviral drugs. Because viruses use the resources of the
host cell for replication and the production of new virus proteins, it is difficult to block their activities without
damaging the host. However, we do have some effective antiviral drugs, such as those used to treat HIV and
influenza. Some antiviral drugs are specific for a particular virus and others have been used to control and
reduce symptoms for a wide variety of viral diseases. For most viruses, these drugs can inhibit the virus by
blocking the actions of one or more of its proteins. It is important to note that the targeted proteins be encoded
by viral genes and that these molecules are not present in a healthy host cell. In this way, viral growth is inhibited
without damaging the host.
Antivirals have been developed to treat genital herpes (herpes simplex II) and influenza. For genital herpes,
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drugs such as acyclovir can reduce the number and duration of episodes of active viral disease, during which
patients develop viral lesions in their skin cells. As the virus remains latent in nervous tissue of the body for life,
this drug is not curative but can make the symptoms of the disease more manageable. For influenza, drugs like
Tamiflu (oseltamivir) (Figure 21.15) can reduce the duration of “flu” symptoms by one or two days, but the drug
does not prevent symptoms entirely. Tamiflu works by inhibiting an enzyme (viral neuraminidase) that allows
new virions to leave their infected cells. Thus, Tamiflu inhibits the spread of virus from infected to uninfected
cells. Other antiviral drugs, such as Ribavirin, have been used to treat a variety of viral infections, although its
mechanism of action against certain viruses remains unclear.
Figure 21.15 Action of an antiviral drug, (a) Tamiflu inhibits a viral enzyme called neuraminidase (NA) found in the
influenza viral envelope, (b) Neuraminidase cleaves the connection between viral hemagglutinin (HA), also found in the
viral envelope, and glycoproteins on the host cell surface. Inhibition of neuraminidase prevents the virus from detaching
from the host cell, thereby blocking further infection, (credit a: modification of work by M. Eickmann)
By far, the most successful use of antivirals has been in the treatment of the retrovirus HIV, which causes a
disease that, if untreated, is usually fatal within 10 to 12 years after infection. Anti-HIV drugs have been able to
control viral replication to the point that individuals receiving these drugs survive for a significantly longer time
than the untreated.
Anti-HIV drugs inhibit viral replication at many different phases of the HIV replicative cycle (Figure 21.16). Drugs
have been developed that inhibit the fusion of the HIV viral envelope with the plasma membrane of the host cell
(fusion inhibitors), the conversion of its RNA genome into double-stranded DNA (reverse transcriptase inhibitors,
like AZT), the integration of the viral DNA into the host genome (integrase inhibitors), and the processing of viral
proteins (protease inhibitors).
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Chapter 211 Viruses
Preintegration
complex
Host immune cell
* Viral DNAis
formed by reverse
transcription.
—Viral RNA
~~—Reverse
transcriptase
w Viral DNAis
transported across the
nucleus and integrates
into the host ONA
* ntegrase
•Viral DNA
V New viral RNA is
used as genomic RNA
and to make viral
proteins.
Co-receptor
HIV fuses to
the host cell
surface.
r HIV RNA, reverse
transcriptase, mtegrase
and other viral proteins
enter the host cell
HIV
gp120
(OCRS or CXCR4)
Mature virion
' The virus matures
by protease,
releasing individual
HIV proteins.
' New viral HNA ^
and proteins move to
the cell surface and a
new, immature HIV
forms.
Figure 21.16 Life cycle of HIV. HIV, an enveloped, icosahedral virus, attaches to the CD4 receptor of an immune cell
and fuses with the cell membrane. Viral contents are released into the cell, where viral enzymes convert the single-
stranded RNA genome into DNA and incorporate it into the host genome, (credit: NIAID, NIH)
Unfortunately, when any of these drugs are used individually, the high mutation rate of the virus allows it to easily
and rapidly develop resistance to the drug, limiting the drug’s effectiveness. The breakthrough in the treatment
of HIV was the development of HAART, highly active anti-retroviral therapy , which involves a mixture of different
drugs, sometimes called a drug “cocktail." By attacking the virus at different stages of its replicative cycle, it is
much more difficult for the virus to develop resistance to multiple drugs at the same time. Still, even with the
use of combination HAART therapy, there is concern that, over time, the virus will develop resistance to this
therapy. Thus, new anti-HIV drugs are constantly being developed with the hope of continuing the battle against
this highly fatal virus.
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everyday CONNECTION
Applied Virology:
The study of viruses has led to the development of a variety of new ways to treat non-viral diseases.
Viruses have been used in gene therapy. Gene therapy is used to treat genetic diseases such as severe
combined immunodeficiency (SCID), a heritable, recessive disease in which children are born with severely
compromised immune systems. One common type of SCID is due to the lack of an enzyme, adenosine
deaminase (ADA), which breaks down purine bases. To treat this disease by gene therapy, bone marrow
cells are taken from a SCID patient and the ADA gene is inserted. This is where viruses come in, and
their use relies on their ability to penetrate living cells and bring genes in with them. Viruses such as
adenovirus, an upper-respiratory human virus, are modified by the addition of the ADA gene, and the virus
then transports this gene into the cell. The modified cells, now capable of making ADA, are then given
back to the patients in the hope of curing them. Gene therapy using viruses as carriers of genes (viral
vectors), although still experimental, holds promise for the treatment of many genetic diseases. Still, many
technological problems need to be solved for this approach to be a viable method for treating genetic
disease.
Another medical use for viruses relies on their specificity and ability to kill the cells they infect. Oncolytic
viruses are engineered in the laboratory specifically to attack and kill cancer cells. A genetically modified
adenovirus known as H101 has been used since 2005 in clinical trials in China to treat head and neck
cancers. The results have been promising, with a greater short-term response rate to the combination of
chemotherapy and viral therapy than to chemotherapy treatment alone. This ongoing research may herald
the beginning of a new age of cancer therapy, where viruses are engineered to find and specifically kill
cancer cells, regardless of where in the body they may have spread.
A third use of viruses in medicine relies on their specificity and involves using bacteriophages in the
treatment of bacterial infections. Bacterial diseases have been treated with antibiotics since the 1940s.
However, over time, many bacteria have evolved resistance to antibiotics. A good example is methicillin-
resistant Staphylococcus aureus (MRSA, pronounced “mersa"), an infection commonly acquired in
hospitals. This bacterium is resistant to a variety of antibiotics, making it difficult to treat. The use of
bacteriophages specific for such bacteria would bypass their resistance to antibiotics and specifically kill
them. Although phage therapy is in use in the Republic of Georgia to treat antibiotic-resistant bacteria,
its use to treat human diseases has not been approved in most countries. However, the safety of the
treatment was confirmed in the United States when the U.S. Food and Drug Administration approved
spraying meats with bacteriophages to destroy the food pathogen Listeria. As more and more antibiotic-
resistant strains of bacteria evolve, the use of bacteriophages might be a potential solution to the problem,
and the development of phage therapy is of much interest to researchers worldwide.
21.4 | Other Acellular Entities: Prions and Viroids
By the end of this section, you will be able to do the following:
• Describe prions and their basic properties
• Define viroids and their targets of infection
Prions and viroids are pathogens (agents with the ability to cause disease) that have simpler structures than
viruses but, in the case of prions, still can produce deadly diseases.
Prions
Prions, so-called because they are proteinaceous, are infectious particles—smaller than viruses—that contain
no nucleic acids (neither DNA nor RNA). Historically, the idea of an infectious agent that did not use nucleic
acids was considered impossible, but pioneering work by Nobel Prize-winning biologist Stanley Prusiner has
convinced the majority of biologists that such agents do indeed exist.
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Chapter 211 Viruses
Fatal neurodegenerative diseases, such as kuru in humans and bovine spongiform encephalopathy (BSE) in
cattle (commonly known as “mad cow disease") were shown to be transmitted by prions. The disease was
spread by the consumption of meat, nervous tissue, or internal organs between members of the same species.
Kuru, native to humans in Papua New Guinea, was spread from human to human via ritualistic cannibalism.
BSE, originally detected in the United Kingdom, was spread between cattle by the practice of including cattle
nervous tissue in feed for other cattle. Individuals with kuru and BSE show symptoms of loss of motor control
and unusual behaviors, such as uncontrolled bursts of laughter with kuru, followed by death. Kuru was controlled
by inducing the population to abandon its ritualistic cannibalism.
On the other hand, BSE was initially thought to only affect cattle. Cattle dying of the disease were shown to have
developed lesions or “holes” in the brain, causing the brain tissue to resemble a sponge. Later on in the outbreak,
however, it was shown that a similar encephalopathy in humans, known as variant Creutzfeldt-Jakob disease
(CJD), could be acquired from eating beef from animals infected with BSE, sparking bans by various countries
on the importation of British beef and causing considerable economic damage to the British beef industry (Figure
21.17). BSE still exists in various areas, and although a rare disease, individuals that acquire CJD are difficult
to treat. The disease can be spread from human to human by blood, so many countries have banned blood
donation from regions associated with BSE.
The cause of spongiform encephalopathies, such as kuru and BSE, is an infectious structural variant of a normal
cellular protein called PrP (prion protein). It is this variant that constitutes the prion particle. PrP exists in two
forms, PrP c , the normal form of the protein, and PrP sc , the infectious form. Once introduced into the body, the
PrP sc contained within the prion binds to PrP c and converts it to PrP sc . This leads to an exponential increase
of the PrP sc protein, which aggregates. PrP sc is folded abnormally, and the resulting conformation (shape)
is directly responsible for the lesions seen in the brains of infected cattle. Thus, although not without some
detractors among scientists, the prion seems likely to be an entirely new form of infectious agent, the first one
found whose transmission is not reliant upon genes made of DNA or RNA.
° ° o
Endogenous PrP c q O q
O °,
PrP sc
Spontaneous
generation of PrP sc
Conversion of mutant
PrP into PrP sc
Inoculation of PrP sc
Interaction
between
PrP c and
PrpSC
••
Conversion of PrP into PrP sc
* *•
& Accumulation of PrP sc
• •
(a) (b)
Figure 21.17 Mad Cow Disease in humans, (a) Endogenous normal prion protein (PrP c ) is converted into the disease-
causing form (PrP sc ) when it encounters this variant form of the protein. PrP sc may arise spontaneously in brain tissue,
especially if a mutant form of the protein is present, or it may occur via the spread of misfolded prions consumed in
food into brain tissue, (b) This prion-infected brain tissue, visualized using light microscopy, shows the vacuoles that
give it a spongy texture, typical of transmissible spongiform encephalopathies, (credit b: modification of work by Dr. Al
Jenny, USDA APHIS; scale-bar data from Matt Russell)
Viroids
Viroids are plant pathogens: small, single-stranded, circular RNA particles that are much simpler than a virus.
They do not have a capsid or outer envelope, but like viruses can reproduce only within a host ceil. Viroids
do not, however, manufacture any proteins, and they only produce a single, specific RNA molecule. Human
diseases caused by viroids have yet to be identified.
Viroids are known to infect plants (Figure 21.18) and are responsible for crop failures and the loss of millions of
dollars in agricultural revenue each year. Some of the plants they infect include potatoes, cucumbers, tomatoes,
chrysanthemums, avocados, and coconut palms.
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Figure 21.18 These potatoes have been infected by the potato spindle tuber viroid (PSTV), which is typically spread
when infected knives are used to cut healthy potatoes, which are then planted, (credit: Pamela Roberts, University of
Florida Institute of Food and Agricultural Sciences, USDA ARS)
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Chapter 211 Viruses
ca eer connection
Virologist
Virology is the study of viruses, and a virologist is an individual trained in this discipline. Training in virology
can lead to many different career paths. Virologists are actively involved in academic research and teaching
in colleges and medical schools. Some virologists treat patients or are involved in the generation and
production of vaccines. They might participate in epidemiologic studies (Figure 21.19) or become science
writers, to name just a few possible careers.
Figure 21.19 This virologist is engaged in fieldwork, sampling eggs from this nest for avian influenza, (credit: Don
Becker, USGS EROS, U.S. Fish and Wildlife Service)
If you think you may be interested in a career in virology, find a mentor in the field. Many large medical
centers have departments of virology, and smaller hospitals usually have virology labs within their
microbiology departments. Volunteer in a virology lab for a semester or work in one over the summer.
Discussing the profession and getting a first-hand look at the work will help you decide whether a career in
virology is right for you. The American Society of Virology’s website (http:// 0 penstaxc 0 llege. 0 rg/l/asv) is
a good resource for information regarding training and careers in virology.
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KEY TERMS
acellular lacking cells
acute disease disease where the symptoms rise and fall within a short period of time
asymptomatic disease disease where there are no symptoms and the individual is unaware of being infected
unless lab tests are performed
attenuation weakening of a virus during vaccine development
AZT anti-HIV drug that inhibits the viral enzyme reverse transcriptase
back mutation when a live virus vaccine reverts back to it disease-causing phenotype
bacteriophage virus that infects bacteria
budding method of exit from the cell used in certain animal viruses, where virions leave the cell individually by
capturing a piece of the host plasma membrane
capsid protein coating of the viral core
capsomere protein subunit that makes up the capsid
cell necrosis cell death
chronic infection describes when the virus persists in the body for a long period of time
cytopathic causing cell damage
envelope lipid bilayer that encircles some viruses
fusion method of entry by some enveloped viruses, where the viral envelope fuses with the plasma membrane
of the host cell
gall appearance of a plant tumor
gene therapy treatment of genetic disease by adding genes, using viruses to carry the new genes inside the
cell
group I virus virus with a dsDNA genome
group II virus virus with an ssDNA genome
group III virus virus with a dsRNA genome
group IV virus virus with an ssRNA genome with positive polarity
group V virus virus with an ssRNA genome with negative polarity
group VI virus virus with an ssRNA genome converted into dsDNA by reverse transcriptase
group VII virus virus with a single-stranded mRNA converted into dsDNA for genome replication
horizontal transmission transmission of a disease between unrelated individuals
hyperplasia abnormally high cell growth and division
hypoplasia abnormally low cell growth and division
intermittent symptom symptom that occurs periodically
latency virus that remains in the body for a long period of time but only causes intermittent symptoms
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Chapter 211 Viruses
lysis bursting of a cell
lysogenic cycle type of virus replication in which the viral genome is incorporated into the genome of the host
cell
lytic cycle type of virus replication in which virions are released through lysis, or bursting, of the cell
matrix protein envelope protein that stabilizes the envelope and often plays a role in the assembly of progeny
virions
negative polarity ssRNA viruses with genomes complementary to their mRNA
oncogenic virus virus that has the ability to cause cancer
oncolytic virus virus engineered to specifically infect and kill cancer cells
pathogen agent with the ability to cause disease
permissive cell type that is able to support productive replication of a virus
phage therapy treatment of bacterial diseases using bacteriophages specific to a particular bacterium
positive polarity ssRNA virus with a genome that contains the same base sequences and codons found in their
mRNA
prion infectious particle that consists of proteins that replicate without DNA or RNA
productive viral infection that leads to the production of new virions
prophage phage DNA that is incorporated into the host cell genome
p r p c normal prion protein
p r psc infectious form of a prion protein
replicative intermediate dsRNA intermediate made in the process of copying genomic RNA
reverse transcriptase enzyme found in Baltimore groups VI and VII that converts single-stranded RNA into
double-stranded DNA
vaccine weakened solution of virus components, viruses, or other agents that produce an immune response
vertical transmission transmission of disease from parent to offspring
viral receptor glycoprotein used to attach a virus to host cells via molecules on the cell
virion individual virus particle outside a host cell
viroid plant pathogen that produces only a single, specific RNA
virus core contains the virus genome
CHAPTER SUMMARY
21.1 Viral Evolution, Morphology, and Classification
Viruses are tiny, noncellular entities that usually can be seen only with an electron microscope. Their genomes
contain either DNA or RNA—never both—and they replicate either by using the replication proteins of a host
cell or by using proteins encoded in the viral genome. Viruses are diverse, infecting archaea, bacteria, fungi,
plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an
outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements
of the classification of viruses. The most commonly used classification method, the Baltimore classification,
categorizes viruses based on how they produce their mRNA.
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21.2 Virus Infections and Hosts
Plant viruses may be transmitted either vertically from parent reproductive cells or horizontally through
damaged plant tissues. Viruses of plants are responsible for significant economic damage in both crop plants
and plants used for ornamentation. Animal viruses enter their hosts through several types of virus-host cell
interactions and cause a variety of infections. Viral infections can be either acute, with a brief period of infection
terminated by host immune responses, or chronic, in which the infection persists. Persistent infections may
cause chronic symptoms (hepatitis C), intermittent symptoms (latent viruses such a herpes simplex virus 1), or
even be effectively asymptomatic (human herpesviruses 6 and 7). Oncogenic viruses in animals have the
ability to cause cancer by interfering with the regulation of the host cell cycle.
21.3 Prevention and Treatment of Viral Infections
Viruses cause a variety of diseases in humans. Many of these diseases can be prevented by the use of viral
vaccines, which stimulate protective immunity against the virus without causing major disease. Viral vaccines
may also be used in active viral infections, boosting the ability of the immune system to control or destroy the
virus. A series of antiviral drugs that target enzymes and other protein products of viral genes have been
developed and used with mixed success. Combinations of anti-HIV drugs have been used to effectively control
the virus, extending the lifespans of infected individuals. Viruses have many uses in medicines, such as in the
treatment of genetic disorders, cancer, and bacterial infections.
21.4 Other Acellular Entities: Prions and Viroids
Prions are infectious agents that consist of protein, but no DNA or RNA, and seem to produce their deadly
effects by duplicating their shapes and accumulating in tissues. They are thought to contribute to several
progressive brain disorders, including mad cow disease and Creutzfeldt-Jakob disease. Viroids are single-
stranded RNA pathogens that infect plants. Their presence can have a severe impact on the agriculture
industry.
VISUAL CONNECTION QUESTIONS
1. Figure 21.5 Which of the following statements
about virus structure is true?
a. All viruses are encased in a viral membrane.
b. The capsomere is made up of small protein
subunits called capsids.
c. DNA is the genetic material in all viruses.
d. Glycoproteins help the virus attach to the
host cell.
2. Figure 21.8 Influenza virus is packaged in a viral
envelope that fuses with the plasma membrane. This
way, the virus can exit the host cell without killing it.
REVIEW QUESTIONS
4. Which statement is true?
a. A virion contains DNA and RNA.
b. Viruses are acellular.
c. Viruses replicate outside of the cell.
d. Most viruses are easily visualized with a
light microscope.
5. The viral_play(s) a role in attaching a
virion to the host cell.
a. core
b. capsid
c. envelope
d. both b and c
6. Viruses_.
What advantage does the virus gain by keeping the
host cell alive?
3. Figure 21.10 Which of the following statements is
false?
a. In the lytic cycle, new phages are produced
and released into the environment.
b. In the lysogenic cycle, phage DNA is
incorporated into the host genome.
c. An environmental stressor can cause the
phage to initiate the lysogenic cycle.
d. Cell lysis only occurs in the lytic cycle.
a. all have a round shape
b. cannot have a long shape
c. do not maintain any shape
d. vary in shape
7. The observation that the bacteria genus
Chlamydia contains species that can only survive as
intracellular parasites supports which viral origin
hypothesis?
a. Progressive
b. Regressive
c. Self-replicating
d. Virus-first
8. A scientist discovers a new virus with a linear,
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RNA genome surrounded by a helical capsid. The
virus is most likely a member of which family based
on structure classification?
a. Rabies virus
b. Herpesviruses
c. Retroviruses
d. Influenza viruses
9. Which statement is not true of viral replication?
a. A lysogenic cycle kills the host cell.
b. There are six basic steps in the viral
replication cycle.
c. Viral replication does not affect host cell
function.
d. Newly released virions can infect adjacent
cells.
10. Which statement is true of viral replication?
a. In the process of apoptosis, the cell
survives.
b. During attachment, the virus attaches at
specific sites on the cell surface.
c. The viral capsid helps the host cell produce
more copies of the viral genome.
d. mRNA works outside of the host cell to
produce enzymes and proteins.
11. Which statement is true of reverse transcriptase?
a. It is a nucleic acid.
b. It infects cells.
c. It transcribes RNA to make DNA.
d. It is a lipid.
12. Oncogenic virus cores can be_.
a. RNA
b. DNA
c. neither RNA nor DNA
d. either RNA or DNA
13. Which is true of DNA viruses?
a. They use the host cell’s machinery to
produce new copies of their genome.
b. They all have envelopes.
c. They are the only kind of viruses that can
cause cancer.
d. They are not important plant pathogens.
14. A bacteriophage can infect_.
a. the lungs
b. viruses
c. prions
d. bacteria
15. People with the CCR5A32 mutation of a T-cell
surface protein can be exposed to some strains of
CRITICAL THINKING QUESTIONS
22. The first electron micrograph of a virus (tobacco
mosaic virus) was produced in 1939. Before that
HIV-1 without becoming sick. What step of the virus
life cycle is likely to be inhibited with this mutation?
a. Release
b. Reverse transcription
c. Uncoating
d. Attachment
16. An apple grower notices that several of his apple
trees with fungi growing on their trunks have
developed necrotic ring spots, while other trees in the
orchard that lack fungi appear healthy. What is the
most likely conclusion the farmer can make about the
virus infecting his apple trees?
a. The apple trees were infected by horizontal
transmission.
b. The fungi carry disease.
c. The fungi attract disease-carrying insects.
d. The apple trees were infected by vertical
transmission.
17. Which of the following is NOT used to treat active
viral disease?
a. Vaccines
b. Antiviral drugs
c. Antibiotics
d. Phage therapy
18. Vaccines_.
a. are similar to viroids
b. are only needed once
c. kill viruses
d. stimulate an immune response
19. A patient presents at the clinic with an acute viral
infection. Assays that analyze the viral life cycle
classify the virus into Group V with a segmented
genome. Which virus is the most likely diagnosis for
the patient?
a. Rabies virus
b. Picornavirus
c. HIV-1
d. influenza A virus
20. Which of the following is not associated with
prions?
a. Replicating shapes
b. Mad cow disease
c. DNA
d. Toxic proteins
21. Which statement is true of viroids?
a. They are single-stranded RNA particles.
b. They reproduce only outside of the cell.
c. They produce proteins.
d. They affect both plants and animals.
time, how did scientists know that viruses existed if
they could not see them? (Hint: Early scientists called
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viruses “filterable agents.”)
23. Varicella-zoster virus is a double-stranded DNA
virus that causes chickenpox. How does its genome
structure provide an evolutionary advantage over a
single-stranded DNA virus?
24. Classify the Rabies virus (a rhabdovirus family
member) and HIV-1 with both the Baltimore and
genomic structure systems. Compare your results.
What conclusions can be made about these two
different methods?
25. Why can’t dogs catch the measles?
26. One of the first and most important targets for
drugs to fight infection with HIV (a retrovirus) is the
reverse transcriptase enzyme. Why?
27. In this section, you were introduced to different
types of viruses and viral diseases. Briefly discuss
the most interesting or surprising thing you learned
about viruses.
28. Although plant viruses cannot infect humans,
what are some of the ways in which they affect
humans?
29. A bacteriophage with a lytic life cycle develops a
mutation that allows it to now also go through the
lysogenic cycle. How would this provide an
evolutionary advantage over the other
bacteriophages that can only spread through lytic
cycles?
30. Why is immunization after being bitten by a rabid
animal so effective and why aren’t people vaccinated
for rabies like dogs and cats are?
31. The vaccine Gardasil that targets human
papillomavirus (HPV), the etiological agent of genital
warts, was developed after the anti-HPV medication
podofilox. Why would doctors still want a vaccine
created after anti-viral medications were available?
32. Prions are responsible for variant Creutzfeldt-
Jakob Disease, which has resulted in over 100
human deaths in Great Britain during the last 10
years. How do humans contract this disease?
33. How are viroids like viruses?
34. A botanist notices that a tomato plant looks
diseased. How could the botanist confirm that the
agent causing disease is a viroid, and not a virus?
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22 | PROKARYOTES:
BACTERIA AND
ARCHAEA
Figure 22.1 Certain prokaryotes can live in extreme environments such as the Morning Glory pool, a hot spring in
Yellowstone National Park. The spring’s vivid blue color is from the prokaryotes that thrive in its very hot waters, (credit:
modification of work by Jon Sullivan)
Chapter Outline
22.1: Prokaryotic Diversity
22.2: Structure of Prokaryotes: Bacteria and Archaea
22.3: Prokaryotic Metabolism
22.4: Bacterial Diseases in Humans
22.5: Beneficial Prokaryotes
Introduction
In the recent past, scientists grouped living things into five kingdoms—animals, plants, fungi, protists, and
prokaryotes—based on several criteria, such as the absence or presence of a nucleus and other membrane-
bound organelles, the absence or presence of cell walls, multicellularity, and so on. In the late 20 th century, the
pioneering work of Carl Woese and others compared sequences of small-subunit ribosomal RNA (SSU rRNA),
which resulted in a more fundamental way to group organisms on Earth. Based on differences in the structure
of cell membranes and in rRNA, Woese and his colleagues proposed that all life on Earth evolved along three
lineages, called domains. The domain Bacteria comprises all organisms in the kingdom Bacteria, the domain
Archaea comprises the rest of the prokaryotes, and the domain Eukarya comprises all eukaryotes—including
organisms in the kingdoms Animalia, Plantae, Fungi, and Protista.
Two of the three domains—Bacteria and Archaea—are prokaryotic. Prokaryotes were the first inhabitants on
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Earth, appearing 3.5 to 3.8 billion years ago. These organisms are abundant and ubiquitous; that is, they are
present everywhere. In addition to inhabiting moderate environments, they are found in extreme conditions: from
boiling springs to permanently frozen environments in Antarctica; from salty environments like the Dead Sea to
environments under tremendous pressure, such as the depths of the ocean; and from areas without oxygen,
such as a waste management plant, to radioactively contaminated regions, such as Chernobyl. Prokaryotes
reside in the human digestive system and on the skin, are responsible for certain illnesses, and serve an
important role in the preparation of many foods.
22.1 1 Prokaryotic Diversity
By the end of this section, you will be able to do the following:
• Describe the evolutionary history of prokaryotes
• Discuss the distinguishing features of extremophiles
• Explain why it is difficult to culture prokaryotes
Prokaryotes are ubiquitous. They cover every imaginable surface where there is sufficient moisture, and
they also live on and inside virtually all other living things. In the typical human body, prokaryotic cells
outnumber human body cells by about ten to one. They comprise the majority of living things in all ecosystems.
Some prokaryotes thrive in environments that are inhospitable for most living things. Prokaryotes recycle
nutrients —essential substances (such as carbon and nitrogen)—and they drive the evolution of new
ecosystems, some of which are natural and others man-made. Prokaryotes have been on Earth since long
before multicellular life appeared. Indeed, eukaryotic cells are thought to be the descendants of ancient
prokaryotic communities.
Prokaryotes, the First Inhabitants of Earth
When and where did cellular life begin? What were the conditions on Earth when life began? We now know
that prokaryotes were likely the first forms of cellular life on Earth, and they existed for billions of years before
plants and animals appeared. The Earth and its moon are dated at about 4.54 billion years in age. This estimate
is based on evidence from radiometric dating of meteorite material together with other substrate material from
Earth and the moon. Early Earth had a very different atmosphere (contained less molecular oxygen) than it does
today and was subjected to strong solar radiation; thus, the first organisms probably would have flourished where
they were more protected, such as in the deep ocean or far beneath the surface of the Earth. Strong volcanic
activity was common on Earth at this time, so it is likely that these first organisms—the first prokaryotes—were
adapted to very high temperatures. Because early Earth was prone to geological upheaval and volcanic eruption,
and was subject to bombardment by mutagenic radiation from the sun, the first organisms were prokaryotes that
must have withstood these harsh conditions.
Microbial Mats
Microbial mats or large biofilms may represent the earliest forms of prokaryotic life on Earth; there is fossil
evidence of their presence starting about 3.5 billion years ago. It is remarkable that cellular life appeared on
Earth only a billion years after the Earth itself formed, suggesting that pre-cellular “life” that could replicate itself
had evolved much earlier. A microbial mat is a multi-layered sheet of prokaryotes (Figure 22.2) that includes
mostly bacteria, but also archaeans. Microbial mats are only a few centimeters thick, and they typically grow
where different types of materials interface, mostly on moist surfaces. The various types of prokaryotes that
comprise them carry out different metabolic pathways, and that is the reason for their various colors. Prokaryotes
in a microbial mat are held together by a glue-like sticky substance that they secrete called extracellular matrix.
The first microbial mats likely obtained their energy from chemicals found near hydrothermal vents. A
hydrothermal vent is a breakage or fissure in the Earth’s surface that releases geothermally heated water.
With the evolution of photosynthesis about three billion years ago, some prokaryotes in microbial mats came to
use a more widely available energy source—sunlight—whereas others were still dependent on chemicals from
hydrothermal vents for energy and food.
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(a) (b)
Figure 22.2 A microbial mat. (a) This microbial mat, about one meter in diameter, is growing over a hydrothermal vent
in the Pacific Ocean in a region known as the “Pacific Ring of Fire." The mat's colony of bacteria helps retain microbial
nutrients. Chimneys such as the one indicated by the arrow allow gases to escape, (b) In this micrograph, bacteria
are visualized using fluorescence microscopy, (credit a: modification of work by Dr. Bob Embley, NOAA PMEL, Chief
Scientist; credit b: modification of work by Ricardo Murga, Rodney Donlan, CDC; scale-bar data from Matt Russell)
Stromatolites
Fossilized microbial mats represent the earliest record of life on Earth. A stromatolite is a sedimentary
structure formed when minerals are precipitated out of water by prokaryotes in a microbial mat (Figure 22.3).
Stromatolites form layered rocks made of carbonate or silicate. Although most stromatolites are artifacts from the
past, there are places on Earth where stromatolites are still forming. For example, growing stromatolites have
been found in the Anza-Borrego Desert State Park in San Diego County, California.
(a)
Figure 22.3 Stromatolites, (a) These living stromatolites
stromatolites, found in Glacier National Park, Montana, arc
b: P. Carrara, NPS)
(*>)
are located in Shark Bay, Australia, (b) These fossilized
nearly 1.5 billion years old. (credit a: Robert Young; credit
The Ancient Atmosphere
Evidence indicates that during the first two billion years of Earth’s existence, the atmosphere was anoxic,
meaning that there was no molecular oxygen. Therefore, only those organisms that can grow without
oxygen —anaerobic organisms —were able to live. Autotrophic organisms that convert solar energy into chemical
energy are called phototrophs, and they appeared within one billion years of the formation of Earth. Then,
cyanobacteria, also known as “blue-green algae,” evolved from these simple phototrophs at least one billion
years later. It was the ancestral cyanobacteria (Figure 22.4) that began the “oxygenation” of the atmosphere:
Increased atmospheric oxygen allowed the evolution of more efficient 02 -utilizing catabolic pathways. It also
opened up the land to increased colonization, because some O 2 is converted into O 3 (ozone) and ozone
effectively absorbs the ultraviolet light that could have otherwise caused lethal mutations in DNA. The current
evidence suggests that the increase in O 2 concentrations allowed the evolution of other life forms.
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Figure 22.4 Cyanobacteria. This hot spring in Yellowstone National Park flows toward the foreground. Cyanobacteria
in the spring are green, and as water flows down the gradient, the intensity of the color increases as cell density
increases. The water is cooler at the edges of the stream than in the center, causing the edges to appear greener,
(credit: Graciela Brelles-Marino)
Microbes Are Adaptable: Life in Moderate and Extreme Environments
Some organisms have developed strategies that allow them to survive harsh conditions. Almost all prokaryotes
have a cell wall, a protective structure that allows them to survive in both hypertonic and hypotonic aqueous
conditions. Some soil bacteria are able to form endospores that resist heat and drought, thereby allowing the
organism to survive until favorable conditions recur. These adaptations, along with others, allow bacteria to
remain the most abundant life form in all terrestrial and aquatic ecosystems.
Prokaryotes thrive in a vast array of environments: Some grow in conditions that would seem very normal to
us, whereas others are able to thrive and grow under conditions that would kill a plant or an animal. Bacteria
and archaea that are adapted to grow under extreme conditions are called extremophiles, meaning “lovers of
extremes." Extremophiles have been found in all kinds of environments: the depths of the oceans, hot springs,
the Arctic and the Antarctic, in very dry places, deep inside Earth, in harsh chemical environments, and in high
radiation environments (Figure 22.5), just to mention a few. Because they have specialized adaptations that
allow them to live in extreme conditions, many extremophiles cannot survive in moderate environments. There
are many different groups of extremophiles: They are identified based on the conditions in which they grow
best, and several habitats are extreme in multiple ways. For example, a soda lake is both salty and alkaline, so
organisms that live in a soda lake must be both alkaliphiles and halophiles (Table 22.1). Other extremophiles,
like radioresistant organisms, do not prefer an extreme environment (in this case, one with high levels of
radiation), but have adapted to survive in it (Figure 22.5). Organisms like these give us a better understanding of
prokaryotic diversity and open up the possibility of finding new prokaryotic species that may lead to the discovery
of new therapeutic drugs or have industrial applications.
Extremophiles and Their Preferred Conditions
Extremophile
Conditions for Optimal Growth
Acidophiles
pH 3 or below
Alkaliphiles
pH 9 or above
Thermophiles
Temperature 60-80 °C (140-176 °F)
Hyperthermophiles
Temperature 80-122 °C (176-250 °F)
Psychrophiles
Temperature of -15-10 °C (5-50 °F) or lower
Halophiles
Salt concentration of at least 0.2 M
Osmophiles
High sugar concentration
Table 22.1
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Figure 22.5 Radiation-tolerant prokaryotes. Deinococcus radiodurans , visualized in this false color transmission
electron micrograph, is a prokaryote that can tolerate very high doses of ionizing radiation. It has developed DNA repair
mechanisms that allow it to reconstruct its chromosome even if it has been broken into hundreds of pieces by radiation
or heat, (credit: modification of work by Michael Daly; scale-bar data from Matt Russell)
Prokaryotes in the Dead Sea
One example of a very harsh environment is the Dead Sea, a hypersaline basin that is located between Jordan
and Israel. Hypersaline environments are essentially concentrated seawater, in the Dead Sea, the sodium
concentration is 10 times higher than that of seawater, and the water contains high levels of magnesium (about
40 times higher than in seawater) that would be toxic to most living things. Iron, calcium, and magnesium,
elements that form divalent ions (Fe 2+ , Ca 2+ , and Mg 2+ ), produce what is commonly referred to as “hard" water.
Taken together, the high concentration of divalent cations, the acidic pH (6.0), and the intense solar radiation flux
make the Dead Sea a unique, and uniquely hostile, ecosystem (Figure 22.6).
What sort of prokaryotes do we find in the Dead Sea? The extremely salt-tolerant bacterial mats include
Halobacterium, Haloferax volcanii (which is found in other locations, not only the Dead Sea), Halorubrum
sodomense , and Halobaculum gomorrense, and the archaean Haloarcula marismortui , among others.
(a) (b)
Figure 22.6 Halophilic prokaryotes, (a) The Dead Sea is hypersaline. Nevertheless, salt-tolerant bacteria thrive in this
sea. (b) These halobacteria cells can form salt-tolerant bacterial mats, (credit a: Julien Menichini; credit b: NASA;
scale-bar data from Matt Russell)
Unculturable Prokaryotes and the Viable-but-Non-Culturable State
The process of culturing bacteria is complex and is one of the greatest discoveries of modern science. German
physician Robert Koch is credited with discovering the techniques for pure culture, including staining and using
1. Bodaker, I, Itai, S, Suzuki, MT, Feingersch, R, Rosenberg, M, Maguire, ME, Shimshon, B, and others. Comparative community genomics
in the Dead Sea: An increasingly extreme environment. The ISME Journal 4 (2010): 399-407, doi:10.1038/ismei.2009.141 . published online
24 December 2009.
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growth media. Microbiologists typically grow prokaryotes in the laboratory using an appropriate culture medium
containing all the nutrients needed by the target organism. The medium can be liquid, broth, or solid. After an
incubation time at the right temperature, there should be evidence of microbial growth (Figure 22.7). Koch's
assistant Julius Petri invented the Petri dish, whose use persists in today’s laboratories. Koch worked primarily
with the Mycobacterium tuberculosis bacterium that causes tuberculosis and developed guidelines, called
Koch's postulates, to identify the organisms responsible for specific diseases. Koch's postulates continue to
be widely used in the medical community. Koch’s postulates include that an organism can be identified as the
cause of disease when it is present in all infected samples and absent in all healthy samples, and it is able to
reproduce the infection after being cultured multiple times. Today, cultures remain a primary diagnostic tool in
medicine and other areas of molecular biology.
Figure 22.7 Bacteria growing on blood agar plates. In these agar plates, the growth medium is supplemented with
red blood cells. Blood agar becomes transparent in the presence of hemolytic Streptococcus, which destroys red
blood cells and is used to diagnose Streptococcus infections. The plate on the left is inoculated with non-hemolytic
Staphylococcus (large white colonies), and the plate on the right is inoculated with hemolytic Streptococcus (tiny clear
colonies). If you look closely at the right plate, you can see that the agar surrounding the bacteria has turned clear,
(credit: Bill Branson, NCI)
Koch's postulates can be fully applied only to organisms that can be isolated and cultured. Some prokaryotes,
however, cannot grow in a laboratory setting. In fact, over 99 percent of bacteria and archaea are unculturable.
For the most part, this is due to a lack of knowledge as to what to feed these organisms and how to grow them;
they may have special requirements for growth that remain unknown to scientists, such as needing specific
micronutrients, pH, temperature, pressure, co-factors, or co-metabolites. Some bacteria cannot be cultured
because they are obligate intracellular parasites and cannot be grown outside a host cell.
In other cases, culturable organisms become unculturable under stressful conditions, even though the same
organism could be cultured previously. Those organisms that cannot be cultured but are not dead are in a
viable-but-non-culturable (VBNC) state. The VBNC state occurs when prokaryotes respond to environmental
stressors by entering a dormant state that allows their survival. The criteria for entering into the VBNC state are
not completely understood. In a process called resuscitation, the prokaryote can go back to “normal” life when
environmental conditions improve.
Is the VBNC state an unusual way of living for prokaryotes? In fact, most of the prokaryotes living in the soil
or in oceanic waters are non-culturable. It has been said that only a small fraction, perhaps one percent, of
prokaryotes can be cultured under laboratory conditions. If these organisms are non-culturable, then how is it
known whether they are present and alive? Microbiologists use molecular techniques, such as the polymerase
chain reaction (PCR), to amplify selected portions of DNA of prokaryotes, e.g., 16S rRNA genes, demonstrating
their existence. (Recall that PCR can make billions of copies of a DNA segment in a process called
amplification.)
The Ecology of Biofilms
Some prokaryotes may be unculturable because they require the presence of other prokaryotic species. Until a
couple of decades ago, microbiologists used to think of prokaryotes as isolated entities living apart. This model,
however, does not reflect the true ecology of prokaryotes, most of which prefer to live in communities where
they can interact. As we have seen, a biofilm is a microbial community (Figure 22.8) held together in a gummy-
textured matrix that consists primarily of polysaccharides secreted by the organisms, together with some proteins
and nucleic acids. Biofilms typically grow attached to surfaces. Some of the best-studied biofilms are composed
of prokaryotes, although fungal biofilms have also been described, as well as some composed of a mixture of
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fungi and bacteria.
Biofilms are present almost everywhere: they can cause the clogging of pipes and readily colonize surfaces in
industrial settings. In recent, large-scale outbreaks of bacterial contamination of food, biofilms have played a
major role. They also colonize household surfaces, such as kitchen counters, cutting boards, sinks, and toilets,
as well as places on the human body, such as the surfaces of our teeth.
Interactions among the organisms that populate a biofilm, together with their protective exopolysaccharidic
(EPS) environment, make these communities more robust than free-living, or planktonic, prokaryotes. The sticky
substance that holds bacteria together also excludes most antibiotics and disinfectants, making biofilm bacteria
hardier than their planktonic counterparts. Overall, biofilms are very difficult to destroy because they are resistant
to many common forms of sterilization.
visual
CONNECTION
Figure 22.8 Development of a biofilm. Five stages of biofilm development are shown. During stage 1, initial
attachment, bacteria adhere to a solid surface via weak van der Waals interactions (forces produced by induced
electrical interactions between atoms). During stage 2, irreversible attachment, hairlike appendages called pili
permanently anchor the bacteria to the surface. During stage 3, maturation I, the biofilm grows through cell
division and recruitment of other bacteria. An extracellular matrix composed primarily of polysaccharides holds the
biofilm together. During stage 4, maturation II, the biofilm continues to grow and takes on a more complex shape.
During stage 5, dispersal, the biofilm matrix is partly broken down, allowing some bacteria to escape and colonize
another surface. Micrographs of a Pseudomonas aeruginosa biofilm in each of the stages of development are
shown, (credit: D. Davis, Don Monroe, PLoS)
Compared to free-floating bacteria, bacteria in biofilms often show increased resistance to antibiotics and
detergents. Why do you think this might be the case?
22.2 | Structure of Prokaryotes: Bacteria and Archaea
By the end of this section, you will be able to do the following:
• Describe the basic structure of a typical prokaryote
• Describe important differences in structure between Archaea and Bacteria
There are many differences between prokaryotic and eukaryotic cells. The name "prokaryote" suggests that
prokaryotes are defined by exclusion—they are not eukaryotes, or organisms whose cells contain a nucleus
and other internal membrane-bound organelles. However, all cells have four common structures: the plasma
membrane, which functions as a barrier for the cell and separates the cell from its environment; the cytoplasm,
a complex solution of organic molecules and salts inside the cell; a double-stranded DNA genome, the
informational archive of the cell; and ribosomes, where protein synthesis takes place. Prokaryotes come in
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various shapes, but many fall into three categories: cocci (spherical), bacilli (rod-shaped), and spirilli (spiral¬
shaped) (Figure 22.9).
(a) (b) (c)
Figure 22.9 Common prokaryotic cell types. Prokaryotes fall into three basic categories based on their shape,
visualized here using scanning electron microscopy: (a) cocci, or spherical (a pair is shown); (b) bacilli, or rod-shaped;
and (c) spirilli, or spiral-shaped, (credit a: modification of work by Janice Haney Carr, Dr. Richard Facklam, CDC; credit
c: modification of work by Dr. David Cox; scale-bar data from Matt Russell)
The Prokaryotic Cell
Recall that prokaryotes are unicellular organisms that lack membrane-bound organelles or other internal
membrane-bound structures (Figure 22.10). Their chromosome—usually single—consists of a piece of circular,
double-stranded DNA located in an area of the cell called the nucleoid. Most prokaryotes have a cell wall
outside the plasma membrane. The cell wall functions as a protective layer, and it is responsible for the
organism’s shape. Some bacterial species have a capsule outside the cell wall. The capsule enables the
organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and makes
pathogens more resistant to our immune responses. Some species also have flagella (singular, flagellum) used
for locomotion, and pili (singular, pilus) used for attachment to surfaces including the surfaces of other cells.
Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.
(DNA)
Figure 22.10 The features of a typical prokaryotic cell. Flagella, capsules, and pili are not found in all prokaryotes.
Recall that prokaryotes are divided into two different domains, Bacteria and Archaea, which together with
Eukarya, comprise the three domains of life (Figure 22.11).
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Figure 22.11 The three domains of living organisms. Bacteria and Archaea are both prokaryotes but differ enough to
be placed in separate domains. An ancestor of modern Archaea is believed to have given rise to Eukarya, the third
domain of life. Major groups of Archaea and Bacteria are shown.
Characteristics of bacterial phyla are described in Figure 22.12 and Figure 22.13. Major bacterial phyla
include the Proteobacteria, the Chlamydias, the Spirochaetes, the photosynthetic Cyanobacteria, and the
Gram-positive bacteria. The Proteobacteria are in turn subdivided into several classes, from the Alpha- to the
Epsilon proteobacteria. Eukaryotic mitochondria are thought to be the descendants of alphaproteobacteria, while
eukaryotic chloroplasts are derived from cyanobacteria. Archaeal phyla are described in Figure 22.14.
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Chapter 22 | Prokaryotes: Bacteria and Archaea
Bacteria of Phylum Proteobacteria
Class
Representative organisms
Representative micrograph
Alpha Proteobacteria
Some species are photoautotrophic
but some are symbionts of plants
and animals and others are
pathogens. Eukaryotic mitochondria
are thought be derived from bacteria
in this group.
Rhizobium
Nitrogen-fixing endosymbiont
associated with the roots of legumes
Rickettsia
Obligate intracellular parasite that
causes typhus and Rocky Mountain
Spotted Fever (but not rickets,
which is caused by Vitamin C
deficiency)
5 pm
Rickettsia rickettsia, stained red, grow
inside a host cell.
Beta Proteobacteria
This group of bacteria is diverse.
Some species play an important
role in the nitrogen cycle.
Nitrosomas
Species from this group oxidize
ammonia into nitrite.
Spirillum minus
Causes rat-bite fever
Spirillum minus
Gamma Proteobacteria
Many are beneficial symbionts that
populate the human gut, but others
are familiar human pathogens.
Some species from this subgroup
oxidize sulfur compounds.
Escherichia coti
Normally beneficial microbe of
the human gut, but some strains
cause disease
Salmonella
Certain strains cause food
poisoning or typhoid fever
Yersinia pestis
Causative agent of Bubonic plague
Psuedomonas aeruginosa
Causes lung infections
Vibrio cholera
Causative agent of cholera
Chromatium
Sulfur-producing bacteria that
oxidize sulfur, producing H 2 S
Vibrio cholera
Delta Proteobacteria
Some species generate a
spore-forming fruiting body in
adverse conditions. Others
reduce sulfate and sulfur.
Myxobacteria
Generate spore-forming fruiting
bodies in adverse conditions
Desulfovibrio vulgaris
Aneorobic, sulfate-reducing
bacterium
£
500 nm
desulfovibrio vulgan
s
Epsilon Proteobacteria
Many species inhabit the digestive
tract of animals as symbionts or
pathogens. Bacteria from this group
have been found in deep-sea
hydrothermal vents and cold seep
habitats.
Campylobacter
Causes blood poisoning and
intestinal inflammation
Heliobacter pylori
Causes stomach ulcers
m
ZtSs
Campylobacter
Figure 22.12 The Proteobacteria. Phylum Proteobacteria is one of up to 52 bacteria phyla. Proteobacteria is further
subdivided into five classes, Alpha through Epsilon, (credit “Rickettsia rickettsia”: modification of work by CDC; credit
“Spirillum minus”: modification of work by Wolframm Adlassnig; credit “Vibrio cholera": modification of work by Janice
Haney Carr, CDC; credit “Desulfovibrio vulgaris”: modification of work by Graham Bradley; credit “Campylobacter”:
modification of work by De Wood, Pooley, USDA, ARS, EMU; scale-bar data from Matt Russell)
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Chapter 22 | Prokaryotes: Bacteria and Archaea
599
Bacteria: Chlamydia, Spirochaetae, Cyanobacteria, and Gram-positive
Phylum
Representative organisms
Representative micrograph
Chlamydias
All members of this group are
obligate intracellular parasites of animal
cells. Cells walls lack peptidoglycan.
Chlamydia trachomatis
Common sexually transmitted
disease that can lead to blindness
10 pm
In this pap smear, Chlamydia trachomatis
appear as pink inclusions inside cells.
Spirochetes
Most members of this species,
which has spiral-shaped cells, are
free-living aneaerobes, but some are
pathogenic. Flagella run lengthwise in the
periplasmic space between the inner and
outer membrane.
Treponema pallidum
Causative agent of syphilis
Borrelia burgdorferi
Causative agent of Lyme disease
reponema pallidurr
Cyanobacteria
Also known as blue-green algae,
these bacteria obtain their energy through
photosynthesis. They are ubiquitous,
found in terrestrial, marine, and freshwater
environments. Eukaryotic chloroplasts are
thought be derived from bacteria in this
group.
Prochlorococcus
Believed to be the most abundant
photosynthetic organism on earth;
responsible for generating half
the world's oxygen
mM
Phormidium
Gram-positive Bacteria
Soil-dwelling members of this subgroup
decompose organic matter. Some species
cause disease. They have a thick cell wall
and lack an outer membrane.
Bacillus anthracis
Causes anthrax
Clostridium botulinum
Causes Botulism
Clostridium difficile
Causes diarrhea during antibiotic
therapy
Streptomyces
Many antibiotics, including
streptomyocin, are derived from
these bacteria.
Mycoplasmas
These tiny bacteria, the smallest
known, lack a cell wall.
Some are free-living, and some
are pathogenic.
pgp
Clostridium difficile
Figure 22.13 Other bacterial phyla. Chlamydia, Spirochetes, Cyanobacteria, and Gram-positive bacteria are described
in this table. Note that bacterial shape is not phylum-dependent; bacteria within a phylum may be cocci, rod-shaped,
or spiral, (credit “Chlamydia trachomatis”: modification of work by Dr. Lance Liotta Laboratory, NCI; credit “Treponema
pallidum modification of work by Dr. David Cox, CDC; credit "Phormidium": modification of work by USGS; credit
“Clostridium difficile”: modification of work by Lois S. Wiggs, CDC; scale-bar data from Matt Russell)
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Chapter 22 | Prokaryotes: Bacteria and Archaea
Archaea
Phylum
Representative organisms
Representative micrograph
Euryarchaeota
This phylum includes
methanogens, which produce
methane as a metabolic waste
product, and halobacteria,
which live in an extreme saline
environment.
Methanogens
Methane production causes
flatulence in humans and other
animals.
Halobacteria
Large blooms of this salt-loving
archaea appear reddish due to
the presence of bacterirhodopsin
in the membrane.
Bacteriorhodopsin is related to
the retinal pigment rhodopsin.
Crenarchaeota
Members of the ubiquitous
phylum play an important role
in the fixation of carbon. Many
members of this group are
sulfur-dependent extremophiles.
Some are thermophilic or
hyperthermophilic.
Sulfolobus
Members of this genus grow
in volcanic springs at
temperatures between 75° and
80°C and at a pH between
2 and 3.
Halobacterium strain NRC-1
Sulfolobus being infected by bacteriophage
Nanoarchaeota
This group currently contains
only one species,
Nanoarchaeum equitans.
Nanoarchaeum equitans
This species was isolated from
the bottom of the Atlantic Ocean
and from a hydrothermal vent
at Yellowstone National Park.
It is an obligate symbiont with
Ignicoccus, another species of
archaea.
Korarchaeota
Members of this phylum,
considered to be one of the
most primitive forms of life,
have only been found in the
Obsidian Pool, a hot spring at
Yellowstone National Park.
No members of this species
have been cultivated.
Nanoarchaeum equitans (small dark spheres) are
in contact with their larger host, Ignicoccus.
This image shows a variety of korarchaeota species
from the Obsidian Pool at Yellowstone National Park.
Figure 22.14 Archaeal phyla. Archaea are separated into four phyla: the Korarchaeota, Euryarchaeota,
Crenarchaeota, and Nanoarchaeota. (credit “Halobacterium’’: modification of work by NASA; credit “Nanoarchaeotum
equitans": modification of work by Karl O. Stetter; credit “Korarchaeota": modification of work by Office of Science of
the U.S. Dept, of Energy; scale-bar data from Matt Russell)
The Plasma Membrane of Prokaryotes
The prokaryotic plasma membrane is a thin lipid bilayer (6 to 8 nanometers) that completely surrounds the
cell and separates the inside from the outside. Its selectively permeable nature keeps ions, proteins, and
other molecules within the cell and prevents them from diffusing into the extracellular environment, while
other molecules may move through the membrane. Recall that the general structure of a cell membrane is a
phospholipid bilayer composed of two layers of lipid molecules. In archaeal cell membranes, isoprene (phytanyl)
chains linked to glycerol replace the fatty acids linked to glycerol in bacterial membranes. Some archaeal
membranes are lipid monolayers instead of bilayers (Figure 22.15).
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Chapter 22 | Prokaryotes: Bacteria and Archaea
601
/vwww
-vwww
/wwwv
AA/WVW
I 41
I 41
sA^SAjaA/sAa/UvU
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Phospholipid bilayer from
Bacteria and Eukarya
Phospholipid bilayer from
Archaea
Figure 22.15 Bacterial and archaeal phospholipids. Archaeal phospholipids differ from those found in Bacteria and
Eukarya in two ways. First, they have branched phytanyl sidechains instead of linear ones. Second, an ether bond
instead of an ester bond connects the lipid to the glycerol.
The Cell Wall of Prokaryotes
The cytoplasm of prokaryotic cells has a high concentration of dissolved solutes. Therefore, the osmotic
pressure within the cell is relatively high. The cell wall is a protective layer that surrounds some cells and gives
them shape and rigidity. It is located outside the cell membrane and prevents osmotic lysis (bursting due to
increasing volume). The chemical composition of the cell wall varies between Archaea and Bacteria, and also
varies between bacterial species.
Bacterial cell walls contain peptidoglycan, composed of polysaccharide chains that are cross-linked by unusual
peptides containing both L- and D-amino acids including D-glutamic acid and D-alanine. (Proteins normally
have only L-amino acids; as a consequence, many of our antibiotics work by mimicking D-amino acids and
therefore have specific effects on bacterial cell-wall development.) There are more than 100 different forms of
peptidoglycan. S-layer (surface layer) proteins are also present on the outside of cell walls of both Archaea and
Bacteria.
Bacteria are divided into two major groups: Gram positive and Gram negative, based on their reaction to Gram
staining. Note that all Gram-positive bacteria belong to one phylum; bacteria in the other phyla (Proteobacteria,
Chlamydias, Spirochetes, Cyanobacteria, and others) are Gram-negative. The Gram staining method is named
after its inventor, Danish scientist Hans Christian Gram (1853-1938). The different bacterial responses to the
staining procedure are ultimately due to cell wall structure. Gram-positive organisms typically lack the outer
membrane found in Gram-negative organisms (Figure 22.16). Up to 90 percent of the cell-wall in Gram-positive
bacteria is composed of peptidoglycan, and most of the rest is composed of acidic substances called teichoic
acids. Teichoic acids may be covalently linked to lipids in the plasma membrane to form iipoteichoic acids.
Lipoteichoic acids anchor the cell wall to the cell membrane. Gram-negative bacteria have a relatively thin cell
wall composed of a few layers of peptidoglycan (only 10 percent of the total cell wall), surrounded by an outer
envelope containing lipopolysaccharides (LPS) and lipoproteins. This outer envelope is sometimes referred to
as a second lipid bilayer. The chemistry of this outer envelope is very different, however, from that of the typical
lipid bilayer that forms plasma membranes.
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Chapter 22 | Prokaryotes: Bacteria and Archaea
visual
a CONNECTION
-Lipoteichoic acid Lipopolysaccharide
Lipoprotein
cell wall
,Porin
Plasma
II I fill HM!: ffflflfll fill I membrane
Cytoplasm
Gram-positive bacteria
Ul 111111
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Membrane Phospholipid
protein
Gram-negative bacteria
Outer
membrane
Periplasmic
space
Inner
membrane
Figure 22.16 Cell walls in Gram-positive and Gram-negative bacteria. Bacteria are divided into two major groups:
Gram positive and Gram negative. Both groups have a cell wall composed of peptidoglycan: in Gram-positive
bacteria, the wall is thick, whereas in Gram-negative bacteria, the wall is thin. In Gram-negative bacteria, the cell
wall is surrounded by an outer membrane that contains lipopolysaccharides and lipoproteins. Porins are proteins
in this cell membrane that allow substances to pass through the outer membrane of Gram-negative bacteria. In
Gram-positive bacteria, lipoteichoic acid anchors the cell wall to the cell membrane, (credit: modification of work
by "Franciscosp2"/Wikimedia Commons)
Which of the following statements is true?
a. Gram-positive bacteria have a single cell wall anchored to the cell membrane by lipoteichoic acid.
b. Porins allow entry of substances into both Gram-positive and Gram-negative bacteria.
c. The cell wall of Gram-negative bacteria is thick, and the cell wall of Gram-positive bacteria is thin.
d. Gram-negative bacteria have a cell wall made of peptidoglycan, whereas Gram-positive bacteria have
a cell wall made of lipoteichoic acid.
Archaean cell walls do not have peptidoglycan. There are four different types of archaean cell walls. One type
is composed of pseudopeptidoglycan, which is similar to peptidoglycan in morphology but contains different
sugars in the polysaccharide chain. The other three types of cell walls are composed of polysaccharides,
glycoproteins, or pure protein. Other differences between Bacteria and Archaea are seen in Table 22.2. Note
that features related to DNA replication, transcription and translation in Archaea are similar to those seen in
eukaryotes.
Differences and Similarities between Bacteria and Archaea
Structural Characteristic
Bacteria
Archaea
Cell type
Prokaryotic
Prokaryotic
Cell morphology
Variable
Variable
Cell wall
Contains peptidoglycan
Does not contain peptidoglycan
Cell membrane type
Lipid bilayer
Lipid bilayer or lipid monolayer
Plasma membrane lipids
Fatty acids-glycerol ester
Phytanyl-glycerol ethers
Chromosome
Typically circular
Typically circular
Replication origins
Single
Multiple
Table 22.2
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603
Differences and Similarities between Bacteria and Archaea
Structural Characteristic
Bacteria
Archaea
RNA polymerase
Single
Multiple
Initiator tRNA
Formyl-methionine
Methionine
Streptomycin inhibition
Sensitive
Resistant
Calvin cycle
Yes
No
Table 22.2
Reproduction
Reproduction in prokaryotes is asexual and usually takes place by binary fission. (Recall that the DNA of a
prokaryote is a single, circular chromosome.) Prokaryotes do not undergo mitosis; instead, the chromosome is
replicated and the two resulting copies separate from one another, due to the growth of the cell. The prokaryote,
now enlarged, is pinched inward at its equator and the two resulting cells, which are clones, separate. Binary
fission does not provide an opportunity for genetic recombination or genetic diversity, but prokaryotes can share
genes by three other mechanisms.
In transformation, the prokaryote takes in DNA shed by other prokaryotes into its environment. If a
nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into
its own chromosome, it too may become pathogenic. In transduction, bacteriophages, the viruses that infect
bacteria, may move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a
recombinant organism. Archaea also have viruses that may translocate genetic material from one individual to
another. In conjugation, DNA is transferred from one prokaryote to another by means of a pilus, which brings
the organisms into contact with one another, and provides a channel for transfer of DNA. The DNA transferred
can be in the form of a plasmid or as a composite molecule, containing both plasmid and chromosomal DNA.
These three processes of DNA exchange are shown in Figure 22.17.
Reproduction can be very rapid: a few minutes for some species. This short generation time coupled with
mechanisms of genetic recombination and high rates of mutation result in the rapid evolution of prokaryotes,
allowing them to respond to environmental changes (such as the introduction of an antibiotic) very quickly.
(a) Transformation (b) Transduction (c) Conjugation
Figure 22.17 Gene transfer mechanisms in prokaryotes. There are three mechanisms by which prokaryotes can
exchange DNA. In (a) transformation, the cell takes up prokaryotic DNA directly from the environment. The DNA
may remain separate as plasmid DNA or be incorporated into the host genome. In (b) transduction, a bacteriophage
injects DNA into the cell that contains a small fragment of DNA from a different prokaryote. In (c) conjugation, DNA is
transferred from one cell to another via a mating bridge, or pilus, that connects the two cells after the sex pilus draws
the two bacteria close enough to form the bridge.
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V /
e olution CONNECTION
The Evolution of Prokaryotes
How do scientists answer questions about the evolution of prokaryotes? Unlike with animals, artifacts in
the fossil record of prokaryotes offer very little information. Fossils of ancient prokaryotes look like tiny
bubbles in rock. Some scientists turn to genetics and to the principle of the molecular clock, which holds
that the more recently two species have diverged, the more similar their genes (and thus proteins) will be.
Conversely, species that diverged long ago will have more genes that are dissimilar.
Scientists at the NASA Astrobiology Institute and at the European Molecular Biology Laboratory
collaborated to analyze the molecular evolution of 32 specific proteins common to 72 species of
[ 2 ]
prokaryotes. The model they derived from their data indicates that three important groups of
bacteria—Actinobacteria, Deinococcus, and Cyanobacteria (collectively called Terrabacteria by the
authors)—were the first to colonize land. Actinobacteria are a group of very common Gram-positive bacteria
that produce branched structures like fungal mycelia, and include species important in decomposition of
organic wastes. You will recall that Deinococcus is a genus of bacterium that is highly resistant to ionizing
radiation. It has a thick peptidoglycan layer in addition to a second external membrane, so it has features of
both Gram-positive and Gram-negative bacteria.
Cyanobacteria are photosynthesizers, and were probably responsible for the production of oxygen on the
ancient earth. The timelines of divergence suggest that bacteria (members of the domain Bacteria) diverged
from common ancestral species between 2.5 and 3.2 billion years ago, whereas the Archaea diverged
earlier: between 3.1 and 4.1 billion years ago. Eukarya later diverged from the archaean line. The work
further suggests that stromatolites that formed prior to the advent of cyanobacteria (about 2.6 billion years
ago) photosynthesized in an anoxic environment and that because of the modifications of the Terrabacteria
for land (resistance to drying and the possession of compounds that protect the organism from excess light),
photosynthesis using oxygen may be closely linked to adaptations to survive on land.
22.3 | Prokaryotic Metabolism
By the end of this section, you will be able to do the following:
• Identify the macronutrients needed by prokaryotes, and explain their importance
• Describe the ways in which prokaryotes get energy and carbon for life processes
• Describe the roles of prokaryotes in the carbon and nitrogen cycles
Prokaryotes are metabolically diverse organisms, in many cases, a prokaryote may be placed into a species
clade by its defining metabolic features: Can it metabolize lactose? Can it grow on citrate? Does it produce
H 2 S? Does it ferment carbohydrates to produce acid and gas? Can it grow under anaerobic conditions? Since
metabolism and metabolites are the product of enzyme pathways, and enzymes are encoded in genes, the
metabolic capabilities of a prokaryote are a reflection of its genome. There are many different environments
on Earth with various energy and carbon sources, and variable conditions to which prokaryotes may be
able to adapt. Prokaryotes have been able to live in every environment from deep-water volcanic vents to
Antarctic ice by using whatever energy and carbon sources are available. Prokaryotes fill many niches on Earth,
including involvement in nitrogen and carbon cycles, photosynthetic production of oxygen, decomposition of
dead organisms, and thriving as parasitic, commensal, or mutualistic organisms inside multicellular organisms,
including humans. The very broad range of environments that prokaryotes occupy is possible because they have
diverse metabolic processes.
2. Battistuzzi, FU, Feijao, A, and Hedges, SB. A genomic timescale of prokaryote evolution: Insights into the origin of methanogenesis,
phototrophy, and the colonization of land. BioMed Central: Evolutionary Biology 4 (2004): 44, doi:10.1186/1471-2148-4-44.
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605
Needs of Prokaryotes
The diverse environments and ecosystems on Earth have a wide range of conditions in terms of temperature,
available nutrients, acidity, salinity, oxygen availability, and energy sources. Prokaryotes are very well equipped
to make their living out of a vast array of nutrients and environmental conditions. To live, prokaryotes need a
source of energy, a source of carbon, and some additional nutrients.
Macronutrients
Cells are essentially a well-organized assemblage of macromolecules and water. Recall that macromolecules
are produced by the polymerization of smaller units called monomers. For cells to build all of the molecules
required to sustain life, they need certain substances, collectively called nutrients. When prokaryotes grow
in nature, they must obtain their nutrients from the environment. Nutrients that are required in large amounts
are called macronutrients, whereas those required in smaller or trace amounts are called micronutrients. Just
a handful of elements are considered macronutrients—carbon, hydrogen, oxygen, nitrogen, phosphorus, and
sulfur. (A mnemonic for remembering these elements is the acronym CHONPS.)
Why are these macronutrients needed in large amounts? They are the components of organic compounds in
cells, including water. Carbon is the major element in all macromolecules: carbohydrates, proteins, nucleic acids,
lipids, and many other compounds. Carbon accounts for about 50 percent of the composition of the cell, in
contrast, nitrogen represents only 12 percent of the total dry weight of a typical cell. Nitrogen is a component of
proteins, nucleic acids, and other cell constituents. Most of the nitrogen available in nature is either atmospheric
nitrogen (N 2 ) or another inorganic form. Diatomic (N 2 ) nitrogen, however, can be converted into an organic form
only by certain microorganisms, called nitrogen-fixing organisms. Both hydrogen and oxygen are part of many
organic compounds and of water. Phosphorus is required by all organisms for the synthesis of nucleotides and
phospholipids. Sulfur is part of the structure of some amino acids such as cysteine and methionine, and is
also present in several vitamins and coenzymes. Other important macronutrients are potassium (K), magnesium
(Mg), calcium (Ca), and sodium (Na). Although these elements are required in smaller amounts, they are very
important for the structure and function of the prokaryotic cell.
Micronutrients
In addition to these macronutrients, prokaryotes require various metallic elements in small amounts. These
are referred to as micronutrients or trace elements. For example, iron is necessary for the function of the
cytochromes involved in electron-transport reactions. Some prokaryotes require other elements—such as boron
(B), chromium (Cr), and manganese (Mn)—primarily as enzyme cofactors.
The Ways in Which Prokaryotes Obtain Energy
Prokaryotes are classified both by the way they obtain energy, and by the carbon source they use for producing
organic molecules. These categories are summarized in Table 22.3. Prokaryotes can use different sources of
energy to generate the ATP needed for biosynthesis and other cellular activities. Phototrophs (or phototrophic
organisms) obtain their energy from sunlight. Phototrophs trap the energy of light using chlorophylls, or in a
few cases, bacterial rhodopsin. (Rhodopsin-using phototrophs, oddly, are phototrophic, but not photosynthetic,
since they do not fix carbon.) Chemotrophs (or chemosynthetic organisms) obtain their energy from chemical
compounds. Chemotrophs that can use organic compounds as energy sources are called chemoorganotrophs.
Those that can use inorganic compounds, like sulfur or iron compounds, as energy sources are called
chemolithotrophs.
Energy-producing pathways may be either aerobic, using oxygen as the terminal electron acceptor, or
anaerobic, using either simple inorganic compounds or organic molecules as the terminal electron acceptor.
Since prokaryotes lived on Earth for nearly a billion years before photosynthesis produced significant amounts
of oxygen for aerobic respiration, many species of both Bacteria and Archaea are anaerobic and their metabolic
activities are important in the carbon and nitrogen cycles discussed below.
The Ways in Which Prokaryotes Obtain Carbon
Prokaryotes not only can use different sources of energy, but also different sources of carbon compounds.
Autotrophic prokaryotes synthesize organic molecules from carbon dioxide. In contrast, heterotrophic
prokaryotes obtain carbon from organic compounds. To make the picture more complex, the terms that describe
how prokaryotes obtain energy and carbon can be combined. Thus, photoautotrophs use energy from sunlight,
and carbon from carbon dioxide and water, whereas chemoheterotrophs obtain both energy and carbon from
an organic chemical source. Chemolithoautotrophs obtain their energy from inorganic compounds, and they
build their complex molecules from carbon dioxide. Finally, prokaryotes that get their energy from light, but their
carbon from organic compounds, are photoheterotrophs. The table below (Table 22.3) summarizes carbon and
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Chapter 22 | Prokaryotes: Bacteria and Archaea
energy sources in prokaryotes.
Carbon and Energy Sources in Prokaryotes
Energy Sources Carbon Sources
Light
Chemicals
Carbon dioxide
Organic compounds
Phototrophs
Chemotrophs
Autotrophs
Heterotrophs
Organic chemicals
inorganic chemicals
Chemo-organotrophs
Chemolithotrophs
Table 22.3
Role of Prokaryotes in Ecosystems
Prokaryotes are ubiquitous: There is no niche or ecosystem in which they are not present. Prokaryotes play
many roles in the environments they occupy. The roles they play in the carbon and nitrogen cycles are vital
to life on Earth. In addition, the current scientific consensus suggests that metabolically interactive prokaryotic
communities may have been the basis for the emergence of eukaryotic cells.
Prokaryotes and the Carbon Cycle
Carbon is one of the most important macronutrients, and prokaryotes play an important role in the carbon cycle
(Figure 22.18). The carbon cycle traces the movement of carbon from inorganic to organic compounds and
back again. Carbon is cycled through Earth’s major reservoirs: land, the atmosphere, aquatic environments,
sediments and rocks, and biomass. In a way, the carbon cycle echoes the role of the “four elements" first
proposed by the ancient Greek philosopher, Empedocles: fire, water, earth, and air. Carbon dioxide is removed
from the atmosphere by land plants and marine prokaryotes, and is returned to the atmosphere via the
respiration of chemoorganotrophic organisms, including prokaryotes, fungi, and animals. Although the largest
carbon reservoir in terrestrial ecosystems is in rocks and sediments, that carbon is not readily available.
Participants in the carbon cycle are roughly divided among producers, consumers, and decomposers of organic
carbon compounds. The primary producers of organic carbon compounds from CO 2 are land plants and
photosynthetic bacteria. A large amount of available carbon is found in living land plants. A related source of
carbon compounds is humus, which is a mixture of organic materials from dead plants and prokaryotes that
have resisted decomposition. (The term "humus," by the way, is the root of the word "human.") Consumers
such as animals and other heterotrophs use organic compounds generated by producers and release carbon
dioxide to the atmosphere. Other bacteria and fungi, collectively called decomposers, carry out the breakdown
(decomposition) of plants and animals and their organic compounds. Most carbon dioxide in the atmosphere is
derived from the respiration of microorganisms that decompose dead animals, plants, and humus.
in aqueous environments and their anoxic sediments, there is another carbon cycle taking place. In this case, the
cycle is based on one-carbon compounds, in anoxic sediments, prokaryotes, mostly archaea, produce methane
(CH 4 ). This methane moves into the zone above the sediment, which is richer in oxygen and supports bacteria
called methane oxidizers that oxidize methane to carbon dioxide, which then returns to the atmosphere.
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Figure 22.18 The carbon cycle. Prokaryotes play a significant role in continuously moving carbon through the
biosphere, (credit: modification of work by John M. Evans and Howard Perlman, USGS)
Prokaryotes and the Nitrogen Cycle
Nitrogen is a very important element for life because it is a major constituent of proteins and nucleic acids.
It is a macronutrient, and in nature, it is recycled from organic compounds to ammonia, ammonium ions,
nitrate, nitrite, and nitrogen gas by many processes, many of which are carried out only by prokaryotes. As
illustrated in Figure 22.19, prokaryotes are key to the nitrogen cycle. The largest pool of nitrogen available in
the terrestrial ecosystem is gaseous nitrogen (N2) from the air, but this nitrogen is not usable by plants, which
are primary producers. Gaseous nitrogen is transformed, or “fixed” into more readily available forms, such as
ammonia (NH3), through the process of nitrogen fixation. Nitrogen-fixing bacteria include Azotobacter in soil
and the ubiquitous photosynthetic cyanobacteria. Some nitrogen fixing bacteria, like Rhizobium, live in symbiotic
relationships in the roots of legumes. Another source of ammonia is ammonification, the process by which
ammonia is released during the decomposition of nitrogen-containing organic compounds. The ammonium ion
is progressively oxidized by different species of bacteria in a process called nitrification. The nitrification process
begins with the conversion of ammonium to nitrite (NO2'), and continues with the conversion of nitrite to nitrate.
Nitrification in soils is carried out by bacteria belonging to the genera Nitrosomas, Nitrobacter, and Nitrospira.
Most nitrogen in soil is in the form of ammonium (NH4 + ) or nitrate (NO3'). Ammonia and nitrate can be used by
plants or converted to other forms.
Ammonia released into the atmosphere, however, represents only 15 percent of the total nitrogen released; the
rest is as N2 and N2O (nitrous oxide). Ammonia is catabolized anaerobically by some prokaryotes, yielding N2
as the final product. Denitrifying bacteria reverse the process of nitrification, reducing the nitrate from soils to
gaseous compounds such as N2O, NO, and N2.
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visual
CONNECTION
Nitrogen (N 2 ) in the
atmosphere Vs
V-
\W
ifi m
vift T
Plants
Assimilation |
Denitrifying
bacteria
Nitrogen-fixing
bacteria in
root nodules
of legumes
Nitrogen-fixing
soil bacteria
Nitrifying
bacteria
Figure 22.19 The nitrogen cycle. Prokaryotes play a key role in the nitrogen cycle, (credit: Environmental
Protection Agency)
Which of the following statements about the nitrogen cycle is false?
a. Nitrogen-fixing bacteria exist on the root nodules of legumes and in the soil.
b. Denitrifying bacteria convert nitrates (NO3") into nitrogen gas (N2).
c. Ammonification is the process by which ammonium ion (NH 4 + ) is released from decomposing organic
compounds.
d. Nitrification is the process by which nitrites (NO 2 ’) are converted to ammonium ion (NH 4 1 ").
22.4 | Bacterial Diseases in Humans
By the end of this section, you will be able to do the following:
• Identify bacterial diseases that caused historically important plagues and epidemics
• Describe the link between biofilms and foodborne diseases
• Explain how overuse of antibiotics may be creating “super bugs"
• Explain the importance of MRSA with respect to the problems of antibiotic resistance
To a prokaryote, humans may be just another housing opportunity. Unfortunately, the tenancy of some species
can have harmful effects and cause disease. Bacteria or other infectious agents that cause harm to their human
hosts are called pathogens. Devastating pathogen-borne diseases and plagues, both viral and bacterial in
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nature, have affected humans and their ancestors for millions of years. The true cause of these diseases was not
understood until modern scientific thought developed, and many people thought that diseases were a “spiritual
punishment." Only within the past several centuries have people understood that staying away from afflicted
persons, disposing of the corpses and personal belongings of victims of illness, and sanitation practices reduced
their own chances of getting sick.
Epidemiologists study how diseases are transmitted and how they affect a population. Often, they must following
the course of an epidemic —a disease that occurs in an unusually high number of individuals in a population at
the same time. In contrast, a pandemic is a widespread, and usually worldwide, epidemic. An endemic disease
is a disease that is always present, usually at low incidence, in a population.
Long History of Bacterial Disease
There are records about infectious diseases as far back as 3000 B.C. A number of significant pandemics caused
by bacteria have been documented over several hundred years. Some of the most memorable pandemics led to
the decline of cities and entire nations.
In the 21 st century, infectious diseases remain among the leading causes of death worldwide, despite advances
made in medical research and treatments in recent decades. A disease spreads when the pathogen that causes
it is passed from one person to another. For a pathogen to cause disease, it must be able to reproduce in the
host’s body and damage the host in some way.
The Plague of Athens
In 430 B.C., the Plague of Athens killed one-quarter of the Athenian troops who were fighting in the great
Peloponnesian War and weakened Athens’s dominance and power. The plague impacted people living in
overcrowded Athens as well as troops aboard ships that had to return to Athens. The source of the plague
may have been identified recently when researchers from the University of Athens were able to use DNA from
teeth recovered from a mass grave. The scientists identified nucleotide sequences from a pathogenic bacterium,
Salmonella enterica serovar Typhi (Figure 22.20), which causes typhoid fever. This disease is commonly seen
in overcrowded areas and has caused epidemics throughout recorded history.
3. Papagrigorakis MJ, Synodinos PN, and Yapijakis C. Ancient typhoid epidemic reveals possible ancestral strain of Salmonella enterica
serovar Typhi. Infect Genet Evol 7 (2007): 126-7, Epub 2006 Jun.
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Figure 22.20 Salmonella enterlca. Salmonella enterlca serovar Typhi, the causative agent of Typhoid fever, is a
Gram-negative, rod-shaped gamma proteobacterium. Typhoid fever, which is spread through feces, causes intestinal
hemorrhage, high fever, delirium, and dehydration. Today, between 16 and 33 million cases of this re-emerging disease
occur annually, resulting in over 200,000 deaths. Carriers of the disease can be asymptomatic. In a famous case in the
early 1900s, a cook named Mary Mallon (“Typhoid Mary”) unknowingly spread the disease to over fifty people, three of
whom died. Other serotypes of Salmonella cause food poisoning, (credit: modification of work by NCI, CDC)
Bubonic Plagues
From 541 to 750, the Plague of Justinian, an outbreak of what was likely bubonic plague, eliminated one-quarter
to one-half of the human population in the eastern Mediterranean region. The population in Europe dropped by
50 percent during this outbreak. Astoundingly, bubonic plague would strike Europe more than once!
Bubonic plague is caused by the bacterium Yersinia pestis. One of the most devastating pandemics attributed
to bubonic plague was the Black Death (1346 to 1361). It is thought to have originated in China and spread
along the Silk Road, a network of land and sea trade routes, to the Mediterranean region and Europe, carried
by fleas living on black rats that were always present on ships. The Black Death was probably named for the
tissue necrosis (Figure 22.21c) that can be one of the symptoms. The "buboes" of bubonic plague were painfully
swollen areas of lymphatic tissue. A pneumonic form of the plague, spread by the coughing and sneezing of
infected individuals, spreads directly from human to human and can cause death within a week. The pneumonic
form was responsible for the rapid spread of the Black Death in Europe. The Black Death reduced the world’s
population from an estimated 450 million to about 350 to 375 million. Bubonic plague struck London yet again
in the mid-1600s (Figure 22.21). In modern times, approximately 1,000 to 3,000 cases of plague arise globally
each year, and a “sylvatic” form of plague, carried by fleas living on rodents such as prairie dogs and black
footed ferrets, infects 10 to 20 people annually in the American Southwest. Although contracting bubonic plague
before antibiotics meant almost certain death, the bacterium responds to several types of modern antibiotics,
and mortality rates from plague are now very low.
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(a) (b) (c)
Figure 22.21 The Black Death. The (a) Great Plague of London killed an estimated 200,000 people, or about 20
percent of the city’s population. The causative agent, the (b) bacterium Yersinia pestis, is a Gram-negative, rod-shaped
bacterium from the class Gammaproteobacteria. The disease is transmitted through the bite of an infected flea, which
is carried on a rodent. Symptoms include swollen lymph nodes, fever, seizure, vomiting of blood, and (c) gangrene,
(credit b: Rocky Mountain Laboratories, NIAID, NIH; scale-bar data from Matt Russell; credit c: Textbook of Military
Medicine, Washington, D.C., U.S. Dept, of the Army, Office of the Surgeon General, Borden Institute)
LINK TQ LEARNING
Watch a video (http:// 0 penstaxc 0 llege. 0 rg/l/black_death) on the modern understanding of the Black
Death—bubonic plague in Europe during the 14 th century.
Migration of Diseases to New Populations
One of the negative consequences of human exploration was the accidental “biological warfare” that resulted
from the transport of a pathogen into a population that had not previously been exposed to it. Over the centuries,
Europeans tended to develop genetic immunity to endemic infectious diseases, but when European conquerors
reached the western hemisphere, they brought with them disease-causing bacteria and viruses, which triggered
epidemics that completely devastated many diverse populations of Native Americans, who had no natural
resistance to many European diseases. It has been estimated that up to 90 percent of Native Americans
died from infectious diseases after the arrival of Europeans, making conquest of the New World a foregone
conclusion.
Emerging and Re-emerging Diseases
The distribution of a particular disease is dynamic. Changes in the environment, the pathogen, or the host
population can dramatically impact the spread of a disease. According to the World Health Organization (WHO),
an emerging disease (Figure 22.22) is one that has appeared in a population for the first time, or that may have
existed previously but is rapidly increasing in incidence or geographic range. This definition also includes re-
emerging diseases that were previously under control. Approximately 75 percent of recently emerging infectious
diseases affecting humans are zoonotic diseases. Zoonoses are diseases that primarily infect animals but can
be transmitted to humans; some are of viral origin and some are of bacterial origin. Brucellosis is an example of
a prokaryotic zoonosis that is re-emerging in some regions, and necrotizing fasciitis (commonly known as flesh¬
eating bacteria) has been increasing in virulence for the last 80 years for unknown reasons.
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Vancomycin-resistant Multidrug-resistant
Staphylococcus aureus tuberculosis
Figure 22.22 Emerging diseases. The map shows regions where bacterial diseases are emerging or re-emerging,
(credit: modification of work by NIH)
Some of the present emerging diseases are not actually new, but are diseases that were catastrophic in the past
(Figure 22.23). They devastated populations and became dormant for a while, just to come back, sometimes
more virulent than before, as was the case with bubonic plague. Other diseases, like tuberculosis, were never
eradicated but were under control in some regions of the world until coming back, mostly in urban centers with
high concentrations of immunocompromised people. WHO has identified certain diseases whose worldwide re-
emergence should be monitored. Among these are three viral diseases (dengue fever, yellow fever, and zika),
and three bacterial diseases (diphtheria, cholera, and bubonic plague). The war against infectious diseases has
no foreseeable end.
Life Cycle of the Ixodes scapularis Tick
Winter
(a) (b)
Figure 22.23 Lyme Disease. Lyme disease often, but not always, results in (a) a characteristic bullseye rash. The
disease is caused by a (b) Gram-negative spirochete bacterium of the genus Borrelia. The bacteria (c) infect ticks,
which in turn infect mice. Deer are the preferred secondary host, but the ticks also may feed on humans. Untreated,
the disease causes chronic disorders in the nervous system, eyes, joints, and heart. The disease is named after
Lyme, Connecticut, where an outbreak occurred in 1995 and has subsequently spread. The disease is not new,
however. Genetic evidence suggests that Otzi the Iceman, a 5,300-year-old mummy found in the Alps, was infected
with Borrelia. (credit a: James Gathany, CDC; credit b: CDC; scale-bar data from Matt Russell)
Foodborne Diseases
Prokaryotes are everywhere: They readily colonize the surface of any type of material, and food is not an
exception. Most of the time, prokaryotes colonize food and food-processing equipment in the form of a biofilm,
as we have discussed earlier. Outbreaks of bacterial infection related to food consumption are common.
A foodborne disease (commonly called “food poisoning”) is an illness resulting from the consumption the
pathogenic bacteria, viruses, or other parasites that contaminate food. Although the United States has one of
the safest food supplies in the world, the U.S. Centers for Disease Control and Prevention (CDC) has reported
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that “76 million people get sick, more than 300,000 are hospitalized, and 5,000 Americans die each year from
foodborne illness.”
The characteristics of foodborne illnesses have changed over time. In the past, it was relatively common to
hear about sporadic cases of botulism, the potentially fatal disease produced by a toxin from the anaerobic
bacterium Clostridium botulinum. Some of the most common sources for this bacterium were non-acidic canned
foods, homemade pickles, and processed meat and sausages. The can, jar, or package created a suitable
anaerobic environment where Clostridium could grow. Proper sterilization and canning procedures have reduced
the incidence of this disease.
While people may tend to think of foodborne illnesses as associated with animal-based foods, most cases are
now linked to produce. There have been serious, produce-related outbreaks associated with raw spinach in
the United States and with vegetable sprouts in Germany, and these types of outbreaks have become more
common. The raw spinach outbreak in 2006 was produced by the bacterium E. coli serotype 0157:H7. A
serotype is a strain of bacteria that carries a set of similar antigens on its cell surface, and there are often many
different serotypes of a bacterial species. Most E. coli are not particularly dangerous to humans, but serotype
0157:H7 can cause bloody diarrhea and is potentially fatal.
All types of food can potentially be contaminated with bacteria. Recent outbreaks of Salmonella reported by the
CDC occurred in foods as diverse as peanut butter, alfalfa sprouts, and eggs. A deadly outbreak in Germany
in 2010 was caused by E. coli contamination of vegetable sprouts (Figure 22.24). The strain that caused the
outbreak was found to be a new serotype not previously involved in other outbreaks, which indicates that E. coli
is continuously evolving. Outbreaks of listeriosis, due to contamination of meats, raw cheeses, and frozen or
fresh vegetables with Listeria monocytogenes, are becoming more frequent.
(a) (b) (c)
Figure 22.24 Foodborne pathogens, (a) Vegetable sprouts grown at an organic farm were the cause of an (b) E.
coli outbreak that killed 32 people and sickened 3,800 in Germany in 2011. The strain responsible, E. coli O104:H4,
produces Shiga toxin, a substance that inhibits protein synthesis in the host cell. The toxin (c) destroys red blood cells
resulting in bloody diarrhea. Deformed red blood cells clog the capillaries of the kidney, which can lead to kidney failure,
as happened to 845 patients in the 2011 outbreak. Kidney failure is usually reversible, but some patients experience
kidney problems years later, (credit c: NIDDK, NIH)
Biofilms and Disease
Recall that biofilms are microbial communities that are very difficult to destroy. They are responsible for
diseases such as Legionnaires’ disease, otitis media (ear infections), and various infections in patients with
cystic fibrosis. They produce dental plaque and colonize catheters, prostheses, transcutaneous and orthopedic
devices, contact lenses, and internal devices such as pacemakers. They also form in open wounds and burned
tissue, in healthcare environments, biofilms grow on hemodialysis machines, mechanical ventilators, shunts,
and other medical equipment. In fact, 65 percent of all infections acquired in the hospital (nosocomial infections)
are attributed to biofilms. Biofilms are also related to diseases contracted from food because they colonize the
surfaces of vegetable leaves and meat, as well as food-processing equipment that isn’t adequately cleaned.
Biofilm infections develop gradually and may not cause immediate symptoms. They are rarely resolved by host
defense mechanisms. Once an infection by a biofilm is established, it is very difficult to eradicate, because
biofilms tend to be resistant to most methods used to control microbial growth, including antibiotics. The matrix
that attaches the cells to a substrate and to other another protects the cells from antibiotics or drugs. In addition,
since biofilms grow slowly, they are less responsive to agents that interfere with cell growth. It has been reported
that biofilms can resist up to 1,000 times the antibiotic concentrations used to kill the same bacteria when they
are free-living or planktonic. An antibiotic dose that large would harm the patient; therefore, scientists are working
on new ways to get rid of biofilms.
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Antibiotics: Are We Facing a Crisis?
The word antibiotic comes from the Greek anti meaning “against" and bios meaning “life.” An antibiotic is
a chemical, produced either by microbes or synthetically, that is hostile to or prevents the growth of other
organisms. Today’s media often address concerns about an antibiotic crisis. Are the antibiotics that easily treated
bacterial infections in the past becoming obsolete? Are there new “superbugs”—bacteria that have evolved to
become more resistant to our arsenal of antibiotics? Is this the beginning of the end of antibiotics? All these
questions challenge the healthcare community.
One of the main causes of antibiotic resistance in bacteria is overexposure to antibiotics. The imprudent and
excessive use of antibiotics has resulted in the natural selection of resistant forms of bacteria. The antibiotic kills
most of the infecting bacteria, and therefore only the resistant forms remain. These resistant forms reproduce,
resulting in an increase in the proportion of resistant forms over non-resistant ones, in addition to transmission
of resistance genes to progeny, lateral transfer of resistance genes on plasmids can rapidly spread these genes
through a bacterial population. A major misuse of antibiotics is in patients with viral infections like colds or the
flu, against which antibiotics are useless. Another problem is the excessive use of antibiotics in livestock. The
routine use of antibiotics in animal feed promotes bacterial resistance as well, in the United States, 70 percent of
the antibiotics produced are fed to animals. These antibiotics are given to livestock in low doses, which maximize
the probability of resistance developing, and these resistant bacteria are readily transferred to humans.
LINK TQ LEARNING
Watch a recent news report (http:// 0 penstaxc 0 llege. 0 rg/l/antibi 0 tics) on the problem of routine antibiotic
administration to livestock and antibiotic-resistant bacteria.
One of the Superbugs: MRS A
The imprudent use of antibiotics has paved the way for the expansion of resistant bacterial populations. For
example, Staphylococcus aureus, often called “staph,” is a common bacterium that can live in the human
body and is usually easily treated with antibiotics. However, a very dangerous strain, methicillin-resistant
Staphylococcus aureus (MRSA) has made the news over the past few years (Figure 22.25). This strain is
resistant to many commonly used antibiotics, including methicillin, amoxicillin, penicillin, and oxacillin. MRSA
can cause infections of the skin, but it can also infect the bloodstream, lungs, urinary tract, or sites of injury.
While MRSA infections are common among people in healthcare facilities, they have also appeared in healthy
people who haven’t been hospitalized, but who live or work in tight populations (like military personnel and
prisoners). Researchers have expressed concern about the way this latter source of MRSA targets a much
younger population than those residing in care facilities. The Journal of the American Medical Association
reported that, among MRSA-afflicted persons in healthcare facilities, the average age is 68, whereas people with
“community-associated MRSA” ( CA-MRSA) have an average age of 23. [41
4. Naimi, TS, LeDell, KH, Como-Sabetti, K, et al. Comparison of community- and health care-associated methicillin-resistant
Staphylococcus aureus infection. JAMA 290 (2003): 2976-84, doi: 10.1001/jama.290.22.2976.
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Figure 22.25 MRSA. This scanning electron micrograph shows methicillin-resistant Staphylococcus aureus bacteria,
commonly known as MRSA. S. aureus is not always pathogenic, but can cause diseases such as food poisoning and
skin and respiratory infections, (credit: modification of work by Janice Haney Carr; scale-bar data from Matt Russell)
In summary, the medical community is facing an antibiotic crisis. Some scientists believe that after years of
being protected from bacterial infections by antibiotics, we may be returning to a time in which a simple bacterial
infection could again devastate the human population. Researchers are developing new antibiotics, but it takes
many years of research and clinical trials, plus financial investments in the millions of dollars, to generate an
effective and approved drug.
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ca eer connection
Epidemiologist
Epidemiology is the study of the occurrence, distribution, and determinants of health and disease in a
population. It is, therefore, part of public health. An epidemiologist studies the frequency and distribution of
diseases within human populations and environments.
Epidemiologists collect data about a particular disease and track its spread to identify the original mode
of transmission. They sometimes work in close collaboration with historians to try to understand the way
a disease evolved geographically and over time, tracking the natural history of pathogens. They gather
information from clinical records, patient interviews, surveillance, and any other available means. That
information is used to develop strategies, such as vaccinations (Figure 22.26), and design public health
policies to reduce the incidence of a disease or to prevent its spread. Epidemiologists also conduct rapid
investigations in case of an outbreak to recommend immediate measures to control it.
An epidemiologist has a bachelor’s degree, plus a master’s degree in public health (MPH). Many
epidemiologists are also physicians (and have an M.D. or D.O degree), or they have a Ph.D. in an
associated field, such as biology or microbiology.
Figure 22.26 Vaccination. Vaccinations can slow the spread of communicable diseases, (credit: modification of
work by Daniel Paquet)
22.5 | Beneficial Prokaryotes
By the end of this section, you will be able to do the following:
• Explain the need for nitrogen fixation and how it is accomplished
• Describe the beneficial effects of bacteria that colonize our skin and digestive tracts
• Identify prokaryotes used during the processing of food
• Describe the use of prokaryotes in bioremediation
Fortunately, only a few species of prokaryotes are pathogenic! Prokaryotes also interact with humans and other
organisms in a number of ways that are beneficial. For example, prokaryotes are major participants in the carbon
and nitrogen cycles. They produce or process nutrients in the digestive tracts of humans and other animals.
Prokaryotes are used in the production of some human foods, and also have been recruited for the degradation
of hazardous materials. In fact, our life would not be possible without prokaryotes!
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Cooperation between Bacteria and Eukaryotes: Nitrogen Fixation
Nitrogen is a very important element to living things, because it is part of nucleotides and amino acids that are
the building blocks of nucleic acids and proteins, respectively. Nitrogen is usually the most limiting element in
terrestrial ecosystems, with atmospheric nitrogen, N 2 , providing the largest pool of available nitrogen. However,
eukaryotes cannot use atmospheric, gaseous nitrogen to synthesize macromolecules. Fortunately, nitrogen
can be “fixed,” meaning it is converted into a more accessible form—ammonia (NH 3 )—either biologically or
abiotically.
Abiotic nitrogen fixation occurs as a result of physical processes such as lightning or by industrial processes.
Biological nitrogen fixation (BNF) is exclusively carried out by prokaryotes: soil bacteria, cyanobacteria,
and Frankia spp. (filamentous bacteria interacting with actinorhizal plants such as alder, bayberry, and sweet
fern). After photosynthesis, BNF is the most important biological process on Earth. The overall nitrogen fixation
equation below represents a series of redox reactions (Pi stands for inorganic phosphate).
N 2 + 16ATP + 8e" + 8H + -> 2NH 3 + 16ADP + 16Pi + H 2
The total fixed nitrogen through BNF is about 100 to 180 million metric tons per year, which contributes about 65
percent of the nitrogen used in agriculture.
Cyanobacteria are the most important nitrogen fixers in aquatic environments. In soil, members of the genera
Clostridium and Azotobacter are examples of free-living, nitrogen-fixing bacteria. Other bacteria live
symbiotically with legume plants, providing the most important source of fixed nitrogen. Symbionts may fix
more nitrogen in soils than free-living organisms by a factor of 10. Soil bacteria, collectively called rhizobia,
are able to symbiotically interact with legumes to form nodules, specialized structures where nitrogen fixation
occurs (Figure 22.27). Nitrogenase, the enzyme that fixes nitrogen, is inactivated by oxygen, so the nodule
provides an oxygen-free area for nitrogen fixation to take place. The oxygen is sequestered by a form of
plant hemoglobin called leghemoglobin, which protects the nitrogenase, but releases enough oxygen to support
respiratory activity.
Symbiotic nitrogen fixation provides a natural and inexpensive plant fertilizer: It reduces atmospheric nitrogen
to ammonia, which is easily usable by plants. The use of legumes is an excellent alternative to chemical
fertilization and is of special interest to sustainable agriculture, which seeks to minimize the use of chemicals and
conserve natural resources. Through symbiotic nitrogen fixation, the plant benefits from using an endless source
of nitrogen: the atmosphere. The bacteria benefit from using photosynthates (carbohydrates produced during
photosynthesis) from the plant and having a protected niche. In addition, the soil benefits from being naturally
fertilized. Therefore, the use of rhizobia as biofertilizers is a sustainable practice.
Why are legumes so important? Some, like soybeans, are key sources of agricultural protein. Some of the most
important legumes consumed by humans are soybeans, peanuts, peas, chickpeas, and beans. Other legumes,
such as alfalfa, are used to feed cattle.
Figure 22.27 Nitrogen-fixation nodules on soybean roots. Soybean ( Glycine max ) is a legume that interacts
symbiotically with the soil bacterium Bradyrhizobium japonicum to form specialized structures on the roots called
nodules where nitrogen fixation occurs, (credit: USDA)
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Microbes on the Human Body
The commensal bacteria that inhabit our skin and gastrointestinal tract do a host of good things for us. They
protect us from pathogens, help us digest our food, and produce some of our vitamins and other nutrients.
These activities have been known for a long time. More recently, scientists have gathered evidence that
these bacteria may also help regulate our moods, influence our activity levels, and even help control weight
by affecting our food choices and absorption patterns. The Human Microbiome Project has begun the
process of cataloging our normal bacteria (and archaea) so we can better understand these functions.
A particularly fascinating example of our normal flora relates to our digestive systems. People who take
high doses of antibiotics tend to lose many of their normal gut bacteria, allowing a naturally antibiotic-
resistant species called Clostridium difficile to overgrow and cause severe gastric problems, especially
chronic diarrhea (Figure 22.28). Obviously, trying to treat this problem with antibiotics only makes it worse.
However, it has been successfully treated by giving the patients fecal transplants from healthy donors to
reestablish the normal intestinal microbial community. Clinical trials are underway to ensure the safety and
effectiveness of this technique.
Figure 22.28 Clostridium difficile. This scanning electron micrograph shows Clostridium difficile, a Gram-positive,
rod-shaped bacterium that causes severe diarrhea. Infection commonly occurs after the normal gut fauna are
eradicated by antibiotics, and in the hospital can be deadly to seriously ill patients, (credit: modification of work by
CDC, HHS; scale-bar data from Matt Russell)
Scientists are also discovering that the absence of certain key microbes from our intestinal tract may set
us up for a variety of problems. This seems to be particularly true regarding the appropriate functioning of
the immune system. There are intriguing findings that suggest that the absence of these microbes is an
important contributor to the development of allergies and some autoimmune disorders. Research is currently
underway to test whether adding certain microbes to our internal ecosystem may help in the treatment of
these problems, as well as in treating some forms of autism.
Early Biotechnology: Cheese, Bread, Wine, Beer, and Yogurt
According to the United Nations Convention on Biological Diversity, biotechnology is “any technological
application that uses biological systems, living organisms, or derivatives thereof, to make or modify products
[5]
or processes for specific use." The concept of “specific use” involves some sort of commercial application.
Genetic engineering, artificial selection, antibiotic production, and cell culture are current topics of study in
biotechnology and will be described in later chapters. However, humans were using prokaryotes before the term
biotechnology was even coined. Some of the products of this early biotechnology are as familiar as cheese,
bread, wine, beer, and yogurt, which employ both bacteria and other microbes, such as yeast, a fungus (Figure
22.29).
5. http://www.cbd.int/convention/articles/?a=cbd-02 (http://openstax.Org/l/UN_convention) , United Nations Convention on Biological
Diversity: Article 2: Use of Terms.
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<c) (d)
Figure 22.29 Some foods produced by microorganisms. Some of the products derived from the use of prokaryotes
in early biotechnology include (a) cheese, (b) wine, (c) beer and bread, and (d) yogurt, (credit bread: modification of
work by F. Rodrigo/Wikimedia Commons; credit wine: modification of work by Jon Sullivan; credit beer and bread:
modification of work by Kris Miller; credit yogurt: modification of work by Jon Sullivan)
Cheese production began around 4,000 to 7,000 years ago when humans began to breed animals and process
their milk. Fermentation in this case preserves nutrients: Milk will spoil relatively quickly, but when processed
as cheese, it is more stable. As for beer, the oldest records of brewing are about 6,000 years old and were
an integral part of the Sumerian culture. Evidence indicates that the Sumerians discovered fermentation by
chance. Wine has been produced for about 4,500 years, and evidence suggests that cultured milk products, like
yogurt, have existed for at least 4,000 years.
Using Prokaryotes to Clean up Our Planet: Bioremediation
Microbial bioremediation is the use of prokaryotes (or microbial metabolism) to remove pollutants.
Bioremediation has been used to remove agricultural chemicals (e.g., pesticides, fertilizers) that leach from
soil into groundwater and the subsurface. Certain toxic metals and oxides, such as selenium and arsenic
compounds, can also be removed from water by bioremediation. The reduction of SeC> 4~ 2 to SeC> 3~ 2 and to Se°
(metallic selenium) is a method used to remove selenium ions from water. Mercury (Hg) is an example of a toxic
metal that can be removed from an environment by bioremediation. As an active ingredient of some pesticides,
mercury is used in industry and is also a by-product of certain processes, such as battery production. Methyl
mercury is usually present in very low concentrations in natural environments, but it is highly toxic because it
accumulates in living tissues. Several species of bacteria can carry out the biotransformation of toxic mercury
into nontoxic forms. These bacteria, such as Pseudomonas aeruginosa, can convert Hg +2 into Hg°, which is
nontoxic to humans.
One of the most useful and interesting examples of the use of prokaryotes for bioremediation purposes is the
cleanup of oil spills. The significance of prokaryotes to petroleum bioremediation has been demonstrated in
several oil spills in recent years, such as the Exxon Valdez spill in Alaska (1989) (Figure 22.30), the Prestige
oil spill in Spain (2002), the spill into the Mediterranean from a Lebanon power plant (2006), and more recently,
the BP oil spill in the Gulf of Mexico (2010). In the case of oil spills in the ocean, ongoing natural bioremediation
tends to occur, since there are oil-consuming bacteria in the ocean prior to the spill. In addition to these naturally
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occurring oil-degrading bacteria, humans select and engineer bacteria that possess the same capability with
increased efficacy and spectrum of hydrocarbon compounds that can be processed. Bioremediation is enhanced
by the addition of inorganic nutrients that help bacteria to grow.
Some hydrocarbon-degrading bacteria feed on hydrocarbons in the oil droplet, breaking down the hydrocarbons
into smaller subunits. Some species, such as Alcanivorax borkumensis, produce surfactants that solubilize
the oil (making it soluble in water), whereas other bacteria degrade the oil into carbon dioxide. Under ideal
conditions, it has been reported that up to 80 percent of the nonvolatile components in oil can be degraded within
one year of the spill. Other oil fractions containing aromatic and highly branched hydrocarbon chains are more
difficult to remove and remain in the environment for longer periods of time.
Figure 22.30 Prokaryotes and bioremediation, (a) Cleaning up oil after the Exxon Valdez spill in Alaska, workers hosed
oil from beaches and then used a floating boom to corral the oil, which was finally skimmed from the water surface.
Some species of bacteria are able to solubilize and degrade the oil. (b) One of the most catastrophic consequences
of oil spills is the damage to fauna, (credit a: modification of work by NOAA; credit b: modification of work by
GOLUBENKOV, NGO: Saving Taman)
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Chapter 22 | Prokaryotes: Bacteria and Archaea
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KEY TERMS
acidophile organism with optimal growth pH of three or below
alkaliphile organism with optimal growth pH of nine or above
ammonification process by which ammonia is released during the decomposition of nitrogen-containing
organic compounds
anaerobic refers to organisms that grow without oxygen
anoxic without oxygen
antibiotic biological substance that, in low concentration, is antagonistic to the growth of prokaryotes
biofilm microbial community that is held together by a gummy-textured matrix
biological nitrogen fixation conversion of atmospheric nitrogen into ammonia exclusively carried out by
prokaryotes
bioremediation use of microbial metabolism to remove pollutants
biotechnology any technological application that uses living organisms, biological systems, or their derivatives
to produce or modify other products
Black Death devastating pandemic that is believed to have been an outbreak of bubonic plague caused by the
bacterium Yersinia pestis
botulism disease produced by the toxin of the anaerobic bacterium Clostridium botulinum
CA-MRSA MRSA acquired in the community rather than in a hospital
capsule external structure that enables a prokaryote to attach to surfaces and protects it from dehydration
chemotroph organism that obtains energy from chemical compounds
conjugation process by which prokaryotes move DNA from one individual to another using a pilus
cyanobacteria bacteria that evolved from early phototrophs and oxygenated the atmosphere; also known as
blue-green algae
decomposer organism that carries out the decomposition of dead organisms
denitrification transformation of nitrate from soil to gaseous nitrogen compounds such as N 2 O, NO, and N 2
emerging disease disease making an initial appearance in a population or that is increasing in incidence or
geographic range
endemic disease disease that is constantly present, usually at low incidence, in a population
epidemic disease that occurs in an unusually high number of individuals in a population at the same time
extremophile organism that grows under extreme or harsh conditions
foodborne disease any illness resulting from the consumption of contaminated food, or of the pathogenic
bacteria, viruses, or other parasites that contaminate food
Gram negative bacterium whose cell wall contains little peptidoglycan but has an outer membrane
Gram positive bacterium that contains mainly peptidoglycan in its cell walls
halophile organism that require a salt concentration of at least 0.2 M
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hydrothermal vent fissure in Earth’s surface that releases geothermally heated water
hyperthermophile organism that grows at temperatures between 80-122 °C
microbial mat multi-layered sheet of prokaryotes that may include bacteria and archaea
MRSA (methicillin-resistant Staphylococcus aureus) very dangerous Staphylococcus aureus strain resistant to
multiple antibiotics
nitrification conversion of ammonium into nitrite and nitrate in soils
nitrogen fixation process by which gaseous nitrogen is transformed, or “fixed” into more readily available forms
such as ammonia
nodule novel structure on the roots of certain plants (legumes) that results from the symbiotic interaction
between the plant and soil bacteria, and is the site of nitrogen fixation
nutrient essential substances for growth, such as carbon and nitrogen
osmophile organism that grows in a high sugar concentration
pandemic widespread, usually worldwide, epidemic disease
peptidoglycan material composed of polysaccharide chains cross-linked to unusual peptides
phototroph organism that is able to make its own food by converting solar energy to chemical energy
pilus surface appendage of some prokaryotes used for attachment to surfaces including other prokaryotes
pseudopeptidoglycan component of archaea cell walls that is similar to peptidoglycan in morphology but
contains different sugars
psychrophile organism that grows at temperatures of -15 °C or lower
radioresistant organism that grows in high levels of radiation
resuscitation process by which prokaryotes that are in the VBNC state return to viability
S-layer surface-layer protein present on the outside of cell walls of archaea and bacteria
serotype strain of bacterium that carries a set of similar antigens on its cell surface, often many in a bacterial
species
stromatolite layered sedimentary structure formed by precipitation of minerals by prokaryotes in microbial mats
teichoic acid polymer associated with the cell wall of Gram-positive bacteria
thermophile organism that lives at temperatures between 60-80 °C
transduction process by which a bacteriophage moves DNA from one prokaryote to another
transformation process by which a prokaryote takes in DNA found in its environment that is shed by other
prokaryotes
viable-but-non-culturable (VBNC) state survival mechanism of bacteria facing environmental stress
conditions
zoonosis disease that primarily infects animals that is transmitted to humans
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CHAPTER SUMMARY
22.1 Prokaryotic Diversity
Prokaryotes existed for billions of years before plants and animals appeared. Hot springs and hydrothermal
vents may have been the environments in which life began. Microbial mats are thought to represent the earliest
forms of life on Earth. A microbial mat is a multi-layered sheet of prokaryotes that grows at interfaces between
different types of material, mostly on moist surfaces. Fossilized microbial mats are called stromatolites and
consist of laminated organo-sedimentary structures formed by precipitation of minerals by prokaryotes. They
represent the earliest fossil record of life on Earth.
During the first two billion years, the atmosphere was anoxic and only anaerobic organisms were able to live.
Cyanobacteria evolved from early phototrophs and began the oxygenation o the atmosphere. The increase in
oxygen concentration allowed the evolution of other life forms.
Bacteria and archaea grow in virtually every environment. Those that survive under extreme conditions are
called extremophiles (extreme lovers). Some prokaryotes cannot grow in a laboratory setting, but they are not
dead. They are in the viable-but-non-culturable (VBNC) state. The VBNC state occurs when prokaryotes enter
a dormant state in response to environmental stressors. Most prokaryotes are colonial and prefer to live in
communities where interactions take place. A biofilm is a microbial community held together in a gummy-
textured matrix.
22.2 Structure of Prokaryotes: Bacteria and Archaea
Prokaryotes (domains Archaea and Bacteria) are single-celled organisms that lack a nucleus. They have a
single piece of circular DNA in the nucleoid area of the cell. Most prokaryotes have a cell wall that lies outside
the boundary of the plasma membrane. Some prokaryotes may have additional structures such as a capsule,
flagella, and pili. Bacteria and Archaea differ in the lipid composition of their cell membranes and the
characteristics of the cell wall. In archaeal membranes, phytanyl units, rather than fatty acids, are linked to
glycerol. Some archaeal membranes are lipid monolayers instead of bilayers.
The cell wall is located outside the cell membrane and prevents osmotic lysis. The chemical composition of cell
walls varies between species. Bacterial cell walls contain peptidoglycan. Archaean cell walls do not have
peptidoglycan, but they may have pseudopeptidoglycan, polysaccharides, glycoproteins, or protein-based cell
walls. Bacteria can be divided into two major groups: Gram positive and Gram negative, based on the Gram
stain reaction. Gram-positive organisms have a thick peptidoglycan layer fortified with teichoic acids. Gram¬
negative organisms have a thin cell wall and an outer envelope containing lipopolysaccharides and
lipoproteins.
Prokaryotes can transfer DNA from one cell to another by three mechanisms: transformation (uptake of
environmental DNA), transduction (transfer of genomic DNA via viruses), and conjugation (transfer of DNA by
direct cell contact).
22.3 Prokaryotic Metabolism
As the oldest living inhabitants of Earth, prokaryotes are also the most metabolically diverse; they flourish in
many different environments with various energy and carbon sources, variable temperature, pH, pressure,
oxygen and water availability. Nutrients required in large amounts are called macronutrients, whereas those
required in trace amounts are called micronutrients or trace elements. Macronutrients include C, H, O, N, P, S,
K, Mg, Ca, and Na. In addition to these macronutrients, prokaryotes require various metallic elements for
growth and enzyme function. Prokaryotes use different sources of energy to assemble macromolecules from
smaller molecules. Phototrophs obtain their energy from sunlight, whereas chemotrophs obtain energy from
chemical compounds. Energy-producing pathways may be either aerobic or anaerobic.
Prokaryotes play roles in the carbon and nitrogen cycles. Producers capture carbon dioxide from the
atmosphere and convert it to organic compounds. Consumers (animals and other chemoorganotrophic
organisms) use organic compounds generated by producers and release carbon dioxide into the atmosphere
by respiration. Carbon dioxide is also returned to the atmosphere by the microbial decomposers of dead
organisms. Nitrogen also cycles in and out of living organisms, from organic compounds to ammonia,
ammonium ions, nitrite, nitrate, and nitrogen gas. Prokaryotes are essential for most of these conversions.
Gaseous nitrogen is transformed into ammonia through nitrogen fixation. Ammonia is anaerobically catabolized
by some prokaryotes, yielding N 2 as the final product. Nitrification is the conversion of ammonium into nitrite.
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Nitrification in soils is carried out by bacteria. Denitrification is also performed by bacteria and transforms nitrate
from soils into gaseous nitrogen compounds, such as N 2 O, NO, and N 2 .
22.4 Bacterial Diseases in Humans
Some prokaryotes are human pathogens. Devastating diseases and plagues have been among us since early
times and remain among the leading causes of death worldwide. Emerging diseases are those rapidly
increasing in incidence or geographic range. They can be new or re-emerging diseases (previously under
control). Many emerging diseases affecting humans originate in animals (zoonoses), such as brucellosis. A
group of re-emerging bacterial diseases recently identified by WHO for monitoring include bubonic plague,
diphtheria, and cholera. Foodborne diseases result from the consumption of food contaminated with food,
pathogenic bacteria, viruses, or parasites.
Some bacterial infections have been associated with biofilms: Legionnaires’ disease, otitis media, and infection
of patients with cystic fibrosis. Biofilms can grow on human tissues, like dental plaque; colonize medical
devices; and cause infection or produce foodborne disease by growing on the surfaces of food and food¬
processing equipment. Biofilms are resistant to most of the methods used to control microbial growth. The
excessive use of antibiotics has resulted in a major global problem, since resistant forms of bacteria have been
selected over time. A very dangerous strain, methicillin-resistant Staphylococcus aureus (MRSA), has wreaked
havoc recently across the world.
22.5 Beneficial Prokaryotes
Pathogens are only a small percentage of all prokaryotes. In fact, prokaryotes provide essential services to
humans and other organisms. Nitrogen, which is not usable by eukaryotes in its plentiful atmospheric form, can
be “fixed,” or converted into ammonia (NH 3 ) either biologically or abiotically. Biological nitrogen fixation (BNF) is
exclusively carried out by prokaryotes, and constitutes the second most important biological process on Earth.
Although some terrestrial nitrogen is fixed by free-living bacteria, most BNF comes from the symbiotic
interaction between soil rhizobia and the roots of legume plants.
Human life is only possible due to the action of microbes, both those in the environment and those species that
call us home. Internally, they help us digest our food, produce vital nutrients for us, protect us from pathogenic
microbes, and help train our immune systems to function properly.
Microbial bioremediation is the use of microbial metabolism to remove pollutants. Bioremediation has been
used to remove agricultural chemicals that leach from soil into groundwater and the subsurface. Toxic metals
and oxides, such as selenium and arsenic compounds, can also be removed by bioremediation. Probably one
of the most useful and interesting examples of the use of prokaryotes for bioremediation purposes is the
cleanup of oil spills.
VISUAL CONNECTION QUESTIONS
1. Figure 22.8 Compared to free-floating bacteria,
bacteria in biofilms often show increased resistance
to antibiotics and detergents. Why do you think this
might be the case?
2. Figure 22.16 Which of the following statements is
true?
a. Gram-positive bacteria have a single cell
wall anchored to the cell membrane by
lipoteichoic acid.
b. Porins allow entry of substances into both
Gram-positive and Gram-negative bacteria.
c. The cell wall of Gram-negative bacteria is
thick, and the cell wall of Gram-positive
bacteria is thin.
d. Gram-negative bacteria have a cell wall
made of peptidoglycan, whereas Gram¬
positive bacteria have a cell wall made of
lipoteichoic acid.
3. Figure 22.19 Which of the following statements
about the nitrogen cycle is false?
a. Nitrogen fixing bacteria exist on the root
nodules of legumes and in the soil.
b. Denitrifying bacteria convert nitrates (NO 3 ')
into nitrogen gas (N 2 ).
c. Ammonification is the process by which
ammonium ion (NH 4 + ) is released from
decomposing organic compounds.
d. Nitrification is the process by which nitrites
(NO 2 ’) are converted to ammonium ion
(NH 4 + ).
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REVIEW QUESTIONS
4. The first forms of life on Earth were thought to
be_.
a. single-celled plants
b. prokaryotes
c. insects
d. large animals such as dinosaurs
5. Microbial mats_.
a. are the earliest forms of life on Earth
b. obtained their energy and food from
hydrothermal vents
c. are multi-layered sheets of prokaryotes
including mostly bacteria but also archaea
d. all of the above
6. The first organisms that oxygenated the
atmosphere were
a. cyanobacteria
b. phototrophic organisms
c. anaerobic organisms
d. all of the above
7. Halophiles are organisms that require_.
a. a salt concentration of at least 0.2 M
b. high sugar concentration
c. the addition of halogens
d. all of the above
8. Many of the first prokaryotes to be cultured in a
scientific lab were human or animal pathogens. Why
would these species be more readily cultured than
non-pathogenic prokaryotes?
a. Pathogenic prokaryotes are hardier than
non-pathogenic prokaryotes.
b. Non-pathogenic prokaryotes require more
supplements in their growth media.
c. Most of the necessary culture conditions
could be inferred for pathogenic
prokaryotes.
d. Pathogenic bacteria can grow as free
bacteria, but non-pathogenic bacteria only
grow as parts of large colonies.
9. The presence of a membrane-enclosed nucleus is
a characteristic of_.
a. prokaryotic cells
b. eukaryotic cells
c. all cells
d. viruses
10. Which of the following consist of prokaryotic
cells?
a. bacteria and fungi
b. archaea and fungi
c. protists and animals
d. bacteria and archaea
11. The cell wall is_.
a. interior to the cell membrane
b. exterior to the cell membrane
c. a part of the cell membrane
d. interior or exterior, depending on the
particular cell
12. Organisms most likely to be found in extreme
environments are_.
a. fungi
b. bacteria
c. viruses
d. archaea
13. Prokaryotes stain as Gram-positive or Gram¬
negative because of differences in the cell_.
a. wall
b. cytoplasm
c. nucleus
d. chromosome
14. Pseudopeptidoglycan is a characteristic of the
walls of_.
a. eukaryotic cells
b. bacterial prokaryotic cells
c. archaean prokaryotic cells
d. bacterial and archaean prokaryotic cells
15. The lipopolysaccharide layer (LPS) is a
characteristic of the wall of_.
a. archaean cells
b. Gram-negative bacteria
c. bacterial prokaryotic cells
d. eukaryotic cells
16. Which of the following elements is not a
micronutrient?
a. boron
b. calcium
c. chromium
d. manganese
17. Prokaryotes that obtain their energy from
chemical compounds are called_.
a. phototrophs
b. auxotrophs
c. chemotrophs
d. lithotrophs
18. Ammonification is the process by which_.
a. ammonia is released during the
decomposition of nitrogen-containing
organic compounds
b. ammonium is converted to nitrite and nitrate
in soils
c. nitrate from soil is transformed to gaseous
nitrogen compounds such as NO, N 2 O, and
N 2
d. gaseous nitrogen is fixed to yield ammonia
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Chapter 22 | Prokaryotes: Bacteria and Archaea
19. Plants use carbon dioxide from the air and are
therefore called_.
a. consumers
b. producers
c. decomposer
d. carbon fixers
20. Cyanobacteria harness energy from the sun
through photosynthesis, and oxidize water to provide
electrons for energy generation. Thus, we classify
cyanobacteria as_.
a. photolithotrophs
b. photoautotrophs
c. chemolithoautotrophs
d. chemo-organotrophs
21. A disease that is constantly present in a
population is called_.
a. pandemic
b. epidemic
c. endemic
d. re-emerging
22. Which of the statements about biofilms is
incorrect?
a. Biofilms are considered responsible for
diseases such as cystic fibrosis.
b. Biofilms produce dental plaque, and
colonize catheters and prostheses.
c. Biofilms colonize open wounds and burned
tissue.
d. All statements are incorrect.
23. Which of these statements is true?
a. An antibiotic is any substance produced by
a organism that is antagonistic to the growth
of prokaryotes.
b. An antibiotic is any substance produced by
a prokaryote that is antagonistic to the
growth of other viruses.
c. An antibiotic is any substance produced by
a prokaryote that is antagonistic to the
growth of eukaryotic cells.
d. An antibiotic is any substance produced by
a prokaryote that prevents growth of the
same prokaryote.
24. A person in England arrives at a medical clinic
with a fever and swollen lymph nodes shortly after
returning from a visit to New Mexico. For which
bacteria should the doctor test the patient?
CRITICAL THINKING QUESTIONS
30. Describe briefly how you would detect the
presence of a non-culturable prokaryote in an
environmental sample.
31. Why do scientists believe that the first organisms
on Earth were extremophiles?
a. Salmonella enterica
b. Borrelia burgdorferi
c. Clostridium botulinum
d. Yersinia pestis
25. MRSA has emerged as a serious infectious
disease, with the first case of methicillin-resistant S.
aureus being detected in 1961. Why are medical
professionals so concerned when antibiotics exist
that can kill MRSA?
a. MRSA can transfer methicillin-resistance to
other bacteria.
b. Patients are not treated with correct
antibiotics rapidly enough to prevent serious
illness.
c. MRSA could acquire additional antibiotic
resistance genes from other bacteria to
become a “super bug.”
d. All of the above.
26. Which of these occurs through symbiotic nitrogen
fixation?
a. The plant benefits from using an endless
source of nitrogen.
b. The soil benefits from being naturally
fertilized.
c. Bacteria benefit from using photosynthates
from the plant.
d. All of the above occur.
27. Synthetic compounds found in an organism but
not normally produced or expected to be present in
that organism are called_.
a. pesticides
b. bioremediators
c. recalcitrant compounds
d. xenobiotics
28. Bioremediation includes_.
a. the use of prokaryotes that can fix nitrogen
b. the use of prokaryotes to clean up pollutants
c. the use of prokaryotes as natural fertilizers
d. All of the above
29. in addition to providing yogurt with its unique
flavor and texture, lactic acid-producing bacteria also
provide which additional benefit during food
production?
a. Providing xenobiotics
b. Lowering the pH to kill pathogenic bacteria
c. Pasteurizing milk products
d. Breaking down lactose for lactose-intolerant
individuals
32. A new bacterial species is discovered and
classified as an endolith, an extremophile that lives
inside rock. If the bacteria were discovered in the
permafrost of Antarctica, describe two extremophile
features the bacteria must possess.
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33. Mention three differences between bacteria and
archaea.
34. Explain the statement that both types, bacteria
and archaea, have the same basic structures, but
built from different chemical components.
35. A scientist isolates a new species of prokaryote.
He notes that the specimen is a bacillus with a lipid
bilayer and cell wall that stains positive for
peptidoglycan. Its circular chromosome replicates
from a single origin of replication. Is the specimen
most likely an Archaea, a Gram-positive bacterium,
or a Gram-negative bacterium? How do you know?
36. Think about the conditions (temperature, light,
pressure, and organic and inorganic materials) that
you may find in a deep-sea hydrothermal vent. What
type of prokaryotes, in terms of their metabolic needs
(autotrophs, phototrophs, chemotrophs, etc.), would
you expect to find there?
37. Farmers continually rotate the crops grown in
different fields to maintain nutrients in the soil. How
would planting soybeans in a field the year after the
field was used to grow carrots help maintain nitrogen
in the soil?
38. Imagine a region of soil became contaminated,
killing bacteria that decompose dead plants and
animals. How would this effect the carbon cycle in
the area? Be specific in stating where carbon would
accumulate in the cycle.
39. Explain the reason why the imprudent and
excessive use of antibiotics has resulted in a major
global problem.
40. Researchers have discovered that washing
spinach with water several times does not prevent
foodborne diseases due to E. coli. How can you
explain this fact?
41. Your friend believes that prokaryotes are always
detrimental and pathogenic. How would you explain
to them that they are wrong?
42. Many people use antimicrobial soap to kill
bacteria on their hands. However, overuse may
actually increase the risk of infection. How could this
occur?
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Chapter 23 | Protists
629
23 | PROTISTS
(a) (b) (c)
Figure 23.1 Protists range from the microscopic, single-celled (a) Acanthocystis turfacea and the (b) ciliate
Tetrahymena thermophila, both visualized here using light microscopy, to the enormous, multicellular (c) kelps
(Chromalveolata) that extend for hundreds of feet in underwater “forests.” (credit a: modification of work by Yuiuji Tsukii;
credit b: modification of work by Richard Robinson, Public Library of Science; credit c: modification of work by Kip
Evans, NOAA; scale-bar data from Matt Russell)
Chapter Outline
23.1: Eukaryotic Origins
23.2: Characteristics of Protists
23.3: Groups of Protists
23.4: Ecology of Protists
Introduction
Humans have been familiar with macroscopic organisms (organisms big enough to see with the unaided eye)
since before there was a written history, and it is likely that most cultures distinguished between animals and
land plants, and most probably included the macroscopic fungi as plants. Therefore, it became an interesting
challenge to deal with the world of microorganisms once microscopes were developed a few centuries ago.
Many different naming schemes were used over the last couple of centuries, but it has become the most
common practice to refer to eukaryotes that are not land plants, animals, or fungi as protists.
This name was first suggested by Ernst Haeckel in the late nineteenth century. It has been applied in many
contexts and has been formally used to represent a kingdom-level taxon called Protista. However, many modern
systematists (biologists who study the relationships among organisms) are beginning to shy away from the idea
of formal ranks such as kingdom and phylum. Instead, they are naming taxa as groups of organisms thought
to include all the descendants of a last common ancestor (monophyletic group). During the past two decades,
the field of molecular genetics has demonstrated that some protists are more related to animals, plants, or fungi
than they are to other protists. Therefore, not including animals, plants, and fungi make the kingdom Protista a
paraphyletic group, or one that does not include all descendents of its common ancestor. For this reason, protist
lineages originally classified into the kingdom Protista continue to be examined and debated. In the meantime,
the term “protist” still is used informally to describe this tremendously diverse group of eukaryotes.
Most protists are microscopic, unicellular organisms that are abundant in soil, freshwater, brackish, and marine
environments. They are also common in the digestive tracts of animals and in the vascular tissues of plants.
Others invade the cells of other protists, animals, and plants. Not all protists are microscopic. Some have huge,
macroscopic cells, such as the plasmodia (giant amoebae) of myxomycete slime molds or the marine green alga
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Chapter 23 | Protists
Caulerpa, which can have single cells that can be several meters in size. Some protists are multicellular, such as
the red, green, and brown seaweeds. It is among the protists that one finds the wealth of ways that organisms
can grow.
23.1 1 Eukaryotic Origins
By the end of this section, you will be able to do the following:
• List the unifying characteristics of eukaryotes
• Describe what scientists know about the origins of eukaryotes based on the last common ancestor
• Explain the endosymbiotic theory
Organisms are classified into three domains: Archaea, Bacteria, and Eukarya. The first two lineages comprise all
prokaryotic cells, and the third contains all eukaryotes. A very sparse fossil record prevents us from determining
what the first members of each of these lineages looked like, so it is possible that all the events that led to the
last common ancestor of extant eukaryotes will remain unknown. However, comparative biology of extant (living)
organisms and the limited fossil record provide some insight into the evolution of Eukarya.
The earliest fossils found appear to be those of domain Bacteria, most likely cyanobacteria. They are about 3.5
to 3.8 billion years old and are recognizable because of their relatively complex structure and, for prokaryotes,
relatively large cells. Most other prokaryotes have small cells, 1 or 2 pm in size, and would be difficult to pick
out as fossils. Fossil stromatolites suggest that at least some prokaryotes lived in interactive communities, and
evidence from the structure of living eukaryotic cells suggests that it was similar ancestral interactions that gave
rise to the eukaryotes. Most living eukaryotes have cells measuring 10 pm or greater. Structures this size, which
might be fossilized remains of early eukaryotes, appear in the geological record in deposits dating to about 2.1
billion years ago.
Characteristics of Eukaryotes
Data from these fossils, as well as from the study of living genomes, have led comparative biologists to conclude
that living eukaryotes are all descendants of a single common ancestor. Mapping the characteristics found in all
major groups of eukaryotes reveals that the following characteristics are present in at least some of the members
of each major lineage, or during some part of their life cycle, and therefore must have been present in the last
common ancestor.
1. Cells with nuclei surrounded by a nuclear envelope with nuclear pores: This is the single characteristic
that is both necessary and sufficient to define an organism as a eukaryote. All extant eukaryotes have cells
with nuclei.
2. Mitochondria: Most extant eukaryotes have "typical" mitochondria, although some eukaryotes have very
reduced mitochondrial “remnants” and a few lack detectable mitochondria.
3. Cytoskeleton of microtubules and microfilaments: Eukaryotic cells possess the structural and motility
components called actin microfilaments and microtubules. All extant eukaryotes have these cytoskeletal
elements.
4. Flagella and cilia: Organelles associated with cell motility. Some extant eukaryotes lack flagella and/
or cilia, but their presence in related lineages suggests that they are descended from ancestors that
possessed these organelles.
5. Chromosomes organized by histones: Each eukaryotic chromosome consists of a linear DNA molecule
coiled around basic (alkaline) proteins called histones. The few eukaryotes with chromosomes lacking
histones clearly evolved from ancestors that had them.
6. Mitosis: A process of nuclear division in which replicated chromosomes are divided and separated using
elements of the cytoskeleton. Mitosis is universally present in eukaryotes.
7. Sexual reproduction: A meiotic process of nuclear division and genetic recombination unique to
eukaryotes. During this process, diploid nuclei at one stage of the life cycle undergo meiosis to yield haploid
nuclei, which subsequently fuse together (karyogamy) to create a diploid zygote nucleus.
8. Cell walls: It might be reasonable to conclude that the last common ancestor could make cell walls during
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Chapter 23 | Protists
631
some stage of its life cycle, simple because cell walls were present in their prokaryote precursors. However,
not enough is known about eukaryotes’ cell walls and their development to know how much homology exists
between those of prokaryotes and eukaryotes. If the last common ancestor could make cell walls, it is clear
that this ability must have been lost in many groups.
Endosymbiosis and the Evolution of Eukaryotes
Before we discuss the origins of eukaryotes, it is first important to understand that all extant eukaryotes are likely
the descendants of a chimera-like organism that was a composite of a host cell and the cell(s) of an alpha-
proteobacterium that “took up residence” inside it. This major theme in the origin of eukaryotes is known as
endosymbiosis, one cell engulfing another such that the engulfed cell survives and both cells benefit. Over
many generations, a symbiotic relationship can result in two organisms that depend on each other so completely
that neither could survive on its own. Endosymbiotic events likely contributed to the origin of the last common
ancestor of today’s eukaryotes and to later diversification in certain lineages of eukaryotes (Figure 23.5). Similar
endosymbiotic associations are not uncommon in living eukaryotes. Before explaining this further, it is necessary
to consider metabolism in prokaryotes.
Prokaryotic Metabolism
Many important metabolic processes arose in prokaryotes; however, some of these processes, such as nitrogen
fixation, are never found in eukaryotes. The process of aerobic respiration is found in all major lineages
of eukaryotes, and it is localized in the mitochondria. Aerobic respiration is also found in many lineages of
prokaryotes, but it is not present in all of them, and a great deal of evidence suggests that such anaerobic
prokaryotes never carried out aerobic respiration nor did their ancestors.
While today’s atmosphere is about 20 percent molecular oxygen (O 2 ), geological evidence shows that it
originally lacked O 2 . Without oxygen, aerobic respiration would not be expected, and living things would have
relied on anaerobic respiration or the process of fermentation instead. At some point before about 3.8 billion
years ago, some prokaryotes began using energy from sunlight to power anabolic processes that reduce carbon
dioxide to form organic compounds. That is, they evolved the ability to photosynthesize. Hydrogen, derived from
various sources, was “captured” using light-powered reactions to reduce fixed carbon dioxide in the Calvin cycle.
The group of Gram-negative bacteria that gave rise to cyanobacteria used water as the hydrogen source and
released O 2 as a “waste” product.
Eventually, the amount of photosynthetic oxygen built up in some environments to levels that posed a risk to
living organisms, since it can damage many organic compounds. Various metabolic processes evolved that
protected organisms from oxygen, one of which, aerobic respiration, also generated high levels of ATP. It
became widely present among prokaryotes, including in a free-living group we now call alpha-proteobacteria.
Organisms that did not acquire aerobic respiration had to remain in oxygen-free environments. Originally,
oxygen-rich environments were likely localized around places where cyanobacteria were abundant and active,
but by about 2 billion years ago, geological evidence shows that oxygen was building up to higher concentrations
in the atmosphere. Oxygen levels similar to today’s levels only arose within the last 700 million years.
Recall that the first fossils that we believe to be eukaryotes date to about 2 billion years old, so they seemed
to have evolved and diversified rapidly as oxygen levels were increasing. Also, recall that all extant eukaryotes
descended from an ancestor with mitochondria. These organelles were first observed by light microscopists
in the late 1800s, where they appeared to be somewhat worm-shaped structures that seemed to be moving
around in the cell. Some early observers suggested that they might be bacteria living inside host cells, but these
hypotheses remained unknown or rejected in most scientific communities.
Endosymbiotic Theory
As cell biology developed in the twentieth century, it became clear that mitochondria were the organelles
responsible for producing ATP using aerobic respiration, in which oxygen was the final electron acceptor. In
the 1960s, American biologist Lynn Margulis of Boston University developed the endosymbiotic theory, which
states that eukaryotes may have been a product of one cell engulfing another, one living within another, and
coevolving over time until the separate cells were no longer recognizable as such and shared genetic control of a
mutualistic metabolic pathway to produce ATP. In 1967, Margulis introduced new data to support her work on the
theory and substantiated her findings through microbiological evidence. Although Margulis’s work initially was
met with resistance, this basic component of this once-revolutionary hypothesis is now widely accepted, with
work progressing on uncovering the steps involved in this evolutionary process and the key players involved.
While the metabolic organelles and genes responsible for many energy-harvesting processes appear to have
had their origins in bacteria, our nuclear genes and the molecular machinery responsible for replication and
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expression appear to be more closely related to those found in the Archaea. Much remains to be clarified about
how this relationship occurred; this continues to be an exciting field of discovery in biology. For instance, it is
not known whether the endosymbiotic event that led to mitochondria occurred before or after the host cell had a
nucleus. Such organisms would be among the extinct precursors of the last common ancestor of eukaryotes.
Mitochondria
One of the major features distinguishing prokaryotes from eukaryotes is the presence of mitochondria, or their
reduced derivatives, in virtually all eukaryotic cells. Eukaryotic cells may contain anywhere from one to several
thousand mitochondria, depending on the cell’s level of energy consumption, in humans being most abundant in
the liver and skeletal muscles. Each mitochondrion measures 1 to 10 or greater micrometers in length and exists
in the cell as an organelle that can be ovoid to worm-shaped to intricately branched (Figure 23.2). However,
although they may have originated as free-living aerobic organisms, mitochondria can no longer survive and
reproduce outside the cell.
Mitochondria have several features that suggest their relationship to alpha-proteobacteria (Figure 23.5). Alpha-
proteobacteria are a large group of bacteria that includes species symbiotic with plants, disease organisms
that can infect humans via ticks, and many free-living species that use light for energy. Mitochondria have their
own genomes, with a circular chromosome stabilized by attachments to the inner membrane. Mitochondria also
have special ribosomes and transfer RNAs that resemble these same components in prokaryotes. An intriguing
feature of mitochondria is that many of them exhibit minor differences from the universal genetic code. However,
many of the genes for respiratory proteins are now relocated in the nucleus. When these genes are compared
to those of other organisms, they appear to be of alpha-proteobacterial origin. In some eukaryotic groups,
such genes are found in the mitochondria, whereas in other groups, they are found in the nucleus. This has
been interpreted as evidence that over evolutionary time, genes have been transferred from the endosymbiont
chromosome to those of the host genome. This apparent “loss” of genes by the endosymbiont is probably one
explanation why mitochondria cannot live without a host.
Another line of evidence supporting the idea that mitochondria were derived by endosymbiosis comes from the
structure of the mitochondrian itself. Most mitochondria are shaped like alpha-proteobacteria and are surrounded
by two membranes; the inner membrane is bacterial in nature whereas the outer membrane is eukaryotic in
nature. This is exactly what one would expect if one membrane-bound organism was engulfed into a vacuole by
another membrane-bound organism. The outer mitochondrial membrane was derived by the enclosing vesicle,
while the inner membrane was derived from the plasma membrane of the endosymbiont. The mitochondrial
inner membrane is extensive and involves substantial infoldings called cristae that resemble the textured,
outer surface of alpha-proteobacteria. The matrix and inner membrane are rich with the enzymes necessary for
aerobic respiration.
Figure 23.2 Mitochondria. In this transmission electron micrograph of mitochondria in a mammalian lung cell, the
cristae, infoldings of the mitochondrial inner membrane, can be seen in cross-section, (credit: Louise Howard)
The third line of evidence comes from the production of new mitochondria. Mitochondria divide independently
by a process that resembles binary fission in prokaryotes. Mitochondria arise only from previous mitochondria;
they are not formed from scratch (de novo) by the eukaryotic cell. Mitochondria may fuse together; and they may
be moved around inside the cell by interactions with the cytoskeleton. They reproduce within their enclosing cell
and are distributed with the cytoplasm when a cell divides or two cells fuse. Therefore, although these organelles
are highly integrated into the eukaryotic cell, they still reproduce as if they were independent organisms within
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the cell. However, their reproduction is synchronized with the activity and division of the cell. These features all
support the theory that mitochondria were once free-living prokaryotes.
Some living eukaryotes are anaerobic and cannot survive in the presence of too much oxygen. However, a few
appear to lack organelles that could be recognized as mitochondria. In the 1970s and on into the early 1990s,
many biologists suggested that some of these eukaryotes were descended from ancestors whose lineages
had diverged from the lineage of mitochondrion-containing eukaryotes before endosymbiosis occurred. Later
findings suggest that reduced organelles are found in most, if not all, anaerobic eukaryotes, and that virtually all
eukaryotes appear to carry some genes in their nuclei that are of mitochondrial origin.
In addition to the aerobic generation of ATP, mitochondria have several other metabolic functions. One of
these functions is to generate clusters of iron and sulfur that are important cofactors of many enzymes. Such
functions are often associated with the reduced mitochondrion-derived organelles of anaerobic eukaryotes. The
protist Monocercomonoldes, an inhabitant of vertebrate digestive tracts, appears to be an exception; it has
no mitochondria and its genome contains neither genes derived from mitochondria nor nuclear genes related
to mitochondrial maintenance. However, it is related to other protists with reduced mitochondria and probably
represents an end-point in mitochondrial reduction. Although most biologists accept that the last common
ancestor of eukaryotes had mitochondria, it appears that the complex relationship between mitochondria and
their host cell continues to evolve.
Plastids
Some groups of eukaryotes are photosynthetic. Their cells contain, in addition to the standard eukaryotic
organelles, another kind of organelle called a plastid. When such cells are carrying out photosynthesis, their
plastids are rich in the pigment chlorophyll a and a range of other pigments, called accessory pigments, which
are involved in harvesting energy from light. Photosynthetic plastids are called chloroplasts (Figure 23.3).
(a) (b)
Figure 23.3 Chloroplasts. (a) This chloroplast cross-section illustrates its elaborate inner membrane organization.
Stacks of thylakoid membranes compartmentalize photosynthetic enzymes and provide scaffolding for chloroplast
DNA. (b) In this micrograph of Elodea sp., the chloroplasts can be seen as small green spheres, (credit b: modification
of work by Brandon Zierer; scale-bar data from Matt Russell)
Like mitochondria, plastids appear to have an endosymbiotic origin. This hypothesis was also proposed and
championed with the first direct evidence by Lynn Margulis. We now know that plastids are derived from
cyanobacteria that lived inside the cells of an ancestral, aerobic, heterotrophic eukaryote. This is called primary
endosymbiosis, and plastids of primary origin are surrounded by two membranes. However, the best evidence
is that the acquisition of cyanobacterial endosymbionts has happened twice in the history of eukaryotes, in
one case, the common ancestor of the major lineage/supergroup Archaeplastida took on a cyanobacterial
endosymbiont; in the other, the ancestor of the small amoeboid rhizarian taxon, Paulinella, took on a different
cyanobacterial endosymbiont. Almost all photosynthetic eukaryotes are descended from the first event, and only
a couple of species are derived from the other, which in evolutionary terms, appears to be more recent.
Cyanobacteria are a group of Gram-negative bacteria with all the conventional structures of the group. However,
unlike most prokaryotes, they have extensive, internal membrane-bound sacs called thylakoids. Chlorophyll
is a component of these membranes, as are many of the proteins of the light reactions of photosynthesis.
Cyanobacteria also have the peptidoglycan wall and lipopolysaccharide layer associated with Gram-negative
bacteria.
Chloroplasts of primary endosymbiotic origin have thylakoids, a circular DNA chromosome, and ribosomes
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similar to those of cyanobacteria. As in mitochondria, each chloroplast is surrounded by two membranes. The
outer membrane is thought to be derived from the enclosing vacuole of the host, and the inner membrane
is thought to be derived from the plasma membrane of the cyanobacterial endosymbiont. In the group of
Archaeplastida called the glaucophytes and in the rhizarian Paulinella, a thin peptidoglycan layer is still present
between the outer and inner plastid membranes. All other plastids lack this relict of the cyanobacterial wall.
There is also, as with the case of mitochondria, strong evidence that many of the genes of the endosymbiont
were transferred to the nucleus. Plastids, like mitochondria, cannot live independently outside the host. In
addition, like mitochondria, plastids are derived from the division of other plastids and never built from scratch.
Researchers have suggested that the endosymbiotic event that led to Archaeplastida occurred 1 to 1.5 billion
years ago, at least 5 hundred million years after the fossil record suggests that eukaryotes were present.
Not all plastids in eukaryotes are derived directly from primary endosymbiosis. Some of the major groups
of algae became photosynthetic by secondary endosymbiosis, that is, by taking in either green algae or red
algae (both from Archaeplastida) as endosymbionts (Figure 23.4). Numerous microscopic and genetic studies
have supported this conclusion. Secondary plastids are surrounded by three or more membranes, and some
secondary plastids even have clear remnants of the nucleus (nucleomorphs) of endosymbiotic algae. There are
even cases where tertiary or higher-order endosymbiotic events are the best explanations for the features of
some eukaryotic plastids.
Figure 23.4 Algae, (a) Red algae and (b) green algae (seen here by light microscopy) share similar DNA sequences
with photosynthetic cyanobacteria. Scientists speculate that, in a process called endosymbiosis, an ancestral
prokaryote engulfed a photosynthetic cyanobacterium that evolved into modern-day chloroplasts. (credit a:
modification of work by Ed Bierman; credit b: modification of work by G. Fahnenstiel, NOAA; scale-bar data from Matt
Russell)
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visual
CONNECTION
The ENDOSYMBIOTIC THEORY
Proto-eukaryote
2 In a first endosymbiotic event,
the ancestral eukaryote
consumed aerobic bacteria
that evolved into mitochondria.
3 In a second endosymbiotic
event, the early eukaryote
consumed photosynthetic
bacteria that evolved into
chloroplasts.
Endoplasmic
reticulum
Photosynthetic
bacterium
1 Infoldings in the plasma
membrane of an ancestral
prokaryote gave rise to
endomembrane components,
including a nucleus and
endoplasmic reticulum.
Nucleus
Modern photosynthetic
eukaryote
Mitochondrion
Modern heterotrophic eukaryote
Figure 23.5 The Endosymbiotic Theory. The first eukaryote may have originated from an ancestral prokaryote
that had undergone membrane proliferation, compartmentalization of cellular function (into a nucleus, lysosomes,
and an endoplasmic reticulum), and the establishment of endosymbiotic relationships with an aerobic prokaryote,
and, in some cases, a photosynthetic prokaryote, to form mitochondria and chloroplasts, respectively.
What evidence is there that mitochondria were incorporated into the ancestral eukaryotic cell before
chloroplasts?
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V /
e olution CONNECTION
Secondary Endosymbiosis in Chlorarachniophytes
Endosymbiosis involves one cell engulfing another to produce, over time, a coevolved relationship in which
neither cell could survive alone. The chloroplasts of red and green algae, for instance, are derived from the
engulfment of a photosynthetic cyanobacterium by an ancestral prokaryote.
This evidence suggests the possibility that an ancestral cell (already containing a photosynthetic
endosymbiont) was engulfed by another eukaryote cell, resulting in a secondary endosymbiosis. Molecular
and morphological evidence suggest that the chlorarachniophyte protists are derived from a secondary
endosymbiotic event. Chlorarachniophytes are rare algae indigenous to tropical seas and sand. They
are classified into the Rhizarian supergroup. Chlorarachniophytes are reticulose amoebae, extending thin
cytoplasmic strands that interconnect them with other chlorarachniophytes in a cytoplasmic network. These
protists are thought to have originated when a eukaryote engulfed a green alga, the latter of which had
previously established an endosymbiotic relationship with a photosynthetic cyanobacterium (Figure 23.6).
Cyanobacterium
Primary
endosymbiosis
Heterotrophic
eukaryote
One of the three
membranes
surrounding the
plastid is lost.
Vestigial nucleus Plastid
endosymbiosis
Figure 23.6 Secondary endosymbiosis. The hypothesized process of several endosymbiotic events leading to
the evolution of chlorarachniophytes is shown. In a primary endosymbiotic event, a heterotrophic eukaryote
consumed a cyanobacterium. In a secondary endosymbiotic event, the cell resulting from primary endosymbiosis
was consumed by a second cell. The resulting organelle became a plastid in modern chlorarachniophytes.
Several lines of evidence support that chlorarachniophytes evolved from secondary endosymbiosis. The
chloroplasts contained within the green algal endosymbionts still are capable of photosynthesis, making
chlorarachniophytes photosynthetic. The green algal endosymbiont also exhibits a vestigial nucleus. In fact,
it appears that chlorarachniophytes are the products of an evolutionarily recent secondary endosymbiotic
event. The plastids of chlorarachniophytes are surrounded by four membranes'. The first two correspond
to the inner and outer membranes of the photosynthetic cyanobacterium, the third corresponds to plasma
membrane of the green alga, and the fourth corresponds to the vacuole that surrounded the green
alga when it was engulfed by the chlorarachniophyte ancestor. In other lineages that involved secondary
endosymbiosis, only three membranes can be identified around plastids. This is currently interpreted as a
sequential loss of a membrane during the course of evolution.
The process of secondary endosymbiosis is not unique to chlorarachniophytes. Secondary plastids are also
found in the Excavates and the Chromalveolates. In the Excavates, secondary endosymbiosis of green
algae led to euglenid protists, while in the Chromalveolates, secondary endosymbiosis of red algae led to
the evolution of plastids in dinoflagellates, apicomplexans, and stramenopiles.
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23.2 | Characteristics of Protists
By the end of this section, you will be able to do the following:
• Describe the cell structure characteristics of protists
• Describe the metabolic diversity of protists
• Describe the life cycle diversity of protists
There are over 100,000 described living species of protists, and it is unclear how many undescribed species may
exist. Since many protists live as commensals or parasites in other organisms and these relationships are often
species-specific, there is a huge potential for protist diversity that matches the diversity of their hosts. Because
the name "protist" serves as a catchall term for eukaryotic organisms that are not animal, plant, or fungi, it is not
surprising that very few characteristics are common to all protists. On the other hand, familiar characteristics of
plants and animals are foreshadowed in various protists.
Cell Structure
The cells of protists are among the most elaborate of all cells. Multicellular plants, animals, and fungi are
embedded among the protists in eukaryotic phylogeny. In most plants and animals and some fungi, complexity
arises out of multicellularity, tissue specialization, and subsequent interaction because of these features.
Although a rudimentary form of multicellularity exists among some of the organisms labelled as “protists,” those
that have remained unicellular show how complexity can evolve in the absence of true multicellularity, with the
differentiation of cellular morphology and function. A few protists live as colonies that behave in some ways
as a group of free-living cells and in other ways as a multicellular organism. Some protists are composed of
enormous, multinucleate, single cells that look like amorphous blobs of slime, or in other cases, like ferns. In
some species of protists, the nuclei are different sizes and have distinct roles in protist cell function.
Single protist cells range in size from less than a micrometer to three meters in length to hectares! Protist cells
may be enveloped by animal-like cell membranes or plant-like cell walls. Others are encased in glassy silica-
based shells or wound with pellicles of interlocking protein strips. The pellicle functions like a flexible coat of
armor, preventing the protist from being torn or pierced without compromising its range of motion.
Metabolism
Protists exhibit many forms of nutrition and may be aerobic or anaerobic. Those that store energy by
photosynthesis belong to a group of photoautotrophs and are characterized by the presence of chloroplasts.
Other protists are heterotrophic and consume organic materials (such as other organisms) to obtain nutrition.
Amoebas and some other heterotrophic protist species ingest particles by a process called phagocytosis, in
which the cell membrane engulfs a food particle and brings it inward, pinching off an intracellular membranous
sac, or vesicle, called a food vacuole (Figure 23.7). In some protists, food vacuoles can be formed anywhere
on the body surface, whereas in others, they may be restricted to the base of a specialized feeding structure.
The vesicle containing the ingested particle, the phagosome, then fuses with a lysosome containing hydrolytic
enzymes to produce a phagolysosome, and the food particle is broken down into small molecules that can
diffuse into the cytoplasm and be used in cellular metabolism. Undigested remains ultimately are expelled from
the cell via exocytosis.
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Phagocytosis
Food particle Food vacuole Exocytic vesicle containing
Lysosome containing
digestive enzymes
Figure 23.7 Phagocytosis. The stages of phagocytosis include the engulfment of a food particle, the digestion of the
particle using hydrolytic enzymes contained within a lysosome, and the expulsion of undigested materials from the cell.
Subtypes of heterotrophs, called saprobes, absorb nutrients from dead organisms or their organic wastes. Some
protists can function as mixotrophs, obtaining nutrition by photoautotrophic or heterotrophic routes, depending
on whether sunlight or organic nutrients are available.
Motility
The majority of protists are motile, but different types of protists have evolved varied modes of movement
(Figure 23.8). Some protists have one or more flagella, which they rotate or whip. Others are covered in rows or
tufts of tiny cilia that they beat in a coordinated manner to swim. Still others form cytoplasmic extensions called
pseudopodia anywhere on the cell, anchor the pseudopodia to a substrate, and pull themselves forward. Some
protists can move toward or away from a stimulus, a movement referred to as taxis. For example, movement
toward light, termed phototaxis, is accomplished by coupling their locomotion strategy with a light-sensing organ.
Paramecium
Amoeba
Euglena
Figure 23.8 Locomotor organelles in protists. Protists use various methods for transportation, (a) Paramecium waves
hair-like appendages called cilia to propel itself, (b) Amoeba uses lobe-like pseudopodia to anchor itself to a solid
surface and pull itself forward, (c) Euglena uses a whip-like tail called a flagellum to propel itself.
Life Cycles
Protists reproduce by a variety of mechanisms. Most undergo some form of asexual reproduction, such as binary
fission, to produce two daughter cells. In protists, binary fission can be divided into transverse or longitudinal,
depending on the axis of orientation; sometimes Paramecium exhibits this method. Some protists such as the
true slime molds exhibit multiple fission and simultaneously divide into many daughter cells. Others produce tiny
buds that go on to divide and grow to the size of the parental protist.
Sexual reproduction, involving meiosis and fertilization, is common among protists, and many protist species
can switch from asexual to sexual reproduction when necessary. Sexual reproduction is often associated with
periods when nutrients are depleted or environmental changes occur. Sexual reproduction may allow the protist
to recombine genes and produce new variations of progeny, some of which may be better suited to surviving
changes in a new or changing environment. However, sexual reproduction is often associated with resistant
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cysts that are a protective, resting stage. Depending on habitat of the species, the cysts may be particularly
resistant to temperature extremes, desiccation, or low pH. This strategy allows certain protists to “wait out”
stressors until their environment becomes more favorable for survival or until they are carried (such as by wind,
water, or transport on a larger organism) to a different environment, because cysts exhibit virtually no cellular
metabolism.
Protist life cycles range from simple to extremely elaborate. Certain parasitic protists have complicated life cycles
and must infect different host species at different developmental stages to complete their life cycle. Some protists
are unicellular in the haploid form and multicellular in the diploid form, a strategy employed by animals. Other
protists have multicellular stages in both haploid and diploid forms, a strategy called alternation of generations,
analogous to that used by plants.
Habitats
Nearly all protists exist in some type of aquatic environment, including freshwater and marine environments,
damp soil, and even snow. Several protist species are parasites that infect animals or plants. A few protist
species live on dead organisms or their wastes, and contribute to their decay.
23.3 | Groups of Protists
By the end of this section, you will be able to do the following:
• Describe representative protist organisms from each of the six presently recognized supergroups of
eukaryotes
• Identify the evolutionary relationships of plants, animals, and fungi within the six presently recognized
supergroups of eukaryotes
• Identify defining features of protists in each of the six supergroups of eukaryotes.
In the span of several decades, the Kingdom Protista has been disassembled because sequence analyses
have revealed new genetic (and therefore evolutionary) relationships among these eukaryotes. Moreover,
protists that exhibit similar morphological features may have evolved analogous structures because of similar
selective pressures—rather than because of recent common ancestry. This phenomenon, called convergent
evolution, is one reason why protist classification is so challenging. The emerging classification scheme groups
the entire domain Eukarya into six “supergroups" that contain all of the protists as well as animals, plants,
and fungi that evolved from a common ancestor (Figure 23.9). Each of the supergroups is believed to be
monophyletic, meaning that all organisms within each supergroup are believed to have evolved from a single
common ancestor, and thus all members are most closely related to each other than to organisms outside
that group. There is still evidence lacking for the monophyly of some groups. Each supergroup can be viewed
as representing one of many variants on eukaryotic cell structure. In each group, one or more of the defining
characters of the eukaryotic cell—the nucleus, the cytoskeleton, and the endosymbiotic organelles—may have
diverged from the "typical" pattern.
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the domain Eukarya is divided into six supergroups. Within each supergroup are multiple kingdoms. Although each
supergroup is believed to be monophyletic, the dotted lines suggest evolutionary relationships among the supergroups
that continue to be debated.
Keep in mind that the classification scheme presented here represents just one of several hypotheses, and the
true evolutionary relationships are still to be determined. The six supergroups may be modified or replaced by
a more appropriate hierarchy as genetic, morphological, and ecological data accumulate. When learning about
protists, it is helpful to focus less on the nomenclature and more on the commonalities and differences that
illustrate how each group has exploited the possibilities of eukaryotic life.
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Archaeplastida
Molecular evidence supports the hypothesis that all Archaeplastida are descendents of an endosymbiotic
relationship between a heterotrophic protist and a cyanobacterium. The protist members of the group include
the red algae and green algae. It was from a common ancestor of these protists that the land plants evolved,
since their closest relatives are found in this group. The red and green algae include unicellular, multicellular,
and colonial forms. A variety of algal life cycles exists, but the most complex is alternation of generations, in
which both haploid and diploid stages are multicellular. A diploid sporophyte contains cells that undergo meiosis
to produce haploid spores. The spores germinate and grow into a haploid gametophyte, which then makes
gametes by mitosis. The gametes fuse to form a zygote that grows into a diploid sporophyte. Alternation of
generations is seen in some species of Archaeplastid algae, as well as some species of Stramenopiles (Figure
23.10). In some species, the gametophyte and sporophyte look quite different, while in others they are nearly
indistinguishable.
Glaucophytes
Glaucophytes are a small group of Archaeplastida interesting because their chloroplasts retain remnants of the
peptidoglycan cell wall of the ancestral cyanobacterial endosymbiont (Figure 23.10).
Figure 23.10 Glaucocystis. (credit: By ja:User:NEON / commons:User:NEONJa - Own work, CC BY-SA 2.5,
https://commons.wikimedia.org/w/index.php?curid=1706641 (http:// 0 penstax. 0 rg/l/Glauc 0 cystis) )
Red Algae
Red algae, or rhodophytes lack flagella, and are primarily multicellular, although they range in size from
microscopic, unicellular protists to large, multicellular forms grouped into the informal seaweed category. Red
algae have a second cell wall outside an inner cellulose cell wall. Carbohydrates in this wall are the source of
agarose used for electrophoresis gels and agar for solidifying bacterial media. The "red" in the red algae comes
from phycoerythrins, accessory photopigments that are red in color and obscure the green tint of chlorophyll in
some species. Other protists classified as red algae lack phycoerythrins and are parasites. Both the red algae
and the glaucophytes store carbohydrates in the cytoplasm rather than in the plastid. Red algae are common
in tropical waters where they have been detected at depths of 260 meters. Other red algae exist in terrestrial
or freshwater environments. The red algae life cycle is an unusual alternation of generations that includes two
sporophyte phases, with meiosis occurring only in the second sporophyte.
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Green Algae: Chlorophytes and Charophytes
The most abundant group of algae is the green algae. The green algae exhibit features similar to those of the
land plants, particularly in terms of chloroplast structure. In both green algae and plants, carbohydrates are
stored in the plastid. That this group of protists shared a relatively recent common ancestor with land plants is
well supported. The green algae are subdivided into the chlorophytes and the charophytes. The charophytes
are the closest living relatives to land plants and resemble them in morphology and reproductive strategies. The
familiar Spirogyra is a charophyte. Charophytes are common in wet habitats, and their presence often signals a
healthy ecosystem.
The chlorophytes exhibit great diversity of form and function. Chlorophytes primarily inhabit freshwater and damp
soil, and are a common component of plankton. Chlamydomonas is a simple, unicellular chlorophyte with a pear-
shaped morphology and two opposing, anterior flagella that guide this protist toward light sensed by its eyespot.
More complex chlorophyte species exhibit haploid gametes and spores that resemble Chlamydomonas.
The chlorophyte Volvox is one of only a few examples of a colonial organism, which behaves in some ways like
a collection of individual cells, but in other ways like the specialized cells of a multicellular organism (Figure
23.11). Volvox colonies contain 500 to 60,000 cells, each with two flagella, contained within a hollow, spherical
matrix composed of a gelatinous glycoprotein secretion. Individual cells in a Volvox colony move in a coordinated
fashion and are interconnected by cytoplasmic bridges. Only a few of the cells reproduce to create daughter
colonies, an example of basic cell specialization in this organism. Daughter colonies are produced with their
flagella on the inside and have to evert as they are released.
Figure 23.11 Volvox. Volvox aureus is a green alga in the supergroup Archaeplastida. This species exists as a colony,
consisting of cells immersed in a gel-like matrix and intertwined with each other via hair-like cytoplasmic extensions,
(credit: Dr. Ralf Wagner)
True multicellular organisms, such as the sea lettuce, Ulva, are also represented among the chlorophytes. In
addition, some chlorophytes exist as large, multinucleate, single cells. Species in the genus Caulerpa exhibit
flattened fern-like foliage and can reach lengths of 3 meters (Figure 23.12). Caulerpa species undergo nuclear
division, but their cells do not complete cytokinesis, remaining instead as massive and elaborate single cells.
Figure 23.12 A multinucleate alga. Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially
thousands of nuclei, (credit: NOAA). An interesting question is how a single cell can produce such complex shapes.
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LINK TQ LEARNING
Take a look at this video to see cytoplasmic streaming in a green alga.
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66555/1.3/#eip-
id!164992)
Amoebozoa
Like the Archaeplastida, the Amoebozoa include species with single cells, species with large multinucleated
cells, and species that have multicellular phases. Amoebozoan cells characteristically exhibit pseudopodia that
extend like tubes or flat lobes. These pseudopods project outward from anywhere on the cell surface and can
anchor to a substrate. The protist then transports its cytoplasm into the pseudopod, thereby moving the entire
cell. This type of motion is similar to the cytoplasmic streaming used to move organelles in the Archaeplastida,
and is also used by other protists as a means of locomotion or as a method to distribute nutrients and oxygen.
The Amoebozoa include both free-living and parasitic species.
Gymnomoebae
The Gymnamoeba or lobose amoebae include both naked amoebae like the familiar Amoeba proteus and
shelled amoebae, whose bodies protrude like snails from their protective tests. Amoeba proteus is a large
amoeba about 500 pm in diameter but is dwarfed by the multinucleate amoebae Pelomyxa, which can be 10
times its size. Although Pelomyxa may have hundreds of nuclei, it has lost its mitochondria, but replaced them
with bacterial endosymbionts. The secondary loss or modification of mitochondria is a feature also seen in other
protist groups.
Figure 23.13 Amoeba. Amoebae with tubular and lobe-shaped pseudopodia are seen under a microscope. These
isolates would be morphologically classified as amoebozoans.
Slime Molds
A subset of the amoebozoans, the slime molds, has several morphological similarities to fungi that are thought
to be the result of convergent evolution. For instance, during times of stress, some slime molds develop into
spore-generating fruiting bodies, much like fungi.
The slime molds are categorized on the basis of their life cycles into plasmodial or cellular types. Plasmodial
slime molds are composed of large, multinucleate cells and move along surfaces like an amorphous blob of
slime during their feeding stage (Figure 23.14). Food particles are lifted and engulfed into the slime mold as it
glides along. The "dog vomit" slime mold seen in Figure 23.14 is a particularly colorful specimen and its ability
to creep about might well trigger suspicion of alien invasion. Upon maturation, the plasmodium takes on a net-
like appearance with the ability to form fruiting bodies, or sporangia, during times of stress. Haploid spores are
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produced by meiosis within the sporangia, and spores can be disseminated through the air or water to potentially
land in more favorable environments. If this occurs, the spores germinate to form ameboid or flagellate haploid
cells that can combine with each other and produce a diploid zygotic slime mold to complete the life cycle.
Plasmodial Slime Mold Life Cycle
Germination gives rise to
cells that can convert between
amoeboid and flagelated forms.
Flagellated
I
The plasmodium matures
and sporangia form.
Mature
plasmodium
Mitosis without cytokinesis
results in a single- celled
multinudeate mass called a
plasmodium
Figure 23.14 Plasmodial slime molds. The life cycle of the plasmodial slime mold is shown. The brightly colored
plasmodium in the inset photo is a single-celled, multinudeate mass, (credit: modification of work by Dr. Jonatha Gott
and the Center for RNA Molecular Biology, Case Western Reserve University)
The cellular slime molds function as independent amoeboid cells when nutrients are abundant. When food is
depleted, cellular slime molds aggregate into a mass of cells that behaves as a single unit, called a slug. Some
cells in the slug contribute to a 2-3-millimeter stalk, drying up and dying in the process. Cells atop the stalk
form an asexual fruiting body that contains haploid spores (Figure 23.15). As with plasmodial slime molds, the
spores are disseminated and can germinate if they land in a moist environment. One representative genus of
the cellular slime molds is Dictyostelium, which commonly exists in the damp soil of forests.
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Figure 23.15 Cellular Slime Mold. The image shows several stages in the life cycle of Dictyostetium discoideum,
including aggregated cells, mobile slugs and their transformation into fruiting bodies with a cluster of spores supported
by a stalk, (credit: By Usman Bashir (Own work) [CC BY-SA 4.0 (http://creativecommons. 0 rg/licenses/by-sa/ 4 .O
(http:// 0 penstax. 0 rg/l/CCBY) )], via Wikimedia Commons)
LINK TQ LEARNING
View this video to see the formation of a fruiting body by a cellular slime mold.
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66555/1.3/#eip-
idll65237746084)
Opisthokonta
The Opisthokonts are named for the single posterior flagellum seen in flagellated cells of the group. The flagella
of other protists are anterior and their movement pulls the cells along, while the opisthokonts are pushed. Protist
members of the opisthokonts include the animal-like choanoflagellates, which are believed to resemble the
common ancestor of sponges and perhaps, all animals. Choanoflagellates include unicellular and colonial forms
(Figure 23.16), and number about 244 described species. In these organisms, the single, apical flagellum is
surrounded by a contractile collar composed of microvilli. The collar is used to filter and collect bacteria for
ingestion by the protist. A similar feeding mechanism is seen in the collar cells of sponges, which suggests a
possible connection between choanoflagellates and animals.
The Mesomycetozoa form a small group of parasites, primarily of fish, and at least one form that can parasitize
humans. Their life cycles are poorly understood. These organisms are of special interest, because they appear
to be so closely related to animals. In the past, they were grouped with fungi and other protists based on their
morphology.
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Figure 23.16 A Colonial Choanoflagellate. (credit: By Dhzanette (http://en.wikipedia.org/wiki/Choanoflagellate
(http:// 0 penstax. 0 rg/l/ch 0 an 0 ) ) [Public domain], via Wikimedia Commons)
The previous supergroups are all the products of primary endosymbiontic events and their organelles—nucleus,
mitochondria, and chloroplasts—are what would be considered "typical," i.e., matching the diagrams you would
find in an introductory biology book. The next three supergroups all contain at least some photosynthetic
members whose chloroplasts were derived by secondary endosymbiosis. They also show some interesting
variations in nuclear structure, and modification of mitochondria or chloroplasts.
Rhizaria
The Rhizaria supergroup includes many of the amoebas with thin threadlike, needle-like or root-like pseudopodia
(Figure 23.17), rather than the broader lobed pseudopodia of the Amoebozoa. Many rhizarians make elaborate
and beautiful tests—armor-like coverings for the body of the cell—composed of calcium carbonate, silicon,
or strontium salts. Rhizarians have important roles in both carbon and nitrogen cycles. When rhizarians die,
and their tests sink into deep water, the carbonates are out of reach of most decomposers, locking carbon
dioxide away from the atmosphere. In general, this process by which carbon is transported deep into the ocean
is described as the biological carbon pump, because carbon is “pumped” to the ocean depths where it is
inaccessible to the atmosphere as carbon dioxide. The biological carbon pump is a crucial component of the
carbon cycle that maintains lower atmospheric carbon dioxide levels. Foraminiferans are unusual in that they are
the only eukaryotes known to participate in the nitrogen cycle by denitrification, an activity usually served only
by prokaryotes.
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Figure 23.17 Rhizaria. Ammonia tepida, a Rhizaria species viewed here using phase contrast light microscopy,
exhibits many threadlike pseudopodia. It also has a chambered calcium carbonate shell or test, (credit: modification of
work by Scott Fay, UC Berkeley; scale-bar data from Matt Russell)
Foraminiferans
Foraminiferans, or forams, are unicellular heterotrophic protists, ranging from approximately 20 micrometers to
several centimeters in length, and occasionally resembling tiny snails (Figure 23.18). As a group, the forams
exhibit porous shells, called tests that are built from various organic materials and typically hardened with
calcium carbonate. The tests may house photosynthetic algae, which the forams can harvest for nutrition.
Foram pseudopodia extend through the pores and allow the forams to move, feed, and gather additional
building materials. Typically, forams are associated with sand or other particles in marine or freshwater habitats.
Foraminiferans are also useful as indicators of pollution and changes in global weather patterns.
Figure 23.18 Foraminiferan Tests. These shells from foraminifera sank to the sea floor, (credit: Deep East 2001,
NOAA/OER)
Radiolarians
A second subtype of Rhizaria, the radiolarians, exhibit intricate exteriors of glassy silica with radial or bilateral
symmetry (Figure 23.19). Needle-like pseudopods supported by microtubules radiate outward from the cell
bodies of these protists and function to catch food particles. The shells of dead radiolarians sink to the ocean
floor, where they may accumulate in 100 meter-thick depths. Preserved, sedimented radiolarians are very
common in the fossil record.
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Figure 23.19 Radiolarian shell. This fossilized radiolarian shell was imaged using a scanning electron microscope,
(credit: modification of work by Hannes Grobe, Alfred Wegener Institute; scale-bar data from Matt Russell)
Cercozoa
The Cercozoa are both morphologically and metabolically diverse, and include both naked and shelled forms.
The Chlorarachniophytes (Figure 23.20) are photosynthetic, having acquired chloroplasts by secondary
endosymbiosis. The chloroplast contains a remnant of the chlorophyte endosymbiont nucleus, sandwiched
between the two sets of chloroplast membranes. Vampyrellids or "vampire amoebae," as their name suggests,
obtain their nutrients by thrusting a pseudopod into the interior of other cells and sucking out their contents.
Figure 23.20 A Chlorarachniophyte. This rhizarian is mixotrophic, and can obtain nutrients both by photosynthesis
and by trapping various microorganisms with its network of pseudopodia, (credit: By ja:User:NEON /
commons:User:NEONJa (Own work) [CC BY-SA 2.5 (http://creativecommons.Org/licenses/by-sa/2.5
(http://openstax.Org/l/CCBY_25) ) or CC BY-SA 2.5 (http://creativecommons.Org/licenses/by-sa/2.5
(http:// 0 penstax. 0 rg/l/CCBY 25) )], via Wikimedia Commons)
Chromalveolata
Current evidence suggests that species classified as chromalveolates are derived from a common ancestor that
engulfed a photosynthetic red algal cell, which itself had already evolved chloroplasts from an endosymbiotic
relationship with a photosynthetic prokaryote. Therefore, the ancestor of chromalveolates is believed to have
resulted from a secondary endosymbiotic event. However, some chromalveolates appear to have lost red alga-
derived plastid organelles or lack plastid genes altogether. Therefore, this supergroup should be considered
a hypothesis-based working group that is subject to change. Chromalveolates include very important
photosynthetic organisms, such as diatoms, brown algae, and significant disease agents in animals and plants.
The chromalveolates can be subdivided into alveolates and stramenopiles.
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Alveolates: Dinoflagellates, Apicomplexians, and Ciliates
A large body of data supports that the alveolates are derived from a shared common ancestor. The alveolates
are named for the presence of an alveolus, or membrane-enclosed sac, beneath the cell membrane. The
exact function of the alveolus is unknown, but it may be involved in osmoregulation. The alveolates are further
categorized into some of the better-known protists: the dinoflagellates, the apicomplexans, and the ciliates.
Dinoflagellates exhibit extensive morphological diversity and can be photosynthetic, heterotrophic, or
mixotrophic. The chloroplast of photosynthetic dinoflagellates was derived by secondary endosymbiosis of
a red alga. Many dinoflagellates are encased in interlocking plates of cellulose. Two perpendicular flagella
fit into the grooves between the cellulose plates, with one flagellum extending longitudinally and a second
encircling the dinoflagellate (Figure 23.21). Together, the flagella contribute to the characteristic spinning motion
of dinoflagellates. These protists exist in freshwater and marine habitats, and are a component of plankton, the
typically microscopic organisms that drift through the water and serve as a crucial food source for larger aquatic
organisms.
Figure 23.21 Dinoflagellates. The dinoflagellates exhibit great diversity in shape. Many are encased in cellulose armor
and have two flagella that fit in grooves between the plates. Movement of these two perpendicular flagella causes a
spinning motion.
Dinoflagellates have a nuclear variant called a dinokaryon. The chromosomes in the dinokaryon are highly
condensed throughout the cell cycle and do not have typical histones. Mitosis in dinoflagellates is closed, that is,
the spindle separates the chromosomes from outside of the nucleus without breakdown of the nuclear envelope.
Some dinoflagellates generate light, called bioluminescence, when they are jarred or stressed. Large numbers
of marine dinoflagellates (billions or trillions of cells per wave) can emit light and cause an entire breaking wave
to twinkle or take on a brilliant blue color (Figure 23.22). For approximately 20 species of marine dinoflagellates,
population explosions (also called blooms) during the summer months can tint the ocean with a muddy red color.
This phenomenon is called a red tide, and it results from the abundant red pigments present in dinoflagellate
plastids. In large quantities, these dinoflagellate species secrete an asphyxiating toxin that can kill fish, birds, and
marine mammals. Red tides can be massively detrimental to commercial fisheries, and humans who consume
these protists may become poisoned.
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Figure 23.22 Dinoflagellate bioluminescence. Bioluminescence is emitted from dinoflagellates in a breaking wave, as
seen from the New Jersey coast, (credit: “catalano82”/Flickr)
The apicomplexan protists are named for a structure called an apical complex (Figure 23.23), which appears
to be a highly modified secondary chloroplast. The apicoplast genome is similar to those of dinoflagellate
chloroplasts. The apical complex is specialized for entry and infection of host cells. Indeed, all apicomplexans
are parasitic. This group includes the genus Plasmodium, which causes malaria in humans. Apicomplexan life
cycles are complex, involving multiple hosts and stages of sexual and asexual reproduction.
Apicomplexan protist
10 Parasite differentiates
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Figure 23.23 Apicomplexa. (a) Apicomplexans are parasitic protists. They have a characteristic apical complex that
enables them to infect host cells, (b) Plasmodium, the causative agent of malaria, has a complex life cycle typical of
apicomplexans. (credit b: modification of work by CDC)
The ciliates, which include Paramecium and Tetrahymena, are a group of protists 10 to 3,000 micrometers
in length that are covered in rows, tufts, or spirals of tiny cilia. By beating their cilia synchronously or in
waves, ciliates can coordinate directed movements and ingest food particles. Certain ciliates have fused cilia-
based structures that function like paddles, funnels, or fins. Ciliates also are surrounded by a pellicle, providing
protection without compromising agility. The genus Paramecium includes protists that have organized their cilia
into a plate-like primitive mouth, called an oral groove, which is used to capture and digest bacteria (Figure
23.24). Food captured in the oral groove enters a food vacuole, where it combines with digestive enzymes.
Waste particles are expelled by an exocytic vesicle that fuses at a specific region on the cell membrane, called
the anal pore. In addition to a vacuole-based digestive system, Paramecium also uses contractile vacuoles,
which are osmoregulatory vesicles that fill with water as it enters the cell by osmosis and then contract to
squeeze water from the cell. Ciliates therefore exhibit considerable structural complexity without having achieved
multicellularity.
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Figure 23.24 Paramecium. Paramecium has a primitive mouth (called an oral groove) to ingest food, and an anal
pore to eliminate waste. Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to
move, (credit “paramecium micrograph": modification of work by NIH; scale-bar data from Matt Russell)
LINK TQ LEARNING
Watch the video of the contractile vacuole of Paramecium expelling water to keep the cell osmotically
balanced. (This multimedia resource will open in a browser.)(http://cnx.org/content/m66555/1.3/#eip-
id!165792853951)
Paramecium has two nuclei, a macronucleus and a micronucleus, in each cell. The micronucleus is essential
for sexual reproduction, and is in many ways a typical eukaryotic nucleus, except that its genes are not
transcribed. The transcribed nucleus is the macronucleus, which directs asexual binary fission and all other
biological functions. The macronucleus is a multiploid nucleus constructed from the micronucleus during sexual
reproduction. Periodic reconstruction of the macronucleus is necessary because the macronucleus divides
amitotically, and thus becomes genetically unbalanced over a period of successive cell replications. Paramecium
and most other ciliates reproduce sexually by conjugation. This process begins when two different mating
types of Paramecium make physical contact and join with a cytoplasmic bridge (Figure 23.25). The diploid
micronucleus in each cell then undergoes meiosis to produce four haploid micronuclei. Three of these
degenerate in each cell, leaving one micronucleus that then undergoes mitosis, generating two haploid
micronuclei. The cells each exchange one of these haploid nuclei and move away from each other. Fusion of
the haploid micronuclei generates a completely novel diploid pre-micronucleus in each conjugative cell. This
pre-micronucleus undergoes three rounds of mitosis to produce eight copies, and the original macronucleus
disintegrates. Four of the eight pre-micronuclei become full-fledged micronuclei, whereas the other four perform
multiple rounds of DNA replication. The copies of the micronuclear chromosomes are severely edited to
form hundreds of smaller chromosomes that contain only the protein coding genes. Each of these smaller
chromosomes gets new telomeres as the macronucleus differentiates. Two cycles of cell division then yield four
new Paramecia from each original conjugative cell.
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visual
CONNECTION
Macronucleus
Cytoplasmic
bridge
V
■h
TWo rounds of
cell division produce
four daughter cells.
Two different mating
types form a
cytoplasmic bridge.
Meiosis produces four
haploid micronuclei,
three of which
disintegrate.
Sexual Reproduction in Paramecium
The original
macronucleus
disintegrates, and four
of the micronuclei
become macronuclei.
Each remaining
micronucleus divides
by mitosis.
The micronucleus
undergoes three
rounds of mitosis,
producing eight
micronuclei.
The haploid
micronuclei fuse,
forming a diploid
micronucleus.
The conjugate pair
swaps micronuclei.
Figure 23.25 Conjugation in Paramecium. The complex process of sexual reproduction in Paramecium creates
eight daughter cells from two original cells. Each cell has a macronucleus and a micronucleus. During sexual
reproduction, the macronucleus dissolves and is replaced by a micronucleus, (credit “micrograph”: modification of
work by Ian Sutton; scale-bar data from Matt Russell)
Which of the following statements about Paramecium sexual reproduction is false?
a. The macronuclei are derived from micronuclei.
b. Both mitosis and meiosis occur during sexual reproduction.
c. The conjugate pair swaps macronucleii.
d. Each parent produces four daughter cells.
Stramenopiles: Diatoms, Brown Algae, Golden Algae and Oomycetes
The other subgroup of chromalveolates, the stramenopiles, includes photosynthetic marine algae and
heterotrophic protists. The chloroplast of these algae is derived from red alga. The identifying feature of this
group is the presence of a textured, or “hairy,” flagellum. Many stramenopiles also have an additional flagellum
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Chapter 23 | Protists
653
that lacks hair-like projections (Figure 23.26). Members of this subgroup range in size from single-celled diatoms
to the massive and multicellular kelp.
Stramenopile
Figure 23.26 Stramenopile flagella. This stramenopile cell has a single hairy flagellum and a secondary smooth
flagellum.
The diatoms are unicellular photosynthetic protists that encase themselves in intricately patterned, glassy cell
walls composed of silicon dioxide in a matrix of organic particles (Figure 23.27). These protists are a component
of freshwater and marine plankton. Most species of diatoms reproduce asexually, although some instances of
sexual reproduction and sporulation also exist. Some diatoms exhibit a slit in their silica shell, called a raphe. By
expelling a stream of mucopolysaccharides from the raphe, the diatom can attach to surfaces or propel itself in
one direction.
Figure 23.27 Diatoms. Assorted diatoms, visualized here using light microscopy, live among annual sea ice in
McMurdo Sound, Antarctica. Diatoms range in size from 2 to 200 pm. (credit: Prof. Gordon T. Taylor, Stony Brook
University, NSF, NOAA)
During periods of nutrient availability, diatom populations bloom to numbers greater than can be consumed by
aquatic organisms. The excess diatoms die and sink to the sea floor where they are not easily reached by
saprobes that feed on dead organisms. As a result, the carbon dioxide that the diatoms had consumed and
incorporated into their cells during photosynthesis is not returned to the atmosphere. Along with rhizarians and
other shelled protists, diatoms help to maintain a balanced carbon cycle.
Like diatoms, golden algae are largely unicellular, although some species can form large colonies. Their
characteristic gold color results from their extensive use of carotenoids, a group of photosynthetic pigments that
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are generally yellow or orange in color. Golden algae are found in both freshwater and marine environments,
where they form a major part of the plankton community.
The brown algae are primarily marine, multicellular organisms that are known colloquially as seaweeds. Giant
kelps are a type of brown alga. Some brown algae have evolved specialized tissues that resemble terrestrial
plants, with root-like holdfasts, stem-like stipes, and leaf-like blades that are capable of photosynthesis. The
stipes of giant kelps are enormous, extending in some cases for 60 meters. Like the green algae, brown algae
have a variety of life cycles, including alternation of generations. In the brown algae genus Laminaria, haploid
spores develop into multicellular gametophytes, which produce haploid gametes that combine to produce diploid
organisms that then become multicellular organisms with a different structure from the haploid form (Figure
23.28).
KHtETlfo cONNFCTION
Figure 23.28 Alternation of generations in a brown alga. Several species of brown algae, such as the Laminaria
shown here, have evolved life cycles in which both the haploid (gametophyte) and diploid (sporophyte) forms
are multicellular. The gametophyte is different in structure than the sporophyte. (credit “laminaria photograph”:
modification of work by Claire Fackler, CINMS, NOAA Photo Library)
Which of the following statements about the Laminaria life cycle is false?
a. In zoospores form in the sporangia.
b. The sporophyte is the 2 n plant.
c. The gametophyte is diploid.
d. Both the gametophyte and sporophyte stages are multicellular.
The water molds, oomycetes (“egg fungus”), were so-named based on their fungus-like morphology, but
molecular data have shown that the water molds are not closely related to fungi. The oomycetes are
characterized by a cellulose-based cell wall and an extensive network of filaments that allow for nutrient uptake.
As diploid spores, many oomycetes have two oppositely directed flagella (one hairy and one smooth) for
locomotion. The oomycetes are nonphotosynthetic and include many saprobes and parasites. The saprobes
appear as white fluffy growths on dead organisms (Figure 23.29). Most oomycetes are aquatic, but some
parasitize terrestrial plants. One plant pathogen is Phytophthora infestans, the causative agent of late blight of
potatoes, such as occurred in the nineteenth century Irish potato famine.
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Figure 23.29 Oomycetes. A saprobic oomycete engulfs a dead insect, (credit: modification of work by Thomas
Bresson)
Excavata
Many of the protist species classified into the supergroup Excavata are asymmetrical, single-celled organisms
with a feeding groove “excavated” from one side. This supergroup includes heterotrophic predators,
photosynthetic species, and parasites. Its subgroups are the diplomonads, parabasalids, and euglenozoans.
The group includes a variety of modified mitochondria, as well as chloroplasts derived from green algae by
secondary endosymbiosis. Many of the euglenozoans are free-living, but most diplomonads and parabasalids
are symbionts or parasites.
Diplomonads
Among the Excavata are the diplomonads, which include the intestinal parasite, Ciardia lamblia (Figure
23.30). Until recently, these protists were believed to lack mitochondria. Mitochondrial remnant organelles,
called mitosomes, have since been identified in diplomonads, but although these mitosomes are essentially
nonfunctional as respiratory organelles, they do function in iron and sulfur metabolism. Diplomonads exist in
anaerobic environments and use alternative pathways, such as glycolysis, to generate energy. Each diplomonad
cell has two similar, but not identical haploid nuclei. Diplomonads have four pairs of locomotor flagella that are
fairly deeply rooted in basal bodies that lie between the two nuclei.
Figure 23.30 Giardia. The mammalian intestinal parasite Giardia lamblia, visualized here using scanning electron
microscopy, is a waterborne protist that causes severe diarrhea when ingested, (credit: modification of work by Janice
Carr, CDC; scale-bar data from Matt Russell)
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Parabasalids
A second Excavata subgroup, the parabasalids, are named for the parabasal apparatus, which consists of a
Golgi complex associated with cytoskeletal fibers. Other cytoskeletal features include an axostyle, a bundle
of fibers that runs the length of the cell and may even extend beyond it. Parabasalids move with flagella and
membrane rippling, and these and other cytoskeletal modifications may assist locomotion. Like the diplomonads,
the parabasalids exhibit modified mitochondria. In parabasalids these structures function anaerobically and are
called hydrogenosomes because they produce hydrogen gas as a byproduct.
The parabasalid Trichomonas vaginalis causes trichomoniasis, a sexually transmitted disease in humans, which
appears in an estimated 180 million cases worldwide each year. Whereas men rarely exhibit symptoms during
an infection with this protist, infected women may become more susceptible to secondary infection with human
immunodeficiency virus (HIV) and may be more likely to develop cervical cancer. Pregnant women infected with
T. vaginalis are at an increased risk of serious complications, such as pre-term delivery.
Some of the most complex of the parabasalids are those that colonize the rumen of ruminant animals and the
guts of termites. These organisms can digest cellulose, a metabolic talent that is unusual among eukaryotic cells.
They have multiple flagella arranged in complex patterns and some additionally recruit spirochetes that attach to
their surface to act as accessory locomotor structures.
Termite gut endosymbionts
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66555/1.3/#med-
idll67232288213)
Euglenozoans
Euglenozoans includes parasites, heterotrophs, autotrophs, and mixotrophs, ranging in size from 10 to 500
pm. Euglenoids move through their aquatic habitats using two long flagella that guide them toward light
sources sensed by a primitive ocular organ called an eyespot. The familiar genus, Euglena, encompasses
some mixotrophic species that display a photosynthetic capability only when light is present. The chloroplast
of Euglena descends from a green alga by secondary endosymbiosis. In the dark, the chloroplasts of Euglena
shrink up and temporarily cease functioning, and the cells instead take up organic nutrients from their
environment. Euglena has a tough pellicle composed of bands of protein attached to the cytoskeleton. The
bands spiral around the cell and give Euglena its exceptional flexibility.
The human parasite, Trypanosoma brucei, belongs to a different subgroup of Euglenozoa, the kinetoplastids.
The kinetoplastid subgroup is named after the kinetoplast, a large modified mitochondrion carrying multiple
circular DNAs. This subgroup includes several parasites, collectively called trypanosomes, which cause
devastating human diseases and infect an insect species during a portion of their life cycle. T. brucei develops in
the gut of the tsetse fly after the fly bites an infected human or other mammalian host. The parasite then travels
to the insect salivary glands to be transmitted to another human or other mammal when the infected tsetse
fly consumes another blood meal. T. brucei is common in central Africa and is the causative agent of African
sleeping sickness, a disease associated with severe chronic fatigue, coma, and can be fatal if left untreated.
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Tsetse fly Stages
Human Stages
6 T. brucei enters
the salivary gland
and multiplies
1 Tsetse fly takes a blood
meal and injects T. brucei
into the bloodstream.
T. brucei transforms
into an infectious
stage.
m
In the midgut of the
fly, T. brucei multiplies
by binary fission.
2 T. brucei multiplies by
binary fission in blood,
lymph, and spinal fluid.
Figure 23.31 Sleeping sickness. Trypanosoma brucei , the causative agent of sleeping sickness, spends part of its life
cycle in the tsetse fly and part in humans, (credit: modification of work by CDC)
LINK TQ LEAENING
Watch this video to see T. brucei swimming. (This multimedia resource will open in a browser.) (http://
cnx.org/content/m66555/1.3/#eip-idll67232288213)
23.4 | Ecology of Protists
By the end of this section, you will be able to do the following:
• Describe the role that protists play in the ecosystem
• Describe important pathogenic species of protists
Protists function in various ecological niches. Whereas some protist species are essential components of the
food chain and generators of biomass, others function in the decomposition of organic materials. Still other
protists are dangerous human pathogens or causative agents of devastating plant diseases.
Primary Producers/Food Sources
Protists are essential sources of food and provide nutrition for many other organisms. In some cases, as
with zooplankton, protists are consumed directly. Alternatively, photosynthetic protists serve as producers of
nutrition for other organisms. Paramecium bursaria and several other species of ciliates are mixotrophic due
to a symbiotic relationship with green algae. This is a temporary version of the secondarily endosymbiotic
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chloroplast found in Euglena. But these symbiotic associations are not restricted to protists. For instance,
photosynthetic dinoflagellates called zooxanthellae provide nutrients for the coral polyps (Figure 23.32) that
house them, giving corals a boost of energy to secrete their calcium carbonate skeleton, in turn, the corals
provide the protist with a protected environment and the compounds needed for photosynthesis. This type of
symbiotic relationship is important in nutrient-poor environments. Without dinoflagellate symbionts, corals lose
algal pigments in a process called coral bleaching, and they eventually die. This explains why reef-building corals
typically do not reside in waters deeper than 20 meters: insufficient light reaches those depths for dinoflagellates
to photosynthesize.
Figure 23.32 Coral with symbiotic dinoflagellates. Coral polyps obtain nutrition through a symbiotic relationship with
dinoflagellates.
The protists and their products of photosynthesis are essential—directly or indirectly—to the survival of
organisms ranging from bacteria to mammals (Figure 23.33). As primary producers, protists feed a large
proportion of the world’s aquatic species. (On land, terrestrial plants serve as primary producers.) In fact,
approximately 25 percent of the world’s photosynthesis is conducted by photosynthetic protists, particularly
dinoflagellates, diatoms, and multicellular algae.
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Figure 23.33 Protists contribute to the food chain. Virtually all aquatic organisms depend directly or indirectly on
protists for food, (credit “mollusks”: modification of work by Craig Stihler, USFWS; credit “crab”: modification of work
by David Berkowitz; credit “dolphin": modification of work by Mike Baird; credit “fish”: modification of work by Tim
Sheerman-Chase; credit “penguin": modification of work by Aaron Logan)
Protists do not create food sources only for sea-dwelling organisms. Recall that certain anaerobic parabasalid
species exist in the digestive tracts of termites and wood-eating cockroaches, where they contribute an essential
step in the digestion of cellulose ingested by these insects as they consume wood.
Human Pathogens
As we have seen, a pathogen is anything that causes disease. Parasitic organisms live in or on a host
organism and harm the organism. A small number of protists are serious pathogenic parasites that must
infect other organisms to survive and propagate. For example, protist parasites include the causative agents
of malaria, African sleeping sickness, amoebic encephalitis, and waterborne gastroenteritis in humans. Other
protist pathogens prey on plants, effecting massive destruction of food crops.
Plasmodium Species
in 2015 WHO reported over 200 million cases of malaria, mostly in Africa, South America, and southern Asia.
However, it is not well known that malaria was also a prevalent and debilitating disease of the North Central
region of the United States, particularly Michigan, with its thousands of lakes and numerous swamps. Prior to the
civil war, and the drainage of many swamps, virtually everyone who immigrated to Michigan picked up malaria
(ague as it was called in the late 1800s), and the pale, sallow, bloated faces of that period were the rule. The
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only healthy faces were worn by those immigrants who had just arrived. In fact, there were more deaths due to
malaria in Michigan than those from the Civil War.
We now know that malaria is caused by several species of the apicomplexan protist genus Plasmodium.
Members of Plasmodium must sequentially require both a mosquito and a vertebrate to complete their life cycle.
In vertebrates, the parasite develops in liver cells (the exoerythrocytic stage) and goes on to infect red blood
cells (the erythrocytic stage), bursting from and destroying the blood cells with each asexual replication cycle
(Figure 23.34). Of the four Plasmodium species known to infect humans, P. falciparum accounts for 50 percent
of all malaria cases and is the primary (and deadliest) cause of disease-related fatalities in tropical regions of the
world. In 2015, it was estimated that malaria caused over 400,000 deaths, mostly in African children. During the
course of malaria, P. falciparum can infect and destroy more than one-half of a human’s circulating blood cells,
leading to severe anemia. In response to waste products released as the parasites burst from infected blood
cells, the host immune system mounts a massive inflammatory response with episodes of delirium-inducing
fever (paroxysms) as parasites lyse red blood cells, spilling parasite waste into the bloodstream. P. falciparum
is transmitted to humans by the African mosquito, Anopheles gambiae. Techniques to kill, sterilize, or avoid
exposure to this highly aggressive mosquito species are crucial to malaria control. Ironically, a type of genetic
control has arisen in parts of the world where malaria is endemic. Possession of one copy of the HbS beta
globin allele results in malaria resistance. Unfortunately, this allele also has an unfortunate second effect; when
homozygous it causes sickle cell disease.
Figure 23.34 Malaria parasite. Red blood cells are shown to be infected with P. falciparum, the causative agent of
malaria. In this light microscopic image taken using a 100x oil immersion lens, the ring-shaped P. falciparum stains
purple, (credit: modification of work by Michael Zahniser; scale-bar data from Matt Russell)
LINK TQ LEARNING
This movie (http:// 0 penstaxc 0 llege. 0 rg/l/malaria) depicts the pathogenesis of Plasmodium falciparum, the
causative agent of malaria.
Trypanosomes
Trypanosoma brucei (Figure 23.35), transmitted by tsetse flies ( Clossina spp) in Africa, and related flies in South
America, is an flagellated endoparasite responsible for the deadly disease nagana in cattle and horses, and for
African sleeping sickness in humans. This trypanosome confounds the human immune system by changing its
thick layer of surface glycoproteins with each infectious cycle. (The glycoproteins are identified by the immune
system as foreign antigens, and a specific antibody defense is mounted against the parasite.) However, T. brucei
has thousands of possible antigens, and with each subsequent generation, the protist switches to a glycoprotein
coating with a different molecular structure. In this way, T. brucei is capable of replicating continuously without
the immune system ever succeeding in clearing the parasite. Without treatment, T. brucei attacks red blood
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cells, causing the patient to lapse into a coma and eventually die. During epidemic periods, mortality from the
disease can be high. Greater surveillance and control measures lead to a reduction in reported cases; some of
the lowest numbers reported in 50 years (fewer than 10,000 cases in all of sub-Saharan Africa) have happened
since 2009.
LINK TQ LEARNING
This movie (http:// 0 penstaxc 0 llege. 0 rg/l/African sleep)
brucei, the causative agent of African sleeping sickness.
discusses the pathogenesis of Trypanosoma
In Latin America, another species of trypanosome, T. cruzi, is responsible for Chagas disease. T. cruzi infections
are mainly caused by a blood-sucking “kissing bug” in the genus Triatoma. These “true bugs” bite the host during
the night and then defecate on the wound, transmitting the trypanosome to the victim. The victim scratches the
wound, further inoculating the site with trypanosomes at the location of the bite. After about 10 weeks, individuals
enter the chronic phase but most never develop further symptoms. In about 30 percent of cases, however, the
trypanosome causes further damage, especially to the heart and digestive system tissues in the chronic phase
of infection, leading to malnutrition and heart failure due to abnormal heart rhythms. An estimated 10 million
people are infected with Chagas disease, and it caused 10,000 deaths in 2008.
V
\
# _
y 5 nm
Figure 23.35 Trypanosomes. Trypanosomes are shown among red blood cells, (credit: modification of work by Dr.
Myron G. Shultz; scale-bar data from Matt Russell)
Plant Parasites
Protist parasites of terrestrial plants include agents that destroy food crops. The oomycete Plasmopara viticola
parasitizes grape plants, causing a disease called downy mildew (Figure 23.36). Grape plants infected with P.
viticola appear stunted and have discolored, withered leaves. The spread of downy mildew nearly collapsed the
French wine industry in the nineteenth century.
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ikvp - r-/
t
A
jjSy Downy \
* Mildew
T
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Powdery
Mildew
Figure 23.36 Protist plant infections. Both downy and powdery mildews on this grape leaf are caused by an infection
of P. viticola. (credit: modification of work by USDA)
Phytophthora infestans is an oomycete responsible for potato late blight, which causes potato stalks and stems
to decay into black slime (Figure 23.37). Widespread potato blight caused by P. infestans precipitated the well-
known Irish potato famine in the nineteenth century that claimed the lives of approximately 1 million people and
led to the emigration of at least 1 million more from Ireland. Late blight continues to plague potato crops in
certain parts of the United States and Russia, wiping out as much as 70 percent of crops when no pesticides are
applied.
Figure 23.37 Potato blight. These unappetizing remnants result from an infection with P. infestans, the causative agent
of potato late blight, (credit: USDA)
Protist Decomposers
The fungus-like protist saprobes are specialized to absorb nutrients from nonliving organic matter, such as dead
organisms or their wastes. For instance, many types of oomycetes grow on dead animals or algae. Saprobic
protists have the essential function of returning inorganic nutrients to the soil and water. This process allows for
new plant growth, which in turn generates sustenance for other organisms along the food chain. Indeed, without
saprobe species, such as protists, fungi, and bacteria, life would cease to exist as all organic carbon became
“tied up” in dead organisms.
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KEY TERMS
biological carbon pump process by which inorganic carbon is fixed by photosynthetic species that then die
and fall to the sea floor where they cannot be reached by saprobes and their carbon dioxide consumption
cannot be returned to the atmosphere
bioluminescence generation and emission of light by an organism, as in dinoflagellates
contractile vacuole vesicle that fills with water (as it enters the cell by osmosis) and then contracts to squeeze
water from the cell; an osmoregulatory vesicle
cytoplasmic streaming movement of cytoplasm into an extended pseudopod such that the entire cell is
transported to the site of the pseudopod
endosymbiosis engulfment of one cell within another such that the engulfed cell survives, and both cells
benefit; the process responsible for the evolution of mitochondria and chloroplasts in eukaryotes
endosymbiotic theory theory that states that eukaryotes may have been a product of one cell engulfing
another, one living within another, and evolving over time until the separate cells were no longer
recognizable as such
hydrogenosome organelle carried by parabasalids (Excavata) that functions anaerobically and outputs
hydrogen gas as a byproduct; likely evolved from mitochondria
kinetoplast mass of DNA carried within the single, oversized mitochondrion, characteristic of kinetoplastids
(phylum: Euglenozoa)
mitosome nonfunctional organelle carried in the cells of diplomonads (Excavata) that likely evolved from a
mitochondrion
mixotroph organism that can obtain nutrition by autotrophic or heterotrophic means, usually facultatively
pellicle outer cell covering composed of interlocking protein strips that function like a flexible coat of armor,
preventing cells from being torn or pierced without compromising their range of motion
phagolysosome cellular body formed by the union of a phagosome containing the ingested particle with a
lysosome that contains hydrolytic enzymes
plankton diverse group of mostly microscopic organisms that drift in marine and freshwater systems and serve
as a food source for larger aquatic organisms
plastid one of a group of related organelles in plant cells that are involved in the storage of starches, fats,
proteins, and pigments
raphe slit in the silica shell of diatoms through which the protist secretes a stream of mucopolysaccharides for
locomotion and attachment to substrates
test porous shell of a foram that is built from various organic materials and typically hardened with calcium
carbonate
CHAPTER SUMMARY
23.1 Eukaryotic Origins
The oldest fossil evidence of eukaryotes is about 2 billion years old. Fossils older than this all appear to be
prokaryotes. It is probable that today’s eukaryotes are descended from an ancestor that had a prokaryotic
organization. The last common ancestor of today’s Eukarya had several characteristics, including cells with
nuclei and an endomembrane system (which includes the nuclear envelope). Its chromosomes were linear and
contained DNA associated with histones. The nuclear genome seems to be descended from an archaean
ancestor. This ancestor would have had a cytoskeleton and divided its chromosomes mitotically.
The ancestral cytoskeletal system included the ability to make cilia/flagella during at least part of its life cycle. It
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was aerobic because it had mitochondria derived from an aerobic alpha-proteobacterium that lived inside a
host cell. Whether this host had a nucleus at the time of the initial symbiosis remains unknown. The last
common ancestor may have had a cell wall for at least part of its life cycle, but more data are needed to
confirm this hypothesis. Today’s eukaryotes are very diverse in their shapes, organization, life cycles, and
number of cells per individual.
23.2 Characteristics of Protists
Protists are extremely diverse in terms of their biological and ecological characteristics, partly because they are
an artificial assemblage of phylogenetically unrelated groups. Protists display highly varied cell structures,
several types of reproductive strategies, virtually every possible type of nutrition, and varied habitats. Most
single-celled protists are motile, but these organisms use diverse structures for transportation.
23.3 Groups of Protists
The process of classifying protists into meaningful groups is ongoing, but genetic data in the past 20 years
have clarified many relationships that were previously unclear or mistaken. The majority view at present is to
order all eukaryotes into six supergroups: Archaeplastida, Amoebozoa, Opisthokonta, Rhizaria,
Chromalveolata, and Excavata. The goal of this classification scheme is to create clusters of species that all
are derived from a common ancestor. At present, the monophyly of some of the supergroups are better
supported by genetic data than others. Although tremendous variation exists within the supergroups,
commonalities at the morphological, physiological, and ecological levels can be identified.
23.4 Ecology of Protists
Protists function at several levels of the ecological food web: as primary producers, as direct food sources, and
as decomposers. In addition, many protists are parasites of plants and animals and can cause deadly human
diseases or destroy valuable crops.
VISUAL CONNECTION QUESTIONS
1. Figure 23.5 What evidence is there that
mitochondria were incorporated into the ancestral
eukaryotic cell before chloroplasts?
2. Figure 23.25 Which of the following statements
about Paramecium sexual reproduction is false?
a. The macronuclei are derived from
micronuclei.
b. Both mitosis and meiosis occur during
sexual reproduction.
c. The conjugate pair swaps macronuclei.
d. Each parent produces four daughter cells
REVIEW QUESTIONS
4. What event is thought to have contributed to the
evolution of eukaryotes?
a. global warming
b. glaciation
c. volcanic activity
d. oxygenation of the atmosphere
5. Which characteristic is shared by prokaryotes and 7. Which of these protists is believed to have evolved
eukaryotes? following a secondary endosymbiosis?
a.
cytoskeleton
a.
green algae
b.
nuclear envelope
b.
cyanobacteria
c.
DNA-based genome
c.
red algae
d.
mitochondria
d.
chlorarachniophytes
6. Mitochondria most likely evolved by
a. a photosynthetic cyanobacterium
b. cytoskeletal elements
c. endosymbiosis
d. membrane proliferation
3. Figure 23.28 Which of the following statements
about the Laminaria life cycle is false?
a. In zoospores form in the sporangia.
b. The sporophyte is the 2n plant.
c. The gametophyte is diploid.
d. Both the gametophyte and sporophyte
stages are multicellular.
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8. In 2016, scientists published the genome of
Monocercomonoides, and demonstrated that this
organism has no detectable mitochondrial genes.
However, its genome was arranged in linear
chromosomes wrapped around histones which are
contained within the nucleus. Monocercomonoides is
therefore a(n)_.
a. Bacteria
b. Archea
c. Eukaryote
d. Endosymbiont
9. Which of the following observations about a
bacterium would distinguish it from the last eukaryotic
common ancestor?
a. A double-stranded DNA genome
b. Lack of a membrane-bound structure
surrounding the genome
c. Fatty acids in the lipid bilayer of the plasma
membrane
d. Enclosed by a cell wall
10. Protists that have a pellicle are surrounded by
a. silica dioxide
b. calcium carbonate
c. carbohydrates
d. proteins
11. Protists with the capabilities to perform
photosynthesis and to absorb nutrients from dead
organisms are called_.
a. photoautotrophs
b. mixotrophs
c. saprobes
d. heterotrophs
12. Which of these locomotor organs would likely be
the shortest?
a. a flagellum
b. acilium
c. an extended pseudopod
d. a pellicle
13. Alternation of generations describes which of the
following?
a. The haploid form can be multicellular; the
diploid form is unicellular.
b. The haploid form is unicellular; the diploid
form can be multicellular.
c. Both the haploid and diploid forms can be
multicellular.
d. Neither the haploid nor the diploid forms can
be multicellular.
14. The amoeba E. histolytica is a pathogen that
forms liver abscesses in infected individuals. Its
metabolic classification is most likely_.
a. Anaerobic heterotroph
b. Mixotroph
c. Aerobic phototroph
d. Phagocytic autotroph
15. Which protist group exhibits mitochondrial
remnants with reduced functionality?
a. slime molds
b. diatoms
c. parabasalids
d. dinoflagellates
16. Conjugation between two Paramecia produces
_total daughter cells.
” a. 2
b. 4
c. 8
d. 16
17. What is the function of the raphe in diatoms?
a. locomotion
b. defense
c. capturing food
d. photosynthesis
18. What genus of protists appears to contradict the
statement that unicellularity restricts cell size?
a. Dictyostelium
b. Ulva
c. Plasmodium
d. Caulerpa
19. A marine biologist analyzing water samples
notices a protist with a calcium carbonate shell that
moves by pseudopodia extension. The protist is likely
to be closely related to which species?
a. Fuligo septica (Dog Vomit slime mold)
b. Circogonia icosahedra (Radiolarian)
c. Euglena viridis
d. Ammonia tepida
20. An example of carbon fixation is_.
a. photosynthesis
b. decomposition
c. phagocytosis
d. parasitism
21. Which parasitic protist evades the host immune
system by altering its surface proteins with each
generation?
a. Paramecium caudatum
b. Trypanosoma brucei
c. Plasmodium falciparum
d. Phytophthora infestans
22. Which of the following is not a way that protists
contribute to the food web?
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a. They fix carbon into organic molecules.
b. They occupy the apex producer niche.
c. They enter symbiotic relationships with
animals.
d. They recycle nutrients back into the carbon
and nitrogen cycles.
CRITICAL THINKING QUESTIONS
23. Describe the hypothesized steps in the origin of
eukaryotic cells.
24. Some aspects of eukaryotes are more similar to
Archaea, while other aspects of eukaryotic cell
composition appear more closely related to Bacteria.
Explain how endosymbiosis could resolve this
paradox.
25. Explain in your own words why sexual
reproduction can be useful if a protist’s environment
changes.
26. Ciardia lamblia is a cyst-forming protist parasite
that causes diarrhea if ingested. Given this
information, against what type(s) of environments
might G. lamblia cysts be particularly resistant?
27. Explain how the definition of protists ensures that
the kingdom Protista includes a wide diversity of
cellular structures. Provide an example of two
different structures that perform the same function for
their respective protist.
28. The chlorophyte (green algae) genera Ulva and
Caulerpa both have macroscopic leaf-like and stem¬
like structures, but only Ulva species are considered
truly multicellular. Explain why.
29. Why might a light-sensing eyespot be ineffective
for an obligate saprobe? Suggest an alternative
organ for a saprobic protist.
30. Opisthokonta includes animals and fungi, as well
as protists. Describe the key feature of this phylum,
and an example of how an organism in each kingdom
uses this feature.
31. Describe two ways in which paramecium differs
from the projected traits of the last eukaryotic
common ancestor.
32. How does killing Anopheles mosquitoes affect the
Plasmodium protists?
33. Without treatment, why does African sleeping
sickness invariably lead to death?
34. Describe how increasing stress to the ocean
would affect a food chain containing zooxanthellae,
corals, parrotfish, and sharks.
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24 | FUNGI
(a) (b) (c)
Figure 24.1 Many species of fungus produce the familiar mushroom (a) which is a reproductive structure. This (b)
coral fungus displays brightly colored fruiting bodies. This electron micrograph shows (c) the spore-bearing structures
of Aspergillus, a type of toxic fungus found mostly in soil and plants, (credit “mushroom": modification of work by Chris
Wee; credit “coral fungus”: modification of work by Cory Zanker; credit “Aspergillus": modification of work by Janice
Haney Carr, Robert Simmons, CDC; scale-bar data from Matt Russell)
Chapter Outline
24.1: Characteristics of Fungi
24.2: Classifications of Fungi
24.3: Ecology of Fungi
24.4: Fungal Parasites and Pathogens
24.5: Importance of Fungi in Human Life
Introduction
The word fungus comes from the Latin word for mushrooms. Indeed, the familiar mushroom is a reproductive
structure used by many types of fungi. However, there are also many fungus species that don't produce
mushrooms at all. Being eukaryotes, a typical fungal cell contains a true nucleus and many membrane-bound
organelles. The kingdom Fungi includes an enormous variety of living organisms collectively referred to as
Eucomycota, or true Fungi. While scientists have identified about 100,000 species of fungi, this is only a fraction
of the 1.5 million species of fungus likely present on Earth. Edible mushrooms, yeasts, black mold, and the
producer of the antibiotic penicillin, Penicillium notatum, are all members of the kingdom Fungi, which belongs
to the domain Eukarya.
Fungi, once considered plant-like organisms, are more closely related to animals than plants. Fungi are not
capable of photosynthesis: they are heterotrophic because they use complex organic compounds as sources of
energy and carbon. Fungi share a few other traits with animals. Their cell walls are composed of chitin, which is
found in the exoskeletons of arthropods. Fungi produce a number of pigments, including melanin,also found in
the hair and skin of animals. Like animals, fungi also store carbohydrates as glycogen. However, like bacteria,
fungi absorb nutrients across the cell surface and act as decomposers, helping to recycle nutrients by breaking
down organic materials to simple molecules.
Some fungal organisms multiply only asexually, whereas others undergo both asexual reproduction and sexual
reproduction with alternation of generations. Most fungi produce a large number of spores, which are haploid
cells that can undergo mitosis to form multicellular, haploid individuals.
Fungi often interact with other organisms, forming beneficial or mutualistic associations. For example, most
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terrestrial plants form symbiotic relationships with fungi. The roots of the plant connect with the underground
parts of the fungus, which form mycorrhizae. Through mycorrhizae, the fungus and plant exchange nutrients
and water, greatly aiding the survival of both species. Alternatively, lichens are an association between a fungus
and its photosynthetic partner (usually an alga).
Fungi also cause serious infections in plants and animals. For example, Dutch elm disease, which is caused by
the fungus Ophiostoma ulmi, is a particularly devastating type of fungal infestation that destroys many native
species of elm (Ulmus sp.) by infecting the tree’s vascular system. The elm bark beetle acts as a vector,
transmitting the disease from tree to tree. Accidentally introduced in the 1900s, the fungus decimated elm trees
across the continent. Many European and Asiatic elms are less susceptible to Dutch elm disease than American
elms.
In humans, fungal infections are generally considered challenging to treat. Unlike bacteria, fungi do not respond
to traditional antibiotic therapy, since they are eukaryotes. Fungal infections may prove deadly for individuals
with compromised immune systems.
Fungi have many commercial applications. The food industry uses yeasts in baking, brewing, and cheese and
wine making. Many industrial compounds are byproducts of fungal fermentation. Fungi are the source of many
commercial enzymes and antibiotics.
24.1 1 Characteristics of Fungi
By the end of this section, you will be able to do the following:
• List the characteristics of fungi
• Describe the composition of the mycelium
• Describe the mode of nutrition of fungi
• Explain sexual and asexual reproduction in fungi
Although humans have used yeasts and mushrooms since prehistoric times, until recently, the biology of fungi
was poorly understood. In fact, up until the mid-20th century, many scientists classified fungi as plants! Fungi,
like plants, are mostly sessile and seemingly rooted in place. They possess a stem-like structure similar to
plants, as well as having a root-like fungal mycelium in the soil. In addition, their mode of nutrition was poorly
understood. Progress in the field of fungal biology was the result of mycology: the scientific study of fungi.
Based on fossil evidence, fungi appeared in the pre-Cambrian era, about 450 million years ago. Molecular
biology analysis of the fungal genome demonstrates that fungi are more closely related to animals than plants.
Under some current systematic phylogenies, they continue to be a polyphyletic group of organisms that share
characteristics, rather than sharing a single common ancestor.
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ca eer connection
Mycologist
Mycologists are biologists who study fungi. Historically, mycology was a branch of microbiology, and
many mycologists start their careers with a degree in microbiology. To become a mycologist, a bachelor's
degree in a biological science (preferably majoring in microbiology) and a master's degree in mycology are
minimally necessary. Mycologists can specialize in taxonomy and fungal genomics, molecular and cellular
biology, plant pathology, biotechnology, or biochemistry. Some medical microbiologists concentrate on the
study of infectious diseases caused by fungi, called mycoses. Mycologists collaborate with zoologists and
plant pathologists to identify and control difficult fungal infections, such as the devastating chestnut blight,
the mysterious decline in frog populations in many areas of the world, or the deadly epidemic called white
nose syndrome, which is decimating bats in the Eastern United States.
Government agencies hire mycologists as research scientists and technicians to monitor the health of crops,
national parks, and national forests. Mycologists are also employed in the private sector by companies
that develop chemical and biological control products or new agricultural products, and by companies that
provide disease control services. Because of the key role played by fungi in the fermentation of alcohol
and the preparation of many important foods, scientists with a good understanding of fungal physiology
routinely work in the food technology industry. Oenology, the science of wine making, relies not only on the
knowledge of grape varietals and soil composition, but also on a solid understanding of the characteristics
of the wild yeasts that thrive in different wine-making regions. It is possible to purchase yeast strains isolated
from specific grape-growing regions. The great French chemist and microbiologist, Louis Pasteur, made
many of his essential discoveries working on the humble brewer’s yeast, thus discovering the process of
fermentation.
Cell Structure and Function
Fungi are eukaryotes, and as such, have a complex cellular organization. As eukaryotes, fungal cells contain
a membrane-bound nucleus. The DNA in the nucleus is wrapped around histone proteins, as is observed in
other eukaryotic cells. A few types of fungi have accessory genomic structures comparable to bacterial plasmids
(loops of DNA); however, the horizontal transfer of genetic information that occurs between one bacterium
and another rarely occurs in fungi. Fungal cells also contain mitochondria and a complex system of internal
membranes, including the endoplasmic reticulum and Golgi apparatus.
Unlike plant cells, fungal cells do not have chloroplasts or chlorophyll. Many fungi display bright colors arising
from other cellular pigments, ranging from red to green to black. The poisonous Amanita muscaria (fly agaric)
is recognizable by its bright red cap with white patches (Figure 24.2). Pigments in fungi are associated with the
cell wall and play a protective role against ultraviolet radiation. Some fungal pigments are toxic to humans.
Figure
(credit:
24.2 Amanita. The poisonous Amanita muscaria is native to temperate and boreal regions of North America.
Christine Majul)
Like plant cells, fungal cells have a thick cell wall. The rigid layers of fungal cell walls contain complex
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polysaccharides called chitin and glucans. Chitin (N-acetyl-D-glucosamine), also found in the exoskeleton of
arthropods such as insects, gives structural strength to the cell walls of fungi. The wall protects the cell from
desiccation and some predators. Fungi have plasma membranes similar to those of other eukaryotes, except
that the structure is stabilized by ergosterol'. a steroid molecule that replaces the cholesterol found in animal cell
membranes. Most members of the kingdom Fungi are nonmotile. However, flagella are produced by the spores
and gametes in the primitive Phylum Chytridiomycota.
Growth
The vegetative body of a fungus is a unicellular or multicellular thallus. Unicellular fungi are called yeasts.
Multicellular fungi produce threadlike hyphae (singular hypha). Dimorphic fungi can change from the unicellular
to multicellular state depending on environmental conditions. Saccharomyces cerevisiae (baker’s yeast) and
Candida species (the agents of thrush, a common fungal infection) are examples of unicellular fungi (Figure
24.3).
Figure 24.3 Candida albicans. Candida albicans is a yeast cell and the agent of candidiasis and thrush. This organism
has a similar morphology to coccus bacteria; however, yeast is a eukaryotic organism (note the nucleus), (credit:
modification of work by Dr. Godon Roberstad, CDC; scale-bar data from Matt Russell)
Most fungi are multicellular organisms. They display two distinct morphological stages: the vegetative and
reproductive. The vegetative stage consists of a tangle of hyphae, whereas the reproductive stage can be more
conspicuous. The mass of hyphae is a mycelium (Figure 24.4). It can grow on a surface, in soil or decaying
material, in a liquid, or even on living tissue. Although individual hyphae must be observed under a microscope,
the mycelium of a fungus can be very large, with some species truly being “the fungus humongous." The giant
Armillaria solidipes (honey mushroom) is considered the largest organism on Earth, spreading across more than
2,000 acres of underground soil in eastern Oregon; it is estimated to be at least 2,400 years old.
Figure 24.4 A fungal mycelium. The mycelium of the fungus Neotestudina rosati can be pathogenic to humans. The
fungus enters through a cut or scrape and develops a mycetoma, a chronic subcutaneous infection, (credit: CDC)
Most fungal hyphae are divided into separate cells by endwalls called septa (singular, septum) (Figure 24.5a,
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c). in most phyla of fungi, tiny holes in the septa allow for the rapid flow of nutrients and small molecules from
cell to cell along the hypha. They are described as perforated septa. The hyphae in bread molds (which belong
to the Phylum Zygomycota) are not separated by septa. Instead, they are formed by large cells containing many
nuclei (multinucleate), an arrangement described as coenocytic hyphae (Figure 24.5b).
(a)
(b)
(O
Figure 24.5 Fungal hyphae. Fungal hyphae may be (a) septated or (b) coenocytic (coeno- = "common"; -cytic = "cell")
with many nuclei present in a single hypha. A bright field light micrograph of (c) Phialophora richardsiae shows septa
that divide the hyphae. (credit c: modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt Russell)
Fungi thrive in environments that are moist and slightly acidic, and can grow with or without light. They vary in
their oxygen requirement. Most fungi are obligate aerobes, requiring oxygen to survive. Other species, such as
members of the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes, in that they only use
anaerobic respiration because oxygen will disrupt their metabolism or kill them. Yeasts are intermediate, being
facultative anaerobes. This means that they grow best in the presence of oxygen using aerobic respiration,
but can survive using anaerobic respiration when oxygen is not available. The alcohol produced from yeast
fermentation is used in wine and beer production.
Nutrition
Like animals, fungi are heterotrophs; they use complex organic compounds as a source of carbon, rather than
fix carbon dioxide from the atmosphere as do some bacteria and most plants. In addition, fungi do not fix
nitrogen from the atmosphere. Like animals, they must obtain it from their diet. However, unlike most animals,
which ingest food and then digest it internally in specialized organs, fungi perform these steps in the reverse
order; digestion precedes ingestion. First, exoenzymes are transported out of the hyphae, where they process
nutrients in the environment. Then, the smaller molecules produced by this external digestion are absorbed
through the large surface area of the mycelium. As with animal cells, the polysaccharide of storage is glycogen,
a branched polysaccaride, rather than amylopectin, a less densely branched polysaccharide, and amylose, a
linear polysaccharide, as found in plants.
Fungi are mostly saprobes (saprophyte is an equivalent term): organisms that derive nutrients from decaying
organic matter. They obtain their nutrients from dead or decomposing organic material derived mainly from
plants. Fungal exoenzymes are able to break down insoluble compounds, such as the cellulose and lignin
of dead wood, into readily absorbable glucose molecules. The carbon, nitrogen, and other elements are thus
released into the environment. Because of their varied metabolic pathways, fungi fulfill an important ecological
role and are being investigated as potential tools in bioremediation of chemically damaged ecosystems. For
example, some species of fungi can be used to break down diesel oil and polycyclic aromatic hydrocarbons
(PAHs). Other species take up heavy metals, such as cadmium and lead.
Some fungi are parasitic, infecting either plants or animals. Smut and Dutch elm disease affect plants, whereas
athlete’s foot and candidiasis (thrush) are medically important fungal infections in humans. In environments poor
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in nitrogen, some fungi resort to predation of nematodes (small non-segmented roundworms). In fact, species
of Arthrobotrys fungi have a number of mechanisms to trap nematodes: One mechanism involves constricting
rings within the network of hyphae. The rings swell when they touch the nematode, gripping it in a tight hold. The
fungus then penetrates the tissue of the worm by extending specialized hyphae called haustoria. Many parasitic
fungi possess haustoria, as these structures penetrate the tissues of the host, release digestive enzymes within
the host's body, and absorb the digested nutrients.
Reproduction
Fungi reproduce sexually and/or asexually. Perfect fungi reproduce both sexually and asexually, while the so-
called imperfect fungi reproduce only asexually (by mitosis).
In both sexual and asexual reproduction, fungi produce spores that disperse from the parent organism by either
floating on the wind or hitching a ride on an animal. Fungal spores are smaller and lighter than plant seeds. For
example, the giant puffball mushroom bursts open and releases trillions of spores in a massive cloud of what
looks like finely particulate dust. The huge number of spores released increases the likelihood of landing in an
environment that will support growth (Figure 24.6).
(a) (b)
Figure 24.6 Puffball and spores. The (a) giant puffball mushroom releases (b) a cloud of spores when it reaches
maturity, (credit a: modification of work by Roger Griffith; credit b: modification of work by Pearson Scott Foresman,
donated to the Wikimedia Foundation)
Asexual Reproduction
Fungi reproduce asexually by fragmentation, budding, or producing spores. Fragments of hyphae can grow new
colonies. Somatic cells in yeast form buds. During budding (an expanded type of cytokinesis), a bulge forms on
the side of the cell, the nucleus divides mitotically, and the bud ultimately detaches itself from the mother cell
(Figure 24.7).
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Figure 24.7 Budding in Histoplasma. The dark cells in this bright field light micrograph are the pathogenic yeast
Histoplasma capsulatum, seen against a backdrop of light blue tissue. Histoplasma primarily infects lungs but can
spread to other tissues, causing histoplasmosis, a potentially fatal disease, (credit: modification of work by Dr. Libero
Ajello, CDC; scale-bar data from Matt Russell)
The most common mode of asexual reproduction is through the formation of asexual spores, which are produced
by a single individual thallus (through mitosis) and are genetically identical to the parent thallus (Figure 24.8).
Spores allow fungi to expand their distribution and colonize new environments. They may be released from the
parent thallus either outside or within a special reproductive sac called a sporangium.
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Figure 24.8 Generalized fungal life cycle. Fungi may have both asexual and sexual stages of reproduction.
There are many types of asexual spores. Conidiospores are unicellular or multicellular spores that are released
directly from the tip or side of the hypha. Other asexual spores originate in the fragmentation of a hypha to form
single cells that are released as spores; some of these have a thick wall surrounding the fragment. Yet others
bud off the vegetative parent cell. In contrast to conidiospores, sporangiospores are produced directly from a
sporangium (Figure 24.9).
25 pm
Figure 24.9 Sporangiospores. This bright field light micrograph shows the release of spores from a sporangium at the
end of a hypha called a sporangiophore. The organism is a Mucor sp. fungus, a mold often found indoors, (credit:
modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt Russell)
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Sexual Reproduction
Sexual reproduction introduces genetic variation into a population of fungi. In fungi, sexual reproduction often
occurs in response to adverse environmental conditions. During sexual reproduction, two mating types are
produced. When both mating types are present in the same mycelium, it is called homothallic, or self-fertile.
Heterothallic mycelia require two different, but compatible, mycelia to reproduce sexually.
Although there are many variations in fungal sexual reproduction, all include the following three stages (Figure
24.8). First, during plasmogamy (literally, “marriage or union of cytoplasm"), two haploid cells fuse, leading to
a dikaryotic stage where two haploid nuclei coexist in a single cell. During karyogamy (“nuclear marriage”), the
haploid nuclei fuse to form a diploid zygote nucleus. Finally, meiosis takes place in the gametangia (singular,
gametangium) organs, in which gametes of different mating types are generated. At this stage, spores are
disseminated into the environment.
Review the characteristics of fungi by visiting this interactive site (http:// 0 penstaxc 0 llege. 0 rg/l/
fungi kingdom) from Wisconsin-online.
24.2 | Classifications of Fungi
By the end of this section, you will be able to do the following:
• Identify fungi and place them into the five major phyla according to current classification
• Describe each phylum in terms of major representative species and patterns of reproduction
The kingdom Fungi contains five major phyla that were established according to their mode of sexual
reproduction or using molecular data. Polyphyletic, unrelated fungi that reproduce without a sexual cycle, were
once placed for convenience in a sixth group, the Deuteromycota, called a “form phylum,” because superficially
they appeared to be similar. However, most mycologists have discontinued this practice. Rapid advances in
molecular biology and the sequencing of 18S rRNA (ribosomal RNA) continue to show new and different
relationships among the various categories of fungi.
The five true phyla of fungi are the Chytridiomycota (Chytrids), the Zygomycota (conjugated fungi), the
Ascomycota (sac fungi), the Basidiomycota (club fungi) and the recently described Phylum Glomeromycota
(Figure 24.10).
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Figure 24.10 Fungal phyla. Note: “-mycota” is used to designate a phylum while “-mycetes” formally denotes a class
or is used informally to refer to all members of the phylum.
Chytridiomycota: The Chytrids
The only class in the Phylum Chytridiomycota is the Chytridiomycetes. The chytrids are the simplest and most
primitive Eumycota, or true fungi. The evolutionary record shows that the first recognizable chytrids appeared
during the late pre-Cambrian period, more than 500 million years ago. Like all fungi, chytrids have chitin in their
cell walls, but one group of chytrids has both cellulose and chitin in the cell wall. Most chytrids are unicellular;
however, a few form multicellular organisms and hyphae, which have no septa between cells (coenocytic). The
Chytrids are the only fungi that have retained flagella. They produce both gametes and diploid zoospores that
swim with the help of a single flagellum. An unusual feature of the chytrids is that both male and female gametes
are flagellated.
The ecological habitat and cell structure of chytrids have much in common with protists. Chytrids usually live in
aquatic environments, although some species live on land. Some species thrive as parasites on plants, insects,
or amphibians (Figure 24.11), while others are saprobes. The chytrid species Allomyces is well characterized as
an experimental organism. Its reproductive cycle includes both asexual and sexual phases. Allomyces produces
diploid or haploid flagellated zoospores in a sporangium.
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30 |im
Figure 24.11 Chytrids. The chytrid Batrachochytrium dendrobatidis is seen in these light micrographs as transparent
spheres growing on (a) a freshwater arthropod (water mite) and (b) algae. This chytrid causes skin diseases in many
species of amphibians, resulting in species decline and extinction, (credit: modification of work by Johnson ML, Speare
R„ CDC)
Zygomycota: The Conjugated Fungi
The zygomycetes are a relatively small group of fungi belonging to the Phylum Zygomycota. They include
the familiar bread mold, Rhizopus stolonifer, which rapidly propagates on the surfaces of breads, fruits, and
vegetables. Most species are saprobes, living off decaying organic material; a few are parasites, particularly
of insects. Zygomycetes play a considerable commercial role. For example, the metabolic products of some
species of Rhizopus are intermediates in the synthesis of semi-synthetic steroid hormones.
Zygomycetes have a thallus of coenocytic hyphae in which the nuclei are haploid when the organism is in the
vegetative stage. The fungi usually reproduce asexually by producing sporangiospores (Figure 24.12). The
black tips of bread mold are the swollen sporangia packed with black spores (Figure 24.13). When spores
land on a suitable substrate, they germinate and produce a new mycelium. Sexual reproduction starts when
environmental conditions become unfavorable. Two opposing mating strains (type + and type -) must be in close
proximity for gametangia from the hyphae to be produced and fuse, leading to karyogamy. Each zygospore
can contain several diploid nuclei. The developing diploid zygospores have thick coats that protect them from
desiccation and other hazards. They may remain dormant until environmental conditions are favorable. When
the zygospore germinates, it undergoes meiosis and produces haploid spores, which will, in turn, grow into a
new organism. This form of sexual reproduction in fungi is called conjugation (although it differs markedly from
conjugation in bacteria and protists), giving rise to the name “conjugated fungi”.
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Zygomycete Life Cycle
Germination
Germination:
Mycelia form. If the two mating
types (+ and -) are in close
proximity, extensions called
gametangia form
between them.
Mycelia
(in)
Plasmogamy:
Fusion between
+ and - mating
types results in a
zygosporangium
with multiple haploid
nuclei. The
zygosporangium
forms a thick,
protective
coat.
Sporangium
(in)
Zygosporangium
Meiosis and
germination:
A sporangium grows
on a short stalk.
Haploid spores are
formed inside.
Karyogamy:
The nuclei fuse to
form a zygote with
multiple diploid nuclei.
Zygote
(2 n)
Figure 24.12 Zygomycete life cycle. Zygomycetes have asexual and sexual phases in their life cycles. In the asexual
phase, spores are produced from haploid sporangia by mitosis (not shown). In the sexual phase, plus and minus
haploid mating types conjugate to form a heterokaryotic zygosporangium. Karyogamy then produces a diploid zygote.
Diploid cells in the zygote undergo meiosis and germinate to form a haploid sporangium, which releases the next
generation of haploid spores.
(a) (b)
Figure 24.13 Rhizopus spores. Asexual sporangia grow at the end of stalks, which appear as (a) white fuzz seen on
this bread mold, Rhizopus stolonifer. The black tips (b) of bread mold are the spore-containing sporangia, (credit b:
modification of work by "polandeze'VFlickr)
Ascomycota: The Sac Fungi
The majority of known fungi belong to the Phylum Ascomycota, which is characterized by the formation of an
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ascus (plural, asci), a sac-like structure that contains haploid ascospores. Filamentous ascomycetes produce
hyphae divided by perforated septa, allowing streaming of cytoplasm from one cell to another. Conidia and asci,
which are used respectively for asexual and sexual reproduction, are usually separated from the vegetative
hyphae by blocked (non-perforated) septa. Many ascomycetes are of commercial importance. Some play a
beneficial role for humanity, such as the yeasts used in baking, brewing, and wine fermentation, and directly as
food delicacies such as truffles and morels. Aspergillus oryzae is used in the fermentation of rice to produce
sake. Other ascomycetes parasitize plants and animals, including humans. For example, fungal pneumonia
poses a significant threat to AIDS patients who have a compromised immune system. Ascomycetes not only
infest and destroy crops directly; they also produce poisonous secondary metabolites that make crops unfit for
consumption.
Asexual reproduction is frequent and involves the production of conidiophores that release haploid
conidiospores (Figure 24.14). Sexual reproduction starts with the development of special hyphae from either
one of two types of mating strains (Figure 24.14). The “male” strain produces an antheridium and the “female”
strain develops an ascogonium. At fertilization, the antheridium and the ascogonium combine in plasmogamy,
without nuclear fusion. Special dikaryotic ascogenous (ascus-producing) hyphae arise from this dikaryon, in
which each cell has pairs of nuclei: one from the “male” strain and one from the “female” strain. In each ascus,
two haploid nuclei fuse in karyogamy. Thousands of asci fill a fruiting body called the ascocarp. The diploid
nucleus in each ascus gives rise to haploid nuclei by meiosis, and spore walls form around each nucleus. The
spores in each ascus contain the meiotic products of a single diploid nucleus. The ascospores are then released,
germinate, and form hyphae that are disseminated in the environment and start new mycelia (Figure 24.15).
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visual
CONNECTION
Ascomycete Life Cycle
Germination
Plasmogamy
and mitosis:
The ascogonium and
antheridium fuse. Mitosis
and cell division result in
the formation of many
dikaryotic hyphae, which
form a fruiting body
called the ascocarp.
Asci form at the tips
of these hyphae.
Mitosis
Asexual
Reproduction
Conidiophore
Dispersal and
germination
Ascospores
(in)
Ascus
Sexual
Reproduction
Mitosis and
cell division:
Eight haploid
ascospores
are formed.
Ascocarp
Karyogamy:
The nuclei in the
asci fuse to form
a diploid zygote.
Zygote
(2n)
Meiosis:
An ascus with four
haploid nuclei is
formed.
Figure 24.14 Ascomycete life cycle. The lifecycle of an ascomycete is characterized by the production of asci
during the sexual phase. In each ascus, the four nuclei produced by meiosis divide once mitotically for a total of
eight haploid ascospores. The haploid phase is the predominant phase of the life cycle in Ascomycetes.
Which of the following statements is true?
a. A dikaryotic ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight
ascospores.
b. A diploid ascus that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight
ascospores.
c. A haploid zygote that forms in the ascocarp undergoes karyogamy, meiosis, and mitosis to form eight
ascospores.
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d. A dikaryotic ascus that forms in the ascocarp undergoes plasmogamy, meiosis, and mitosis to form
eight ascospores.
Figure 24.15 Ascospores. The bright field light micrograph shows ascospores being released from asci in the fungus
Talaromyces flavus var. flavus. (credit: modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt
Russell)
Basidiomycota: The Club Fungi
The fungi in the Phylum Basidiomycota are easily recognizable under a light microscope by their club-
shaped fruiting bodies called basidia (singular, basidium), which are the swollen terminal cells of hyphae. The
basidia, which are the reproductive organs of these fungi, are often contained within the familiar mushroom,
commonly seen in fields after rain, on the supermarket shelves, and growing on your lawn (Figure 24.16). These
mushroom-producing basidiomycetes are sometimes referred to as “gill fungi" because of the presence of gill¬
like structures on the underside of the cap. The gills are actually compacted hyphae on which the basidia are
borne. This group also includes shelf fungi, which cling to the bark of trees like small shelves. In addition, the
basidiomycota include smuts and rusts, which are important plant pathogens. Most edible fungi belong to the
Phylum Basidiomycota; however, some basidiomycota are inedible and produce deadly toxins. For example,
Cryptococcus neoformans causes severe respiratory illness. The infamous death cap mushroom ( Amanita
phailoides) is related to the fly agaric seen at the beginning of the previous section.
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Figure 24.16 Fairy ring. The fruiting bodies of a basidiomycete form a ring in a meadow, commonly called “fairy ring.”
The best-known fairy ring fungus has the scientific name Marasmius oreades. The body of this fungus, its mycelium, is
underground and grows outward in a circle. As it grows, the mycelium depletes the soil of nitrogen, causing the mycelia
to grow away from the center and leading to the “fairy ring” of fruiting bodies where there is adequate soil nitrogen.
(Credit: "Cropcircles"/Wikipedia Commons)]
The lifecycle of basidiomycetes includes alternation of generations (Figure 24.17). Most fungi are haploid
through most of their life cycles, but the basidiomycetes produce both haploid and dikaryotic mycelia, with
the dikaryotic phase being dominant. (Note: The dikaryotic phase is technically not diploid, since the nuclei
remain unfused until shortly before spore production.) In the basidiomycetes, sexual spores are more common
than asexual spores. The sexual spores form in the club-shaped basidium and are called basidiospores. In
the basidium, nuclei of two different mating strains fuse (karyogamy), giving rise to a diploid zygote that then
undergoes meiosis. The haploid nuclei migrate into four different chambers appended to the basidium, and then
become basidiospores.
Each basidiospore germinates and generates monokaryotic haploid hyphae. The mycelium that results is called
a primary mycelium. Mycelia of different mating strains can combine and produce a secondary mycelium that
contains haploid nuclei of two different mating strains. This is the dominant dikaryotic stage of the basidiomycete
life cycle. Thus, each cell in this mycelium has two haploid nuclei, which will not fuse until formation of the
basidium. Eventually, the secondary mycelium generates a basidiocarp, a fruiting body that protrudes from the
ground—this is what we think of as a mushroom. The basidiocarp bears the developing basidia on the gills under
its cap.
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683
visual
a CONNECTION
Basidiomycete Life Cycle
Germination:
Myceliaform. There are
two mating types (+ and -).
Basidiospores (n)
©@®
©i®
Plasmogamy:
Fusion between
+ and - mating
types results in
formation of a
dikaryotic
mycelium.
Mycelia (In)
Mating type
Dispersal and
germination
Cell division:
Four
basidiospores
are formed.
Basidium with
four nuclei
(in)
Meiosis:
Four haploid nuclei
are formed in the
basidium.
Mitosis:
Under the right
environmental
conditions, a
basidiocarp forms. Gills
of the basidiocarp contain
cells called basidia.
Basidia
Zygote
(2n)
Karyogamy:
Basidia form
diploid nuclei.
Basidiocarp
Figure 24.17 Basidiomycete life cycle. The lifecycle of a basidiomycete has alternate generations with haploid and
dikaryotic mycelia. Haploid primary mycelia fuse to form a dikaryotic secondary mycelium, which is the dominant
stage of the life cycle, and produces the basidiocarp.
Which of the following statements is true?
a. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
b. The result of the plasmogamy step is four basidiospores.
c. Karyogamy results directly in the formation of mycelia.
d. A basidiocarp is the fruiting body of a mushroom-producing fungus.
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Asexual Ascomycota and Basidiomycota
Imperfect fungi —those that do not display a sexual phase—were formerly classified in the form phylum
Deuteromycota, an invalid taxon no longer used in the present, ever-developing classification of organisms.
While Deuteromycota was once a classification taxon, recent molecular analysis has shown that some of the
members classified in this group belong to the Ascomycota (Figure 24.18) or the Basidiomycota. Because some
members of this group have not yet been appropriately classified, they are less well described in comparison to
members of other fungal taxa. Most imperfect fungi live on land, with a few aquatic exceptions. They form visible
mycelia with a fuzzy appearance and are commonly known as mold.
50 rim
Figure 24.18 Aspergillus. Aspergillus niger is an asexually reproducing fungus (phylum Ascomycota) commonly found
as a food contaminant. The spherical structure in this light micrograph is an asexual conidiophore. Molecular studies
have placed Aspergillus with the ascomycetes and sexual cycles have been identified in some species, (credit:
modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt Russell)
The fungi in this group have a large impact on everyday human life. The food industry relies on them for ripening
some cheeses. The blue veins in Roquefort cheese and the white crust on Camembert are the result of fungal
growth. The antibiotic penicillin was originally discovered on an overgrown Petri plate, on which a colony of
Penicillium fungi had killed the bacterial growth surrounding it. Other fungi in this group cause serious diseases,
either directly as parasites (which infect both plants and humans), or as producers of potent toxic compounds,
as seen in the aflatoxins released by fungi of the genus Aspergillus.
Glomeromycota
The Glomeromycota is a newly established phylum that comprises about 230 species, all of which are involved
in close associations with the roots of trees. Fossil records indicate that trees and their root symbionts share
a long evolutionary history. It appears that nearly all members of this family form arbuscular mycorrhizae:
the hyphae interact with the root cells forming a mutually beneficial association in which the plants supply the
carbon source and energy in the form of carbohydrates to the fungus, and the fungus supplies essential minerals
from the soil to the plant. The exception is Geosiphon pyriformis, which hosts the cyanobacterium Nostoc as an
endosymbiont.
The glomeromycetes do not reproduce sexually and do not survive without the presence of plant roots. Although
they have coenocytic hyphae like the zygomycetes, they do not form zygospores. DNA analysis shows that all
glomeromycetes probably descended from a common ancestor, making them a monophyletic lineage.
24.3 | Ecology of Fungi
By the end of this section, you will be able to do the following:
• Describe the role of fungi in various ecosystems
• Describe mutualistic relationships of fungi with plant roots and photosynthetic organisms
• Describe the beneficial relationship between some fungi and insects
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Fungi play a crucial role in the constantly changing “balance” of ecosystems. They colonize most habitats on
Earth, preferring dark, moist conditions. They can thrive in seemingly hostile environments, such as the tundra,
thanks to a most successful symbiosis with photosynthetic organisms like algae to produce lichens. Within their
communities, fungi are not as obvious as are large animals or tall treas. Like bacteria, they act behind the scene
as major decomposers. With their versatile metabolism, fungi break down organic matter, which would otherwise
not be recycled.
Habitats
Although fungi are primarily associated with humid and cool environments that provide a supply of organic
matter, they colonize a surprising diversity of habitats, from seawater to human skin and mucous membranes.
Chytrids are found primarily in aquatic environments. Other fungi, such as Coccidioides immitis, which causes
pneumonia when its spores are inhaled, thrive in the dry and sandy soil of the southwestern United States.
Fungi that parasitize coral reefs live in the ocean. However, most members of the Kingdom Fungi grow on the
forest floor, where the dark and damp environment is rich in decaying debris from plants and animals, in these
environments, fungi play a major role as decomposers and recyclers, making it possible for members of the
other kingdoms to be supplied with nutrients and live.
Decomposers and Recyclers
The food web would be incomplete without organisms that decompose organic matter (Figure 24.19). Some
elements—such as nitrogen and phosphorus—are required in large quantities by biological systems, and yet
are not abundant in the environment. The action of fungi releases these elements from decaying matter, making
them available to other living organisms. Trace elements present in low amounts in many habitats are essential
for growth, and would remain tied up in rotting organic matter if fungi and bacteria did not return them to the
environment via their metabolic activity.
Figure 24.19 Bracket fungi. Fungi are an important part of ecosystem nutrient cycles. These bracket fungi growing on
the side of a tree are the fruiting structures of a basidiomycete. They receive their nutrients through their hyphae, which
invade and decay the tree trunk, (credit: Cory Zanker)
The ability of fungi to degrade many large and insoluble molecules is due to their mode of nutrition. As seen
earlier, digestion precedes ingestion. Fungi produce a variety of exoenzymes to digest nutrients. The enzymes
are either released into the substrate or remain bound to the outside of the fungal cell wall. Large molecules are
broken down into small molecules, which are transported into the cell by a system of protein carriers embedded
in the cell membrane. Because the movement of small molecules and enzymes is dependent on the presence
of water, active growth depends on a relatively high percentage of moisture in the environment.
As saprobes, fungi help maintain a sustainable ecosystem for the animals and plants that share the same
habitat. In addition to replenishing the environment with nutrients, fungi interact directly with other organisms in
beneficial, and sometimes damaging, ways (Figure 24.20).
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Figure 24.20 Shelf fungi. Shelf fungi, so called because they grow on trees in a stack, attack and digest the trunk or
branches of a tree. While some shelf fungi are found only on dead trees, others can parasitize living trees and cause
eventual death, so they are considered serious tree pathogens, (credit: Cory Zanker)
Mutualistic Relationships
Symbiosis is the ecological interaction between two organisms that live together. This definition does not
describe the type or quality of the interaction. When both members of the association benefit, the symbiotic
relationship is called mutualistic. Fungi form mutualistic associations with many types of organisms, including
cyanobacteria, algae, plants, and animals.
Fungus/Plant Mutualism
One of the most remarkable associations between fungi and plants is the establishment of mycorrhizae.
Mycorrhiza, which is derived from the Greek words myco meaning fungus and rhizo meaning root, refers to
the fungal partner of a mutualistic association between vascular plant roots and their symbiotic fungi. Nearly 90
percent of all vascular plant species have mycorrhizal partners, in a mycorrhizal association, the fungal mycelia
use their extensive network of hyphae and large surface area in contact with the soil to channel water and
minerals from the soil into the plant, in exchange, the plant supplies the products of photosynthesis to fuel the
metabolism of the fungus.
There are several basic types of mycorrhizae. Ectomycorrhizae (“outside” mycorrhizae) depend on fungi
enveloping the roots in a sheath (called a mantle). Hyphae grow from the mantle into the root and envelope
the outer layers of the root cells in a network of hyphae called a Hartig net (Figure 24.21). The fungal partner
can belong to the Ascomycota, Basidiomycota or Zygomycota. Endomycorrhizae ("inside" mycorrhizae), also
called arbuscular mycorrhizae , are produced when the fungi grow inside the root in a branched structure called
an arbuscule (from the Latin for “little trees”). The fungal partners of endomycorrhizal associates all belong to
the Glomeromycota. The fungal arbuscules penetrate root cells between the cell wall and the plasma membrane
and are the site of the metabolic exchanges between the fungus and the host plant (Figure 24.21b and Figure
24.22b). Orchids rely on a third type of mycorrhiza. Orchids are epiphytes that typically produce very small
airborne seeds without much storage to sustain germination and growth. Their seeds will not germinate without a
mycorrhizal partner (usually a Basidiomycete). After nutrients in the seed are depleted, fungal symbionts support
the growth of the orchid by providing necessary carbohydrates and minerals. Some orchids continue to be
mycorrhizal throughout their life cycle.
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visual
CONNECTION
Figure 24.21 Two types of mycorrhizae. (a) Ectomycorrhizae and (b) arbuscular or endomycorrhizae have
different mechanisms for interacting with the roots of plants, (credit b: MS Turmel, University of Manitoba, Plant
Science Department)
If symbiotic fungi were absent from the soil, what impact do you think this would have on plant growth?
Figure 24.22 Mycorrhizae. The (a) infection of Pinus radiata (Monterey pine) roots by the hyphae of Amanita muscaria
(fly amanita) causes the pine tree to produce many small, branched rootlets. The Amanita hyphae cover these small
roots with a white mantle, (b) Spores (the round bodies) and hyphae (thread-like structures) are evident in this light
micrograph of an arbuscular mycorrhiza by a fungus on the root of a corn plant, (credit a: modification of work by Randy
Molina, USDA; credit b: modification of work by Sara Wright, USDA-ARS; scale-bar data from Matt Russell)
Other examples of fungus-plant mutualism include the endophytes: fungi that live inside tissue without
damaging the host plant. Endophytes release toxins that repel herbivores, or confer resistance to environmental
stress factors, such as infection by microorganisms, drought, or heavy metals in soil.
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V /
e olution CONNECTION
Coevolution of Land Plants and Mycorrhizae
As we have seen, mycorrhizae are the fungal partners of a mutually beneficial symbiotic association that
coevolved between roots of vascular plants and fungi. A well-supported theory proposes that fungi were
instrumental in the evolution of the root system in plants and contributed to the success of Angiosperms.
The bryophytes (mosses and liverworts), which are considered the most ancestral plants and the first to
survive and adapt on land, have simple underground rhizoids, rather than a true root system, and therefore
cannot survive in dry areas. However, some bryophytes have arbuscular mycorrhizae and some do not.
True roots first appeared in the ancestral vascular plants: Vascular plants that developed a system of thin
extensions from their roots would have had a selective advantage over nonvascular plants because they
had a greater surface area of contact with the fungal partners than did the rhizoids of mosses and liverworts.
The first true roots would have allowed vascular plants to obtain more water and nutrients in the ground.
Fossil records indicate that fungi actually preceded the invasion of ancestral freshwater plants onto dry land.
The first association between fungi and photosynthetic organisms on land involved moss-like plants and
endophytes. These early associations developed before roots appeared in plants. Slowly, the benefits of the
endophyte and rhizoid interactions for both partners led to present-day mycorrhizae: About 90 percent of
today’s vascular plants have associations with fungi in their rhizosphere.
The fungi involved in mycorrhizae display many characteristics of ancestral fungi; they produce simple
spores, show little diversification, do not have a sexual reproductive cycle, and cannot live outside of a
mycorrhizal association. The plants benefited from the association because mycorrhizae allowed them to
move into new habitats and allowed the increased uptake of nutrients, which gave them an enormous
selective advantage over plants that did not establish symbiotic relationships.
Lichens
Lichens display a range of colors and textures (Figure 24.23) and can survive in the most unusual and hostile
habitats. They cover rocks, gravestones, tree bark, and the ground in the tundra where plant roots cannot
penetrate. Lichens can survive extended periods of drought, when they become completely desiccated, and then
rapidly become active once water is available again.
LINK
T a
LEARNING
Explore the world of lichens using this site (http:// 0 penstaxc 0 llege. 0 rg/l/lichenland) from Oregon State
University.
(a) (b) (c)
Figure 24.23 Lichens. Lichens have many forms. They may be (a) crust-like, (b) hair-like, or (c) leaf-like, (credit a:
modification of work by Jo Naylor; credit b: modification of work by "djpmapleferryman"/Flickr; credit c: modification of
work by Cory Zanker)
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It is important to note that lichens are not a single organism, but rather another wonderful example of a
mutualism, in which a fungus (usually a member of the Ascomycota or Basidiomycota) lives in a physical and
physiological relationship with a photosynthetic organism (a eukaryotic alga or a prokaryotic cyanobacterium)
(Figure 24.24). Generally, neither the fungus nor the photosynthetic organism can survive alone outside of the
symbiotic relationship. The body of a lichen, referred to as a thallus, is formed of hyphae wrapped around the
photosynthetic partner. The photosynthetic organism provides carbon and energy in the form of carbohydrates.
Some cyanobacteria additionally fix nitrogen from the atmosphere, contributing nitrogenous compounds to the
association. In return, the fungus supplies minerals and protection from dryness and excessive light by encasing
the algae in its mycelium. The fungus also attaches the lichen to its substrate.
<HS@9BS£
Figure 24.24 Structure of a lichen. This cross-section of a lichen thallus shows the (a) upper cortex of fungal hyphae,
which provides protection; the (b) algal zone where photosynthesis occurs, the (c) medulla of fungal hyphae, and the
(d) lower cortex, which also provides protection and may have (e) rhizines to anchor the thallus to the substrate.
The thallus of lichens grows very slowly, expanding its diameter a few millimeters per year. Both the fungus
and the alga participate in the formation of dispersal units, called soredia—clusters of algal cells surrounded by
mycelia. Soredia are dispersed by wind and water and form new lichens.
Lichens are extremely sensitive to air pollution, especially to abnormal levels of nitrogenous and sulfurous
compounds. The U.S. Forest Service and National Park Service can monitor air quality by measuring the relative
abundance and health of the lichen population in an area. Lichens fulfill many ecological roles. Caribou and
reindeer eat lichens, and they provide cover for small invertebrates that hide in the mycelium. In the production
of textiles, weavers used lichens to dye wool for many centuries until the advent of synthetic dyes. The pigments
used in litmus paper are also extracted from lichens.
LINK TQ LEARNING
Lichens are used to monitor the quality of air. Read more on this site (http:// 0 penstaxc 0 llege. 0 rg/l/
lichen_monitrng) from the United States Forest Service.
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Fungus/Animal Mutualism
Fungi have evolved mutualisms with numerous insects in Phylum Arthropoda: joint-legged invertebrates with
a chitinous exoskeleton. Arthropods depend on the fungus for protection from predators and pathogens, while
the fungus obtains nutrients and a way to disseminate spores into new environments. The association between
species of Basidiomycota and scale insects is one example. The fungal mycelium covers and protects the insect
colonies. The scale insects foster a flow of nutrients from the parasitized plant to the fungus.
in a second example, leaf-cutter ants of Central and South America literally farm fungi. They cut disks of
leaves from plants and pile them up in subterranean gardens (Figure 24.25). Fungi are cultivated in these disk
gardens, digesting the cellulose in the leaves that the ants cannot break down. Once smaller sugar molecules
are produced and consumed by the fungi, the fungi in turn become a meal for the ants. The insects also patrol
their garden, preying on competing fungi. Both ants and fungi benefit from this mutualistic association. The
fungus receives a steady supply of leaves and freedom from competition, while the ants feed on the fungi they
cultivate.
Figure 24.25 Leaf-cutter ant. A leaf-cutter ant transports a leaf that will feed a farmed fungus, (credit: Scott Bauer,
USDA-ARS)
Fungivores
Animal dispersal is important for some fungi because an animal may carry fungal spores considerable distances
from the source. Fungal spores are rarely completely degraded in the gastrointestinal tract of an animal, and
many are able to germinate when they are passed in the feces. Some “dung fungi” actually require passage
through the digestive system of herbivores to complete their lifecycle. The black truffle—a prized gourmet
delicacy—is the fruiting body of an underground ascomycete. Almost all truffles are ectomycorrhizal, and are
usually found in close association with trees. Animals eat truffles and disperse the spores. In Italy and France,
truffle hunters use female pigs to sniff out truffles (female pigs are attracted to truffles because the fungus
releases a volatile compound closely related to a pheromone produced by male pigs.)
24.4 | Fungal Parasites and Pathogens
By the end of this section, you will be able to do the following:
• Describe some fungal parasites and pathogens of plants
• Describe the different types of fungal infections in humans
• Explain why antifungal therapy is hampered by the similarity between fungal and animal cells
Parasitism describes a symbiotic relationship in which one member of the association benefits at the expense
of the other. Both parasites and pathogens harm the host; however, pathogens cause disease, damage to host
tissues or physiology, whereas parasites usually do not, but can cause serious damage and death by competition
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for nutrients or other resources. Commensalism occurs when one member benefits without affecting the other.
Plant Parasites and Pathogens
The production of sufficient high-quality crops is essential to human existence. Unfortunately, plant diseases
have ruined many crops throughout human agricultural history, sometimes creating widespread famine. Many
plant pathogens are fungi that cause tissue decay and the eventual death of the host (Figure 24.26). In addition
to destroying plant tissue directly, some plant pathogens spoil crops by producing potent toxins that can further
damage and kill the host plant. Fungi are also responsible for food spoilage and the rotting of stored crops. For
example, the fungus Claviceps purpurea causes ergot, a disease of cereal crops (especially of rye). Although
the fungus reduces the yield of cereals, the effects of the ergot's alkaloid toxins on humans and animals are
of much greater significance. In animals, the disease is referred to as ergotism. The most common signs and
symptoms are convulsions, hallucination, gangrene, and loss of milk in cattle. The active ingredient of ergot
is lysergic acid, which is a precursor of the drug LSD. Smuts, rusts, and powdery or downy mildew are other
examples of common fungal pathogens that affect crops.
(a) (b)
(c) {<*)
Figure 24.26 Fungal pathogens. Some fungal pathogens include (a) green mold on grapefruit, (b) powdery mildew on
a zinnia, (c) stem rust on a sheaf of barley, and (d) grey rot on grapes. In wet conditions Botrytis cinerea, the fungus
that causes grey rot, can destroy a grape crop. However, controlled infection of grapes by Botrytis results in noble rot,
a condition that produces strong and much-prized dessert wines, (credit a: modification of work by Scott Bauer, USDA-
ARS; credit b: modification of work by Stephen Ausmus, USDA-ARS; credit c: modification of work by David Marshall,
USDA-ARS; credit d: modification of work by Joseph Smilanick, USDA-ARS)
Aflatoxins are toxic, carcinogenic compounds released by fungi of the genus Aspergiiius. Periodically, harvests
of nuts and grains are tainted by aflatoxins, leading to massive recall of produce. This sometimes ruins producers
and causes food shortages in developing countries.
Animal and Human Parasites and Pathogens
Fungi can affect animals, including humans, in several ways. A mycosis is a fungal disease that results from
infection and direct damage due to the growth and infiltration of the fungus. Fungi attack animals directly by
colonizing and destroying tissues. Mycotoxicosis is the poisoning of humans (and other animals) by foods
contaminated by fungal toxins (mycotoxins). Mycetismus specifically describes the ingestion of preformed
toxins in poisonous mushrooms. In addition, individuals who display hypersensitivity to molds and spores may
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develop strong and dangerous allergic reactions. Fungal infections are generally very difficult to treat because,
unlike bacteria, fungi are eukaryotes. Antibiotics only target prokaryotic cells, whereas compounds that kill fungi
also harm the eukaryotic animal host.
Many fungal infections are superficial ; that is, they occur on the animal’s skin. Termed cutaneous (“skin”)
mycoses, they can have devastating effects. For example, the decline of the world’s frog population in recent
years is caused (in part) by the chytrid fungus Batrachochytrium dendrobatidis. This deadly fungus infects the
skin of frogs and presumably interferes with cutaneous gaseous exchange, which is essential for amphibian
survival. Similarly, more than a million bats in the United States have been killed by white-nose syndrome, which
appears as a white ring around the mouth of the bat. It is caused by the cold-loving fungus Pseudogymnoascus
destructans, which disseminates its deadly spores in caves where bats hibernate. Mycologists are researching
the transmission, mechanism, and control of P. destructans to stop its spread.
Fungi that cause the superficial mycoses of the epidermis, hair, and nails rarely spread to the underlying tissue
(Figure 24.27). These fungi are often misnamed “dermatophytes”, from the Greek words dermis meaning skin
and phyte meaning plant, although they are not plants. Dermatophytes are also called “ringworms" because of
the red ring they cause on skin. They secrete extracellular enzymes that break down keratin (a protein found in
hair, skin, and nails), causing conditions such as athlete’s foot and jock itch. These conditions are usually treated
with over-the-counter topical creams and powders, and are easily cleared. More persistent superficial mycoses
may require prescription oral medications.
(a) (b) (c)
Figure 24.27 Fungal diseases of humans, (a) Ringworm presents as a red ring on skin; (b) Trichophyton violaceum,
shown in this bright field light micrograph, causes superficial mycoses on the scalp; (c) Histoplasma capsulatum is an
ascomycete that infects airways and causes symptoms similar to influenza, (credit a: modification of work by Dr. Lucille
K. Georg, CDC; credit b: modification of work by Dr. Lucille K. Georg, CDC; credit c: modification of work by M. Renz,
CDC; scale-bar data from Matt Russell)
Systemic mycoses spread to internal organs, most commonly entering the body through the respiratory system.
For example, coccidioidomycosis (often called valley fever) is commonly found in the southwestern United
States, but as far north as Washington, where the fungus resides in the dust. Once inhaled, the spores
develop in the lungs and cause symptoms similar to those of tuberculosis. Histoplasmosis is caused by the
dimorphic fungus Histoplasma capsulatum. in its human host, Histoplasma grows as a yeast, causing pulmonary
infections, and in rarer cases, swelling of the membranes of the brain and spinal cord. Treatment of these and
many other fungal diseases requires the use of antifungal medications that have serious side effects.
Opportunistic mycoses are fungal infections that are either common in all environments, or part of the normal
biota. They mainly affect individuals who have a compromised immune system. Patients in the late stages of
AIDS suffer from opportunistic mycoses that can be life threatening. The yeast Candida sp., a common member
of the natural biota, can grow unchecked and infect the vagina or mouth (oral thrush) if the pH of the surrounding
environment, the person’s immune defenses, or the normal population of bacteria are altered.
Mycetismus can occur when poisonous mushrooms are eaten. It causes a number of human fatalities during
mushroom-picking season. Many edible fruiting bodies of fungi resemble highly poisonous relatives, and
amateur mushroom hunters are cautioned to carefully inspect their harvest and avoid eating mushrooms of
doubtful origin. The adage “there are bold mushroom pickers and old mushroom pickers, but are there no old,
bold mushroom pickers” is unfortunately true.
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Dutch Elm Disease
Question: Do trees resistant to Dutch elm disease secrete antifungal compounds?
Hypothesis: Construct a hypothesis that addresses this question.
Background: Dutch elm disease is a fungal infestation that affects many species of elm ( Ulmus ) in North
America. The fungus infects the vascular system of the tree, which blocks water flow within the plant and
mimics drought stress. Accidently introduced to the United States in the early 1930s, it decimated American
elm shade trees across the continent. It is caused by the fungus Ophiostoma ulmi. The elm bark beetle
acts as a vector and transmits the disease from tree to tree. Many European and Asiatic elms are less
susceptible to the disease than are American elms.
Test the hypothesis: A researcher testing this hypothesis might do the following. Inoculate several Petri
plates containing a medium that supports the growth of fungi with fragments of Ophiostoma mycelium. Cut
(with a metal punch) several disks from the vascular tissue of susceptible varieties of American elms and
resistant European and Asiatic elms. Include control Petri plates inoculated with mycelia without plant tissue
to verify that the medium and incubation conditions do not interfere with fungal growth. As a positive control,
add paper disks impregnated with a known fungicide to Petri plates inoculated with the mycelium.
Incubate the plates for a set number of days to allow fungal growth and spreading of the mycelium over the
surface of the plate. Record the diameter of the zone of clearing, if any, around the tissue samples and the
fungicide control disk.
Record your observations in the following table.
Results of Antifungal Testing of Vascular Tissue from Different Species of
Elm
Disk Zone of Inhibition (mm)
Distilled Water
Fungicide
Tissue from Susceptible Elm #1
Tissue from Susceptible Elm #2
Tissue from Resistant Elm #1
Tissue from Resistant Elm #2
Table 24.1
Analyze the data and report the results. Compare the effect of distilled water to the fungicide. These are
negative and positive controls that validate the experimental setup. The fungicide should be surrounded by
a clear zone where the fungus growth was inhibited. Is there a difference among different species of elm?
Draw a conclusion: Was there antifungal activity as expected from the fungicide? Did the results support
the hypothesis? If not, how can this be explained? There are several possible explanations.
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24.5 | Importance of Fungi in Human Life
By the end of this section, you will be able to do the following:
• Describe the importance of fungi to the balance of the environment
• Summarize the role of fungi in agriculture and food and beverage preparation
• Describe the importance of fungi in the chemical and pharmaceutical industries
• Discuss the role of fungi as model organisms
Although we often think of fungi as organisms that cause disease and rot food, they are vitally important to
human life on many levels. As we have seen, fungi influence the well-being of human populations on a large
scale because they are part of the nutrient cycle in ecosystems. They have other ecosystem roles as well. As
animal pathogens, fungi help to control the population of damaging pests. These fungi are very specific to the
insects they attack, and do not infect animals or plants. Fungi are currently under investigation as potential
microbial insecticides, with several already on the market. For example, the fungus Beauveria bassiana is being
tested as a possible biological control agent for the recent spread of emerald ash borer a beetle that feeds on
ash trees. It has been released in Michigan, Illinois, Indiana, Ohio, West Virginia, and Maryland (Figure 24.28).
Figure 24.28 Fungal insect control. The emerald ash borer ( Agrilus ptanipennis ) is an insect that attacks ash trees. It
is in turn parasitized by a pathogenic fungus ( Beauveria bassiana) that holds promise as a biological insecticide. The
parasitic fungus appears as white fuzz on the body of the insect, (credit: Houping Liu, USDA Agricultural Research
Service)
The mycorrhizal relationship between fungi and plant roots is essential for the productivity of farm land. Without
the fungal partner in root systems, 80-90 percent of trees and grasses would not survive. Mycorrhizal fungal
inoculants are available as soil amendments from gardening supply stores and are promoted by supporters of
organic agriculture.
We also eat some types of fungi. Mushrooms figure prominently in the human diet. Morels, shiitake mushrooms,
chanterelles, and truffles are considered delicacies (Figure 24.29). The humble meadow mushroom, Agaricus
campestris, appears in many dishes. Molds of the genus Penicillium ripen many cheeses. They originate in the
natural environment such as the caves of Roquefort, France, where wheels of sheep milk cheese are stacked in
order to capture the molds responsible for the blue veins and pungent taste of the cheese.
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Figure 24.29 Edible fungi. The morel mushroom (a) is an ascomycete greatly appreciated for its delicate taste, (credit:
Jason Hollinger). Basidiocarps of Agaricus ready for an omelet (credit: Mary Anne Clark)
Fermentation—of grains to produce beer, and of fruits to produce wine—is an ancient art that humans in most
cultures have practiced for millennia. Wild yeasts are acquired from the environment and used to ferment sugars
into CO 2 and ethyl alcohol under anaerobic conditions. It is now possible to purchase isolated strains of wild
yeasts from different wine-making regions. Louis Pasteur was instrumental in developing a reliable strain of
brewer’s yeast, Saccharomyces cerevisiae, for the French brewing industry in the late 1850s. This was one of
the first examples of biotechnology patenting.
Many secondary metabolites of fungi are of great commercial importance. Antibiotics are naturally produced
by fungi to kill or inhibit the growth of bacteria, limiting their competition in the natural environment, important
antibiotics, such as penicillin and the cephalosporins, are isolated from fungi. Valuable drugs isolated from fungi
include the immunosuppressant drug cyclosporine (which reduces the risk of rejection after organ transplant),
the precursors of steroid hormones, and ergot alkaloids used to stop bleeding. Psilocybin is a compound found in
fungi such as Psilocybe semilanceata and Cymnopilus junonius, which have been used for their hallucinogenic
properties by various cultures for thousands of years.
As simple eukaryotic organisms, fungi are important model research organisms. Many advances in modern
genetics were achieved by the use of the red bread mold Neurospora crassa. Additionally, many important
genes originally discovered in S. cerevisiae served as a starting point in discovering analogous human genes.
As a eukaryotic organism, the yeast cell produces and modifies proteins in a manner similar to human cells,
as opposed to the bacterium Escherichia coli, which lacks the internal membrane structures and enzymes to
tag proteins for export. This makes yeast a much better organism for use in recombinant DNA technology
experiments. Like bacteria, yeasts grow easily in culture, have a short generation time, and are amenable to
genetic modification.
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KEY TERMS
arbuscular mycorrhiza mycorrhizal association in which the fungal hyphae enter the root cells and form
extensive networks
Arbuscular mycorrhizae mycorrhizae commonly involving Glomeromycetes in which the fungal hyphae
penetrate the cell walls of the plant root cells (but not the cell membranes)
ascocarp fruiting body of ascomycetes
Ascomycota (also, sac fungi) phylum of fungi that store spores in a sac called ascus
basidiocarp fruiting body that protrudes from the ground and bears the basidia
Basidiomycota (also, club fungi) phylum of fungi that produce club-shaped structures (basidia) that contain
spores
basidium club-shaped fruiting body of basidiomycetes
Chytridiomycota (also, chytrids) primitive phylum of fungi that live in water and produce gametes with flagella
coenocytic hypha single hypha that lacks septa and contains many nuclei
commensalism symbiotic relationship in which one member benefits while the other member is not affected
Deuteromycota former form phylum of fungi that do not have a known sexual reproductive cycle (presently
members of two phyla: Ascomycota and Basidiomycota)
ectomycorrhiza mycorrhizal fungi that surround the roots with a mantle and have a Hartig net that extends into
the roots between cells
Ectomycorrhizae mycorrhizae in which the fungal hyphae do not penetrate the root cells of the plant
facultative anaerobes organisms that can perform both aerobic and anaerobic respiration and can survive in
oxygen-rich and oxygen-poor environment
Glomeromycota phylum of fungi that form symbiotic relationships with the roots of trees
haustoria modified hyphae on many parasitic fungi that penetrate the tissues of their hosts, release digestive
enzymes, and/or absorb nutrients from the host
heterothallic describes when only one mating type is present in an individual mycelium
homothallic describes when both mating types are present in mycelium
hypha fungal filament composed of one or more cells
karyogamy fusion of nuclei
lichen close association of a fungus with a photosynthetic alga or bacterium that benefits both partners
mold tangle of visible mycelia with a fuzzy appearance
mycelium mass of fungal hyphae
mycetismus ingestion of toxins in poisonous mushrooms
mycology scientific study of fungi
mycorrhiza mutualistic association between fungi and vascular plant roots
mycorrhizae a mutualistic relationship between a plant and a fungus. Mycorrhizae are connections between
fungal hyphae, which provide soil minerals to the plant, and plant roots, which provide carbohydrates to the
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fungus
mycosis fungal infection
mycotoxicosis poisoning by a fungal toxin released in food
obligate aerobes organisms, such as humans, that must perform aerobic respiration to survive
obligate anaerobes organisms that only perform anaerobic respiration and often cannot survive in the
presence of oxygen
parasitism symbiotic relationship in which one member of the association benefits at the expense of the other
plasmogamy fusion of cytoplasm
saprobe organism that derives nutrients from decaying organic matter; also saprophyte
septa cell wall division between hyphae
soredia clusters of algal cells and mycelia that allow lichens to propagate
sporangium reproductive sac that contains spores
spore a haploid cell that can undergo mitosis to form a multicellular, haploid individual
thallus vegetative body of a fungus
yeast general term used to describe unicellular fungi
Zygomycota (also, conjugated fungi) phylum of fungi that form a zygote contained in a zygospore
zygospore structure with thick cell wall that contains the zygote in zygomycetes
CHAPTER SUMMARY
24.1 Characteristics of Fungi
Fungi are eukaryotic organisms that appeared on land more than 450 million years ago, but clearly have an
evolutionary history far greater. They are heterotrophs and contain neither photosynthetic pigments such as
chlorophyll, nor organelles such as chloroplasts. Because fungi feed on decaying and dead matter, they are
termed saprobes. Fungi are important decomposers that release essential elements into the environment.
External enzymes called exoenzymes digest nutrients that are absorbed by the body of the fungus, which is
called a thallus. A thick cell wall made of chitin surrounds the cell. Fungi can be unicellular as yeasts, or
develop a network of filaments called a mycelium, which is often described as mold. Most species multiply by
asexual and sexual reproductive cycles and display an alternation of generations, in one group of fungi, no
sexual cycle has been identified. Sexual reproduction involves plasmogamy (the fusion of the cytoplasm),
followed by karyogamy (the fusion of nuclei). Following these processes, meiosis generates haploid spores.
24.2 Classifications of Fungi
Chytridiomycota (chytrids) are considered the most ancestral group of fungi. They are mostly aquatic, and their
gametes are the only fungal cells known to have flagella. They reproduce both sexually and asexually; the
asexual spores are called zoospores. Zygomycota (conjugated fungi) produce non-septate hyphae with many
nuclei. Their hyphae fuse during sexual reproduction to produce a zygospore in a zygosporangium.
Ascomycota (sac fungi) form spores in sacs called asci during sexual reproduction. Asexual reproduction is
their most common form of reproduction. In the Basidiomycota (club fungi), the sexual phase predominates,
producing showy fruiting bodies that contain club-shaped basidia, within which spores form. Most familiar
mushrooms belong to this division. Fungi that have no known sexual cycle were originally classified in the “form
phylum" Deuteromycota, but many have been classified by comparative molecular analysis with the
Ascomycota and Basidiomycota. Glomeromycota form tight associations (called mycorrhizae) with the roots of
plants.
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24.3 Ecology of Fungi
Fungi have colonized nearly all environments on Earth, but are frequently found in cool, dark, moist places with
a supply of decaying material. Fungi are saprobes that decompose organic matter. Many successful mutualistic
relationships involve a fungus and another organism. Many fungi establish complex mycorrhizal associations
with the roots of plants. Some ants farm fungi as a supply of food. Lichens are a symbiotic relationship between
a fungus and a photosynthetic organism, usually an alga or cyanobacterium. The photosynthetic organism
provides energy from stored carbohydrates, while the fungus supplies minerals and protection. Some animals
that consume fungi help disseminate spores over long distances.
24.4 Fungal Parasites and Pathogens
Fungi establish parasitic relationships with plants and animals. Fungal diseases can decimate crops and spoil
food during storage. Compounds produced by fungi can be toxic to humans and other animals. Mycoses are
infections caused by fungi. Superficial mycoses affect the skin, whereas systemic mycoses spread through the
body. Fungal infections are difficult to cure, since fungi, like their hosts, are eukaryotic, and cladistically related
closely to Kingdom Animalia.
24.5 Importance of Fungi in Human Life
Fungi are important to everyday human life. Fungi are important decomposers in most ecosystems. Mycorrhizal
fungi are essential for the growth of most plants. Fungi, as food, play a role in human nutrition in the form of
mushrooms, and also as agents of fermentation in the production of bread, cheeses, alcoholic beverages, and
numerous other food preparations. Secondary metabolites of fungi are used as medicines, such as antibiotics
and anticoagulants. Fungi are model organisms for the study of eukaryotic genetics and metabolism.
VISUAL CONNECTION QUESTIONS
1. Figure 24.14 Which of the following statements is
true?
a. A dikaryotic ascus that forms in the
ascocarp undergoes karyogamy, meiosis,
and mitosis to form eight ascospores.
b. A diploid ascus that forms in the ascocarp
undergoes karyogamy, meiosis, and mitosis
to form eight ascospores.
c. A haploid zygote that forms in the ascocarp
undergoes karyogamy, meiosis, and mitosis
to form eight ascospores.
d. A dikaryotic ascus that forms in the
ascocarp undergoes plasmogamy, meiosis,
and mitosis to form eight ascospores.
REVIEW QUESTIONS
4. Which polysaccharide is usually found in the cell filament is called a
wall of fungi?
a. thallus
a. starch
b. hypha
b. glycogen
c. mycelium
c. chitin
d. septum
d. cellulose
7. During sexual reproduction, a homothallic
5. Which of these organelles is not found in a fungal
mycelium contains
cell?
a. all septated hyphae
a. chloroplast
b. all haploid nuclei
b. nucleus
c. both mating types
c. mitochondrion
d. none of the above
d. Golgi apparatus
8. The life cycles of perfect fungi are most similar
6. The wall dividing individual cells in a fungal which other organism?
2. Figure 24.17 Which of the following statements is
true?
a. A basidium is the fruiting body of a
mushroom-producing fungus, and it forms
four basidiocarps.
b. The result of the plasmogamy step is four
basidiospores.
c. Karyogamy results directly in the formation
of mycelia.
d. A basidiocarp is the fruiting body of a
mushroom-producing fungus.
3. Figure 24.21 If symbiotic fungi are absent from the
soil, what impact do you think this would have on
plant growth?
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a. Hydra that undergo asexual budding
b. Diploid-dominant pea plants
c. Haploid-dominant green algae
d. Bacteria undergoing binary fission
9. The most primitive phylum of fungi is the
a. Chytridiomycota
b. Zygomycota
c. Glomeromycota
d. Ascomycota
10. Members of which phylum produce a club-
shaped structure that contains spores?
a. Chytridiomycota
b. Basidiomycota
c. Glomeromycota
d. Ascomycota
11. Members of which phylum establish a successful
symbiotic relationship with the roots of trees?
a. Ascomycota
b. Deuteromycota
c. Basidiomycota
d. Glomeromycota
12. The fungi that do not reproduce sexually used to
be classified as_.
a. Ascomycota
b. Deuteromycota
c. Basidiomycota
d. Glomeromycota
13. A scientist discovers a new species of fungus that
introduces genetic diversity during reproduction by
creating a diploid zygote. This new species cannot
belong to which modern phylum of fungi?
a. Zygomycota
b. Glomeromycota
c. Chytridiomycota
d. Deuteromycota
14. What term describes the close association of a
fungus with the root of a tree?
a. a rhizoid
b. a lichen
c. a mycorrhiza
d. an endophyte
15. Why are fungi important decomposers?
a. They produce many spores.
b. They can grow in many different
environments.
c. They produce mycelia.
d. They recycle carbon and inorganic minerals
by the process of decomposition.
16. Consider an ecosystem where all the fungi not
involved in mycorrhizae are eliminated. How would
this affect nitrogen intake by plants?
a. Nitrogen intake would increase.
b. Nitrogen intake would not change.
c. Nitrogen intake would decrease.
d. Nitrogen intake would stop.
17. A fungus that climbs up a tree reaching higher
elevation to release its spores in the wind and does
not receive any nutrients from the tree or contribute
to the tree’s welfare is described as a_.
a. commensal
b. mutualist
c. parasite
d. pathogen
18. A fungal infection that affects nails and skin is
classified as_.
a. systemic mycosis
b. mycetismus
c. superficial mycosis
d. mycotoxicosis
19. The targets for anti-fungal drugs are much more
limited than antibiotics or anti-viral medications.
Why?
a. There are more bacteria and viruses than
fungi.
b. Fungi can only be targeted during sexual
reproduction, while bacteria and viruses can
be targeted at any point in their lifespan.
c. Fungi cause topical infections, while viruses
and bacteria cause systemic infections.
d. Human cells are much more similar to fungi
cells than bacteria or viruses.
20. Yeast is a facultative anaerobe. This means that
alcohol fermentation takes place only if:
a. the temperature is close to 37°C
b. the atmosphere does not contain oxygen
c. sugar is provided to the cells
d. light is provided to the cells
21. The advantage of yeast cells over bacterial cells
to express human proteins is that:
a. yeast cells grow faster
b. yeast cells are easier to manipulate
genetically
c. yeast cells are eukaryotic and modify
proteins similarly to human cells
d. yeast cells are easily lysed to purify the
proteins
22. Why are fungal insecticides an attractive
alternative to chemical pesticides for growing food
crops?
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a. Human consumption of fungal insecticides
would not make a person sick, but ingestion
of chemical pesticides can be harmful to
humans.
b. A single fungal insecticide would kill a wider
variety of insects than a chemical pesticide.
c. Fungal insecticides can eliminate both
harmful insects and plant pathogens, while
chemical pesticides only kill insects.
d. Fungal insecticides will decompose dying
plants, enhancing the nitrogen content of
the soil, while chemical pesticides are not
decomposers.
CRITICAL THINKING QUESTIONS
23. What are the evolutionary advantages for an
organism to reproduce both asexually and sexually?
24. Compare plants, animals, and fungi, considering
these components: cell wall, chloroplasts, plasma
membrane, food source, and polysaccharide storage.
Be sure to indicate fungi’s similarities and differences
to plants and animals.
25. Why is the large surface area of the mycelium
essential for nutrient acquisition by fungi?
26. What is the advantage for a basidiomycete to
produce a showy and fleshy fruiting body?
27. For each of the four groups of perfect fungi
(Chytridiomycota, Zygomycota, Ascomycota, and
Basidiomycota), compare the body structure and
features, and provide an example.
28. Why does protection from light actually benefit
the photosynthetic partner in lichens?
29. Ambrosia bark beetles carry Ambrosiella fungal
spores to trees, then bore holes and lay their eggs
with the fungus. When the new larvae hatch, they eat
the fungus that has germinated in the holes. Describe
how this relationship can be classified as mutualistic.
30. Ecologists often attempt to introduce new plants
to restore degraded land. In an arid climate,
scientists recommend introducing plants with
arbuscular mycorrhizae. How would the mycorrhizae
increase the plants’ survival compared to plants
without mycorrhizae?
31. Why can superficial mycoses in humans lead to
bacterial infections?
32. Explain how the Red Queen Hypothesis
describes the continuously evolving relationship
between red grapes and Botrytis cinerea.
33. Historically, artisanal breads were produced by
capturing wild yeasts from the air. Prior to the
development of modern yeast strains, the production
of artisanal breads was long and laborious because
many batches of dough ended up being discarded.
Can you explain this fact?
34. How would treating an area of a forest with a
broad-spectrum fungicide alter the carbon and
nitrogen cycles in the area?
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25 | SEEDLESS PLANTS
Figure 25.1 Seedless plants, like these horsetails (Equisetum sp.), thrive in damp, shaded environments under a tree
canopy where dryness is rare, (credit: modification of work by Jerry Kirkhart)
Chapter Outline
25.1: Early Plant Life
25.2: Green Algae: Precursors of Land Plants
25.3: Bryophytes
25.4: Seedless Vascular Plants
Introduction
An incredible variety of seedless plants populates the terrestrial landscape. Mosses may grow on a tree trunk,
and horsetails may display their jointed stems and spindly leaves across the forest floor. Today, seedless
plants represent only a small fraction of the plants in our environment; yet, 300 million years ago, seedless
plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Their
decomposition created large deposits of coal that we mine today.
Current evolutionary thought holds that all plants—some green algae as well as land plants—are monophyletic;
that is, they are descendants of a single common ancestor. The evolutionary transition from water to land
imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse
reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants have
developed adaptations that allow them to populate even the most arid habitats on Earth, full independence from
water did not happen in all plants. Most seedless plants still require a moist environment for reproduction.
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25.1 1 Early Plant Life
By the end of this section, you will be able to do the following:
• Discuss the challenges to plant life on land
• Describe the adaptations that allowed plants to colonize the land
• Describe the timeline of plant evolution and the impact of land plants on other living things
The kingdom Plantae constitutes large and varied groups of organisms. There are more than 300,000 species of
catalogued plants. Of these, more than 260,000 are seed plants. Mosses, ferns, conifers, and flowering plants
are all members of the plant kingdom. Land plants arose within the Archaeplastida, which includes the red algae
(Rhodophyta) and two groups of green algae, Chlorophyta and Charaphyta. Most biologists also consider at
least some green algae to be plants, although others exclude all algae from the plant kingdom. The reason
for this disagreement stems from the fact that only green algae, the Chlorophytes and Charophytes, share
common characteristics with land plants (such as using chlorophyll a and b plus carotene in the same proportion
as plants). These characteristics are absent from other types of algae.
V / _
e olution CONNECTION
Algae and Evolutionary Paths to Photosynthesis
Some scientists consider all algae to be plants, while others assert that only the green algae belong in
the kingdom Plantae. Still others include only the Charophytes among the plants. These divergent opinions
are related to the different evolutionary paths to photosynthesis selected for in different types of algae.
While all algae are photosynthetic—that is, they contain some form of a chloroplast—they didn’t all become
photosynthetic via the same path.
The ancestors to the Archaeplastida became photosynthetic by forming an endosymbiotic relationship with a
green, photosynthetic bacterium about 1.65 billion years ago. That algal line evolved into the red and green
algae, and eventually into the modern mosses, ferns, gymnosperms, and angiosperms. Their evolutionary
trajectory was relatively straight and monophyletic. In contrast, algae outside of the Archaeplastida, e.g.,
the brown and golden algae of the stramenopiles, and so on—all became photosynthetic by secondary, or
even tertiary, endosymbiotic events; that is, they engulfed cells that already contained an endosymbiotic
cyanobacterium. These latecomers to photosynthesis are parallels to the Archaeplastida in terms of
autotrophy, but they did not expand to the same extent as the Archaeplastida, nor did they colonize the land.
Scientists who solely track evolutionary straight lines (that is, monophyly), consider only the Charophytes
as plants. The common ancestor of Charophytes and land plants excludes the other members of the
Archaeplastida. Charophytes also share other features with the land plants. These will be discussed in more
detail in another section.
LINK TQ LEARNING
Go to this interactive website (http:// 0 penstaxc 0 llege. 0 rg/l/char 0 phytes) to get a more in-depth view of
the Charophytes.
Plant Adaptations to Life on Land
As organisms adapted to life on land, they had to contend with several challenges in the terrestrial environment.
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Water has been described as “the stuff of life.” The cell’s interior is a thick soup: in this medium, most small
molecules dissolve and diffuse, and the majority of the chemical reactions of metabolism take place. Desiccation,
or drying out, is a constant danger for an organism exposed to air. Even when parts of a plant are close to a
source of water, the aerial structures are likely to dry out. Water also provides buoyancy to organisms. On land,
plants need to develop structural support in a medium that does not give the same lift as water. The organism is
also subject to bombardment by mutagenic radiation, because air does not filter out ultraviolet rays of sunlight.
Additionally, the male gametes must reach the female gametes using new strategies, because swimming is no
longer possible. Therefore, both gametes and zygotes must be protected from desiccation. The successful land
plants developed strategies to deal with all of these challenges. Not all adaptations appeared at once. Some
species never moved very far from the aquatic environment, whereas others went on to conquer the driest
environments on Earth.
To balance these survival challenges, life on land offers several advantages. First, sunlight is abundant. Water
acts as a filter, altering the spectral quality of light absorbed by the photosynthetic pigment chlorophyll. Second,
carbon dioxide is more readily available in air than in water, since it diffuses faster in air. Third, land plants
evolved before land animals; therefore, until dry land was colonized by animals, no predators threatened plant
life. This situation changed as animals emerged from the water and fed on the abundant sources of nutrients
in the established flora. In turn, plants developed strategies to deter predation: from spines and thorns to toxic
chemicals.
Early land plants, like the early land animals, did not live very far from an abundant source of water and
developed survival strategies to combat dryness. One of these strategies is called tolerance. Many mosses, for
example, can dry out to a brown and brittle mat, but as soon as rain or a flood makes water available, mosses
will absorb it and are restored to their healthy green appearance. Another strategy is to colonize environments
with high humidity, where droughts are uncommon. Ferns, which are considered an early lineage of plants, thrive
in damp and cool places such as the understory of temperate forests. Later, plants moved away from moist or
aquatic environments using resistance to desiccation, rather than tolerance. These plants, like cacti, minimize
the loss of water to such an extent they can survive in extremely dry environments.
The most successful adaptation solution was the development of new structures that gave plants the advantage
when colonizing new and dry environments. Four major adaptations contribute to the success of terrestrial
plants. The first adaptation is that the life cycle in all land plants exhibits the alternation of generations, a
sporophyte in which the spores are formed and a gametophyte that produces gametes. Second is an apical
meristem tissue in roots and shoots. Third is the evolution of a waxy cuticle to resist desiccation (absent from
some mosses). Finally cell walls with lignin to support structures off the ground. These adaptations all contribute
to the success of the land plants, but are noticeably lacking in the closely related green algae—another reason
for the debate over their placement in the plant kingdom. They are also not all found in the mosses, which can
be regarded as representing an intermediate stage in adaptation to land.
Alternation of Generations
All sexually reproducing organisms have both haploid and diploid cells in their life cycles. In organisms with
haplontic life cycles, the haploid stage is dominant, while in organisms with a diplontic life cycle, the diploid
stage is the dominant life stage. Dominant in this context means both the stage in which the organism spends
most of its time, and the stage in which most mitotic cell reproduction occurs—the multicellular stage. In haplontic
life cycles, the only diploid cell is the zygote, which undergoes immediate meiosis to restore the haploid state.
In diplontic life cycles, the only haploid cells are the gametes, which combine to restore the diploid state at their
earliest convenience. Humans, for example, are diplontic.
Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular
stages (Figure 25.2). This type of life cycle, which is found in all plants, is described as haplodiplontic.
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n Gametophyte
2n Sporophyte
Figure 25.2 Alternation of generations between the In gametophyte and 2n sporophyte is shown. Mitosis occurs in
both gametophyte and sporophyte generations. Diploid sporophytes produce haploid spores by meiosis, while haploid
gametophytes produce gametes by mitosis, (credit: Peter Coxhead)
In alternation of generations, the multicellular haploid form, known as a gametophyte, is followed in the
developmental sequence by a multicellular diploid form, the sporophyte. The gametophyte gives rise to the
gametes (reproductive cells) by mitosis. This can be the most obvious phase of the life cycle of the plant, as in
the mosses, or it can occur in a microscopic structure, such as a pollen grain, in the seed plants. The evolution
of the land plants is marked by increasing prominence of the sporophyte generation. The sporophyte stage
is barely noticeable in non-vascular plants (the collective term for the plants that include the liverworts and
mosses). In the seed plants, the sporophyte phase can be a towering tree, as in sequoias and pines.
Protection of the embryo is a major requirement for land plants. The vulnerable embryo must be sheltered
from desiccation and other environmental hazards. In both seedless and seed plants, the female gametophyte
provides protection and nutrients to the embryo as it develops into the new sporophyte. This distinguishing
feature of land plants gave the group its alternate name of embryophytes.
Sporangia in Seedless Plants
The sporophyte of seedless plants is diploid and results from syngamy (fusion) of two gametes. The sporophyte
bears the sporangia (singular, sporangium). The term “sporangia" literally means “a vessel for spores,” as it
is a reproductive sac in which spores are formed (Figure 25.3). Inside the multicellular sporangia, the diploid
sporocytes, or mother cells, produce haploid spores by meiosis, during which the 2 n chromosome number is
reduced to In (note that in many plants, chromosome number is complicated by polyploidy: for example, durum
wheat is tetraploid, bread wheat is hexaploid, and some ferns are 1000-ploid). The spores are later released by
the sporangia and disperse in the environment. When the haploid spore germinates in a hospitable environment,
it generates a multicellular gametophyte by mitosis. The gametophyte supports the zygote formed from the
fusion of gametes and the resulting young sporophyte (vegetative form). The cycle then begins anew.
Figure 25.3 Sporangia. Spore-producing sacs called sporangia grow at the ends of long, thin stalks in this photo of the
moss Esporangios bryum. (credit: Javier Martin)
Plants that produce only one type of spore are called homosporous and the resultant gametophyte produces
both male and female gametes, usually on the same individual. Non-vascular plants are homosporous, and the
gametophyte is the dominant generation in the life cycle. Plants that produce two types of spores are called
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heterosporous. The male spores are called microspores, because of their smaller size, and develop into the
male gametophyte; the comparatively larger megaspores develop into the female gametophyte. A few seedless
vascular plants and all seed plants are heterosporous, and the sporophyte is the dominant generation.
The spores of seedless plants are surrounded by thick cell walls containing a tough polymer known as
sporopollenin. As the name suggests, it is also found in the walls of pollen grains. This complex substance is
characterized by long chains of organic molecules related to fatty acids and carotenoids: hence the yellow color
of most pollen. Sporopollenin is unusually resistant to chemical and biological degradation. In seed plants, in
which pollen is the male gametophyte, the toughness of sporopollenin explains the existence of well-preserved
pollen fossils. Sporopollenin was once thought to be an innovation of land plants; however, the charophyte
Coleochaetes also forms spores that contain sporopollenin.
Gametangia in Seedless Plants
Gametangia (singular, gametangium) are structures observed on multicellular haploid gametophytes. In the
gametangia, precursor cells give rise to gametes by mitosis. The male gametangium ( antheridium) releases
sperm. Seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment
to the archegonium: the female gametangium. The embryo develops inside the archegonium as the
sporophyte. Gametangia are prominent in seedless plants, but are absent or rudimentary in seed plants.
Apical Meristems
Shoots and roots of plants increase in length through rapid cell division in a tissue called the apical meristem,
which is a small mitotically active zone of cells found at the shoot tip or root tip (Figure 25.4). The apical
meristem is made of undifferentiated cells that continue to proliferate throughout the life of the plant.
Meristematic cells give rise to all the specialized tissues of the organism. Elongation of the shoots and roots
allows a plant to access additional space and resources: light in the case of the shoot, and water and minerals
in the case of roots. A separate meristem, called the lateral meristem, produces cells that increase the diameter
of tree trunks.
Apical meristem
Root cap
Figure 25.4 Apical meristem at a root tip. Addition of new cells in a root occurs at the apical meristem. Subsequent
enlargement of these cells causes the organ to grow and elongate. The root cap protects the fragile apical meristem
as the root tip is pushed through the soil by cell elongation.
Additional Land Plant Adaptations
As plants adapted to dry land and became independent from the constant presence of water in damp habitats,
new organs and structures made their appearance. Early land plants did not grow more than a few inches off
the ground, competing for light on these low mats. By developing a shoot and growing taller, individual plants
captured more light. Because air offers substantially less support than water, land plants incorporated more rigid
molecules in their stems (and later, tree trunks). In small plants such as single-celled algae, simple diffusion
suffices to distribute water and nutrients throughout the organism. However, for plants to evolve larger forms,
the evolution of a conductive tissue for the distribution of water and solutes was a prerequisite. The evolution of
vascular tissue in plants met both of these needs. The vascular system contains two types of conductive tissue:
xylem and phloem. Xylem conducts water and minerals absorbed from the soil up to the shoot, while phloem
transports food derived from photosynthesis throughout the entire plant. In xylem, the cells walls are reinforced
with lignin, whose tough hydrophobic polymers help prevent the seepage of water across the xylem cell walls.
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Lignin also adds to the strength of these tissues in supporting the plant. The vascular tissues extend into the
root of land plants. The root system evolved to take up water and minerals from the soil, and to anchor the
increasingly taller shoot in the soil.
In land plants, a waxy, waterproof cover called a cuticle protects the leaves and stems from desiccation.
However, the cuticle also prevents intake of carbon dioxide needed for the synthesis of carbohydrates through
photosynthesis. To overcome this, stomata or pores that open and close to regulate traffic of gases and water
vapor appeared in plants as they moved away from moist environments into drier habitats.
Water filters ultraviolet-B (UVB) light, which is harmful to all organisms, especially those that must absorb light
to survive. This filtering does not occur for land plants. Exposure to damaging radiation presented an additional
challenge to land colonization, which was met by the evolution of biosynthetic pathways for the synthesis of
protective flavonoids and other pigments that absorb UV wavelengths of light and protect the aerial parts of
plants from photodynamic damage.
Plants cannot avoid being eaten by animals. Instead, they synthesize a large range of poisonous secondary
metabolites: complex organic molecules such as alkaloids, whose noxious smells and unpleasant taste deter
animals. These toxic compounds can also cause severe diseases and even death, thus discouraging predation.
Humans have used many of these compounds for centuries as drugs, medications, or spices. In contrast, as
plants co-evolved with animals, the development of sweet and nutritious metabolites lured animals into providing
valuable assistance in dispersing pollen grains, fruit, or seeds. Plants have been enlisting animals to be their
helpers in this way for hundreds of millions of years.
Evolution of Land Plants
No discussion of the evolution of plants on land can be undertaken without a brief review of the timeline of the
geological eras. The early era, known as the Paleozoic, is divided into six periods. It starts with the Cambrian
period, followed by the Ordovician, Silurian, Devonian, Carboniferous, and Permian. The major event to mark
the Ordovician, more than 500 million years ago, was the colonization of land by the ancestors of modern land
plants. Fossilized cells, cuticles, and spores of early land plants have been dated as far back as the Ordovician
period in the early Paleozoic era. The oldest-known vascular plants have been identified in deposits from the
Devonian. One of the richest sources of information is the Rhynie chert, a sedimentary rock deposit found in
Rhynie, Scotland (Figure 25.5), where embedded fossils of some of the earliest vascular plants have been
identified.
Figure 25.5 Early vascular plant fossils. This Rhynie chert (a) contains fossilized material from vascular plants.
Reconstruction of Cooksonia (b), the plant forms inside the circle, (credit b: modification of work by Peter Coxhead
based on original image by “Smith609"/Wikimedia Commons; scale-bar data from Matt Russell)
Paleobotanists distinguish between extinct species, as fossils, and extant species, which are still living. The
extinct vascular plants most probably lacked true leaves and roots and formed low vegetation mats similar
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in size to modern-day mosses, although some could reach one meter in height. The later genus Cooksonia ,
which flourished during the Silurian, has been extensively studied from well-preserved examples. Imprints
of Cooksonia show slender branching stems ending in what appear to be sporangia. From the recovered
specimens, it is not possible to establish for certain whether Cooksonia possessed vascular tissues. Fossils
indicate that by the end of the Devonian period, ferns, horsetails, and seed plants populated the landscape,
giving rising to trees and forests. This luxuriant vegetation helped enrich the atmosphere with oxygen, making
it easier for air-breathing animals to colonize dry land. Plants also established early symbiotic relationships with
fungi, creating mycorrhizae: a relationship in which the fungal network of filaments increases the efficiency of the
plant root system, and the plants provide the fungi with byproducts of photosynthesis.
career connection
Paleobotanist
How organisms acquired traits that allow them to colonize new environments—and how the contemporary
ecosystem is shaped—are fundamental questions of evolution. Paleobotany (the study of extinct plants)
addresses these questions through the analysis of fossilized specimens retrieved from field studies,
reconstituting the morphology of organisms that disappeared long ago. Paleobotanists trace the evolution of
plants by following the modifications in plant morphology: shedding light on the connection between existing
plants by identifying common ancestors that display the same traits. This field seeks to find transitional
species that bridge gaps in the path to the development of modern organisms. Fossils are formed when
organisms are trapped in sediments or environments where their shapes are preserved. Paleobotanists
collect fossil specimens in the field and place them in the context of the geological sediments and other
fossilized organisms surrounding them. The activity requires great care to preserve the integrity of the
delicate fossils and the layers of rock in which they are found.
One of the most exciting recent developments in paleobotany is the use of analytical chemistry and
molecular biology to study fossils. Preservation of molecular structures requires an environment free of
oxygen, since oxidation and degradation of material through the activity of microorganisms depend on its
presence. One example of the use of analytical chemistry and molecular biology is the identification of
oleanane, a compound that deters pests. Up to this point, oleanane appeared to be unique to flowering
plants; however, it has now been recovered from sediments dating from the Permian, much earlier than
the current dates given for the appearance of the first flowering plants. Paleobotanists can also study fossil
DNA, which can yield a large amount of information, by analyzing and comparing the DNA sequences of
extinct plants with those of living and related organisms. Through this analysis, evolutionary relationships
can be built for plant lineages.
Some paleobotanists are skeptical of the conclusions drawn from the analysis of molecular fossils. For
example, the chemical materials of interest degrade rapidly when exposed to air during their initial isolation,
as well as in further manipulations. There is always a high risk of contaminating the specimens with
extraneous material, mostly from microorganisms. Nevertheless, as technology is refined, the analysis of
DNA from fossilized plants will provide invaluable information on the evolution of plants and their adaptation
to an ever-changing environment.
The Major Divisions of Land Plants
The green algae and land plants are grouped together into a subphylum called the Streptophyta, and thus are
called Streptophytes. In a further division, land plants are classified into two major groups according to the
absence or presence of vascular tissue, as detailed in Figure 25.6. Plants that lack vascular tissue, which is
formed of specialized cells for the transport of water and nutrients, are referred to as non-vascular plants.
Liverworts, mosses, and hornworts are seedless, non-vascular plants that likely appeared early in land plant
evolution. Vascular plants developed a network of cells that conduct water and solutes. The first vascular
plants appeared in the late Ordovician (500 to 435 MYA) and were probably similar to lycophytes, which
include club mosses (not to be confused with the mosses) and the pterophytes (ferns, horsetails, and whisk
ferns). Lycophytes and pterophytes are referred to as seedless vascular plants, because they do not produce
seeds. The seed plants, or spermatophytes, form the largest group of all existing plants, and hence dominate
the landscape. Seed plants include gymnosperms, most notably conifers, which produce “naked seeds,” and
the most successful of all plants, the flowering plants (angiosperms). Angiosperms protect their seeds inside
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chambers at the center of a flower; the walls of the chamber later develop into a fruit.
visual
CONNECTION
STREPTOPHYTES: THE GREEN PLANTS
Charophytes
Embryophytes: The Land Plants
Non-vascular
Vascular
Seedless Plants
Seedless Plants
Seed Plants
Bryophytes
Lycophytes
Pterophytes
Spermatophytes
Liver¬
worts
Horn-
worts
Mosses
Club
Mosses
Whisk
Ferns
Gymno¬
sperms
Angio¬
sperms
Quillworts
Horsetails
Spike
Mosses
Ferns
Figure 25.6 Streptophytes. This table shows the major divisions of green plants.
Which of the following statements about plant divisions is false?
a. Lycophytes and pterophytes are seedless vascular plants.
b. All vascular plants produce seeds.
c. All non-vascular embryophytes are bryophytes.
d. Seed plants include angiosperms and gymnosperms.
25.2 | Green Algae: Precursors of Land Plants
By the end of this section, you will be able to do the following:
• Describe the traits shared by green algae and land plants
• Explain why charophytes are considered the closest algal relative to land plants
• Explain how current phylogenetic relationships are reshaped by comparative analysis of DNA
sequences
Streptophytes
Until recently, all photosynthetic eukaryotes were classified as members of the kingdom Plantae. The brown
and golden algae, however, are now reassigned to the protist supergroup Chromalveolata. This is because
apart from their ability to capture light energy and fix C02, they lack many structural and biochemical traits
that are characteristic of plants. The plants are now classified, along with the red and green algae, in the
protist supergroup Archaeplastida. Green algae contain the same carotenoids and chlorophyll a and b as land
plants, whereas other algae have different accessory pigments and types of chlorophyll molecules in addition
to chlorophyll a. Both green algae and land plants also store carbohydrates as starch. Their cells contain
chloroplasts that display a dizzying variety of shapes, and their cell walls contain cellulose, as do land plants.
Which of the green algae to include among the plants has not been phylogenetically resolved.
Green algae fall into two major groups, the chlorophytes and the charophytes. The chlorophytes include the
genera Chlorella, Chlamydomonas , the “sea lettuce" Ulva, and the colonial alga Volvox. The charophytes include
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desmids, as well as the genera Spirogyra, Coleochaete, and Chara. There are familiar green algae in both
groups. Some green algae are single cells, such as Chlamydomonas and desmids, which adds to the ambiguity
of green algae classification, because plants are multicellular. Other green algae, like Volvox, form colonies, and
some, like Ulva are multicellular (Figure 25.7). Spirogyra is a long filament of colonial cells. Most members of
this genus live in fresh water, brackish water, seawater, or even in snow patches. A few green algae can survive
on soil, provided it is covered by a thin film of moisture within which they can live. Periodic dry spells provide a
selective advantage to algae that can survive water stress.
(a) Spirogya
(b) Desmid
(c) Chlamydomonas (d) Ulva
Figure 25.7 Green algae. Charophyta include (a) Spirogyra and (b) desmids. Chlorophyta include (c) Chlamydomonas,
and (d) Ulva. Desmids and Chlamydomonas are single-celled organisms, Spirogyra forms chains of cells, and Ulva
forms multicellular structures resembling leaves, although the cells are not differentiated as they are in higher plants
(credit b: modification of work by Derek Keats; credit c: modification of work by Dartmouth Electron Microscope Facility,
Dartmouth College; credit d: modification of work by Holger Krisp; scale-bar data from Matt Russell)
The chlorophytes and the charophytes differ in a few respects that, in addition to molecular analysis, place the
land plants as a sister group of the charophytes. First, cells in charophytes and the land plants divide along cell
plates called phragmoplasts, in which microtubules parallel to the spindle serve as guides for the vesicles of
the forming cell plate. In the chlorophytes, the cell plate is organized by a phycoplast, in which the microtubules
are perpendicular to the spindle. Second, only the charophytes and the land plants have plasmodesmata,
or intercellular channels that allow the transfer of materials from cell to cell. In the chlorophytes, intercellular
connections do not persist in mature multicellular forms. Finally, both charophytes and the land plants show
apical growth —growth from the tips of the plant rather than throughout the plant body. Consequently, land plants
and the charophytes are now part of a new monophyletic group called Streptophyta.
Reproduction of Green Algae
Green algae reproduce both asexually, by fragmentation or dispersal of spores, or sexually, by producing
gametes that fuse during fertilization. In a single-celled organism such as Chlamydomonas, there is no mitosis
after fertilization. In the multicellular Ulva, a sporophyte grows by mitosis after fertilization (and thus exhibits
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alternation of generations). Both Chlamydomonas and Ulva produce flagellated gametes.
Charophytes
The charophytes include several different algal orders that have each been suggested to be the closest relatives
of the land plants: the Charales, the Zygnematales, and the Coleochaetales. The Charales can be traced back
420 million years. They live in a range of freshwater habitats and vary in size from a few millimeters to a meter
in length. The representative genus is Chara (Figure 25.8), often called muskgrass or skunkweed because of
its unpleasant smell. Large cells form the thallus'. the main stem of the alga. Branches arising from the nodes
are made of smaller cells. Male and female reproductive structures are found on the nodes, and the sperm have
flagella. Although Chara looks superficially like some land plants, a major difference is that the stem has no
supportive tissue. However, the Charales exhibit a number of traits that are significant for adaptation to land life.
They produce the compounds lignin and sporopollenin, and form plasmodesmata that connect the cytoplasm of
adjacent cells. Although the life cycle of the Charales is haplontic (the main form is haploid, and diploid zygotes
are formed but have a brief existence), the egg, and later, the zygote, form in a protected chamber on the haploid
parent plant.
Figure 25.8 Chara. The representative alga, Chara, is a noxious weed in Florida, where it clogs waterways, (credit:
South Florida Information Access, U.S. Geological Survey)
The Coleochaetes are branched or disclike multicellular forms. They can produce both sexually and asexually,
but the life cycle is basically haplontic. Recent extensive DNA sequence analysis of charophytes indicates that
the Zygnematales are more closely related to the embryophytes than the Charales or the Coleochaetales. The
Zygnematales include the familiar genus Spirogyra, as well as the desmids. As techniques in DNA analysis
improve and new information on comparative genomics arises, the phylogenetic connections between the
charophytes and the land plants will continued to be examined to produce a satisfactory solution to the mystery
of the origin of land plants.
25.3 | Bryophytes
By the end of this section, you will be able to do the following:
• Identify the main characteristics of bryophytes
• Describe the distinguishing traits of liverworts, hornworts, and mosses
• Chart the development of land adaptations in the bryophytes
• Describe the events in the bryophyte lifecycle
Bryophytes are the closest extant relatives of early terrestrial plants. The first bryophytes (liverworts) most likely
appeared in the Ordovician period, about 450 million years ago. Because they lack lignin and other resistant
structures, the likelihood of bryophytes forming fossils is rather small. Some spores protected by sporopollenin
have survived and are attributed to early bryophytes. By the Silurian period (435 MYA), however, vascular plants
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had spread through the continents. This compelling fact is used as evidence that non-vascular plants must have
preceded the Silurian period.
More than 25,000 species of bryophytes thrive in mostly damp habitats, although some live in deserts. They
constitute the major flora of inhospitable environments like the tundra, where their small size and tolerance
to desiccation offer distinct advantages. They generally lack lignin and do not have actual tracheids (xylem
cells specialized for water conduction). Rather, water and nutrients circulate inside specialized conducting cells.
Although the term non-tracheophyte is more accurate, bryophytes are commonly called non-vascular plants.
In a bryophyte, all the conspicuous vegetative organs—including the photosynthetic leaf-like structures, the
thallus (“plant body”), stem, and the rhizoid that anchors the plant to its substrate—belong to the haploid
organism or gametophyte. The male gametes formed by bryophytes swim with a flagellum, so fertilization
is dependent on the presence of water. The bryophyte embryo also remains attached to the parent plant,
which protects and nourishes it. The sporophyte that develops from the embryo is barely noticeable. The
sporangium —the multicellular sexual reproductive structure in which meiosis produces haploid spores—is
present in bryophytes and absent in the majority of algae. This is also a characteristic of land plants.
The bryophytes are divided into three phyla: the liverworts or Hepaticophyta, the hornworts or Anthocerotophyta,
and the mosses or true Bryophyta.
Liverworts
Liverworts (Hepaticophyta) are currently classified as the plants most closely related to the ancestor of vascular
plants that adapted to terrestrial environments. In fact, liverworts have colonized every terrestrial habitat on Earth
and diversified to more than 7000 existing species (Figure 25.9). Lobate liverworts form a flat thallus, with lobes
that have a vague resemblance to the lobes of the liver (Figure 25.10), which accounts for the name given to
the phylum. Leafy liverworts have tiny leaflike structures attached to a stalk. Several leafy liverworts are shown
in Figure 25.9.
Figure 25.9 Liverworts. This 1904 drawing shows the variety of forms of Hepaticophyta.
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Chapter 25 | Seedless Plants
Figure 25.10 Liverwort gametophyte. A liverwort, Lunularia cruciata, displays its lobate, flat thallus. The organism in
the photograph is in the gametophyte stage, but has not yet produced gametangia. Lunularia gametophytes produce
crescent-shaped gemmae (circled), which contain asexual spores. The tiny white dots on the surface of the thallus are
air pores.
Openings in the thallus that allow the movement of gases may be observed in liverworts (Figure 25.10).
However, these are not stomata, because they do not actively open and close by the action of guard cells.
Instead, the thallus takes up water over its entire surface and has no cuticle to prevent desiccation, which
explains their preferred wet habitats. Figure 25.11 represents the lifecycle of a lobate liverwort. Haploid spores
germinate into flattened thalli attached to the substrate by thin, single-celled filaments. Stalk-like structures
(i gametophores ) grow from the thallus and carry male and female gametangia, which may develop on separate,
individual plants, or on the same plant, depending on the species. Flagellated male gametes develop within
antheridia (male gametangia). The female gametes develop within archegonia (female gametangia). Once
released, the male gametes swim with the aid of their flagella to an archegonium, and fertilization ensues.
The zygote grows into a small sporophyte still contained in the archegonium. The diploid zygote will give rise,
by meiosis, to the next generation of haploid spores, which can be disseminated by wind or water. In many
liverworts, spore dispersal is facilitated by elaters —long single cells that suddenly change shape as they dry
out and throw adjacent spores out of the spore capsule. Liverwort plants can also reproduce asexually, by the
breaking of “branches” or the spreading of leaf fragments called gemmae. In this latter type of reproduction, the
gemmae —small, intact, complete pieces of plant that are produced in a cup on the surface of the thallus (shown
in Figure 25.11 and Figure 25.12)— are splashed out of the cup by raindrops. The gemmae then land nearby
and develop into gametophytes.
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Archegonium
Mature sporophyte
Egg(ln)/J (2n) j'
Seta
/ Spores T-/— \
V3
* ^Sperm(ln) ■
(in)
Rhizoids
The sperm swims into
the archegonium and
fertilizes the egg.
The embryo grows into "
a slender stalk called a seta.
Meiosis produces spores.
The spore grow
into a thallus with
rhizoids.
Archegonia
Antheridia
Antheridial head
(in)
Rhizoids
Archegonial head
(in)
Gemma Cup
(Asexual
reproduction)
Female
Gametophyte
Male
Gametophyte
Liverwort Lite Cycle
Figure 25.11 Reproductive cycle of liverworts. The life cycle of a typical lobate liverwort is shown. This image shows a
liverwort in which antheridia and archegonia are produced on separate gametophytes. (credit: modification of work by
Mariana Ruiz Villareal)
Hornworts
The defining characteristic of the hornworts ( Anthocerotophyta ) is the narrow, pipe-like sporophyte. Hornworts
have colonized a variety of habitats on land, although they are never far from a source of moisture. The short,
blue-green gametophyte is the dominant phase of the life cycle of a hornwort. The sporophytes emerge from the
parent gametophyte and continue to grow throughout the life of the plant (Figure 25.12).
Figure 25.12 Hornwort sporophytes. Hornworts grow a tall and slender sporophyte. (credit: modification of work by
Jason Hollinger)
Stomata (air pores that can be opened and closed) appear in the hornworts and are abundant on the sporophyte.
Photosynthetic cells in the thallus each contain a single chloroplast. Meristem cells at the base of the plant
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Chapter 25 | Seedless Plants
keep dividing and adding to the height of the sporophyte. This growth pattern is unique to the hornworts. Many
hornworts establish symbiotic relationships with cyanobacteria that fix nitrogen from the environment.
The lifecycle of hornworts (Figure 25.13) follows the general pattern of alternation of generations. The
gametophytes grow as flat thalli on the soil with embedded male and female gametangia. Flagellated sperm
swim to the archegonia and fertilize eggs. The zygote develops into a long and slender sporophyte that
eventually splits open down the side, releasing spores. Thin branched cells called pseudoelaters surround the
spores and help propel them farther in the environment. The haploid spores germinate and give rise to the next
generation of gametophytes.
k Pseudoelater
•Stoma
Antheridium
Archegonium
Sperm swim into the
archegonium and fertilize
the egg, producing a 2 n
embryo.
'Sporophyte (2n)
Rhizoids
Thallus*
Gametophyte (
plant (In)
Protonema (In)
Within the
sporophyte
meiosis
produces
In spores.
Each spore
grows into
a In
gametophyte
Meristem
^Foot
The embryo
develops into
a slender
sporophyte.
Figure 25.13 Reproductive cycle of hornworts. The alternation of generation in hornworts is shown, (credit:
modification of work by “Smith609”/Wikimedia Commons based on original work by Mariana Ruiz Villareal)
Mosses
The mosses are the most numerous of the non-vascular plants. More than 10,000 species of mosses have been
catalogued. Their habitats vary from the tundra, where they are the main vegetation, to the understory of tropical
forests. In the tundra, the mosses’ shallow rhizoids allow them to fasten to a substrate without penetrating the
frozen soil. Mosses slow down erosion, store moisture and soil nutrients, and provide shelter for small animals
as well as food for larger herbivores, such as the musk ox. Mosses are very sensitive to air pollution and are
used to monitor air quality. They are also sensitive to copper salts, so these salts are a common ingredient of
compounds marketed to eliminate mosses from lawns.
Mosses form diminutive gametophytes, which are the dominant phase of the lifecycle. Green, flat structures with
a simple midrib—resembling true leaves, but lacking stomata and vascular tissue—are attached in a spiral to a
central stalk. Mosses have stomata only on the sporophyte. Water and nutrients are absorbed directly through
the leaflike structures of the gametophyte. Some mosses have small branches. A primitive conductive system
that carries water and nutrients runs up the gametophyte's stalk, but does not extend into the leaves. Additionally,
mosses are anchored to the substrate—whether it is soil, rock, or roof tiles—by multicellular rhizoids, precursors
of roots. They originate from the base of the gametophyte, but are not the major route for the absorption of water
and minerals. The lack of a true root system explains why it is so easy to rip moss mats from a tree trunk. The
mosses therefore occupy a threshold position between other bryophytes and the vascular plants.
The moss lifecycle follows the pattern of alternation of generations as shown in Figure 25.14. The most
familiar structure is the haploid gametophyte, which germinates from a haploid spore and forms first a
protonema —usually, a tangle of single-celled filaments that hug the ground. Cells akin to an apical meristem
actively divide and give rise to a gametophore, consisting of a photosynthetic stem and foliage-like structures.
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Male and female gametangia develop at the tip of separate gametophores. The antheridia (male organs)
produce many sperm, whereas the archegonia (the female organs) each form a single egg at the base (venter)
of a flask-shaped structure. The archegonium produces attractant substances and at fertilization, the sperm
swims down the neck to the venter and unites with the egg inside the archegonium. The zygote, protected by
the archegonium, divides and grows into a sporophyte, still attached by its foot to the gametophyte.
visual
CONNECTION
Life Cycle of a Typical Moss
Fertilization
Mature Gametophytes
In
Non-vascular “leaves^
'Jon-vascular “Sternt
' Rhizoids- "Vy-.
Male Female
In
Calyptra
Operculum
Capsule^ _
Meiospores
Seta r -^(spores)
rv
Spore case Rhizoid
Meiosis
Figure 25.14 Reproductive cycle of mosses. This illustration shows the life cycle of mosses, (credit: modification
of work by Mariana Ruiz Villareal)
Which of the following statements about the moss life cycle is false?
a. The mature gametophyte is haploid.
b. The sporophyte produces haploid spores.
c. The calyptra buds to form a mature gametophyte.
d. The zygote is housed in the venter.
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Chapter 25 | Seedless Plants
The moss sporophyte is dependent on the gametophyte for nutrients. The slender seta (plural, setae), as seen
in Figure 25.15, contains tubular cells that transfer nutrients from the base of the sporophyte (the foot) to the
sporangium or capsule.
Figure 25.15 Moss sporophyte. This photograph shows the long slender stems, called setae, connected to capsules
of the moss Thamnobryum alopecurum. The operculum and remnants of the calyptra are visible in some capsules,
(credit: modification of work by Hermann Schachner)
Spore mother cells in the sporangium undergo meiosis to produce haploid spores. The sporophyte has several
features that protect the developing spores and aid in their dispersal. The calyptra, derived from the walls of
the archegonium, covers the sporangium. A structure called the operculum is at the tip of the spore capsule.
The calyptra and operculum fall off when the spores are ready for dispersal. The peristome, tissue around the
mouth of the capsule, is made of triangular, close-fitting units like little “teeth.” The peristome opens and closes,
depending on moisture levels, and periodically releases spores.
25.4 | Seedless Vascular Plants
By the end of this section, you will be able to do the following:
• Identify the new traits that first appear in seedless tracheophytes
• Discuss how each trait is important for adaptation to life on land
• Identify the classes of seedless tracheophytes
• Describe the life cycle of a fern
• Explain the role of seedless plants in the ecosystem
The vascular plants, or tracheophytes, are the dominant and most conspicuous group of land plants. More than
260,000 species of tracheophytes represent more than 90 percent of Earth’s vegetation. Several evolutionary
innovations explain their success and their ability to spread to all habitats.
Bryophytes may have been successful at the transition from an aquatic habitat to land, but they are still
dependent on water for reproduction, and must absorb moisture and nutrients through the gametophyte surface.
The lack of roots for absorbing water and minerals from the soil, as well as a lack of lignin-reinforced conducting
cells, limit bryophytes to small sizes. Although they may survive in reasonably dry conditions, they cannot
reproduce and expand their habitat range in the absence of water. Vascular plants, on the other hand, can
achieve enormous heights, thus competing successfully for light. Photosynthetic organs become leaves, and
pipe-like cells or vascular tissues transport water, minerals, and fixed carbon organic compounds throughout the
organism.
Throughout plant evolution, there is a progressive increase in the dominance of the sporophyte generation. In
seedless vascular plants, the diploid sporophyte is the dominant phase of the life cycle. The gametophyte is now
less conspicuous, but still independent of the sporophyte. Seedless vascular plants still depend on water during
fertilization, as the flagellated sperm must swim on a layer of moisture to reach the egg. This step in reproduction
explains why ferns and their relatives are more abundant in damp environments.
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Vascular Tissue: Xylem and Phloem
The first plant fossils that show the presence of vascular tissue date to the Silurian period, about 430 million
years ago. The simplest arrangement of conductive cells shows a pattern of xylem at the center surrounded
by phloem. Xylem is the tissue responsible for the storage and long-distance transport of water and nutrients,
as well as the transfer of water-soluble growth factors from the organs of synthesis to the target organs. The
tissue consists of conducting cells, known as tracheids, and supportive filler tissue, called parenchyma. Xylem
conductive cells incorporate the compound lignin into their walls, and are thus described as lignified. Lignin itself
is a complex polymer: It is impermeable to water and confers mechanical strength on vascular tissue. With their
rigid cell walls, the xylem cells provide support to the plant and allow it to achieve impressive heights. Tall plants
have a selective advantage by being able to reach unfiltered sunlight and disperse their spores or seeds away
from the parent plant, thus expanding the species’ range. By growing higher than other plants, tall trees cast
their shadows on shorter plants and thereby outcompete them for water and precious nutrients in the soil.
Phloem is the second type of vascular tissue; it transports sugars, proteins, and other solutes throughout the
plant. Phloem cells are divided into sieve elements (conducting cells) and cells that support the sieve elements.
Together, xylem and phloem tissues form the vascular system of plants (Figure 25.16).
Figure 25.16 Vascular bundles in celery. This cross section of a celery stalk shows a number of vascular bundles.
The xylem is on the inner part of each bundle, (credit: fir0002 | flagstaffotos.com.au [GFDL 1.2 (http://www.gnu.org/
licenses/old-licenses/fdl-1.2.html)], via Wikimedia Commons. Image modified from source.)
Roots: Support for the Plant
Roots are not well-preserved in the fossil record. Nevertheless, it seems that roots appeared later in evolution
than vascular tissue. The development of an extensive network of roots represented a significant new feature
of vascular plants. Thin rhizoids attached bryophytes to the substrate, but these rather flimsy filaments did not
provide a strong anchor for the plant; nor did they absorb substantial amounts of water and nutrients. In contrast,
roots, with their prominent vascular tissue system, transfer water and minerals from the soil to the rest of the
plant. The extensive network of roots that penetrates deep into the soil to reach sources of water also stabilizes
plants by acting as a ballast or anchor. The majority of roots establish a symbiotic relationship with fungi, forming
mutualistic mycorrhizae, which benefit the plant by greatly increasing the surface area for absorption of water,
soil minerals, and nutrients.
Leaves, Sporophylls, and Strobili
A third innovation marks the seedless vascular plants. Accompanying the prominence of the sporophyte and the
development of vascular tissue, the appearance of true leaves improved their photosynthetic efficiency. Leaves
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capture more sunlight with their increased surface area by employing more chloroplasts to trap light energy and
convert it to chemical energy, which is then used to fix atmospheric carbon dioxide into carbohydrates. The
carbohydrates are exported to the rest of the plant by the conductive cells of phloem tissue.
The existence of two types of leaf morphology— microphylls and megaphylls —suggests that leaves evolved
independently in several groups of plants. Microphylls ("little leaves") are small and have a simple vascular
system. The first microphylls in the fossil record can be dated to 350 million years ago in the late Silurian. A
single unbranched vein —a bundle of vascular tissue made of xylem and phloem—runs through the center of
the leaf. Microphylls may have originated from the flattening of lateral branches, or from sporangia that lost their
reproductive capabilities. Microphylls are seen in club mosses. Microphylls probably preceded the development
of megaphylls ("big leaves"), which are larger leaves with a pattern of multiple veins. Megaphylls most likely
appeared independently several times during the course of evolution. Their complex networks of veins suggest
that several branches may have combined into a flattened organ, with the gaps between the branches being
filled with photosynthetic tissue. Megaphylls are seen in ferns and more derived vascular plants.
In addition to photosynthesis, leaves play another role in the life of the plants. Pine cones, mature fronds of
ferns, and flowers are all sporophylls —leaves that were modified structurally to bear sporangia. Strobili are
cone-like structures that contain sporangia. They are prominent in conifers, where they are commonly known as
pine cones.
Ferns and Other Seedless Vascular Plants
By the late Devonian period, plants had evolved vascular tissue, well-defined leaves, and root systems. With
these advantages, plants increased in height and size. During the Carboniferous period (360 to 300 MYA),
swamp forests of club mosses and horsetails—some specimens reaching heights of more than 30 m (100
ft)—covered most of the land. These forests gave rise to the extensive coal deposits that gave the Carboniferous
its name. In seedless vascular plants, the sporophyte became the dominant phase of the life cycle.
Water is still required as a medium of sperm transport during the fertilization of seedless vascular plants, and
most favor a moist environment. Modern-day seedless tracheophytes include club mosses, horsetails, ferns, and
whisk ferns.
Phylum Lycopodiophyta: Club Mosses
The club mosses, or phylum Lycopodiophyta, are the earliest group of seedless vascular plants. They
dominated the landscape of the Carboniferous, growing into tall trees and forming large swamp forests.
Today’s club mosses are diminutive, evergreen plants consisting of a stem (which may be branched) and
microphylls (Figure 25.17). The phylum Lycopodiophyta consists of close to 1,200 species, including the
quillworts ( Isoetales ), the club mosses ( Lycopodiales ), and spike mosses ( Selaginellales ), none of which are true
mosses or bryophytes.
Lycophytes follow the pattern of alternation of generations seen in the bryophytes, except that the sporophyte
is the major stage of the life cycle. Some lycophytes, like the club moss Lycopodium, produce gametophytes
that are independent of the sporophyte, developing underground or in other locations where they can form
mycorrhizal associations with fungi. In many club mosses, the sporophyte gives rise to sporophylls arranged in
strobili, cone-like structures that give the class its name. Sporangia develop within the chamber formed by each
sporophyll.
Lycophytes can be homosporous (spores of the same size) or heterosporous (spores of different sizes). The
spike moss Selaginella is a heterosporous lycophyte. The same strobilus will contain microsporangia, which
produce spores that will develop into the male gametophyte, and megasporangia, which produce spores that will
develop into the female gametophyte. Both gametophytes develop within the protective strobilus.
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Chapter 25 | Seedless Plants
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Figure 25.17 Lycopodium. In the club mosses such as Lycopodium clavatum, sporangia are arranged in clusters
called strobili. The generic name means "wolf-foot" from the resemblance of the branched sporophyte to a paw. The
specific epithet clavatum refers to the club-shaped strobilus, and reflects the common name of the phylum, (credit:
Cory Zanker)
Phylum Monilophyta: Class Equisetopsida (Horsetails)
Horsetails, whisk ferns, and ferns belong to the phylum Monilophyta, with horsetails placed in the class
Equisetopsida. The single genus Equisetum is the survivor of a large group of plants, known as Arthrophyta,
which produced large trees and entire swamp forests in the Carboniferous. The plants are usually found in damp
environments and marshes (Figure 25.18).
Figure 25.18 Horsetails. Horsetails, named for the brushy appearance of the sporophyte, thrive in a marsh, (credit:
Myriam Feldman)
The stem of a horsetail is characterized by the presence of joints or nodes, hence the name Arthrophyta (arthro-
= "joint"; -phyta = "plant"). Leaves and branches come out as whorls from the evenly spaced joints. The needle-
shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem
(Figure 25.19).
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Chapter 25 | Seedless Plants
Figure 25.19 The jointed stem of a horsetail. Thin leaves originating at the joints are noticeable on the horsetail plant.
Because silica deposited in the cell walls made these plants abrasive, horsetails were once used as scrubbing brushes
and were nicknamed scouring rushes, (credit: Myriam Feldman)
Silica collected by in the epidermal cells contributes to the stiffness of horsetail plants, but underground stems
known as rhizomes anchor the plants to the ground. Modern-day horsetails are homosporous. The spores are
attached to elaters—as we have seen, these are coiled threads that spring open in dry weather and casts
the spores to a location distant from the parent plants. The spores then germinate to produce small bisexual
gametophytes.
Phylum Monilophyta: Class Psilotopsida (Whisk Ferns)
While most ferns form large leaves and branching roots, the whisk ferns, class Psilotopsida, lack both roots and
leaves, probably lost by reduction. Photosynthesis takes place in their green stems, which branch dichotomously.
Small yellow knobs form at the tip of a branch or at branch nodes and contain the sporangia (Figure 25.20).
Spores develop into gametophytes that are only a few millimeters across, but which produce both male and
female gametangia. Whisk ferns were considered early pterophytes. However, recent comparative DNA analysis
suggests that this group may have lost both vascular tissue and roots through evolution, and is more closely
related to ferns.
Figure 25.20 Psiiotum. The whisk fern Psitotum nudum has conspicuous green stems with knob-shaped sporangia,
(credit: Forest & Kim Starr)
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Chapter 25 | Seedless Plants
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Phylum Monilophyta: Class Polypodiopsida (True Ferns)
With their large fronds, the true ferns are perhaps the most readily recognizable seedless vascular plants. They
are also considered to be the most advanced seedless vascular plants and display characteristics commonly
observed in seed plants. More than 20,000 species of ferns live in environments ranging from the tropics
to temperate forests. Although some species survive in dry environments, most ferns are restricted to moist,
shaded places. Ferns made their appearance in the fossil record during the Devonian period (420 MYA) and
expanded during the Carboniferous (360 to 300 MYA).
The dominant stage of the life cycle of a fern is the sporophyte, which consists of large compound leaves called
fronds. Fronds may be either finely divided or broadly lobed. Fronds fulfill a double role; they are photosynthetic
organs that also carry reproductive organs. The stem may be buried underground as a rhizome, from which
adventitious roots grow to absorb water and nutrients from the soil; or, they may grow above ground as a trunk
in tree ferns (Figure 25.21). Adventitious organs are those that grow in unusual places, such as roots growing
from the side of a stem.
Figure 25.21 A tree fern. Some specimens of this short tree-fern species can grow very tall, (credit: Adrian Pingstone)
The tip of a developing fern frond is rolled into a crazier, or fiddlehead (Figure 25.22). Fiddleheads unroll as the
frond develops.
(a) (b)
Figure 25.22 Fern fiddleheads. Croziers, or fiddleheads, are the tips of fern fronds, (credit a: modification of work by
Cory Zanker; credit b: modification of work by Myriam Feldman)
On the underside of each mature fern frond are groups of sporangia called sori (Figure 25.23a). Most ferns
are homosporous. Spores are produced by meiosis and are released into the air from the sporangium. Those
that land on a suitable substrate germinate and form a heart-shaped gametophyte, or prothallus, which is
attached to the ground by thin filamentous rhizoids (Figure 25.23b). Gametophytes produce both antheridia and
archegonia. Like the sperm cells of other pterophytes, fern sperm have multiple flagella and must swim to the
archegonium, which releases a chemoattractant to guide them. The zygote develops into a fern sporophyte,
which emerges from the archegonium of the gametophyte. Maturation of antheridia and archegonia at different
times encourages cross-fertilization. The full life cycle of a fern is depicted in Figure 25.24.
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Chapter 25 ] Seedless Plants
(a) (b)
Figure 25.23 Fern reproductive stages. Sori (a) appear as small bumps on the underside of a fern frond, (credit:
Myriam Feldman), (b) Fern gametophyte and young sporophyte. The sporophyte and gametophyte are labeled, (credit:
modification of work by "Vlmastra'VWikimedia Commons)
visual
CONNECTION
MEIOSIS
jr
Spores
Sporongia
Germination
Gametophyte
Diploid 2/i
Sporophyte
FERTILIZATION
Archegonium
^99 / Sporophyte
Antheridium
Mitosis
Zygote
Mitosis
Haploid In
Figure 25.24 Reproductive cycle of a fern. This life cycle of a fern shows alternation of generations with a
dominant sporophyte stage, (credit "fern": modification of work by Cory Zanker; credit "gametophyte": modification
of work by "Vlmastra'VWikimedia Commons)
Which of the following statements about the fern life cycle is false?
a. Sporangia produce haploid spores.
b. The sporophyte grows from a gametophyte.
c. The sporophyte is diploid and the gametophyte is haploid.
d. Sporangia form on the underside of the gametophyte.
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Chapter 25 | Seedless Plants
723
LINK
T &
LEARNING
To see an animation of the life cycle of a
(http:// 0 penstaxc 0 llege. 0 rg/l/fern_life_cycle) .
fern and to test your knowledge, go to the website
ca eer connection
Landscape Designer
Looking at the ornamental arrangement of flower beds and fountains typical of the grounds of royal castles
and historic houses of Europe, it’s clear that the gardens’ creators knew about more than art and design.
They were also familiar with the biology of the plants they chose. Landscape design also has strong roots
in the United States’ tradition. A prime example of early American classical design is Monticello, Thomas
Jefferson’s private estate. Among his many interests, Jefferson maintained a strong passion for botany.
Landscape layout can encompass a small private space like a backyard garden, public gathering places
such as Central Park in New York City, or an entire city plan like Pierre L’Enfant’s design for Washington,
DC.
A landscape designer will plan traditional public spaces—such as botanical gardens, parks, college
campuses, gardens, and larger developments—as well as natural areas and private gardens. The
restoration of natural places encroached on by human intervention, such as wetlands, also requires the
expertise of a landscape designer.
With such an array of necessary skills, a landscape designer’s education should include a solid background
in botany, soil science, plant pathology, entomology, and horticulture. Coursework in architecture and design
software is also required for the completion of the degree. The successful design of a landscape rests on an
extensive knowledge of plant growth requirements such as light and shade, moisture levels, compatibility of
different species, and susceptibility to pathogens and pests. Mosses and ferns will thrive in a shaded area,
where fountains provide moisture; cacti, on the other hand, would not fare well in that environment. The
future growth of individual plants must be taken into account, to avoid crowding and competition for light
and nutrients. The appearance of the space overtime is also of concern. Shapes, colors, and biology must
be balanced for a well-maintained and sustainable green space. Art, architecture, and biology blend in a
beautifully designed and implemented landscape (Figure 25.25).
Figure 25.25 This landscaped border at a college campus was designed by students in the horticulture and
landscaping department of the college, (credit: Myriam Feldman)
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Chapter 25 | Seedless Plants
The Importance of Seedless Plants
Mosses and liverworts are often the first macroscopic organisms to colonize an area, both in a primary
succession—where bare land is settled for the first time by living organisms, or in a secondary
succession—where soil remains intact after a catastrophic event wipes out many existing species. Their spores
are carried by the wind, birds, or insects. Once mosses and liverworts are established, they provide food and
shelter for other plant species. In a hostile environment, like the tundra where the soil is frozen, bryophytes grow
well because they do not have roots and can dry and rehydrate quickly once water is again available. Mosses
are at the base of the food chain in the tundra biome. Many species—from small herbivorous insects to musk
oxen and reindeer—depend on mosses for food. In turn, predators feed on the herbivores, which are the primary
consumers. Some reports indicate that bryophytes make the soil more amenable to colonization by other plants.
Because they establish symbiotic relationships with nitrogen-fixing cyanobacteria, mosses replenish the soil with
nitrogen.
By the end of the nineteenth century, scientists had observed that lichens and mosses were becoming
increasingly rare in urban and suburban areas. Because bryophytes have neither a root system for absorption
of water and nutrients, nor a cuticular layer that protects them from desiccation, pollutants in rainwater readily
penetrate their tissues as they absorb moisture and nutrients through their entire exposed surfaces. Therefore,
pollutants dissolved in rainwater penetrate plant tissues readily and have a larger impact on mosses than on
other plants. The disappearance of mosses can be considered a biological indicator for the level of pollution in
the environment.
Ferns contribute to the environment by promoting the weathering of rock, accelerating the formation of topsoil,
and slowing down erosion as rhizomes spread throughout the soil. The water ferns of the genus Azolla harbor
nitrogen-fixing cyanobacteria and restore this important nutrient to aquatic habitats.
Seedless plants have historically played a role in human life with uses as tools, fuel, and medicine. For example,
dried peat moss, Sphagnum, is commonly used as fuel in some parts of Europe and is considered a renewable
resource. Sphagnum bogs (Figure 25.26) are cultivated with cranberry and blueberry bushes. In addition, the
ability of Sphagnum to hold moisture makes the moss a common soil conditioner. Even florists use blocks of
Sphagnum to maintain moisture for floral arrangements!
Figure 25.26 Sphagnum moss. Sphagnum acutifolium is dried peat moss and can be used as fuel, (credit: Ken
Goulding)
The attractive fronds of ferns make them a favorite ornamental plant. Because they thrive in low light, they are
well suited as house plants. More importantly, fiddleheads of bracken fern ( Pteridium aquilinum) are a traditional
spring food of Native Americans, and are popular as aside dish in French cuisine. The licorice fern, Polypodium
glycyrrhiza, is part of the diet of the Pacific Northwest coastal tribes, owing in part to the sweetness of its
rhizomes. It has a faint licorice taste and serves as a sweetener. The rhizome also figures in the pharmacopeia
of Native Americans for its medicinal properties and is used as a remedy for sore throat.
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Chapter 25 | Seedless Plants
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LINK TQ LEARNING
Go to this website (http:// 0 penstaxc 0 llege. 0 rg/l/fiddleheads) to learn how to identify fern species based
upon their fiddleheads.
By far the greatest impact of seedless vascular plants on human life, however, comes from their extinct
progenitors. The tall club mosses, horsetails, and tree-like ferns that flourished in the swampy forests of the
Carboniferous period gave rise to large deposits of coal throughout the world. Coal provided an abundant source
of energy during the Industrial Revolution, which had tremendous consequences on human societies, including
rapid technological progress and growth of large cities, as well as the degradation of the environment. Coal is
still a prime source of energy and also a major contributor to global warming.
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Chapter 25 | Seedless Plants
KEY TERMS
adventitious describes an organ that grows in an unusual place, such as a roots growing from the side of a
stem
antheridium male gametangium
archegonium female gametangium
capsule case of the sporangium in mosses
charophyte other term for green algae; considered the closest relative of land plants
club mosses earliest group of seedless vascular plants
diplontic diploid stage is the dominant stage
embryophyte other name for land plant; embryo is protected and nourished by the sporophyte
extant still-living species
extinct no-longer-existing species
fern seedless vascular plant that produces large fronds; the most advanced group of seedless vascular plants
gametangium structure on the gametophyte in which gametes are produced
gemma (plural, gemmae) leaf fragment that spreads for asexual reproduction
haplodiplodontic haploid and diploid stages alternate
haplontic haploid stage is the dominant stage
heterosporous produces two types of spores
homosporous produces one type of spore
hornworts group of non-vascular plants in which stomata appear
horsetail seedless vascular plant characterized by joints
lignin complex polymer impermeable to water
liverworts most primitive group of the non-vascular plants
lycophyte club moss
megaphyll larger leaves with a pattern of branching veins
megaspore female spore
microphyll small size and simple vascular system with a single unbranched vein
microspore male spore
mosses group of bryophytes in which a primitive conductive system appears
non-vascular plant plant that lacks vascular tissue, which is formed of specialized cells for the transport of
water and nutrients
peat moss Sphagnum
peristome tissue that surrounds the opening of the capsule and allows periodic release of spores
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Chapter 25 | Seedless Plants
727
phloem tissue responsible for transport of sugars, proteins, and other solutes
protonema tangle of single-celled filaments that forms from the haploid spore
rhizoids thin filaments that anchor the plant to the substrate
seedless vascular plant plant that does not produce seeds
seta stalk that supports the capsule in mosses
sporocyte diploid cell that produces spores by meiosis
sporophyll leaf modified structurally to bear sporangia
sporopollenin tough polymer surrounding the spore
streptophytes group that includes green algae and land plants
strobili cone-like structures that contain the sporangia
tracheophyte vascular plant
vascular plant plant containing a network of cells that conducts water and solutes through the organism
vein bundle of vascular tissue made of xylem and phloem
whisk fern seedless vascular plant that lost roots and leaves by reduction
xylem tissue responsible for long-distance transport of water and nutrients
CHAPTER SUMMARY
25.1 Early Plant Life
Land plants acquired traits that made it possible to colonize land and survive out of the water. All land plants
share the following characteristics: alternation of generations, with the haploid plant called a gametophyte, and
the diploid plant called a sporophyte; formation of haploid spores in a sporangium; formation of gametes in a
gametangium; protection of the embryo; and an apical meristem. Vascular tissues, roots, leaves, cuticle cover,
and a tough outer layer that protects the spores contributed to the adaptation of plants to dry land. Land plants
appeared about 500 million years ago in the Ordovician period.
25.2 Green Algae: Precursors of Land Plants
Charophytes share more traits with land plants than do other algae, according to structural features and DNA
analysis. Within the charophytes, the Charales, the Coleochaetales, and the Zygnematales have been each
considered as sharing the closest common ancestry with the land plants. Charophytes form sporopollenin and
precursors of lignin, phragmoplasts, and have flagellated sperm. They do not exhibit alternation of generations.
25.3 Bryophytes
Seedless non-vascular plants are small, having the gametophyte as the dominant stage of the lifecycle.
Without a vascular system and roots, they absorb water and nutrients on all their exposed surfaces.
Collectively known as bryophytes, the three main groups include the liverworts, the hornworts, and the mosses.
Liverworts are the most primitive plants and are closely related to the first land plants. Hornworts developed
stomata and possess a single chloroplast per cell. Mosses have simple conductive cells and are attached to
the substrate by rhizoids. They colonize harsh habitats and can regain moisture after drying out. The moss
sporangium is a complex structure that allows release of spores away from the parent plant.
25.4 Seedless Vascular Plants
The seedless vascular plants show several features important to living on land: vascular tissue, roots, and
leaves. Vascular systems consist of xylem tissue, which transports water and minerals, and phloem tissue,
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Chapter 25 | Seedless Plants
which transports sugars and proteins. With the development of the vascular system, leaves appeared to act as
large photosynthetic organs, and roots to access water from the ground. Small uncomplicated leaves are
termed microphylls. Large leaves with vein patterns are termed megaphylls. Modified leaves that bear
sporangia are called sporophylls. Some sporophylls are arranged in cone structures called strobili.
The support and conductive properties of vascular tissues have allowed the sporophyte generation of vascular
plants to become increasingly dominant. The seedless vascular plants include club mosses, which are the most
primitive; whisk ferns, which lost leaves and roots by reductive evolution; and horsetails and ferns. Ferns are
the most advanced group of seedless vascular plants. They are distinguished by large leaves called fronds and
small sporangia-containing structures called sori, which are found on the underside of the fronds.
Both mosses and ferns play an essential role in the balance of the ecosystems. Mosses are pioneering species
that colonize bare or devastated environments and make it possible for succession to occur. They contribute to
the enrichment of the soil and provide shelter and nutrients for animals in hostile environments. Mosses are
important biological indicators of environmental pollution. Ferns are important for providing natural habitats, as
soil stabilizers, and as decorative plants. Both mosses and ferns are part of traditional medical practice. In
addition to culinary, medical, and decorative purposes, mosses and ferns can be used as fuels, and ancient
seedless plants were important contributors to the fossil fuel deposits that we now use as an energy resource.
VISUAL CONNECTION QUESTIONS
1. Figure 25.6 Which of the following statements
about plant divisions is false?
a. Lycophytes and pterophytes are seedless
vascular plants.
b. All vascular plants produce seeds.
c. All non-vascular embryophytes are
bryophytes.
d. Seed plants include angiosperms and
gymnosperms.
2. Figure 25.14 Which of the following statements
about the moss life cycle is false?
REVIEW QUESTIONS
4. The land plants are probably descendants of which
of these groups?
a. green algae
b. red algae
c. brown algae
d. angiosperms
5. Alternation of generations means that plants
produce:
a. only haploid multicellular organisms
b. only diploid multicellular organisms
c. only diploid multicellular organisms with
single-celled haploid gametes
d. both haploid and diploid multicellular
organisms
6. Which of the following traits of land plants allows
them to grow in height?
a. alternation of generations
b. waxy cuticle
c. tracheids
d. sporopollenin
7. How does a haplontic plant population maintain
a. The mature gametophyte is haploid.
b. The sporophyte produces haploid spores.
c. The calyptra buds to form a mature
gametophyte.
d. The zygote is housed in the venter.
3. Figure 25.24 Which of the following statements
about the fern life cycle is false?
a. Sporangia produce haploid spores.
b. The sporophyte grows from a gametophyte.
c. The sporophyte is diploid and the
gametophyte is haploid.
d. Sporangia form on the underside of the
gametophyte.
genetic diversity?
a. Zygotes are produced by random fusion.
b. Gametes are created through meiosis.
c. Diploid spores undergo independent
assortment during mitosis.
d. The zygote undergoes meiosis to generate
a haploid sporophyte.
8. What characteristic of Charales would enable them
to survive a dry spell?
a. sperm with flagella
b. phragmoplasts
c. sporopollenin
d. chlorophyll a
9. Which one of these characteristics is present in
land plants and not in Charales?
a. alternation of generations
b. flagellated sperm
c. phragmoplasts
d. plasmodesmata
10. A scientist sequences the genome of Chara, red
algae, and a tomato plant. What result would support
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Chapter 25 | Seedless Plants
729
the conclusion that Charophytes should be included
in the Plantae kingdom?
a. The Chara genome is more similar to the
red algae than the tomato plant.
b. All three genomes are distinctly different.
c. The Chara genome is more similar to the
tomato plant genome than the red algae
genome.
d. The tomato plant genome is distinct from
the red algae genome.
11. Which of the following features does not support
the inclusion of Charophytes in the Plantae kingdom?
a. Charophyte chloroplasts contain chlorophyll
a and b.
b. Charophyte plant cell walls contain
plasmodesmata to allow transfer between
cells within multicellular organisms.
c. Charophytes do not exhibit growth
throughout the entire plant body.
d. Charophytes are multicellular organisms
that lack vascular tissue.
12. Which of the following structures is not found in
bryophytes?
a. a cellulose cell wall
b. chloroplast
c. sporangium
d. root
13. Stomata appear in which group of plants?
a. Charales
b. liverworts
c. hornworts
d. mosses
14. The chromosome complement in a moss
protonema is:
a. In
b. 2 n
c. 3 n
d. varies with the size of the protonema
15. Why do mosses grow well in the Arctic tundra?
a. They grow better at cold temperatures.
b. They do not require moisture.
c. They do not have true roots and can grow
on hard surfaces.
d. There are no herbivores in the tundra.
16. A botanist travels to an area that has experienced
a long, severe drought. While examining the
bryophytes in the area, he notices that many are in
the same life-cycle stage. Which life-cycle stage
should be the most common?
CRITICAL THINKING QUESTIONS
24. Why did land plants lose some of the accessory
a. zygote
b. gametophyte
c. sporophyte
d. archegonium
17. Microphylls are characteristic of which types of
plants?
a. mosses
b. liverworts
c. club mosses
d. ferns
18. A plant in the understory of a forest displays a
segmented stem and slender leaves arranged in a
whorl. It is probably a_.
a. club moss
b. whisk fern
c. fern
d. horsetail
19. The following structures are found on the
underside of fern leaves and contain sporangia:
a. sori
b. rhizomes
c. megaphylls
d. microphylls
20. The dominant organism in fern is the_.
a. sperm
b. spore
c. gamete
d. sporophyte
21. What seedless plant is a renewable source of
energy?
a. club moss
b. horsetail
c. sphagnum moss
d. fern
22. How do mosses contribute to returning nitrogen
to the soil?
a. Mosses fix nitrogen from the air.
b. Mosses harbor cyanobacteria that fix
nitrogen.
c. Mosses die and return nitrogen to the soil.
d. Mosses decompose rocks and release
nitrogen.
23. The production of megaphylls by many different
species of plants is an example of_.
a. parallel evolution
b. analogy
c. divergent evolution
d. homology
pigments present in brown and red algae?
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Chapter 25 | Seedless Plants
25. What is the difference between extant and
extinct?
26. Describe at least two challenges that cactuses
had to overcome that cattails did not.
27. Describe a minimum of two ways that plants
changed the land environment to support the
emergence of land animals.
28. To an alga, what is the main advantage of
producing drought-resistant structures?
29. in areas where it rains often, mosses grow on
roofs. How do mosses survive on roofs without soil?
30. What are the three classes of bryophytes?
31. Describe two adaptations that are present in
mosses, but not hornworts or liverworts, which reflect
steps of evolution toward land plants.
32. Bryophytes form a monophyletic group that
transitions between green algae and vascular plants.
Describe at least one similarity and one difference
between bryophyte reproduction and green algae
reproduction.
33. How did the development of a vascular system
contribute to the increase in size of plants?
34. Which plant is considered the most advanced
seedless vascular plant and why?
35. Ferns are simultaneously involved in promoting
rock weathering, while preventing soil erosion.
Explain how a single plant can perform both these
functions, and how these functions are beneficial to
its ecosystem.
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26 | SEED PLANTS
(a) (b) (c) (d)
Figure 26.1 Seed plants dominate the landscape and play an enormous and integral role in the success of all human
societies. Here are a few examples: (a) Palm trees grow along the shoreline, serving numerous purposes for food,
shelter, and even transportation; (b) wheat is an important crop grown throughout most of the world; (c) the fruit of the
cotton plant produces fibers that are woven into fabric; (d) the potent alkaloids of the beautiful opium poppy have long
influenced human life both as a medicinal remedy and as a dangerously addictive drug, (credit a: modification of work
by Ryan Kozie; credit b: modification of work by Stephen Ausmus; credit c: modification of work by David Nance; credit
d: modification of work by Jolly Janner)
Chapter Outline
26.1: Evolution of Seed Plants
26.2: Gymnosperms
26.3: Angiosperms
26.4: The Role of Seed Plants
Introduction
The lush palms on tropical shorelines do not depend on water for the dispersal of their pollen, fertilization, or
the survival of the zygote—unlike mosses, liverworts, and ferns living within the same terrain. These palms are
seed plants, which have broken free from the need to rely on water for their reproductive needs. The seed plants
play an integral role in all aspects of life on the planet, shaping the physical terrain, influencing the climate, and
maintaining life as we know it. For millennia, human societies have depended on seed plants for nutrition and
medicinal compounds. Somewhat more recently, seed plants have served as a source of manufactured products
such as timber and paper, dyes, and textiles. As an example, multiple uses have been found for each of the
plants shown above. Palms provide materials including rattans, oils, and dates. Grains like wheat are grown to
feed both human and animal populations or fermented to produce alcoholic beverages. The fruit of the cotton
flower is harvested as a boll, with its fibers transformed into clothing or pulp for paper. The showy opium poppy
is valued both as an ornamental flower and as a source of potent opiate compounds.
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26.1 1 Evolution of Seed Plants
By the end of this section, you will be able to do the following:
• Describe the two major innovations that allowed seed plants to reproduce in the absence of water
• Explain when seed plants first appeared and when gymnosperms became the dominant plant group
• Discuss the purpose of pollen grains and seeds
• Describe the significance of angiosperms bearing both flowers and fruit
The first plants to colonize land were most likely related to the ancestors of modern day mosses (bryophytes),
which are thought to have appeared about 500 million years ago. They were followed by liverworts (also
bryophytes) and primitive vascular plants—the pterophytes—from which modern ferns are descended. The life
cycle of bryophytes and pterophytes is characterized by the alternation of generations, which is also exhibited in
the gymnosperms and angiosperms. However, what sets bryophytes and pterophytes apart from gymnosperms
and angiosperms is their reproductive requirement for water. The completion of the bryophyte and pterophyte
life cycle requires water because the male gametophyte releases flagellated sperm, which must swim to reach
and fertilize the female gamete or egg. After fertilization, the zygote undergoes cellular division and grows into a
diploid sporophyte, which in turn will form sporangia or "spore vessels." In the sporangia, mother cells undergo
meiosis and produce the haploid spores. Release of spores in a suitable environment will lead to germination
and a new generation of gametophytes.
In seed plants, the evolutionary trend led to a dominant sporophyte generation accompanied by a corresponding
reduction in the size of the gametophyte from a conspicuous structure to a microscopic cluster of cells enclosed
in the tissues of the sporophyte. Whereas lower vascular plants, such as club mosses and ferns, are mostly
homosporous (producing only one type of spore), all seed plants, or spermatophytes, are heterosporous,
producing two types of spores: megaspores (female) and microspores (male). Megaspores develop into female
gametophytes that produce eggs, and microspores mature into male gametophytes that generate sperm.
Because the gametophytes mature within the spores, they are not free-living, as are the gametophytes of other
seedless vascular plants.
Ancestral heterosporous seedless plants, represented by modern-day plants such as the spike moss
Selaginella, are seen as the evolutionary forerunners of seed plants. In the life cycle of Selaginella, both
male and female sporangia develop within the same stem-like strobilus. In each male sporangium, multiple
microspores are produced by meiosis. Each microspore produces a small antheridium contained within a spore
case. As it develops it is released from the strobilus, and a number of flagellated sperm are produced that
then leave the spore case. In the female sporangium, a single megaspore mother cell undergoes meiosis to
produce four megaspores. Gametophytes develop within each megaspore, consisting of a mass of tissue that
will later nourish the embryo and a few archegonia. The female gametophyte may remain within remnants of
the spore wall in the megasporangium until after fertilization has occurred and the embryo begins to develop.
This combination of an embryo and nutritional cells is a little different from the organization of a seed, since the
nutritive endosperm in a seed is formed from a single cell rather than multiple cells.
Both seeds and pollen distinguish seed plants from seedless vascular plants. These innovative structures
allowed seed plants to reduce or eliminate their dependence on water for gamete fertilization and development of
the embryo, and to conquer dry land. Pollen grains are male gametophytes, which contain the sperm (gametes)
of the plant. The small haploid (In) cells are encased in a protective coat that prevents desiccation (drying
out) and mechanical damage. Pollen grains can travel far from their original sporophyte, spreading the plant’s
genes. Seeds offer the embryo protection, nourishment, and a mechanism to maintain dormancy for tens or even
thousands of years, ensuring that germination can occur when growth conditions are optimal. Seeds therefore
allow plants to disperse the next generation through both space and time. With such evolutionary advantages,
seed plants have become the most successful and familiar group of plants.
Both adaptations expanded the colonization of land begun by the bryophytes and their ancestors. Fossils
place the earliest distinct seed plants at about 350 million years ago. The first reliable record of gymnosperms
dates their appearance to the Pennsylvanian period, about 319 million years ago (Figure 26.2). Gymnosperms
were preceded by progymnosperms, the first naked seed plants, which arose about 380 million years ago.
Progymnosperms were a transitional group of plants that superficially resembled conifers (cone bearers)
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because they produced wood from the secondary growth of the vascular tissues; however, they still reproduced
like ferns, releasing spores into the environment. At least some species were heterosporous. Progymnosperms,
like the extinct Archaeopteris (not to be confused with the ancient bird Archaeopteryx), dominated the forests
of the late Devonian period. However, by the early (Triassic, c. 240 MYA) and middle (Jurassic, c. 205 MYA)
Mesozoic era, the landscape was dominated by the true gymnosperms. Angiosperms surpassed gymnosperms
by the middle of the Cretaceous (c. 100 MYA) in the late Mesozoic era, and today are the most abundant and
biologically diverse plant group in most terrestrial biomes.
Flowering Plants
Gymnosperms
Pterophytes
Lycophytes
Liverworts and Mosses
Figure 26.2 Plant timeline. Various plant species evolved in different eras, (credit: United States Geological Survey)
Figure modified from source.
Evolution of Gymnosperms
The fossil plant Elkinsia polymorpha, a "seed fern" from the Devonian period—about 400 million years ago—is
considered the earliest seed plant known to date. Seed ferns (Figure 26.3) produced their seeds along their
branches, in structures called cupules that enclosed and protected the ovule—the female gametophyte and
associated tissues—which develops into a seed upon fertilization. Seed plants resembling modern tree ferns
became more numerous and diverse in the coal swamps of the Carboniferous period.
Figure 26.3 Seed fern leaf. This fossilized leaf is from Glossopteris, a seed fern that thrived during the Permian age
(290-240 million years ago), (credit: D.L. Schmidt, USGS)
Fossil records indicate the first gymnosperms (progymnosperms) most likely originated in the Paleozoic era,
during the middle Devonian period: about 390 million years ago. The previous Mississippian and Pennsylvanian
periods, were wet and dominated by giant fern trees. But the following Permian period was dry, which gave
a reproductive edge to seed plants, which are better adapted to survive dry spells. The Ginkgoales, a group
EON
ERA
PERIOD
Millions o!
YEARS AGO
Cenozoic
Quaternary
— 1.6 --
— 66 —
— 138 —
— 205 —
-- 240 --
-- 290 —
— 330 --
— 360 —
---410 —
— 435 -
— 500 --
— 570-
— - 2500 - -
3800? -
Tertiary
Cretaceous
Mesozoic
Jurassic
Triassic
Phanerozoic
Permian
Pennsylvanian
Mississippian
Paleozoic
Devonian
Silurian
Ordotician
Cambrian
Proterozoic
Lute Proterozoic
Middle Proterozoic
FaH\ Prolenooic
Archean
Laic Arvlicun
Middle Arc bean
Farit Arcltcan
Pre-Archean
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Chapter 26 | Seed Plants
of gymnosperms with only one surviving species—the Ginkgo biloba —were the first gymnosperms to appear
during the lower Jurassic. Gymnosperms expanded in the Mesozoic era (about 240 million years ago),
supplanting ferns in the landscape, and reaching their greatest diversity during this time. The Jurassic period
was as much the age of the cycads (palm-tree-like gymnosperms) as the age of the dinosaurs. Ginkgoales and
the more familiar conifers also dotted the landscape. Although angiosperms (flowering plants) are the major form
of plant life in most biomes, gymnosperms still dominate some ecosystems, such as the taiga (boreal forests)
and the alpine forests at higher mountain elevations (Figure 26.4) because of their adaptation to cold and dry
growth conditions.
Figure 26.4 Conifers. This boreal forest (taiga) has low-lying plants and conifer trees, (credit: L.B. Brubaker, NOAA)
Seeds and Pollen as an Evolutionary Adaptation to Dry Land
Bryophyte and fern spores are haploid cells dependent on moisture for rapid development of multicellular
gametophytes. In the seed plants, the female gametophyte consists of just a few cells: the egg and some
supportive cells, including the endosperm-producing cell that will support the growth of the embryo. After
fertilization of the egg, the diploid zygote produces an embryo that will grow into the sporophyte when the seed
germinates. Storage tissue to sustain growth of the embryo and a protective coat give seeds their superior
evolutionary advantage. Several layers of hardened tissue prevent desiccation, and free the embryo from the
need for a constant supply of water. Furthermore, seeds remain in a state of dormancy—induced by desiccation
and the hormone abscisic acid —until conditions for growth become favorable. Whether blown by the wind,
floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus
avoiding competition with the parent plant.
Pollen grains (Figure 26.5) are male gametophytes containing just a few cells and are distributed by wind, water,
or an animal pollinator. The whole structure is protected from desiccation and can reach the female organs
without depending on water. After reaching a female gametophyte, the pollen grain grows a tube that will deliver
a male nucleus to the egg cell. The sperm of modern gymnosperms and all angiosperms lack flagella, but in
cycads, Ginkgo, and other primitive gymnosperms, the sperm are still motile, and use flagella to swim to the
female gamete; however, they are delivered to the female gametophyte enclosed in a pollen grain. The pollen
grows or is taken into a fertilization chamber, where the motile sperm are released and swim a short distance to
an egg.
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Chapter 26 | Seed Plants
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Figure 26.5 Pollen fossils. This fossilized pollen is from a Buckbean fen core found in Yellowstone National Park,
Wyoming. The pollen is magnified 1,054 times, (credit: R.G. Baker, USGS; scale-bar data from Matt Russell)
Evolution of Angiosperms
The roughly 200 million years between the appearance of the gymnosperms and the flowering plants gives us
some appreciation for the evolutionary experimentation that ultimately produced flowers and fruit. Angiosperms
(“seed in a vessel") produce a flower containing male and/or female reproductive structures. Fossil evidence
(Figure 26.6) indicates that flowering plants first appeared about 125 million years ago in the Lower Cretaceous
(late in the Mesozoic era), and were rapidly diversifying by about 100 million years ago in the Middle Cretaceous.
Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been
attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm
leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The
same geological period is also marked by the appearance of many modern groups of insects, suggesting that
pollinating insects played a key role in the evolution of flowering plants.
New data in comparative genomics and paleobotany (the study of ancient plants) have shed some light on
the evolution of angiosperms. Although the angiosperms appeared after the gymnosperms, they are probably
not derived from gymnosperm ancestors. Instead, the angiosperms form a sister clade (a species and its
descendents) that developed in parallel with the gymnosperms. The two innovative structures of flowers and
fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic
variability and range. There is no current consensus on the origin of the angiosperms. Paleobotanists debate
whether angiosperms evolved from small woody bushes, or were related to the ancestors of tropical grasses.
Both views draw support from cladistics, and the so-called woody magnoliid hypothesis —which proposes
that the early ancestors of angiosperms were shrubs like modern magnolia—also offers molecular biological
evidence.
The most primitive living angiosperm is considered to be Amborella trichopoda, a small plant native to the
rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. trichopoda has shown
that it is related to all existing flowering plants and belongs to the oldest confirmed branch of the angiosperm
family tree. The nuclear genome shows evidence of an ancient whole-genome duplication. The mitochondrial
genome is large and multichromosomal, containing elements from the mitochondrial genomes of several other
species, including algae and a moss. A few other angiosperm groups, called basal angiosperms, are viewed as
having ancestral traits because they branched off early from the phylogenetic tree. Most modern angiosperms
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are classified as either monocots or eudicots, based on the structure of their leaves and embryos. Basal
angiosperms, such as water lilies, are considered more ancestral in nature because they share morphological
traits with both monocots and eudicots.
Figure 26.6 Ficus imprint. This leaf imprint shows a Ficus speciosissima, an angiosperm that flourished during the
Cretaceous period, (credit: W. T. Lee, USGS)
Flowers and Fruits as an Evolutionary Adaptation
Angiosperms produce their gametes in separate organs, which are usually housed in a flower. Both fertilization
and embryo development take place inside an anatomical structure that provides a stable system of sexual
reproduction largely sheltered from environmental fluctuations. With about 300,000 species, flowering plants are
the most diverse phylum on Earth after insects, which number about 1,200,000 species. Flowers come in a
bewildering array of sizes, shapes, colors, smells, and arrangements. Most flowers have a mutualistic pollinator,
with the distinctive features of flowers reflecting the nature of the pollination agent. The relationship between
pollinator and flower characteristics is one of the great examples of coevolution.
Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken,
developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Not
all fruits develop completely from an ovary; such “false fruits" or pseudocarps, develop from tissues adjacent
to the ovary. Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, green
peppers, corn, and avocados are all examples of fruits. Along with pollen and seeds, fruits also act as agents
of dispersal. Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the
seeds through their digestive systems, then deposit the seeds in another location. Cockleburs are covered with
stiff, hooked spines that can hook into fur (or clothing) and hitch a ride on an animal for long distances. The
cockleburs that clung to the velvet trousers of an enterprising Swiss hiker, George de Mestral, inspired his
invention of the loop and hook fastener he named Velcro.
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V /
e olution CONNECTION
Building Phylogenetic Trees with Analysis of DNA Sequence
Alignments
All living organisms display patterns of relationships derived from their evolutionary history. Phylogeny is the
science that describes the relative connections between organisms, in terms of ancestral and descendant
species. Phylogenetic trees, such as the plant evolutionary history shown in Figure 26.7, are tree-like
branching diagrams that depict these relationships. Species are found at the tips of the branches. Each
branching point, called a node, is the point at which a single taxonomic group (taxon), such as a species,
separates into two or more species.
Figure 26.7 Plant phylogeny. This phylogenetic tree shows the evolutionary relationships of plants.
Phylogenetic trees have been built to describe the relationships between species since the first sketch of
a tree that appeared in Darwin's Origin of Species. Traditional methods involve comparison of homologous
anatomical structures and embryonic development, assuming that closely related organisms share
anatomical features that emerge during embryo development. Some traits that disappear in the adult are
present in the embryo; for example, an early human embryo has a postanal tail, as do all members of
the Phylum Chordata. The study of fossil records shows the intermediate stages that link an ancestral
form to its descendants. However, many of the approaches to classification based on the fossil record
alone are imprecise and lend themselves to multiple interpretations. As the tools of molecular biology
and computational analysis have been developed and perfected in recent years, a new generation of
tree-building methods has taken shape. The key assumption is that genes for essential proteins or RNA
structures, such as the ribosomal RNAs, are inherently conserved because mutations (changes in the DNA
sequence) could possibly compromise the survival of the organism. DNA from minute samples of living
organisms or fossils can be amplified by polymerase chain reaction (PCR) and sequenced, targeting the
regions of the genome that are most likely to be conserved between species. The genes encoding the 18S
ribosomal RNA from the small subunit and plastid genes are frequently chosen for DNA alignment analysis.
Once the sequences of interest are obtained, they are compared with existing sequences in databases
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such as GenBank, which is maintained by The National Center for Biotechnology Information. A number
of computational tools are available to align and analyze sequences. Sophisticated computer analysis
programs determine the percentage of sequence identity or homology. Sequence homology can be used
to estimate the evolutionary distance between two DNA sequences and reflect the time elapsed since
the genes separated from a common ancestor. Molecular analysis has revolutionized phylogenetic trees.
In some cases, prior results from morphological studies have been confirmed: for example, confirming
Amborella trichopoda as the most primitive angiosperm known. However, some groups and relationships
have been rearranged as a result of DNA analysis.
26.2 | Gymnosperms
By the end of this section, you will be able to do the following:
• Discuss the type of seeds produced by gymnosperms, as well as other characteristics of gymnosperms
• Identify the geological era dominated by the gymnosperms and describe the conditions to which they
were adapted
• List the four groups of modern-day gymnosperms and provide examples of each
• Describe the life cycle of a typical gymnosperm
Gymnosperms, meaning “naked seeds,” are a diverse group of seed plants. According to the "anthophyte"
hypothesis, the angiosperms are a sister group of one group of gymnosperms (the Gnetales), which makes
the gymnosperms a paraphyletic group. Paraphyletic groups are those in which not all descendants of a single
common ancestor are included in the group. However, the "netifer" hypothesis suggests that the gnetophytes
are sister to the conifers, making the gymnosperms monophyletic and sister to the angiosperms. Further
molecular and anatomical studies may clarify these relationships. Characteristics of the gymnosperms include
naked seeds, separate female and male gametes, pollination by wind, and tracheids (which transport water and
solutes in the vascular system).
Gymnosperm seeds are not enclosed in an ovary; rather, they are only partially sheltered by modified leaves
called sporophylls. You may recall the term strobilus (plural = strobili) describes a tight arrangement of
sporophylls around a central stalk, as seen in pine cones. Some seeds are enveloped by sporophyte tissues
upon maturation. The layer of sporophyte tissue that surrounds the megasporangium, and later, the embryo, is
called the integument.
Gymnosperms were the dominant phylum in the Mesozoic era. They are adapted to live where fresh water
is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, they are still the prominent
phylum in the coniferous biome or taiga, where the evergreen conifers have a selective advantage in cold and
dry weather. Evergreen conifers continue low levels of photosynthesis during the cold months, and are ready to
take advantage of the first sunny days of spring. One disadvantage is that conifers are more susceptible than
deciduous trees to leaf infestations because most conifers do not lose their leaves all at once. They cannot,
therefore, shed parasites and restart with a fresh supply of leaves in spring.
The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte in which
reduced male and female gametophytes reside. All gymnosperms are heterosporous. The male and female
reproductive organs can form in cones or strobili. Male and female sporangia are produced either on the same
plant, described as monoecious (“one home” or bisexual), or on separate plants, referred to as dioecious
(“two homes" or unisexual) plants. The life cycle of a conifer will serve as our example of reproduction in
gymnosperms.
Life Cycle of a Conifer
Pine trees are conifers (coniferous = cone bearing) and carry both male and female sporophylls on the same
mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous
and generate two different types of spores (male microspores and female megaspores). Male and female
spores develop in different strobili, with small male cones and larger female cones. In the male cones, or
staminate cones, the microsporocytes undergo meiosis and the resultant haploid microspores give rise to male
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gametophytes or “pollen grains” by mitosis. Each pollen grain consists of just a few haploid cells enclosed in a
tough wall reinforced with sporopollenin. In the spring, large amounts of yellow pollen are released and carried
by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube
growth. The pollen tube develops slowly, and the generative cell in the pollen grain produces two haploid sperm
or generative nuclei by mitosis. At fertilization, one of the haploid sperm nuclei will unite with the haploid nucleus
of an egg cell.
Female cones, or ovulate cones, contain two ovules per scale. Each ovule has a narrow passage that
opens near the base of the sporophyll. This passage is the micropyle, through which a pollen tube will later
grow. One megaspore mother cell, or megasporocyte, undergoes meiosis in each ovule. Three of the four
cells break down; only a single surviving cell will develop into a female multicellular gametophyte, which
encloses archegonia (an archegonium is a reproductive organ that contains a single large egg). As the female
gametophyte begins to develop, a sticky pollination drop traps windblown pollen grains near the opening of the
micropyle. A pollen tube is formed and grows toward the developing gametophyte. One of the generative or
sperm nuclei from the pollen tube will enter the egg and fuse with the egg nucleus as the egg matures. Upon
fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent
plant. Although several eggs may be formed and even fertilized, there is usually a single surviving embryo in
each ovule. Fertilization and seed development is a long process in pine trees: it may take up to two years after
pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the
sporophyte tissue, the gametophyte tissue that will provide nutrients, and the embryo itself.
Figure 26.8 illustrates the life cycle of a conifer. The sporophyte (2n) phase is the longest phase in the life of
a gymnosperm. The gametophytes (In)—produced by microspores and megaspores—are reduced in size. It
may take more than a year between pollination and fertilization while the pollen tube grows towards the growing
female gametophyte (In), which develops from a single megaspore. The slow growth of the pollen tube allows
the female gametophyte time to produce eggs (In).
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visual
CONNECTION
Sporophyte (2 n)
(mature tree)
Female cones grow in the
upper branches where they
may be fertilized by pollen
blown on the wind
from the male cones.
Male cones grow in the
lower branches.
o
Seeds are dispersed
and grow into
mature trees.
Pollen Pollen Ovule
grain \ tube
Scale
Generative (sperm)
nuclei (In)
Megaspore
(in)
A pollen tube forms, allowing the pollen to
migrate toward the female gametophyte.
Upon fertilization, a diploid zygote forms.
Figure 26.8 Conifer life cycle. This image shows the life cycle of a conifer. Pollen from male cones blows up into
upper branches, where it fertilizes female cones. The megaspore shown in the image develops into the female
gametophyte as the pollen tube slowly grows toward it, eventually fusing with the egg and delivering a male
nucleus, which combines with the female nucleus of the mature egg.
At what stage does the diploid zygote form?
a. when the female cone begins to bud from the tree
b. at fertilization
c. when the seeds drop from the tree
d. when the pollen tube begins to grow
LINK
T &
LEARNING
Watch this video to see the process of seed production in gymnosperms. (This multimedia resource will
open in a browser.) (http://cnx.Org/content/m66573/l.3/#eip-idl089843)
Diversity of Gymnosperms
Modern gymnosperms are classified into four phyla. Coniferophyta, Cycadophyta, and Ginkgophyta are similar
in their pattern of seed development and also in their production of secondary cambium (cells that generate the
vascular system of the trunk or stem and are partially specialized for water transportation). However, the three
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Chapter 26 | Seed Plants
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phyla are not closely related phylogenetically to each other. Gnetophyta are considered the closest group to
angiosperms because they produce true xylem tissue, with vessels as well as the tracheids found in the rest of
the gymnosperms. It is possible that vessel elements arose independently in the two groups
Conifers (Coniferophyta)
Conifers are the dominant phylum of gymnosperms, with the greatest variety of species (Figure 26.9). Typical
conifers are tall trees that bear scale-like or needle-like leaves. Water evaporation from leaves is reduced by their
narrow shape and a thick cuticle. Snow easily slides off needle-shaped leaves, keeping the snow load light, thus
reducing broken branches. Such adaptations to cold and dry weather explain the predominance of conifers at
high altitudes and in cold climates. Conifers include familiar evergreen trees such as pines, spruces, firs, cedars,
sequoias, and yews. A few species are deciduous and lose their leaves in fall. The bald cypress, dawn redwood,
European larch and the tamarack (Figure 26.9c) are examples of deciduous conifers. Many coniferous trees
are harvested for paper pulp and timber. The wood of conifers is more primitive than the wood of angiosperms;
it contains tracheids, but no vessel elements, and is therefore referred to as “softwood.”
Figure 26.9 Conifers. Conifers are the dominant form of vegetation in cold or arid environments and at high altitudes.
Shown here are the (a) evergreen spruce Picea sp., (b) juniper Juniperus sp., (c) coastal redwood or sequoia Sequoia
sempervirens, and (d) the tamarack Larix larcinia. Notice the deciduous yellow leaves of the tamarack, (credit a:
modification of work by Rosendahl; credit b: modification of work by Alan Levine; credit c: modification of work by
Wendy McCormic; credit d: modification of work by Micky Zlimen)
Cycads
Cycads thrive in mild climates, and are often mistaken for palms because of the shape of their large, compound
leaves. Cycads bear large strobili or cones (Figure 26.10), and may be pollinated by beetles rather than wind,
which is unusual for a gymnosperm. Large cycads dominated the landscape during the age of dinosaurs in the
Mesozoic, but only a hundred or so smaller species persisted to modern times. They face possible extinction,
and several species are protected through international conventions. Because of their attractive shape, they are
often used as ornamental plants in gardens in the tropics and subtropics.
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Figure 26.10 Cycad. This cycad, Encephalartos ferox, has large cones and broad, fern-like leaves, (credit: Wendy
Cutler)
Ginkgophytes
The single surviving species of the ginkgophytes group is Ginkgo biloba (Figure 26.11). Its fan-shaped
leaves—unique among seed plants because they feature a dichotomous venation pattern—turn yellow in
autumn and fall from the tree. For centuries, G. biloba was cultivated by Chinese Buddhist monks in monasteries,
which ensured its preservation. It is planted in public spaces because it is unusually resistant to pollution. Male
and female organs are produced on separate plants. Typically, gardeners plant only male trees because the
seeds produced by the female plant have an off-putting smell of rancid butter.
Figure 26.11 Ginkgo. This plate from the 1870 book Flora Japonica, Sectio Prima (Tafelband) depicts the leaves and
fruit of Ginkgo biloba, as drawn by Philipp Franz von Siebold and Joseph Gerhard Zuccarini.
Gnetophytes
The phylogenetic position of the gnetophytes is not currently resolved. Their possession of vessel elements
suggests they are the closest relative to modern angiosperms. However, molecular analysis places them closer
to the conifers. The three living genera are quite dissimilar: Ephedra, Gnetum, and Welwitschia (Figure 26.12),
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which may indicate that the group is not monophyletic. Like angiosperms, they have broad leaves. Ephedra
(Figure 26.12a) occurs in dry areas of the West Coast of the United States and Mexico. Ephedra’s small, scale¬
like leaves are the source of the compound ephedrine , which is used in medicine as a potent decongestant.
Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use
is restricted to prescription drugs. Gnetum species (Figure 26.12b) are found in some parts of Africa, South
America, and Southeast Asia, and include trees, shrubs and vines. Welwitschia (Figure 26.12c) is found
in the Namib desert, and is possibly the oddest member of the group. It produces only two leaves, which
grow continuously throughout the life of the plant (some plants are hundreds of years old). Like the ginkgos,
Welwitschia produces male and female gametes on separate plants.
(a) Ephedra (b) Gnetum (c) Welwitschia
Figure 26.12 (a) Ephedra viridis, known by the common name Mormon tea, grows on the West Coast of the United
States and Mexico, (b) Gnetum gnemon grows in Malaysia, (c) The large Welwitschia mirabilis can be found in the
Namibian desert, (credit a: modification of work by USDA; credit b: modification of work by Malcolm Manners; credit c:
modification of work by Derek Keats)
LINK TQ LEARNING
Watch this BBC video describing the amazing strangeness of Welwitschia. (This multimedia
resource will open in a browser.) (http://cnx.Org/content/m66573/l.3/#eip-id2212644)
26.3 | Angiosperms
By the end of this section, you will be able to do the following:
• Explain why angiosperms are the dominant form of plant life in most terrestrial ecosystems
• Describe the main parts of a flower and their functions
• Detail the life cycle of a typical gymnosperm and angiosperm
• Discuss the similarities and differences between the two main groups of flowering plants
From their humble and still obscure beginning during the early Jurassic period, the angiosperms—or flowering
plants—have evolved to dominate most terrestrial ecosystems (Figure 26.13). With more than 300,000 species,
the angiosperm phylum (Anthophyta) is second only to insects in terms of diversification.
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Figure 26.13 Flowers. These flowers grow in a botanical garden border in Bellevue, WA. Flowering plants dominate
terrestrial landscapes. The vivid colors of flowers and enticing fragrance of flowers are adaptations to pollination by
animals like insects, birds, and bats, (credit: Myriam Feldman)
The success of angiosperms is due to two novel reproductive structures: flowers and fruits. The function of
the flower is to ensure pollination, often by arthropods, as well as to protect a developing embryo. The colors
and patterns on flowers offer specific signals to many pollinating insects or birds and bats that have coevolved
with them. For example, some patterns are visible only in the ultraviolet range of light, which can be seen by
arthropod pollinators. For some pollinators, flowers advertise themselves as a reliable source of nectar. Flower
scent also helps to select its pollinators. Sweet scents tend to attract bees and butterflies and moths, but some
flies and beetles might prefer scents that signal fermentation or putrefaction. Flowers also provide protection for
the ovule and developing embryo inside a receptacle. The function of the fruit is seed protection and dispersal.
Different fruit structures or tissues on fruit—such as sweet flesh, wings, parachutes, or spines that grab—reflect
the dispersal strategies that help spread seeds.
Flowers
Flowers are modified leaves, or sporophylls, organized around a central receptacle. Although they vary greatly in
appearance, virtually all flowers contain the same structures: sepals, petals, carpels, and stamens. The peduncle
typically attaches the flower to the plant proper. A whorl of sepals (collectively called the calyx) is located at
the base of the peduncle and encloses the unopened floral bud. Sepals are usually photosynthetic organs,
although there are some exceptions. For example, the corolla in lilies and tulips consists of three sepals and
three petals that look virtually identical. Petals, collectively the corolla, are located inside the whorl of sepals
and may display vivid colors to attract pollinators. Sepals and petals together form the perianth. The sexual
organs, the female gynoecium and male androecium are located at the center of the flower. Typically, the sepals,
petals, and stamens are attached to the receptacle at the base of the gynoecium, but the gynoecium may also
be located deeper in the receptacle, with the other floral structures attached above it.
As illustrated in Figure 26.14, the innermost part of a perfect flower is the gynoecium, the location in the flower
where the eggs will form. The female reproductive unit consists of one or more carpels, each of which has a
stigma, style, and ovary. The stigma is the location where the pollen is deposited either by wind or a pollinating
arthropod. The sticky surface of the stigma traps pollen grains, and the style is a connecting structure through
which the pollen tube will grow to reach the ovary. The ovary houses one or more ovules, each of which will
ultimately develop into a seed. Flower structure is very diverse, and carpels may be singular, multiple, or fused.
(Multiple fused carpels comprise a pistil.) The androecium, or male reproductive region is composed of multiple
stamens surrounding the central carpel. Stamens are composed of a thin stalk called a filament and a sac-like
structure called the anther. The filament supports the anther, where the microspores are produced by meiosis
and develop into haploid pollen grains, or male gametophytes.
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Androecium
Figure 26.14 Flower structure. This image depicts the structure of a perfect flower. Perfect flowers produce both male
and female floral organs. The flower shown has only one carpel, but some flowers have a cluster of carpels. Together,
all the carpels make up the gynoecium. (credit: modification of work by Mariana Ruiz Villareal)
The Life Cycle of an Angiosperm
The adult or sporophyte phase is the main phase of an angiosperm’s life cycle (Figure 26.15). Like
gymnosperms, angiosperms are heterosporous. Therefore, they produce microspores, which will generate
pollen grains as the male gametophytes, and megaspores, which will form an ovule that contains female
gametophytes. Inside the anther’s microsporangia, male sporocytes divide by meiosis to generate haploid
microspores, which, in turn, undergo mitosis and give rise to pollen grains. Each pollen grain contains two cells:
one generative cell that will divide into two sperm and a second cell that will become the pollen tube cell.
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visual
CONNECTION
Micropyle
Microsporangium
Anther
Microgametophyte
(pollen)
Pollination and Fertilization
Figure 26.15 Angiosperm life cycle. The life cycle of an angiosperm is shown. Anthers and carpels are structures
that shelter the actual gametophytes: the pollen grain and embryo sac. Double fertilization is a process unique to
angiosperms. (credit: modification of work by Mariana Ruiz Villareal)
Question: if a flower lacked a megasporangium, what type of gamete would not form? If the flower lacked a
microsporangium, what type of gamete would not form?
The ovule, sheltered within the ovary of the carpel, contains the megasporangium protected by two layers of
integuments and the ovary wall. Within each megasporangium, a diploid megasporocyte undergoes meiosis,
generating four haploid megaspores—three small and one large. Only the large megaspore survives; it divides
mitotically three times to produce eight nuclei distributed among the seven cells of the female gametophyte or
embryo sac. Three of these cells are located at each pole of the embryo sac. The three cells at one pole become
the egg and two synergids. The three cells at the opposite pole become antipodal cells. The center cell contains
the remaining two nuclei (polar nuclei). This cell will eventually produce the endosperm of the seed. The mature
embryo sac then contains one egg cell, two synergids or “helper” cells, three antipodal cells (which eventually
degenerate), and a central cell with two polar nuclei. When a pollen grain reaches the stigma, a pollen tube
extends from the grain, grows down the style, and enters through the micropyle: an opening in the integuments
of the ovule. The two sperm are deposited in the embryo sac.
A double fertilization event then occurs. One sperm and the egg combine, forming a diploid zygote —the
future embryo. The other sperm fuses with the polar nuclei, forming a triploid cell that will develop into the
endosperm— the tissue that serves as a food reserve for the developing embryo. The zygote develops into an
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embryo with a radicle, or small root, and one (monocot) or two (dicot) leaf-like organs called cotyledons. This
difference in the number of embryonic leaves is the basis for the two major groups of angiosperms: the monocots
and the eudicots. Seed food reserves are stored outside the embryo, in the form of complex carbohydrates,
lipids, or proteins. The cotyledons serve as conduits to transmit the broken-down food reserves from their
storage site inside the seed to the developing embryo. The seed consists of a toughened layer of integuments
forming the coat, the endosperm with food reserves, and at the center, the well-protected embryo.
Most angiosperms have perfect flowers, which means that each flower carries both stamens and carpels
(Figure 26.15). In monoecious plants, male (staminate) and female (pistillate) flowers are separate, but carried
on the same plant. Sweetgums (Liquidambar spp.) and beeches ( Betula spp. are monoecious (Figure 26.16).
In dioecious plants, male and female flowers are found on separate plants. Willows ( Salix spp.) and poplars
(Populus spp.) are dioecious. In spite of the predominance of perfect flowers, only a few species of angiosperms
self-pollinate. Both anatomical and environmental barriers promote cross-pollination mediated by a physical
agent (wind or water), or an animal, such as an insect or bird. Cross-pollination increases genetic diversity in a
species.
Figure 26.16 Beech inflorescences. The female inflorescence is at the upper left. The male inflorescence is at
the lower right, (credit: Stephen J. Baskauf, 2002. http://bioimages.vanderbilt.edu/baskauf/10593
(http:// 0 penstax. 0 rg/l/betula) . Morphbank :: Biological Imaging (http://www.morphbank.net/ (http://openstax.org/
l/morphbank) , 29 June 2017). Florida State University, Department of Scientific Computing, Tallahassee, FL
32306-4026 USA)
Fruit
As the seed develops, the walls of the ovary thicken and form the fruit. The seed forms in an ovary, which also
enlarges as the seeds grow. Many foods commonly called vegetables are actually fruits. Eggplants, zucchini,
string beans, tomatoes, and bell peppers are all technically fruits because they contain seeds and are derived
from the thick ovary tissue. Acorns are true nuts, and winged maple “helicopter seeds” or whirligigs (whose
botanical name is samara ) are also fruits. Botanists classify fruit into more than two dozen different categories,
only a few of which are actually fleshy and sweet.
Mature fruit can be fleshy or dry. Fleshy fruit include the familiar berries, peaches, apples, grapes, and tomatoes.
Rice, wheat, and nuts are examples of dry fruit. Another subtle distinction is that not all fruits are derived from
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just the ovary. For instance, strawberries are derived from the ovary as well as the receptacle, and apples are
formed from the ovary and the pericarp, or hypanthium. Some fruits are derived from separate ovaries in a single
flower, such as the raspberry. Other fruits, such as the pineapple, form from clusters of flowers. Additionally,
some fruits, like watermelon and orange, have rinds. Regardless of how they are formed, fruits are an agent of
seed dispersal. The variety of shapes and characteristics reflect the mode of dispersal. Wind carries the light
dry fruits of trees and dandelions. Water transports floating coconuts. Some fruits attract herbivores with their
color or scent, or as food. Once eaten, tough, undigested seeds are dispersed through the herbivore’s feces
(endozoochory ). Other fruits have burrs and hooks to cling to fur and hitch rides on animals ( epizoochory ).
Diversity of Angiosperms
Angiosperms are classified in a single phylum: the Anthophyta. Modern angiosperms appear to be a
monophyletic group, which as you may recall means that they originated from a single ancestor. Within the
angiosperms are three major groups: basal angiosperms, monocots, and dicots. Basal angiosperms are a group
of plants that are believed to have branched off before the separation of the monocots and eudicots, because
they exhibit traits from both groups. They are categorized separately in most classification schemes. The basal
angiosperms include Amborella, water lilies, the Magnoliids (magnolia trees, laurels, and spice peppers), and
a group called the Austrobaileyales, which includes the star anise. The monocots and dicots are differentiated
on the basis of the structure of the cotyledons, pollen grains, and other structures. Monocots include grasses
and lilies, and the dicots form a multi-branched group that includes (among many others) roses, cabbages,
sunflowers, and mints.
Basal Angiosperms
The Magnoliidae are represented by the magnolias, laurels, and peppers. Magnolias are tall trees bearing dark,
shiny leaves, and large, fragrant flowers with many parts, and are considered archaic (Figure 26.17). In the outer
whorl of the magnolia flower the sepals and petals are undifferentiated and are collectively called tepals. The
reproductive parts are arranged in a spiral around a cone-shaped receptacle, with the carpels located above the
stamens (Figure 26.17). The aggregate fruit, with one seed formed from each carpel, is seen in Figure 26.18d.
Laurel trees produce fragrant leaves and small, inconspicuous flowers. The Laurales grow mostly in warmer
climates and are small trees and shrubs. Familiar plants in this group include the bay laurel, cinnamon, spice
bush (Figure 26.18a), and avocado tree.
Figure 26.17 Magnolia grandiflora. A cluster of carpels can be seen above the stamens, which have shed their
pollen and begun to drop from the inflorescence. In the flower, the sepals and petals are undifferentiated and are
collectively called tepals. (credit: lanare Sevi. http://bioimages.vanderbilt.edu/baskauf/10949 (http://openstax.org/
l/grandiflora) )
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(c) (d)
Figure 26.18 Basal angiosperms. The (a) common spicebush belongs to the Laurales, the same family as cinnamon
and bay laurel. The fruit of (b) the Piper nigrum plant is black pepper, the main product that was traded along
spice routes. Notice the small, unobtrusive, clustered flowers. The leaf venation resembles that of both the monocots
(parallel) and the dicots (branched), (c) Water lilies, Nymphaea lotus. Although the leaves of the plant float on the
surface of the water, their roots are in the underlying soil at the bottom of the lake. The aggregate fruit of a magnolia
(d). The fruit is in its final stage, with its red seeds just starting to appear, (credit a: modification of work by Cory Zanker;
credit b: modification of work by Franz Eugen Kohler; credit c: modification of work by Rl/Wikimedia Commons, d:
modification of work by "Coastside2"/Wikimedia Commons).
Monocots
Plants in the monocot group are primarily identified by the presence of a single cotyledon in the seedling. Other
anatomical features shared by monocots include veins that run parallel to and along the length of the leaves, and
flower parts that are arranged in a three- or six-fold symmetry. True woody tissue is rarely found in monocots.
In palm trees, vascular and parenchyma tissues produced by the primary and secondary thickening meristems
form the trunk. The pollen from the first angiosperms was likely monosulcate, containing a single furrow or
pore through the outer layer. This feature is still seen in the modern monocots. Vascular tissue of the stem is
scattered, not arranged in any particular pattern, but is organized in a ring in the roots. The root system consists
of multiple fibrous roots, with no major tap root. Adventitious roots often emerge from the stem or leaves. The
monocots include familiar plants such as the true lilies (Liliopsida), orchids, yucca, asparagus, grasses, and
palms. Many important crops are monocots, such as rice and other cereals, corn, sugar cane, and tropical fruits
like bananas and pineapples (Figure 26.19a,b,c).
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Monocots
(a) Rice
(b) Wheat
(c) Bananas
Dicots
(d) Cabbage (e) Beans (f) Peaches
Figure 26.19 Monocot and dicot crop plants. The world’s major crops are flowering plants, (a) Rice, (b) wheat, and
(c) bananas are monocots, while (d) cabbage, (e) beans, and (f) peaches are dicots, (credit a: modification of work
by David Nance, USDA ARS; credit b, c: modification of work by Rosendahl; credit d: modification of work by Bill
Tarpenning, USDA; credit e: modification of work by Scott Bauer, USDA ARS; credit f: modification of work by Keith
Weller, USDA)
Eudicots
Eudicots, or true dicots, are characterized by the presence of two cotyledons in the developing shoot. Veins
form a network in leaves, and flower parts come in four, five, or many whorls. Vascular tissue forms a ring in
the stem; in monocots, vascular tissue is scattered in the stem. Eudicots can be herbaceous (not woody), or
produce woody tissues. Most eudicots produce pollen that is trisulcate or triporate, with three furrows or pores.
The root system is usually anchored by one main root developed from the embryonic radicle. Eudicots comprise
two-thirds of all flowering plants. The major differences between monocots and eudicots are summarized in
Table 26.1. However, some species may exhibit characteristics usually associated with the other group, so
identification of a plant as a monocot or a eudicot is not always straightforward.
Comparison of Structural Characteristics of Monocots and Eudicots
Characteristic
Monocot
Eudicot
Cotyledon
One
Two
Veins in Leaves
Parallel
Network (branched)
Stem Vascular Tissue
Scattered
Arranged in ring pattern
Roots
Network of fibrous roots
Tap root with many lateral roots
Pollen
Monosulcate
Trisulcate
Flower Parts
Three or multiple of three
Four, five, multiple of four or five and whorls
Table 26.1
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26.4 | The Role of Seed Plants
By the end of this section, you will be able to do the following:
• Explain how angiosperm diversity is due, in part, to multiple complex interactions with animals
• Describe ways in which pollination occurs
• Discuss the roles that plants play in ecosystems and how deforestation threatens plant biodiversity
Without seed plants, life as we know it would not be possible. Plants play a key role in the maintenance of
terrestrial ecosystems through the stabilization of soils, cycling of carbon, and climate moderation. Large tropical
forests release oxygen and act as carbon dioxide “sinks.” Seed plants provide shelter to many life forms, as well
as food for herbivores, thereby indirectly feeding carnivores. Plant secondary metabolites are used for medicinal
purposes and industrial production. Virtually all animal life is dependent on plants for survival.
Animals and Plants: Herbivory
Coevolution of flowering plants and insects is a hypothesis that has received much attention and support,
especially because both angiosperms and insects diversified at about the same time in the middle Mesozoic.
Many authors have attributed the diversity of plants and insects to both pollination and herbivory, or the
consumption of plants by insects and other animals. Herbivory is believed to have been as much a driving force
as pollination. Coevolution of herbivores and plant defenses is easily and commonly observed in nature. Unlike
animals, most plants cannot outrun predators or use mimicry to hide from hungry animals (although mimicry
has been used to entice pollinators). A sort of arms race exists between plants and herbivores. To “combat”
herbivores, some plant seeds—such as acorn and unripened persimmon—are high in alkaloids and therefore
unsavory to some animals. Other plants are protected by bark, although some animals developed specialized
mouth pieces to tear and chew vegetal material. Spines and thorns (Figure 26.20) deter most animals, except
for mammals with thick fur, and some birds have specialized beaks to get past such defenses.
(a) (b)
Figure 26.20 Plant defenses, (a) Spines and (b) thorns are examples of plant defenses, (credit a: modification of work
by Jon Sullivan; credit b: modification of work by I. Sacek, Sr.)
Herbivory has been exploited by seed plants for their own benefit. The dispersal of fruits by herbivorous animals
is a striking example of mutualistic relationships. The plant offers to the herbivore a nutritious source of food in
return for spreading the plant’s genetic material to a wider area.
An extreme example of coevolution (discovered by Dan Jansen) between an animal and a plant is exemplified
by Mexican acacia trees and their attendant acacia ants Pseudomyrmex spp. (this is termed myrmecophytism).
The trees support the ants with shelter and food: The ants nest in the hollows of large thorns produced by the
tree and feed on sugary secretions produced at the ends of the leaves. The sugar pellets also help to keep
the ants from interfering with insect pollinators. In return, ants discourage herbivores, both invertebrates and
vertebrates, by stinging and attacking leaf-eaters and insects ovipositing on the plants. The ants also help to
remove potential plant pathogens, such as fungal growths. Another case of insect-plant coevolution is found in
bracken fern (Pteridium aquinilum), whose subspecies are found throughout the world. Bracken ferns produce
a number of “secondary plant compounds” in their adult fronds that serve as defensive compounds against
nonadapted insect attack (these compounds include cyanogenic glucosides, tannins, and phenolics). However,
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during the “fiddlehead” or crozier stage, bracken secretes nutritious sugary and proteinaceous compounds from
special “nectaries” that attract ants and even species of jumping spiders, all of which defend the plant’s croziers
until they are fully unfolded. These opportunistic groups of protective arthropods greatly reduce the damage that
otherwise would occur during the early stages of growth.
Animals and Plants: Pollination
Flowers pollinated by wind are usually small, feathery, and visually inconspicuous. Grasses are a successful
group of flowering plants that are wind pollinated. They produce large amounts of powdery pollen carried over
large distances by the wind. Some large trees such as oaks, maples, and birches are also wind pollinated.
LINK TQ LEARNING
Explore this website (http://openstaxcollege.Org/l/pollinators2) for additional information on pollinators.
More than 80 percent of angiosperms depend on animals for pollination (technically the transfer of pollen from
the anther to the stigma). Consequently, plants have developed many adaptations to attract pollinators. With
over 200,000 different plants dependent on animal pollination, the plant needs to advertise to its pollinators with
some specificity. The specificity of specialized plant structures that target animals can be very surprising. It is
possible, for example, to determine the general type of pollinators favored by a plant by observing the flower’s
physical characteristics. Many bird or insect-pollinated flowers secrete nectar, which is a sugary liquid. They
also produce both fertile pollen, for reproduction, and sterile pollen rich in nutrients for birds and insects. Many
butterflies and bees can detect ultraviolet light, and flowers that attract these pollinators usually display a pattern
of ultraviolet reflectance that helps them quickly locate the flower's center. In this manner, pollinating insects
collect nectar while at the same time are dusted with pollen (Figure 26.21). Large, red flowers with little smell and
a long funnel shape are preferred by hummingbirds, who have good color perception, a poor sense of smell, and
need a strong perch. White flowers that open at night attract moths. Other animals—such as bats, lemurs, and
lizards—can also act as pollinating agents. Any disruption to these interactions, such as the disappearance of
bees, for example as a consequence of colony collapse disorders, can lead to disaster for agricultural industries
that depend heavily on pollinated crops.
Figure 26.21 Pollination. As a bee collects nectar from a flower, it is dusted by pollen, which it then disperses to other
flowers, (credit: John Severns)
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scientific method CONNECTION
Testing Attraction of Flies by Rotting Flesh Smell
Question: Will flowers that offer cues to bees attract carrion flies if sprayed with compounds that smell like
rotten flesh?
Background: Visitation of flowers by pollinating flies is a function mostly of smell. Flies are attracted by
rotting flesh and carrions. The putrid odor seems to be the major attractant. The polyamines putrescine and
cadaverine, which are the products of protein breakdown after animal death, are the source of the pungent
smell of decaying meat. Some plants strategically attract flies by synthesizing polyamines similar to those
generated by decaying flesh and thereby attract carrion flies.
Flies seek out dead animals because they normally lay their eggs on them and their maggots feed on the
decaying flesh, interestingly, time of death can be determined by a forensic entomologist based on the
stages and type of maggots recovered from cadavers.
Hypothesis: Because flies are drawn to other organisms based on smell and not sight, a flower that is
normally attractive to bees because of its colors will attract flies if it is sprayed with polyamines similar to
those generated by decaying flesh.
Test the hypothesis:
1. Select flowers usually pollinated by bees. White petunia may be a good choice.
2. Divide the flowers into two groups, and while wearing eye protection and gloves, spray one group with
a solution of either putrescine or cadaverine. (Putrescine dihydrochloride is typically available in 98
percent concentration; this can be diluted to approximately 50 percent for this experiment.)
3. Place the flowers in a location where flies are present, keeping the sprayed and unsprayed flowers
separated.
4. Observe the movement of the flies for one hour. Record the number of visits to the flowers using a table
similar to Table 26.2. Given the rapid movement of flies, it may be beneficial to use a video camera to
record the fly-flower interaction. Replay the video in slow motion to obtain an accurate record of the
number of fly visits to the flowers.
5. Repeat the experiment four more times with the same species of flower, but using different specimens.
6. Repeat the entire experiment with a different type of flower that is normally pollinated by bees.
Results of Number of Visits by Flies to Sprayed and Control/Unsprayed
Flowers
Trial # Sprayed Flowers Unsprayed Flowers
1
2
3
4
5
Table 26.2
Analyze your data: Review the data you have recorded. Average the number of visits that flies made to
sprayed flowers over the course of the five trials (on the first flower type) and compare and contrast them to
the average number of visits that flies made to the unsprayed/control flowers. Can you draw any conclusions
regarding the attraction of the flies to the sprayed flowers?
For the second flower type used, average the number of visits that flies made to sprayed flowers over the
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course of the five trials and compare and contrast them to the average number of visits that flies made to the
unsprayed/control flowers. Can you draw any conclusions regarding the attraction of the flies to the sprayed
flowers?
Compare and contrast the average number of visits that flies made to the two flower types. Can you draw
any conclusions about whether the appearance of the flower had any impact on the attraction of flies? Did
smell override any appearance differences, or were the flies attracted to one flower type more than another?
Form a conclusion: Do the results support the hypothesis? If not, how can your observations be explained?
The Importance of Seed Plants in Human Life
Seed plants are the foundation of human diets across the world (Figure 26.22). Many societies eat almost
exclusively vegetarian fare and depend solely on seed plants for their nutritional needs. A few crops (rice,
wheat, and potatoes) dominate the agricultural landscape. Many crops were developed during the agricultural
revolution, when human societies made the transition from nomadic hunter-gatherers to horticulture and
agriculture. Cereals, rich in carbohydrates, provide the staple of many human diets. Beans and nuts supply
proteins. Fats are derived from crushed seeds, as is the case for peanut and rapeseed (canola) oils, or fruits
such as olives. Animal husbandry also consumes large quantities of crop plants.
Staple crops are not the only food derived from seed plants. Various fruits and vegetables provide nutrient
macromolecules, vitamins, minerals, and fiber. Sugar, to sweeten dishes, is produced from the monocot
sugarcane and the eudicot sugar beet. Drinks are made from infusions of tea leaves, chamomile flowers,
crushed coffee beans, or powdered cocoa beans. Spices come from many different plant parts: saffron and
cloves are stamens and buds, black pepper and vanilla are seeds, the bark of a bush in the Laurales family
supplies cinnamon, and the herbs that flavor many dishes come from dried leaves and fruit, such as the pungent
red chili pepper. The volatile oils of a number of flowers and bark provide the scent of perfumes.
Additionally, no discussion of seed plant contribution to human diet would be complete without the mention
of alcohol. Fermentation of plant-derived sugars and starches is used to produce alcoholic beverages in all
societies. In some cases, the beverages are derived from the fermentation of sugars from fruit, as with wines
and, in other cases, from the fermentation of carbohydrates derived from seeds, as with beers. The sharing of
foods and beverages also contributes to human social ritual.
Seed plants have many other uses, including providing wood as a source of timber for construction, fuel, and
material to build furniture. Most paper is derived from the pulp of coniferous trees. Fibers of seed plants such
as cotton, flax, and hemp are woven into cloth. Textile dyes, such as indigo, were mostly of plant origin until the
advent of synthetic chemical dyes.
Lastly, it is more difficult to quantify the benefits of ornamental seed plants. These grace private and public
spaces, adding beauty and serenity to human lives and inspiring painters and poets alike.
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(C) (d)
Figure 26.22 Human uses of plants. Humans rely on plants for a variety of reasons, (a) Cacao beans were introduced
to Europe from the New World, where they were used by Mesoamerican civilizations. Combined with sugar, another
plant product, chocolate is a popular food, (b) Flowers like the tulip are cultivated for their beauty, (c) Quinine, extracted
from cinchona trees, is used to treat malaria, to reduce fever, and to alleviate pain, (d) This violin is made of wood,
(credit a: modification of work by "Everjean'VFlickr; credit b: modification of work by Rosendahl; credit c: modification
of work by Franz Eugen Kohler)
The medicinal properties of plants have been known to human societies since ancient times. There are
references to the use of plants’ curative properties in Egyptian, Babylonian, and Chinese writings from 5,000
years ago. Many modern synthetic therapeutic drugs are derived or synthesized de novo from plant secondary
metabolites. It is important to note that the same plant extract can be a therapeutic remedy at low concentrations,
become an addictive drug at higher doses, and can potentially kill at high concentrations. Table 26.3 presents a
few drugs, their plants of origin, and their medicinal applications.
Plant Origin of Medicinal Compounds and Medical Applications
Plant
Compound
Application
Deadly nightshade
(.Atropa belladonna
)
Atropine
Dilate eye pupils for eye exams
Table 26.3
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Plant Origin of Medicinal Compounds and Medical Applications
Plant
Compound
Application
Foxglove ( Digitalis
purpurea )
Digitalis
Heart disease, stimulates heart beat
Yam ( Dioscorea
spp.)
Steroids
Steroid hormones: contraceptive pill and cortisone
Ephedra ( Ephedra
spp.)
Ephedrine
Decongestant and bronchiole dilator
Pacific yew ( Taxus
brevifolia)
Taxol
Cancer chemotherapy; inhibits mitosis
Opium poppy
(Papaver
somniferum )
Opioids
Analgesic (reduces pain without loss of consciousness) and narcotic
(reduces pain with drowsiness and loss of consciousness) in higher
doses
Quinine tree
(■Cinchona spp.)
Quinine
Antipyretic (lowers body temperature) and antimalarial
Willow ( Salix spp.)
Salicylic acid
(aspirin)
Analgesic and antipyretic
Table 26.3
career connection
Ethnobotanist
The relatively new field of ethnobotany studies the interaction between a particular culture and the plants
native to the region. Seed plants have a large influence on day-to-day human life. Not only are plants
the major source of food and medicine, they also influence many other aspects of society, from clothing
to industry. The medicinal properties of plants were recognized early on in human cultures. From the
mid-1900s, synthetic chemicals began to supplant plant-based remedies.
Pharmacognosy is the branch of pharmacology that focuses on medicines derived from natural sources.
With massive globalization and industrialization, it is possible that much human knowledge of plants and
their medicinal purposes will disappear with the cultures that fostered them. This is where ethnobotanists
come in. To learn about and understand the use of plants in a particular culture, an ethnobotanist must bring
in knowledge of plant life and an understanding and appreciation of diverse cultures and traditions. The
Amazon forest is home to an incredible diversity of vegetation and is considered an untapped resource of
medicinal plants; yet, both the ecosystem and its indigenous cultures are threatened with extinction.
To become an ethnobotanist, a person must acquire a broad knowledge of plant biology, ecology, and
sociology. Not only are the plant specimens studied and collected, but also the stories, recipes, and
traditions that are linked to them. For ethnobotanists, plants are not viewed solely as biological organisms to
be studied in a laboratory, but as an integral part of human culture. The convergence of molecular biology,
anthropology, and ecology make the field of ethnobotany a truly multidisciplinary science.
Biodiversity of Plants
Biodiversity ensures a resource for new food crops and medicines. Plant life balances ecosystems, protects
watersheds, mitigates erosion, moderates our climate, and provides shelter for many animal species. Threats to
plant diversity, however, come from many sources. The explosion of the human population, especially in tropical
countries where birth rates are highest and economic development is in full swing, is leading to devastating
human encroachment into forested areas. To feed the growing population, humans need to obtain arable land,
so there has been and continues to be massive clearing of trees. The need for more energy to power larger
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cities and economic growth therein leads to the construction of dams, the consequent flooding of ecosystems,
and increased emissions of pollutants. Other threats to tropical forests come from poachers, who log trees for
their precious wood. Ebony and Brazilian rosewood, both on the endangered list, are examples of tree species
driven almost to extinction by indiscriminate logging. This unfortunate practice continues unabated today largely
due to lack of population control and political willpower.
The number of plant species becoming extinct is increasing at an alarming rate. Because ecosystems are in
a delicate balance, and seed plants maintain close symbiotic relationships with animals—whether predators or
pollinators—the disappearance of a single plant can lead to the extinction of connected animal species. A real
and pressing issue is that many plant species have not yet been catalogued, and so their place in the ecosystem
is unknown. These unknown species are threatened by logging, habitat destruction, and loss of pollinators.
They may become extinct before we have the chance to begin to understand the possible impacts from their
disappearance. Efforts to preserve biodiversity take several lines of action, from preserving heirloom seeds to
barcoding species. Heirloom seeds come from plants that were traditionally grown in human populations, as
opposed to the seeds used for large-scale agricultural production. Barcoding is a technique in which one or
more short gene sequences, taken from a well-characterized portion of DNA found in most genomes, are used
to identify a species through DNA analysis.
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KEY TERMS
anther sac-like structure at the tip of the stamen in which pollen grains are produced
Anthophyta phylum to which angiosperms belong
barcoding molecular biology technique in which one or more short gene sequences taken from a well-
characterized portion of the genome is used to identify a species
basal angiosperms a group of plants that probably branched off before the separation of monocots and
eudicots
calyx whorl of sepals
carpel single unit of the pistil
conifer dominant phylum of gymnosperms with the greatest variety of trees
corolla collection of petals
cotyledon primitive leaf that develops in the zygote; monocots have one cotyledon, and dicots have two
cotyledons
crop cultivated plant
cycad gymnosperm that grows in tropical climates and resembles a palm tree; member of the phylum
Cycadophyta
dicot (also, eudicot) related group of angiosperms whose embryos possess two cotyledons
dioecious describes a species in which the male and female reproductive organs are carried on separate
specimens
filament thin stalk that links the anther to the base of the flower
flower branches specialized for reproduction found in some seed-bearing plants, containing either specialized
male or female organs or both male and female organs
fruit thickened tissue derived from ovary wall that protects the embryo after fertilization and facilitates seed
dispersal
ginkgophyte gymnosperm with one extant species, the Ginkgo biloba: a tree with fan-shaped leaves
gnetophyte gymnosperm shrub with varied morphological features that produces vessel elements in its woody
tissues; the phylum includes the genera Ephedra, Gnetum, and Welwitschia
gymnosperm seed plant with naked seeds (seeds exposed on modified leaves or in cones)
gynoecium (also, carpel) structure that constitutes the female reproductive organ
heirloom seed seed from a plant that was grown historically, but has not been used in modern agriculture on a
large scale
herbaceous grass-like plant noticeable by the absence of woody tissue
herbivory consumption of plants by insects and other animals
integument layer of sporophyte tissue that surrounds the megasporangium, and later, the embryo
megasporocyte megaspore mother cell; larger spore that germinates into a female gametophyte in a
heterosporous plant
microsporocyte smaller spore that produces a male gametophyte in a heterosporous plant
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monocot related group of angiosperms that produce embryos with one cotyledon and pollen with a single ridge
monoecious describes a species in which the male and female reproductive organs are on the same plant
nectar liquid rich in sugars produced by flowers to attract animal pollinators
ovary chamber that contains and protects the ovule or female megasporangium
ovulate cone cone containing two ovules per scale
ovule female gametophyte
perianth part of the plant consisting of the calyx (sepals) and corolla (petals)
petal modified leaf interior to the sepals; colorful petals attract animal pollinators
pistil fused group of carpels
pollen grain structure containing the male gametophyte of the plant
pollen tube extension from the pollen grain that delivers sperm to the egg cell
pollination transfer of pollen from the anther to the stigma
progymnosperm transitional group of plants that resembled conifers because they produced wood, yet still
reproduced like ferns
seed structure containing the embryo, storage tissue, and protective coat
sepal modified leaf that encloses the bud; outermost structure of a flower
spermatophyte seed plant; from the Greek sperm (seed) and phyte (plant)
stamen structure that contains the male reproductive organs
stigma uppermost structure of the carpel where pollen is deposited
strobilus plant structure with a tight arrangement of sporophylls around a central stalk, as seen in cones or
flowers; the male strobilus produces pollen, and the female strobilus produces eggs
style long, thin structure that links the stigma to the ovary
CHAPTER SUMMARY
26.1 Evolution of Seed Plants
Seed plants appeared about one million years ago, during the Carboniferous period. Two major innovations
were seeds and pollen. Seeds protect the embryo from desiccation and provide it with a store of nutrients to
support the early growth of the sporophyte. Seeds are also equipped to delay germination until growth
conditions are optimal. Pollen allows seed plants to reproduce in the absence of water. The gametophytes of
seed plants shrank, while the sporophytes became prominent structures and the diploid stage became the
longest phase of the life cycle.
In the gymnosperms, which appeared during the drier Permian period and became the dominant group during
the Triassic, pollen was dispersed by wind, and their naked seeds developed in the sporophylls of a strobilus.
Angiosperms bear both flowers and fruit. Flowers expand the possibilities for pollination, especially by insects,
who have coevolved with the flowering plants. Fruits offer additional protection to the embryo during its
development, and also assist with seed dispersal. Angiosperms appeared during the Mesozoic era and have
become the dominant plant life in terrestrial habitats.
26.2 Gymnosperms
Gymnosperms are heterosporous seed plants that produce naked seeds. They appeared in the Paleozoic
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period and were the dominant plant life during the Mesozoic. Modern-day gymnosperms belong to four phyla.
The largest phylum, Coniferophyta, is represented by conifers, the predominant plants at high altitude and
latitude. Cycads (phylum Cycadophyta) resemble palm trees and grow in tropical climates. Ginkgophyta is
represented today by a single species, Ginkgo biloba. The last phylum, Gnetophyta, is a diverse group of
plants that produce vessel elements in their wood.
26.3 Angiosperms
Angiosperms are the dominant form of plant life in most terrestrial ecosystems, comprising about 90 percent of
all plant species. Most crops and ornamental plants are angiosperms. Their success comes from two
innovative structures that protect reproduction from variability in the environment: the flower and the fruit.
Flowers were derived from modified leaves; their color and fragrance encourages species-specific pollination.
The main parts of a flower are the sepals and petals, which protect the reproductive parts: the stamens and the
carpels. The stamens produce the male gametes in pollen grains. The carpels contain the female gametes (the
eggs inside the ovules), which are within the ovary of a carpel. The walls of the ovary thicken after fertilization,
ripening into fruit that ensures dispersal by wind, water, or animals.
The angiosperm life cycle is dominated by the sporophyte stage. Double fertilization is an event unique to
angiosperms. One sperm in the pollen fertilizes the egg, forming a diploid zygote, while the other combines with
the two polar nuclei, forming a triploid cell that develops into a food storage tissue called the endosperm.
Flowering plants are divided into two main groups, the monocots and eudicots, according to the number of
cotyledons in the seedlings. Basal angiosperms belong to an older lineage than monocots and eudicots.
26.4 The Role of Seed Plants
Angiosperm diversity is due in part to multiple interactions with animals. Flerbivory has favored the
development of defense mechanisms in plants, and avoidance of those defense mechanisms in animals.
Conversely, seed dispersal can be aided by animals that eat plant fruits. Pollination (the transfer of pollen to a
carpel) is mainly carried out by wind and animals, and angiosperm fruits and seeds have evolved numerous
adaptations to capture the wind or attract specific classes of animals.
Plants play a key role in ecosystems. They are a source of food and medicinal compounds, and provide raw
materials for many industries. Rapid deforestation and industrialization, however, threaten plant biodiversity. In
turn, this threatens the ecosystem.
VISUAL CONNECTION QUESTIONS
1. Figure 26.8 At what stage does the diploid zygote
form?
a. when the female cone begins to bud from
the tree
b. at fertilization
c. when the seeds drop from the tree
d. when the pollen tube begins to grow
REVIEW QUESTIONS
3. Seed plants are_.
a. all homosporous
b. mostly homosporous with some
heterosporous
c. mostly heterosporous with some
homosporous
d. all heterosporous
4. Besides the seed, what other major structure
diminishes a plant’s reliance on water for
reproduction?
6. Which of the following structures widens the
geographic range of a species and is an agent of
a.
flower
b.
fruit
c.
pollen
d.
spore
5. In which of the following geological periods would
gymnosperms dominate the landscape?
a. Carboniferous
b. Permian
c. Triassic
d. Eocene (present)
2. Figure 26.15 If a flower lacked a
megasporangium, what type of gamete would not
form? If the flower lacked a microsporangium, what
type of gamete would not form?
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dispersal?
a. seed
b. flower
c. leaf
d. root
7. Which of the following traits characterizes
gymnosperms?
a. The plants carry exposed seeds on modified
leaves.
b. Reproductive structures are located in a
flower.
c. After fertilization, the ovary thickens and
forms a fruit.
d. The gametophyte is the longest phase of
the life cycle.
8. Megasporocytes will eventually produce which of
the following?
a. pollen grain
b. sporophytes
c. male gametophytes
d. female gametophytes
9. What is the ploidy of the following structures:
gametophyte, seed, spore, sporophyte?
a. In, In, 2n, 2n
b. In, 2n, In, 2n
c. 2n, In, 2n, In
d. 2n, 2n, In, In
10. In the northern forests of Siberia, a tall tree is
most likely a:
a. conifer
b. cycad
c. Ginkgo biloba
d. gnetophyte
11. Which of the following structures in a flower is not
directly involved in reproduction?
a. the style
b. the stamen
c. the sepal
d. the anther
12. Pollen grains develop in which structure?
CRITICAL THINKING QUESTIONS
19. The Triassic Period was marked by the increase
in number and variety of angiosperms. Insects also
diversified enormously during the same period. Can
you propose the reason or reasons that could foster
coevolution?
20. What role did the adaptations of seed and pollen
play in the development and expansion of seed
plants?
21. The Mediterranean landscape along the sea
shore is dotted with pines and cypresses. The
a. the anther
b. the stigma
c. the filament
d. the carpel
13. In the course of double fertilization, one sperm
cell fuses with the egg and the second one fuses with
a. the synergids
b. the polar nuclei of the center cell
c. the egg as well
d. the antipodal cells
14. Corn develops from a seedling with a single
cotyledon, displays parallel veins on its leaves, and
produces monosulcate pollen. It is most likely:
a. a gymnosperm
b. a monocot
c. a eudicot
d. a basal angiosperm
15. Which of the following plant structures is not a
defense against herbivory?
a. thorns
b. spines
c. nectar
d. alkaloids
16. White and sweet-smelling flowers with abundant
nectar are probably pollinated by
a. bees and butterflies
b. flies
c. birds
d. wind
17. Abundant and powdery pollen produced by small,
indistinct flowers is probably transported by:
a. bees and butterflies
b. flies
c. birds
d. wind
18. Plants are a source of_.
a. food
b. fuel
c. medicine
d. all of the above
weather is not cold, and the trees grow at sea level.
What evolutionary adaptation of conifers makes them
suitable to the Mediterranean climate?
22. What are the four modern-day phyla of
gymnosperms?
23. Some cycads are considered endangered
species and their trade is severely restricted.
Customs officials stop suspected smugglers who
claim that the plants in their possession are palm
trees, not cycads. How would a botanist distinguish
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between the two types of plants?
24. What are the two structures that allow
angiosperms to be the dominant form of plant life in
most terrestrial ecosystems?
25. Biosynthesis of nectar and nutrient-rich pollen is
energetically very expensive for a plant. Yet, plants
funnel large amounts of energy into animal
pollination. What are the evolutionary advantages
that offset the cost of attracting animal pollinators?
26. What is biodiversity and why is it important to an
ecosystem?
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27 | INTRODUCTION TO
ANIMAL DIVERSITY
Figure 27.1 The leaf chameleon (Brookesia micra) was discovered in northern Madagascar in 2012. At just over one
inch long, it is the smallest known chameleon, (credit: modification of work by Frank Glaw, et al., PLOS)
Chapter Outline
27.1: Features of the Animal Kingdom
27.2: Features Used to Classify Animals
27.3: Animal Phylogeny
27.4: The Evolutionary History of the Animal Kingdom
Introduction
Animal evolution began in the ocean over 600 million years ago with tiny creatures that probably do not resemble
any living organism today. Since then, animals have evolved into a highly diverse kingdom. Although over one
million extant (currently living) species of animals have been identified, scientists are continually discovering
more species as they explore ecosystems around the world. The number of extant species is estimated to be
between 3 and 30 million.
But what is an animal? While we can easily identify dogs, birds, fish, spiders, and worms as animals, other
organisms, such as corals and sponges, are not as easy to classify. Animals vary in complexity—from sea
sponges to crickets to chimpanzees—and scientists are faced with the difficult task of classifying them within
a unified system. They must identify traits that are common to all animals as well as traits that can be used to
distinguish among related groups of animals. The animal classification system characterizes animals based on
their anatomy, morphology, evolutionary history, features of embryological development, and genetic makeup.
This classification scheme is constantly developing as new information about species arises. Understanding and
classifying the great variety of living species help us better understand how to conserve the diversity of life on
earth.
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27.1 1 Features of the Animal Kingdom
By the end of this section, you will be able to do the following:
• List the features that distinguish the kingdom Animalia from other kingdoms
• Explain the processes of animal reproduction and embryonic development
• Describe the roles that Hox genes play in development
Two different groups within the Domain Eukaryota have produced complex multicellular organisms: The plants
arose within the Archaeplastida, whereas the animals (and their close relatives, the fungi) arose within the
Opisthokonta. However, plants and animals not only have different life styles, they also have different cellular
histories as eukaryotes. The opisthokonts share the possession of a single posterior flagellum in flagellated cells,
e.g., sperm cells.
Most animals also share other features that distinguish them from organisms in other kingdoms. All animals
require a source of food and are therefore heterotrophic , ingesting other living or dead organisms. This feature
distinguishes them from autotrophic organisms, such as most plants, which synthesize their own nutrients
through photosynthesis. As heterotrophs, animals may be carnivores, herbivores, omnivores, or parasites
(Figure 27.2a,b). As with plants, almost all animals have a complex tissue structure with differentiated and
specialized tissues. The necessity to collect food has made most animals motile, at least during certain life
stages. The typical life cycle in animals is diplontic (like you, the diploid state is multicellular, whereas the haploid
state is gametic, such as sperm or egg). We should note that the alternation of generations characteristic of
the land plants is typically not found in animals. In animals whose life histories include several to multiple body
forms (e.g., insect larvae or the medusae of some Cnidarians), all body forms are diploid. Animal embryos pass
through a series of developmental stages that establish a determined and fixed body plan. The body plan refers
to the morphology of an animal, determined by developmental cues.
(a) (b)
Figure 27.2 Heterotrophy. All animals are heterotrophs and thus derive energy from a variety of food sources. The
(a) black bear is an omnivore, eating both plants and animals. The (b) heartworm Dirofilaria immitis is a parasite that
derives energy from its hosts. It spends its larval stage in mosquitoes and its adult stage infesting the heart of dogs
and other mammals, as shown here, (credit a: modification of work by USDA Forest Service; credit b: modification of
work by Clyde Robinson)
Complex Tissue Structure
Many of the specialized tissues of animals are associated with the requirements and hazards of seeking
and processing food. This explains why animals typically have evolved special structures associated with
specific methods of food capture and complex digestive systems supported by accessory organs. Sensory
structures help animals navigate their environment, detect food sources (and avoid becoming a food source
for other animals!). Movement is driven by muscle tissue attached to supportive structures like bone or chitin,
and is coordinated by neural communication. Animal cells may also have unique structures for intercellular
communication (such as gap junctions). The evolution of nerve tissues and muscle tissues has resulted in
animals’ unique ability to rapidly sense and respond to changes in their environment. This allows animals to
survive in environments where they must compete with other species to meet their nutritional demands.
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The tissues of animals differ from those of the other major multicellular eukaryotes, plants and fungi, because
their cells don't have cell walls. However, cells of animal tissues may be embedded in an extracellular matrix
(e.g., mature bone cells reside within a mineralized organic matrix secreted by the cells). In vertebrates, bone
tissue is a type of connective tissue that supports the entire body structure. The complex bodies and activities
of vertebrates demand such supportive tissues. Epithelial tissues cover and protect both external and internal
body surfaces, and may also have secretory functions. Epithelial tissues include the epidermis of the integument,
the lining of the digestive tract and trachea, as well as the layers of cells that make up the ducts of the liver
and glands of advanced animals, for example. The different types of tissues in true animals are responsible for
carrying out specific functions for the organism. This differentiation and specialization of tissues is part of what
allows for such incredible animal diversity.
Just as there are multiple ways to be a eukaryote, there are multiple ways to be a multicellular animal. The
animal kingdom is currently divided into five monophyletic clades: Parazoa or Porifera (sponges), Placozoa (tiny
parasitic creatures that resemble multicellular amoebae), Cnidaria (jellyfish and their relatives), Ctenophora (the
comb jellies), and Bilateria (all other animals). The Placozoa ("flat animal") and Parazoa (“beside animal") do not
have specialized tissues derived from germ layers of the embryo; although they do possess specialized cells that
act functionally like tissues. The Placozoa have only four cell types, while the sponges have nearly two dozen.
The three other clades do include animals with specialized tissues derived from the germ layers of the embryo. In
spite of their superficial similarity to Cnidarian medusae, recent molecular studies indicate that the Ctenophores
are only distantly related to the Cnidarians, which together with the Bilateria constitute the Eumetazoa ("true
animals"). When we think of animals, we usually think of Eumetazoa, since most animals fall into this category.
Watch a presentation (http://0penstaxc0llege.0rg/l/saving_life) by biologist E.O. Wilson on the importance
of diversity.
Animal Reproduction and Development
Most animals are diploid organisms, meaning that their body (somatic) cells are diploid and haploid reproductive
(gamete) cells are produced through meiosis. Some exceptions exist: for example, in bees, wasps, and ants, the
male is haploid because it develops from unfertilized eggs. Most animals undergo sexual reproduction. However,
a few groups, such as cnidarians, flatworms, and roundworms, may also undergo asexual reproduction, in which
offspring originate from part of the parental body.
Processes of Animal Reproduction and Embryonic Development
During sexual reproduction, the haploid gametes of the male and female individuals of a species combine in a
process called fertilization. Typically, both male and female gametes are required: the small, motile male sperm
fertilizes the typically much larger, sessile female egg. This process produces a diploid fertilized egg called a
zygote.
Some animal species—including sea stars and sea anemones—are capable of asexual reproduction. The most
common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, where
part of a parent individual can separate and grow into a new individual. This type of asexual reproduction
produces genetically identical offspring, which would appear to be disadvantageous from the perspective of
evolutionary adaptability, simply because of the potential buildup of deleterious mutations.
In contrast, a form of uniparental reproduction found in some insects and a few vertebrates is called
parthenogenesis (or “virgin beginning”). In this case, progeny develop from a gamete, but without fertilization.
Because of the nutrients stored in eggs, only females produce parthenogenetic offspring. In some insects,
unfertilized eggs develop into new male offspring. This type of sex determination is called haplodiploidy, since
females are diploid (with both maternal and paternal chromosomes) and males are haploid (with only maternal
chromosomes). A few vertebrates, e.g., some fish, turkeys, rattlesnakes, and whiptail lizards, are also capable
of parthenogenesis. In the case of turkeys and rattlesnakes, parthenogenetically reproducing females also
produce only male offspring, but not because the males are haploid. In birds and rattlesnakes, the female is the
heterogametic (ZW) sex, so the only surviving progeny of post-meiotic parthenogenesis would be ZZ males. In
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the whiptail lizards, on the other hand, only female progeny are produced by parthenogenesis. These animals
may not be identical to their parent, although they have only maternal chromosomes. However, for animals that
are limited in their access to mates, uniparental reproduction can ensure genetic propagation.
In animals, the zygote progresses through a series of developmental stages, during which primary germ
layers (ectoderm, endoderm, and mesoderm) are established and reorganize to form an embryo. During this
process, animal tissues begin to specialize and organize into organs and organ systems, determining their future
morphology and physiology.
Animal development begins with cleavage, a series of mitotic cell divisions, of the zygote (Figure 27.3).
Cleavage differs from somatic cell division in that the egg is subdivided by successive cleavages into smaller
and smaller cells, with no actual cell growth. The cells resulting from subdivision of the material of the egg in
this way are called blastomeres. Three cell divisions transform the single-celled zygote into an eight-celled
structure. After further cell division and rearrangement of existing cells, a solid morula is formed, followed by
a hollow structure called a blastula. The blastula is hollow only in invertebrates whose eggs have relatively
small amounts of yolk. In very yolky eggs of vertebrates, the yolk remains undivided, with most cells forming an
embryonic layer on the surface of the yolk (imagine a chicken embryo growing over the egg’s yolk), which serve
as food for the developing embryo.
Further cell division and cellular rearrangement leads to a process called gastrulation. Gastrulation results in
two important events: the formation of the primitive gut (archenteron) or digestive cavity, and the formation of
the embryonic germ layers, as we have discussed above. These germ layers are programmed to develop into
certain tissue types, organs, and organ systems during a process called organogenesis. Diploblastic organisms
have two germ layers, endoderm and ectoderm. Endoderm forms the wall of the digestive tract, and ectoderm
covers the surface of the animal. In triploblastic animals, a third layer forms: mesoderm, which differentiates into
various structures between the ectoderm and endoderm, including the lining of the body cavity.
Figure 27.3 Development of a simple embryo. During embryonic development, the zygote undergoes a series of
mitotic cell divisions, or cleavages, that subdivide the egg into smaller and smaller blastomeres. Note that the 8-cell
stage and the blastula are about the same size as the original zygote. In many invertebrates, the blastula consists of
a single layer of cells around a hollow space. During a process called gastrulation, the cells from the blastula move
inward on one side to form an inner cavity. This inner cavity becomes the primitive gut (archenteron) of the gastrula
("little gut") stage. The opening into this cavity is called the blastopore, and in some invertebrates it is destined to form
the mouth.
Some animals produce larval forms that are different from the adult. In insects with incomplete metamorphosis,
such as grasshoppers, the young resemble wingless adults, but gradually produce larger and larger wing
buds during successive molts, until finally producing functional wings and sex organs during the last molt.
Other animals, such as some insects and echinoderms, undergo complete metamorphosis in which the embryo
develops into one or more feeding larval stages that may differ greatly in structure and function from the
adult (Figure 27.4). The adult body then develops from one or more regions of larval tissue. For animals
with complete metamorphosis, the larva and the adult may have different diets, limiting competition for food
between them. Regardless of whether a species undergoes complete or incomplete metamorphosis, the series
of developmental stages of the embryo remains largely the same for most members of the animal kingdom.
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(a) Incomplete metamorphosis
Figure 27.4 Insect metamorphosis, (a) The grasshopper undergoes incomplete metamorphosis, (b) The butterfly
undergoes complete metamorphosis, (credit: S.E. Snodgrass, USDA)
LINK TQ LEARNING
Watch the following video (http://0penstaxc0llege.0rg/l/embry0_ev0l) to see how human embryonic
development (after the blastula and gastrula stages of development) reflects evolution.
The Role of Homeobox (Hox) Genes in Animal Development
Since the early nineteenth century, scientists have observed that many animals, from the very simple to the
complex, shared similar embryonic morphology and development. Surprisingly, a human embryo and a frog
embryo, at a certain stage of embryonic development, look remarkably alike! For a long time, scientists did not
understand why so many animal species looked similar during embryonic development but were very different
as adults. They wondered what dictated the developmental direction that a fly, mouse, frog, or human embryo
would take. Near the end of the twentieth century, a particular class of genes was discovered that had this very
job. These genes that determine animal structure are called “homeotic genes,” and they contain DNA sequences
called homeoboxes. Genes with homeoboxes encode protein transcription factors. One group of animal genes
containing homeobox sequences is specifically referred to as Hox genes. This cluster of genes is responsible
for determining the general body plan, such as the number of body segments of an animal, the number and
placement of appendages, and animal head-tail directionality. The first Hox genes to be sequenced were those
from the fruit fly ( Drosophila melanogaster). A single Hox mutation in the fruit fly can result in an extra pair
of wings or even legs growing from the head in place of antennae (this is because antennae and legs are
embryologic homologous structures and their appearance as antennae or legs is dictated by their origination
within specific body segments of the head and thorax during development). Now, Hox genes are known from
virtually all other animals as well.
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While there are a great many genes that play roles in the morphological development of an animal, including
other homeobox-containing genes, what makes Hox genes so powerful is that they serve as “master control
genes" that can turn on or off large numbers of other genes. Hox genes do this by encoding transcription factors
that control the expression of numerous other genes. Hox genes are homologous across the animal kingdom,
that is, the genetic sequences of Hox genes and their positions on chromosomes are remarkably similar across
most animals because of their presence in a common ancestor, from worms to flies, mice, and humans (Figure
27.5). In addition, the order of the genes reflects the anterior-posterior axis of the animal's body. One of the
contributions to increased animal body complexity is that Hox genes have undergone at least two and perhaps
as many as four duplication events during animal evolution, with the additional genes allowing for more complex
body types to evolve. All vertebrates have four (or more) sets of Hox genes, while invertebrates have only one
set.
visual
CONNECTION
Figure 27.5 Hox genes. Hox genes are highly conserved genes encoding transcription factors that determine the
course of embryonic development in animals. In vertebrates, the genes have been duplicated into four clusters on
different chromosomes: Hox-A , Hox-B, Hox-C, and Hox-D. Genes within these clusters are expressed in certain
body segments at certain stages of development. Shown here is the homology between Hox genes in mice and
humans. Note how Hox gene expression, as indicated with orange, pink, blue, and green shading, occurs in the
same body segments in both the mouse and the human. While at least one copy of each Hox gene is present in
humans and other vertebrates, some Hox genes are missing in some chromosomal sets.
If a Hox 13 gene in a mouse was replaced with a Hox 1 gene, how might this alter animal development?
Two of the five clades within the animal kingdom do not have Hox genes: the Ctenophora and the Porifera.
In spite of the superficial similarities between the Cnidaria and the Ctenophora, the Cnidaria have a number
of Hox genes, but the Ctenophora have none. The absence of Hox genes from the ctenophores has led
to the suggestion that they might be “basal" animals, in spite of their tissue differentiation. Ironically, the
Placozoa, which have only a few cell types, do have at least one Hox gene. The presence of a Hox gene in
the Placozoa, in addition to similarities in the genomic organization of the Placozoa, Cnidaria and Bilateria,
has led to the inclusion of the three groups in a “Parahoxozoa" clade. However, we should note that at this
time the reclassification of the Animal Kingdom is still tentative and requires much more study.
27.2 | Features Used to Classify Animals
By the end of this section, you will be able to do the following:
• Explain the differences in animal body plans that support basic animal classification
• Compare and contrast the embryonic development of protostomes and deuterostomes
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Scientists have developed a classification scheme that categorizes all members of the animal kingdom, although
there are exceptions to most “rules” governing animal classification (Figure 27.6). Animals have been
traditionally classified according to two characteristics: body plan and developmental pathway. The major feature
of the body plan is its symmetry: how the body parts are distributed along the major body axis. Symmetrical
animals can be divided into roughly equivalent halves along at least one axis. Developmental characteristics
include the number of germ tissue layers formed during development, the origin of the mouth and anus, the
presence or absence of an internal body cavity, and other features of embryological development, such as larval
types or whether or not periods of growth are interspersed with molting.
visual
CONNECTION
Figure 27.6 Animal phylogeny. The phylogenetic tree of animals is based on morphological, fossil, and genetic
evidence. The Ctenophora and Porifera are both considered to be basal because of the absence of Hox genes in
this group, but how they are related to the “Parahoxozoa" (Placozoa + Eumetazoa) or to each other, continues to
be a matter of debate.
Which of the following statements is false?
a. Eumetazoans have specialized tissues and parazoans don’t.
b. Lophotrochozoa and Ecdysozoa are both Bilataria.
c. Acoela and Cnidaria both possess radial symmetry.
d. Arthropods are more closely related to nematodes than they are to annelids.
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Animal Characterization Based on Body Symmetry
At a very basic level of classification, true animals can be largely divided into three groups based on the type of
symmetry of their body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Asymmetry is seen
in two modern clades, the Parazoa (Figure 27.7a) and Placozoa. (Although we should note that the ancestral
fossils of the Parazoa apparently exhibited bilateral symmetry.) One clade, the Cnidaria (Figure 27.7b,c),
exhibits radial or biradial symmetry: Ctenophores have rotational symmetry (Figure 27. 7e). Bilateral symmetry
is seen in the largest of the clades, the Bilateria (Figure 27. 7d); however the Echinodermata are bilateral as
larvae and metamorphose secondarily into radial adults. All types of symmetry are well suited to meet the unique
demands of a particular animal’s lifestyle.
Radial symmetry is the arrangement of body parts around a central axis, as is seen in a bicycle wheel or pie.
It results in animals having top and bottom surfaces but no left and right sides, nor front or back. If a radially
symmetrical animal is divided in any direction along the oral/aboral axis (the side with a mouth is “oral side,” and
the side without a mouth is the “aboral side"), the two halves will be mirror images. This form of symmetry marks
the body plans of many animals in the phyla Cnidaria, including jellyfish and adult sea anemones (Figure 27.7b,
c). Radial symmetry equips these sea creatures (which may be sedentary or only capable of slow movement or
floating) to experience the environment equally from all directions. Bilaterally symmetrical animals, like butterflies
(Figure 27. 7d) have only a single plane along which the body can be divided into equivalent halves. The
Ctenophora (Figure 27. 7e), although they look similar to jellyfish, are considered to have rotational symmetry
rather than radial or biradial symmetry because division of the body into two halves along the oral/aboral axis
divides them into two copies of the same half, with one copy rotated 180°, rather than two mirror images.
(a) (b) (c)
(d) (e)
Figure 27.7 Symmetry in animals. The (a) sponge is asymmetrical. The (b) jellyfish and (c) anemone are radially
symmetrical, the (d) butterfly is bilaterally symmetrical. Rotational symmetry (e) is seen in the ctenophore Berne,
shown swimming open-mouthed, (credit a: modification of work by Andrew Turner; credit b: modification of work by
Robert Freiburger; credit c: modification of work by Samuel Chow; credit d: modification of work by Cory Zanker; credit
e: modification of work by NOAA)
Bilateral symmetry involves the division of the animal through a midsagittal plane, resulting in two superficially
mirror images, right and left halves, such as those of a butterfly (Figure 27. 7d), crab, or human body. Animals
with bilateral symmetry have a “head” and “tail” (anterior vs. posterior), front and back (dorsal vs. ventral), and
right and left sides (Figure 27.8). All Eumetazoa except those with secondary radial symmetry are bilaterally
symmetrical. The evolution of bilateral symmetry that allowed for the formation of anterior and posterior (head
and tail) ends promoted a phenomenon called cephalization, which refers to the collection of an organized
nervous system at the animal’s anterior end. In contrast to radial symmetry, which is best suited for stationary or
limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. In evolutionary terms,
this simple form of symmetry promoted active and controlled directional mobility and increased sophistication of
resource-seeking and predator-prey relationships.
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Figure 27.8 Bilateral symmetry. The bilaterally symmetrical human body can be divided by several planes.
Animals in the phylum Echinodermata (such as sea stars, sand dollars, and sea urchins) display modified radial
symmetry as adults, but as we have noted, their larval stages (such as the bipinnaria) initially exhibit bilateral
symmetry until they metamorphose in animals with radial symmetry (this is termed secondary radial symmetry).
Echinoderms evolved from bilaterally symmetrical animals; thus, they are classified as bilaterally symmetrical.
LINK TQ LEARNING
Watch this video to see a quick sketch of the different types of body symmetry. (This multimedia
resource will open in a browser.) (http://cnx.Org/content/m66578/l.3/#eip-idll65785284264)
Animal Characterization Based on Features of Embryological
Development
Most animal species undergo a separation of tissues into germ layers during embryonic development. Recall
that these germ layers are formed during gastrulation, and that each germ layer typically gives rise to specific
types of embryonic tissues and organs. Animals develop either two or three embryonic germ layers (Figure
27.9). The animals that display radial, biradial, or rotational symmetry develop two germ layers, an inner layer
(endoderm or mesendoderm) and an outer layer (ectoderm). These animals are called diploblasts, and have a
nonliving middle layer between the endoderm and ectoderm (although individual cells may be distributed through
this middle layer, there is no coherent third layer of tissue). The four clades considered to be diploblastic have
different levels of complexity and different developmental pathways, although there is little information about
development in Placozoa. More complex animals (usually those with bilateral symmetry) develop three tissue
layers: an inner layer (endoderm), an outer layer (ectoderm), and a middle layer (mesoderm). Animals with three
tissue layers are called triploblasts.
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CONNECTION
Diploblast
Triploblast
Endoderm
O Q
Ectoderm
Non-living layer
Mesoderm
Figure 27.9 Diploblastic and triploblastic embryos. During embryogenesis, diploblasts develop two embryonic
germ layers: an ectoderm and an endoderm or mesendoderm. Triploblasts develop a third layer—the
mesoderm—which arises from mesendoderm and resides between the endoderm and ectoderm.
Which of the following statements about diploblasts and triploblasts is false?
a. Animals that display only radial symmetry during their lifespans are diploblasts.
b. Animals that display bilateral symmetry are triploblasts.
c. The endoderm gives rise to the lining of the digestive tract and the respiratory tract.
d. The mesoderm gives rise to the central nervous system.
Each of the three germ layers is programmed to give rise to specific body tissues and organs, although there
are variations on these themes. Generally speaking, the endoderm gives rise to the lining of the digestive tract
(including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and
lungs of the respiratory tract, along with a few other structures. The ectoderm develops into the outer epithelial
covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the
third germ layer; it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all
specialized muscle tissues (including the cardiac tissues and muscles of the intestines), connective tissues such
as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen. Diploblastic
animals may have cell types that serve multiple functions, such as epitheliomuscular cells, which serve as a
covering as well as contractile cells.
Presence or Absence of a Coelom
Further subdivision of animals with three germ layers (triploblasts) results in the separation of animals that may
develop an internal body cavity derived from mesoderm, called a coelom, and those that do not. This epithelial
cell-lined coelomic cavity, usually filled with fluid, lies between the visceral organs and the body wall. It houses
many organs such as the digestive, urinary, and reproductive systems, the heart and lungs, and also contains
the major arteries and veins of the circulatory system. In mammals, the body cavity is divided into the thoracic
cavity, which houses the heart and lungs, and the abdominal cavity, which houses the digestive organs. In the
thoracic cavity further subdivision produces the pleural cavity, which provides space for the lungs to expand
during breathing, and the pericardial cavity, which provides room for movements of the heart. The evolution of
the coelom is associated with many functional advantages. For example, the coelom provides cushioning and
shock absorption for the major organ systems that it encloses. In addition, organs housed within the coelom can
grow and move freely, which promotes optimal organ development and placement. The coelom also provides
space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.
Triploblasts that do not develop a coelom are called acoelomates, and their mesoderm region is completely
filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the
phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates (or
coelomates) (Figure 27.10). In such cases, a true coelom arises entirely within the mesoderm germ layer
and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting
and holding them in position while allowing them some freedom of movement. Annelids, mollusks, arthropods,
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echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom
lined partly by mesoderm and partly by endoderm. Although still functionally a coelom, these are considered
“false" coeloms, and so we call these animals pseudocoelomates. The phylum Nematoda (roundworms) is an
example of a pseudocoelomate. True coelomates can be further characterized based on other features of their
early embryological development.
Flatworm: Pseudobiceros bedfordi Annelid: Clycera Nematode: Heterodera glycines
mollusks,
arthropods,
echinoderms,
chordates)
Figure 27.10 Body cavities. Triploblasts may be (a) acoelomates, (b) eucoelomates, or (c) pseudocoelomates.
Acoelomates have no body cavity. Eucoelomates have a body cavity within the mesoderm, called a coelom, in which
both the gut and the body wall are lined with mesoderm. Pseudocoelomates also have a body cavity, but only the body
wall is lined with mesoderm, (credit a: modification of work by Jan Derk; credit b: modification of work by NOAA; credit
c: modification of work by USDA, ARS)
Embryonic Development of the Mouth
Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in
the origin of the mouth. When the primitive gut forms, the opening that first connects the gut cavity to the outside
of the embryo is called the blastopore. Most animals have openings at both ends of the gut: mouth at one end
and anus at the other. One of these openings will develop at or near the site of the blastopore, in Protostomes
("mouth first"), the mouth develops at the blastopore (Figure 27.11). In Deuterostomes ("mouth second"), the
mouth develops at the other end of the gut (Figure 27.11) and the anus develops at the site of the blastopore.
Protostomes include arthropods, mollusks, and annelids. Deuterostomes include more complex animals such as
chordates but also some “simple" animals such as echinoderms. Recent evidence has challenged this simple
view of the relationship between the location of the blastopore and the formation of the mouth, however, and
the theory remains under debate. Nevertheless, these details of mouth and anus formation reflect general
differences in the organization of protostome and deuterostome embryos, which are also expressed in other
developmental features.
One of these differences between protostomes and deuterostomes is the method of coelom formation, beginning
from the gastrula stage. Since body cavity formation tends to accompany the formation of the mesoderm, the
mesoderm of protostomes and deuterostomes forms differently. The coelom of most protostomes is formed
through a process called schizocoely. The mesoderm in these organisms is usually the product of specific
blastomeres, which migrate into the interior of the embryo and form two clumps of mesodermal tissue. Within
each clump, cavities develop and merge to form the hollow opening of the coelom. Deuterostomes differ in that
their coelom forms through a process called enterocoely. Here, the mesoderm develops as pouches that are
pinched off from the endoderm tissue. These pouches eventually fuse and expand to fill the space between the
gut and the body wall, giving rise to the coelom.
Another difference in organization of protostome and deuterostome embryos is expressed during cleavage.
Protostomes undergo spiral cleavage, meaning that the cells of one pole of the embryo are rotated, and thus
misaligned, with respect to the cells of the opposite pole. This is due to the oblique angle of cleavage relative
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to the two poles of the embryo. Deuterostomes undergo radial cleavage, where the cleavage axes are either
parallel or perpendicular to the polar axis, resulting in the parallel (up-and-down) alignment of the cells between
the two poles.
Protostomes Deuterostomes
Figure 27.11 Protostomes and deuterostomes. Eucoelomates can be divided into two groups based on their early
embryonic development. In protostomes, the mouth forms at or near the site of the blastopore and the body cavity
forms by splitting the mesodermal mass during the process of schizocoely. In deuterostomes, the mouth forms at a
site opposite the blastopore end of the embryo and the mesoderm pinches off to form the coelom during the process
of enterocoely.
A second distinction between the types of cleavage in protostomes and deuterostomes relates to the fate of the
resultant blastomeres (cells produced by cleavage). In addition to spiral cleavage, protostomes also undergo
determinate cleavage. This means that even at this early stage, the developmental fate of each embryonic
cell is already determined. A given cell does not have the ability to develop into any cell type other than its
original destination. Removal of a blastomere from an embryo with determinate cleavage can result in missing
structures, and embryos that fail to develop. In contrast, deuterostomes undergo indeterminate cleavage, in
which cells are not yet fully committed at this early stage to develop into specific cell types. Removal of individual
blastomeres from these embryos does not result in the loss of embryonic structures. In fact, twins (clones)
can be produced as a result from blastomeres that have been separated from the original mass of blastomere
cells. Unlike protostomes, however, if some blastomeres are damaged during embryogenesis, adjacent cells
are able to compensate for the missing cells, and the embryo is not damaged. These cells are referred to as
undetermined cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem
cells, which have the ability to develop into any cell type until their fate is programmed at a later developmental
stage.
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V /
e olution CONNECTION
The Evolution of the Coelom
One of the first steps in the classification of animals is to examine the animal’s body. One structure that
is used in classification of animals is the body cavity or coelom. The body cavity develops within the
mesoderm, so only triploblastic animals can have body cavities. Therefore body cavities are found only
within the Bilateria. In other animal clades, the gut is either close to the body wall or separated from it by
a jelly-like material. The body cavity is important for two reasons. Fluid within the body cavity protects the
organs from shock and compression. In addition, since in triploblastic embryos, most muscle, connective
tissue, and blood vessels develop from mesoderm, these tissues developing within the lining of the body
cavity can reinforce the gut and body wall, aid in motility, and efficiently circulate nutrients.
To recap what we have discussed above, animals that do not have a coelom are called acoelomates. The
major acoelomate group in the Bilateria is the flatworms, including both free-living and parasitic forms such
as tapeworms. In these animals, mesenchyme fills the space between the gut and the body wall. Although
two layers of muscle are found just under the epidermis, there is no muscle or other mesodermal tissue
around the gut. Flatworms rely on passive diffusion for nutrient transport across their body.
In pseudocoelomates, there is a body cavity between the gut and the body wall, but only the body wall
has mesodermal tissue. In these animals, the mesoderm forms, but does not develop cavities within
it. Major pseudocoelomate phyla are the rotifers and nematodes. Animals that have a true coelom are
called eucoelomates] all vertebrates, as well as molluscs, annelids, arthropods, and echinoderms, are
eucoelomates. The coelom develops within the mesoderm during embryogenesis. Of the major bilaterian
phyla, the molluscs, annelids, and arthropods are schizocoels, in which the mesoderm splits to form the
body cavity, while the echinoderms and chordates are enterocoels, in which the mesoderm forms as two or
more buds off of the gut. These buds separate from the gut and coalesce to form the body cavity. In the
vertebrates, mammals have a subdivided body cavity, with the thoracic cavity separated from the abdominal
cavity. The pseudocoelomates may have had eucoelomate ancestors and may have lost their ability to form
a complete coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the
coelom—has had a large evolutionary impact on the various species of the animal kingdom.
27.3 | Animal Phylogeny
By the end of this section, you will be able to do the following:
• Interpret the metazoan phylogenetic tree
• Describe the types of data that scientists use to construct and revise animal phylogeny
• List some of the relationships within the modern phylogenetic tree that have been discovered as a result
of modern molecular data
Biologists strive to understand the evolutionary history and relationships of members of the animal kingdom,
and all of life, for that matter. The study of phylogeny (the branching sequence of evolution) aims to determine
the evolutionary relationships between phyla. Currently, most biologists divide the animal kingdom into 35 to 40
phyla. Scientists develop phylogenetic trees, which serve as hypotheses about which species have evolved from
which ancestors.
Recall that until recently, only morphological characteristics and the fossil record were used to determine
phylogenetic relationships among animals. Scientific understanding of the distinctions and hierarchies between
anatomical characteristics provided much of this knowledge. Used alone, however, this information can be
misleading. Morphological characteristics (such as skin color, body shape, etc.) may evolve multiple times, and
independently, through evolutionary history. Analogous characteristics may appear similar between animals,
but their underlying evolution may be very different. With the advancement of molecular technologies, modern
phylogenetics is now informed by genetic and molecular analyses, in addition to traditional morphological and
fossil data. With a growing understanding of genetics, the animal evolutionary tree has changed substantially
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and continues to change as new DNA and RNA analyses are performed on additional animal species.
Constructing an Animal Phylogenetic Tree
The current understanding of evolutionary relationships among animal, or Metazoa, phyla begins with the
distinction between animals with true differentiated tissues, called Eumetazoa, and animal phyla that do not
have true differentiated tissues, such as the sponges ( Porifera) and the Placozoa. Similarities between the
feeding cells of sponges (choanocytes) and choanoflagellate protists (Figure 27.12) have been used to suggest
that Metazoa evolved from a common ancestral organism that resembled the moderncolonial choanoflagellates.
Choanoflagellate
(protist)
Sponge choanocyte cell
(animal)
Sponge
Figure 27.12 Choanoflagellates and choanocytes. Cells of the protist choanoflagellate clade closely resemble sponge
choanocyte cells. Beating of choanocyte flagella draws water through the sponge so that nutrients can be extracted
and waste removed.
Eumetazoa are subdivided into radially symmetrical animals and bilaterally symmetrical animals, and are
thus classified into the clades Bilateria and Radiata, respectively. As mentioned earlier, the cnidarians and
ctenophores are animal phyla with true radial, biradial, or rotational symmetry. All other Eumetazoa are members
of the Bilateria clade. The bilaterally symmetrical animals are further divided into deuterostomes (including
chordates and echinoderms) and two distinct clades of protostomes (including ecdysozoans and
lophotrochozoans) (Figure 27. 13a, b). Ecdysozoa includes nematodes and arthropods; they are so named for
a commonly found characteristic among the group: the physiological process of exoskeletal molting followed by
the “stripping” of the outer cuticular layer, called ecdysis. Lophotrochozoa is named for two structural features,
each common to certain phyla within the clade. Some lophotrochozoan phyla are characterized by a larval stage
called trochophore larvae, and other phyla are characterized by the presence of a feeding structure called a
lophophore (thus, the shorter term, “lopho-trocho-zoa").
Figure 27.13 Ecdysozoa. Animals that molt their exoskeletons, such as these (a) Madagascar hissing cockroaches,
are in the clade Ecdysozoa. (b) Phoronids are in the clade Lophotrochozoa. The tentacles are part of a feeding
structure called a lophophore. (credit a: modification of work by Whitney Cranshaw, Colorado State University,
Bugwood.org; credit b: modification of work by NOAA)
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LINK TQ LEARNING
Explore an interactive tree (http://openstaxcollege.Org/l/tree_of_life 2 ) of life here. Zoom and click to learn
more about the organisms and their evolutionary relationships.
Modern Advances in Phylogenetic Understanding Come from
Molecular Analyses
The phylogenetic groupings are continually being debated and refined by evolutionary biologists. Each year, new
evidence emerges that further alters the relationships described by a phylogenetic tree diagram.
LINK TQ LEARNING
Watch the following video (http://0penstaxc0llege.0rg/l/build phylogeny) to learn how biologists use
genetic data to determine relationships among organisms.
Nucleic acid and protein analyses have greatly modified and refined the modern phylogenetic animal tree. These
data come from a variety of molecular sources, such as mitochondrial DNA, nuclear DNA, ribosomal RNA
(rRNA), and certain cellular proteins. Many evolutionary relationships in the modern tree have only recently
been determined from the molecular evidence. For example, a previously classified group of animals called
lophophorates, which included brachiopods and bryozoans, were long-thought to be primitive deuterostomes.
Extensive molecular analysis using rRNA data found these animals are actually protostomes, more closely
related to annelids and mollusks. This discovery allowed for the distinction of the protostome clade
Lophotrochozoa. Molecular data have also shed light on some differences within the lophotrochozoan group,
and the placement of the Platyhelminthes is particularly problematic. Some scientists believe that the phyla
Platyhelminthes and Rotifera should actually belong to their own clade of protostomes termed Platyzoa.
Molecular research similar to the discoveries that brought about the distinction of the lophotrochozoan clade
has also revealed a dramatic rearrangement of the relationships between mollusks, annelids, arthropods, and
nematodes, and as a result, a new ecdysozoan clade was formed. Due to morphological similarities in their
segmented body types, annelids and arthropods were once thought to be closely related. However, molecular
evidence has revealed that arthropods are actually more closely related to nematodes, now comprising the
ecdysozoan clade, and annelids are more closely related to mollusks, brachiopods, and other phyla in the
lophotrochozoan clade. These two clades now make up the protostomes.
Another change to former phylogenetic groupings because of modern molecular analyses includes the
emergence of an entirely new phylum of worm called Acoelomorpha. These acoel flatworms were long thought
to belong to the phylum Platyhelminthes because of their similar “flatworm” morphology. However, molecular
analyses revealed this to be a false relationship and originally suggested that acoels represented living species
of some of the earliest divergent bilaterians. More recent research into the acoelomorphs has called this
hypothesis into question and suggested that the acoels are more closely related to deuterostomes. The
placement of this new phylum remains disputed, but scientists agree that with sufficient molecular data, their true
phylogeny will be determined.
Another example of phylogenetic reorganization involves the identification of the Ctenophora as the basal clade
of the animal kingdom. Ctenophora, or comb jellies, were once considered to be a sister group of the Cnidaria,
and the sponges (Porifera) were placed as the basal animal group, sister to other animals. The presence of
nerve and muscle cells in both the Ctenophores and the Cnidaria and their absence in the Porifera strengthened
this view of the relationships among simple animal forms. However, recent molecular analysis has shown that
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many of the genes that support neural development in other animals are absent from the Ctenophore genome.
The muscle cells are restricted to the mouth and tentacles and are derived from cells in the mesoglea. The
mitochondrial genome of the Ctenophores is small and lacks many genes found in other animal mitochondrial
genomes. These features plus the absence of Hox genes from the Ctenophores have been used to argue that
the Ctenophores should be considered basal or as a sister group of the Porifera, and that the evolution of
specialized nerve and muscle tissue may have occurred more than once in the history of animal life. Although
Ctenophores have been shown as basal to other animals in the phylogeny presented in Chapter 27.2, debate
on this issue is likely to continue as Ctenophores are more closely studied.
Changes to the phylogenetic tree can be difficult to track and understand, and are evidence of the process
of science. Data and analytical methods play a significant role in the development of phylogenies. For this
reason - because molecular analysis and reanalysis are not complete -- we cannot necessarily dismiss a
former phylogenetic tree as inaccurate. A recent reanalysis of molecular evidence by an international group of
evolutionary biologists refuted the proposition that comb jellies are the phylogenetically oldest extant metazoan
group. The study, which relied on more sophisticated methods of analyzing the original genetic data, reaffirms
the traditional view that the sponges were indeed the first phylum to diverge from the common ancestor of
metazoans. The ongoing discussion concerning the location of sponges and comb jellies on the animal “family
tree” is an example of what drives science forward.
27.4 | The Evolutionary History of the Animal Kingdom
By the end of this section, you will be able to do the following:
• Describe the features that characterized the earliest animals and approximately when they appeared on
earth
• Explain the significance of the Cambrian period for animal evolution and the changes in animal diversity
that took place during that time
• Describe some of the unresolved questions surrounding the Cambrian explosion
• Discuss the implications of mass animal extinctions that have occurred in evolutionary history
Many questions regarding the origins and evolutionary history of the animal kingdom continue to be researched
and debated, as new fossil and molecular evidence change prevailing theories. Some of these questions include
the following: How long have animals existed on Earth? What were the earliest members of the animal kingdom,
and what organism was their common ancestor? While animal diversity increased during the Cambrian period of
the Paleozoic era, 530 million years ago, modern fossil evidence suggests that primitive animal species existed
much earlier.
Pre-Cambrian Animal Life
The time before the Cambrian period is known as the Ediacaran Period (from about 635 million years ago to
543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (Figure 27.14). Ediacaran
fossils were first found in the Ediacaran hills of Southern Australia. There are no living representatives of these
species, which have left impressions that look like those of feathers or coins (Figure 27.15). It is believed that
early animal life, termed Ediacaran biota, evolved from protists at this time.
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EON
ERA
PERIOD
MILLIONS Of
YE ARS AGO
Phanerozoic
Cenozoic
yualoman
Tertian
— 1.6 —
— 66 —
— 138 --
— 205 --
— 240 --
— 290 --
Mesozoic
Cretaceous
Jurassic
Triass ic
Paleozoic
Permian
Pennsylvanian
Mississippiun
Devonian
Silurian
Ordovician
Cambrian
— 360
— 410 -
--435
— 500
— 540 -
— 2500-
Proterozoic
l.alr Proterozoic
Middle Proterozoic
t»rlv Proterozoic
F.diacaran
635-543 MYA
Archcan
1 air Vrrhean
Middle A it bean
Earlv Are bean
Pre-Archean
(a)
Figure 27.14 An evolutionary timeline, (a) Earth’s history is divided into eons, eras, and periods. Note that the
Ediacaran period starts in the Proterozoic eon and ends in the Cambrian period of the Phanerozoic eon. (b) Stages on
the geological time scale are represented as a spiral, (credit: modification of work by USGS)
Most Ediacaran biota were just a few mm or cm long, but some of the feather-like forms could reach lengths of
over a meter. Recently there has been increasing scientific evidence suggesting that more varied and complex
animal species lived during this time, and likely even before the Ediacaran period.
Fossils believed to represent the oldest animals with hard body parts were recently discovered in South
Australia. These sponge-like fossils, named Coronacollina acula, date back as far as 560 million years, and are
believed to show the existence of hard body parts and spicules that extended 20-40 cm from the thimble-shaped
body (estimated about 5 cm long). Other fossils from the Ediacaran period are shown in Figure 27.15a, b, c.
(a) (b) (c)
Figure 27.15 Ediacaran fauna. Fossils of (a) Cyclomedusa (up to 20 cm), (b) Dickinsonia (up to 1.4 m), (and
(c) Spriggina (up to 5 cm) date to the Ediacaran period (543-635 MYA). (credit: modification of work by
“Smith6097Wikimedia Commons)
Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this
claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like
creatures, irregularly shaped and with internal tubes or canals. These ancient fossils from South Australia date
back 650 million years, actually placing the putative animal before the great ice age extinction event that marked
the transition between the Cryogenian period and the Ediacaran period. Until this discovery, most scientists
believed that there was no animal life prior to the Ediacaran period. Many scientists now believe that animals
may in fact have evolved during the Cryogenian period.
The Cambrian Explosion of Animal Life
If the fossils of the Ediacaran and Cryogenian periods are enigmatic, those of the following Cambrian period
are far less so, and include body forms similar to those living today. The Cambrian period, occurring between
approximately 542-488 million years ago, marks the most rapid evolution of new animal phyla and animal
diversity in Earth’s history. The rapid diversification of animals that appeared during this period, including most
of the animal phyla in existence today, is often referred to as the Cambrian explosion (Figure 27.16). Animals
resembling echinoderms, mollusks, worms, arthropods, and chordates arose during this period. What may have
been a top predator of this period was an arthropod-like creature named Anomalocaris, over a meter long,
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with compound eyes and spiky tentacles. Obviously, all these Cambrian animals already exhibited complex
structures, so their ancestors must have existed much earlier.
Figure 27.16 Fauna of the Burgess Shale. An artist’s rendition depicts some organisms from the Cambrian period.
Anomalocaris is seen in the upper left quadrant of the picture.
One of the most dominant species during the Cambrian period was the trilobite, an arthropod that was among
the first animals to exhibit a sense of vision (Figure 27.17a,b,c,d). Trilobites were somewhat similar to modern
horseshoe crabs. Thousands of different species have been identified in fossil sediments of the Cambrian period;
not a single species survives today.
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Figure 27.17 Trilobites. These fossils (a-d) belong to trilobites, extinct arthropods that appeared in the early Cambrian
period, 525 million years ago, and disappeared from the fossil record during a mass extinction at the end of the
Permian period, about 250 million years ago.
The cause of the Cambrian explosion is still debated, and in fact, it may be that a number of interacting causes
ushered in this incredible explosion of animal diversity. For this reason, there are a number of hypotheses
that attempt to answer this question. Environmental changes may have created a more suitable environment
for animal life. Examples of these changes include rising atmospheric oxygen levels (Figure 27.18) and large
increases in oceanic calcium concentrations that preceded the Cambrian period. Some scientists believe that
an expansive, continental shelf with numerous shallow lagoons or pools provided the necessary living space
for larger numbers of different types of animals to coexist. There is also support for hypotheses that argue that
ecological relationships between species, such as changes in the food web, competition for food and space,
and predator-prey relationships, were primed to promote a sudden massive coevolution of species. Yet other
hypotheses claim genetic and developmental reasons for the Cambrian explosion. The morphological flexibility
and complexity of animal development afforded by the evolution of Hox control genes may have provided the
necessary opportunities for increases in possible animal morphologies at the time of the Cambrian period.
Hypotheses that attempt to explain why the Cambrian explosion happened must be able to provide valid reasons
for the massive animal diversification, as well as explain why it happened when it did. There is evidence that both
supports and refutes each of the hypotheses described above, and the answer may very well be a combination
of these and other theories.
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Oxygen Content of Earth’s Atmosphere
Millions of years before present
Figure 27.18 Atmospheric oxygen over time. The oxygen concentration in Earth’s atmosphere rose sharply around
300 million years ago.
However, unresolved questions about the animal diversification that took place during the Cambrian period
remain. For example, we do not understand how the evolution of so many species occurred in such a short
period of time. Was there really an “explosion” of life at this particular time? Some scientists question the
validity of this idea, because there is increasing evidence to suggest that more animal life existed prior to the
Cambrian period and that other similar species’ so-called explosions (or radiations) occurred later in history as
well. Furthermore, the vast diversification of animal species that appears to have begun during the Cambrian
period continued well into the following Ordovician period. Despite some of these arguments, most scientists
agree that the Cambrian period marked a time of impressively rapid animal evolution and diversification of body
forms that is unmatched for any other time period.
LINK TQ LEARNING
View an animation of what ocean life may have been like during the Cambrian explosion. (This multimedia
resource will open in a browser.) (http://cnx.Org/content/m66586/l.3/#eip-idll69840612792)
Post-Cambrian Evolution and Mass Extinctions
The periods that followed the Cambrian during the Paleozoic Era are marked by further animal evolution and
the emergence of many new orders, families, and species. As animal phyla continued to diversify, new species
adapted to new ecological niches. During the Ordovician period, which followed the Cambrian period, plant life
first appeared on land. This change allowed formerly aquatic animal species to invade land, feeding directly on
plants or decaying vegetation. Continual changes in temperature and moisture throughout the remainder of the
Paleozoic Era due to continental plate movements encouraged the development of new adaptations to terrestrial
existence in animals, such as limbed appendages in amphibians and epidermal scales in reptiles.
Changes in the environment often create new niches (diversified living spaces) that invite rapid speciation
and increased diversity. On the other hand, cataclysmic events, such as volcanic eruptions and meteor strikes
that obliterate life, can result in devastating losses of diversity to some clades, yet provide new opportunities
for others to “fill in the gaps” and speciate. Such periods of mass extinction (Figure 27.19) have occurred
repeatedly in the evolutionary record of life, erasing some genetic lines while creating room for others to evolve
into the empty niches left behind. The end of the Permian period (and the Paleozoic Era) was marked by the
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largest mass extinction event in Earth’s history, a loss of an estimated 95 percent of the extant species at that
time. Some of the dominant phyla in the world’s oceans, such as the trilobites, disappeared completely. On land,
the disappearance of some dominant species of Permian reptiles made it possible for a new line of reptiles
to emerge, the dinosaurs. The warm and stable climatic conditions of the ensuing Mesozoic Era promoted an
explosive diversification of dinosaurs into every conceivable niche in land, air, and water. Plants, too, radiated
into new landscapes and empty niches, creating complex communities of producers and consumers, some of
which became very large on the abundant food available.
Another mass extinction event occurred at the end of the Cretaceous period, bringing the Mesozoic Era to an
end. Skies darkened and temperatures fell after a large meteor impact and tons of volcanic ash ejected into
the atmosphere blocked incoming sunlight. Plants died, herbivores and carnivores starved, and the dinosaurs
ceded their dominance of the landscape to the more warm-blooded mammals. In the following Cenozoic Era,
mammals radiated into terrestrial and aquatic niches once occupied by dinosaurs, and birds—the warm-blooded
direct descendants of one line of the ruling reptiles—became aerial specialists. The appearance and dominance
of flowering plants in the Cenozoic Era created new niches for pollinating insects, as well as for birds and
mammals. Changes in animal species diversity during the late Cretaceous and early Cenozoic were also
promoted by a dramatic shift in Earth’s geography, as continental plates slid over the crust into their current
positions, leaving some animal groups isolated on islands and continents, or separated by mountain ranges
or inland seas from other competitors. Early in the Cenozoic, new ecosystems appeared, with the evolution of
grasses and coral reefs. Late in the Cenozoic, further extinctions followed by speciation occurred during ice ages
that covered high latitudes with ice and then retreated, leaving new open spaces for colonization.
LINK TQ LEARNING
Watch the following video (http:// 0 penstaxc 0 llege. 0 rg/l/mass_extincti 0 n) to learn more about the mass
extinctions.
Figure 27.19 Extinctions. Mass extinctions have occurred repeatedly over geological time.
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ca eer connection
Paleontologist
Natural history museums contain the fossils of extinct animals as well as information about how these
animals evolved, lived, and died. Paleontologists are scientists who study prehistoric life. They use fossils
to observe and explain how life evolved on Earth and how species interacted with each other and with
the environment. A paleontologist needs to be knowledgeable in mathematics, biology, ecology, chemistry,
geology, and many other scientific disciplines. A paleontologist’s work may involve field studies: searching
for and studying fossils. In addition to digging for and finding fossils, paleontologists also prepare fossils
for further study and analysis. Although dinosaurs are probably the first animals that come to mind when
thinking about ancient life, paleontologists study a variety of life forms, from plants, fungi and invertebrates
to the vertebrate fishes, amphibians, reptiles, birds and mammals.
An undergraduate degree in earth science or biology is a good place to start toward the career path of
becoming a paleontologist. Most often, a graduate degree is necessary. Additionally, work experience in a
museum or in a paleontology lab is useful.
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KEY TERMS
acoelomate animal without a body cavity
bilateral symmetry type of symmetry in which there is only one plane of symmetry, so the left and right halves
of an animal are mirror images
blastopore opening into the archenteron that forms during gastrulation
blastula 16-32 cell stage of development of an animal embryo
body plan morphology or defining shape of an organism
Cambrian explosion time during the Cambrian period (542-488 million years ago) when most of the animal
phyla in existence today evolved
cleavage cell divisions subdividing a fertilized egg (zygote) to form a multicellular embryo
coelom lined body cavity
Cryogenian period geologic period (850-630 million years ago) characterized by a very cold global climate
determinate cleavage cleavage pattern in which developmental fate of each blastomere is tightly defined
deuterostome blastopore develops into the anus, with the second opening developing into the mouth
diploblast animal that develops from two germ layers
Ecdysozoa clade of protostomes that exhibit exoskeletal molting (ecdysis)
Ediacaran period geological period (630-542 million years ago) when the oldest definite multicellular
organisms with tissues evolved
enterocoely mesoderm of deuterostomes develops as pouches that are pinched off from endodermal tissue,
cavity contained within the pouches becomes coelom
eucoelomate animal with a body cavity completely lined with mesodermal tissue
Eumetazoa group of animals with true differentiated tissues
gastrula stage of animal development characterized by the formation of the digestive cavity
germ layer collection of cells formed during embryogenesis that will give rise to future body tissues, more
pronounced in vertebrate embryogenesis
Hox gene (also, homeobox gene) master control gene that can turn on or off large numbers of other genes
during embryogenesis
indeterminate cleavage cleavage pattern in which individual blastomeres have the character of "stem cells,"
and are not yet predetermined to develop into specific cell types
Lophotrochozoa clade of protostomes that exhibit a trochophore larvae stage or a lophophore feeding
structure
mass extinction event or environmental condition that wipes out the majority of species within a relatively short
geological time period
Metazoa group containing all animals
organogenesis formation of organs in animal embryogenesis
Parazoa group of animals without true differentiated tissues
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protostome blastopore develops into the mouth of protostomes, with the second opening developing into the
anus
pseudocoelomate animal with a body cavity located between the mesoderm and endoderm
radial cleavage cleavage axes are parallel or perpendicular to the polar axis, resulting in the alignment of cells
between the two poles
radial symmetry type of symmetry with multiple planes of symmetry, with body parts (rays) arranged around a
central disk
schizocoely during development of protostomes, a solid mass of mesoderm splits apart and forms the hollow
opening of the coelom
spiral cleavage cells of one pole of the embryo are rotated or misaligned with respect to the cells of the
opposite pole
triploblast animal that develops from three germ layers
CHAPTER SUMMARY
27.1 Features of the Animal Kingdom
Animals constitute an incredibly diverse kingdom of organisms. Although animals range in complexity from
simple sea sponges to human beings, most members of the animal kingdom share certain features. Animals
are eukaryotic, multicellular, heterotrophic organisms that ingest their food and usually develop into motile
creatures with a fixed body plan. A major characteristic unique to the animal kingdom is the presence of
differentiated tissues, such as nerve, muscle, and connective tissues, which are specialized to perform specific
functions. Most animals undergo sexual reproduction, leading to a series of developmental embryonic stages
that are relatively similar across the animal kingdom. A class of transcriptional control genes called Hox genes
directs the organization of the major animal body plans, and these genes are strongly homologous across the
animal kingdom.
27.2 Features Used to Classify Animals
Organisms in the animal kingdom are classified based on their body morphology, their developmental
pathways, and their genetic affinities. The relationships between the Eumetazoa and more basal clades
(Ctenophora, Porifera, and Placozoa) are still being debated. The Eumetazoa ("true animals") are divided into
those with radial versus bilateral symmetry. Generally, the simpler and often nonmotile animals display radial
symmetry, which allows them to explore their environment in all directions. Animals with radial symmetry are
also generally characterized by the development of two embryological germ layers, the endoderm and
ectoderm, whereas animals with bilateral symmetry are generally characterized by the development of a third
embryologic germ layer, the mesoderm. Animals with three germ layers, called triploblasts, are further
characterized by the presence or absence of an internal body cavity called a coelom. The presence of a
coelom affords many advantages, and animals with a coelom may be termed true coelomates or
pseudocoelomates, depending the extent to which mesoderm lines the body cavity. Coelomates are further
divided into one of two groups called protostomes and deuterostomes, based on a number of developmental
characteristics, including differences in zygote cleavage, the method of coelom formation, and the rigidity of the
developmental fate of blastomeres.
27.3 Animal Phylogeny
Scientists are interested in the evolutionary history of animals and the evolutionary relationships among them.
There are three main sources of data that scientists use to create phylogenetic evolutionary tree diagrams that
illustrate such relationships: morphological information (which includes developmental morphologies), fossil
record data, and, most recently, molecular data. The details of the modern phylogenetic tree change frequently
as new data are gathered, and molecular data has recently contributed to many substantial modifications of the
understanding of relationships between animal phyla.
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27.4 The Evolutionary History of the Animal Kingdom
The most rapid documented diversification and evolution of animal species in all of history occurred during the
Cambrian period of the Paleozoic Era, a phenomenon known as the Cambrian explosion. Until recently,
scientists believed that there were only very few tiny and simplistic animal species in existence before this
period. However, recent fossil discoveries have revealed that additional, larger, and more complex animals
existed during the Ediacaran period, and even possibly earlier, during the Cryogenian period. Still, the
Cambrian period undoubtedly witnessed the emergence of the majority of animal phyla that we know today,
although many questions remain unresolved about this historical phenomenon.
The remainder of the Paleozoic Era is marked by the growing appearance of new classes, families, and
species, and the early colonization of land by certain marine animals and semiaquatic arthropods, both
freshwater and marine. The evolutionary history of animals is also marked by numerous major extinction
events, each of which wiped out a majority of extant species. Some species of most animal phyla survived
these extinctions, allowing the phyla to persist and continue to evolve into species that we see today.
VISUAL CONNECTION QUESTIONS
1. Figure 27.5 If a Hox 13 gene in a mouse was
replaced with a Hox 1 gene, how might this alter
animal development?
2. Figure 27.6 Which of the following statements is
false?
a. Eumetazoans have specialized tissues and
parazoans don’t.
b. Lophotrochozoa and Ecdysozoa are both
Bilataria.
c. Acoela and Cnidaria both possess radial
symmetry.
d. Arthropods are more closely related to
nematodes than they are to annelids.
REVIEW QUESTIONS
4. Which of the following is not a feature common to
most animals?
a. development into a fixed body plan
b. asexual reproduction
c. specialized tissues
d. heterotrophic nutrient sourcing
5. During embryonic development, unique cell layers
develop into specific groups of tissues or organs
during a stage called_.
a. the blastula stage
b. the germ layer stage
c. the gastrula stage
d. the organogenesis stage
6. Which of the following phenotypes would most
likely be the result of a Hox gene mutation?
a. abnormal body length or height
b. two different eye colors
c. the contraction of a genetic illness
d. two fewer appendages than normal
7. Which of the following organisms is most likely to
be a diploblast?
3. Figure 27.9 Which of the following statements
about diploblasts and triploblasts is false?
a. Animals that display radial symmetry are
diploblasts.
b. Animals that display bilateral symmetry are
triploblasts.
c. The endoderm gives rise to the lining of the
digestive tract and the respiratory tract.
d. The mesoderm gives rise to the central
nervous system.
a. sea star
b. shrimp
c. jellyfish
d. insect
8. Which of the following is not possible?
a. radially symmetrical diploblast
b. diploblastic eucoelomate
c. protostomic coelomate
d. bilaterally symmetrical deuterostome
9. An animal whose development is marked by radial
cleavage and enterocoely is_.
a. a deuterostome
b. an annelid or mollusk
c. either an acoelomate or eucoelomate
d. none of the above
10. Consulting the modern phylogenetic tree of
animals, which of the following would not constitute a
clade?
a. deuterostomes
b. lophotrochozoans
c. Parazoa
d. Bilateria
11. Which of the following is thought to be the most
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closely related to the common animal ancestor?
a. fungal cells
b. protist cells
c. plant cells
d. bacterial cells
12. As with the emergence of the Acoelomorpha
phylum, it is common for_data to misplace
animals in close relation to other species, whereas
_data often reveals a different and more
accurate evolutionary relationship.
a. molecular: morphological
b. molecular: fossil record
c. fossil record : morphological
d. morphological : molecular
13. Which of the following periods is the earliest
during which animals may have appeared?
a. Ordovician period
b. Cambrian period
c. Ediacaran period
d. Cryogenian period
14. What type of data is primarily used to determine
the existence and appearance of early animal
species?
a. molecular data
b. fossil data
c. morphological data
d. embryological development data
CRITICAL THINKING QUESTIONS
19. Why might the evolution of specialized tissues be
important for animal function and complexity?
20. Describe and give examples of how humans
display all of the features common to the animal
kingdom.
21. How have Hox genes contributed to the diversity
of animal body plans?
22. Using the following terms, explain what
classifications and groups humans fall into, from the
most general to the most specific: symmetry, germ
layers, coelom, cleavage, embryological
development.
23. Explain some of the advantages brought about
through the evolution of bilateral symmetry and
15. The time between 542-488 million years ago
marks which period?
a. Cambrian period
b. Silurian period
c. Ediacaran period
d. Devonian period
16. Until recent discoveries suggested otherwise,
animals existing before the Cambrian period were
believed to be:
a. small and ocean-dwelling
b. small and nonmotile
c. small and soft-bodied
d. small and radially symmetrical or
asymmetrical
17. Plant life first appeared on land during which of
the following periods?
a. Cambrian period
b. Ordovician period
c. Silurian period
d. Devonian period
18. Approximately how many mass extinction events
occurred throughout the evolutionary history of
animals?
a. 3
b. 4
c. 5
d. more than 5
coelom formation.
24. Describe at least two major changes to the
animal phylogenetic tree that have come about due
to molecular or genetic findings.
25. How is it that morphological data alone might
lead scientists to group animals into erroneous
evolutionary relationships?
26. Briefly describe at least two theories that attempt
to explain the cause of the Cambrian explosion.
27. How is it that most, if not all, of the extant animal
phyla today evolved during the Cambrian period if so
many massive extinction events have taken place
since then?
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Chapter 28 | Invertebrates
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28 | INVERTEBRATES
Figure 28.1 Nearly 97 percent of animal species are invertebrates, including this sea star (Astropecten articulatus)
common to the eastern and southern coasts of the United States (credit: modification of work by Mark Walz)
Chapter Outline
28.1: Phylum Porifera
28.2: Phylum Cnidaria
28.3: Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans
28.4: Superphylum Lophotrochozoa: Molluscs and Annelids
28.5: Superphylum Ecdysozoa: Nematodes and Tardigrades
28.6: Superphylum Ecdysozoa: Arthropods
28.7: Superphylum Deuterostomia
Introduction
A brief look at any magazine pertaining to our natural world, such as National Geographic , would show a rich
variety of vertebrates, especially mammals and birds. To most people, these are the animals that attract our
attention. Concentrating on vertebrates, however, gives us a rather biased and limited view of animal diversity,
because it ignores nearly 97 percent of the animal kingdom—the invertebrates—animals that lack a cranium and
a defined vertebral column or spine.
The invertebrate animal phyla exhibit an enormous variety of cells and tissues adapted for specific purposes, and
frequently these tissues are unique to their phyla. These specializations show the range of cellular differentiation
possible within the clade Opisthokonta, which has both unicellular and multicellular members. Cellular and
structural specializations include cuticles for protection, spines and tiny harpoons for defense, toothy structures
for feeding, and wings for flight. An exoskeleton may be adapted for movement or for the attachment of muscles
as in the clams and insects. Secretory cells can produce venom, mucus, or digestive enzymes. The body plans
of some phyla, such as those of the molluscs, annelids, arthropods, and echinoderms, have been modified and
adapted throughout evolution to produce thousands of different forms. Perhaps you will find it amazing that an
enormous number of both aquatic and terrestrial invertebrates—perhaps millions of species—have not yet been
scientifically classified. As a result, the phylogenetic relationships among the invertebrates are constantly being
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updated as new information is collected about the organisms of each phylum.
28.1 1 Phylum Porifera
By the end of this section, you will be able to do the following:
• Describe the organizational features of the simplest multicellular organisms
• Explain the various body forms and bodily functions of sponges
As we have seen, the vast majority of invertebrate animals do not possess a defined bony vertebral
endoskeleton, or a bony cranium. However, one of the most ancestral groups of deuterostome invertebrates, the
Echinodermata, do produce tiny skeletal “bones” called ossicles that make up a true endoskeleton, or internal
skeleton, covered by an epidermis.
We will start our investigation with the simplest of all the invertebrates—animals sometimes classified within
the clade Parazoa (“beside the animals”). This clade currently includes only the phylum Placozoa (containing a
single species, Trichoplax adhaerens), and the phylum Porifera, containing the more familiar sponges (Figure
28.2). The split between the Parazoa and the Eumetazoa (all animal clades above Parazoa) likely took place
over a billion years ago.
We should reiterate here that the Porifera do not possess “true" tissues that are embryologically homologous
to those of all other derived animal groups such as the insects and mammals. This is because they do not
create a true gastrula during embryogenesis, and as a result do not produce a true endoderm or ectoderm.
But even though they are not considered to have true tissues, they do have specialized cells that perform
specific functions like tissues (for example, the external “pinacoderm” of a sponge acts like our epidermis). Thus,
functionally, the poriferans can be said to have tissues; however, these tissues are likely not embryologically
homologous to our own.
Sponge larvae (e.g, parenchymula and amphiblastula) are flagellated and able to swim; however, adults are
non-motile and spend their life attached to a substratum. Since water is vital to sponges for feeding, excretion,
and gas exchange, their body structure facilitates the movement of water through the sponge. Various canals,
chambers, and cavities enable water to move through the sponge to allow the exchange of food and waste as
well as the exchange of gases to nearly all body cells.
Figure 28.2 Sponges. Sponges are members of the phylum Porifera, which contains the simplest invertebrates, (credit:
Andrew Turner)
Morphology of Sponges
There are at least 5,000 named species of sponges, likely with thousands more yet to be classified. The
morphology of the simplest sponges takes the shape of an irregular cylinder with a large central cavity, the
spongocoel, occupying the inside of the cylinder (Figure 28.3). Water enters into the spongocoel through
numerous pores, or ostia, that create openings in the body wall. Water entering the spongocoel is expelled via a
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large common opening called the osculum. However, we should note that sponges exhibit a range of diversity in
body forms, including variations in the size and shape of the spongocoel, as well as the number and arrangement
of feeding chambers within the body wall. In some sponges, multiple feeding chambers open off of a central
spongocoel and in others, several feeding chambers connecting to one another may lie between the entry pores
and the spongocoel.
While sponges do not exhibit true tissue-layer organization, they do have a number of functional “tissues”
composed of different cell types specialized for distinct functions. For example, epithelial-like cells called
pinacocytes form the outermost body, called a pinacoderm, that serves a protective function similar that of
our epidermis. Scattered among the pinacoderm are the ostia that allow entry of water into the body of the
sponge. These pores have given the sponges their phylum name Porifera—pore-bearers, in some sponges,
ostia are formed by porocytes, single tube-shaped cells that act as valves to regulate the flow of water into
the spongocoel. In other sponges, ostia are formed by folds in the body wall of the sponge. Between the outer
layer and the feeding chambers of the sponge is a jelly-like substance called the mesohyl, which contains
collagenous fibers. Various cell types reside within the mesohyl, including amoebocytes, the “stem cells" of
sponges, and sclerocytes, which produce skeletal materials. The gel-like consistency of mesohyl acts like an
endoskeleton and maintains the tubular morphology of sponges.
The feeding chambers inside the sponge are lined by choanocytes ("collar cells"). The structure of a choanocyte
is critical to its function, which is to generate a directed water current through the sponge and to trap and
ingest microscopic food particles by phagocytosis. These feeding cells are similar in appearance to unicellular
choanoflageilates (Protista). This similarity suggests that sponges and choanoflagellates are closely related
and likely share common ancestry. The body of the choanocyte is embedded in mesohyl and contains all the
organelles required for normal cell function. Protruding into the “open space" inside the feeding chamber is
a mesh-like collar composed of microvilli with a single flagellum in the center of the column. The beating of
the flagella from all choanocytes draws water into the sponge through the numerous ostia, into the spaces
lined by choanocytes, and eventually out through the osculum (or osculi, if the sponge consists of a colony of
attached sponges). Food particles, including waterborne bacteria and unicellular organisms such as algae and
various animal-like protists, are trapped by the sieve-like collar of the choanocytes, slide down toward the body
of the cell, and are ingested by phagocytosis. Choanocytes also serve another surprising function: They can
differentiate into sperm for sexual reproduction, at which time they become dislodged from the mesohyl and
leave the sponge with expelled water through the osculum.
Watch this video to see the movement of water through the sponge body. (This multimedia resource will
open in a browser.) (http://cnx.Org/content/m66394/l.3/#eip-id7448133)
The amoebocytes (derived from stem-cell-like archaeocytes), are so named because they move throughout
the mesohyl in an amoeba-like fashion. They have a variety of functions: In addition to delivering nutrients from
choanocytes to other cells within the sponge, they also give rise to eggs for sexual reproduction. (The eggs
remain in the mesohyl, whereas the sperm cells are released into the water.) The amoebocytes can differentiate
into other cell types of the sponge, such as collenocytes and lophocytes, which produce the collagen-like protein
that support the mesohyl. Amoebocytes can also give rise to sclerocytes, which produce spicules (skeletal
spikes of silica or calcium carbonate) in some sponges, and spongocytes, which produce the protein spongin in
the majority of sponges. These different cell types in sponges are shown in Figure 28.3.
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visual
CONNECTION
Osculum
Mesohyl
Lophocyte or collenocyte Pinacocyte
secretes collagen.
Choanocyte
generates water current
and filters food particles
from water.
forms the outer
i covering of the
/ sponge; may
phagocytize large
food particles.
Oocyte
egg cell
Porocyte
controls water
flow through
ostia.
■ Amoebocyte
delivers nutrients
to cells, and
differentiates into
other cell types.
• Sclerocyte
secretes silica
spicules.
(a) Basic sponge body plan (b) Some sponge cell types
Figure 28.3 Simple sponge body plan and cell types. The sponge’s (a) basic body plan and (b) some of the
specialized cell types found in sponges are shown.
Which of the following statements is false?
a. Choanocytes have flagella that propel water through the body.
b. Pinacocytes can transform into any cell type.
c. Lophocytes secrete collagen.
d. Porocytes control the flow of water through pores in the sponge body.
LINK TO LEARNING
Take an up-close tour (http:// 0 penstaxc 0 llege. 0 rg/l/sp 0 nge_ride) through the sponge and its cells.
As we’ve seen, most sponges are supported by small bone-like spicules (usually tiny pointed structures made
of calcium carbonate or silica) in the mesohyl. Spicules provide support for the body of the sponge, and may
also deter predation. The presence and composition of spicules form the basis for differentiating three of the
four classes of sponges (Figure 28.4). Sponges in class Calcarea produce calcium carbonate spicules and no
spongin; those in class Hexactinellida produce six-rayed siliceous (glassy) spicules and no spongin; and those in
class Demospongia contain spongin and may or may not have spicules; if present, those spicules are siliceous.
Sponges in this last class have been used as bath sponges. Spicules are most conspicuously present in the
glass sponges, class Hexactinellida. Some of the spicules may attain gigantic proportions. For example, relative
to typical glass sponge spicules, whose size generally ranges from 3 to 10 mm, some of the basal spicules of
the hexactinellid Monorhaphis chuni are enormous and grow up to 3 meters long! The glass sponges are also
unusual in that most of their body cells are fused together to form a multinucleate syncytium. Because their
cells are interconnected in this way, the hexactinellid sponges have no mesohyl. A fourth class of sponges, the
Sclerospongiae, was described from species discovered in underwater tunnels. These are also called coralline
sponges after their multilayered calcium carbonate skeletons. Dating based on the rate of deposition of the
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skeletal layers suggests that some of these sponges are hundreds of years old.
(a) (b) (c)
Figure 28.4 Several classes of sponges, (a) Clathrina clathrus belongs to class Calcarea, (b) Staurocalyptus spp.
(common name: yellow Picasso sponge) belongs to class Hexactinellida, and (c) Acarnus erithacus belongs to class
Demospongia. (credit a: modification of work by Parent Gery; credit b: modification of work by Monterey Bay Aquarium
Research Institute, NOAA; credit c: modification of work by Sanctuary Integrated Monitoring Network, Monterey Bay
National Marine Sanctuary, NOAA)
LINK
%
LEARNING
Use the Interactive Sponge Guide (http:// 0 penstaxc 0 llege. 0 rg/l/id_sp 0 nges) to identify species of
sponges based on their external form, mineral skeleton, fiber, and skeletal architecture.
Physiological Processes in Sponges
Sponges, despite being simple organisms, regulate their different physiological processes through a variety of
mechanisms. These processes regulate their metabolism, reproduction, and locomotion.
Digestion
Sponges lack complex digestive, respiratory, circulatory, and nervous systems. Their food is trapped as water
passes through the ostia and out through the osculum. Bacteria smaller than 0.5 microns in size are trapped
by choanocytes, which are the principal cells engaged in feeding, and are ingested by phagocytosis. However,
particles that are larger than the ostia may be phagocytized at the sponge's surface by pinacocytes. In some
sponges, amoebocytes transport food from cells that have ingested food particles to those that do not. in
sponges, in spite of what looks like a large digestive cavity, all digestion is intracellular. The limit of this type of
digestion is that food particles must be smaller than individual sponge cells.
All other major body functions in the sponge (gas exchange, circulation, excretion) are performed by diffusion
between the cells that line the openings within the sponge and the water that is passing through those openings.
All cell types within the sponge obtain oxygen from water through diffusion. Likewise, carbon dioxide is released
into seawater by diffusion. In addition, nitrogenous waste produced as a byproduct of protein metabolism is
excreted via diffusion by individual cells into the water as it passes through the sponge.
Some sponges host green algae or cyanobacteria as endosymbionts within archeocytes and other cells, it may
be a surprise to learn that there are nearly 150 species of carnivorous sponges, which feed primarily on tiny
crustaceans, snaring them through sticky threads or hooked spicules!
Although there is no specialized nervous system in sponges, there is intercellular communication that can
regulate events like contraction of the sponge's body or the activity of the choanocytes.
Reproduction
Sponges reproduce by sexual as well as asexual methods. The typical means of asexual reproduction is either
fragmentation (during this process, a piece of the sponge breaks off, settles on a new substrate, and develops
into a new individual), or budding (a genetically identical outgrowth grows from the parent and eventually
detaches or remains attached to form a colony). An atypical type of asexual reproduction is found only in
freshwater sponges and occurs through the formation of gemmules. Gemmules are environmentally resistant
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structures produced by adult sponges (e.g., in the freshwater sponge Spongilla). In gemmules, an inner layer
of archeocytes (amoebocytes) is surrounded by a pneumatic cellular layer that may be reinforced with spicules.
In freshwater sponges, gemmules may survive hostile environmental conditions like changes in temperature,
and then serve to recolonize the habitat once environmental conditions improve and stabilize. Gemmules are
capable of attaching to a substratum and generating a new sponge. Since gemmules can withstand harsh
environments, are resistant to desiccation, and remain dormant for long periods, they are an excellent means of
colonization for a sessile organism.
Sexual reproduction in sponges occurs when gametes are generated. Oocytes arise by the differentiation of
amoebocytes and are retained within the spongocoel, whereas spermatozoa result from the differentiation of
choanocytes and are ejected via the osculum. Sponges are monoecious (hermaphroditic), which means that
one individual can produce both gametes (eggs and sperm) simultaneously. In some sponges, production of
gametes may occur throughout the year, whereas other sponges may show sexual cycles depending upon water
temperature. Sponges may also become sequentially hermaphroditic , producing oocytes first and spermatozoa
later. This temporal separation of gametes produced by the same sponge helps to encourage cross-fertilization
and genetic diversity. Spermatozoa carried along by water currents can fertilize the oocytes borne in the mesohyl
of other sponges. Early larval development occurs within the sponge, and free-swimming larvae (such as
flagellated parenchymula ) are then released via the osculum.
Locomotion
Sponges are generally sessile as adults and spend their lives attached to a fixed substratum. They do not
show movement over large distances like other free-swimming marine invertebrates. However, sponge cells are
capable of creeping along substrata via organizational plasticity, i.e., rearranging their cells. Under experimental
conditions, researchers have shown that sponge cells spread on a physical support demonstrate a leading edge
for directed movement. It has been speculated that this localized creeping movement may help sponges adjust
to microenvironments near the point of attachment. It must be noted, however, that this pattern of movement has
been documented in laboratories, it remains to be observed in natural sponge habitats.
LINK TQ LEARNING
Watch this BBC video (http:// 0 penstaxc 0 llege. 0 rg/l/sea_sp 0 nges) showing the array of sponges seen
along the Cayman Wall during a submersible dive.
28.2 | Phylum Cnidaria
By the end of this section, you will be able to do the following:
• Compare structural and organization characteristics of Porifera and Cnidaria
• Describe the progressive development of tissues and their relevance to animal complexity
• identify the two general body forms found in the Cnidaria
• Describe the identifying features of the major cnidarian classes
Phylum Cnidaria includes animals that exhibit radial or biradial symmetry and are diploblastic, meaning that
they develop from two embryonic layers, ectoderm and endoderm. Nearly all (about 99 percent) cnidarians are
marine species.
Whereas the defining cell type for the sponges is the choanocyte, the defining cell type for the cnidarians is the
cnidocyte, or stinging cell. These cells are located around the mouth and on the tentacles, and serve to capture
prey or repel predators. Cnidocytes have large stinging organelles called nematocysts, which usually contain
barbs at the base of a long coiled thread. The outer wall of the cell has a hairlike projection called a cnidocil,
which is sensitive to tactile stimulation. If the cnidocils are touched, the hollow threads evert with enormous
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acceleration, approaching 40,000 times that of gravity. The microscopic threads then either entangle the prey
or instantly penetrate the flesh of the prey or predator, releasing toxins (including neurotoxins and pore-forming
toxins that can lead to cell lysis) into the target, thereby immobilizing it or paralyzing it (see Figure 28.5).
(a) Nematocyst with stored (b) Nematocyst after
thread and barb firing
Figure 28.5 Cnidocytes. Animals from the phylum Cnidaria have stinging cells called cnidocytes. Cnidocytes contain
large organelles called (a) nematocysts that store a coiled thread and barb, the nematocyst. When the hairlike cnidocil
on the cell surface is touched, even lightly, (b) the thread, barb, and a toxin are fired from the organelle.
LINK TQ LEARNING
View this video (https://www.openstaxcollege.Org/l/nematocyst) animation showing two anemones
engaged in a battle.
Two distinct body plans are found in Cnidarians: the polyp or tuliplike "stalk" form and the medusa or "bell"
form. (Figure 28.6). An example of the polyp form is found in the genus Hydra, whereas the most typical form
of medusa is found in the group called the “sea jellies” (jellyfish). Polyp forms are sessile as adults, with a
single opening (the mouth/anus) to the digestive cavity facing up with tentacles surrounding it. Medusa forms
are motile, with the mouth and tentacles hanging down from an umbrella-shaped bell.
Mesoglea
(a) Medusa (b) Polyp
Figure 28.6 Cnidarian body forms. Cnidarians have two distinct body plans, the medusa (a) and the polyp (b). All
cnidarians have two membrane layers, with a jelly-like mesoglea between them.
Some cnidarians are dimorphic, that is, they exhibit both body plans during their life cycle. In these species,
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the polyp serves as the asexual phase, while the medusa serves as the sexual stage and produces gametes.
However, both body forms are diploid.
An example of cnidarian dimorphism can be seen in the colonial hydroid Obelia. The sessile asexual colony
has two types of polyps, shown in Figure 28.7. The first is the gastrozooid, which is adapted for capturing prey
and feeding. In Obelia, all polyps are connected through a common digestive cavity called a coenosarc. The
other type of polyp is the gonozooid, adapted for the asexual budding and the production of sexual medusae.
The reproductive buds from the gonozooid break off and mature into free-swimming medusae, which are either
male or female (dioecious). Each medusa has either several testes or several ovaries in which meiosis occurs
to produce sperm or egg cells. Interestingly, the gamete-producing cells do not arise within the gonad itself,
but migrate into it from the tissues in the gonozooid. This separate origin of gonad and gametes is common
throughout the eumetazoa. The gametes are released into the surrounding water, and after fertilization, the
zygote develops into a blastula, which soon develops into a ciliated, bilaterally symmetrical planula larva. The
planula swims freely for a while, but eventually attaches to a substrate and becomes a single polyp, from which
a new colony of polyps is formed by budding.
Gastrozooid Gonozooid
Figure 28.7 Obelia. The colonial sessile form of Obelia geniculata has two types of polyps: gastrozooids, which are
adapted for capturing prey, and gonozooids, which asexually bud to produce medusae.
LINK TQ LEARNING
Click here to follow an Obelia life cycle (http:// 0 penstaxc 0 llege. 0 rg/l/ 0 belia) animation and quiz.
All cnidarians are diploblastic and thus have two “epithelial" layers in the body that are derived from the
endoderm and ectoderm of the embryo. The outer layer (from ectoderm) is called the epidermis and lines the
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outside of the animal, whereas the inner layer (from endoderm) is called the gastrodermis and lines the digestive
cavity. In the planula larva, a layer of ectoderm surrounds a solid mass of endoderm, but as the polyp develops,
the digestive or gastrovascular cavity opens within the endoderm. A non-living, jelly-like mesoglea lies between
these two epithelial layers. In terms of cellular complexity, cnidarians show the presence of differentiated cell
types in each tissue layer, such as nerve cells, contractile epithelial cells, enzyme-secreting cells, and nutrient¬
absorbing cells, as well as the presence of intercellular connections. However, with a few notable exceptions
such as statocysts and rhopalia (see below), the development of organs or organ systems is not advanced in
this phylum.
The nervous system is rudimentary, with nerve cells organized in a network scattered across the body. This
nerve net may show the presence of groups of cells that form nerve plexi (singular: plexus) or nerve cords.
Organization of the nervous system in the motile medusa is more complex than that of the sessile polyp, with
a nerve ring around the edge of the medusa bell that controls the action of the tentacles. Cnidarian nerve
cells show mixed characteristics of motor and sensory neurons. The predominant signaling molecules in these
primitive nervous systems are peptides, which perform both excitatory and inhibitory functions. Despite the
simplicity of the nervous system, it is remarkable that it coordinates the complicated movement of the tentacles,
the drawing of captured prey to the mouth, the digestion of food, and the expulsion of waste.
The gastrovascular cavity has only one opening that serves as both a mouth and an anus; this arrangement
is called an incomplete digestive system. In the gastrovascular cavity, extracellular digestion occurs as food is
taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb
nutrients. However, some intracellular digestion also occurs. The gastrovascular cavity distributes nutrients
throughout the body of the animal, with nutrients passing from the digestive cavity across the mesoglea to the
epidermal cells. Thus, this cavity serves both digestive and circulatory functions.
Cnidarian cells exchange oxygen and carbon dioxide by diffusion between cells in the epidermis and water in
the environment, and between cells in the gastrodermis and water in the gastrovascular cavity. The lack of a
circulatory system to move dissolved gases limits the thickness of the body wall and necessitates a non-living
mesoglea between the layers. In the cnidarians with a thicker mesoglea, a number of canals help to distribute
both nutrients and gases. There is neither an excretory system nor organs, and nitrogenous wastes simply
diffuse from the cells into the water outside the animal or into the gastrovascular cavity.
The phylum Cnidaria contains about 10,000 described species divided into two monophyletic clades: the
Anthozoa and the Medusozoa. The Anthozoa include the corals, sea fans, sea whips, and the sea anemones.
The Medusozoa include several classes of Cnidaria in two clades: The Hydrozoa include sessile forms, some
medusoid forms, and swimming colonial forms like the Portuguese man-of-war. The other clade contains various
types of jellies including both Scyphozoa and Cubozoa. The Anthozoa contain only sessile polyp forms, while
the Medusozoa include species with both polyp and medusa forms in their life cycle.
Class Anthozoa
The class Anthozoa ("flower animals") includes sea anemones (Figure 28.8), sea pens, and corals, with an
estimated number of 6,100 described species. Sea anemones are usually brightly colored and can attain a size
of 1.8 to 10 cm in diameter. Individual animals are cylindrical in shape and are attached directly to a substrate.
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Outer
epidermis
Tentacles (contain cnidocytes,
or stinging cells, that bear the
nematocysts)
Mouth
(a) (b)
Figure 28.8 Sea anemone. The sea anemone is shown (a) photographed and (b) in a diagram illustrating its
morphology, (credit a: modification of work by "Dancing With Ghosts'VFlickr; credit b: modification of work by NOAA)
The mouth of a sea anemone is surrounded by tentacles that bear cnidocytes. The slit-like mouth opening
and flattened pharynx are lined with ectoderm. This structure of the pharynx makes anemones bilaterally
symmetrical. A ciliated groove called a siphonoglyph is found on two opposite sides of the pharynx and directs
water into it. The pharynx is the muscular part of the digestive system that serves to ingest as well as egest food,
and may extend for up to two-thirds the length of the body before opening into the gastrovascular cavity. This
cavity is divided into several chambers by longitudinal septa called mesenteries. Each mesentery consists of a
fold of gastrodermal tissue with a layer of mesoglea between the sheets of gastrodermis. Mesenteries do not
divide the gastrovascular cavity completely, and the smaller cavities coalesce at the pharyngeal opening. The
adaptive benefit of the mesenteries appears to be an increase in surface area for absorption of nutrients and gas
exchange, as well as additional mechanical support for the body of the anemone.
Sea anemones feed on small fish and shrimp, usually by immobilizing their prey with nematocysts. Some sea
anemones establish a mutualistic relationship with hermit crabs when the crab seizes and attaches them to their
shell. In this relationship, the anemone gets food particles from prey caught by the crab, and the crab is protected
from the predators by the stinging cells of the anemone. Some species of anemone fish, or clownfish, are also
able to live with sea anemones because they build up an acquired immunity to the toxins contained within the
nematocysts and also secrete a protective mucus that prevents them from being stung.
The structure of coral polyps is similar to that of anemones, although the individual polyps are usually smaller
and part of a colony, some of which are massive and the size of small buildings. Coral polyps feed on smaller
planktonic organisms, including algae, bacteria, and invertebrate larvae. Some anthozoans have symbiotic
associations with dinoflagellate algae called zooxanthellae. The mutually beneficial relationship between
zooxanthellae and modern corals—which provides the algae with shelter—gives coral reefs their colors and
supplies both organisms with nutrients. This complex mutualistic association began more than 210 million years
ago, according to a new study by an international team of scientists. That this symbiotic relationship arose during
a time of massive worldwide coral-reef expansion suggests that the interconnection of algae and coral is crucial
for the health of coral reefs, which provide habitat for roughly one-fourth of all marine life. Reefs are threatened
by a trend in ocean warming that has caused corals to expel their zooxanthellae algae and turn white, a process
called coral bleaching.
Anthozoans remain polypoid (note that this term is easily confused with "polyploid") throughout their lives
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and can reproduce asexually by budding or fragmentation, or sexually by producing gametes. Male or female
gametes produced by a polyp fuse to give rise to a free-swimming planula larva. The larva settles on a suitable
substratum and develops into a sessile polyp.
Class Scyphozoa
Class Scyphozoa ("cup animals") includes all (and only) the marine jellies, with about 200 known species. The
medusa is the prominent stage in the life cycle, although there is a polyp stage in the life cycle of most species.
Most jellies range from 2 to 40 cm in length but the largest scyphozoan species, Cyanea capillata, can reach a
size of two meters in diameter. Scyphozoans display a characteristic bell-like morphology (Figure 28.9).
(a) (b)
Figure 28.9 A sea jelly. A jelly is shown (a) photographed and (b) in a diagram illustrating its morphology, (credit a:
modification of work by "Jimg944"/Flickr; credit b: modification of work by Mariana Ruiz Villareal)
In the sea jelly, a mouth opening is present on the underside of the animal, surrounded by hollow tentacles
bearing nematocysts. Scyphozoans live most of their life cycle as free-swimming, solitary carnivores. The mouth
leads to the gastrovascular cavity, which may be sectioned into four interconnected sacs, called diverticuli. In
some species, the digestive system may branch further into radial canals. Like the septa in anthozoans, the
branched gastrovascular cells serve two functions: to increase the surface area for nutrient absorption and
diffusion, and to support the body of the animal.
In scyphozoans, nerve cells are organized in a nerve net that extends over the entire body, with a nerve ring
around the edge of the bell. Clusters of sensory organs called rhopalia may be present in pockets in the edge of
the bell. Jellies have a ring of muscles lining the dome of the body, which provides the contractile force required
to swim through water, as well as to draw in food from the water as they swim. Scyphozoans have separate
sexes. The gonads are formed from the gastrodermis and gametes are expelled through the mouth. Planula
larvae are formed by external fertilization; they settle on a substratum in a polypoid form. These polyps may
bud to form additional polyps or begin immediately to produce medusa buds. In a few species, the planula larva
may develop directly into the medusa. The life cycle (Figure 28.10) of most scyphozoans includes both sexual
medusoid and asexual polypoid body forms.
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Figure 28.10 Scyphozoan life cycle. The lifecycle of most jellyfish includes two stages: the medusa stage and the
polyp stage. The polyp reproduces asexually by budding, and the medusa reproduces sexually, (credit "medusa":
modification of work by Francesco Crippa)
Class Cubozoa
This class includes jellies that have a box-shaped medusa, or a bell that is square in cross-section, and are
colloquially known as “box jellyfish.” These species may achieve sizes of 15 to 25 cm, but typically members of
the Cubozoa are not as large as those of the Scyphozoa. However, cubozoans display overall morphological and
anatomical characteristics that are similar to those of the scyphozoans. A prominent difference between the two
classes is the arrangement of tentacles. The cubozoans contain muscular pads called pedalia at the corners
of the square bell canopy, with one or more tentacles attached to each pedalium. In some cases, the digestive
system may extend into the pedalia. Nematocysts may be arranged in a spiral configuration along the tentacles;
this arrangement helps to effectively subdue and capture prey. Cubozoans include the most venomous of all the
cnidarians (Figure 28.11).
These animals are unusual in having image-forming eyes, including a cornea, lens, and retina. Because these
structures are made from a number of interactive tissues, they can be called true organs. Eyes are located in
four clusters between each pair of pedalia. Each cluster consists of four simple eye spots plus two image-forming
eyes oriented in different directions. How images formed by these very complex eyes are processed remains
a mystery, since cubozoans have extensive nerve nets but no distinct brain. Nontheless, the presence of eyes
helps the cubozoans to be active and effective hunters of small marine animals like worms, arthropods, and fish.
Cubozoans have separate sexes and fertilization occurs inside the female. Planula larvae may develop inside
the female or be released, depending on species. Each planula develops into a polyp. These polyps may bud to
form more polyps to create a colony; each polyp then transforms into a single medusa.
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(a) (b) (c)
Figure 28.11 A cubozoan. The (a) tiny cubozoan jelly Malo kingi is thimble-shaped and, like all cubozoan jellies, (b) has
four muscular pedalia to which the tentacles attach. M. kingi is one of two species of jellies known to cause Irukandji
syndrome, a condition characterized by excruciating muscle pain, vomiting, increased heart rate, and psychological
symptoms. Two people in Australia, where Irukandji jellies are most commonly found, are believed to have died from
Irukandji stings, (c) A sign on a beach in northern Australia warns swimmers of the danger, (credit c: modification of
work by Peter Shanks)
Class Hydrozoa
Hydrozoa is a diverse group that includes nearly 3,200 species; most are marine, although some freshwater
species are known (Figure 28.12). Most species exhibit both polypoid and medusoid forms in their lifecycles,
although the familiar Hydra has only the polyp form. The medusoid form has a muscular veil or velum below
the margin of the bell and for this reason is called a hydromedusa. In contrast, the medusoid form of Scyphozoa
lacks a velum and is termed a scyphomedusa.
The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined
by the gastrodermis. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between
them. A mouth opening, surrounded by tentacles, is present at the oral end of the animal. Many hydrozoans
form sessile, branched colonies of specialized polyps that share a common, branching gastrovascular cavity
(coenosarc), such as is found in the colonial hydroid Obelia.
Free-floating colonial species called siphonophores contain both medusoid and polypoid individuals that are
specialized for feeding, defense, or reproduction. The distinctive rainbow-hued float of the Portuguese man o’
war ( Physalia physalis) creates a pneumatophore with which it regulates buoyancy by filling and expelling carbon
monoxide gas. At first glance, these complex superorganisms appear to be a single organism; but the reality is
that even the tentacles are actually composed of zooids laden with nematocysts. Thus, although it superficially
resembles atypical medusozoan jellyfish, P. physalis is a free-floating hydrozoan colony, each specimen is made
up of many hundreds of organisms, each specialized for a certain function, including motility and buoyancy,
feeding, reproduction and defense. Although they are carnivorous and feed on many soft bodied marine animals,
P. physalis lack stomachs and instead have specialized polyps called gastrozooids that they use to digest their
prey in the open water.
Physalia has male and female colonies, which release their gametes into the water. The zygote develops into
a single individual, which then buds asexually to form a new colony. Siphonophores include the largest known
floating cnidarian colonies such as Praya dubia, whose chain of zoids can get up to 50 meters (165 feet) long.
Other hydrozoan species are solitary polyps (Hydra) or solitary hydromedusae (Gonionemus). One defining
characteristic shared by the hydrozoans is that their gonads are derived from epidermal tissue, whereas in all
other cnidarians they are derived from gastrodermal tissue.
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(a) Obelia
(b) Physalia physalis (Portuguese Man O' War)
(c) Velella bae (d) Hydra
Figure 28.12 Hydrozoans. The polyp colony Obelia (a), siphonophore colonies Physalia (b) physalis, known as the
Portuguese man o‘ war and Velella bae (c), and the solitary polyp Hydra (d) have different body shapes but all belong
to the family Hydrozoa. (credit b: modification of work by NOAA; scale-bar data from Matt Russell)
28.3 | Superphylum Lophotrochozoa: Flatworms,
Rotifers, and Nemerteans
By the end of this section, you will be able to do the following:
• Describe the unique anatomical and morphological features of flatworms, rotifers, and Nemertea
• Identify an important extracoelomic cavity found in Nemertea
• Explain the key features of Platyhelminthes and their importance as parasites
Animals belonging to superphylum Lophotrochozoa are triploblastic (have three germ layers) and unlike the
cnidarians, they possess an embryonic mesoderm sandwiched between the ectoderm and endoderm. These
phyla are also bilaterally symmetrical, meaning that a longitudinal section will divide them into right and left sides
that are superficially symmetrical. In these phyla, we also see the beginning of cephalization, the evolution of
a concentration of nervous tissues and sensory organs in the head of the organism—exactly where a mobile
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bilaterally symmetrical organism first encounters its environment.
Lophotrochozoa are also protostomes, in which the blastopore, or the point of invagination of the ectoderm
(outer germ layer), becomes the mouth opening into the alimentary canal. This developmental pattern is called
protostomy or “first mouth.” Protostomes include acoelomate, pseudocoelomate, and eucoelomate phyla. The
coelom is a cavity that separates the ectoderm from the endoderm. In acoelomates, a solid mass of mesoderm
is sandwiched between the ectoderm and endoderm and does not form a cavity or “coelom,” leaving little room
for organ development; in pseudocoelomates, there is a cavity or pseudocoelom that replaces the blastocoel
(the cavity within the blastula), but it is only lined by mesoderm on the outside of the cavity, leaving the gut tube
and organs unlined; in eucoelomates, the cavity that obliterates the blastocoel as the coelom develops is lined
both on the outside of the cavity (parietal layer) and also on the inside of the cavity, around the gut tube and the
internal organs (visceral layer).
Eucoelmate protostomes are schizocoels, in which mesoderm-producing cells typically migrate into the
blastocoel during gastrulation and multiply to form a solid mass of cells. Cavities then develop within the cell
mass to form the coelom. Since the forming body cavity splits the mesoderm, this protostomic coelom is termed a
schizocoelom. As we will see later in this chapter, chordates, the phylum to which we belong, generally develop
a coelom by enterocoely: pouches of mesoderm pinch off the invaginating primitive gut, or archenteron, and then
fuse to form a complete coelom. We should note here that a eucoelomate can form its “true coelom” by either
schizocoely or enterocoely. The process that produces the coelom is different and of taxonomic importance, but
the result is the same: a complete, mesodermally lined coelom.
Most organisms placed in the superphylum Lophotrochozoa possess either a lophophore feeding apparatus
or a trochophore larvae (thus the contracted name, “lopho-trocho-zoa"). The lophophore is a feeding structure
composed of a set of ciliated tentacles surrounding the mouth. A trochophore is a free-swimming larva
characterized by two bands of cilia surrounding a top-like body. Some of the phyla classified as Lophotrochozoa
may be missing one or both of these defining structures. Nevertheless their placement with the Lophotrochozoa
is upheld when ribosomal RNA and other gene sequences are compared. The systematics of this complex group
is still unclear and much more work remains to resolve the cladistic relationships among them.
Phylum Platyhelminthes
The flatworms are acoelomate organisms that include many free-living and parasitic forms. The flatworms
possess neither a lophophore nor trochophore larvae, although the larvae of one group of flatworms, the
Polycladida (named after its many-branched digestive tract), are considered to be homologous to trochophore
larvae. Spiral cleavage is also seen in the polycladids and other basal flatworm groups. The developmental
pattern of some of the free-living forms is obscured by a phenomenon called "blastomere anarchy" in which
a sort of temporary feeding larva forms, followed by a regrouping of cells within the embryo that gives
rise to a second-stage embryo. However, both the monophyly of the flatworms and their placement in the
Lophotrochozoa has been supported by molecular analyses.
The Platyhelminthes consist of two monophyletic lineages: the Catenulida and the Rhabditophora. The
Catenulida, or "chain worms," is a small clade of just over 100 species. These worms typically reproduce
asexually by budding. However, the offspring do not fully detach from the parents and the formation resembles
a chain in appearance. All of the flatworms discussed here are part of the Rhabditophora ("rhabdite bearers").
Rhabdites are rodlike structures discharged in the mucus produced by some free-living flatworms; Eucoelmate
protostomes are schizocoels, in which mesoderm-producing cells typically migrate into the blastocoel during
gastrulation likely serve in both defense and to provide traction for ciliary gliding along the substrate. Unlike
free-living flatworms, many species of trematodes and cestodes are parasitic, including important parasites of
humans.
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Figure 28.13 Flatworms exhibit significant diversity, (a) A blue Pseudoceros flatworm (Pseudoceros bifurcus)\ (b) gold
speckled flatworm (Thysanozoon nigropapillosum). (credit a: modification of work by Stephen Childs; b: modification
of work by Pril Fish.)
Flatworms have three embryonic tissue layers that give rise to epidermal tissues (from ectoderm), the lining of
the digestive system (from endoderm), and other internal tissues (from mesoderm). The epidermal tissue is a
single layer of cells or a layer of fused cells ( syncytium) that covers two layers of muscle, one circular and
the other longitudinal. The mesodermal tissues include mesenchymal cells that contain collagen and support
secretory cells that produce mucus and other materials at the surface. Because flatworms are acoelomates,
the mesodermal layer forms a solid mass between the outer epidermal surface and the cavity of the digestive
system.
Physiological Processes of Flatworms
The free-living species of flatworms are predators or scavengers. Parasitic forms feed by absorbing nutrients
provided by their hosts. Most flatworms, such as the planarian shown in Figure 28.14, have a branching
gastrovascular cavity rather than a complete digestive system. In such animals, the “mouth” is also used to expel
waste materials from the digestive system, and thus also serves as an anus. (A few species may have a second
anal pore or opening.) The gut may be a simple sac or highly branched. Digestion is primarily extracellular, with
digested materials taken into the cells of the gut lining by phagocytosis. One parasitic group, the tapeworms
(cestodes), lacks a digestive system altogether, and absorb digested food from the host.
Flatworms have an excretory system with a network of tubules attached to flame cells, whose cilia beat to direct
waste fluids concentrated in the tubules out of the body through excretory pores. The system is responsible
for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of
a pair of lateral nerve cords running the length of the body with transverse connections between them. Two
large cerebral ganglia—concentrations of nerve cell bodies at the anterior end of the worm—are associated with
photosensory and chemosensory cells. There is neither a circulatory nor a respiratory system, with gas and
nutrient exchange dependent on diffusion and cell-to-cell junctions. This necessarily limits the thickness of the
body in these organisms, constraining them to be “flat” worms. Most flatworm species are monoecious (both
male and female reproductive organs are found in the same individual), and fertilization is typically internal.
Asexual reproduction by fission is common in some groups.
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Transverse
nerve Intestine Pharynx
Figure 28.14 Planaria, a free-living flatworm. The planarian is a flatworm that has a gastrovascular cavity with one
opening that serves as both mouth and anus. The excretory system is made up of flame cells and tubules connected
to excretory pores on both sides of the body. The nervous system is composed of two interconnected nerve cords
running the length of the body, with cerebral ganglia and eyespots at the anterior end.
Diversity of Flatworms
The flatworms have been traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and
Cestoda (Figure 28.15). However, the relationships among members of these classes has recently been
reassessed, with the turbellarians in particular now viewed as paraphyletic, since its descendants may also
include members of the other three classes. Members of the clade or class Rhabditophora are now dispersed
among multiple orders of Platyhelminthes, the most familiar of these being the Polycladida, which contains
the large marine flatworms; the Tricladida (which includes Dugesia [“planaria”] and Planaria and its relatives);
and the major parasitic orders: Monogenea (fish ectoparasites), Trematoda (flukes), and Cestoda (tapeworms),
which together form a monophyletic clade.
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(a) Class Turbellaria (b) Class Monogenea
(c) Class Trematoda (d) Class Cestoda
Figure 28.15 Traditional flatworm classes. Phylum Platyhelminthes was previously divided into four classes, (a) Class
Turbellaria includes the free-living polycladid Bedford’s flatworm ( Pseudobiceros bedfordi), which is about 8 to 10 cm
in length, (b) The parasitic class Monogenea includes Dactylogyrus spp, commonly called gill flukes, which are about
0.2 mm in length and have two anchors, indicated by arrows used to attach the parasite on to the gills of host fish, (c)
The class Trematoda includes Fascioloides magna (right) and Fasciola hepatica (two specimens on left, also known
as the common liver fluke), (d) Class Cestoda includes tapeworms such as this Taenia saginata, infects both cattle
and humans, and can reach 4 to 10 meters in length; the specimen shown here is about four meters long, (credit a:
modification of work by Jan Derk; credit d: modification of work by CDC)
Most free-living flatworms are marine polycladids, although tricladid species live in freshwater or moist terrestrial
environments, and there are a number of members from other orders in both environments. The ventral
epidermis of free-living flatworms is ciliated, which facilitates their locomotion. Some free-living flatworms are
capable of remarkable feats of regeneration in which an individual may regrow its head or tail after being
severed, or even several heads if the planaria is cut lengthwise.
The monogeneans are ectoparasites , mostly of fish, with simple life cycles that consist of a free-swimming larva
that attaches to a fish, prior to its transformation to the ectoparasitic adult form. The parasite has only one host
and that host is usually very specific. The worms may produce enzymes that digest the host tissues, or they
may simply graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the male
gametes develop first and so cross-fertilization is quite common.
The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans.
Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs, and one
or more secondary hosts in which asexual reproduction occurs. The primary host is usually a vertebrate and
the secondary host is almost always a mollusk, in which multiple larvae are produced asexually. Trematodes,
which attached internally to the host via an oral and medial sucker, are responsible for serious human diseases
including schistosomiasis, caused by several species of the blood fluke, Schistosoma spp. Various forms of
schistosomiasis infect an estimated 200 million people in the tropics, leading to organ damage, secondary
infection by bacteria, and chronic symptoms like fatigue. Infection occurs when the human enters the water and
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metacercaria larvae, released from the snail host, locate and penetrate the skin. The parasite infects various
organs in the body and feeds on red blood cells before reproducing.
Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect
the snail host. The eggs, which have a barb on them, can damage the vascular system of the human host,
causing ulceration, abscesses, and bloody diarrhea, wherever they reside, thereby allowing other pathogens
to cause secondary infections, in fact, it is the parasite’s eggs that produce most of the main ill effects of
schistosomiasis. Many eggs do not make the transit through the veins of the host for elimination, and are swept
by blood flow back to the liver and other locations, where they can cause severe inflammation, in the liver, the
errant eggs may impede circulation and cause cirrhosis. Control is difficult in impoverished areas in unsanitary,
crowded conditions, and prognosis is poor in people with heavy infections of Schistosoma japonicum, without
early treatment.
The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates (Figure 28.16). Tapeworms, such
as those of Taenia spp, live in the intestinal tract of the primary host and remain fixed using a sucker or hooks
on the anterior end, or scolex, of the tapeworm body, which is essentially a colony of similar subunits called
proglottids. Each proglottid may contain an excretory system with flame cells, along with reproductive structures,
both male and female. Because they are so long and flat, tapeworms do not need a digestive system; instead,
they absorb nutrients from the food matter surrounding them in the host’s intestine by diffusion.
Proglottids are produced at the scolex and gradually migrate to the end of the tapeworm; at this point, they
are “mature” and all structures except fertilized eggs have degenerated. Most reproduction occurs by cross¬
fertilization between different worms in the same host, but may also occur between proglottids. The mature
proglottids detach from the body of the worm and are released into the feces of the organism. The eggs are
eaten by an intermediate host, typically another vertebrate. The juvenile worm infects the intermediate host and
takes up residence, usually in muscle tissue. When the muscle tissue is consumed by the primary host, the
cycle is completed. There are several tapeworm parasites of humans that are transmitted by eating uncooked or
poorly cooked pork, beef, or fish.
4 Embryos develop into
larvae in muscle.
Figure 28.16 Tapeworm life cycle. Tapeworm (Taenia spp.) infections occur when humans consume raw or
undercooked infected meat, (credit: modification of work by CDC)
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Phylum Rotifera
The rotifers ("wheel-bearer") belong to a group of microscopic (about 100 pm to 2 mm) mostly aquatic animals
that get their name from the corona —a pair of ciliated feeding structures that appear to rotate when viewed
under the light microscope (Figure 28.17). Although their taxonomic status is currently in flux, one treatment
places the rotifers in three classes: Bdelloidea, Monogononta, and Seisonidea. In addition, the parasitic “spiny
headed worms” currently in phylum Acanthocephala, appear to be modified rotifers and will probably be placed
into the group in the near future. Undoubtedly the rotifers will continue to be revised as more phylogenetic
evidence becomes available.
The pseudocoelomate body of a rotifer is remarkably complex for such a small animal (roughly the size of a
Paramecium ) and is divided into three sections: a head (which contains the corona), a trunk (which contains
most of the internal organs), and the foot. A cuticle, rigid in some species and flexible in others, covers the body
surface. They have both skeletal muscle associated with locomotion and visceral muscles associated with the
gut, both composed of single cells. Rotifers are typically free-swimming or planktonic (drifting) organisms, but the
toes or extensions of the foot can secrete a sticky material to help them adhere to surfaces. The head contains
a number of eyespots and a bilobed “brain,” with nerves extending into the body.
(a) Bdelloidea (a) Monogonota
Figure 28.17 Rotifers. Shown are examples from two of the three classes of rotifer, (a) Species from the class
Bdelloidea are characterized by a large corona. The whole animals in the center of this scanning electron micrograph
are shown surrounded by several sets of jaws from the mastax of rotifers, (b) Polyarthra, from the largest rotifer class
Monogononta, has a smaller corona than bdelloid rotifers, and a single gonad, which give the class its name, (credit a:
modification of work by Diego Fontaneto; credit b: modification of work by U.S. EPA; scale-bar data from Cory Zanker)
Rotifers are commonly found in freshwater and some saltwater environments throughout the world. As filter
feeders, they will eat dead material, algae, and other microscopic living organisms, and are therefore very
important components of aquatic food webs. A rotifer's food is directed toward the mouth by the current created
from the movement of the coronal cilia. The food particles enter the mouth and travel first to the mastax —a
muscular pharynx with toothy jaw-like structures. Examples of the jaws of various rotifers are seen in Figure
28.17a. Masticated food passes near digestive and salivary glands, into the stomach, and then to the intestines.
Digestive and excretory wastes are collected in a cloacal bladder before being released out the anus.
LINK TQ LEARNING
Watch this video (http:// 0 penstaxc 0 llege. 0 rg/l/r 0 tifers) to see rotifers feeding.
About 2,200 species of rotifers have been identified. Figure 28.18 shows the anatomy of a rotifer belonging
to class Bdelloidea. Some rotifers are dioecious organisms and exhibit sexual dimorphism (males and females
have different forms). In many dioecious species, males are short-lived and smaller with no digestive system
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and a single testis. Many rotifer species exhibit haplodiploidy, a method of sex determination in which a fertilized
egg develops into a female and an unfertilized egg develops into a male. However, reproduction in the bdelloid
rotifers is exclusively parthenogenetic and appears to have been so for millions of years: Thus, all bdelloid
rotifers and their progeny are female! The bdelloids may compensate for this genetic insularity by borrowing
genes from the DNA of other species. Up to 10% of a bdelloid genome comprises genes imported from
related species. Some rotifer eggs are capable of extended dormancy for protection during harsh environmental
conditions.
Mouth
Corona ■
Cerebral ganglion
Digestive gland
1WI
Mastax
Pseudocoel
Stomach
■ Intestine
■ Anus
Figure 28.18 A bdelloid rotifer. This illustration shows the anatomy of a bdelloid rotifer.
Phylum Nemertea
The Nemertea are colloquially known as ribbon worms or proboscis worms. Most species of phylum Nemertea
are marine and predominantly benthic (bottom dwellers), with an estimated 900 known species. However,
nemerteans have been recorded in freshwater and very damp terrestrial habitats as well. Most nemerteans
are carnivores, feeding on worms, clams, and crustaceans. Some species are scavengers, and some, like
Malacobdella grossa, have also evolved commensal relationships with mollusks. Economically important
species have at times devastated commercial fishing of clams and crabs. Nemerteans have almost no predators
and two species are sold as fish bait.
Morphology
Nemerteans vary in size from 1 cm to several meters. They show bilateral symmetry and remarkable contractile
properties. Because of their contractility, they can change their morphological presentation in response to
environmental cues. Animals in phylum Nemertea are soft and unsegmented animals, with a morphology like a
flattened tube. (Figure 28.19).
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Figure 28.19 A proboscis worm. The proboscis worm (Parborlasia corrugatus) is a scavenger that combs the sea floor
for food. The species is a member of the phylum Nemertea. The specimen shown here was photographed in the Ross
Sea, Antarctica, (credit: Henry Kaiser, National Science Foundation)
A unique characteristic of this phylum is the presence of an eversible proboscis enclosed in a pocket called a
rhynchocoel (not part of the animal's actual coelom). The proboscis is located dorsal to the gut and serves
as a harpoon or tentacle for food capture. In some species it is ornamented with barbs. The rhynchocoel is
a fluid-filled cavity that extends from the head to nearly two-thirds of the length of the gut in these animals
(Figure 28.20). The proboscis may be extended by hydrostatic pressure generated by contraction of muscle of
the rhynchocoel and retracted by a retractor muscle attached to the rear wall of the rhynchocoel.
Sensory Cerebral Mouth Proboscis Anus
Figure 28.20 The anatomy of a Nemertean is shown.
LINK TQ LEARNING
Watch this video (https://www. 0 penstaxc 0 llege. 0 rg/l/nemertean) to see a nemertean attack a polychaete
with its proboscis.
Digestive System
The nemerteans, which are primarily predators of annelids and crustaceans, have a well-developed digestive
system. A mouth opening that is ventral to the rhynchocoel leads into the foregut, followed by the intestine. The
intestine is present in the form of diverticular pouches and ends in a rectum that opens via an anus. Gonads are
interspersed with the intestinal diverticular pouches and open outward via genital pores.
Nemerteans are sometimes classified as acoels, but because their closed circulatory system is derived from the
coelomic cavity of the embryo, they may be regarded as coelomic. Their circulatory system consists of a closed
loop formed by a connected pair of lateral blood vessels. Some species may also have a dorsal vessel or cross¬
connecting vessels in addition to lateral ones. Although the circulatory fluid contains cells, it is often colorless.
However, the blood cells of some species bear hemoglobin as well as other yellow or green pigments. The blood
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vessels are contractile, although there is usually no regular circulatory pathway, and movement of blood is also
facilitated by the contraction of muscles in the body wall. The circulation of fluids in the rhychocoel is more or less
independent of the blood circulation, although blind branches from the blood vessels into the rhyncocoel wall
can mediate exchange of materials between them. A pair of protonephridia, or excretory tubules, is present in
these animals to facilitate osmoregulation. Gaseous exchange occurs through the skin.
Nervous System
Nemerteans have a "brain" composed of four ganglia situated at the anterior end, around the rhynchocoel.
Paired longitudinal nerve cords emerge from the brain ganglia and extend to the posterior end. Additional nerve
cords are found in some species. Interestingly, the brain can contain hemoglobin, which acts as an oxygen
reserve. Ocelli or eyespots are present in pairs, in multiples of two in the anterior portion of the body. It is
speculated that the eyespots originate from neural tissue and not from the epidermis.
Reproduction
Nemerteans, like flatworms, have excellent powers of regeneration, and asexual reproduction by fragmentation
is seen in some species. Most animals in phylum Nemertea are dioecious, although freshwater species may
be hermaphroditic. Stem cells that become gametes aggregate within gonads placed along the digestive tract.
Eggs and sperm are released into the water, and fertilization occurs externally. Like most lophotrochozoan
protostomes, cleavage is spiral, and development is usually direct, although some species have a trochophore-
like larva, in which a young worm is constructed from a series of imaginal discs that begin as invaginations from
the body surface of the larva.
28.4 | Superphylum Lophotrochozoa: Molluscs and
Annelids
By the end of this section, you will be able to do the following:
• Describe the unique anatomical and morphological features of molluscs and annelids
• Describe the formation of the coelom
• Identify an important extracoelomic cavity in molluscs
• Describe the major body regions of Mollusca and how they vary in different molluscan classes
• Discuss the advantages of true body segmentation
• Describe the features of animals classified in phylum Annelida
The annelids and the mollusks are the most familiar of the lophotrochozoan protostomes. They are also more
“typical" lophotrochozoans, since both groups include aquatic species with trochophore larvae, which unite both
taxa in common ancestry. These phyla show how a flexible body plan can lead to biological success in terms
of abundance and species diversity. The phylum Mollusca has the second greatest number of species of all
animal phyla with nearly 100,000 described extant species, and about 80,000 described extinct species. In fact,
it is estimated that about 25 percent of all known marine species are mollusks! The annelids and mollusca are
both bilaterally symmetrical, cephalized, triploblastic, schizocoelous eucoeolomates They include animals you
are likely to see in your backyard or on your dinner plate!
Phylum Mollusca
The name “Mollusca" means “soft" body, since the earliest descriptions of molluscs came from observations of
“squishy,” unshelled cuttlefish. Molluscs are predominantly a marine group of animals; however, they are also
known to inhabit freshwater as well as terrestrial habitats. This enormous phylum includes chitons, tusk shells,
snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses. Molluscs
display a wide range of morphologies in each class and subclass, but share a few key characteristics (Figure
28.21). The chief locomotor structure is usually a muscular foot. Most internal organs are contained in a region
called the visceral mass. Overlying the visceral mass is a fold of tissue called the mantle; within the cavity
formed by the mantle are respiratory structures called gills, that typically fold over the visceral mass. The mouths
of most mollusks, except bivalves (e.g., clams) contain a specialized feeding organ called a radula, an abrasive
tonguelike structure. Finally, the mantle secretes a calcium-carbonate-hardened shell in most molluscs, although
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this is greatly reduced in the class Cephalopoda, which contains the octopuses and squids.
visual
CONNECTION
Mantle Coelom Intestine Gonad Shell
Figure 28.21 Molluscan body regions. There are many species and variations of molluscs; this illustration
shows the anatomy of an aquatic gastropod. In a terrestrial gastropod, the mantle cavity itself would serve as a
respiratory organ.
Which of the following statements about the anatomy of a mollusc is false?
1. Most molluscs have a radula for grinding food.
2. A digestive gland is connected to the stomach.
3. The tissue beneath the shell is called the mantle.
4. The digestive system includes a gizzard, a stomach, a digestive gland, and the intestine.
The muscular foot is the ventral-most organ, whereas the mantle is the limiting dorsal organ that folds over the
visceral mass. The foot, which is used for locomotion and anchorage, varies in shape and function, depending
on the type of mollusk under study. In shelled mollusks, the foot is usually the same size as the opening of the
shell. The foot is both retractable and extendable. In the class Cephalopoda (“head-foot"), the foot takes the form
of a funnel for expelling water at high velocity from the mantle cavity; and the anterior margin of the foot has
been modified into a circle of arms and tentacles.
The visceral mass is present above the foot, in the visceral hump. This mass contains digestive, nervous,
excretory, reproductive, and respiratory systems. Molluscan species that are exclusively aquatic have gills that
extend into the mantle cavity, whereas some terrestrial species have "lungs" formed from the lining of the mantle
cavity. Mollusks are schizocoelous eucoelomates, but the coelomic cavity in adult animals has been largely
reduced to a cavity around the heart. However, a reduced coelom sometimes surrounds the gonads, part of the
kidneys, and intestine as well. This overall coelomic reduction makes the mantle cavity the major internal body
chamber.
Most mollusks have a special rasp-like organ, the radula, which bears chitinous filelike teeth. The radula is
present in all groups except the bivalves, and serves to shred or scrape food before it enters the digestive
tract. The mantle (also known as the pallium ) is the dorsal epidermis in mollusks; all mollusks except some
cephalopods are specialized to secrete a calcareous shell that protects the animal's soft body.
Most mollusks are dioecious animals and fertilization occurs externally, although this is not the case in terrestrial
mollusks, such as snails and slugs, or in cephalopods. In most aquatic mollusks, the zygote hatches and
produces a trochophore larva, with several bands of cilia around a toplike body, and an additional apical tuft
of cilia. In some species, the trochophore may be followed by additional larval stages, such as a veliger larvae,
before the final metamorphosis to the adult form. Most cephalopods develop directly into small versions of their
adult form.
Classification of Phylum Mollusca
Phylum Mollusca comprises a very diverse group of organisms that exhibits a dramatic variety of forms, ranging
from chitons to snails to squids, the latter of which typically show a high degree of intelligence. This variability is a
consequence of modification of the basic body regions, especially the foot and mantle. The phylum is organized
into eight classes: Caudofoveata, Solenogastres, Monoplacophora, Polyplacophora, Gastropoda, Cephalopoda,
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Bivalvia, and Scaphopoda. Although each molluscan class appears to be monophyletic, their relationship to one
another is unclear and still being reviewed.
Both the Caudofoveata and the Solenogastres include shell-less, worm-like animals primarily found in benthic
marine habitats. Although these animals lack a calcareous shell, they get some protection from calcareous
spicules embedded in a cuticle that covers their epidermis. The mantle cavity is reduced, and both groups lack
eyes, tentacles, and nephridia (excretory organs). The Caudofoveata possess a radula, but the Solenogastres
do not have a radula or gills. The foot is also reduced in the Solenogastres and absent from the Caudofoveata.
Long thought to be extinct, the first living specimens of Monoplacophora, Neopilina galatheae, were discovered
in 1952 on the ocean bottom near the west coast of Costa Rica. Today there are over 25 described species.
Members of class Monoplacophora (“bearing one plate") possess a single, cap-like shell that covers the dorsal
body. The morphology of the shell and the underlying animal can vary from circular to ovate. They have a simple
radula, a looped digestive system, multiple pairs of excretory organs, and a pair of gonads. Multiple gills are
located between the foot and the edge of the mantle.
Animals in class Polyplacophora (“bearing many plates") are commonly known as “chitons" and bear eight limy
plates that make up the dorsal shell (Figure 28.22). These animals have a broad, ventral foot that is adapted
for suction onto rocks and other substrates, and a mantle that extends beyond the edge of the shell. Calcareous
spines on the exposed mantle edge provide protection from predators. Respiration is facilitated by multiple pairs
of gills in the mantle cavity. Blood from the gills is collected in a posterior heart, and then sent to the rest of
the body in a hemocoel —an open circulation system in which the blood is contained in connected chambers
surrounding various organs rather than within individual blood vessels. The radula, which has teeth composed of
an ultra-hard magnetite, is used to scrape food organisms off rocky surfaces. Chiton teeth have been shown to
exhibit the greatest hardness and stiffness of any biomineral material reported to date, being as much as three-
times harder than human enamel and the calcium carbonate-based shells of mollusks.
The nervous system is rudimentary with only buccal or “cheek” ganglia present at the anterior end. Multiple tiny
sensory structures, including photosensors, extend from the mantle into channels in the upper layer of the shell.
These structures are called esthetes and are unique to the chitons. Another sensory structure under the radula
is used to sample the feeding environment. A single pair of nephridia is used for the excretion of nitrogenous
wastes.
Figure 28.22 A chiton. This chiton from the class Polyplacaphora has the eight-plated shell for which its class is
named, (credit: Jerry Kirkhart)
Class Bivalvia (“two-valves") includes clams, oysters, mussels, scallops, geoducks, and shipworms. Some
bivalves are almost microscopic, while others, in the genus Tridacna, may be one meter in length and weigh
225 kilograms. Members of this class are found in marine as well as freshwater habitats. As the name suggests,
bivalves are enclosed in two-part valves or shells (Figure 28.23a) fused on the dorsal side by hinge ligaments
as well as shell teeth on the ventral side that keep the two halves aligned. The two shells, which consist of an
outer organic layer, a middle prismatic layer, and a very smooth nacreous layer, are joined at the oldest part
of the shell called the umbo. Anterior and posterior adductor and abductor muscles close and open the shell
respectively.
The overall body of the bivalve is laterally flattened; the foot is wedge-shaped; and the head region is poorly
developed (with no obvious mouth). Bivalves are filter-feeders, and a radula is absent in this class of mollusks.
The mantle cavity is fused along the edges except for openings for the foot and for the intake and expulsion of
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water, which is circulated through the mantle cavity by the actions of the incurrent and excurrent siphons. During
water intake by the incurrent siphon, food particles are captured by the paired posterior gills (ctenidia) and then
carried by the movement of cilia forward to the mouth. Excretion and osmoregulation are performed by a pair
of nephridia. Eyespots and other sensory structures are located along the edge of the mantle in some species.
The "eyes" are especially conspicuous in scallops (Figure 28.23b). Three pairs of connected ganglia regulate
activity of different body structures.
Figure 28.23 Bivalves. These mussels (a), found in the intertidal zone in Cornwall, England, show the bivalve shell.
The scallop Argopecten irradians (b) has a fluted shell and conspicuous eyespots. (credit (a): Mark A. Wilson,
credit (b) Rachael Norris and Marina Freudzon. https://c 0 mm 0 ns.wikimedia. 0 rg/w/index.php?curid=17251065
(http:// 0 penstax. 0 rg/l/scall 0 p_eyes) )
One of the functions of the mantle is to secrete the shell. Some bivalves, like oysters and mussels, possess the
unique ability to secrete and deposit a calcareous nacre or “mother of pearl" around foreign particles that may
enter the mantle cavity. This property has been commercially exploited to produce pearls.
LINK TQ LEARNING
Watch the animations of bivalves feeding: View the process in clams (http:// 0 penstaxc 0 llege. 0 rg/l/clams)
and mussels (http:// 0 penstaxc 0 llege. 0 rg/l/mussels) at these sites.
More than half of molluscan species are in the class Gastropoda (“stomach foot"), which includes well-known
mollusks like snails, slugs, conchs, cowries, limpets, and whelks. Aquatic gastropods include both marine and
freshwater species, and all terrestrial mollusks are gastropods. Gastropoda includes shell-bearing species as
well as species without shells. Gastropod bodies are asymmetrical and usually present a coiled shell (Figure
28.24a). Shells may be planospiral (like a garden hose wound up), commonly seen in garden snails, or
conispiral, (like a spiral staircase), commonly seen in marine conches. Cowrie shells have a polished surface
because the mantle extends up over the top of the shell as it is secreted.
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Figure 28.24 Gastropods. Snails(a) and slugs(b) are both gastropods, but slugs lack a shell, (credit a: modification of
work by Murray Stevenson; credit b: modification of work by Rosendahl)
A key characteristic of some gastropods is the embryonic development of torsion. During this process, the
mantle and visceral mass are rotated around the perpendicular axis over the center of the foot to bring the anal
opening forward just behind the head (Figure 28.25), creating a very peculiar situation. The left gill, kidney, and
heart atrium are now on the right side, whereas the original right gill, kidney, and heart atrium are on the left
side. Even stranger, the nerve cords have been twisted and contorted into a figure-eight pattern. Because of the
space made available by torsion in the mantle cavity, the animal’s sensitive head end can now be withdrawn into
the protection of the shell, and the tougher foot (and sometimes the protective covering or operculum) forms a
barrier to the outside. The strange arrangement that results from torsion poses a serious sanitation problem by
creating the possibility of wastes being washed back over the gills, causing fouling. There is actually no really
perfect explanation for the embryonic development of torsion, and some groups that formerly exhibited torsion
in their ancestral groups are now known to have reversed the process.
Gastropods also have a foot that is modified for crawling. Most gastropods have a well-defined head with
tentacles and eyes. A complex radula is used to scrape up food particles, in aquatic gastropods, the mantle
cavity encloses the gills (ctenidia), but in land gastropods, the mantle itself is the major respiratory structure,
acting as a kind of lung. Nephridia (“kidneys”) are also found in the mantle cavity.
Figure 28.25 Torsion in gastropods. During embryonic development of some gastropods, the visceral mass undergoes
torsion, or counterclockwise rotation of the visceral anatomical features. As a result, the anus of the adult animal is
located over the head. Although torsion is always counterclockwise, the shell may coil in either direction; thus coiling
of a shell is not the same as torsion of the visceral mass.
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everyday CONNECTION
Can Snail Venom Be Used as a Pharmacological Painkiller?
Marine snails of the genus Conus (Figure 28.26) attack prey with a venomous stinger, modified from the
radula. The toxin released, known as conotoxin, is a peptide with internal disulfide linkages. Conotoxins can
bring about paralysis in humans, indicating that this toxin attacks neurological targets. Some conotoxins
have been shown to block neuronal ion channels. These findings have led researchers to study conotoxins
for possible medical applications.
Conotoxins are an exciting area of potential pharmacological development, since these peptides may be
possibly modified and used in specific medical conditions to inhibit the activity of specific neurons. For
example, conotoxins or modifications of them may be used to induce paralysis in muscles in specific health
applications, similar to the use of botulinum toxin. Since the entire spectrum of conotoxins, as well as their
mechanisms of action, is not completely known, the study of their potential applications is still in its infancy.
Most research to date has focused on their use to treat neurological diseases. They have also shown
some efficacy in relieving chronic pain, and the pain associated with conditions like sciatica and shingles.
The study and use of biotoxins—toxins derived from living organisms—are an excellent example of the
application of biological science to modern medicine.
Figure 28.26 Conus. Members of the genus Conus produce neurotoxins that may one day have medical uses.
The tube above the eyes is a siphon used both to circulate water over the gills and to sample the water for
chemical evidence of prey nearby. Note the eyes below the siphon. The proboscis, through which the venomous
harpoon is projected, is located between the eyes, (credit: David Burdick, NOAA)
Class Cephalopoda (“head foot” animals), includes octopuses, squids, cuttlefish, and nautiluses. Cephalopods
include both animals with shells as well as animals in which the shell is reduced or absent. In the shell-bearing
Nautilus, the spiral shell is multi-chambered. These chambers are filled with gas or water to regulate buoyancy.
A siphuncle runs through the chambers, and it is this tube that regulates the amount of water and gases
(nitrogen, carbon dioxide, and oxygen mixture) present in the chambers. Ammonites and other nautiloid shells
are commonly seen in the fossil record. The shell structure in squids and cuttlefish is reduced and is present
internally in the form of a squid pen and cuttlefish bone, respectively. Cuttle bone is sold in pet stores to help
smooth the beaks of birds and also to provide birds such as egg-laying chickens and quail with an inexpensive
natural source of calcium carbonate. Examples of cephalopods are shown in Figure 28.27.
Cephalopods can display vivid and rapidly changing coloration, almost like flashing neon signs. Typically these
flashing displays are seen in squids and octopuses, where they may be used for camouflage and possibly
as signals for mating displays. We should note, however, that researchers are not entirely sure if squid can
actually see color, or see color in the same way as we do. We know that pigments in the skin are contained
in special pigment cells ( chromatophores), which can expand or contract to produce different color patterns.
But chromatophores can only make yellow, red, brown, and black pigmentation; however, underneath them is a
whole different set of elements called iridophores and leucophores that reflect light and can make blue, green,
and white. It is possible that squid skin might actually be able to detect some light on its own, without even
needing its eyes!
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All animals in this class are carnivorous predators and have beak-like jaws in addition to the radula. Cephalopods
include the most intelligent of the mollusks, and have a well-developed nervous system along with image-forming
eyes. Unlike other mollusks, they have a closed circulatory system, in which the blood is entirely contained in
vessels rather than in a hemocoel.
The foot is lobed and subdivided into arms and tentacles. Suckers with chitinized rings are present on the arms
and tentacles of octopuses and squid. Siphons are well developed and the expulsion of water is used as their
primary mode of locomotion, which resembles jet propulsion. Gills (ctenidia) are attached to the wall of the
mantle cavity and are serviced by large blood vessels, each with its own heart. A pair of nephridia is present
within the mantle cavity for water balance and excretion of nitrogenous wastes. Cephalopods such as squids
and octopuses also produce sepia or a dark ink, which contains melanin. The ink gland is located between the
gills and can be released into the excurrent water stream. Ink clouds can be used either as a “smoke screen” to
hide the animal from predators during a quick attempt at escape, or to create a fake image to distract predators.
Cephalopods are dioecious. Members of a species mate, and the female then lays the eggs in a secluded and
protected niche. Females of some species care for the eggs for an extended period of time and may end up
dying during that time period. While most other aquatic mollusks produce trochophore larvae, cephalopod eggs
develop directly into a juvenile without an intervening larval stage.
(c) «)
Figure 28.27 Cephalopods. The (a) nautilus, (b) giant cuttlefish, (c) reef squid, and (d) blue-ring octopus are all
members of the class Cephalopoda, (credit a: modification of work by J. Baecker; credit b: modification of work by
Adrian Mohedano; credit c: modification of work by Silke Baron; credit d: modification of work by Angell Williams)
Members of class Scaphopoda (“boat feet”) are known colloquially as “tusk shells” or “tooth shells,” as evident
when examining Dentalium, one of the few remaining scaphopod genera (Figure 28.28). Scaphopods are
usually buried in sand with the anterior opening exposed to water. These animals have a single conical shell,
which is open on both ends. The head is not well developed, but the mouth, containing a radula, opens among
a group of tentacles that terminate in ciliated bulbs used to catch and manipulate prey. Scaphopods also have a
foot similar to that seen in bivalves. Ctenidia are absent in these animals; the mantle cavity forms a tube open at
both ends and serves as the respiratory structure in these animals.
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Figure 28.28 Tooth shells. Antalis vulgaris shows the classic Dentaliidae shape that gives these animals their common
name of "tusk shell." (credit: Georges Jansoone)
Phylum Annelida
Phylum Annelida comprises the true, segmented worms. These animals are found in marine, terrestrial, and
freshwater habitats, but the presence of water or humidity is a critical factor for their survival in terrestrial
habitats. The annelids are often called “segmented worms” due to their key characteristic of metamerism, or
true segmentation. Approximately 16,500 species have been described in phylum Annelida, which includes
polychaete worms (marine annelids with multiple appendages), and oligochaetes (earthworms and leeches).
Some animals in this phylum show parasitic and commensal symbioses with other species in their habitat.
Morphology
Annelids display bilateral symmetry and are worm-like in overall morphology. The name of the phylum is derived
from the Latin word annullus, which means a small ring, an apt description of the ring-like segmentation of
the body. Annelids have a body plan with metameric segmentation, in which several internal and external
morphological features are repeated in each body segment. Metamerism allows animals to become bigger by
adding “compartments,” while making their movement more efficient. The overall body can be divided into head,
body, and pygidium (or tail). During development, the segments behind the head arise sequentially from a growth
region anterior to the pygidium, a pattern called teloblastic growth. In the Oligochaetes, the clitellum is a
reproductive structure that generates mucus to aid sperm transfer and also produces a “cocoon,” within which
fertilization occurs; it appears as a permanent, fused band located on the anterior third of the animal (Figure
28.29).
Figure 28.29 The clitellum of an earthworm. The clitellum, seen here as a protruding segment with different coloration
than the rest of the body, is a structure that aids in oligochaete reproduction, (credit: Rob Hille)
Anatomy
The epidermis is protected by a collagenous, external cuticle, which is much thinner than the cuticle found in
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the ecdysozoans and does not require periodic shedding for growth. Circular as well as longitudinal muscles are
located interior to the epidermis. Chitinous bristles called setae (or chaetae) are anchored in the epidermis, each
with its own muscle. In the polychaetes, the setae are borne on paired appendages called parapodia.
Most annelids have a well-developed and complete digestive system. Feeding mechanisms vary widely across
the phylum. Some polychaetes are filter-feeders that use feather-like appendages to collect small organisms.
Others have tentacles, jaws, or an eversible pharynx to capture prey. Earthworms collect small organisms from
soil as they burrow through it, and most leeches are blood-feeders armed with teeth or a muscular proboscis.
In earthworms, the digestive tract includes a mouth, muscular pharynx, esophagus, crop, and muscular gizzard.
The gizzard leads to the intestine, which ends in an anal opening in the terminal segment. A cross-sectional
view of a body segment of an earthworm is shown in Figure 28.30; each segment is limited by a membranous
septum that divides the coelomic cavity into a series of compartments.
Most annelids possess a closed circulatory system of dorsal and ventral blood vessels that run parallel to the
alimentary canal as well as capillaries that service individual tissues. In addition, the dorsal and ventral vessels
are connected by transverse loops in every segment. Some polychaetes and leeches have an open system
in which the major blood vessels open into a hemocoel. In many species, the blood contains hemoglobin, but
not contained in cells. Annelids lack a well-developed respiratory system, and gas exchange occurs across the
moist body surface. In the polychaetes, the parapodia are highly vascular and serve as respiratory structures.
Excretion is facilitated by a pair of metanephridia (a type of primitive “kidney" that consists of a convoluted tubule
and an open, ciliated funnel) that is present in every segment toward the ventral side. Annelids show well-
developed nervous systems with a ring of fused ganglia present around the pharynx. The nerve cord is ventral
in position and bears enlarged nodes or ganglia in each segment.
Dorsal blood vessel
Nephridium
Intestine / Ventral blood vessel
Ventral nerve cord
Figure 28.30 Segmental anatomy of an earthworm. This schematic drawing shows the basic anatomy of annelids in a
cross-sectional view.
Annelids may be either monoecious with permanent gonads (as in earthworms and leeches) or dioecious
with temporary or seasonal gonads (as in polychaetes). However, cross-fertilization is preferred even in
hermaphroditic animals. Earthworms may show simultaneous mutual fertilization when they are aligned for
copulation. Some leeches change their sex over their reproductive lifetimes. In most polychaetes, fertilization
is external and development includes a trochophore larva, which then metamorphoizes to the adult form. In
oligochaetes, fertilization is typically internal and the fertilized eggs develop in a cocoon produced by the
clitellum; development is direct. Polychaetes are excellent regenerators and some even reproduce asexually by
budding or fragmentation.
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LINK TQ LEARNING
This combination video and animation (http://shapeoflife.org/video/animation/annelid-animation-body-
plan) provides a close-up look at annelid anatomy.
Classification of Phylum Annelida
Phylum Annelida contains the class Polychaeta (the polychaetes) and the class Oligochaeta (the earthworms,
leeches, and their relatives). The earthworms and the leeches form a monophyletic clade within the polychaetes,
which are therefore paraphyletic as a group.
There are more than 22,000 different species of annelids, and more than half of these are marine polychaetes
("many bristles"). In the polychaetes, bristles are arranged in clusters on their parapodia—fleshy, flat, paired
appendages that protrude from each segment. Many polychaetes use their parapodia to crawl along the sea
floor, but others are adapted for swimming or floating. Some are sessile, living in tubes. Some polychaetes live
near hydrothermal vents. These deepwater tubeworms have no digestive tract, but have a symbiotic relationship
with bacteria living in their bodies.
Earthworms are the most abundant members of the class Oligochaeta ("few bristles"), distinguished by the
presence of a permanent clitellum as well as the small number of reduced chaetae on each segment. (Recall
that oligochaetes do not have parapodia.) The oligochaete subclass Hirudinea, includes leeches such as
the medicinal leech, Hirudo medicinalis, which is effective at increasing blood circulation and breaking up
blood clots, and thus can be used to treat some circulatory disorders and cardiovascular diseases. Their use
goes back thousands of years. These animals produce a seasonal clitellum, unlike the permanent clitellum
of other oligochaetes. A significant difference between leeches and other annelids is the lack of setae and
the development of suckers at the anterior and posterior ends, which are used to attach to the host animal.
Additionally, in leeches, the segmentation of the body wall may not correspond to the internal segmentation of
the coelomic cavity. This adaptation possibly helps the leeches to elongate when they ingest copious quantities
of blood from host vertebrates, a condition in which they are said to be “engorged.” The subclass Brachiobdella
includes tiny leechlike worms that attach themselves to the gills or body surface of crayfish.
(a) (b) (c)
Figure 28.31 Annelid groups. The (a) earthworm, (b) leech, and (c) featherduster are all annelids. The earthworm and
leech are oligochaetes, while the featherduster worm is a tube-dwelling filter-feeding polychaete. (credit a: modification
of work by S. Shepherd; credit b: modification of work by “Sarah G...7Flickr; credit c: modification of work by Chris
Gotschalk, NOAA)
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28.5 | Superphylum Ecdysozoa: Nematodes and
Tardigrades
By the end of this section, you will be able to do the following:
• Describe the structural organization of nematodes
• Describe the importance of Caenorhabditis elegans in research
• Describe the features of Tardigrades
Superphylum Ecdysozoa
The superphylum Ecdysozoa contains an incredibly large number of species. This is because it contains two of
the most diverse animal groups: phylum Nematoda (the roundworms) and phylum Arthropoda (the arthropods).
The most prominent distinguishing feature of ecdysozoans is the cuticle—a tough, but flexible exoskeleton that
protects these animals from water loss, predators, and other dangers of the external environment. One small
phylum within the Ecdysozoa, with exceptional resistance to desiccation and other environmental hazards, is
the Tardigrada. The nematodes, tardigrades, and arthropods all belong to the superphylum Ecdysozoa, which is
believed to be monophyletic —a clade consisting of all evolutionary descendants from one common ancestor.
All members of this superphylum periodically go through a molting process that culminates in ecdysis—the
actual shedding of the old exoskeleton. (The term “ecdysis" translates roughly as “take off" or “strip.”) During the
molting process, old cuticle is replaced by a new cuticle, which is secreted beneath it, and which will last until
the next growth period.
Phylum Nematoda
The Nematoda, like other members of the superphylum Ecdysozoa, are triploblastic and possess an embryonic
mesoderm that is sandwiched between the ectoderm and endoderm. They are also bilaterally symmetrical,
meaning that a longitudinal section will divide them into right and left sides that are superficially symmetrical. In
contrast with flatworms, nematodes are pseudocoelomates and show a tubular morphology and circular cross-
section. Nematodes include both free-living and parasitic forms.
In 1914, N.A. Cobb said, “In short, if all the matter in the universe except the nematodes were swept away, our
world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find
its mountains, hills, vales, rivers, lakes and oceans represented by a thin film of nematodes...” To paraphrase
Cobb, nematodes are so abundant that if all the non-nematode matter of the biosphere were removed, there
[i]
would still remain a shadow of the former world outlined by nematodes! The phylum Nematoda includes more
than 28,000 species with an estimated 16,000 being parasitic in nature. However, nematologists believe there
may be over one million unclassified species.
The name Nematoda is derived from the Greek word “Nemos,” which means “thread,” and includes all true
roundworms. Nematodes are present in all habitats, typically with each species occurring in great abundance.
The free-living nematode, Caenorhabditis elegans, has been extensively used as a model system for many
different avenues of biological inquiry in laboratories all over the world.
Morphology
The cylindrical body form of the nematodes is seen in Figure 28.32. These animals have a complete digestive
system with a distinct mouth and anus, whereas only one opening is present in the digestive tract of flatworms.
The mouth opens into a muscular pharynx and intestine, which leads to a rectum and anal opening at the
posterior end. The epidermis can be either a single layer of cells or a syncytium —a multinucleated tissue that
in this case is formed by the fusion of many single cells. The cuticle of nematodes is rich in collagen and a
polymer called chitin, which forms a protective armor outside the epidermis. The cuticle extends into both ends
of the digestive tract, the pharynx, and rectum. In the head, an anterior mouth opening is composed of three
(or six) “lips” as well as teeth derived from the cuticle (in some species). Some nematodes may present other
modifications of the cuticle such as rings, head shields, or warts. These external rings, however, do not reflect
true internal body segmentation, which as we have seen is a hallmark of phylum Annelida. The attachment of
1. Stoll, N. R., “This wormy world. 1947,” Journal of Parasitology 85(3) (1999): 392-396.
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the muscles of nematodes differs from that of most animals: they have a longitudinal layer only, and their direct
attachment to the dorsal and ventral nerve cords creates a strong muscular contraction that results in a whiplike,
almost spastic, body movement.
(a)
Figure 28.32 Nematode morphology. Scanning electron micrograph shows (a) the soybean cyst nematode
(Heterodera glycines) and a nematode egg. (b) A schematic representation shows the anatomy of a typical nematode,
(credit a: modification of work by USDA ARS; scale-bar data from Matt Russell)
Excretory System
In nematodes, specialized excretory systems are not well developed. Nitrogenous wastes, largely in the form
of ammonia , are released directly across the body wall. In some nematodes, osmoregulation and salt balance
are performed by simple excretory cells or glands that may be connected to paired canals that release wastes
through an anterior pore. In marine nematodes, the excretory cells are called renette cells, which are unique to
nematodes.
Nervous system
Most nematodes have four longitudinal nerve cords that run along the length of the body in dorsal, ventral, and
lateral positions. The ventral nerve cord is better developed than the dorsal and lateral cords. Nonetheless,
all nerve cords fuse at the anterior end, to form a pharyngeal nerve ring around the pharynx, which acts as
the head ganglion or the “brain” of the roundworm. A similar fusion forms a posterior ganglion at the tail. In C.
eiegans, the nervous system accounts for nearly one-third of the total number of cells in the animal!
Reproduction
Nematodes employ a variety of reproductive strategies ranging from monoecious to dioecious to
parthenogenetic, depending upon the species. C. eiegans is a mostly monoecious species with both self¬
fertilizing hermaphrodites and some males. In the hermaphrodites, ova and sperm develop at different times in
the same gonad. Ova are contained in a uterus and amoeboid sperm are contained in a spermatheca ("sperm
receptacle"). The uterus has an external opening known as the vulva. The female genital pore is near the middle
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Chapter 28 | Invertebrates
823
of the body, whereas the male genital pore is nearer to the tip. In anatomical males, specialized structures called
copulatory spicules at the tail of the male keep him in place and open the vulva of the female into which the
amoeboid sperm travel into the spermatheca.
Fertilization is internal, and embryonic development starts very soon after fertilization. The embryo is released
from the vulva during the gastrulation stage. The embryonic development stage lasts for 14 hours; development
then continues through four successive larval stages with molting and ecdysis taking place between each
stage—LI, L2, L3, and L4—ultimately leading to the development of a young adult worm. Adverse environmental
conditions such as overcrowding or lack of food can result in the formation of an intermediate larval stage known
as the dauer larva. An unusual feature of some nematodes is eutely: the body of a given species contains a
specific number of cells as the consequence of a rigid developmental pathway.
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everyday CONNECTION
C. elegans: The Model System for Linking Developmental Studies
with Genetics
If biologists wanted to research how nicotine dependence develops in the body, how lipids are regulated, or
observe the attractant or repellant properties of certain odors, they would clearly need to design three very
different experiments. However, they might only need one subject of study: Caenorhabditis elegans. The
nematode C. elegans was brought into the focus of mainstream biological research by Dr. Sydney Brenner.
Since 1963, Dr. Brenner and scientists worldwide have used this animal as a model system to study many
different physiological and developmental mechanisms.
C. elegans is a free-living nematode found in soil. Only about a millimeter long, it can be cultured on agar
plates (10,000 worms/plate!), feeding on the common intestinal bacterium Escherichia coli (another long¬
term resident of biological laboratories worldwide), and therefore can be readily grown and maintained in
a laboratory. The biggest asset of this nematode is its transparency, which helps researchers to observe
and monitor changes within the animal with ease. It is also a simple organism with about 1,000 cells and
a genome of only 20,000 genes. Its chromosomes are organized into five pairs of autosomes plus a pair
of sex chromosomes, making it an ideal candidate with which to study genetics. Since every cell can be
visualized and identified, this organism is useful for studying cellular phenomena like cell-to-cell interactions,
cell-fate determinations, cell division, apoptosis (cell death), and intracellular transport.
Another tremendous asset is the short life cycle of this worm (Figure 28.33). It takes only three days
to achieve the “egg to adult to daughter egg”; therefore, the developmental consequences of genetic
changes can be quickly identified. The total life span of C. elegans is two to three weeks; hence, age-
related phenomena are also easy to observe. There are two sexes in this species: hermaphrodites (XX)
and males (XO). However, anatomical males are relatively infrequently obtained from matings between
hermaphrodites, since their XO chromosome composition requires meiotic nondisjunction when both
parents are XX. Another feature that makes C. elegans an excellent experimental model is that the position
and number of the 959 cells present in adult hermaphrodites of this organism is constant. This feature is
extremely significant when studying cell differentiation, cell-to-cell communication, and apoptosis. Lastly, C.
elegans is also amenable to genetic manipulations using molecular methods, rounding off its usefulness as
a model system.
Biologists worldwide have created information banks and groups dedicated to research using C. elegans.
Their findings have led, for example, to better understandings of cell communication during development,
neuronal signaling, and insight into lipid regulation (which is important in addressing health issues like the
development of obesity and diabetes). In recent years, studies have enlightened the medical community
with a better understanding of polycystic kidney disease. This simple organism has led biologists to complex
and significant findings, growing the field of science in ways that touch the everyday world.
Hexosamine cycling modulates
insulin signaling in C. elegans
ogt-1 knockout:
| Dauer = Insulin hypersensitive
oga-1 knockout:
'f Dauer = Insulin resistant
L3 (7 hrs)
(a) (b)
Figure 28.33 Caenorhabditis elegans. (a) This light micrograph shows the bodies of a group of roundworms.
These hermaphrodites consist of exactly 959 cells, (b) The life cycle of C. elegans has four juvenile stages (LI
through L4) and an adult stage. Under ideal conditions, the nematode spends a set amount of time at each
juvenile stage, but under stressful conditions, it may enter a dauer state that does not age significantly and is
somewhat analogous to the diapausing state of some insects, (credit a: modification of work by “snickclunk’VFlickr:
credit b: modification of work by NIDDK, NIH; scale-bar data from Matt Russell)
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Chapter 28 | Invertebrates
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Parasitic Nematodes
A number of common parasitic nematodes serve as prime examples of parasitism (endoparasitism). These
economically and medically important animals exhibit complex life cycles that often involve multiple hosts, and
they can have significant medical and veterinary impacts. Here is a partial list of nasty nematodes: Humans
may become infected by Dracunculus medinensis, known as guinea worms, when they drink unfiltered water
containing copepods (Figure 28.34), an intermediate crustacean host. Hookworms, such as Ancylostoma and
Necator, infest the intestines and feed on the blood of mammals, especially of dogs, cats, and humans. Trichina
worms ( Trichinella ) are the causal organism of trichinosis in humans, often resulting from the consumption of
undercooked pork; Trichinella can infect other mammalian hosts as well. Ascaris , a large intestinal roundworm,
steals nutrition from its human host and may create physical blockage of the intestines. The filarial worms, such
as Dirofilaria and Wuchereria, are commonly vectored by mosquitoes, which pass the infective agents among
mammals through their blood-sucking activity. One species, Wuchereria bancrofti, infects the lymph nodes of
over 120 million people worldwide, usually producing a non-lethal but deforming condition called elephantiasis.
in this disease, parts of the body often swell to gigantic proportions due to obstruction of lymphatic drainage,
inflammation of lymphatic tissues, and resulting edema. Dirofilaria immitis, a blood-infective parasite, is the
notorious dog heartworm species.
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(a)
Figure 28.34 Life cycle of the guinea worm. The guinea worm Dracunculus medinensis infects about 3.5 million people
annually, mostly in Africa, (a) Here, the worm is wrapped around a stick so it can be slowly extracted, (b) Infection
occurs when people consume water contaminated by infected copepods, but this can easily be prevented by simple
filtration systems, (credit: modification of work by CDC)
Phylum Tardigrada
The tardigrades ("slow-steppers") comprise a phylum of inconspicuous little animals living in marine, freshwater,
or damp terrestrial environments throughout the world. They are commonly called "water bears" because of
their plump bodies and the large claws on their stubby legs. There are over 1,000 species, most of which are
less than 1 mm in length. A chitinous cuticle covers the body surface and may be divided into plates (Figure
28.35). Tardigrades are known for their ability to enter a state called cryptobiosis, which provides them with
are resistance to multiple environmental challenges, including desiccation, very low temperatures, vacuum, high
pressure, and radiation. They can suspend their metabolic activity for years, and survive the loss of up to 99%
of their water content. Their remarkable resistance has recently been attributed to unique proteins that replace
water in their cells and protect their internal cell structure and their DNA from damage.
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Chapter 28 | Invertebrates
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Figure 28.35 Scanning electronmicrograph of Mitnesium tardigradum. (credit: Schokraie E, Warnken U, Hotz-
Wagenblatt A, Grohme MA, Hengherr S, et al. (2012) -https://commons.wikimedia. 0 rg/w/
index.php?curid=22716809 (http:// 0 penstax. 0 rg/l/waterbear) )
Morphology and Physiology
Tardigrades have cylindrical bodies, with four pairs of legs terminating in a number of claws. The cuticle is
periodically shed, including the cuticular covering of the claws. The first three pairs of legs are used for walking,
and the posterior pair for clinging to the substrate. A circular mouth leads to a muscular pharynx and salivary
glands. Tardigrades feed on plants, algae, or small animals. Plant cells are pierced with a chitinous stylet and
the cellular contents are then sucked into the gut by the muscular pharynx. Bands of single muscle cells are
attached to the various points of the epidermis and extend into the legs to provide ambulatory movement. The
major body cavity is a hemocoel, but there are no specialized circulatory structures for moving the blood, nor
are there specialized respiratory structures. Malpighian tubules in the hemocoel remove metabolic wastes and
transport them to the gut. A dorsal brain is connected to a ventral nerve cord with segmental ganglia associated
with the appendages. Sensory structures are greatly reduced, but there is a pair of simple eyespots on the head,
and sensory cilia or bristles concentrated toward the head end of the animal.
Reproduction
Most tardigrades are dioecious, and males and females each have a single gonad. Mating usually occurs at the
time of a molt and fertilization is external. Eggs may be deposited in a molted cuticle or attached to other objects.
Development is direct, and the animal may molt a dozen times during its lifetime. In tardigrades, like nematodes,
development produces a fixed number of cells, with the actual number of cells being dependent on the species.
Further growth occurs by enlarging the cells, not by multiplying them.
28.6 | Superphylum Ecdysozoa: Arthropods
By the end of this section, you will be able to do the following:
• Compare the internal systems and appendage specializations of phylum Arthropoda
• Discuss the environmental importance of arthropods
• Discuss the reasons for arthropod success and abundance
The superphylum Ecdysozoa also includes the phylum Arthropoda, one of the most successful clades of animals
on the planet. Arthropods are coelomate organisms characterized by a sturdy chitinous exoskeleton and jointed
appendages. There are well over a million arthropod species described, and systematists believe that there are
millions of species awaiting proper classification. Like other Ecdysozoa, all arthropods periodically go through
the physiological process of molting, followed by ecdysis (the actual shedding of the exoskeleton), as they
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grow. Arthropods are eucoelomate, protostomic organisms, often with incredibly complicated life cycles.
Phylum Arthropoda
The name “arthropoda” means “jointed feet.” The name aptly describes the invertebrates included in this phylum.
Arthropods have probably always dominated the animal kingdom in terms of number of species and likely will
continue to do so: An estimated 85 percent of all known species are included in this phylum! In effect, life on
Earth could conceivably be called the Age of Arthropods beginning nearly 500 million years ago.
The principal characteristics of all the animals in this phylum are the structural and functional segmentation
of the body and the presence of jointed appendages. Arthropods have an exoskeleton made principally of
chitin—a waterproof, tough polysaccharide composed of N-acetylglucosamine. Phylum Arthropoda is the most
speciose clade in the animal world (Table 28.1), and insects form the single largest class within this phylum. For
comparison, refer to the approximate numbers of species in the phyla listed below.
Phylum
# species
Ctenophora
100
Porifera
5,000
Cnidaria
11,000
Platyhelminthes
25,000
Rotifera
2,000
Nemertea
1,200
Annelida
22,000
Mollusca
112,000
Nematoda
28,000+
Tardigrades
>1,000
Arthropoda
1,134,000
Echinodermata
7,000
Chordata
100,000
Table 28.1
Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial
habitats. This phylum is further classified into five subphyla: Trilobita (trilobites, all extinct), Chelicerata
(horseshoe crabs, spiders, scorpions, ticks, mites, and daddy longlegs or harvestmen), Myriapoda (millipedes,
centipedes, and their relatives), Crustacea (crabs, lobsters, crayfish, isopods, barnacles, and some
zooplankton), and Hexapoda (insects and their six-legged relatives). Trilobites, an extinct group of arthropods
found chiefly in the pre-Cambrian Era (about 500 million years ago), are probably most closely related to the
Chelicerata. These are identified based on their fossils; they were quite diverse and radiated significantly into
thousands of species before their complete extinction at the end of the Permian about 240 million years ago
(Figure 28.36).
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Chapter 28 | Invertebrates
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Figure 28.36 A trilobite. Trilobites, like the one in this fossil, are an extinct group of arthropods. Their name "trilobite"
refers to the three longitudinal lobes making up the body: right pleural lobe, axial lobe, and left pleural lobe (credit:
Kevin Walsh).
Morphology
Characteristic features of the arthropods include the presence of jointed appendages, body segmentation,
and chitinized exoskeleton. Fusion of adjacent groups of segments gave rise to functional body regions
called tagmata (singular = tagma). Tagmata may be in the form of a head, thorax, and abdomen, or a
cephalothorax and abdomen, or a head and trunk, depending on the taxon. Commonly described tagmata
may be composed of different numbers of segments; for example, the head of most insects results from the
fusion of six ancestral segments, whereas the “head" of another arthropod may be made of fewer ancestral
segments, due to independent evolutionary events. Jointed arthropod appendages, often in segmental pairs,
have been specialized for various functions: sensing their environment (antennae), capturing and manipulating
food (mandibles and maxillae), as well as for walking, jumping, digging, and swimming.
In the arthropod body, a central cavity, called the hemocoel (or blood cavity), is present, and the hemocoel fluids
are moved by contraction of regions of the tubular dorsal blood vessel called "hearts." Groups of arthropods also
differ in the organs used for nitrogenous waste excretion, with crustaceans possessing green glands and insects
using Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while ridding the body
of nitrogenous waste. The nervous system tends to be distributed among the segments, with larger ganglia in
segments with sensory structures or appendages. The ganglia are connected by a ventral nerve cord.
Respiratory systems vary depending on the group of arthropod. Insects and myriapods use a series of tubes
(tracheae) that branch through the body, ending in minute tracheoles. These fine respiratory tubes perform
gas exchange directly between the air and cells within the organism. The major tracheae open to the surface
of the cuticle via apertures called spiracles. We should note that these tracheal systems of ventilation have
evolved independently in hexapods, myriapods, and arachnids. Although the tracheal system works extremely
well in terrestrial environments, it also works well in freshwater aquatic environments: In fact, numerous species
of aquatic insects in both immature and adult stages possess tracheal systems. However, although there are
insects that live on the surface of marine environments, none is strictly marine—meaning that they complete
their entire metamorphosis in saltwater.
In contrast, aquatic crustaceans utilize gills, terrestrial chelicerates employ book lungs, and aquatic chelicerates
use book gills (Figure 28.37). The book lungs of arachnids (scorpions, spiders, ticks, and mites) contain a
vertical stack of hemocoel wall tissue that somewhat resembles the pages of a book. Between each of the
"pages" of tissue is an air space. This allows both sides of the tissue to be in contact with the air at all
times, greatly increasing the efficiency of gas exchange. The gills of crustaceans are filamentous structures that
exchange gases with the surrounding water.
The cuticle is the hard “covering” of an arthropod. It is made up of two layers: the epicuticle, which is a
thin, waxy, water-resistant outer layer containing no chitin, and the layer beneath it, the chitinous procuticle,
which itself is composed of an exocuticle and a lower endocuticle, all supported ultimately by a basement
membrane. The exoskeleton is very protective (it is sometimes difficult to squish a big beetle!), but does not
sacrifice flexibility or mobility. Both the exocuticle (which is secreted before a molt), and an endocuticle, (which
is secreted after a molt), are composed of chitin bound with a protein; chitin is insoluble in water, alkalis, and
weak acids. The procuticle is not only flexible and lightweight, but also provides protection against dehydration
and other biological and physical stresses. Some hexapods, such as the crustaceans, add calcium salts to their
exoskeleton, which increases the strength of the cuticle, but does reduce its flexibility. In some cases, such as
lobsters, the amount of calcium salt deposited within the chitin is extreme. In order to grow, the arthropod must
“shed” the exoskeleton during the physiological process called molting, following by the actual stripping of the
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Chapter 28 | Invertebrates
outer cuticle, called ecdysis (“to strip off”). At first, this seems to be a dangerous method of growth, because
while the new exoskeleton is hardening, the animal is vulnerable to predation; however, molting and ecdysis
also allow for growth and change in morphology, as well as for great diversification in size, simply because the
numbers of molts can be modified through evolution.
The characteristic morphology of representative animals from each subphylum is described below.
Figure 28.37 Arthropod respiratory structures. The book lungs of (a) arachnids are made up of alternating air pockets
and hemocoel tissue shaped like a stack of books (hence the name, “book lung"). The book gills of (b) horseshoe
crabs are similar to book lungs but are external so that gas exchange can occur with the surrounding water, (credit a:
modification of work by Ryan Wilson based on original work by John Henry Comstock; credit b: modification of work
by Angel Schatz)
Subphylum Chelicerata
This subphylum includes animals such as horseshoe crabs, sea spiders, spiders, mites, ticks, scorpions, whip
scorpions, and harvestmen. Chelicerates are predominantly terrestrial, although some freshwater and marine
species also exist. An estimated 77,000 species of chelicerates can be found in almost all terrestrial habitats.
The body of chelicerates is divided into two tagmata: prosoma and opisthosoma, which are basically the
equivalents of a cephalothorax (usually smaller) and an abdomen (usually larger). A distinct “head” tagma is
not usually discernible. The phylum derives its name from the first pair of appendages: the chelicerae (Figure
28.38), which serve as specialized clawlike or fanglike mouthparts. We should note here that chelicerae are
actually modified legs, but they are not the exact serial equivalent of mandibles, which are the modified leglike
chewing mouthparts of insects and crustaceans: The chelicerae are borne on the first segment making up
the prosoma, whereas the mandibles are embryonically on the fourth segment of the mandibulate head. The
chelicerates have secondarily lost their antennae and hence do not have them. Some of the functions of the
antennae (such as touch) are now performed by the second pair of appendages— the pedipalps, which may
also be used for general sensing the environment as well as the manipulation of food. In some species, such
as sea spiders, an additional pair of derived leg appendages, called ovigers, is present between the chelicerae
and pedipalps. Ovigers are used for grooming and by males to carry eggs. In spiders, the chelicerae are often
modified and terminate in fangs that inject venom into their prey before feeding (Figure 28.39).
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Chapter 28 | Invertebrates
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Figure 28.38 Chelicerae. The chelicerae (first set of appendages) are well developed in the scorpion, (credit: Kevin
Walsh)
Most chelicerates ingest food using a preoral cavity formed by the chelicerae and pedipalps. Some chelicerates
may secrete digestive enzymes to pre-digest food before ingesting it. Parasitic chelicerates like ticks and
mites have evolved blood-sucking apparatuses. Members of this subphylum have an open circulatory system
with a heart that pumps blood into the hemocoel. Aquatic species, like horseshoe crabs, have gills, whereas
terrestrial species have either tracheae or book lungs for gaseous exchange. Chelicerate hemolymph contains
hemocyanin a copper-containing oxygen transport protein.
Figure 28.39 Spider. The trapdoor spider, like all spiders, is a member of the subphylum Chelicerata. (credit: Marshal
Hedin)
The nervous system in chelicerates consists of a brain and two ventral nerve cords. Chelicerates are dioecious,
meaning that the sexes are separate. These animals use external fertilization as well as internal fertilization
strategies for reproduction, depending upon the species and its habitat. Parental care for the young ranges from
absolutely none to relatively prolonged care.
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Chapter 28 | Invertebrates
LINK TQ LEARNING
Visit this site (http://openstaxcollege. 0 rg/l/arthropodstory) to click through a lesson on arthropods,
including interactive habitat maps, and more.
Subphylum Myriapoda
Subphylum Myriapoda comprises arthropods with numerous legs. Although the name is misleading, suggesting
that thousands of legs are present in these invertebrates, the number of legs typically varies from 10 to 750.
This subphylum includes 16,000 species; the most commonly found examples are millipedes and centipedes.
Virtually all myriapods are terrestrial animals and prefer a humid environment. Ancient myriapods (or myriapod¬
like arthropods) from the Silurian to the Devonian grew up to 10 feet in length (three meters). Unfortunately, they
are all extinct!
Myriapods are typically found in moist soils, decaying biological material, and leaf litter. Subphylum Myriapoda
is divided into four classes: Chilopoda, Symphyla, Diplopoda, and Pauropoda. Centipedes like Scutigera
coleoptrata (Figure 28.40) are classified as chilopods. These animals bear one pair of legs per segment,
mandibles as mouthparts, and are somewhat dorsoventrally flattened. The legs in the first segment are modified
to form forcipules (poison claws) that deliver poison to prey like spiders and cockroaches, as these animals
are all predatory. Symphyla are similar to centipedes, but lack the poison claws and are vegetarian. Millipedes
bear two pairs of legs per diplosegment—a feature that results from the embryonic fusion of adjacent pairs of
body segments. These arthropods are usually rounder in cross-section than centipedes, and are herbivores or
detritivores. Millipedes have visibly more numbers of legs as compared to centipedes, although they do not have
a thousand legs (Figure 28.40b). The Pauropods are similar to millipedes, but have fewer segments.
(a) (b)
Figure 28.40 Myriapods. The centipede Scutigera coleoptrata (a) has up to 15 pairs of legs. The North American
millipede Narceus americanus (b) bears many legs, although not a thousand, as its name might suggest, (credit a:
modification of work by Bruce Marlin; credit b: modification of work by Cory Zanker)
Subphylum Crustacea
Crustaceans are the most dominant aquatic (both freshwater and marine) arthropods, with the total number of
marine crustaceans standing at about 70,000 species. Krill, shrimp, lobsters, crabs, and crayfish are examples
of crustaceans (Figure 28.41). However, there are also a number of terrestrial crustacean species as well:
Terrestrial species like the wood lice ( Armadillidium spp), also called pill bugs, roly-polies, potato bugs, or
isopods, are also crustaceans. Nonetheless, the number of terrestrial species in this subphylum is relatively low.
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Chapter 28 | Invertebrates
833
(a) (b) (c)
Figure 28.41 Crustaceans. The (a) crab and (b) shrimp krill are both aquatic crustaceans. The pill bug Armadillidium
is a terrestrial crustacean, (credit a: modification of work by William Warby; credit b: modification of work by Jon
Sullivan credit c: modification of work by Franco Folini. httpsd/commons.wikimedia.org/w/index.php?curid=789616
(http:// 0 penstax. 0 rg/l/pillbug) )
Crustaceans typically possess two pairs of antennae, mandibles as mouthparts, and biramous (“two branched”)
appendages, which means that their legs are formed in two parts called endopods and exopods, which appear
superficially distinct from the uniramous (“one branched”) legs of myriapods and hexapods (Figure 28.42). Since
biramous appendages are also seen in the trilobites, biramous appendages represent the ancestral condition in
the arthropods. Currently, we describe various arthropods as having uniramous or biramous appendages, but
these are descriptive only, and do not necessarily reflect evolutionary relationships other than that all jointed legs
of arthropods share common ancestry.
(a) Biramous appendage (crayfish leg) (b) Uniramous appendage (insect leg)
Figure 28.42 Arthropod appendages. Arthropods may have (a) biramous (two-branched) appendages or (b)
uniramous (one-branched) appendages, (credit b: modification of work by Nicholas W. Beeson)
In most crustaceans, the head and thorax is fused to form a cephalothorax (Figure 28.43), which is covered
by a plate called the carapace, thus producing a body plan comprising two tagmata: cephalophorax and
abdomen. Crustaceans have a chitinous exoskeleton that is shed by molting and ecdysis whenever the animal
requires an increase in size or the next stage of development. The exoskeletons of many aquatic species
are also infused with calcium carbonate, which makes them even stronger than those of other arthropods.
Crustaceans have an open circulatory system where blood is pumped into the hemocoel by the dorsally located
heart. Hemocyanin is the major respiratory pigment present in crustaceans, but hemoglobin is found in a few
species and both are dissolved in the hemolymph rather than carried in cells.
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Figure 28.43 Crustacean anatomy. The crayfish is an example of a crustacean. It has a carapace around the
cephalothorax and the heart in the dorsal thorax area, (credit: Jane Whitney)
As in the chelicerates, most crustaceans are dioecious. However, some species like barnacles may be
hermaphrodites. Serial hermaphroditism, where the gonad can switch from producing sperm to ova, is also
exhibited in some species. Fertilized eggs may be held within the female of the species or may be released in
the water. Terrestrial crustaceans seek out damp spaces in their habitats to lay eggs.
Larval stages—nauplius or zoea—are seen in the early development of aquatic crustaceans. A cypris larva is
also seen in the early development of barnacles (Figure 28.44).
(a) Nauplius larva of a tadpole shrimp (b) Cypris larva of a barnacle (c) Zoea larva of a green crab
Figure 28.44 Crustacean larvae. All crustaceans go through different larval stages. Shown are (a) the nauplius larval
stage of a tadpole shrimp, (b) the cypris larval stage of a barnacle, and (c) the zoea larval stage of a green crab, (credit
a: modification of work by USGS; credit b: modification of work by M a . C. Mingorance Rodriguez; credit c: modification
of work by B. Kimmel based on original work by Ernst Haeckel)
Crustaceans possess a brain formed by the fusion of the first three segmental ganglia, as well as two
compound eyes. A ventral nerve cord connects additional segmental ganglia. Most crustaceans are carnivorous,
but herbivorous and detritivorous species, and even endoparasitic species are known. A highly evolved
endoparasitic species, such as Sacculina spp, parasitizes its crab host and ultimately destroys it after it forces
the host to incubate the parasite’s eggs! Crustaceans may also be cannibalistic when extremely high populations
of these organisms are present.
Subphylum Hexapoda
The insects comprise the largest class of arthropods in terms of species diversity as well as in terms of
biomass—at least in terrestrial habitats.
The name Hexapoda describes the presence of six legs (three pairs) in these animals, which differentiates them
from other groups of arthropods that have different numbers of legs. In some cases, however, the number of
legs has been evolutionarily reduced, or the legs have been highly modified to accommodate specific conditions,
such as endoparasitism. Hexapod bodies are organized into three tagmata: head, thorax, and abdomen.
Individual segments of the head have mouthparts derived from jointed legs, and the thorax has three pairs of
jointed appendages, and also wings, in most derived groups. For example, in the pterygotes (winged insects),
in addition to a pair of jointed legs on all three segments comprising the thorax: prothorax, mesothorax, and
metathorax.
Appendages found on other body segments are also evolutionarily derived from modified legs. Typically, the
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head bears an upper “lip” or labrum and mandibles (or derivation of mandibles) that serve as mouthparts;
maxillae, and a lower “lip” called a labium: both of which manipulate food. The head also has one pair of
sensory antennae, as well as sensory organs such as a pair of compound eyes, ocelli (simple eyes), and
numerous sensory hairs. The abdomen usually has 11 segments and bears external reproductive apertures. The
subphylum Hexapoda includes some insects that are winged (such as fruit flies) and others that are secondarily
wingless (such as fleas). The only order of “primitively wingless” insects is the Thysanura, the bristletails. All
other orders are winged or are descendants of formally winged insects.
The evolution of wings is a major, unsolved mystery. Unlike vertebrates, whose “wings” are simply
preadaptations of “arms” that served as the structural foundations for the evolution of functional wings (this has
occurred independently in pterosaurs, dinosaurs [birds], and bats), the evolution of wings in insects is a what
we call a de novo (new) development that has given the pteryogotes domination over the Earth. Winged insects
existed over 425 million years ago, and by the Carboniferous, several orders of winged insects ( Paleoptera),
most of which are now extinct, had evolved. There is good physical evidence that Paleozoic nymphs with
thoracic winglets (perhaps hinged, former gill covers of semi-aquatic species) used these devices on land to
elevate the thoracic temperature (the thorax is where the legs are located) to levels that would enable them to
escape predators faster, find more food resources and mates, and disperse more easily. The thoracic winglets
(which can be found on fossilized insects preceding the advent of truly winged insects) could have easily been
selected for thermoregulatory purposes prior to reaching a size that would have allowed them the capacity for
gliding or actual flapping flight. Even modern insects with broadly attached wings, such as butterflies, use the
basal one-third of their wings (the area next to the thorax) for thermoregulation, and the outer two-thirds for flight,
camouflage, and mate selection.
Many of the common insects we encounter on a daily basis—including ants, beetles, cockroaches, butterflies,
crickets and flies—are examples of Hexapoda. Among these, adult ants, beetles, flies, and butterflies develop
by complete metamorphosis from grub-like or caterpillar-like larvae, whereas adult cockroaches and crickets
develop through a gradual or incomplete metamorphosis from wingless immatures. All growth occurs during the
juvenile stages. Adults do not grow further (but may become larger) after their final molt. Variations in wing, leg,
and mouthpart morphology all contribute to the enormous variety seen in the insects. Insect variability was also
encouraged by their activity as pollinators and their coevolution with flowering plants. Some insects, especially
termites, ants, bees, and wasps, are eusocial, meaning that they live in large groups with individuals assigned
to specific roles or castes, like queen, drone, and worker. Social insects use pheromones— external chemical
signals—to communicate and maintain group structure as well as a cohesive colony.
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CONNECTION
Abdomen
Thorax
Anus
Spirat
Intesti
Antenna
Figure 28.45 Insect anatomy. In this basic anatomy of a hexapod insect, note that insects have a well-developed
digestive system (yellow), a respiratory system (blue), a circulatory system (red), and a nervous system (purple).
Note the multiple "hearts" and the segmental ganglia.
Which of the following statements about insects is false?
a. Insects have both dorsal and ventral blood vessels.
b. Insects have spiracles, openings that allow air to enter into the tracheal system.
c. The trachea is part of the digestive system.
d. Most insects have a well-developed digestive system with a mouth, crop, and intestine.
28.7 | Superphylum Deuterostomia
By the end of this section, you will be able to do the following:
• Describe the distinguishing characteristics of echinoderms
• Describe the distinguishing characteristics of chordates
The phyla Echinodermata and Chordata (the phylum that includes humans) both belong to the superphylum
Deuterostomia. Recall that protostomes and deuterostomes differ in certain aspects of their embryonic
development, and they are named based on which opening of the archenteron (primitive gut tube) develops
first. The word deuterostome comes from the Greek word meaning “mouth second,” indicating that the mouth
develops as a secondary structure opposite the location of the blastopore, which becomes the anus. In
protostomes (“mouth first”), the first embryonic opening becomes the mouth, and the second opening becomes
the anus.
There are a series of other developmental characteristics that differ between protostomes and deuterostomes,
including the type of early cleavage (embryonic cell division) and the mode of formation of the coelom of the
embryo: Protosomes typically exhibit spiral mosaic cleavage whereas deuterostomes exhibit radial regulative
cleavage. In deuterostomes, the endodermal lining of the archenteron usually forms buds called coelomic
pouches that expand and ultimately obliterate the embryonic blastocoel (the cavity within the blastula and early
gastrula) to become the embryonic mesoderm, the third germ layer. This happens when the mesodermal
pouches become separated from the invaginating endodermal layer forming the archenteron, then expand and
fuse to form the coelomic cavity. The resulting coelom is termed an enterocoelom. The archenteron develops
into the alimentary canal, and a mouth opening is formed by invagination of ectoderm at the pole opposite the
blastopore of the gastrula. The blastopore forms the anus of the alimentary system in the juvenile and adult
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forms. Cleavage in most deuterostomes is also indeterminant, meaning that the developmental fates of early
embryonic cells are not decided at that point of embryonic development (this is why we could potentially clone
most deuterostomes, including ourselves).
The deuterostomes consist of two major clades—the Chordata and the Ambulacraria. The Chordata include
the vertebrates and two invertebrate subphyla, the urochordates and the cephalochordates. The Ambulacraria
include the echinoderms and the hemichordates, which were once considered to be a chordate subphylum
(Figure 28.46). The two clades, in addition to being deuterostomes, have some other interesting features in
common. As we have seen, the vast majority of invertebrate animals do not possess a defined bony vertebral
endoskeleton, or a bony cranium. However, one of the most ancestral groups of deuterostome invertebrates,
the Echinodermata, do produce tiny skeletal “bones" called ossicles that make up a true endoskeleton, or
internal skeleton, covered by an epidermis. The Hemichordata (acorn worms and pterobranchs) will not be
covered here, but share with the echinoderms a three-part (tripartite) coelom, similar larval forms, and a derived
metanephridium that rids the animals of nitrogenous wastes. They also share pharyngeal slits with the chordates
(Figure 28.46). In addition, hemichordates have a dorsal nerve cord in the midline of the epidermis, but lack a
neural tube, a true notochord and the endostyle and post-anal tail characteristic of chordates.
Deuterostomes
Ambulacraria
Chordates
-Echinoderms
Hemichordates
-Cephalochordates
-Urochordates
Vertebrates
(a)
Hemichordate
Urchordate
(h)
Figure 28.46 Ambulacraria and Chordata, (a) The major deuterostome taxa. (b) pharyngeal slits in hemichordates and
urochordates. (credit a MAC; credit b modification of Gill Slits By Own work by Zebra.element [Public domain], via
Wikimedia Commons)
Phylum Echinodermata
Echinodermata are named after their “prickly skin” (from the Greek “echinos” meaning “prickly” and “dermos”
meaning “skin"). This phylum is a collection of about 7,000 described living species of exclusively marine,
bottom-dwelling organisms. Sea stars (Figure 28.47), sea cucumbers, sea urchins, sand dollars, and brittle stars
are all examples of echinoderms.
Morphology and Anatomy
Despite the adaptive value of bilaterality for most free-living cephalized animals, adult echinoderms exhibit
pentaradial symmetry (with “arms” typically arrayed in multiples of five around a central axis). Echinoderms have
an endoskeleton made of calcareous ossicles (small bony plates), covered by the epidermis. For this reason, it
is an endoskeleton like our own, not an exoskeleton like that of arthropods. The ossicles may be fused together,
embedded separately in the connective tissue of the dermis, or be reduced to minute spicules of bone as in sea
cucumbers. The spines for which the echinoderms are named are connected to some of the plates. The spines
may be moved by small muscles, but they can also be locked into place for defense. In some species, the spines
are surrounded by tiny stalked claws called pedicellaria, which help keep the animal's surface clean of debris,
protect papulae used in respiration, and sometimes aid in food capture.
The endoskeleton is produced by dermal cells, which also produce several kinds of pigments, imparting vivid
colors to these animals. In sea stars, fingerlike projections (papillae) of dermal tissue extend through the
endoskeleton and function as gills. Some cells are glandular, and may produce toxins. Each arm or section of
the animal contains several different structures: for example, digestive glands, gonads, and the tube feet that
are unique to the echinoderms. In echinoderms like sea stars, every arm bears two rows of tube feet on the oral
side, running along an external ambulacral groove. These tube feet assist in locomotion, feeding, and chemical
sensations, as well as serve to attach some species to the substratum.
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Figure 28.47 Anatomy of a sea star. This diagram of a sea star shows the pentaradial pattern typical of adult
echinoderms, and the water vascular system that is their defining characteristic.
Water Vascular and Hemal Systems
Echinoderms have a unique ambulacral (water vascular) system, derived from part of the coelom, or “body
cavity.” The water vascular system consists of a central ring canal and radial canals that extend along each arm.
Each radial canal is connected to a double row of tube feet, which project through holes in the endoskeleton,
and function as tactile and ambulatory structures. These tube feet can extend or retract based on the volume
of water present in the system of that arm, allowing the animal to move and also allowing it to capture prey
with their suckerlike action. Individual tube feet are controlled by bulblike ampullae. Seawater enters the system
through an aboral madreporite (opposite the oral area where the mouth is located) and passes to the ring
canal through a short stone canal. Water circulating through these structures facilitates gaseous exchange and
provides a hydrostatic source for locomotion and prey manipulation. A hemal system, consisting of oral, gastric,
and aboral rings, as well as other vessels running roughly parallel to the water vascular system, circulates
nutrients. Transport of nutrients and gases is shared by the water vascular and hemal systems in addition to the
visceral body cavity that surrounds the major organs.
Nervous System
The nervous system in these animals is a relatively simple, comprising a circumoral nerve ring at the center
and five radial nerves extending outward along the arms. In addition, several networks of nerves are located
in different parts of the body. However, structures analogous to a brain or large ganglia are not present in
these animals. Depending on the group, echinoderms may have well-developed sensory organs for touch and
chemoreception (e.g., within the tube feet and on tentacles at the tips of the arms), as well as photoreceptors
and statocysts.
Digestive and Excretory Systems
A mouth, located on the oral (ventral) side, opens through a short esophagus to a large, baglike stomach. The
so-called “cardiac" stomach can be everted through the mouth during feeding (for example, when a starfish
everts its stomach into a bivalve prey item to digest the animal— alive —within its own shell!) There are masses
of digestive glands ( pyloric caeca) in each arm, running dorsally along the arms and overlying the reproductive
glands below them. After passing through the pyloric caeca in each arm, the digested food is channeled to a
small anus, if one exists.
Podocytes—cells specialized for ultrafiltration of bodily fluids—are present near the center of the echinoderm
disc, at the junction of the water vascular and hemal systems. These podocytes are connected by an internal
system of canals to the madreporite, where water enters the stone canal. The adult echinoderm typically has a
spacious and fluid-filled coelom. Cilia aid in circulating the fluid within the body cavity, and lead to the fluid-filled
papulae, where the exchange of oxygen and carbon dioxide takes place, as well as the secretion of nitrogenous
waste such as ammonia, by diffusion.
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Reproduction
Echinoderms are dioecious, but males and females are indistinguishable apart from their gametes. Males and
females release their gametes into water at the same time and fertilization is external. The early larval stages
of all echinoderms (e.g., the bipinnaria of asteroid echinoderms such as sea stars) have bilateral symmetry,
although each class of echinoderms has its own larval form. The radially symmetrical adult forms from a cluster
of cells in the larva. Sea stars, brittle stars, and sea cucumbers may also reproduce asexually by fragmentation,
as well as regenerate body parts lost in trauma, even when over 75 percent of their body mass is lost!
Classes of Echinoderms
This phylum is divided into five extant classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea
(sea urchins and sand dollars), Crinoidea (sea lilies or feather stars), and Holothuroidea (sea cucumbers)
(Figure 28.48).
The most well-known echinoderms are members of class Asteroidea, or sea stars. They come in a large variety
of shapes, colors, and sizes, with more than 1,800 species known so far. The key characteristic of sea stars
that distinguishes them from other echinoderm classes includes thick arms that extend from a central disk from
which various body organs branch into the arms. At the end of each arm are simple eye spots and tentacles
that serve as touch receptors. Sea stars use their rows of tube feet not only for gripping surfaces but also for
grasping prey. Most sea stars are carnivores and their major prey are in the phylum Mollusca. By manipulating
its tube feet, a sea star can open molluscan shells. Sea stars have two stomachs, one of which can protrude
through their mouths and secrete digestive juices into or onto prey, even before ingestion. A sea star eating a
clam can partially open the shell, and then evert its stomach into the shell, introducing digestive enzymes into
the interior of the mollusk. This process can both weaken the strong adductor (closing) muscles of a bivalve and
begin the process of digestion.
LINK TQ LEARNING
Explore the sea star’s body plan (http:// 0 penstaxc 0 llege. 0 rg/l/sea_star) up close, watch one move across
the sea floor, and see it devour a mussel.
Brittle stars belong to the class Ophiuroidea ("snake-tails"). Unlike sea stars, which have plump arms, brittle stars
have long, thin, flexible arms that are sharply demarcated from the central disk. Brittle stars move by lashing
out their arms or wrapping them around objects and pulling themselves forward. Their arms are also used for
grasping prey. The water vascular system in ophiuroids is not used for locomotion.
Sea urchins and sand dollars are examples of Echinoidea ("prickly"). These echinoderms do not have arms, but
are hemispherical or flattened with five rows of tube feet that extend through five rows of pores in a continuous
internal shell called a test. Their tube feet are used to keep the body surface clean. Skeletal plates around the
mouth are organized into a complex multipart feeding structure called "Aristotle's lantern." Most echinoids graze
on algae, but some are suspension feeders, and others may feed on small animals or organic detritus —the
fragmentary remains of plants or animals.
Sea lilies and feather stars are examples of Crinoidea. Sea lilies are sessile, with the body attached to a stalk,
but the feather stars can actively move about using leglike cirri that emerge from the aboral surface. Both types
of crinoid are suspension feeders, collecting small food organisms along the ambulacral grooves of their feather¬
like arms. The "feathers" consisted of branched arms lined with tube feet. The tube feet are used to move
captured food toward the mouth. There are only about 600 extant species of crinoids, but they were far more
numerous and abundant in ancient oceans. Many crinoids are deep-water species, but feather stars typically
inhabit shallow areas, especially in substropical and tropical waters.
Sea cucumbers of class Holothuroidea exhibit an extended oral-aboral axis. These are the only echinoderms
that demonstrate “functional" bilateral symmetry as adults, because the extended oral-aboral axis compels the
animal to lie horizontally rather than stand vertically. The tube feet are reduced or absent, except on the side
on which the animal lies. They have a single gonad and the digestive tract is more typical of a bilaterally
symmetrical animal. A pair of gill-like structures called respiratory trees branch from the posterior gut; muscles
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around the cloaca pump water in and out of these trees. There are clusters of tentacles around the mouth.
Some sea cucumbers feed on detritus, while others are suspension feeders, sifting out small organisms with
their oral tentacles. Some species of sea cucumbers are unique among the echinoderms in that cells containing
hemoglobin circulate in the coelomic fluid, the water vascular system and/or the hemal system.
(a) (b) (c)
(d) (e)
Figure 28.48 Classes of echinoderms. Different members of Echinodermata include the (a) sea star of class
Asteroidea, (b) the brittle star of class Ophiuroidea, (c) the sea urchins of class Echinoidea, (d) the sea lilies belonging
to class Crinoidea, and (e) sea cucumbers, representing class Holothuroidea. (credit a: modification of work by Adrian
Pingstone; credit b: modification of work by Joshua Ganderson; credit c: modification of work by Samuel Chow; credit
d: modification of work by Sarah Depper; credit e: modification of work by Ed Bierman)
Phylum Chordata
Animals in the phylum Chordata share five key features that appear at some stage of their development:
a notochord, a dorsal hollow nerve cord, pharyngeal slits, a post-anal tail, and an endostyle/thyroid gland
that secretes iodinated hormones. In some groups, some of these traits are present only during embryonic
development. In addition to containing vertebrate classes, the phylum Chordata contains two clades of
“invertebrates": Urochordata (tunicates, salps, and larvaceans) and Cephalochordata (lancelets). Most tunicates
live on the ocean floor and are suspension feeders. Lancelets are suspension feeders that feed on phytoplankton
and other microorganisms. The invertebrate chordates will be discussed more extensively in the following
chapter.
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KEY TERMS
amoebocyte sponge cell with multiple functions, including nutrient delivery, egg formation, sperm delivery, and
cell differentiation
Annelida phylum of vermiform animals with metamerism
archenteron primitive gut cavity within the gastrula that opens outward via the blastopore
Arthropoda phylum of animals with jointed appendages
biramous referring to two branches per appendage
captacula tentacle-like projection that is present in tusks shells to catch prey
cephalothorax fused head and thorax in some species
chelicera modified first pair of appendages in subphylum Chelicerata
choanocyte (also, collar cell) sponge cell that functions to generate a water current and to trap and ingest food
particles via phagocytosis
Chordata phylum of animals distinguished by their possession of a notochord, a dorsal, hollow nerve cord, an
endostyle, pharyngeal slits, and a post-anal tail at some point in their development
clitellum specialized band of fused segments, which aids in reproduction
Cnidaria phylum of animals that are diploblastic and have radial symmetry
cnidocyte specialized stinging cell found in Cnidaria
conispiral shell shape coiled around a horizontal axis
corona wheel-like structure on the anterior portion of the rotifer that contains cilia and moves food and water
toward the mouth
ctenidium specialized gill structure in mollusks
cuticle (animal) the tough, external layer possessed by members of the invertebrate class Ecdysozoa that is
periodically molted and replaced
cypris larval stage in the early development of crustaceans
Echinodermata phylum of deuterostomes with spiny skin; exclusively marine organisms
enterocoelom coelom formed by fusion of coelomic pouches budded from the endodermal lining of the
archenteron
epidermis outer layer (from ectoderm) that lines the outside of the animal
extracellular digestion food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and
the cells lining the cavity absorb nutrients
gastrodermis inner layer (from endoderm) that lines the digestive cavity
gastrovascular cavity opening that serves as both a mouth and an anus, which is termed an incomplete
digestive system
gemmule structure produced by asexual reproduction in freshwater sponges where the morphology is inverted
hemocoel internal body cavity seen in arthropods
hermaphrodite referring to an animal where both male and female gonads are present in the same individual
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invertebrata (also, invertebrates) category of animals that do not possess a cranium or vertebral column
madreporite pore for regulating entry and exit of water into the water vascular system
mantle (also, pallium) specialized epidermis that encloses all visceral organs and secretes shells
mastax jawed pharynx unique to the rotifers
medusa free-floating cnidarian body plan with mouth on underside and tentacles hanging down from a bell
mesoglea non-living, gel-like matrix present between ectoderm and endoderm in cnidarians
mesohyl collagen-like gel containing suspended cells that perform various functions in the sponge
metamerism series of body structures that are similar internally and externally, such as segments
Mollusca phylum of protostomes with soft bodies and no segmentation
nacre calcareous secretion produced by bivalves to line the inner side of shells as well as to coat intruding
particulate matter
nauplius larval stage in the early development of crustaceans
nematocyst harpoon-like organelle within cnidocyte with pointed projectile and poison to stun and entangle
prey
Nematoda phylum of worm-like animals that are triploblastic, pseudocoelomates that can be free-living or
parasitic
Nemertea phylum of dorsoventrally flattened protostomes known as ribbon worms
osculum large opening in the sponge’s body through which water leaves
ostium pore present on the sponge’s body through which water enters
oviger additional pair of appendages present on some arthropods between the chelicerae and pedipalps
parapodium fleshy, flat, appendage that protrudes in pairs from each segment of polychaetes
pedipalp second pair of appendages in Chelicerata
pilidium larval form found in some nemertine species
pinacocyte epithelial-like cell that forms the outermost layer of sponges and encloses a jelly-like substance
called mesohyl
planospiral shell shape coiled around a vertical axis
planuliform larval form found in phylum Nemertea
polymorphic possessing multiple body plans within the lifecycle of a group of organisms
polyp stalk-like sessile life form of a cnidarians with mouth and tentacles facing upward, usually sessile but may
be able to glide along surface
Porifera phylum of animals with no true tissues, but a porous body with rudimentary endoskeleton
radula tongue-like organ with chitinous ornamentation
rhynchocoel cavity present above the mouth that houses the proboscis
schizocoelom coelom formed by groups of cells that split from the endodermal layer
sclerocyte cell that secretes silica spicules into the mesohyl
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seta/chaeta chitinous projection from the cuticle
siphonophore tubular structure that serves as an inlet for water into the mantle cavity
spicule structure made of silica or calcium carbonate that provides structural support for sponges
spongocoel central cavity within the body of some sponges
trochophore first of the two larval stages in mollusks
uniramous referring to one branch per appendage
veliger second of the two larval stages in mollusks
water vascular system system in echinoderms where water is the circulatory fluid
zoea larval stage in the early development of crustaceans
CHAPTER SUMMARY
28.1 Phylum Porifera
Animals included in phylum Porifera are parazoans because they do not show the formation of true
embryonically derived tissues, although they have a number of specific cell types and “functional” tissues such
as pinacoderm. These organisms show very simple organization, with a rudimentary endoskeleton of spicules
and spongin fibers. Glass sponge cells are connected together in a multinucleated syncytium. Although
sponges are very simple in organization, they perform most of the physiological functions typical of more
complex animals.
28.2 Phylum Cnidaria
Cnidarians represent a more complex level of organization than Porifera. They possess outer and inner tissue
layers that sandwich a noncellular mesoglea between them. Cnidarians possess a well-formed digestive
system and carry out extracellular digestion in a digestive cavity that extends through much of the animal. The
mouth is surrounded by tentacles that contain large numbers of cnidocytes—specialized cells bearing
nematocysts used for stinging and capturing prey as well as discouraging predators. Cnidarians have separate
sexes and many have a lifecycle that involves two distinct morphological forms—medusoid and polypoid—at
various stages in their life cycles. In species with both forms, the medusa is the sexual, gamete-producing
stage and the polyp is the asexual stage. Cnidarian species include individual or colonial polypoid forms,
floating colonies, or large individual medusa forms (sea jellies).
28.3 Superphylum Lophotrochozoa: Flatworms, Rotifers, and Nemerteans
This section describes three phyla of relatively simple invertebrates: one acoelomate, one pseudocoelomate,
and one eucoelomate. Flatworms are acoelomate, triploblastic animals. They lack circulatory and respiratory
systems, and have a rudimentary excretory system. This digestive system is incomplete in most species, and
absent in tapeworms. There are four traditional groups of flatworms, the largely free-living turbellarians, which
include polycladid marine worms and tricladid freshwater species, the ectoparasitic monogeneans, and the
endoparasitic trematodes and cestodes. Trematodes have complex life cycles involving a molluscan secondary
host and a primary host in which sexual reproduction takes place. Cestodes, or tapeworms, infect the digestive
systems of their primary vertebrate hosts.
Rotifers are microscopic, multicellular, mostly aquatic organisms that are currently under taxonomic revision.
The group is characterized by the ciliated, wheel-like corona, located on their head. Food collected by the
corona is passed to another structure unique to this group of organisms—the mastax or jawed pharynx.
The nemerteans are probably simple eucoelomates. These ribbon-shaped animals also bear a specialized
proboscis enclosed within a rhynchocoel. The development of a closed circulatory system derived from the
coelom is a significant difference seen in this species compared to other phyla described here. Alimentary,
nervous, and excretory systems are more developed in the nemerteans than in the flatworms or rotifers.
Embryonic development of nemertean worms proceeds via a planuliform or trochophore-like larval stage.
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28.4 Superphylum Lophotrochozoa: Molluscs and Annelids
Phylum Mollusca is a large, group of protostome schizocoelous invertebrates that occupy marine, freshwater,
and terrestrial habitats. Mollusks can be divided into seven classes, each of which exhibits variations on the
basic molluscan body plan. Two defining features are the mantle, which secretes a protective calcareous shell
in many species, and the radula, a rasping feeding organ found in most classes. Some mollusks have evolved
a reduced shell, and others have no radula. The mantle also covers the body and forms a mantle cavity, which
is quite distinct from the coelomic cavity—typically reduced to the area surrounding the heart, kidneys, and
intestine. In aquatic mollusks, respiration is facilitated by gills (ctenidia) in the mantle cavity. In terrestrial
mollusks, the mantle cavity itself serves as an organ of gas exchange. Mollusks also have a muscular foot,
which is modified in various ways for locomotion or food capture. Most mollusks have separate sexes. Early
development in aquatic species occurs via one or more larval stages, including a trochophore larva, that
precedes a veliger larva in some groups.
Phylum Annelida includes vermiform, segmented animals. Segmentation is metameric (i.e., each segment is
partitioned internally as well as externally, with various structures repeated in each segment). These animals
have well-developed neuronal, circulatory, and digestive systems. The two major groups of annelids are the
polychaetes, which have parapodia with multiple bristles, and oligochaetes, which have no parapodia and
fewer bristles or no bristles. Oligochaetes, which include earthworms and leeches, have a specialized band of
segments known as a clitellum, which secretes a cocoon and protects gametes during reproduction. The
leeches do not have full internal segmentation. Reproductive strategies include separate sexes,
hermaphroditism, and serial hermaphroditism. Polychaetes typically have trochophore larvae, while the
oligochaetes develop more directly.
28.5 Superphylum Ecdysozoa: Nematodes and Tardigrades
The defining feature of the Ecdysozoa is a collagenous/chitinous cuticle that covers the body, and the necessity
to molt the cuticle periodically during growth. Nematodes are roundworms, with a pseudocoel body cavity. They
have a complete digestive system, a differentiated nervous system, and a rudimentory excretory system. The
phylum includes free-living species like Caenorhabditis elegans as well as many species of endoparasitic
organisms such as Ascaris spp. They include dioeceous as well as hermaphroditic species. Embryonic
development proceeds via several larval stages, and most adults have a fixed number of cells.
The tardigrades, sometimes called "water bears," are a widespread group of tiny animals with a segmented
cuticle covering the epidermis and four pairs of clawed legs. Like the nematodes, they are pseudocoelomates
and have a fixed number of cells as adults. Specialized proteins enable them to enter cryptobiosis, a kind of
suspended animation during which they can resist a number of adverse environmental conditions.
28.6 Superphylum Ecdysozoa: Arthropods
Arthropods represent the most successful animal phylum on Earth, both in terms of the number of species and
the number of individuals. As members of the Ecdysozoa, all arthropods have a protective chitinous cuticle that
must be periodically molted and shed during development or growth. Arthropods are characterized by a
segmented body as well as the presence of jointed appendages. In the basic body plan, a pair of appendages
is present per body segment. Within the phylum, traditional classification is based on mouthparts, body
subdivisions, number of appendages, and modifications of appendages present. In aquatic arthropods, the
chitinous exoskeleton may be calcified. Gills, tracheae, and book lungs facilitate respiration. Unique larval
stages are commonly seen in both aquatic and terrestrial groups of arthropods.
28.7 Superphylum Deuterostomia
Echinoderms are deuterostome marine organisms, whose adults show five-fold symmetry. This phylum of
animals has a calcareous endoskeleton composed of ossicles, or body plates. Epidermal spines are attached
to some ossicles and serve in a protective capacity. Echinoderms possess a water-vascular system that serves
both for respiration and for locomotion, although other respiratory structures such as papulae and respiratory
trees are found in some species. A large aboral madreporite is the point of entry and exit for sea water pumped
into the water vascular system. Echinoderms have a variety of feeding techniques ranging from predation to
suspension feeding. Osmoregulation is carried out by specialized cells known as podocytes associated with the
hemal system.
The characteristic features of the Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, a
post-anal tail, and an endostyle/thyroid that secretes iodinated hormones. The phylum Chordata contains two
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clades of invertebrates: Urochordata (tunicates, salps, and larvaceans) and Cephalochordata (lancelets),
together with the vertebrates in the Vertebrata. Most tunicates live on the ocean floor and are suspension
feeders. Lancelets are suspension feeders that feed on phytoplankton and other microorganisms. The sister
taxon of the Chordates is the Ambulacraria, which includes both the Echinoderms and the hemichordates,
which share pharyngeal slits with the chordates.
VISUAL CONNECTION QUESTIONS
1. Figure 28.3 Which of the following statements is
false?
a. Choanocytes have flagella that propel water
through the body.
b. Pinacocytes can transform into any cell
type.
c. Lophocytes secrete collagen.
d. Porocytes control the flow of water through
pores in the sponge body.
2. Figure 28.21 Which of the following statements
about the anatomy of a mollusk is false?
REVIEW QUESTIONS
4. Mesohyl contains:
a. a polysaccharide gel and dead cells.
b. a collagen-like gel and suspended cells for
various functions.
c. spicules composed of silica or calcium
carbonate.
d. multiple pores.
5. The large central opening in the parazoan body is
called the:
a. gemmule.
b. spicule.
c. ostia.
d. osculum.
6. Most sponge body plans are slight variations on a
simple tube-within-a-tube design. Which of the
following is a key limitation of sponge body plans?
a. Sponges lack the specialized cell types
needed to produce more complex body
plans.
b. The reliance on osmosis/diffusion requires a
design that maximizes the surface area to
volume ratio of the sponge.
c. Choanocytes must be protected from the
hostile exterior environment.
d. Spongin cannot support heavy bodies.
7. Cnidocytes are found in_.
a. Mollusks have a radula for grinding food.
b. A digestive gland is connected to the
stomach.
c. The tissue beneath the shell is called the
mantle.
d. The digestive system includes a gizzard, a
stomach, a digestive gland, and the
intestine.
3. Figure 28.45 Which of the following statements
about insects is false?
a. Insects have both dorsal and ventral blood
vessels.
b. Insects have spiracles, openings that allow
air to enter.
c. The trachea is part of the digestive system.
d. Insects have a developed digestive system
with a mouth, crop, and intestine.
a. phylum Porifera
b. phylum Nemertea
c. phylum Nematoda
d. phylum Cnidaria
8. Cubozoans are_.
a. polyps
b. medusoids
c. polymorphs
d. sponges
9. While collecting specimens, a marine biologist
finds a sessile Cnidarian. The medusas that bud from
it swim by contracting a ring of muscle in their bells.
To which class does this specimen belong?
a. Class Hydrozoa
b. Class Cubozoa
c. Class Scyphozoa
d. Class Anthozoa
10. Which group of flatworms are primarily
ectoparasites of fish?
a. monogeneans
b. trematodes
c. cestodes
d. turbellarians
11. The rhynchocoel is a_.
a. circulatory system
b. fluid-filled cavity
c. primitive excretory system
d. proboscis
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12. Annelids have (a):
a. pseudocoelom.
b. true coelom.
c. no coelom.
d. none of the above
13. A mantle and mantle cavity are present in:
a. phylum Echinodermata.
b. phylum Adversoidea.
c. phylum Mollusca.
d. phylum Nemertea.
14. How does segmentation enhance annelid
locomotion?
a. Segmentation creates repeating body
structures so the entire organism functions
in synchrony.
b. Segmentation allows specialization of
different body regions.
c. Neural segmentation allows annelids to
localize sensations.
d. Muscle contractions can be localized to
specific regions of the body to coordinate
movement.
15. The embryonic development in nematodes can
have up to_larval stages.
a. one
b. two
c. three
d. five
16. The nematode cuticle contains_.
a. glucose
b. skin cells
c. chitin
d. nerve cells
17. Crustaceans are_.
a. ecdysozoans
b. nematodes
c. arachnids
d. parazoans
18. Flies are_.
CRITICAL THINKING QUESTIONS
24. Describe the different cell types and their
functions in sponges.
25. Describe the feeding mechanism of sponges and
identify how it is different from other animals.
26. Explain the function of nematocysts in cnidarians.
27. Compare the structural differences between
Porifera and Cnidaria.
28. Compare the differences in sexual reproduction
between Porifera and Cubozoans. How does the
difference in fertilization provide an evolutionary
advantage to the Cubozoans?
a. chelicerates
b. hexapods
c. arachnids
d. crustaceans
19. Which of the following is not a key advantage
provided by the exoskeleton of terrestrial arthropods?
a. Prevents dessication
b. Protects internal tissue
c. Provides mechanical support
d. Grows with the arthropod throughout its life
20. Echinoderms have_.
a. triangular symmetry
b. radial symmetry
c. hexagonal symmetry
d. pentaradial symmetry
21. The circulatory fluid in echinoderms is_.
a. blood
b. mesohyl
c. water
d. saline
22. Which of the following features does not
distinguish humans as a member of phylum
Chordata?
a. Human embryos undergo indeterminate
cleavage.
b. A spinal cord runs along an adult human’s
dorsal side.
c. Human embryos exhibit pharyngeal arches
and gill slits.
d. The human coccyx forms from an
embryonic tail.
23. The sister taxon of the Chordata is the_.
a. Mollusca
b. Arthropoda
c. Ambulacraria
d. Rotifera
29. How does the tapeworm body plan support
widespread dissemination of the parasite?
30. Describe the morphology and anatomy of
mollusks.
31. What are the anatomical differences between
nemertines and mollusks?
32. How does a change in the circulatory system
organization support the body designs in
cephalopods compared to other mollusks?
33. Enumerate features of Caenorhabditis elegans
that make it a valuable model system for biologists.
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34. What are the different ways in which nematodes
can reproduce?
35. Why are tardigrades essential to recolonizing
habits following destruction or mass extinction?
36. Describe the various superclasses that phylum
Arthropoda can be divided into.
37. Compare and contrast the segmentation seen in
phylum Annelida with that seen in phylum
Arthropoda.
38. How do terrestrial arthropods of the subphylum
Hexapoda impact the world’s food supply? Provide at
least two positive and two negative effects.
39. Describe the different classes of echinoderms
using examples.
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29 | VERTEBRATES
Figure 29.1 Examples of critically endangered vertebrate species include (a) the Siberian tiger (Panthera tigris), (b)
the mountain gorilla (Gorilla beringei), and (c) the harpy eagle (Harpia harpyja). (The harpy eagle is considered "near
threatened" globally, but critically endangered in much of its former range in Mexico and Central America.) (credit a:
modification of work by Dave Pape; credit b: modification of work by Dave Proffer; credit c: modification of work by
Haui Ared)
Chapter Outline
29.1: Chordates
29.2: Fishes
29.3: Amphibians
29.4: Reptiles
29.5: Birds
29.6: Mammals
29.7: The Evolution of Primates
Introduction
Vertebrates are among the most recognizable organisms of the Animal Kingdom, and more than 62,000
vertebrate species have been identified. The vertebrate species now living represent only a small portion of the
vertebrates that have existed in the past. The best-known extinct vertebrates are the dinosaurs, a unique group
of reptiles, some of which reached sizes not seen before or after in terrestrial animals. In fact, they were the
dominant terrestrial animals for 150 million years, until most of them died out in a mass extinction near the end of
the Cretaceous period (except for the feathered theropod ancestors of modern birds, whose direct descendents
now number nearly 10,000 species). Although it is not known with certainty what caused this mass extinction
(not only of dinosaurs, but of many other groups of organisms), a great deal is known about the anatomy of the
dinosaurs and early birds, given the preservation of numerous skeletal elements, nests, eggs, and embryos in
the fossil record.
The vertebrates exhibit two major innovations in their evolution from the invertebrate chordates. These
innovations may be associated with the whole genome duplications that resulted in a quadruplication of the basic
chordate genome, including the Hox gene loci that regulate the placement of structures along the three axes of
the body. One of the first major steps was the emergence of the quadrupeds in the form of the amphibians. A
second step was the evolution of the amniotic egg, which, similar to the evolution of pollen and seeds in plants,
freed terrestrial animals from their dependence on water for fertilization and embryonic development. Within the
amniotes, modifications of keratinous epidermal structures have given rise to scales, claws, hair, and feathers.
The scales of reptiles sealed their skins against water loss, while hair and feathers provided insulation to support
the evolution of endothermy, as well as served other functions such as camouflage and mate attraction in the
vertebrate lineages that led to birds and mammals.
Currently, a number of vertebrate species face extinction primarily due to habitat loss and pollution. According
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to the International Union for the Conservation of Nature, more than 6,000 vertebrate species are classified as
threatened. Amphibians and mammals are the classes with the greatest percentage of threatened species, with
29 percent of all amphibians and 21 percent of all mammals classified as threatened. Attempts are being made
around the world to prevent the extinction of threatened species. For example, the Biodiversity Action Plan is an
international program, ratified by 188 countries, which is designed to protect species and habitats.
29.1 1 Chordates
By the end of this section, you will be able to do the following:
• Describe the distinguishing characteristics of chordates
• Identify the derived characters of craniates that sets them apart from other chordates
• Describe the developmental fate of the notochord in vertebrates
Vertebrates are members of the kingdom Animalia and the phylum Chordata (Figure 29.2). Recall that animals
that possess bilateral symmetry can be divided into two groups—protostomes and deuterostomes—based on
their patterns of embryonic development. The deuterostomes, whose name translates as “second mouth,”
consist of two major phyla: Echinodermata and Chordata. Echinoderms are invertebrate marine animals that
have pentaradial symmetry and a spiny body covering, a group that includes sea stars, sea urchins, and sea
cucumbers. The most conspicuous and familiar members of Chordata are vertebrates, but this phylum also
includes two groups of invertebrate chordates.
Echinodermata
(sea stars, sea urchins)
Deuterostomes
Chordates (notochord)
y
Chordate ancestor
(possessed notochord)
Craniata (head)
Vertebrata (vertebral column)
Gnathostomes (jaw)
Lungs
Cephalochordata
(lancelets)
Urochordata
(tunicates)
Myxini
(hagfishes)
Petromyzontida
(lampreys)
Chondrichthyes
(sharks, rays, chimaeras)
Actinopterygii
(ray-finned fishes)
Actinistia
(coelacanths)
Dipnoi
(lungfishes)
Four limbs
Amphibia
(frogs, salamanders)
Tetrapods (four legs)
Amniota (amniotic egg)
Mammals (milk)
Reptilia
(turtles, snakes, crocodiles, birds)
Mammalia
(mammals)
Figure 29.2 Deuterostome phylogeny. All chordates are deuterostomes possessing a notochord at some stage of their
life cycle.
Characteristics of Chordata
Animals in the phylum Chordata share five key chacteristics that appear at some stage during their
development: a notochord, a dorsal hollow (tubular) nerve cord, pharyngeal gill arches or slits, a post-anal tail,
and an endostyle/thyroid gland (Figure 29.3). In some groups, some of these key chacteristics are present only
during embryonic development.
The chordates are named for the notochord, which is a flexible, rod-shaped mesodermal structure that is found
in the embryonic stage of all chordates and in the adult stage of some chordate species. It is strengthened with
glycoproteins similar to cartilage and covered with a collagenous sheath. The notocord is located between the
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digestive tube and the nerve cord, and provides rigid skeletal support as well as a flexible location for attachment
of axial muscles. In some chordates, the notochord acts as the primary axial support of the body throughout
the animal’s lifetime. However, in vertebrates (craniates), the notochord is present only during embryonic
development, at which time it induces the development of the neural tube and serves as a support for the
developing embryonic body. The notochord, however, is not found in the postembryonic stages of vertebrates;
at this point, it has been replaced by the vertebral column (that is, the spine).
visual
CONNECTION
Dorsal hollow
Figure 29.3 Chordate features. In chordates, four common features appear at some point during development: a
notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. The endostyle is embedded in the
floor of the pharynx.
Which of the following statements about common features of chordates is true?
1. The dorsal hollow nerve cord is part of the chordate central nervous system.
2. In vertebrate fishes, the pharyngeal slits become the gills.
3. Humans are not chordates because humans do not have a tail.
4. Vertebrates do not have a notochord at any point in their development; instead, they have a vertebral
column.
5. The endostyle secretes steroid hormones.
The dorsal hollow nerve cord is derived from ectoderm that rolls into a hollow tube during development. In
chordates, it is located dorsally to the notochord. In contrast, the nervous system in protostome animal phyla is
characterized by solid nerve cords that are located either ventrally and/or laterally to the gut. In vertebrates, the
neural tube develops into the brain and spinal cord, which together comprise the central nervous system (CNS).
The peripheral nervous system (PNS) refers to the peripheral nerves (including the cranial nerves) lying outside
of the brain and spinal cord.
Pharyngeal slits are openings in the pharynx (the region just posterior to the mouth) that extend to the outside
environment. In organisms that live in aquatic environments, pharyngeal slits allow for the exit of water that
enters the mouth during feeding. Some invertebrate chordates use the pharyngeal slits to filter food out of
the water that enters the mouth. The endostyle is a strip of ciliated mucus-producing tissue in the floor of the
pharynx. Food particles trapped in the mucus are moved along the endostyle toward the gut. The endostyle also
produces substances similar to thyroid hormones and is homologous with the thyroid gland in vertebrates. In
vertebrate fishes, the pharyngeal slits are modified into gill supports, and in jawed fishes, into jaw supports. In
tetrapods (land vertebrates), the slits are highly modified into components of the ear, and tonsils and thymus
glands. In other vertebrates, pharyngeal arches, derived from all three germ layers, give rise to the oral jaw from
the first pharyngeal arch, with the second arch becoming the hyoid and jaw support.
The post-anal tail is a posterior elongation of the body, extending beyond the anus. The tail contains skeletal
elements and muscles, which provide a source of locomotion in aquatic species, such as fishes. In some
terrestrial vertebrates, the tail also helps with balance, courting, and signaling when danger is near. In humans
and other great apes, the post-anal tail is reduced to a vestigial coccyx (“tail bone") that aids in balance during
sitting.
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LINK 1\_. EARNING
Click for a video discussing the evolution of chordates and five characteristics that they share. (This
multimedia resource will open in a browser.) (http://cnx.org/content/m66588/1.3/#eip-
idll65239610289)
Chordates and the Evolution of Vertebrates
Two clades of chordates are invertebrates: Cephalochordata and Urochordata. Members of these groups also
possess the five distinctive features of chordates at some point during their development.
Cephalochordata
Members of Cephalochordata possess a notochord, dorsal hollow tubular nerve cord, pharyngeal slits,
endostyle/thyroid gland, and a post-anal tail in the adult stage (Figure 29.4). The notochord extends into the
head, which gives the subphylum its name. Although the neural tube also extends into the head region, there is
no well-defined brain, and the nervous system is centered around a hollow nerve cord lying above the notochord.
Extinct members of this subphylum include Pikaia, which is the oldest known cephalochordate. Excellently
preserved Pikaia fossils were recovered from the Burgess shales of Canada and date to the middle of the
Cambrian age, making them more than 500 million years old. Its anatomy of Pikaia closely resembles that of the
extant lancelet in the genus Branchiostoma.
The lancelets are named for their bladelike shape. Lancelets are only a few centimeters long and are usually
found buried in sand at the bottom of warm temperate and tropical seas. Cephalochordates are suspension
feeders. A water current is created by cilia in the mouth, and is filtered through oral tentacles. Water from the
mouth then enters the pharyngeal slits, which filter out food particles. The filtered water collects in a gill chamber
called the atrium and exits through the atriopore. Trapped food particles are caught in a stream of mucus
produced by the endostyle in a ventral ciliated fold (or groove) of the pharynx and carried to the gut. Most gas
exchange occurs across the body surface. Sexes are separate and gametes are released into the water through
the atriopore for external fertilization.
Figure 29.4 Cephalochordate anatomy. In the lancelet and other cephalochordates, the notochord extends into the
head region. Adult lancelets retain all five key characteristics of chordates: a notochord, a dorsal hollow nerve cord,
pharyngeal slits, an endostyle, and a post-anal tail.
Urochordata
The 1,600 species of Urochordata are also known as tunicates (Figure 29.5). The name tunicate derives from
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the cellulose-like carbohydrate material, called the tunic, which covers the outer body of tunicates. Although
tunicates are classified as chordates, the adults do not have a notochord, a dorsal hollow nerve cord, or a post-
anal tail, although they do have pharyngeal slits and an endostyle. The “tadpole" larval form, however, possesses
all five structures. Most tunicates are hermaphrodites; their larvae hatch from eggs inside the adult tunicate’s
body. After hatching, a tunicate larva (possessing all five chordate features) swims for a few days until it finds
a suitable surface on which it can attach, usually in a dark or shaded location. It then attaches via the head to
the surface and undergoes metamorphosis into the adult form, at which point the notochord, nerve cord, and
tail disappear, leaving the pharyngeal gill slits and the endostyle as the two remaining features of its chordate
morphology.
Stomach Gonad
(a) (b) (c)
Figure 29.5 Urochordate anatomy, (a) This photograph shows a colony of the tunicate Botrylloides violaceus. (b) The
larval stage of the tunicate possesses all of the features characteristic of chordates: a notochord, a dorsal hollow nerve
cord, pharyngeal slits, an endostyle, and a post-anal tail, (c) In the adult stage, the notochord, nerve cord, and tail
disappear, leaving just the pharyngeal slits and endostyle. (credit: modification of work by Dann Blackwood, USGS)
Adult tunicates may be either solitary or colonial forms, and some species may reproduce by budding. Most
tunicates live a sessile existence on the ocean floor and are suspension feeders. However, chains of thaliacean
tunicates called salps (Figure 29.6) can swim actively while feeding, propelling themselves as they move water
through the pharyngeal slits. The primary foods of tunicates are plankton and detritus. Seawater enters the
tunicate’s body through its incurrent siphon. Suspended material is filtered out of this water by a mucous net
produced by the endostyle and is passed into the intestine via the action of cilia. The anus empties into the
excurrent siphon, which expels wastes and water. Tunicates are found in shallow ocean waters around the world.
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Figure 29.6 Salps. These colonial tunicates feed on phytoplankton. Salps are sequential hermaphrodites, with
younger female colonies fertilized by older male colonies, (credit: Oregon Department of Fish & Wildlife via Wikimedia
Commons)
Subphylum Vertebrata (Craniata)
A cranium is a bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones (Figure
29.7). Most bilaterally symmetrical animals have a head; of these, those that have a cranium comprise the clade
Craniata/Vertebrata, which includes the primitively jawless Myxini (hagfishes), Petromyzontida (lampreys), and
all of the organisms called “vertebrates." (We should note that the Myxini have a cranium but lack a backbone.)
Cranium
Mandible
Figure 29.7 A craniate skull. The subphylum Craniata (or Vertebrata), including this placoderm fish (Dunkleosteus
sp.), are characterized by the presence of a cranium, mandible, and other facial bones, (credit: “Steveoc 86'7Wikimedia
Commons)
Members of the phylum Craniata/Vertebrata display the five characteristic features of the chordates; however,
members of this group also share derived characteristics that distinguish them from invertebrate chordates.
Vertebrates are named for the vertebral column, composed of vertebrae —a series of separate, irregularly
shaped bones joined together to form a backbone (Figure 29.8). Initially, the vertebrae form in segments around
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the embryonic notochord, but eventually replace it in adults. In most derived vertebrates, the notochord becomes
the nucleus pulposus of the intervertebral discs that cushion and support adjacent vertebrae.
Figure 29.8 A vertebrate skeleton. Vertebrata are characterized by the presence of a backbone, such as the one that
runs through the middle of this fish. All vertebrates are in the Craniata clade and have a cranium, (credit: Ernest V.
More; taken at Smithsonian Museum of Natural History, Washington, D.C.)
The relationship of the vertebrates to the invertebrate chordates has been a matter of contention, but although
these cladistic relationships are still being examined, it appears that the Craniata/Vertebrata are a monophyletic
group that shares the five basic chordate characteristics with the other two subphyla, Urochordata and
Cephalochordata. Traditional phylogenies place the cephalochordates as a sister clade to the chordates, a
view that has been supported by most current molecular analyses. This hypothesis is further supported by the
discovery of a fossil in China from the genus Haikouella. This organism seems to be an intermediate form
between cephalochordates and vertebrates. The Haikouella fossils are about 530 million years old and appear
similar to modern lancelets. These organisms had a brain and eyes, as do vertebrates, but lack the skull found
in craniates. This evidence suggests that vertebrates arose during the Cambrian explosion.
Vertebrates are the largest group of chordates, with more than 62,000 living species, which are grouped based
on anatomical and physiological traits. More than one classification and naming scheme is used for these
animals. Here we will consider the traditional groups Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia,
Aves, and Mammalia, which constitute classes in the subphylum Vertebrata/Craniata. Virtually all modern
cladists classify birds within Reptilia, which correctly reflects their evolutionary heritage. Thus, we now have
the nonavian reptiles and the avian reptiles in our reptilian classification. We consider them separately only
for convenience. Further, we will consider hagfishes and lampreys together as jawless fishes, the Agnatha,
although emerging classification schemes separate them into chordate jawless fishes (the hagfishes) and
vertebrate jawless fishes (the lampreys).
Animals that possess jaws are known as gnathostomes, which means “jawed mouth." Gnathostomes include
fishes and tetrapods. Tetrapod literally means “four-footed,” which refers to the phylogenetic history of various
land vertebrates, even though in some of the tetrapods, the limbs may have been modified for purposes other
than walking. Tetrapods include amphibians, reptiles, birds, and mammals, and technically could also refer to
the extinct fishlike groups that gave rise to the tetrapods. Tetrapods can be further divided into two groups:
amphibians and amniotes. Amniotes are animals whose eggs contain four extraembryonic membranes (yolk
sac, amnion, chorion, and allantois) that provide nutrition and a water-retaining environment for their embryos.
Amniotes are adapted for terrestrial living, and include mammals, reptiles, and birds.
29.2 | Fishes
By the end of this section, you will be able to do the following:
• Describe the difference between jawless and jawed fishes
• Discuss the distinguishing features of sharks and rays compared to other modern fishes
Modern fishes include an estimated 31,000 species, by far the most of all clades within the Vertebrata. Fishes
1. Chen, J. Y., Huang, D. Y., and Li, C. W., “An early Cambrian craniate-like chordate,” Nature 402 (1999): 518-522, doi:10.1038/990080.
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were the earliest vertebrates, with jawless species being the earliest forms and jawed species evolving later.
They are active feeders, rather than sessile, suspension feeders. The Agnatha (jawless fishes)—the hagfishes
and lampreys—have a distinct cranium and complex sense organs including eyes, that distinguish them from
the invertebrate chordates, the urochordates and cephalochordates.
Jawless Fishes: Superclass Agnatha
Jawless fishes (Agnatha) are craniates representing an ancient vertebrate lineage that arose over 550 million
years ago. In the past, hagfishes and lampreys were sometimes recognized as separate clades within the
Agnatha, primarily because lampreys were regarded as true vertebrates, whereas hagfishes were not. However,
recent molecular data, both from rRNA and mtDNA, as well as embryological data, provide strong support for
the hypothesis that living agnathans—previously called cyclostomes —are monophyletic, and thus share recent
common ancestry. The discussion below, for convenience, separates the modern “cyclostomes” into the class
Myxini and class Petromyzontida. The defining features of the living jawless fishes are the lack of jaws and
lack of paired lateral appendages (fins). They also lack internal ossification and scales, although these are not
defining features of the clade.
Some of the earliest jawless fishes were the armored ostracoderms (which translates to “shell-skin”): vertebrate
fishes encased in bony armor—unlike present-day jawless fishes, which lack bone in their scales. Some
ostracoderms, also unlike living jawless fishes, may have had paired fins. We should note, however, that the
“ostracoderms” represent an assemblage of heavily armored extinct jawless fishes that may not form a natural
evolutionary group. Fossils of the genus Haikouichthys from China, with an age of about 530 million years, show
many typical vertebrate characteristics including paired eyes, auditory capsules, and rudimentary vertebrae.
Class Myxini: Hagfishes
The class Myxini includes at least 70 species of hagfishes—eel-like scavengers that live on the ocean floor
and feed on living or dead invertebrates, fishes, and marine mammals (Figure 29.9). Although they are almost
completely blind, sensory barbels around the mouth help them locate food by smell and touch. They feed using
keratinized teeth on a movable cartilaginous plate in the mouth, which rasp pieces of flesh from their prey. These
feeding structures allow the gills to be used exclusively for respiration, not for filter feeding as in the urochordates
and cephalochordates. Hagfishes are entirely marine and are found in oceans around the world, except for
the polar regions. Unique slime glands beneath the skin release a milky mucus (through surface pores) that
upon contact with water becomes incredibly slippery, making the animal almost impossible to hold. This slippery
mucus thus allows the hagfish to escape from the grip of predators. Hagfish can also twist their bodies into a
knot, which provides additional leverage to feed. Sometimes hagfish enter the bodies of dead animals and eat
carcasses from the inside out! Interestingly, they do not have a stomach!
Figure 29.9 Hagfish. Pacific hagfish are scavengers that live on the ocean floor, (credit: Linda Snook, NOAA/CBNMS)
Hagfishes have a cartilaginous skull, as well as a fibrous and cartilaginous skeleton, but the major supportive
structure is the notochord that runs the length of the body. In hagfishes, the notochord is not replaced by the
vertebral column, as it is in true vertebrates, and thus they may (morphologically) represent a sister group to the
true vertebrates, making them the most basal clade among the skull-bearing chordates.
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Class Petromyzontida: Lampreys
The class Petromyzontida includes approximately 40 species of lampreys, which are superficially similar to
hagfishes in size and shape. However, lampreys possess extrinsic eye muscles, at least two semicircular canals,
and a true cerebellum, as well as simple vertebral elements, called arcualia —cartilaginous structures arranged
above the notochord. These features are also shared with the gnathostomes —vertebrates with jawed mouths
and paired appendages (see below). Lampreys also have a dorsal tubular nerve cord with a well-differentiated
brain, a small cerebellum, and 10 pairs of nerves. The classification of lampreys is still debated, but they clearly
represent one of the oldest divergences of the vertebrate lineage. Lampreys lack paired appendages, as do the
hagfishes, although they have one or two fleshy dorsal fins. As adults, lampreys are characterized by a rasping
tongue within a toothed, funnel-like sucking mouth. Many species have a parasitic stage of their life cycle during
which they are fish ectoparasites (some call them predators because they attack and eventually fall off) (Figure
29.10).
Figure 29.10 Lamprey. These parasitic sea lampreys, Petromyzon marinus, attach by suction to their lake trout host,
and use their rough tongues to rasp away flesh in order to feed on the trout’s blood, (credit: USGS)
Lampreys live primarily in coastal and freshwater environments, and have a worldwide distribution, except for the
tropics and polar regions. Some species are marine, but all species spawn in fresh water. Interestingly, northern
lampreys in the family Petromyzontidae, have the highest number of chromosomes (164 to 174) among the
vertebrates. Eggs are fertilized externally, and the larvae (called ammocoetes) differ greatly from the adult form,
closely resembling the adult cephalocordate amphioxus. After spending three to 15 years as suspension feeders
in rivers and streams, they attain sexual maturity. Shortly afterward, the adults swim upstream, reproduce, and
die within days.
Gnathostomes: Jawed Fishes
Gnathostomes, or “jaw-mouths,” are vertebrates that possess true jaws—a milestone in the evolution of the
vertebrates. In fact, one of the most significant developments in early vertebrate evolution was the development
of the jaw: a hinged structure attached to the cranium that allows an animal to grasp and tear its food. Jaws were
probably derived from the first pair of gill arches supporting the gills of jawless fishes.
Early gnathostomes also possessed two sets of paired fins, allowing the fishes to maneuver accurately. Pectoral
fins are typically located on the anterior body, and pelvic fins on the posterior. Evolution of the jaw and paired fins
permitted gnathostomes to expand their food options from the scavenging and suspension feeding of jawless
fishes to active predation. The ability of gnathostomes to exploit new nutrient sources probably contributed to
their replacing most jawless fishes during the Devonian period. Two early groups of gnathostomes were the
acanthodians and placoderms (Figure 29.11), which arose in the late Silurian period and are now extinct. Most
modern fishes are gnathostomes that belong to the clades Chondrichthyes and Osteichthyes (which include the
class Actinoptertygii and class Sarcopterygii).
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Figure 29.11 A placoderm. Dunkteosteus was an enormous placoderm from the Devonian period, 380 to 360 million
years ago. It measured up to 10 meters in length and weighed up to 3.6 tons. Its head and neck were armored with
heavy bony plates. Although Dunkleosteus had no true teeth, the edge of the jaw was armed with sharp bony blades,
(credit: Nobu Tamura)
Class Chondrichthyes: Cartilaginous Fishes
The class Chondrichthyes (about 1,000 species) is a morphologically diverse clade, consisting of subclass
Elasmobranchii (sharks [Figure 29.12], rays, and skates, together with the obscure and critically endangered
sawfishes), and a few dozen species of fishes called chimaeras, or “ghost sharks” in the subclass Holocephali.
Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. This clade arose
approximately 370 million years ago in the early or middle Devonian. They are thought to be descended from the
placoderms, which had endoskeletons made of bone; thus, the lighter cartilaginous skeleton of Chondrichthyes
is a secondarily derived evolutionary development. Parts of shark skeleton are strengthened by granules of
calcium carbonate, but this is not the same as bone.
Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for a part or all of their
lives. Most sharks are carnivores that feed on live prey, either swallowing it whole or using their jaws and teeth to
tear it into smaller pieces. Sharks have abrasive skin covered with tooth-like scales called placoid scales. Shark
teeth probably evolved from rows of these scales lining the mouth. A few species of sharks and rays, like the
enormous whale shark (Figure 29.13), are suspension feeders that feed on plankton. The sawfishes have an
extended rostrum that looks like a double-edged saw. The rostrum is covered with electrosensitive pores that
allow the sawfish to detect slight movements of prey hiding in the muddy sea floor. The teeth in the rostrum are
actually modified tooth-like structures called denticles, similar to scales.
Figure 29.12 Shark. Hammerhead sharks tend to school during the day and hunt prey at night, (credit: Masashi
Sugawara)
Sharks have well-developed sense organs that aid them in locating prey, including a keen sense of smell and
the ability to detect electromagnetic fields. Electroreceptors called ampullae of Lorenzini allow sharks to detect
the electromagnetic fields that are produced by all living things, including their prey. (Electroreception has only
been observed in aquatic or amphibious animals and sharks have perhaps the most sensitive electroreceptors
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of any animal.) Sharks, together with most fishes and aquatic and larval amphibians, also have a row of sensory
structures called the lateral line, which is used to detect movement and vibration in the surrounding water, and
is often considered to be functionally similar to the sense of “hearing" in terrestrial vertebrates. The lateral line is
visible as a darker stripe that runs along the length of a fish’s body. Sharks have no mechanism for maintaining
neutral buoyancy and must swim continuously to stay suspended in the water. Some must also swim in order to
ventilate their gills but others have muscular pumps in their mouths to keep water flowing over the gills.
Figure 29.13 Whale shark in the Georgia Aquarium. Whale sharks are filter-feeders and can grow to be over 10 meters
long. Whale sharks, like most other sharks, are ovoviviparous. (credit: modified from Zac Wolf [Own work] [CC BY-SA
2.5 (http://creativecommons.Org/licenses/by-sa/2.5 (http:// 0 penstax. 0 rg/l/CCSA) )], via Wikimedia Commons)
Sharks reproduce sexually, and eggs are fertilized internally. Most species are ovoviviparous'. The fertilized egg
is retained in the oviduct of the mother’s body and the embryo is nourished by the egg yolk. The eggs hatch
in the uterus, and young are born alive and fully functional. Some species of sharks are oviparous: They lay
eggs that hatch outside of the mother’s body. Embryos are protected by a shark egg case or “mermaid’s purse”
(Figure 29.14) that has the consistency of leather. The shark egg case has tentacles that snag in seaweed and
give the newborn shark cover. A few species of sharks, e.g., tiger sharks and hammerheads, are viviparous: the
yolk sac that initially contains the egg yolk and transfers its nutrients to the growing embryo becomes attached
to the oviduct of the female, and nutrients are transferred directly from the mother to the growing embryo. In both
viviparous and ovoviviparous sharks, gas exchange uses this yolk sac transport.
Figure 29.14 Shark egg cases. Shark embryos are clearly visible through these transparent egg cases. The round
structure is the yolk that nourishes the growing embryo, (credit: Jek Bacarisas)
In general, the Chondrichthyes have a fusiform or dorsoventrally flattened body, a heterocercal caudal fin or tail
(unequally sized fin lobes, with the tail vertebrae extending into the larger upper lobe) paired pectoral and pelvic
fins (in males these may be modified as claspers), exposed gill slits ( eiasmobranch ), and an intestine with a
spiral valve that condenses the length of the intestine. They also have three pairs of semicircular canals, and
excellent senses of smell, vibration, vision, and electroreception. A very large lobed liver produces squalene oil
(a lightweight biochemical precursor to steroids) that serves to aid in buoyancy (because with a specific gravity
of 0.855, it is lighter than that of water).
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Rays and skates comprise more than 500 species. They are closely related to sharks but can be distinguished
from sharks by their flattened bodies, pectoral fins that are enlarged and fused to the head, and gill slits on their
ventral surface (Figure 29.15). Like sharks, rays and skates have a cartilaginous skeleton. Most species are
marine and live on the sea floor, with nearly a worldwide distribution.
Unlike the stereotypical sharks and rays, the Holocephali (chimaeras or ratfish) have a diphycercal tail (equally
sized fin lobes, with the tail vertebrae located between them), lack scales (lost secondarily in evolution), and
have teeth modified as grinding plates that are used to feed on mollusks and other invertebrates (Figure
29.15b). Unlike sharks with elasmobranch or naked gills, chimaeras have four pairs of gills covered by an
operculum. Many species have a pearly iridescence and are extremely pretty.
Figure 29.15 Cartilaginous fish, (a) Stingray. This stingray blends into the sandy bottom of the ocean floor. A spotted
ratfish (b) Hydrolagus colliei credit a "Sailnl'VFlickr; (credit: a "SailnT'/Flickr b: Linda Snook / MBNMS [Public domain],
via Wikimedia Commons.)
Osteichthyes: Bony Fishes
Members of the clade Osteichthyes, also called bony fishes, are characterized by a bony skeleton. The vast
majority of present-day fishes belong to this group, which consists of approximately 30,000 species, making it
the largest class of vertebrates in existence today.
Nearly all bony fishes have an ossified skeleton with specialized bone cells (osteocytes) that produce and
maintain a calcium phosphate matrix. This characteristic has been reversed only in a few groups of
Osteichthyes, such as sturgeons and paddlefish, which have primarily cartilaginous skeletons. The skin of bony
fishes is often covered by overlapping scales, and glands in the skin secrete mucus that reduces drag when
swimming and aids the fish in osmoregulation. Like sharks, bony fishes have a lateral line system that detects
vibrations in water.
All bony fishes use gills to breathe. Water is drawn over gills that are located in chambers covered and ventilated
by a protective, muscular flap called the operculum. Many bony fishes also have a swim bladder, a gas-filled
organ derived as a pouch from the gut. The swim bladder helps to control the buoyancy of the fish. In most bony
fish, the gases of the swim bladder are exchanged directly with the blood. The swim bladder is believed to be
homologous to the lungs of lungfish and the lungs of land vertebrates.
Bony fishes are further divided into two extant clades: Class Actinopterygii (ray-finned fishes) and Class
Sarcopterygii (lobe-finned fishes).
Actinopterygii (Figure 29.16a), the ray-finned fishes, include many familiar fishes—tuna, bass, trout, and salmon
among others—and represent about half of all vertebrate species. Ray-finned fishes are named for the fan of
slender bones that supports their fins.
In contrast, the fins of Sarcopterygii (Figure 29.16b) are fleshy and lobed, supported by bones that are similar
in type and arrangement to the bones in the limbs of early tetrapods. The few extant members of this clade
include several species of lungfishes and the less familiar coelacanths, which were thought to be extinct until
living specimens were discovered between Africa and Madagascar. Currently, two species of coelocanths have
been described.
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(a) <b>
Figure 29.16 Osteichthyes. The (a) sockeye salmon and (b) coelacanth are both bony fishes of the Osteichthyes clade.
The coelacanth, sometimes called a lobe-finned fish, was thought to have gone extinct in the Late Cretaceous period,
100 million years ago, until one was discovered in 1938 near the Comoros Islands between Africa and Madagascar,
(credit a: modification of work by Timothy Knepp, USFWS; credit b: modification of work by Robbie Cada)
29.3 | Amphibians
By the end of this section, you will be able to do the following:
• Describe the important difference between the life cycle of amphibians and the life cycles of other
vertebrates
• Distinguish between the characteristics of Urodela, Anura, and Apoda
• Describe the evolutionary history of amphibians
Amphibians are vertebrate tetrapods (“four limbs”), and include frogs, salamanders, and caecilians. The term
“amphibian” loosely translates from the Greek as “dual life,” which is a reference to the metamorphosis that
many frogs and salamanders undergo and the unique mix of aquatic and terrestrial phases that are required in
their life cycle. In fact, they cannot stray far from water because their reproduction is intimately tied to aqueous
environments. Amphibians evolved during the Devonian period and were the earliest terrestrial tetrapods. They
represent an evolutionary transition from water to land that occurred over many millions of years. Thus, the
Amphibia are the only living true vertebrates that have made a transition from water to land in both their ontogeny
(life development) and phylogeny (evolution). They have not changed much in morphology over the past 350
million years!
LINK TQ LEARNING
Watch this series of five Animal Planet videos on tetrapod evolution:
1: The evolution from fish to earliest tetrapod
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66590/1.3/#eip-
idll65240950500)
2: Fish to Earliest Tetrapod
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66590/1.3/#eip-
idll65238909112)
3: The discovery of coelacanth and Acanthostega fossils
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66590/1.3/#eip-
idll65238838011)
4: The number of fingers on “legs”
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66590/1.3/#eip-
idll65241069476)
5: Reconstructing the environment of early tetrapods
(This multimedia resource will open in a browser.) (http://cnx.org/content/m66590/1.3/#eip-
id5626625)
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Characteristics of Amphibians
As tetrapods, most amphibians are characterized by four well-developed limbs. In some species of salamanders,
hindlimbs are reduced or absent, but all caecilians are (secondarily) limbless. An important characteristic of
extant amphibians is a moist, permeable skin that is achieved via mucus glands. Most water is taken in across
the skin rather than by drinking. The skin is also one of three respiratory surfaces used by amphibians. The other
two are the lungs and the buccal (mouth) cavity. Air is taken first into the mouth through the nostrils, and then
pushed by positive pressure into the lungs by elevating the throat and closing the nostrils.
All extant adult amphibians are carnivorous, and some terrestrial amphibians have a sticky tongue used to
capture prey. Amphibians also have multiple small teeth at the edge of the jaws. In salamanders and caecilians,
teeth are present in both jaws, sometimes in multiple rows. In frogs and toads, teeth are seen only in the
upper jaw. Additional teeth, called vomerine teeth, may be found in the roof of the mouth. Amphibian teeth are
pedicellate, which means that the root and crown are calcified, separated by a zone of noncalcified tissue.
Amphibians have image-forming eyes and color vision. Ears are best developed in frogs and toads, which
vocalize to communicate. Frogs use separate regions of the inner ear for detecting higher and lower sounds: the
papilla amphibiorum, which is sensitive to frequencies below 10,000 hertz and unique to amphibians, and the
papilla basilaris, which is sensitive to higher frequencies, including mating calls, transmitted from the eardrum
through the stapes bone. Amphibians also have an extra bone in the ear, the operculum, which transmits low-
frequency vibrations from the forelimbs and shoulders to the inner ear, and may be used for the detection of
seismic signals.
Evolution of Amphibians
The fossil record provides evidence of the first tetrapods: now-extinct amphibian species dating to nearly 400
million years ago. Evolution of tetrapods from lobe-finned freshwater fishes (similar to coelacanths and lungfish)
represented a significant change in body plan from one suited to organisms that respired and swam in water,
to organisms that breathed air and moved onto land; these changes occurred over a span of 50 million years
during the Devonian period.
Aquatic tetrapods of the Devonian period include Ichthyostega and Acanthostega. Both were aquatic, and may
have had both gills and lungs. They also had four limbs, with the skeletal structure of limbs found in present-day
tetrapods, including amphibians. However, the limbs could not be pulled in under the body and would not have
supported their bodies well out of water. They probably lived in shallow freshwater environments, and may have
taken brief terrestrial excursions, much like “walking” catfish do today in Florida. In Ichthyostega, the forelimbs
were more developed than the hind limbs, so it might have dragged itself along when it ventured onto land. What
preceded Acanthostega and Ichthyostega ?
In 2006, researchers published news of their discovery of a fossil of a “tetrapod-like fish,” Tiktaalik roseae, which
seems to be a morphologically “intermediate form” between sarcopterygian fishes having feet-like fins and early
tetrapods having true limbs (Figure 29.17). Tiktaalik likely lived in a shallow water environment about 375 million
[ 2 ]
years ago. Tiktaalik also had gills and lungs, but the loss of some gill elements gave it a neck, which would have
allowed its head to move sideways for feeding. The eyes were on top of the head. It had fins, but the attachment
of the fin bones to the shoulder suggested they might be weight-bearing. Tiktaalik preceded Acanthostega and
Ichthyostega, with their four limbs, by about 10 million years and is considered to be a true intermediate clade
between fish and amphibians.
2. Daeschler, E. B., Shubin, N. H., and Jenkins, F. J. "A Devonian tetrapod-like fish and the evolution of the tetrapod body plan,” Nature 440
(2006): 757-763, doi:10.1038/nature04639, http://www.nature.com/nature/journal/v440/n7085/abs/nature04639.html (http:// 0 penstax. 0 rg/l/
tetrapod) .
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Figure 29.17 Tiktaalik. The recent fossil discovery of Tiktaalik roseae suggests evidence for an animal intermediate to
finned fish and legged tetrapods, sometimes called a "fishapod." (credit: Zina Deretsky, National Science Foundation)
The early tetrapods that moved onto land had access to new nutrient sources and relatively few predators. This
led to the widespread distribution of tetrapods during the early Carboniferous period, a period sometimes called
the “age of the amphibians.”
Modern Amphibians
Amphibia comprises an estimated 6,770 extant species that inhabit tropical and temperate regions around the
world. All living species are classified in the subclass Lissamphibia ("smooth-amphibian"), which is divided into
three clades: Urodela (“tailed"), the salamanders; Anura (“tail-less"), the frogs; and Apoda (“legless ones”), the
caecilians.
Urodela: Salamanders
Salamanders are amphibians that belong to the order Urodela (or Caudata). These animals are probably the
most similar to ancestral amphibians. Living salamanders (Figure 29.18) include approximately 620 species,
some of which are aquatic, others terrestrial, and some that live on land only as adults. Most adult salamanders
have a generalized tetrapod body plan with four limbs and a tail. The placement of their legs makes it difficult to
lift their bodies off the ground and they move by bending their bodies from side to side, called lateral undulation,
in a fish-like manner while “walking” their arms and legs fore-and-aft. It is thought that their gait is similar to that
used by early tetrapods. The majority of salamanders are lungless, and respiration occurs through the skin or
through external gills in aquatic species. Some terrestrial salamanders have primitive lungs; a few species have
both gills and lungs. The giant Japanese salamander, the largest living amphibian, has additional folds in its skin
that enlarge its respiratory surface.
Most salamanders reproduce using an unusual process of internal fertilization of the eggs. Mating between
salamanders typically involves an elaborate and often prolonged courtship. Such a courtship ends in the
deposition of sperm by the males in a packet called a spermatophore, which is subsequently picked up
by the female, thus ultimately fertilization is internal. All salamanders except one, the fire salamander, are
oviparous. Aquatic salamanders lay their eggs in water, where they develop into legless larvae called efts.
Terrestrial salamanders lay their eggs in damp nests, where the eggs are guarded by their mothers. These
embryos go through the larval stage and complete metamorphosis before hatching into tiny adult forms. One
aquatic salamander, the Mexican axolotl, never leaves the larval stage, becoming sexually mature without
metamorphosis.
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Figure 29.18 Salamander. Most salamanders have legs and a tail, but respiration varies among species, (credit:
Valentina Storti)
LINK
T a
LEARNING
View River Monsters: Fish With Arms and Hands? (http:// 0 penstaxc 0 llege. 0 rg/l/river_m 0 nster) to see a
video about an unusually large salamander species.
Anura: Frogs
Frogs (Figure 29.19) are amphibians that belong to the order Anura or Salientia ("jumpers"). Anurans are among
the most diverse groups of vertebrates, with approximately 5,965 species that occur on all of the continents
except Antarctica. Anurans, ranging from the minute New Guinea frog at 7 mm to the huge goliath frog at 32 cm
from tropical Africa, have a body plan that is more specialized for movement. Adult frogs use their hind limbs
and their arrow-like endoskeleton to jump accurately to capture prey on land. Tree frogs have hands adapted for
grasping branches as they climb. In tropical areas, “flying frogs” can glide from perch to perch on the extended
webs of their feet. Frogs have a number of modifications that allow them to avoid predators, including skin
that acts as camouflage. Many species of frogs and salamanders also release defensive chemicals that are
poisonous to predators from glands in the skin. Frogs with more toxic skins have bright warning ( aposematic )
coloration.
Figure 29.19 Tree frog. The Australian green tree frog is a nocturnal predator that lives in the canopies of trees near a
water source.
Frog eggs are fertilized externally, and like other amphibians, frogs generally lay their eggs in moist
environments. Although amphibian eggs are protected by a thick jelly layer, they would still dehydrate quickly in
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a dry environment. Frogs demonstrate a great diversity of parental behaviors, with some species laying many
eggs and exhibiting little parental care, to species that carry eggs and tadpoles on their hind legs or embedded
in their backs. The males of Darwin's frog carry tadpoles in their vocal sac. Many tree frogs lay their eggs off the
ground in a folded leaf located over water so that the tadpoles can drop into the water as they hatch.
The life cycle of most frogs, as other amphibians, consists of two distinct stages: the larval stage followed by
metamorphosis to an adult stage. However, the eggs of frogs in the genus Eleutherodactylus develop directly
into little froglets, guarded by a parent. The larval stage of a frog, the tadpole, is often a filter-feeding herbivore.
Tadpoles usually have gills, a lateral line system, longfinned tails, and lack limbs. At the end of the tadpole stage,
frogs undergo metamorphosis into the adult form (Figure 29.20). During this stage, the gills, tail, and lateral line
system disappear, and four limbs develop. The jaws become larger and are suited for carnivorous feeding, and
the digestive system transforms into the typical short gut of a predator. An eardrum and air-breathing lungs also
develop. These changes during metamorphosis allow the larvae to move onto land in the adult stage.
Figure 29.20 Amphibian metapmorphosis. A juvenile frog metamorphoses into a frog. Here, the frog has started to
develop limbs, but its tadpole tail is still evident.
Apoda: Caecilians
An estimated 185 species comprise the caecilians, a group of amphibians that belong to the order Apoda. They
have no limbs, although they evolved from a legged vertebrate ancestor. The complete lack of limbs makes
them resemble earthworms. This resemblance is enhanced by folds of skin that look like the segments of an
earthworm. However, unlike earthworms, they have teeth in both jaws, and feed on a variety of small organisms
found in soil, including earthworms! Caecilians are adapted for a burrowing or aquatic lifestyle, and they are
nearly blind, with their tiny eyes sometimes covered by skin. Although they have a single lung, they also depend
on cutaneous respiration. These animals are found in the tropics of South America, Africa, and Southern Asia. In
the caecelians, the only amphibians in which the males have copulatory structures, fertilization is internal. Some
caecilians are oviparous, but most bear live young. In these cases, the females help nourish their young with
tissue from their oviduct before birth and from their skin after birth.
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V /
e olution CONNECTION
The Paleozoic Era and the Evolution of Vertebrates
When the vertebrates arose during the Paleozoic Era (542 to 251 MYA), the climate and geography of
Earth was vastly different. The distribution of landmasses on Earth were also very different from that of
today. Near the equator were two large supercontinents, Laurentia and Gondwana, which included most of
today's continents, but in a radically different configuration (Figure 29.21). At this time, sea levels were very
high, probably at a level that hasn’t been reached since. As the Paleozoic progressed, glaciations created
a cool global climate, but conditions warmed near the end of the first half of the Paleozoic. During the latter
half of the Paleozoic, the landmasses began moving together, with the initial formation of a large northern
block called Laurasia, which contained parts of what is now North America, along with Greenland, parts of
Europe, and Siberia. Eventually, a single supercontinent, called Pangaea, was formed, starting in the latter
third of the Paleozoic. Glaciations then began to affect Pangaea’s climate and the distribution of vertebrate
life.
Australia
East
Antarctica?
Indtu
Arabian
tyabiau
Shield
Kalahari
Congo
Rio
Plata
J V
West
Africa
Amazonia
Baltica
/Avalon is'
Siberia
Gondwana
Laurentia
Figure 29.21 Paleozoic continents. During the Paleozoic Era, around 550 million years ago, the continent
Gondwana formed. Both Gondwana and the continent Laurentia were located near the equator.
During the early Paleozoic, the amount of carbon dioxide in the atmosphere was much greater than it is
today. This may have begun to change later, as land plants became more common. As the roots of land
plants began to infiltrate rock and soil began to form, carbon dioxide was drawn out of the atmosphere and
became trapped in the rock. This reduced the levels of carbon dioxide and increased the levels of oxygen in
the atmosphere, so that by the end of the Paleozoic, atmospheric conditions were similar to those of today.
As plants became more common through the latter half of the Paleozoic, microclimates began to emerge
and ecosystems began to change. As plants and ecosystems continued to grow and become more complex,
vertebrates moved from the water to land. The presence of shoreline vegetation may have contributed to
the movement of vertebrates onto land. One hypothesis suggests that the fins of aquatic vertebrates were
used to maneuver through this vegetation, providing a precursor to the movement of fins on land and the
further development of limbs. The late Paleozoic was a time of diversification of vertebrates, as amniotes
emerged and became two different lines that gave rise, on one hand, to synapsids and mammals, and, on
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the other hand, to the codonts, reptiles, dinosaurs, and birds. Many marine vertebrates became extinct near
the end of the Devonian period, which ended about 360 million years ago, and both marine and terrestrial
vertebrates were decimated by a mass extinction in the early Permian period about 250 million years ago.
LINK TQ LEARNING
View Earth’s Paleogeography: Continental Movements Through Time (http:// 0 penstaxc 0 llege. 0 rg/l/
paleogeography) to see changes in Earth as life evolved.
29.4 | Reptiles
By the end of this section, you will be able to do the following:
• Describe the main characteristics of amniotes
• Explain the difference between anapsids, synapsids, and diapsids, and give an example of each
• identify the characteristics of reptiles
• Discuss the evolution of reptiles
The reptiles (including dinosaurs and birds) are distinguished from amphibians by their terrestrially adapted egg,
which is supported by four extraembryonic membranes', the yolk sac, the amnion, the chorion, and the allantois
(Figure 29.22). The chorion and amnion develop from folds in the body wall, and the yolk sac and allantois
are extensions of the midgut and hindgut respectively. The amnion forms a fluid-filled cavity that provides the
embryo with its own internal aquatic environment. The evolution of the extraembryonic membranes led to less
dependence on water for development and thus allowed the amniotes to branch out into drier environments.
In addition to these membranes, the eggs of birds, reptiles, and a few mammals have shells. An amniote
embryo was then enclosed in the amnion, which was in turn encased in an extra-embryonic coelom contained
within the chorion. Between the shell and the chorion was the albumin of the egg, which provided additional fluid
and cushioning. This was a significant development that further distinguishes the amniotes from amphibians,
which were and continue to be restricted to moist environments due their shell-less eggs. Although the shells
of various reptilian amniotic species vary significantly, they all permit the retention of water and nutrients for the
developing embryo. The egg shells of bird (avian reptiles) are hardened with calcium carbonate, making them
rigid, but fragile. The shells of most nonavian reptile eggs, such as turtles, are leathery and require a moist
environment. Most mammals do not lay eggs (except for monotremes such as the echindnas and platypuses).
Instead, the embryo grows within the mother’s body, with the placenta derived from two of the extraembryonic
membranes.
Characteristics of Amniotes
The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the egg provides
protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide
and oxygen. The albumin, or egg white, outside of the chorion provides the embryo with water and protein,
whereas the fattier egg yolk contained in the yolk sac provides nutrients for the embryo, as is the case with the
eggs of many other animals, such as amphibians. Here are the functions of the extraembryonic membranes:
1. Blood vessels in the yolk sac transport yolk nutrients to the circulatory system of the embryo.
2. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external
environment.
3. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration.
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4. The amnion protects the embryo from mechanical shock and supports hydration.
In mammals, the yolk sac is very reduced, but the embryo is still cushioned and enclosed within the amnion. The
placenta, which transports nutrients and functions in gas exchange and waste management, is derived from the
chorion and allantois.
visual
CONNECTION
Shell
Figure 29.22 An amniotic egg. The key features of an amniotic egg are shown.
Which of the following statements about the parts of an egg are false?
1. The allantois stores nitrogenous waste and facilitates respiration.
2. The chorion facilitates gas exchange.
3. The yolk provides food for the growing embryo.
4. The amniotic cavity is filled with albumen.
Additional derived characteristics of amniotes include a waterproof skin, accessory keratinized structures, and
costal (rib) ventilation of the lungs.
Evolution of Amniotes
The first amniotes evolved from tetrapod ancestors approximately 340 million years ago during the
Carboniferous period. The early amniotes quickly diverged into two main lines: synapsids and sauropsids.
Synapsids included the therapsids, a clade from which mammals evolved. Sauropsids were further divided into
anapsids and diapsids. Diapsids gave rise to the reptiles, including the dinosaurs and birds. The key differences
between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal
fenestrae (“windows”) behind each eye (Figure 29.23). Temporal fenestrae are post-orbital openings in the
skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one
(fused ancestrally from two fenestrae), and diapsids have two (although many diapsids such as birds have highly
modified diapsid skulls). Anapsids include extinct organisms and traditionally included turtles. However, more
recent molecular and fossil evidence clearly shows that turtles arose within the diapsid line and secondarily lost
the temporal fenestrae; thus they appear to be anapsids because modern turtles do not have fenestrae in the
temporal bones of the skull. The canonical diapsids include dinosaurs, birds, and all other extinct and living
reptiles.
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Anapsid skull Synapsld skull Dlapsld skull
Figure 29.23 Amniote skulls. Compare the skulls and temporal fenestrae of anapsids, synapsids, and diapsids.
Anapsids have no openings, synapsids have one opening, and diapsids have two openings.
The diapsids in turn diverged into two groups, the Archosauromorpha (“ancient lizard form”) and the
Lepidosauromorpha (“scaly lizard form”) during the Mesozoic period (Figure 29.24). The lepidosaurs include
modern lizards, snakes, and tuataras. The archosaurs include modern crocodiles and alligators, and the extinct
ichthyosaurs (“fish lizards” superficially resembling dolphins), pterosaurs (“winged lizard"), dinosaurs (“terrible
lizard"), and birds. (We should note that clade Dinosauria includes birds, which evolved from a branch of
maniraptoran theropod dinosaurs in the Mesozoic.)
The evolutionarily derived characteristics of amniotes include the amniotic egg and its four extraembryonic
membranes, a thicker and more waterproof skin, and rib ventilation of the lungs (ventilation is performed by
drawing air into and out of the lungs by muscles such as the costal rib muscles and the diaphragm).
visual
CONNECTION
Synapsida/Therapsida
f -
Mammalia (mammals)
Ancestral
amniote
Anapsida
Lepidosauria
Extinct anapsids
r
Pleiosaurs
Ichthyosaurs
V
V.
Sphenodontia
T uataras
Squamata
(lizards and snakes)
--
Diapsida
r
\ _
Archosauria
Ornithischia
V_
Dinosauria
V_
Saurischia
Testudines (turtles)
Crocodilia
(crocodiles, alligators)
Pterosaurs
Ornithischian dinosaurs
Saurischian dinosaurs
Aves (birds)
Figure 29.24 Amniote phylogeny. This chart shows the evolution of amniotes. The placement of Testudines
(turtles) is currently still debated.
Question: Members of the order Testudines have an anapsid-like skull without obvious temporal fenestrae.
However, molecular studies clearly indicate that turtles descended from a diapsid ancestor. Why might this
be the case?
In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves.
However, both birds and mammals are descended from different amniote branches: the synapsids giving rise to
the therapsids and mammals, and the diapsids giving rise to the lepidosaurs and archosaurs. We will consider
both the birds and the mammals as groups distinct from reptiles for the purpose of this discussion with the
understanding that this does not accurately reflect phylogenetic history and relationships.
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Characteristics of Reptiles
Reptiles are tetrapods. Limbless reptiles—snakes and legless lizards—are classified as tetrapods because they
are descended from four-limbed ancestors. Reptiles lay calcareous or leathery eggs enclosed in shells on land.
Even aquatic reptiles return to the land to lay eggs. They usually reproduce sexually with internal fertilization.
Some species display ovoviviparity, with the eggs remaining in the mother’s body until they are ready to hatch.
In ovoviviparous reptiles, most nutrients are supplied by the egg yolk, while the chorioallantois assists with
respiration. Other species are viviparous, with the offspring born alive, with their development supported by a
yolk sac-placenta, a chorioallantoic-placenta, or both.
One of the key adaptations that permitted reptiles to live on land was the development of their scaly skin,
containing the protein keratin and waxy lipids, which reduced water loss from the skin. A number of keratinous
epidermal structures have emerged in the descendants of various reptilian lineages and some have become
defining characters for these lineages: scales, claws, nails, horns, feathers, and hair. Their occlusive skin means
that reptiles cannot use their skin for respiration, like amphibians, and thus all amniotes breathe with lungs. All
reptiles grow throughout their lives and regularly shed their skin, both to accommodate their growth and to rid
themselves of ectoparasites. Snakes tend to shed the entire skin at one time, but other reptiles shed their skins
in patches.
Reptiles ventilate their lungs using various muscular mechanisms to produce negative pressure (low pressure)
within the lungs that allows them to expand and draw in air. In snakes and lizards, the muscles of the spine
and ribs are used to expand or contract the rib cage. Since walking or running interferes with this activity,
the squamates cannot breathe effectively while running. Some squamates can supplement rib movement with
buccal pumping through the nose, with the mouth closed. In crocodilians, the lung chamber is expanded and
contracted by moving the liver, which is attached to the pelvis. Turtles have a special problem with breathing,
because their rib cage cannot expand. However, they can change the pressure around the lungs by pulling their
limbs in and out of the shell, and by moving their internal organs. Some turtles also have a posterior respiratory
sac that opens off the hindgut that aids in the diffusion of gases.
Most reptiles are ectotherms, animals whose main source of body heat comes from the environment; however,
some crocodilians maintain elevated thoracic temperatures and thus appear to be at least regional endotherms.
This is in contrast to true endotherms, which use heat produced by metabolism and muscle contraction
to regulate body temperature over a very narrow temperature range, and thus are properly referred to as
homeotherms. Reptiles have behavioral adaptations to help regulate body temperature, such as basking in
sunny places to warm up through the absorption of solar radiation, or finding shady spots or going underground
to minimize the absorption of solar radiation, which allows them to cool down and prevent overheating. The
advantage of ectothermy is that metabolic energy from food is not required to heat the body; therefore, reptiles
can survive on about 10 percent of the calories required by a similarly sized endotherm. In cold weather, some
reptiles such as the garter snake brumate. Brumation is similar to hibernation in that the animal becomes less
active and can go for long periods without eating, but differs from hibernation in that brumating reptiles are not
asleep or living off fat reserves. Rather, their metabolism is slowed in response to cold temperatures, and the
animal is very sluggish.
Evolution of Reptiles
Reptiles originated approximately 300 million years ago during the Carboniferous period. One of the oldest
known amniotes is Casineria, which had both amphibian and reptilian characteristics. One of the earliest
undisputed reptile fossils was Hylonomus, a lizardlike animal about 20 cm long. Soon after the first amniotes
appeared, they diverged into three groups—synapsids, anapsids, and diapsids—during the Permian period. The
Permian period also saw a second major divergence of diapsid reptiles into stem archosaurs (predecessors of
thecodonts, crocodilians, dinosaurs, and birds) and lepidosaurs (predecessors of snakes and lizards). These
groups remained inconspicuous until the Triassic period, when the archosaurs became the dominant terrestrial
group possibly due to the extinction of large-bodied anapsids and synapsids during the Permian-Triassic
extinction. About 250 million years ago, archosaurs radiated into the pterosaurs and both saurischian “lizard hip”
and ornithischian “bird-hip” dinosaurs (see below).
Although they are sometimes mistakenly called dinosaurs, the pterosaurs were distinct from true dinosaurs
(Figure 29.25). Pterosaurs had a number of adaptations that allowed for flight, including hollow bones (birds
also exhibit hollow bones, a case of convergent evolution). Their wings were formed by membranes of skin that
attached to the long, fourth finger of each arm and extended along the body to the legs.
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Figure 29.25 Pterosaurs. Pterosaurs, such as this Quetzalcoatlus, which existed from the late Triassic to the
Cretaceous period (230 to 65.5 million years ago), possessed wings but are not believed to have been capable of
powered flight. Instead, they may have been able to soar after launching from cliffs, (credit: Mark Witton, Darren Naish)
Archosaurs: Dinosaurs
Dinosaurs (“fearfully-great lizard") include the Saurischia (“lizard-hipped") with a simple, three-pronged pelvis,
and Ornithischia (“bird-hipped”) dinosaurs with a more complex pelvis, superficially similar to that of birds.
However, it is a fact that birds evolved from the saurischian “lizard hipped” lineage, not the ornithischian “bird
hip” lineage. Dinosaurs and their theropod descendants, the birds, are remnants of what was formerly a hugely
diverse group of reptiles, some of which like Argentinosaurus were nearly 40 meters (130 feet) in length and
weighed at least 80,000 kg (88 tons). They were the largest land animals to have lived, challenging and perhaps
exceeding the great blue whale in size, but probably not weight—which could be greater than 200 tons.
Herrerasaurus, a bipedal dinosaur from Argentina, was one of the earliest dinosaurs that walked upright with the
legs positioned directly below the pelvis, rather than splayed outward to the sides as in the crocodilians. The
Ornithischia were all herbivores, and sometimes evolved into crazy shapes, such as ankylosaur “armored tanks”
and horned dinosaurs such as Triceratops. Some, such as Parasaurolophus, lived in great herds and may have
amplified their species-specific calls through elaborate crests on their heads.
Both the ornithischian and saurischian dinosaurs provided parental care for their broods, just as crocodilians
and birds do today. The end of the age of dinosaurs came about 65 million years ago, during the Mesozoic,
coinciding with the impact of a large asteroid (that produced the Chicxulub crater) in what is now the Yucatan
Peninsula of Mexico. Besides the immediate environmental disasters associated with this asteroid impacting the
Earth at about 45,000 miles per hour, the impact may also have helped generate an enormous series of volcanic
eruptions that changed the distribution and abundance of plant life worldwide, as well as its climate. At the end
of the Triassic, massive volcanic activity across North America, South America, Africa, and southwest Europe
ultimately would lead to the break-up of Pangea and the opening of the Atlantic Ocean. The formerly incredibly
diverse dinosaurs (save for the evolving birds) met their extinction during this time period.
Archosaurs: Pterosaurs
More than 200 species of pterosaurs have been described, and in their day, beginning about 230 million years
ago, they were the undisputed rulers of the Mesozoic skies for over 170 million years. Recent fossils suggest
that hundreds of pterosaur species may have lived during any given period, dividing up the environment much
like birds do today. Pterosaurs came in amazing sizes and shapes, ranging in size from that of a small song bird
to that of the enormous Quetzalcoatlus northropi, which stood nearly 6 meters (19 feet) high and had a wingspan
of nearly 14 meters (40 feet). This monstrous pterosaur, named after the Aztec god Quetzalcoatl, the feathered
flying serpent that contributed largely to the creation of humankind, may have been the largest flying animal that
ever evolved!
Some male pterosaurs apparently had brightly colored crests that may have served in sexual displays; some of
these crests were much higher than the actual head! Pterosaurs had ultralight skeletons, with a pteroid bone,
unique to pterosaurs, that strengthened the forewing membrane. Much of their wing span was exaggerated
by a greatly elongated fourth finger that supported perhaps half of the wing. It is tempting to relate to them in
terms of bird characteristics, but in reality, their proportions were decidedly not like birds at all. For example, it is
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common to find specimens, such as Quetzalcoatlus, with a head and neck region that together was three to four
times as large as the torso, in addition, unlike the feathered bird wing, the reptilian wing had a layer of muscles,
connective tissue, and blood vessels, all reinforced with a webbing of fibrous cords.
In contrast to the aerial pterosaurs, the dinosaurs were a diverse group of terrestrial reptiles with more than 1,000
species classified to date. Paleontologists continue to discover new species of dinosaurs. Some dinosaurs were
quadrupeds (Figure 29.26); others were bipeds. Some were carnivorous, whereas others were herbivorous.
Dinosaurs laid eggs, and a number of nests containing fossilized eggs, with intact embryos, have been found. It
is not known with certainty whether dinosaurs were homeotherms or facultative endotherms. However, given that
modern birds are endothermic, the dinosaurs that were the immediate ancestors to birds likely were endothermic
as well. Some fossil evidence exists for dinosaurian parental care, and comparative biology supports this
hypothesis since the archosaur birds and crocodilians both display extensive parental care.
(a) (b)
Figure 29.26 Ornithischian and saurischian Dinosaurs. Edmontonia was an armored dinosaur that lived in the Late
Cretaceous period, 145.5 to 65.6 million years ago. Herrerrasaurus and Eoraptor (b) were late Triassic saurischian
dinosaurs dating to about 230 million years ago. (credit: a Mariana Ruiz Villareal b Zach Tirrell from Plymouth, USA,
Dino Origins)
Dinosaurs dominated the Mesozoic era, which was known as the “Age of Reptiles." The dominance of dinosaurs
lasted until the end of the Cretaceous, the last period of the Mesozoic era. The Cretaceous-Tertiary extinction
resulted in the loss of most of the large-bodied animals of the Mesozoic era. Birds are the only living descendants
of one of the major clades of theropod dinosaurs.
LINK
T a
LEARNING
Visit this site to see a video (http:// 0 penstaxc 0 llege. 0 rg/l/K-T_extincti 0 n) discussing the hypothesis that an
asteroid caused the Cretaceous-Triassic (KT) extinction.
Modern Reptiles
Class Reptilia includes many diverse species that are classified into four living clades. There are the 25 species
of Crocodilia, two species of Sphenodontia, approximately 9,200 Squamata species, and about 325 species of
Testudines.
Crocodilia
Crocodilia (“small lizard”) arose as a distinct lineage by the middle Triassic; extant species include alligators,
crocodiles, gharials, and caimans. Crocodilians (Figure 29.27) live throughout the tropics and subtropics of
Africa, South America, Southern Florida, Asia, and Australia. They are found in freshwater, saltwater, and
brackish habitats, such as rivers and lakes, and spend most of their time in water. Crocodiles are descended
from terrestrial reptiles and can still walk and run well on land. They often move on their bellies in a swimming
motion, propelled by alternate movements of their legs. However, some species can lift their bodies off the
ground, pulling their legs in under the body with their feet rotated to face forward. This mode of locomotion
takes a lot of energy, and seems to be used primarily to clear ground obstacles. Amazingly, some crocodiles
can also gallop, pushing off with their hind legs and moving their hind and forelegs alternately in pairs. Galloping
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crocodiles have been clocked at speeds over 17 kph and, over short distances, in an ambush situation, they can
easily chase down most humans if they are taken by surprise. However, they are short distance runners, not
interested in a long chase, and most fit humans can probably outrun them in a sprint (assuming they respond
quickly to the ambush!).
Figure 29.27 A crocodilian. Crocodilians, such as this Siamese crocodile (Crocodylus siamensis), provide parental
care for their offspring, (credit: Keshav Mukund Kandhadai)
Sphenodontia
Sphenodontia (“wedge tooth”) arose in the early Mesozoic era, when they had a moderate radiation, but
now are represented by only two living species: Sphenodon punctatus and Sphenodon guntheri, both found
on offshore islands in New Zealand (Figure 29.28). The common name "tuatara" comes from a Maori word
describing the crest along its back. Tuataras have a primitive diapsid skull with biconcave vertebrae. They
measure up to 80 centimeters and weigh about 1 kilogram. Although superficially similar to an iguanid lizard,
several unique features of the skull and jaws clearly define them and distinguish this group from the Squamata.
They have no external ears. Tuataras briefly have a third (parietal) eye—with a lens, retina, and cornea—in the
middle of the forehead. The eye is visible only in very young animals; it is soon covered with skin. Parietal eyes
can sense light, but have limited color discrimination. Similar light-sensing structures are also seen in some other
lizards. In their jaws, tuataras have two rows of teeth in the upper jaw that bracket a single row of teeth in the
lower jaw. These teeth are actually projections from the jawbones, and are not replaced as they wear down.
Figure 29.28 A tuatara. This tuatara from New Zealand may resemble a lizard but belongs to a distinct lineage, the
Sphenodontidae family, (credit: Sid Mosdell)
Squamata
The Squamata (“scaly or having scales”) arose in the late Permian, and extant species include lizards and
snakes. Both are found on all continents except Antarctica. Lizards and snakes are most closely related to
tuataras, both groups having evolved from a lepidosaurian ancestor. Squamata is the largest extant clade of
reptiles (Figure 29.29).
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Figure 29.29 A chameleon. This Jackson's chameleon (Trioceros jacksonii) blends in with its surroundings.
Most lizards differ from snakes by having four limbs, although these have often been lost or significantly reduced
in at least 60 lineages. Snakes lack eyelids and external ears, which are both present in lizards. There are
about 6,000 species of lizards, ranging in size from tiny chameleons and geckos, some of which are only a few
centimeters in length, to the Komodo dragon, which is about 3 meters in length.
Some lizards are extravagantly decorated with spines, crests, and frills, and many are brightly colored. Some
lizards, like chameleons (Figure 29.29), can change their skin color by redistributing pigment within
chromatophores in their skins. Chameleons change color both for camouflage and for social signaling. Lizards
have multiple-colored oil droplets in their retinal cells that give them a good range of color vision. Lizards,
unlike snakes, can focus their eyes by changing the shape of the lens. The eyes of chameleons can move
independently. Several species of lizards have a "hidden" parietal eye, similar to that in the tuatara. Both lizards
and snakes use their tongues to sample the environment and a pit in the roof of the mouth, Jacobson's organ, is
used to evaluate the collected sample.
Most lizards are carnivorous, but some large species, such as iguanas, are herbivores. Some predatory lizards
are ambush predators, waiting quietly until their prey is close enough for a quick grab. Others are patient
foragers, moving slowly through their environment to detect possible prey. Lizard tongues are long and sticky
and can be extended at high speed for capturing insects or other small prey. Traditionally, the only venomous
lizards are the Gila monster and the beaded lizard. However, venom glands have also been identified in several
species of monitors and iguanids, but the venom is not injected directly and should probably be regarded as a
toxin delivered with the bite.
Specialized features of the jaw are related to adaptations for feeding that have evolved to feed on relatively large
prey (even though some current species have reversed this trend). Snakes are thought to have descended from
either burrowing or aquatic lizards over 100 million years ago (Figure 29.30). They include about 3,600 species,
ranging in size from 10 centimeter-long thread snakes to 10 meter-long pythons and anacondas. All snakes
are legless, except for boids (e.g., boa constrictors), which have vestigial hindlimbs in the form of pelvic spurs.
Like caecilian amphibians, the narrow bodies of most snakes have only a single functional lung. All snakes are
carnivorous and eat small animals, birds, eggs, fish, and insects.
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Figure 29.30 A nonvenomous snake. The garter snake belongs to the genus Thamnophis, the most widely distributed
reptile genus in North America, (credit: Steve Jurvetson)
Most snakes have a skull that is very flexible, involving eight rotational joints. They also differ from other
squamates by having mandibles (lower jaws) without either bony or ligamentous attachment anteriorly. Having
this connection via skin and muscle allows for great dynamic expansion of the gape and independent motion
of the two sides—both advantages in swallowing big prey. Most snakes are nonvenomous and simply swallow
their prey alive, or subdue it by constriction before swallowing it. Venomous snakes use their venom both to kill
or immobilize their prey, and to help digest it.
Although snakes have no eyelids, their eyes are protected with a transparent scale. Their retinas have both rods
and cones, and like many animals, they do not have receptor pigments for red light. Some species, however,
can see in the ultraviolet, which allows them to track ultraviolet signals in rodent trails. Snakes adjust focus
by moving their heads. They have lost both external and middle ears, although their inner ears are sensitive
to ground vibrations. Snakes have a number of sensory structures that assist in tracking prey. In pit vipers,
like rattlesnakes, a sensory pit between the eye and nostrils is sensitive to infrared (“heat") emissions from
warm-blooded prey. A row of similar pits is located on the upper lip of boids. As noted above, snakes also use
Jacobson's organ for detecting olfactory signals.
Testudines
The turtles, terrapins, and tortoises are members of the clade Testudines (“having a shell") (Figure 29.31), and
are characterized by a bony or cartilaginous shell. The shell in turtles is not just an epidermal covering, but is
incorporated into the skeletal system. The dorsal shell is called the carapace and includes the backbone and
ribs; the ventral shell is called the plastron. Both shells are covered with keratinous plates or scutes, and the
two shells are held together by a bridge. In some turtles, the plastron is hinged to allow the head and legs to be
withdrawn under the shell.
The two living groups of turtles, Pleurodira and Cryptodira, have significant anatomical differences and are most
easily recognized by how they retract their necks. The more common Cryptodira retract their neck in a vertical
S-curve; they appear to simply pull their head backward when retracting. Less common Pleurodira ("side-neck")
retract their neck with a horizontal curve, basically folding their neck to the side.
The Testudines arose approximately 200 million years ago, predating crocodiles, lizards, and snakes. There
are about 325 living species of turtles and tortoises. Like other reptiles, turtles are ectotherms. All turtles are
oviparous, laying their eggs on land, although many species live in or near water. None exhibit parental care.
Turtles range in size from the speckled padloper tortoise at 8 centimeters (3.1 inches) to the leatherback sea
turtle at 200 centimeters (over 6 feet). The term “turtle" is sometimes used to describe only those species of
Testudines that live in the sea, with the terms “tortoise" and “terrapin” used to refer to species that live on land
and in fresh water, respectively.
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Figure 29.31 A tortoise. The African spurred tortoise ( Geochelone sulcata) lives at the southern edge of the Sahara
Desert. It is the third largest tortoise in the world, (credit: Jim Bowen)
29.5 | Birds
By the end of this section, you will be able to do the following:
• Describe the evolutionary history of birds
• Describe the derived characteristics in birds that facilitate flight
With over 10,000 identified species, the birds are the most speciose of the land vertebrate classes. Abundant
research has shown that birds are really an extant clade that evolved from maniraptoran theropod dinosaurs
about 150 million years ago. Thus, even though the most obvious characteristic that seems to set birds apart
from other extant vertebrates is the presence of feathers, we now know that feathers probably appeared in the
common ancestor of both ornithischian and saurischian lineages of dinosaurs. Feathers in these clades are also
homologous to reptilian scales and mammalian hair, according to the most recent research. While the wings of
vertebrates like bats function without feathers, birds rely on feathers, and wings, along with other modifications
of body structure and physiology, for flight, as we shall see.
Characteristics of Birds
Birds are endothermic, and more specifically, homeothermic —meaning that they usually maintain an elevated
and constant body temperature, which is significantly above the average body temperature of most mammals.
This is, in part, due to the fact that active flight—especially the hovering skills of birds such as
hummingbirds—requires enormous amounts of energy, which in turn necessitates a high metabolic rate. Like
mammals (which are also endothermic and homeothermic and covered with an insulating pelage), birds have
several different types of feathers that together keep “heat" (infrared energy) within the core of the body, away
from the surface where it can be lost by radiation and convection to the environment.
Modern birds produce two main types of feathers: contour feathers and down feathers. Contour feathers have
a number of parallel barbs that branch from a central shaft. The barbs in turn have microscopic branches
called barbules that are linked together by minute hooks, making the vane of a feather a strong, flexible,
and uninterrupted surface. In contrast, the barbules of down feathers do not interlock, making these feathers
especially good for insulation, trapping air in spaces between the loose, interlocking barbules of adjacent
feathers to decrease the rate of heat loss by convection and radiation. Certain parts of a bird’s body are covered
in down feathers, and the base of other feathers has a downy portion, whereas newly hatched birds are covered
almost entirely in down, which serves as an excellent coat of insulation, increasing the thermal boundary layer
between the skin and the outside environment.
Feathers not only provide insulation, but also allow for flight, producing the lift and thrust necessary for flying
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birds to become and stay airborne. The feathers on a wing are flexible, so the feathers at the end of the
wing separate as air moves over them, reducing the drag on the wing. Flight feathers are also asymmetrical
and curved, so that air flowing over them generates lift. Two types of flight feathers are found on the wings,
primary feathers and secondary feathers (Figure 29.32). Primary feathers are located at the tip of the wing and
provide thrust as the bird moves its wings downward, using the pectoralis major muscles. Secondary feathers
are located closer to the body, in the forearm portion of the wing, and provide lift, in contrast to primary and
secondary feathers, contour feathers are found on the body, where they help reduce form drag produced by wind
resistance against the body during flight. They create a smooth, aerodynamic surface so that air moves swiftly
over the bird’s body, preventing turbulence and creating ideal aerodynamic conditions for efficient flight.
Primary —
feathers
Secondary —
feathers
(a)
(b)
Figure 29.32 Flight feathers, (a) Primary feathers are located at the wing tip and provide thrust; secondary feathers are
located close to the body and provide lift, (b) Primary and secondary feathers from a common buzzard (Buteo buteo).
(Credit b: Mod. from S. Seyfert https://commons.wikimedia. 0 rg/w/index.php?curid=613813 (http://openstax.org/
l/buzzard feathers) )
Flapping of the entire wing occurs primarily through the actions of the chest muscles: Specifically, the contraction
of the pectoralis major muscles moves the wings downward (downstroke), whereas contraction of the
supracoracoideus muscles moves the wings upward (upstroke) via a tough tendon that passes over the coracoid
bone and the top of the humerus. Both muscles are attached to the keel of the sternum, and these are the
muscles that humans eat on holidays (this is why the back of the bird offers little meat!). These muscles are
highly developed in birds and account for a higher percentage of body mass than in most mammals. The
flight muscles attach to a blade-shaped keel projecting ventrally from the sternum, like the keel of a boat. The
sternum of birds is deeper than that of other vertebrates, which accommodates the large flight muscles. The
flight muscles of birds who are active flyers are rich with oxygen-storing myoglobin. Another skeletal modification
found in most birds is the fusion of the two clavicles (collarbones), forming the furcula or wishbone. The furcula
is flexible enough to bend and provide support to the shoulder girdle during flapping.
An important requirement for flight is a low body weight. As body weight increases, the muscle output required
for flying increases. The largest living bird is the ostrich, and while it is much smaller than the largest mammals, it
is secondarily flightless. For birds that do fly, reduction in body weight makes flight easier. Several modifications
are found in birds to reduce body weight, including pneumatization of bones. Pneumatic bones (Figure 29.33)
are bones that are hollow, rather than filled with tissue; cross struts of bone called trabeculae provide structural
reinforcement. Pneumatic bones are not found in all birds, and they are more extensive in large birds than in
small birds. Not all bones of the skeleton are pneumatic, although the skulls of almost all birds are. The jaw is
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also lightened by the replacement of heavy jawbones and teeth with a beak made of keratin (just as hair, scales,
and feathers are).
Figure 29.33 Pneumatic bone. Many birds have hollow, pneumatic bones, which make flight easier.
Other modifications that reduce weight include the lack of a urinary bladder. Birds possess a cloaca, an external
body cavity into which the intestinal, urinary, and genital orifices empty in reptiles, birds, and the monotreme
mammals. The cloaca allows water to be reabsorbed from waste back into the bloodstream. Thus, uric acid is
not eliminated as a liquid but is concentrated into urate salts, which are expelled along with fecal matter. In this
way, water is not held in a urinary bladder, which would increase body weight. In addition, the females of most
bird species only possess one functional (left) ovary rather than two, further reducing body mass.
The respiratory system of birds is dramatically different from that of reptiles and mammals, and is well adapted
for the high metabolic rate required for flight. To begin, the air spaces of pneumatic bone are sometimes
connected to air sacs in the body cavity, which replace coelomic fluid and also lighten the body. These air sacs
are also connected to the path of airflow through the bird's body, and function in respiration. Unlike mammalian
lungs in which air flows in two directions, as it is breathed in and out, diluting the concentration of oxygen, airflow
through bird lungs is unidirectional (Figure 29.34). Gas exchange occurs in "air capillaries" or microscopic air
passages within the lungs. The arrangement of air capillaries in the lungs creates a counter-current exchange
system with the pulmonary blood. In a counter-current system, the air flows in one direction and the blood
flows in the opposite direction, producing a favorable diffusion gradient and creating an efficient means of gas
exchange. This very effective oxygen-delivery system of birds supports their higher metabolic activity. In effect,
ventilation is provided by the parabronchi (minimally expandible lungs) with thin air sacs located among the
visceral organs and the skeleton. A syrinx (voice box) resides near the junction of the trachea and bronchi. The
syrinx, however, is not homologous to the mammalian larynx, which resides within the upper part of the trachea.
Figure 29.34 Air flow in bird lungs. Avian respiration is an efficient system of gas exchange with air flowing
unidirectionally. A full ventilation cycle takes two breathing cycles. During the first inhalation, air passes from the
trachea into posterior air sacs, then during the first exhalation into the lungs. The second inhalation moves the air in the
lungs to the anterior air sacs, and the second exhalation moves the air in the anterior air sacs out of the body. Overall,
each inhalation moves air into the air sacs, while each exhalation moves fresh air through the lungs and "used" air out
of the body. The air sacs are connected to the hollow interior of bones, (credit: modification of work by L. Shyamal)
Beyond the unique characteristics discussed above, birds are also unusual vertebrates because of a number
of other features. First, they typically have an elongate (very “dinosaurian") S-shaped neck, but a short tail or
pygostyte, produced from the fusion of the caudal vertebrae. Unlike mammals, birds have only one occipital
condyle, allowing them extensive movement of the head and neck. They also have a very thin epidermis without
sweat glands, and a specialized uropygial gland or sebaceous “preening gland" found at the dorsal base of the
tail. This gland is an essential to preening (a virtually continuous activity) in most birds because it produces an
oily substance that birds use to help waterproof their feathers as well as keep them flexible for flight. A number
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of birds, such as pigeons, parrots, hawks, and owls, lack a uropygial gland but have specialized feathers that
“disintegrate” into a powdery down, which serves the same purpose as the oils of the uropygial gland.
Like mammals, birds have 12 pairs of cranial nerves, and a very large cerebellum and optic lobes, but only a
single bone in the middle ear called the columella (the stapes in mammals). They have a closed circulatory
system with two atria and two ventricles, but rather than a “left-bending” aortic arch like that of mammals,
they have a “right-bending" aortic arch, and nucleated red blood cells (unlike the enucleated red blood cells of
mammals).
All these unique and highly derived characteristics make birds one of the most conspicuous and successful
groups of vertebrate animals, filling a range of ecological niches, and ranging in size from the tiny bee
hummingbird of Cuba (about 2 grams) to the ostrich (about 140,000 grams). Their large brains, keen senses,
and the abilities of many species to imitate vocalization and use tools make them some of the most intelligent
vertebrates on Earth.
Evolution of Birds
Thanks to amazing new fossil discoveries in China, the evolutionary history of birds has become clearer, even
though bird bones do not fossilize as well as those of other vertebrates. As we’ve seen earlier, birds are highly
modified diapsids, but rather than having two fenestrations or openings in their skulls behind the eye, the skulls
of modern birds are so specialized that it is difficult to see any trace of the original diapsid condition.
Birds belong to a group of diapsids called the archosaurs, which includes three other groups: living crocodilians,
pterosaurs, and dinosaurs. Overwhelming evidence shows that birds evolved within the clade Dinosauria, which
is further subdivided into two groups, the Saurischia (“lizard hips") and the Ornithischia (“bird hips"). Despite the
names of these groups, it was not the bird-hipped dinosaurs that gave rise to modern birds. Rather, Saurischia
diverged into two groups: One included the long-necked herbivorous dinosaurs, such as Apatosaurus. The
second group, bipedal predators called theropods, gave rise to birds. This course of evolution is highlighted by
numerous similarities between late (maniraptoran) theropod fossils and birds, specifically in the structure of the
hip and wrist bones, as well as the presence of the wishbone, formed by the fusion of the clavicles.
The clade Neornithes includes the avian crown group, which comprises all living birds and the descendants
from their most recent common maniraptoran ancestor. One well-known and important fossil of an animal that
appears “intermediate" between dinosaurs and birds is Archaeopteryx (Figure 29.35), which is from the Jurassic
period (200 to 145 MYA). Archaeopteryx has characteristics of both maniraptoran dinosaurs and modern birds.
Some scientists propose classifying it as a bird, but others prefer to classify it as a dinosaur. Traits in skeletons of
Archaeopteryx like those of a dinosaur included a jaw with teeth and a long bony tail. Like birds, it had feathers
modified for flight, both on the forelimbs and on the tail, a trait associated only with birds among modern animals.
Fossils of older feathered dinosaurs exist, but the feathers may not have had the characteristics of modern flight
feathers.
Figure 29.35 Archaeopteryx, (a) Archaeopteryx lived in the late Jurassic period around 150 million years ago. It had
cuplike thecodont teeth like a dinosaur, but had (b) flight feathers like modern birds, which can be seen in this fossil.
Note the claws on the wings, which are still found in a number of birds, such as the newborn chicks of the South
American Hoatzin.
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The Evolution of Flight in Birds
There are two basic hypotheses that explain how flight may have evolved in birds: the arboreal (“tree”)
hypothesis and the terrestrial (“land") hypothesis. The arboreal hypothesis posits that tree-dwelling precursors
to modern birds jumped from branch to branch using their feathers for gliding before becoming fully capable of
flapping flight. In contrast to this, the terrestrial hypothesis holds that running (perhaps pursuing active prey
such as small cursorial animals) was the stimulus for flight. In this scenario, wings could be used to capture
prey and were preadapted for balance and flapping flight. Ostriches, which are large flightless birds, hold their
wings out when they run, possibly for balance. However, this condition may represent a behavioral relict of the
clade of flying birds that were their ancestors. It seems more likely that small feathered arboreal dinosaurs, were
capable of gliding (and flapping) from tree to tree and branch to branch, improving the chances of escaping
enemies, finding mates, and obtaining prey such as flying insects. This early flight behavior would have also
greatly increased the opportunity for species dispersal.
Although we have a good understanding of how feathers and flight may have evolved, the question of how
endothermy evolved in birds (and other lineages) remains unanswered. Feathers provide insulation, but this is
only beneficial for thermoregulatory purposes if body heat is being produced internally. Similarly, internal heat
production is only viable for the evolution of endothermy if insulation is present to retain that infrared energy.
It has been suggested that one or the other—feathers or endothermy—evolved first in response to some other
selective pressure (e.g., the ability to be active at night, provide camouflage, repel water, or serve as signals
for mate selection). It seems probable that feathers and endothermy coevolved together, the improvement and
evolutionary advancement of feathers reinforcing the evolutionary advancement of endothermy, and so on.
During the Cretaceous period (145 to 66 MYA), a group known as the Enantiornithes was the dominant bird type
(Figure 29.36). Enantiornithes means “opposite birds," which refers to the fact that certain bones of the shoulder
are joined differently than the way the bones are joined in modern birds. Like Archaeopteryx, these birds retained
teeth in their jaws, but did have a shortened tail, and at least some fossils have preserved “fans” of tail feathers.
These birds formed an evolutionary lineage separate from that of modern birds, and they did not survive past the
Cretaceous. Along with the Enantiornithes, however, another group of birds—the Ornithurae ("bird tails"), with a
short, fused tail or pygostyle —emerged from the evolutionary line that includes modern birds. This clade was
also present in the Cretaceous.
After the extinction of Enantiornithes, the Ornithurae became the dominant birds, with a large and rapid
radiation occurring after the extinction of the dinosaurs during the Cenozoic era (66 MYA to the present).
Molecular analysis based on very large data sets has produced our current understanding of the relationships
among living birds. There are three major clades: the Paleognathae, the Galloanserae, and the Neoaves.
The Paleognathae (“old jaw”) or ratites (polyphyletic) are a group of flightless birds including ostriches, emus,
rheas, and kiwis. The Galloanserae include pheasants, ducks, geese and swans. The Neoaves ("new birds")
includes all other birds. The Neoaves themselves have been distributed among five clades: Strisores (nightjars,
swifts, and hummingbirds), Columbaves (turacos, bustards, cuckoos, pigeons, and doves), Gruiformes (cranes),
Aequorlitornithes (diving birds, wading birds, and shorebirds), and Inopinaves (a very large clade of land birds
including hawks, owls, woodpeckers, parrots, falcons, crows, and songbirds). Despite the current classification
scheme, it is important to understand that phylogenetic revisions, even for the extant birds, are still taking place.
3. Prum, RO et al. 2015. A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing. Nature 526: 569 -
573. http://dx.doi.org/10.1038/naturel5697 (http:// 0 penstax. 0 rg/l/bird_phyl 0 geny)
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Figure 29.36 Enantiornithean bird. Shanweiniao cooperorum was a species of Enantiornithes that did not survive past
the Cretaceous period, (credit: Nobu Tamura)
career connection
Veterinarian
Veterinarians are concerned with diseases, disorders, and injuries in animals, primarily vertebrates. They
treat pets, livestock, and animals in zoos and laboratories. Veterinarians often treat dogs and cats, but also
take care of birds, reptiles, rabbits, and other animals that are kept as pets. Veterinarians that work with
farms and ranches care for pigs, goats, cows, sheep, and horses.
Veterinarians are required to complete a degree in veterinary medicine, which includes taking courses in
comparative zoology, animal anatomy and physiology, microbiology, and pathology, among many other
courses in chemistry, physics, and mathematics.
Veterinarians are also trained to perform surgery on many different vertebrate species, which requires an
understanding of the vastly different anatomies of various species. For example, the stomach of ruminants
like cows has four “compartments” versus one compartment for non-ruminants. As we have seen, birds also
have unique anatomical adaptations that allow for flight, which requires additional training and care.
Some veterinarians conduct research in academic settings, broadening our knowledge of animals and
medical science. One area of research involves understanding the transmission of animal diseases to
humans, called zoonotic diseases. For example, one area of great concern is the transmission of the
avian flu virus to humans. One type of avian flu virus, H5N1, is a highly pathogenic strain that has been
spreading in birds in Asia, Europe, Africa, and the Middle East. Although the virus does not cross over easily
to humans, there have been cases of bird-to-human transmission. More research is needed to understand
how this virus can cross the species barrier and how its spread can be prevented.
29.6 | Mammals
By the end of this section, you will be able to do the following:
• Name and describe the distinguishing features of the three main groups of mammals
• Describe the likely line of evolutionary descent that produced mammals
• List some derived features that may have arisen in response to mammals’ need for constant, high-level
metabolism
• Identify the major clades of eutherian mammals
Mammals, comprising about 5,200 species, are vertebrates that possess hair and mammary glands. Several
other characteristics are distinctive to mammals, including certain features of the jaw, skeleton, integument,
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and internal anatomy. Modem mammals belong to three clades: monotremes, marsupials, and eutherians (or
placental mammals).
Characteristics of Mammals
The presence of hair, composed of the protein keratin, is one of the most obvious characteristics of mammals.
Although it is not very extensive or obvious on some species (such as whales), hair has many important
functions for most mammals. Mammals are endothermic, and hair traps a boundary layer of air close to the body,
retaining heat generated by metabolic activity. Along with insulation, hair can serve as a sensory mechanism via
specialized hairs called vibrissae, better known as whiskers. Vibrissae attach to nerves that transmit information
about tactile vibration produced by sound sensation, which is particularly useful to nocturnal or burrowing
mammals. Hair can also provide protective coloration or be part of social signaling, such as when an animal’s
hair stands “on end” to warn enemies, or possibly to make the mammal “look bigger” to predators.
Unlike the skin of birds, the integument (skin) of mammals, includes a number of different types of secretory
glands. Sebaceous glands produce a lipid mixture called sebum that is secreted onto the hair and skin,
providing water resistance and lubrication for hair. Sebaceous glands are located over most of the body. Eccrine
glands produce sweat, or perspiration, which is mainly composed of water, but also contains metabolic waste
products, and sometimes compounds with antibiotic activity. In most mammals, eccrine glands are limited to
certain areas of the body, and some mammals do not possess them at all. However, in primates, especially
humans, sweat glands are located over most of the body surface and figure prominently in regulating the body
temperature through evaporative cooling. Apocrine glands, or scent glands, secrete substances that are used
for chemical communication, such as in skunks. Mammary glands produce milk that is used to feed newborns.
In both monotremes and eutherians, both males and females possess mammary glands, while in marsupials,
mammary glands have been found only in some opossums. Mammary glands likely are modified sebaceous or
eccrine glands, but their evolutionary origin is not entirely clear.
The skeletal system of mammals possesses many unique features. The lower jaw of mammals consists of only
one bone, the dentary, and the jaw hinge connects the dentary to the squamosal (flat) part of the temporal
bone in the skull. The jaws of other vertebrates are composed of several bones, including the quadrate bone
at the back of the skull and the articular bone at the back of the jaw, with the jaw connected between the
quadrate and articular bones. In the ear of other vertebrates, vibrations are transmitted to the inner ear by a
single bone, the stapes. In mammals, the quadrate and articular bones have moved into the middle ear (Figure
29.37). The malleus is derived from the articular bone, whereas the incus originated from the quadrate bone.
This arrangement of jaw and ear bones aids in distinguishing fossil mammals from fossils of other synapsids.
Malleus Incus Stapes
Cranial Bones
Figure 29.37 Mammalian ear bones. Bones of the mammalian middle ear are modified from bones of the jaw and skull
in reptiles. The stapes is found in other vertebrates (e.g., the columella of birds) whereas in mammals, the malleus and
incus are derived from the articular and quadrate bones, respectively, (credit: NCI)
The adductor muscles that close the jaw comprise two major muscles in mammals: the temporalis and the
masseter. Working together, these muscles permit up-and-down and side-to-side movements of the jaw, making
chewing possible—which is unique to mammals. Most mammals have heterodont teeth, meaning that they have
different types and shapes of teeth (incisors, canines, premolars, and molars) rather than just one type and
shape of tooth. Most mammals are also diphyodonts, meaning that they have two sets of teeth in their lifetime:
deciduous or “baby" teeth, and permanent teeth. Most other vertebrates with teeth are polyphyodonts, that is,
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their teeth are replaced throughout their entire life.
Mammals, like birds, possess a four-chambered heart; however, the hearts of birds and mammals are an
example of convergent evolution, since mammals clearly arose independently from different groups of tetrapod
ancestors. Mammals also have a specialized group of cardiac cells (fibers) located in the walls of their right
atrium called the sinoatrial node, or pacemaker, which determines the rate at which the heart beats. Mammalian
erythrocytes (red blood cells) do not have nuclei, whereas the erythrocytes of other vertebrates are nucleated.
The kidneys of mammals have a portion of the nephron called the loop of Henle or nephritic loop, which allows
mammals to produce urine with a high concentration of solutes—higher than that of the blood. Mammals lack
a renal portal system, which is a system of veins that moves blood from the hind or lower limbs and region of
the tail to the kidneys. Renal portal systems are present in all other vertebrates except jawless fishes. A urinary
bladder is present in all mammals.
Unlike birds, the skulls of mammals have two occipital condyles, bones at the base of the skull that articulate
with the first vertebra, as well as a secondary palate at the rear of the pharynx that helps to separate the pathway
of swallowing from that of breathing. Turbinate bones (chonchae in humans) are located along the sides of
the nasal cavity, and help warm and moisten air as it is inhaled. The pelvic bones are fused in mammals, and
there are typically seven cervical vertebrae (except for some edentates and manatees). Mammals have movable
eyelids and fleshy external ears (pinnae), quite unlike the naked external auditory openings of birds. Mammals
also have a muscular diaphragm that is lacking in birds.
Mammalian brains also have certain characteristics that differ from the brains of other vertebrates. In some,
but not all mammals, the cerebral cortex , the outermost part of the cerebrum, is highly convoluted and folded,
allowing for a greater surface area than is possible with a smooth cortex. The optic lobes, located in the midbrain,
are divided into two parts in mammals, while other vertebrates possess a single, undivided lobe. Eutherian
mammals also possess a specialized structure, the corpus callosum, which links the two cerebral hemispheres
together. The corpus callosum functions to integrate motor, sensory, and cognitive functions between the left and
right cerebral cortexes.
Evolution of Mammals
Mammals are synapsids, meaning they have a single, ancestrally fused, postorbital opening in the skull. They
are the only living synapsids, as earlier forms became extinct by the Jurassic period. The early non-mammalian
synapsids can be divided into two groups, the pelycosaurs and the therapsids. Within the therapsids, a group
called the cynodonts are thought to have been the ancestors of mammals (Figure 29.38).
Figure 29.38 Cynodont. Cynodonts ("dog teeth"), which first appeared in the Late Permian period 260 million years
ago, are thought to be the ancestors of modern mammals. Holes in the upper jaws of cynodonts suggest that they had
whiskers, which might also indicate the presence of hair, (credit: Nobu Tamura)
As with birds, a key characteristic of synapsids is endothermy, rather than the ectothermy seen in many other
vertebrates (such as fish, amphibians, and most reptiles). The increased metabolic rate required to internally
modify body temperature likely went hand-in-hand with changes to certain skeletal structures that improved food
processing and ambulation. The later synapsids, which had more evolved characteristics unique to mammals,
possess cheeks for holding food and heterodont teeth, which are specialized for chewing, mechanically breaking
down food to speed digestion, and releasing the energy needed to produce heat. Chewing also requires the
ability to breathe at the same time, which is facilitated by the presence of a secondary palate (comprising the
bony palate and the posterior continuation of the soft palate). The secondary palate separates the area of the
mouth where chewing occurs from the area above where respiration occurs, allowing breathing to proceed
uninterrupted while the animal is chewing. A secondary palate is not found in pelycosaurs but is present in
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cynodonts and mammals. The jawbone also shows changes from early synapsids to later ones. The zygomatic
arch, or cheekbone, is present in mammals and advanced therapsids such as cynodonts, but is not present in
pelycosaurs. The presence of the zygomatic arch suggests the presence of masseter muscles, which close the
jaw and function in chewing.
In the appendicular skeleton, the shoulder girdle of therian mammals is modified from that of other vertebrates
in that it does not possess a procoracoid bone or an interclavicle, and the scapula is the dominant bone.
Mammals evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are
from the early Jurassic period, some 205 million years ago. One group of transitional mammals was the
morganucodonts, small nocturnal insectivores. The jaws of morganucodonts were “transitional,” with features
of both reptilian and mammalian jaws (Figure 29.39). Like modern mammals, the morganucodonts had
differentiated teeth and were diphyodonts. Mammals first began to diversify in the Mesozoic era, from the
Jurassic to the Cretaceous periods. Even some small gliding mammals appear in the fossil record during this
time period. However, most of the Jurassic mammals were extinct by the end of the Mesozoic. During the
Cretaceous period, another radiation of mammals began and continued through the Cenozoic era, about 65
million years ago.
Figure 29.39 A morganucodont. This morganucodont Megazotrodon, an extinct basal mammal, may have been
nocturnal and insectivorous. Inset: Jaw of a morganucodont, showing a double hinge, one between the dentary and
squamosal and one between the articular (yellow) and quadrate (blue) bones. In living mammals, the articular and
quadrate bones have been incorporated into the middle ear. (Credit: By Nordelch [Megazostrodon Natural History
Museum] Wikimedia Commons. Credit inset: Mod from Philcha. https://commons.wikimedia.Org/w/index.php?
curid=3631949 (http:// 0 penstax. 0 rg/l/jawJ 0 int) )
Living Mammals
There are three major groups of living mammals: monotremes (prototheria ), marsupials (metatheria ), and
placental (eutheria ) mammals. The eutherians and the marsupials together comprise a clade of therian
mammals, with the monotremes forming a sister clade to both metatherians and eutherians.
There are very few living species of monotremes: the platypus and four species of echidnas, or spiny anteaters.
The leathery-beaked platypus belongs to the family Ornithorhynchidae (“bird beak"), whereas echidnas belong
to the family Tachyglossidae (“sticky tongue”) (Figure 29.40). The platypus and one species of echidna are
found in Australia, and the other species of echidna are found in New Guinea. Monotremes are unique among
mammals because they lay eggs, rather than giving birth to live young. The shells of their eggs are not like the
hard shells of birds, but have a leathery shell, similar to the shells of reptile eggs. Monotremes retain their eggs
through about two-thirds of the developmental period, and then lay them in nests. A yolk-sac placenta helps
support development. The babies hatch in a fetal state and complete their development in the nest, nourished
by milk secreted by mammary glands opening directly to the skin. Monotremes, except for young platypuses,
do not have teeth. Body temperature in the three monotreme species is maintained at about 30°C, considerably
lower than the average body temperature of marsupial and placental mammals, which are typically between 35
and 38°C.
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(a) (b)
Figure 29.40 Egg-laying mammals, (a) The platypus, a monotreme, possesses a leathery beak and lays eggs rather
than giving birth to live young, (b) The echidna is another monotreme, with long hairs modified into spines, (credit b:
modification of work by Barry Thomas)
Over 2/3 of the approximately 330 living species of marsupials are found in Australia, with the rest, nearly all
various types of opossum, found in the Americas, especially South America. Australian marsupials include the
kangaroo, koala, bandicoot, Tasmanian devil (Figure 29.41), and several other species. Like monotremes, the
embryos of marsupials are nourished during a short gestational period (about a month in kangaroos) by a yolk-
sac placenta, but with no intervening egg shell. Some marsupial embryos can enter an embryonic diapause,
and delay implantation, suspending development until implantation is completed. Marsupial young are also
effectively fetal at birth. Most, but not all, species of marsupials possess a pouch in which the very premature
young reside, receiving milk and continuing their development. In kangaroos, the young joeys continue to nurse
for about a year and a half.
Figure 29.41 A marsupial mammal. The Tasmanian devil is one of several marsupials native to Australia, (credit:
Wayne McLean)
Eutherians (placentals) are the most widespread and numerous of the mammals, occurring throughout
the world. Eutherian mammals are sometimes called “placental mammals” because all species possess a
complex chorioallantoic placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient
exchange. There are about 4,000 species of placental mammals in 18 to 20 orders with various adaptations
for burrowing, flying, swimming, hunting, running, and climbing. In the evolutionary sense, they have been
incredibly successful in form, diversity, and abundance. The eutherian mammals are classified in two major
clades, the Atlantogenata and the Boreoeutheria. The Atlantogeneta include the Afrotheria (e.g., elephants,
hyraxes, and manatees) and the Xenarthra (anteaters, armadillos, and sloths). The Boreoeutheria contain two
large groups, the Euarchontoglires and the Laurasiatheria. Familiar orders in the Euarchontoglires are the
Scandentia (tree shrews), Rodentia (rats, mice, squirrels, porcupines), Lagomorpha (rabbits and hares), and the
Primates (including humans). Major Laurasiatherian orders include the Perissodactyla (e.g., horses and rhinos),
the Cetartiodactyla (e.g., cows, giraffes, pigs, hippos, and whales), the Carnivora (e.g., cats, dogs, and bears),
and the Chiroptera (bats and flying foxes). The two largest orders are the rodents (2,000 species) and bats
(about 1,000 species), which together constitute approximately 60 percent of all eutherian species.
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29.7 | The Evolution of Primates
By the end of this section, you will be able to do the following:
• Describe the derived features that distinguish primates from other animals
• Describe the defining features of the major groups of primates
• identify the major hominin precursors to modern humans
• Explain why scientists are having difficulty determining the true lines of descent in hominids
Order Primates of class Mammalia includes lemurs, tarsiers, monkeys, apes, and humans. Non-human primates
live primarily in the tropical or subtropical regions of South America, Africa, and Asia. They range in size from the
mouse lemur at 30 grams (1 ounce) to the mountain gorilla at 200 kilograms (441 pounds). The characteristics
and evolution of primates are of particular interest to us as they allow us to understand the evolution of our own
species.
Characteristics of Primates
All primate species possess adaptations for climbing trees, as they all descended from tree-dwellers. This
arboreal heritage of primates has resulted in hands and feet that are adapted for climbing, or brachiation
(swinging through trees using the arms). These adaptations include, but are not limited to: 1) a rotating shoulder
joint, 2) a big toe that is widely separated from the other toes (except humans) and thumbs sufficiently separated
from fingers to allow for gripping branches, and 3) stereoscopic vision, two overlapping fields of vision from the
eyes, which allows for the perception of depth and gauging distance. Other characteristics of primates are brains
that are larger than those of most other mammals, claws that have been modified into flattened nails, typically
only one offspring per pregnancy, and a trend toward holding the body upright.
Order Primates is divided into two groups: Strepsirrhini (“turned-nosed”) and Haplorhini (“simple-nosed”)
primates. Strepsirrhines, also called the wet-nosed primates, include prosimians like the bush babies and pottos
of Africa, the lemurs of Madagascar, and the lorises of Southeast Asia. Haplorhines, or dry-nosed primates,
include tarsiers (Figure 29.42) and simians (New World monkeys, Old World monkeys, apes, and humans). In
general, strepsirrhines tend to be nocturnal, have larger olfactory centers in the brain, and exhibit a smaller size
and smaller brain than anthropoids. Haplorhines, with a few exceptions, are diurnal, and depend more on their
vision. Another interesting difference between the strepsirrhines and haplorhines is that strepsirrhines have the
enzymes for making vitamin C, while haplorhines have to get it from their food.
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Figure 29.42 A Philippine tarsier. This tarsier, Carlito syrichta, is one of the smallest primates—about 5 inches
long, from nose to the base of the tail. The tail is not shown, but is about twice the length of the body. Note
the large eyes, each of which is about the same size as the animal's brain, and the long hind legs, (credit: mtoz
(http://creativecommons. 0 rg/licenses/by-sa/ 2 .O (http:// 0 penstax. 0 rg/l/CCSA_ 2 ) ), via Wikimedia Commons)
Evolution of Primates
The first primate-like mammals are referred to as proto-primates. They were roughly similar to squirrels
and tree shrews in size and appearance. The existing fossil evidence (mostly from North Africa) is very
fragmented. These proto-primates remain largely mysterious creatures until more fossil evidence becomes
available. Although genetic evidence suggests that primates diverged from other mammals about 85 MYA, the
oldest known primate-like mammals with a relatively robust fossil record date to about 65 MYA. Fossils like the
proto-primate Plesiadapis (although some researchers do not agree that Plesiadapis was a proto-primate) had
some features of the teeth and skeleton in common with true primates. They were found in North America and
Europe in the Cenozoic and went extinct by the end of the Eocene.
The first true primates date to about 55 MYA in the Eocene epoch. They were found in North America, Europe,
Asia, and Africa. These early primates resembled present-day prosimians such as lemurs. Evolutionary changes
continued in these early primates, with larger brains and eyes, and smaller muzzles being the trend. By the end
of the Eocene epoch, many of the early prosimian species went extinct due either to cooler temperatures or
competition from the first monkeys.
Anthropoid monkeys evolved from prosimians during the Oligocene epoch. By 40 million years ago, evidence
indicates that monkeys were present in the New World (South America) and the Old World (Africa and Asia).
New World monkeys are also called Platyrrhini—a reference to their broad noses (Figure 29.43). Old World
monkeys are called Catarrhini—a reference to their narrow, downward-pointed noses. There is still quite a bit
of uncertainty about the origins of the New World monkeys. At the time the platyrrhines arose, the continents
of South American and Africa had drifted apart. Therefore, it is thought that monkeys arose in the Old World
and reached the New World either by drifting on log rafts or by crossing land bridges. Due to this reproductive
isolation, New World monkeys and Old World monkeys underwent separate adaptive radiations over millions of
years. The New World monkeys are all arboreal, whereas Old World monkeys include both arboreal and ground¬
dwelling species. The arboreal habits of the New World monkeys are reflected in the possession of prehensile
or grasping tails by most species. The tails of Old World monkeys are never prehensile and are often reduced,
and some species have ischial callosities—thickened patches of skin on their seats.
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Figure 29.43 A New World monkey. The howler monkey is native to Central and South America. It makes a call that
sounds like a lion roaring, (credit: Xavi Talleda)
Apes evolved from the catarrhines in Africa midway through the Cenozoic, approximately 25 million years ago.
Apes are generally larger than monkeys and they do not possess a tail. All apes are capable of moving through
trees, although many species spend most their time on the ground. When walking quadrupedally, monkeys walk
on their palms, while apes support the upper body on their knuckles. Apes are more intelligent than monkeys,
and they have larger brains relative to body size. The apes are divided into two groups. The lesser apes
comprise the family Hylobatidae, including gibbons and siamangs. The great apes include the genera Pan
(chimpanzees and bonobos) Gorilla (gorillas), Pongo (orangutans), and Homo (humans) (Figure 29.44).
Gibbon Human Chimpanzee Gorilla Orangutan
Figure 29.44 Primate skeletons. All great apes have a similar skeletal structure, (credit: modification of work by Tim
Vickers)
The very arboreal gibbons are smaller than the great apes; they have low sexual dimorphism (that is, the
sexes are not markedly different in size), although in some species, the sexes differ in color; and they have
relatively longer arms used for swinging through trees (Figure 29.45a). Two species of orangutan are native
to different islands in Indonesia: Borneo (P. pygmaeus) and Sumatra (P. abelii). A third orangutan species,
Pongo tapanuliensis, was reported in 2017 from the Batang Toru forest in Sumatra. Orangutans are arboreal
and solitary. Males are much larger than females and have cheek and throat pouches when mature. Gorillas
all live in Central Africa. The eastern and western populations are recognized as separate species, G. berengei
and G. gorilla. Gorillas are strongly sexually dimorphic, with males about twice the size of females. In older
males, called silverbacks, the hair on the back turns white or gray. Chimpanzees (Figure 29.45b) are the species
considered to be most closely related to humans. However, the species most closely related to the chimpanzee
is the bonobo. Genetic evidence suggests that chimpanzee and human lineages separated 5 to 7 MYA, while
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chimpanzee (Pan troglodytes) and bonobo (Pan paniscus) lineages separated about 2 MYA. Chimpanzees
and bonobos both live in Central Africa, but the two species are separated by the Congo River, a significant
geographic barrier. Bonobos are slighter than chimpanzees, but have longer legs and more hair on their heads.
In chimpanzees, white tail tufts identify juveniles, while bonobos keep their white tail tufts for life. Bonobos also
have higher-pitched voices than chimpanzees. Chimpanzees are more aggressive and sometimes kill animals
from other groups, while bonobos are not known to do so. Both chimpanzees and bonobos are omnivorous.
Orangutan and gorilla diets also include foods from multiple sources, although the predominant food items are
fruits for orangutans and foliage for gorillas.
(a) (b)
Figure 29.45 Lesser and great apes. This white-cheeked gibbon (a) is a lesser ape. In gibbons of this species, females
and infants are buff and males are black. This young chimpanzee (b) is one of the great apes. It possesses a relatively
large brain and has no tail, (credit a: MAC. credit b: modification of work by Aaron Logan)
Human Evolution
The family Hominidae of order Primates includes the hominoids: the great apes and humans (Figure 29.46).
Evidence from the fossil record and from a comparison of human and chimpanzee DNA suggests that humans
and chimpanzees diverged from a common hominoid ancestor approximately six million years ago. Several
species evolved from the evolutionary branch that includes humans, although our species is the only surviving
member. The term hominin is used to refer to those species that evolved after this split of the primate line,
thereby designating species that are more closely related to humans than to chimpanzees. A number of marker
features differentiate humans from the other hominoids, including bipedalism or upright posture, increase in
the size of the brain, and a fully opposable thumb that can touch the little finger. Bipedal hominins include
several groups that were probably part of the modern human lineage— Australopithecus, Homo habilis, and
Homo erectus —and several non-ancestral groups that can be considered “cousins” of modern humans, such as
Neanderthals and Denisovans.
Determining the true lines of descent in hominins is difficult. In years past, when relatively few hominin fossils
had been recovered, some scientists believed that considering them in order, from oldest to youngest, would
demonstrate the course of evolution from early hominins to modern humans. In the past several years, however,
many new fossils have been found, and it is clear that there was often more than one species alive at any one
time and that many of the fossils found (and species named) represent hominin species that died out and are
not ancestral to modern humans.
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f-
r
Hominoidea
Cercopithecoidae
Baboon
Hylobatidae
Gibbon
Pongidae
Orangutan
Hominidae
'-
? Sahelanthropus tchadensis, Ardipithecus
Homininae
Gorillinae
Gorilla
Panini
Chimpanzee
—
- Orrorin
f
Hominini
Australopithecus anamensls
Australopithecus afarensis
Australopithecus bahrelghazali
Australopithecus garhi
Australopithecus africanus
Paranthropus aethiopicus
Parantliropus boisei
Paranthropus robustus
,- Homo gautengensis
- Homo habilis
Homo erectus
, - Homo neanderthalensis
- Homo heidelbergensis
- Homo rhodesiensis
-Hominini
Humans
Figure 29.46 Hominin phylogeny. This chart shows the evolution of modern humans.
Very Early Hominins
Three species of very early hominids have made news in the late 20th and early 21st centuries: Ardipithecus,
Sahelanthropus, and Orrorin. The youngest of the three species, Ardipithecus, was discovered in the 1990s, and
dates to about 4.4 MYA. Although the bipedality of the early specimens was uncertain, several more specimens
of Ardipithecus were discovered in the intervening years and demonstrated that the organism was bipedal. Two
different species of Ardipithecus have been identified, A. ramidus and A. kadabba, whose specimens are older,
dating to 5.6 MYA. However, the status of this genus as a human ancestor is uncertain.
The oldest of the three, Sahelanthropus tchadensis, was discovered in 2001-2002 and has been dated to nearly
seven million years ago. There is a single specimen of this genus, a skull that was a surface find in Chad. The
fossil, informally called “Toumai,” is a mosaic of primitive and evolved characteristics, and it is unclear how this
fossil fits with the picture given by molecular data, namely that the line leading to modern humans and modern
chimpanzees apparently bifurcated about six million years ago. It is not thought at this time that this species was
an ancestor of modern humans.
A younger (c. 6 MYA) species, Orrorin tugenensis, is also a relatively recent discovery, found in 2000. There are
several specimens of Orrorin. Some features of Orrorin are more similar to those of modern humans than are the
australopithicenes, although Orrorin is much older. If Orrorin is a human ancestor, then the australopithicenes
may not be in the direct human lineage. Additional specimens of these species may help to clarify their role.
Early Hominins: Genus Australopithecus
Australopithecus (“southern ape”) is a genus of hominin that evolved in eastern Africa approximately four million
years ago and went extinct about two million years ago. This genus is of particular interest to us as it is
thought that our genus, genus Homo, evolved from a common ancestor shared with Australopithecus about
two million years ago (after likely passing through some transitional states). Australopithecus had a number
of characteristics that were more similar to the great apes than to modern humans. For example, sexual
dimorphism was more exaggerated than in modern humans. Males were up to 50 percent larger than females,
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a ratio that is similar to that seen in modern gorillas and orangutans. In contrast, modern human males are
approximately 15 to 20 percent larger than females. The brain size of Australopithecus relative to its body mass
was also smaller than in modern humans and more similar to that seen in the great apes. A key feature that
Australopithecus had in common with modern humans was bipedalism, although it is likely that Australopithecus
also spent time in trees. Hominin footprints, similar to those of modern humans, were found in Laetoli, Tanzania
and dated to 3.6 million years ago. They showed that hominins at the time of Australopithecus were walking
upright.
There were a number of Australopithecus species, which are often referred to as australopiths. Australopithecus
anamensis lived about 4.2 million years ago. More is known about another early species, Australopithecus
afarensis, which lived between 3.9 and 2.9 million years ago. This species demonstrates a trend in human
evolution: the reduction of the dentition and jaw in size. A. afarensis (Figure 29.47a) had smaller canines and
molars compared to apes, but these were larger than those of modern humans. Its brain size was 380 to 450
cubic centimeters, approximately the size of a modern chimpanzee brain. It also had prognathic jaws, which is
a relatively longer jaw than that of modern humans. In the mid-1970s, the fossil of an adult female A. afarensis
was found in the Afar region of Ethiopia and dated to 3.24 million years ago (Figure 29.48). The fossil, which is
informally called “Lucy,” is significant because it was the most complete australopith fossil found, with 40 percent
of the skeleton recovered.
(a) (b)
Figure 29.47 Australopithicene and modern human skulls. The skull of (a) Australopithecus afarensis, an early hominid
that lived between two and three million years ago, resembled that of (b) modern humans but was smaller with a sloped
forehead, larger teeth, and a prominent jaw.
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4
Figure 29.48 Lucy. This adult female Australopithecus afarensis skeleton, nicknamed Lucy, was discovered in the
mid-1970s, (credit: “1207Wikimedia Commons)
Australopithecus africanus lived between two and three million years ago. It had a slender build and was bipedal,
but had robust arm bones and, like other early hominids, may have spent significant time in trees. Its brain was
larger than that of A. afarensis at 500 cubic centimeters, which is slightly less than one-third the size of modern
human brains. Two other species, Australopithecus bahrelghazali and Australopithecus garhi, have been added
to the roster of australopiths in recent years. A. bahrelghazali is unusual in being the only australopith found in
Central Africa.
A Dead End: Genus Paranthropus
The australopiths had a relatively slender build and teeth that were suited for soft food. In the past several
years, fossils of hominids of a different body type have been found and dated to approximately 2.5 million years
ago. These hominids, of the genus Paranthropus , were muscular, stood 1.3 to 1.4 meters tall, and had large
grinding teeth. Their molars showed heavy wear, suggesting that they had a coarse and fibrous vegetarian diet
as opposed to the partially carnivorous diet of the australopiths. Paranthropus includes Paranthropus robustus of
South Africa, and Paranthropus aethiopicus and Paranthropus boisei of East Africa. The hominids in this genus
went extinct more than one million years ago and are not thought to be ancestral to modern humans, but rather
members of an evolutionary branch on the hominin tree that left no descendants.
Early Hominins: Genus Homo
The human genus, Homo, first appeared between 2.5 and three million years ago. For many years, fossils of a
species called H. habilis were the oldest examples in the genus Homo, but in 2010, a new species called Homo
gautengensis was discovered and may be older. Compared to A. africanus, H. habilis had a number of features
more similar to modern humans. H. habilis had a jaw that was less prognathic than the australopiths and a larger
brain, at 600 to 750 cubic centimeters. However, H. habilis retained some features of older hominin species,
such as long arms. The name H. habilis means “handy man,” which is a reference to the stone tools that have
been found with its remains.
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LINK TQ LEARNING
Watch this video about Smithsonian paleontologist Briana Pobiner explaining the link between hominin eating
of meat and evolutionary trends. (This multimedia resource will open in a browser.) (http://cnx.org/
content/m66594/1.3/#eip-id3050292)
H. erectus appeared approximately 1.8 million years ago (Figure 29.49). It is believed to have originated in East
Africa and was the first hominin species to migrate out of Africa. Fossils of H. erectus have been found in India,
China, Java, and Europe, and were known in the past as “Java Man" or “Peking Man." H. erectus had a number
of features that were more similar to modern humans than those of H. habilis. H. erectus was larger in size than
earlier hominins, reaching heights up to 1.85 meters and weighing up to 65 kilograms, which are sizes similar to
those of modern humans. Its degree of sexual dimorphism was less than in earlier species, with males being 20
to 30 percent larger than females, which is close to the size difference seen in our own species. H. erectus had
a larger brain than earlier species at 775 to 1,100 cubic centimeters, which compares to the 1,130 to 1,260 cubic
centimeters seen in modern human brains. H. erectus also had a nose with downward-facing nostrils similar
to modern humans, rather than the forward-facing nostrils found in other primates. Longer, downward-facing
nostrils allow for the warming of cold air before it enters the lungs and may have been an adaptation to colder
climates. Artifacts found with fossils of H. erectus suggest that it was the first hominin to use fire, hunt, and have
a home base. H. erectus is generally thought to have lived until about 50,000 years ago.
Figure 29.49 Homo erectus. Homo erectus had a prominent brow and a nose that pointed downward rather than
forward.
Humans: Homo sapiens
A number of species, sometimes called archaic Homo sapiens, apparently evolved from H. erectus starting
about 500,000 years ago. These species include Homo heidelbergensis, Homo rhodesiensis, and Homo
neanderthalensis. These archaic H. sapiens had a brain size similar to that of modern humans, averaging 1,200
to 1,400 cubic centimeters. They differed from modern humans by having a thick skull, a prominent brow ridge,
and a receding chin. Some of these species survived until 30,000 to 10,000 years ago, overlapping with modern
humans (Figure 29.50).
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Figure 29.50 Neanderthal. The Homo neanderthalensis used tools and may have worn clothing.
There is considerable debate about the origins of anatomically modern humans or Homo sapiens sapiens. As
discussed earlier, H. erectus migrated out of Africa and into Asia and Europe in the first major wave of migration
about 1.5 million years ago. It is thought that modern humans arose in Africa from H. erectus and migrated out of
Africa about 100,000 years ago in a second major migration wave. Then, modern humans replaced H. erectus
species that had migrated into Asia and Europe in the first wave.
This evolutionary timeline is supported by molecular evidence. One approach to studying the origins of modern
humans is to examine mitochondrial DNA (mtDNA) from populations around the world. Because a fetus develops
from an egg containing its mother’s mitochondria (which have their own, non-nuclear DNA), mtDNA is passed
entirely through the maternal line. Mutations in mtDNA can now be used to estimate the timeline of genetic
divergence. The resulting evidence suggests that all modern humans have mtDNA inherited from a common
ancestor that lived in Africa about 160,000 years ago. Another approach to the molecular understanding of
human evolution is to examine the Y chromosome, which is passed from father to son. This evidence suggests
that all men today inherited a Y chromosome from a male that lived in Africa about 140,000 years ago.
The study of mitochondrial DNA led to the identification of another human species or subspecies, the
Denisovans. DNA from teeth and finger bones suggested two things. First, the mitochondrial DNA was different
from that of both modern humans and Neanderthals. Second, the genomic DNA suggested that the Denisovans
shared a common ancestor with the Neanderthals. Genes from both Neanderthals and Denisovans have been
identified in modern human populations, indicating that interbreeding among the three groups occurred over part
of their range.
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KEY TERMS
Acanthostega one of the earliest known tetrapods
Actinopterygii ray-finned fishes
allantois membrane of the egg that stores nitrogenous wastes produced by the embryo; also facilitates
respiration
amnion membrane of the egg that protects the embryo from mechanical shock and prevents dehydration
amniote animal that produces a terrestrially adapted egg protected by amniotic membranes
Amphibia frogs, salamanders, and caecilians
ampulla of Lorenzini sensory organ that allows sharks to detect electromagnetic fields produced by living
things
anapsid animal having no temporal fenestrae in the cranium
anthropoid monkeys, apes, and humans
Anura frogs
apocrine gland scent gland that secretes substances that are used for chemical communication
Apoda caecilians
Archaeopteryx transition species from dinosaur to bird from the Jurassic period
archosaur modern crocodilian or bird, or an extinct pterosaur or dinosaur
Australopithecus genus of hominins that evolved in eastern Africa approximately four million years ago
brachiation movement through trees branches via suspension from the arms
brumation period of much reduced metabolism and torpor that occurs in any ectotherm in cold weather
caecilian legless amphibian that belongs to the clade Apoda
Casineria one of the oldest known amniotes; had both amphibian and reptilian characteristics
Catarrhini clade of Old World monkeys
Cephalochordata chordate clade whose members possess a notochord, dorsal hollow nerve cord, pharyngeal
slits, and a post-anal tail in the adult stage
Chondrichthyes jawed fish with paired fins and a skeleton made of cartilage
Chordata phylum of animals distinguished by their possession of a notochord, a dorsal hollow nerve cord,
pharyngeal slits, and a post-anal tail at some point during their development
chorion membrane of the egg that surrounds the embryo and yolk sac
contour feather feather that creates an aerodynamic surface for efficient flight
Craniata clade composed of chordates that possess a cranium; includes Vertebrata together with hagfishes
cranium bony, cartilaginous, or fibrous structure surrounding the brain, jaw, and facial bones
Crocodilia crocodiles and alligators
cutaneous respiration gas exchange through the skin
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dentary single bone that comprises the lower jaw of mammals
diapsid animal having two temporal fenestrae in the cranium
diphyodont refers to the possession of two sets of teeth in a lifetime
dorsal hollow nerve cord hollow, tubular structure derived from ectoderm, which is located dorsal to the
notochord in chordates
down feather feather specialized for insulation
eccrine gland sweat gland
Enantiornithes dominant bird group during the Cretaceous period
eutherian mammal mammal that possesses a complex placenta, which connects a fetus to the mother;
sometimes called placental mammals
flight feather feather specialized for flight
frog tail-less amphibian that belongs to the clade Anura
furcula wishbone formed by the fusing of the clavicles
gnathostome jawed fish
Gorilla genus of gorillas
hagfish eel-like jawless fish that live on the ocean floor and are scavengers
heterodont tooth different types of teeth that are modified for different purposes
hominin species that are more closely related to humans than chimpanzees
hominoid pertaining to great apes and humans
Homo genus of humans
Homo sapiens sapiens anatomically modern humans
Hylobatidae family of gibbons
Hylonomus one of the earliest reptiles
lamprey jawless fish characterized by a toothed, funnel-like, sucking mouth
lancelet member of Cephalochordata; named for its blade-like shape
lateral line sense organ that runs the length of a fish’s body; used to detect vibration in the water
lepidosaur modern lizards, snakes, and tuataras
mammal one of the groups of endothermic vertebrates that possesses hair and mammary glands
mammary gland in female mammals, a gland that produces milk for newborns
marsupial one of the groups of mammals that includes the kangaroo, koala, bandicoot, Tasmanian devil, and
several other species; young develop within a pouch
monotreme egg-laying mammal
Myxini hagfishes
Neognathae birds other than the Paleognathae
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Neornithes modem birds
notochord flexible, rod-shaped support structure that is found in the embryonic stage of all chordates and in the
adult stage of some chordates
Ornithorhynchidae clade that includes the duck-billed platypus
Osteichthyes bony fish
ostracoderm one of the earliest jawless fish covered in bone
Paleognathae ratites; flightless birds, including ostriches and emus
Pan genus of chimpanzees and bonobos
Petromyzontidae clade of lampreys
pharyngeal slit opening in the pharynx
Platyrrhini clade of New World monkeys
Plesiadapis oldest known primate-like mammal
pneumatic bone air-filled bone
Pongo genus of orangutans
post-anal tail muscular, posterior elongation of the body extending beyond the anus in chordates
primary feather feather located at the tip of the wing that provides thrust
Primates order of lemurs, tarsiers, monkeys, apes, and humans
prognathic jaw long jaw
prosimian division of primates that includes bush babies and pottos of Africa, lemurs of Madagascar, and
lorises of Southeast Asia
salamander tailed amphibian that belongs to the clade Urodela
Sarcopterygii lobe-finned fish
sauropsid reptile or bird
sebaceous gland in mammals, a skin gland that produce a lipid mixture called sebum
secondary feather feather located at the base of the wing that provides lift
Sphenodontia clade of tuataras
Squamata clade of lizards and snakes
stereoscopic vision two overlapping fields of vision from the eyes that produces depth perception
swim bladder in fishes, a gas filled organ that helps to control the buoyancy of the fish
synapsid mammal having one temporal fenestra
Tachyglossidae clade that includes the echidna or spiny anteater
tadpole larval stage of a frog
temporal fenestra non-orbital opening in the skull that may allow muscles to expand and lengthen
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Testudines order of turtles
tetrapod phylogenetic reference to an organism with a four-footed evolutionary history; includes amphibians,
reptiles, birds, and mammals
theropod dinosaur group ancestral to birds
tunicate sessile chordate that is a member of Urochordata
Urochordata clade composed of tunicates
Urodela salamanders
vertebral column series of separate bones joined together as a backbone
Vertebrata members of the phylum Chordata that possess a backbone
CHAPTER SUMMARY
29.1 Chordates
The five characteristic features of chordates present during some time of their life cycles are a notochord, a
dorsal hollow tubular nerve cord, pharyngeal slits, endostyle/thyroid gland, and a post-anal tail. Chordata
contains two clades of invertebrates: Urochordata (tunicates) and Cephalochordata (lancelets), together with
the vertebrates in the Vertebrata/Craniata. Lancelets are suspension feeders that feed on phytoplankton and
other microorganisms. Most tunicates live on the ocean floor and are suspension feeders. Which of the two
invertebrate chordate clades is more closely related to the vertebrates continues to be debated. Vertebrata is
named for the vertebral column, which is a feature of almost all members of this clade. The name Craniata
(organisms with a cranium) is considered to be synonymous with Vertebrata.
29.2 Fishes
The earliest vertebrates that diverged from the invertebrate chordates were the agnathan jawless fishes, whose
extant members include the hagfishes and lampreys. Hagfishes are eel-like scavengers that feed on dead
invertebrates and other fishes. Lampreys are characterized by a toothed, funnel-like sucking mouth, and most
species are parasitic or predaceous on other fishes. Fishes with jaws (gnathostomes) evolved later. Jaws
allowed early gnathostomes to exploit new food sources.
Gnathostomes include the cartilaginous fishes and the bony fishes, as well as all other tetrapods (amphibians,
reptiles, mammals). Cartilaginous fishes include sharks, rays, skates, and ghost sharks. Most cartilaginous
fishes live in marine habitats, with a few species living in fresh water for part or all of their lives. The vast
majority of present-day fishes belong to the clade Osteichthyes, which consists of approximately 30,000
species. Bony fishes (Osteichthyes) can be divided into two clades: Actinopterygii (ray-finned fishes, virtually all
extant species) and Sarcopterygii (lobe-finned fishes, comprising fewer than 10 extant species, but form the
sister group of the tetrapods).
29.3 Amphibians
As tetrapods, most amphibians are characterized by four well-developed limbs, although some species of
salamanders and all caecilians are limbless. The most important characteristic of extant amphibians is a moist,
permeable skin used for cutaneous respiration, although lungs are found in the adults of many species.
All amphibians are carnivores and possess many small teeth. The fossil record provides evidence of amphibian
species, now extinct, that arose over 400 million years ago as the first tetrapods. Living Amphibia can be
divided into three classes: salamanders (Urodela), frogs (Anura), and caecilians (Apoda). In the majority of
amphibians, development occurs in two distinct stages: a gilled aquatic larval stage that metamorphoses into
an adult stage, acquiring lungs and legs, and losing the tail in Anurans. A few species in all three clades bypass
a free-living larval stage. Various levels of parental care are seen in the amphibians.
29.4 Reptiles
The amniotes are distinguished from amphibians by the presence of a terrestrially adapted egg protected by
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four extra-embryonic membranes. The amniotes include reptiles, birds, and mammals. The early amniotes
diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids (mammals)
and sauropsids. Sauropsids can be further divided into anapsids and diapsids (crocodiles, dinosaurs, birds, and
modern reptiles).
Reptiles are tetrapods that ancestrally had four limbs; however, a number of extant species have secondarily
lost them or greatly reduced them over evolutionary time. For example, limbless reptiles (e.g., snakes) are
classified as tetrapods, because they descended from ancestors with four limbs. One of the key adaptations
that permitted reptiles to live on land was the development of scaly skin containing the protein keratin, which
prevented water loss from the skin. Reptilia includes four living clades of nonavian organisms: Crocodilia
(crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles).
Currently, this classification is paraphyletic, leaving out the birds, which are now classified as avian reptiles in
the class Reptilia.
29.5 Birds
Birds are the most speciose group of land vertebrates and display a number of adaptations related to their
ability to fly, which were first present in their therapod (maniraptoran) ancestors. Birds are endothermic (and
homeothermic), meaning they have a very high metabolism that produces a considerable amount of heat, as
well as structures such as feathers that allow them to retain their own body heat. These adaptations are used
to regulate their internal temperature, making it largely independent of ambient thermal conditions.
Birds have feathers, which allow for insulation and flight, as well as for mating and warning signals. Flight
feathers have a broad and continuously curved vane that produces lift. Some birds have pneumatic bones
containing air spaces that are sometimes connected to air sacs in the body cavity. Airflow through bird lungs
travels in one direction, creating a counter-current gas exchange with the blood.
Birds are highly modified diapsids and belong to a group called the archosaurs. Within the archosaurs, birds
are most likely evolved from theropod (maniraptoran) dinosaurs. One of the oldest known fossils (and best
known) of a “dinosaur-bird” is that of Archaeopteryx, which is dated from the Jurassic period. Modern birds are
now classified into three groups: Paleognathae, Galloanserae, and Neoaves.
29.6 Mammals
Mammals are vertebrates that possess hair and mammary glands. The mammalian integument includes
various secretory glands, including sebaceous glands, eccrine glands, apocrine glands, and mammary glands.
Mammals are synapsids, meaning that they have a single opening in the skull behind the eye. Mammals
probably evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the
early Jurassic period. A key characteristic of synapsids is endothermy, and most mammals are homeothermic.
There are three groups of mammals living today: monotremes, marsupials, and eutherians. Monotremes are
unique among mammals as they lay eggs, rather than giving birth to young. Marsupials give birth to very
immature young, which typically complete their development in a pouch. Eutherian mammals are sometimes
called placental mammals, because all species possess a complex placenta that connects a fetus to the
mother, allowing for gas, fluid, and nutrient exchange. All mammals nourish their young with milk, which is
derived from modified sweat or sebaceous glands.
29.7 The Evolution of Primates
All primate species possess adaptations for climbing trees and probably descended from arboreal ancestors,
although not all living species are arboreal. Other characteristics of primates are brains that are larger, relative
to body size, than those of other mammals, claws that have been modified into flattened nails, typically only
one young per pregnancy, stereoscopic vision, and a trend toward holding the body upright. Primates are
divided into two groups: strepsirrhines, which include most prosimians, and haplorhines, which include simians.
Monkeys evolved from prosimians during the Oligocene epoch. The simian line includes both platyrrhine and
catarrhine branches. Apes evolved from catarrhines in Africa during the Miocene epoch. Apes are divided into
the lesser apes and the greater apes. Hominins include those groups that gave rise to our own species, such
as Australopithecus and H. erectus, and those groups that can be considered “cousins” of humans, such as
Neanderthals and Denisovans. Fossil evidence shows that hominins at the time of Australopithecus were
walking upright, the first evidence of bipedal hominins. A number of species, sometimes called archaic H.
sapiens, evolved from H. erectus approximately 500,000 years ago. There is considerable debate about the
origins of anatomically modern humans or H. sapiens sapiens, and the discussion will continue, as new
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Chapter 29 | Vertebrates
evidence from fossil finds and genetic analysis emerges.
VISUAL CONNECTION QUESTIONS
1. Figure 29.3 Which of the following statements
about common features of chordates is true?
a. The dorsal hollow nerve cord is part of the
chordate central nervous system.
b. in vertebrate fishes, the pharyngeal slits
become the gills.
c. Humans are not chordates because humans
do not have a tail.
d. Vertebrates do not have a notochord at any
point in their development; instead, they
have a vertebral column.
2. Figure 29.22 Which of the following statements
REVIEW QUESTIONS
4. Which of the following is not contained in phylum
Chordata?
a. Cephalochordata
b. Echinodermata
c. Urochordata
d. Vertebrata
5. Which group of invertebrates is most closely
related to vertebrates?
a. cephalochordates
b. echinoderms
c. arthropods
d. urochordates
6. Hagfish, lampreys, sharks, and tuna are all
chordates that can also be classified into which
group?
a. Craniates
b. Vertebrates
c. Cartilaginous fish
d. Cephalocordata
7. Members of Chondrichthyes differ from members
of Osteichthyes by having (a)_.
a. jaw
b. bony skeleton
c. cartilaginous skeleton
d. two sets of paired fins
8. Members of Chondrichthyes are thought to be
descended from fishes that had_.
a. a cartilaginous skeleton
b. a bony skeleton
c. mucus glands
d. slime glands
9. A marine biologist catches a species of fish she
has never seen before. Upon examination, she
determines that the species has a predominantly
cartilaginous skeleton and a swim bladder. If its
pectoral fins are not fused with its head, to which
about the parts of an amniotic egg are false?
a. The allantois stores nitrogenous waste and
facilitates respiration.
b. The chorion facilitates gas exchange.
c. The yolk provides food for the growing
embryo.
d. The amniotic cavity is filled with albumen.
3. Figure 29.24 Members of the order Testudines
have an anapsid-like skull with one opening.
However, molecular studies indicate that turtles
descended from a diapsid ancestor. Why might this
be the case?
category of fish does the specimen belong?
a. Rays
b. Osteichthyes
c. Sharks
d. Hagfish
10. Which of the following is not true of
Acanthostega ?
a. It was aquatic.
b. It had gills.
c. It had four limbs.
d. It laid shelled eggs.
11. Frogs belong to which order?
a. Anura
b. Urodela
c. Caudata
d. Apoda
12. During the Mesozoic period, diapsids diverged
into_.
a. pterosaurs and dinosaurs
b. mammals and reptiles
c. lepidosaurs and archosaurs
d. Testudines and Sphenodontia
13. Squamata includes_.
a. crocodiles and alligators
b. turtles
c. tuataras
d. lizards and snakes
14. Which of the following reptile groups gave rise to
modern birds?
a. Lepidosaurs
b. Pterosaurs
c. Anapsids
d. Archosaurs
15. A bird or feathered dinosaur is_.
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a. Neornithes
b. Archaeopteryx
c. Enantiornithes
d. Paleognathae
16. Which of the following feather types helps to
reduce drag produced by wind resistance during
flight?
a. Flight feathers
b. Primary feathers
c. Secondary feathers
d. Contour feathers
17. Eccrine glands produce_.
a. sweat
b. lipids
c. scents
d. milk
18. Monotremes include:
a. kangaroos.
b. koalas.
c. bandicoots.
d. platypuses.
19. The evolution of which of the following features of
mammals is hardest to trace through the fossil
record?
CRITICAL THINKING QUESTIONS
23. What are the characteristic features of the
chord ates?
24. What is the structural advantage of the notochord
in the human embryo? Be sure to compare the
notochord with the corresponding structure in adults.
25. What can be inferred about the evolution of the
cranium and vertebral column from examining
hagfishes and lampreys?
26. Why did gnathostomes replace most agnathans?
27. Explain why frogs are restricted to a moist
environment.
28. Describe the differences between the larval and
adult stages of frogs.
29. Describe how metamorphosis changes the
structures involved in gas exchange over the life
cycle of animals in the clade Anura, and what
evolutionary advantage this change provides.
30. Describe the functions of the three extra-
embryonic membranes present in amniotic eggs.
31. What characteristics differentiate lizards and
snakes?
a. Jaw structure
b. Mammary glands
c. Middle ear structure
d. Development of hair
20. Which of the following is not an anthropoid?
a. Lemurs
b. Monkeys
c. Apes
d. Humans
21. Which of the following is part of a clade believed
to have died out, leaving no descendants?
a. Paranthropus robustus
b. Australopithecus africanus
c. Homo erectus
d. Homo sapiens sapiens
22. Which of the following human traits is not a
shared characteristic of primates?
a. Hip structure supporting bipedalism
b. Detection and processing of three-color
vision
c. Nails at the end of each digit
d. Enlarged brain area associated with vision,
and reduced area associated with smell
32. Based on how reptiles thermoregulate, which
climates would you predict to have the highest reptile
population density, and why?
33. Explain why birds are thought to have evolved
from theropod dinosaurs.
34. Describe three skeletal adaptations that allow for
flight in birds.
35. How would the chest structure differ between
ostriches, penguins, and terns?
36. Describe three unique features of the mammalian
skeletal system.
37. Describe three characteristics of the mammalian
brain that differ from other vertebrates.
38. How did the evolution of jaw musculature allow
mammals to spread?
39. How did archaic Homo sapiens differ from
anatomically modern humans?
40. Why is it so difficult to determine the sequence of
hominin ancestors that have led to modern Homo
sapiens ?
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Chapter 30 | Plant Form and Physiology
903
30 | PLANT FORM AND
PHYSIOLOGY
Figure 30.1 A locust leaf consists of leaflets arrayed along a central midrib. Each leaflet is a complex photosynthetic
machine, exquisitely adapted to capture sunlight and carbon dioxide. An intricate vascular system supplies the leaf
with water and minerals, and exports the products of photosynthesis, (credit: modification of work by Todd Petit)
Chapter Outline
30.1: The Plant Body
30.2: Stems
30.3: Roots
30.4: Leaves
30.5: Transport of Water and Solutes in Plants
30.6: Plant Sensory Systems and Responses
Introduction
Plants are as essential to human existence as land, water, and air. Without plants, our day-to-day lives would be
impossible because without oxygen from photosynthesis, aerobic life cannot be sustained. From providing food
and shelter to serving as a source of medicines, oils, perfumes, and industrial products, plants provide humans
with numerous valuable resources.
When you think of plants, most of the organisms that come to mind are vascular plants. These plants have
tissues that conduct food and water, and they have seeds. Seed plants are divided into gymnosperms and
angiosperms. Gymnosperms include the needle-leaved conifers—spruce, fir, and pine—as well as less familiar
plants, such as ginkgos and cycads. Their seeds are not enclosed by a fleshy fruit. Angiosperms, also called
flowering plants, constitute the majority of seed plants. They include broadleaved trees (such as maple, oak,
and elm), vegetables (such as potatoes, lettuce, and carrots), grasses, and plants known for the beauty of their
flowers (roses, irises, and daffodils, for example).
While individual plant species are unique, all share a common structure: a plant body consisting of stems,
roots, and leaves. They all transport water, minerals, and sugars produced through photosynthesis through the
plant body in a similar manner. All plant species also respond to environmental factors, such as light, gravity,
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competition, temperature, and predation.
30.1 1 The Plant Body
By the end of this section, you will be able to do the following:
• Describe the shoot organ system and the root organ system
• Distinguish between meristematic tissue and permanent tissue
• Identify and describe the three regions where plant growth occurs
• Summarize the roles of dermal tissue, vascular tissue, and ground tissue
• Compare simple plant tissue with complex plant tissue
Like animals, plants contain cells with organelles in which specific metabolic activities take place. Unlike animals,
however, plants use energy from sunlight to form sugars during photosynthesis. In addition, plant cells have cell
walls, plastids, and a large central vacuole: structures that are not found in animal cells. Each of these cellular
structures plays a specific role in plant structure and function.
LINK TQ LEARNING
Watch Botany Without Borders (http://openstaxcollege. 0 rg/l/botany_wo_bord) , a video produced by the
Botanical Society of America about the importance of plants.
Plant Organ Systems
In plants, just as in animals, similar cells working together form a tissue. When different types of tissues
work together to perform a unique function, they form an organ; organs working together form organ systems.
Vascular plants have two distinct organ systems: a shoot system, and a root system. The shoot system consists
of two portions: the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems, and
the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above
ground, where it absorbs the light needed for photosynthesis. The root system, which supports the plants and
absorbs water and minerals, is usually underground. Figure 30.2 shows the organ systems of a typical plant.
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Chapter 30 | Plant Form and Physiology
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Figure 30.2 The shoot system of a plant consists of leaves, stems, flowers, and fruits. The root system anchors the
plant while absorbing water and minerals from the soil.
Plant Tissues
Plants are multicellular eukaryotes with tissue systems made of various cell types that carry out specific
functions. Plant tissue systems fall into one of two general types: meristematic tissue, and permanent (or
non-meristematic) tissue. Cells of the meristematic tissue are found in meristems, which are plant regions
of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely
differentiated, and they continue to divide and contribute to the growth of the plant. In contrast, permanent
tissue consists of plant cells that are no longer actively dividing.
Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain
meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral
meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in
monocots, at the bases of leaf blades and at nodes (the areas where leaves attach to a stem). This tissue
enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves
to elongate even after repeated mowing.
Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells
take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal,
vascular, and ground tissue. Dermal tissue covers and protects the plant, and vascular tissue transports water,
minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides
a supporting matrix for the vascular tissue, and helps to store water and sugars.
Secondary tissues are either simple (composed of similar cell types) or complex (composed of different cell
types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls
gas exchange. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting
tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the
plant, and includes three different cell types: vessel elements and tracheids (both of which conduct water), and
xylem parenchyma. Phloem tissue, which transports organic compounds from the site of photosynthesis to other
parts of the plant, consists of four different cell types: sieve cells (which conduct photosynthates), companion
cells, phloem parenchyma, and phloem fibers. Unlike xylem conducting cells, phloem conducting cells are alive
at maturity. The xylem and phloem always lie adjacent to each other (Figure 30.3). In stems, the xylem and
the phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular
cylinder.
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Chapter 30 | Plant Form and Physiology
Phloem
Epidermis
(Dermal tissue)
Vascular
bundle
Figure 30.3 This light micrograph shows a cross section of a squash (Curcurbita maxima) stem. Each teardrop-shaped
vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. Xylem
cells, which transport water and nutrients from the roots to the rest of the plant, are dead at functional maturity. Phloem
cells, which transport sugars and other organic compounds from photosynthetic tissue to the rest of the plant, are
living. The vascular bundles are encased in ground tissue and surrounded by dermal tissue, (credit: modification of
work by "(biophotos)"/Flickr; scale-bar data from Matt Russell)
30.2 | Stems
By the end of this section, you will be able to do the following:
• Describe the main function and basic structure of stems
• Compare and contrast the roles of dermal tissue, vascular tissue, and ground tissue
• Distinguish between primary growth and secondary growth in stems
• Summarize the origin of annual rings
• List and describe examples of modified stems
Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds
of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although
the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or
woody in nature. Their main function is to provide support to the plant, holding leaves, flowers and buds; in some
cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be
highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to
transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of
photosynthesis, namely sugars, from the leaves to the rest of the plant.
Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes
(Figure 30.4). Nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between
two nodes is called an internode. The stalk that extends from the stem to the base of the leaf is the petiole. An
axillary bud is usually found in the axil—the area between the base of a leaf and the stem—where it can give
rise to a branch or a flower. The apex (tip) of the shoot contains the apical meristem within the apical bud.
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Chapter 30 | Plant Form and Physiology
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Figure 30.4 Leaves are attached to the plant stem at areas called nodes. An internode is the stem region between two
nodes. The petiole is the stalk connecting the leaf to the stem. The leaves just above the nodes arose from axillary
buds.
Stem Anatomy
The stem and other plant organs arise from the ground tissue, and are primarily made up of simple tissues
formed from three types of cells: parenchyma, collenchyma, and sclerenchyma cells.
Parenchyma cells are the most common plant cells (Figure 30.5). They are found in the stem, the root, the
inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as
photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.
Figure 30.5 The stem of common St John's Wort (Hypericum perforatum ) is shown in cross section in this light
micrograph. The central pith (greenish-blue, in the center) and peripheral cortex (narrow zone 3-5 cells thick just inside
the epidermis) are composed of parenchyma cells. Vascular tissue composed of xylem (red) and phloem tissue (green,
between the xylem and cortex) surrounds the pith, (credit: Rolf-Dieter Mueller)
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Chapter 30 | Plant Form and Physiology
Collenchyma cells are elongated cells with unevenly thickened walls (Figure 30.6). They provide structural
support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the
epidermis. The “strings" of a celery stalk are an example of collenchyma cells.
Figure 30.6 Collenchyma cell walls are uneven in thickness, as seen in this light micrograph. They provide support to
plant structures, (credit: modification of work by Carl Szczerski; scale-bar data from Matt Russell)
Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at
maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls
that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long,
slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma
fibers to make linen and rope (Figure 30.7).
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Chapter 30 | Plant Form and Physiology
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Phloem Xylem Pith
Epidermis Cortex Sclerenchyma
(a)
visual
CONNECTION
(C)
Figure 30.7 The central pith and outer cortex of the (a) flax stem are made up of parenchyma cells. Inside the
cortex is a layer of sclerenchyma cells, which make up the fibers in flax rope and clothing. Humans have grown
and harvested flax for thousands of years. In (b) this drawing, fourteenth-century women prepare linen. The (c)
flax plant is grown and harvested for its fibers, which are used to weave linen, and for its seeds, which are the
source of linseed oil. (credit a: modification of work by Emmanuel Boutet based on original work by Ryan R.
MacKenzie; credit c: modification of work by Brian Dearth; scale-bar data from Matt Russell)
Which layers of the stem are made of parenchyma cells?
a. cortex and pith
b. phloem
c. sclerenchyma
d. xylem
Like the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is
distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival.
Dermal Tissue
The dermal tissue of the stem consists primarily of epidermis, a single layer of cells covering and protecting the
underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark,
which further protects the plant from damage. Epidermal cells are the most numerous and least differentiated
of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which
the exchange of gases takes place (Figure 30.8). Two cells, known as guard cells, surround each leaf stoma,
controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen
and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration
(the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the
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Chapter 30 | Plant Form and Physiology
leaves against predation by herbivores.
Guard cells
Open stoma
(b)
Closed stoma
Guard cells
Stomatal pore
Epidermal cells
(a) (c)
Figure 30.8 Openings called stomata (singular: stoma) allow a plant to take up carbon dioxide and release oxygen and
water vapor. The (a) colorized scanning-electron micrograph shows a closed stoma of a dicot. Each stoma is flanked
by two guard cells that regulate its (b) opening and closing. The (c) guard cells sit within the layer of epidermal cells,
(credit a: modification of work by Louisa Howard, Rippel Electron Microscope Facility, Dartmouth College; credit b:
modification of work by June Kwak, University of Maryland; scale-bar data from Matt Russell)
Vascular Tissue
The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called
vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the
vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the
individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular
bundles are randomly scattered throughout the ground tissue (Figure 30.9).
Dicot stem Monocot stem
Figure 30.9 In (a) dicot stems, vascular bundles are arranged around the periphery of the ground tissue. The xylem
tissue is located toward the interior of the vascular bundle, and phloem is located toward the exterior. Sclerenchyma
fibers cap the vascular bundles. In (b) monocot stems, vascular bundles composed of xylem and phloem tissues are
scattered throughout the ground tissue.
Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types
conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are
lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where
secondary walls are absent. Vessel elements are xylem cells with thinner walls; they are shorter than tracheids.
Each vessel element is connected to the next by means of a perforation plate at the end walls of the element.
Water moves through the perforation plates to travel up the plant.
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Chapter 30 | Plant Form and Physiology
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Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers.
A series of sieve-tube cells (also called sieve-tube elements) are arranged end to end to make up a long
sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one
sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between
two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have
disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support.
The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some
cellular organelles.
Ground Tissue
Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma
cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is
known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.
Growth in Stems
Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also
increase in thickness during their life span. The increase in length of the shoot and the root is referred to
as primary growth, and is the result of cell division in the shoot apical meristem. Secondary growth is
characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral
meristem. Figure 30.10 shows the areas of primary and secondary growth in a plant. Herbaceous plants mostly
undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood”
is noticeable in woody plants; it occurs in some dicots, but occurs very rarely in monocots.
Primary growth Secondary growth
Phloem
_ . . Vascular
Sclerenchyma cambjum
Xylem
Primary
xylem
Epidermis
Cork
Secondary cambium
xylem
Secondary
phloem
Primary Cork
phloem
Pith
Figure 30.10 In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase
in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree
extends from the vascular cambium to the epidermis.
Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called
indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases
when a plant part reaches a particular size.
Primary Growth
Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing
cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary
growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots)
or sunlight (shoots).
The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the
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growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong
apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then
the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by
cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.
LINK TQ LEARNING
Watch this BBC Nature video (http:// 0 penstaxc 0 llege. 0 rg/l/m 0 ti 0 n plants) showing how time-lapse
photography captures plant growth at high speed.
Secondary Growth
The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems,
which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants,
the cork cambium (see Figure 30.10). The vascular cambium is located just outside the primary xylem and to the
interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids
and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside.
The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and
secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost
layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.
In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy
substance known as suberin that can repel water. The bark protects the plant against physical damage and helps
reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward
from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The
periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known
as lenticels, which allow the interior cells to exchange gases with the outside atmosphere (Figure 30.11). This
supplies oxygen to the living and metabolically active cells of the cortex, xylem, and phloem.
Figure 30.11 Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding
atmosphere, (credit: Roger Griffith)
Annual Rings
The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells
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Chapter 30 | Plant Form and Physiology
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of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened.
This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened
cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and
late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in
the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the
cross section of the stem (Figure 30.12). An examination of the number of annual rings and their nature (such
as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during
each season.
Figure 30.12 The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring
for each year of growth. Seasonal changes in weather patterns can also affect the growth rate—note how the rings
vary in thickness, (credit: Adrian Pingstone)
Stem Modifications
Some plant species have modified stems that are especially suited to a particular habitat and environment
(Figure 30.13). A rhizome is a modified stem that grows horizontally underground and has nodes and
internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns.
Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain
stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the
ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon
that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an
example. Tubers are modified stems that may store starch, as seen in the potato (Solarium sp.). Tubers arise
as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on
potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the
appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in
the iris.
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Figure 30.13 Stem modifications enable plants to thrive in a variety of environments. Shown are (a) ginger ( Zingiber
officinale) rhizomes, (b) a carrion flower ( Amorphophallus titanum) corm, (c) Rhodes grass ( Chloris gayana) stolons,
(d) strawberry ( Fragaria ananassa) runners, (e) potato ( Solanum tuberosum) tubers, and (f) red onion ( Allium ) bulbs,
(credit a: modification of work by Maja Dumat; credit c: modification of work by Harry Rose; credit d: modification of
work by Rebecca Siegel; credit e: modification of work by Scott Bauer, USDA ARS; credit f: modification of work by
Stephen Ausmus, USDA ARS)
LINK
T &
LEARNING
Watch botanist Wendy Hodgson, of Desert Botanical Garden in Phoenix, Arizona, explain how agave plants
were cultivated for food hundreds of years ago in the Arizona desert in this video:
(http:// 0 penstaxc 0 llege. 0 rg/l/ancient_cr 0 p) Finding the Roots of an Ancient Crop.
Some aerial modifications of stems are tendrils and thorns (Figure 30.14). Tendrils are slender, twining strands
that enable a plant (like a vine or pumpkin) to seek support by climbing on other surfaces. Thorns are modified
branches appearing as sharp outgrowths that protect the plant; common examples include roses, Osage orange,
and devil’s walking stick.
(a) (b)
Figure 30.14 Found in southeastern United States, (a) buckwheat vine ( Brunnichia ovata) is a weedy plant that climbs
with the aid of tendrils. This one is shown climbing up a wooden stake, (b) Thorns are modified branches, (credit a:
modification of work by Christopher Meloche, USDA ARS; credit b: modification of work by “macrophile’VFlickr)
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915
30.3 | Roots
By the end of this section, you will be able to do the following:
• Identify the two types of root systems
• Describe the three zones of the root tip and summarize the role of each zone in root growth
• Describe the structure of the root
• List and describe examples of modified roots
The roots of seed plants have three major functions: anchoring the plant to the soil, absorbing water and minerals
and transporting them upwards, and storing the products of photosynthesis. Some roots are modified to absorb
moisture and exchange gases. Most roots are underground. Some plants, however, also have adventitious
roots, which emerge above the ground from the shoot.
Types of Root Systems
Root systems are mainly of two types (Figure 30.15). Dicots have a tap root system, while monocots have a
fibrous root system. A tap root system has a main root that grows down vertically, and from which many smaller
lateral roots arise. Dandelions are a good example; their tap roots usually break off when trying to pull these
weeds, and they can regrow another shoot from the remaining root. A tap root system penetrates deep into the
soil. In contrast, a fibrous root system is located closer to the soil surface, and forms a dense network of roots
that also helps prevent soil erosion (lawn grasses are a good example, as are wheat, rice, and corn). Some
plants have a combination of tap roots and fibrous roots. Plants that grow in dry areas often have deep root
systems, whereas plants growing in areas with abundant water are likely to have shallower root systems.
(a) Taproot system (b) Fibrous root system
Figure 30.15 (a) Tap root systems have a main root that grows down, while (b) fibrous root systems consist of many
small roots, (credit b: modification of work by “Austen Squarepants’VFlickr)
Root Growth and Anatomy
Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the
embryo forms the root system. The tip of the root is protected by the root cap, a structure exclusive to roots
and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as
the root pushes through soil. The root tip can be divided into three zones: a zone of cell division, a zone of
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elongation, and a zone of maturation and differentiation (Figure 30.16). The zone of cell division is closest to
the root tip; it is made up of the actively dividing cells of the root meristem. The zone of elongation is where the
newly formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone
of cell maturation where the root cells begin to differentiate into special cell types. All three zones are in the first
centimeter or so of the root tip.
Area of
maturation
Area of
elongation
Area of
cell division
Apical
meristem
Root cap
Vascular
cylinder
Root hair
Figure 30.16 A longitudinal view of the root reveals the zones of cell division, elongation, and maturation. Cell division
occurs in the apical meristem.
The root has an outer layer of cells called the epidermis, which surrounds areas of ground tissue and vascular
tissue. The epidermis provides protection and helps in absorption. Root hairs, which are extensions of root
epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and
minerals.
Inside the root, the ground tissue forms two regions: the cortex and the pith (Figure 30.17). Compared to stems,
roots have lots of cortex and little pith. Both regions include cells that store photosynthetic products. The cortex
is between the epidermis and the vascular tissue, whereas the pith lies between the vascular tissue and the
center of the root.
Xylem
Cortex Pericycle Endodermis Root hairs
(parenchyma cells)
Exodermis
Epidermis
Phloem
Figure 30.17 Staining reveals different cell types in this light micrograph of a wheat ( Triticum ) root cross section.
Sclerenchyma cells of the exodermis and xylem cells stain red, and phloem cells stain blue. Other cell types stain
black. The stele, or vascular tissue, is the area inside endodermis (indicated by a green ring). Root hairs are visible
outside the epidermis, (credit: scale-bar data from Matt Russell)
The vascular tissue in the root is arranged in the inner portion of the root, which is called the stele (Figure
30.18). A layer of cells known as the endodermis separates the stele from the ground tissue in the outer portion
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Chapter 30 | Plant Form and Physiology
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of the root. The endodermis is exclusive to roots, and serves as a checkpoint for materials entering the root’s
vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy
region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal
cells instead of slipping between the cells. This ensures that only materials required by the root pass through the
endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root’s
vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xylem and phloem
of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in
a ring around the pith.
Dicot root Monocot root
Figure 30.18 In (left) typical dicots, the vascular tissue forms an X shape in the center of the root. In (right) typical
monocots, the phloem cells and the larger xylem cells form a characteristic ring around the central pith.
Root Modifications
Root structures may be modified for specific purposes. For example, some roots are bulbous and store starch.
Aerial roots and prop roots are two forms of aboveground roots that provide additional support to anchor the
plant. Tap roots, such as carrots, turnips, and beets, are examples of roots that are modified for food storage
(Figure 30.19).
Figure 30.19 Many vegetables are modified roots.
Epiphytic roots enable a plant to grow on another plant. For example, the epiphytic roots of orchids develop
a spongy tissue to absorb moisture. The banyan tree (Ficus sp.) begins as an epiphyte, germinating in the
branches of a host tree; aerial roots develop from the branches and eventually reach the ground, providing
additional support (Figure 30.20). In screwpine (Pandanus sp.), a palm-like tree that grows in sandy tropical
soils, aboveground prop roots develop from the nodes to provide additional support.
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Figure 30.20 The (a) banyan tree, also known as the strangler fig, begins life as an epiphyte in a host tree.
Aerial roots extend to the ground and support the growing plant, which eventually strangles the host tree. The (b)
screwpine develops aboveground roots that help support the plant in sandy soils, (credit a: modification of work by
"psyberartist'VFlickr; credit b: modification of work by David Eikhoff)
30.4 | Leaves
By the end of this section, you will be able to do the following:
• Identify the parts of a typical leaf
• Describe the internal structure and function of a leaf
• Compare and contrast simple leaves and compound leaves
• List and describe examples of modified leaves
Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are
usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different
colors, caused by other plant pigments that mask the green chlorophyll.
The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species
maximize its chances of survival in a particular habitat. Usually, the leaves of plants growing in tropical
rainforests have larger surface areas than those of plants growing in deserts or very cold conditions, which are
likely to have a smaller surface area to minimize water loss.
Structure of a Typical Leaf
Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Some leaves are
attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant
stem are called sessile leaves. Small green appendages usually found at the base of the petiole are known as
stipules. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce
veins of vascular tissue. The edge of the leaf is called the margin. Figure 30.21 shows the structure of a typical
eudicot leaf.
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Chapter 30 | Plant Form and Physiology
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Figure 30.21 Deceptively simple in appearance, a leaf is a highly efficient structure.
Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation
pattern. Monocots and dicots differ in their patterns of venation (Figure 30.22). Monocots have parallel venation;
the veins run in straight lines across the length of the leaf without converging at a point. In dicots, however, the
veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. One extant plant,
the Ginkgo biloba, has dichotomous venation where the veins fork.
Figure 30.22 (a) Tulip ( Tulipa ), a monocot, has leaves with parallel venation. The netlike venation in this (b) linden
('Tilia cordata) leaf distinguishes it as a dicot. The (c) Ginkgo biloba tree has dichotomous venation, (credit a photo:
modification of work by “Drewboy64”/Wikimedia Commons; credit b photo: modification of work by Roger Griffith;
credit c photo: modification of work by "geishaboy500"/Flickr; credit abc illustrations: modification of work by Agnieszka
Kwiecien)
Leaf Arrangement
The arrangement of leaves on a stem is known as phyllotaxy. The number and placement of a plant’s leaves
will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are
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classified as either alternate, spiral, or opposite. Plants that have only one leaf per node have leaves that are
said to be either alternate—meaning the leaves alternate on each side of the stem in a flat plane—or spiral,
meaning the leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise
at the same point, with the leaves connecting opposite each other along the branch. If there are three or more
leaves connected at a node, the leaf arrangement is classified as whorled.
Leaf Form
Leaves may be simple or compound (Figure 30.23). in a simple leaf, the blade is either completely
undivided—as in the banana leaf—or it has lobes, but the separation does not reach the midrib, as in the maple
leaf. In a compound leaf, the leaf blade is completely divided, forming leaflets, as in the locust tree. Each leaflet
may have its own stalk, but is attached to the rachis. A palmately compound leaf resembles the palm of a
hand, with leaflets radiating outwards from one point. Examples include the leaves of poison ivy, the buckeye
tree, or the familiar houseplant Schefflera sp. (common name “umbrella plant”). Pinnately compound leaves
take their name from their feather-like appearance; the leaflets are arranged along the midrib, as in rose leaves
(Rosa sp.), or the leaves of hickory, pecan, ash, or walnut trees.
Figure 30.23 Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant
(Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be
palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut (Aesculus hippocastanum ),
the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from the midrib, as on a scrub
hickory (Carya floridana). The (d) honey locust has double compound leaves, in which leaflets branch from the veins,
(credit a: modification of work by "BazzaDaRambler'VFlickr; credit b: modification of work by Roberto Verzo; credit c:
modification of work by Eric Dion; credit d: modification of work by Valerie Lykes)
Leaf Structure and Function
The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper
and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower
side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata
(Figure 30.24): openings through which the exchange of gases takes place. Two guard cells surround each
stoma, regulating its opening and closing.
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(a) (b) (c)
Figure 30.24 Visualized at 500x with a scanning electron microscope, several stomata are clearly visible on (a) the
surface of this sumac ( Rhus glabra) leaf. At 5,000x magnification, the guard cells of (b) a single stoma from lyre-leaved
sand cress ( Arabidopsis lyrata) have the appearance of lips that surround the opening. In this (c) light micrograph
cross-section of an A. lyrata leaf, the guard cell pair is visible along with the large, sub-stomatal air space in the leaf,
(credit: modification of work by Robert R. Wise; part c scale-bar data from Matt Russell)
The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions,
the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy
layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from
the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to deter
herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce
the rate of transpiration by blocking air flow across the leaf surface (Figure 30.25).
(a) (b) (c)
Figure 30.25 Trichomes give leaves a fuzzy appearance as in this (a) sundew ( Drosera sp.). Leaf trichomes include (b)
branched trichomes on the leaf of Arabidopsis lyrata and (c) multibranched trichomes on a mature Quercus marllandlca
leaf, (credit a: John Freeland; credit b, c: modification of work by Robert R. Wise; scale-bar data from Matt Russell)
Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or “middle leaf.” The mesophyll
of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy
parenchyma (Figure 30.26). The palisade parenchyma (also called the palisade mesophyll) has column-shaped,
tightly packed cells, and may be present in one, two, or three layers. Below the palisade parenchyma are loosely
arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll).
The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the
outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma
help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal
cells to contain chloroplasts.
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Cuticle
Palisade parenchyma
Spongy parenchyma
Cuticle
Upper
epidermis
Mesophyll
Lower
epidermis
Stoma
cells
(a)
(b)
Figure 30.26 In the (a) leaf drawing, the central mesophyll is sandwiched between an upper and lower epidermis. The
mesophyll has two layers: an upper palisade layer comprised of tightly packed, columnar cells, and a lower spongy
layer, comprised of loosely packed, irregularly shaped cells. Stomata on the leaf underside allow gas exchange. A
waxy cuticle covers all aerial surfaces of land plants to minimize water loss. These leaf layers are clearly visible in
the (b) scanning electron micrograph. The numerous small bumps in the palisade parenchyma cells are chloroplasts.
Chloroplasts are also present in the spongy parenchyma, but are not as obvious. The bumps protruding from the lower
surface of the leave are glandular trichomes, which differ in structure from the stalked trichomes in Figure 30.25. (credit
b: modification of work by Robert R. Wise)
Like the stem, the leaf contains vascular bundles composed of xylem and phloem (Figure 30.27). The xylem
consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the
photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how
large or small, always contains both xylem and phloem tissues.
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Figure 30.27 This scanning electron micrograph shows xylem and phloem in the leaf vascular bundle from the lyre¬
leaved sand cress (Arabidopsis lyrata). (credit: modification of work by Robert R. Wise; scale-bar data from Matt
Russell)
Leaf Adaptations
Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced
in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface
area: two attributes that aid in reducing water loss. In hot climates, plants such as cacti have leaves that are
reduced to spines, which in combination with their succulent stems, help to conserve water. Many aquatic plants
have leaves with wide lamina that can float on the surface of the water, and a thick waxy cuticle on the leaf
surface that repels water.
LINK TQ LEARNING
Watch “The Pale Pitcher Plant" episode of the video (http:// 0 penstaxc 0 llege. 0 rg/l/plants_c 00 l_t 00 ) series
Plants Are Cool, Too, a Botanical Society of America video about a carnivorous plant species found in
Louisiana.
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V /
e olution CONNECTION
Plant Adaptations in Resource-Deficient Environments
Roots, stems, and leaves are structured to ensure that a plant can obtain the required sunlight, water, soil
nutrients, and oxygen resources. Some remarkable adaptations have evolved to enable plant species to
thrive in less than ideal habitats, where one or more of these resources is in short supply.
In tropical rainforests, light is often scarce, since many trees and plants grow close together and block much
of the sunlight from reaching the forest floor. Many tropical plant species have exceptionally broad leaves to
maximize the capture of sunlight. Other species are epiphytes: plants that grow on other plants that serve
as a physical support. Such plants are able to grow high up in the canopy atop the branches of other trees,
where sunlight is more plentiful. Epiphytes live on rain and minerals collected in the branches and leaves
of the supporting plant. Bromeliads (members of the pineapple family), ferns, and orchids are examples of
tropical epiphytes (Figure 30.28). Many epiphytes have specialized tissues that enable them to efficiently
capture and store water.
Figure 30.28 One of the most well known bromeliads is Spanish moss (Tillandsia usneoides), seen here in an
oak tree, (credit: Kristine Paulus)
Some plants have special adaptations that help them to survive in nutrient-poor environments. Carnivorous
plants, such as the Venus flytrap and the pitcher plant (Figure 30.29), grow in bogs where the soil is low
in nitrogen. In these plants, leaves are modified to capture insects. The insect-capturing leaves may have
evolved to provide these plants with a supplementary source of much-needed nitrogen.
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Chapter 30 | Plant Form and Physiology
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(a) (b)
Figure 30.29 The (a) Venus flytrap has modified leaves that can capture insects. When an unlucky insect touches
the trigger hairs inside the leaf, the trap suddenly closes. The opening of the (b) pitcher plant is lined with a
slippery wax. Insects crawling on the lip slip and fall into a pool of water in the bottom of the pitcher, where they
are digested by bacteria. The plant then absorbs the smaller molecules, (credit a: modification of work by Peter
Shanks; credit b: modification of work by Tim Mansfield)
Many swamp plants have adaptations that enable them to thrive in wet areas, where their roots grow
submerged underwater. In these aquatic areas, the soil is unstable and little oxygen is available to reach
the roots. Trees such as mangroves (Rhizophora sp.) growing in coastal waters produce aboveground
roots that help support the tree (Figure 30.30). Some species of mangroves, as well as cypress trees,
have pneumatophores: upward-growing roots containing pores and pockets of tissue specialized for gas
exchange. Wild rice is an aquatic plant with large air spaces in the root cortex. The air-filled tissue—called
aerenchyma—provides a path for oxygen to diffuse down to the root tips, which are embedded in oxygen-
poor bottom sediments.
(a) (b) (c)
Figure 30.30 The branches of (a) mangrove trees develop aerial roots, which descend to the ground and
help to anchor the trees, (b) Cypress trees and some mangrove species have upward-growing roots called
pneumatophores that are involved in gas exchange. Aquatic plants such as (c) wild rice have large spaces in the
root cortex called aerenchyma, visualized here using scanning electron microscopy, (credit a: modification of work
by Roberto Verzo; credit b: modification of work by Duane Burdick; credit c: modification of work by Robert R.
Wise)
LINK TQ LEARNING
Watch Venus Flytraps: Jaws of Death, an extraordinary BBC close-up of the Venus flytrap in action. (This
multimedia resource will open in a browser.) (http://cnx.org/content/m66599/1.3/#eip-
idll68018040099)
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Chapter 30 | Plant Form and Physiology
30.5 | Transport of Water and Solutes in Plants
By the end of this section, you will be able to do the following:
• Define water potential and explain how it is influenced by solutes, pressure, gravity, and the matric
potential
• Describe how water potential, evapotranspiration, and stomatal regulation influence how water is
transported in plants
• Explain how photosynthates are transported in plants
The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates
throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential,
evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To
understand how these processes work, we must first understand the energetics of water potential.
Water Potential
Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation
of potential energy, plants can move water to the top of a 116-meter-tall tree (Figure 30.31a). Plants can also
use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure 30.31b). Plants achieve this
because of water potential.
(a) <b)
Figure 30.31 With heights nearing 116 meters, (a) coastal redwoods (Sequoia sempervirens) are the tallest trees in the
world. Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of
homeowners and city maintenance departments, (credit a: modification of work by Bernt Rostad; credit b: modification
of work by Pedestrians Educating Drivers on Safety, Inc.)
Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy
in any one particular aqueous system, but are very interested in water movement between two systems. In
practical terms, therefore, water potential is the difference in potential energy between a given water sample and
pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter
lJj ( psi ) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The
potential of pure water ( l 4 J w pure H2 °) is, by convenience of definition, designated a value of zero (even though
pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a
plant root, stem, or leaf are therefore expressed relative to 4 J w pure H2 °.
The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called
matrix effects. Water potential can be broken down into its individual components using the following equation:
'P system = * total = ^ + *Pg + «P m
where ip s , H^p, "-Pg, and refer to the solute, pressure, gravity, and matric potentials, respectively. “System”
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can refer to the water potential of the soil water ( l 4 jSOil ), root water ( 4 ’ root ), stem water (i4J stem ), leaf water
(ip'eaf) or t ^ e water j n atmosphere (i4J atmos P here ) : whichever aqueous system is under consideration. As the
individual components change, they raise or lower the total water potential of a system. When this happens,
water moves to equilibrate, moving from the system or compartment with a higher water potential to the system
or compartment with a lower water potential. This brings the difference in water potential between the two
systems (A4 1 ) back to zero (A4 1 = 0). Therefore, for water to move through the plant from the soil to the air (a
process called transpiration), HJ soil must be > 4J root > 4J stem > 4J leaf > ip atmos P here
Water only moves in response to A4\ not in response to the individual components. However, because the
individual components influence the total ^system, by manipulating the individual components (especially < 4 J S ), a
plant can control water movement.
Solute Potential
Solute potential ( l 4 J s), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical
values for cell cytoplasm are -0.5 to -1.0 MPa. Solutes reduce water potential (resulting in a negative HJw) by
consuming some of the potential energy available in the water. Solute molecules can dissolve in water because
water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to
water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no
longer available to do work in the system because it is tied up in the bond. In other words, the amount of available
potential energy is reduced when solutes are added to an aqueous system. Thus, decreases with increasing
solute concentration. Because HJs is one of the four components of ^system or *4 J total, a decrease in 4*s will cause
a decrease in l 4 J total. The internal water potential of a plant cell is more negative than pure water because of
the cytoplasm’s high solute content (Figure 30.32). Because of this difference in water potential water will move
from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called
osmotic potential.
Plant cells can metabolically manipulate I’s (and by extension, ^total) by adding or removing solute molecules.
Therefore, plants have control over ^total via their ability to exert metabolic control over 4^.
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visual
CONNECTION
Adding solute to the
right side lowers iJj Si
causing water to
move to the right
side of the tube.
Applying positive
pressure to the left
side increases ijj p
causing water to
move to the right
side of the tube.
Applying negative
pressure to the left
side lowers ifj pi
causing water to
move to the left side
of the tube.
Figure 30.32 In this example with a semipermeable membrane between two aqueous systems, water will move
from a region of higher to lower water potential until equilibrium is reached. Solutes (4^), pressure ( l 4 J p), and
gravity (4^) influence total water potential for each side of the tube (4 J total r ' 9ht or left ), and therefore, the difference
between Vtotal on each side (A4J). (V m , the potential due to interaction of water with solid substrates, is ignored
in this example because glass is not especially hydrophilic). Water moves in response to the difference in water
potential between two systems (the left and right sides of the tube).
Positive water potential is placed on the left side of the tube by increasing 4^ such that the water level rises
on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so,
how?
Pressure Potential
Pressure potential (4^), also called turgor potential, may be positive or negative (Figure 30.32). Because
pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice
versa. Therefore, a positive 4*p (compression) increases 4*total, and a negative 4^ (tension) decreases *4 J total-
Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are
typically around 0.6-0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Vp of 1.5 MPa
equates to 210 pounds per square inch (1.5 MPa x 140 lb in' 2 MPa' 1 = 210 lb/in" 2 ). As a comparison, most
automobile tires are kept at a pressure of 30-34 psi. An example of the effect of turgor pressure is the wilting of
leaves and their restoration after the plant has been watered (Figure 30.33). Water is lost from the leaves via
transpiration (approaching H^p = 0 MPa at the wilting point) and restored by uptake via the roots.
A plant can manipulate 4>p via its ability to manipulate 4J S and by the process of osmosis. If a plant cell
increases the cytoplasmic solute concentration, 4^ will decline, 4 J total will decline, the A4J between the cell and
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the surrounding tissue will decline, water will move into the cell by osmosis, and HJp will increase. HJp is also
under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate
from the leaf, reducing HJp and ^total of the leaf and increasing MJ between the water in the leaf and the petiole,
thereby allowing water to flow from the petiole into the leaf.
(a) (b)
Figure 30.33 When (a) total water potential (Vtotai) is lower outside the cells than inside, water moves out of the cells
and the plant wilts. When (b) the total water potential is higher outside the plant cells than inside, water moves into the
cells, resulting in turgor pressure (Vp) and keeping the plant erect, (credit: modification of work by Victor M. Vicente
Selvas)
Gravity Potential
Gravity potential (*4 J g) is always negative to zero in a plant with no height. It always removes or consumes
potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total
amount of potential energy in the water in the plant O+'total)- The taller the plant, the taller the water column,
and the more influential becomes. On a cellular scale and in short plants, this effect is negligible and easily
ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of -0.1 MPa
m" 1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the
tallest trees. Plants are unable to manipulate l 4 J g.
Matric Potential
Matric potential (HJm) is always negative to zero. In a dry system, it can be as low as -2 MPa in a dry seed, and
it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential
energy from the system. l 4 J m is similar to solute potential because it involves tying up the energy in an aqueous
system by forming hydrogen bonds between the water and some other component. However, in solute potential,
the other components are soluble, hydrophilic solute molecules, whereas in ^m, the other components are
insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose
in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. HJm is
very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as
the seed takes up water or the soil hydrates. HJm cannot be manipulated by the plant and is typically ignored in
well-watered roots, stems, and leaves.
Movement of Water and Minerals in the Xylem
Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves
from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential.
Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is
expressed as AHT
Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver
of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf-atmosphere
interface; it creates negative pressure (tension) equivalent to -2 MPa at the leaf surface. This value varies
greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and
substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and
transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem
vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion-tension
theory of sap ascent.
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Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of
the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon
dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and
the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of
the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells (Figure 30.34),
thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally
adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like
the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between
vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The
formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant,
causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces
needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem
vessels, making them nonfunctional.
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visual
CONNECTION
Mesophyll
cells
Xylem
Soil particle
Water molecule
Xylem
Low
High
Transpiration draws
water from the leaf.
Cohesion and adhes
draw water up the xy
Negative water potential
draws water into the root.
Atmosphere:
—100 MPa
Leaf at tip of tree:
—1.5 MPa
Stem:—0.6 MPa
Root cells: —0.2 MPa
Figure 30.34 The cohesion-tension theory of sap ascent is shown. Evaporation from the mesophyll cells
produces a negative water potential gradient that causes water to move upwards from the roots through the xylem.
Which of the following statements is false?
a. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the
xylem. Transpiration draws water from the leaf.
b. Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the
phloem. Transpiration draws water from the leaf.
c. Water potential decreases from the roots to the top of the plant.
d. Water enters the plants through root hairs and exits through stoma.
Transpiration —the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that
metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the
difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is
tightly controlled.
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Control of Transpiration
The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the
plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.
Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of
transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface.
Stomata are surrounded by two specialized cells called guard cells, which open and close in response to
environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations.
Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis
and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing
the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water
loss.
Plants have evolved over time to adapt to their local environment and reduce transpiration (Figure 30.35).
Desert plant (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water.
Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered
environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and
morphological leaf adaptations.
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(C) (d)
Figure 30.35 Plants are suited to their local environment, (a) Xerophytes, like this prickly pear cactus ( Opuntia
sp.) and (b) epiphytes such as this tropical Aeschynanthus perrottetii have adapted to very limited water resources.
The leaves of a prickly pear are modified into spines, which lowers the surface-to-volume ratio and reduces water
loss. Photosynthesis takes place in the stem, which also stores water, (b) A. perottetii leaves have a waxy cuticle
that prevents water loss, (c) Goldenrod ( Solidago sp.) is a mesophyte, well suited for moderate environments, (d)
Hydrophytes, like this fragrant water lily ( Nymphaea odorata), are adapted to thrive in aquatic environments, (credit
a: modification of work by Jon Sullivan; credit b: modification of work by L. Shyamal/Wikimedia Commons; credit c:
modification of work by Huw Williams; credit d: modification of work by Jason Hollinger)
Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s
surface. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations
impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly
found in these types of plants.
Transportation of Photosynthates in the Phloem
Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are
converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are
growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are
called photosynthates, which are usually in the form of simple sugars such as sucrose.
Structures that produce photosynthates for the growing plant are referred to as sources. Sugars produced in
sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called
translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called
sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development
and the season.
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The products from the source are usually translocated to the nearest sink through the phloem. For example,
the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct
photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the
flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). The pattern of photosynthate flow
changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots
and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also
directed to tubers for storage.
Translocation: Transport from Source to Sink
Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there
they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by
cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem
sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded
into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring
ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the
uptake of sucrose with a carrier protein called the sucrose-H + symporter.
Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow
for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They
assist with metabolic activities and produce energy for the STEs (Figure 30.36).
Sieve tube
element
Companion
cell
Lateral sieve
area
Sieve tube
plate
Figure 30.36 Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called
sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide
them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells.
Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution
that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high percentage of
sugar decreases MJ S , which decreases the total water potential and causes water to move by osmosis from the
adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes
the bulk flow of phloem from source to sink (Figure 30.37). Sucrose concentration in the sink cells is lower
than in the phloem STEs because the sink sucrose has been metabolized for growth, or converted to starch for
storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube
occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of
low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem
back into the phloem sap.
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Xylem
Phloem
Companion
cell
Source cell
(leaf)
Sink cell
(root)
Figure 30.37 Sucrose is actively transported from source cells into companion cells and then into the sieve-tube
elements. This reduces the water potential, which causes water to enter the phloem from the xylem. The resulting
positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded. Transpiration
causes water to return to the leaves through the xylem vessels.
30.6 | Plant Sensory Systems and Responses
By the end of this section, you will be able to do the following:
• Describe how red and blue light affect plant growth and metabolic activities
• Discuss gravitropism
• Understand how hormones affect plant growth and development
• Describe thigmotropism, thigmonastism, and thigmogenesis
• Explain how plants defend themselves from predators and respond to wounds
Animals can respond to environmental factors by moving to a new location. Plants, however, are rooted in place
and must respond to the surrounding environmental factors. Plants have sophisticated systems to detect and
respond to light, gravity, temperature, and physical touch. Receptors sense environmental factors and relay
the information to effector systems—often through intermediate chemical messengers—to bring about plant
responses.
Plant Responses to Light
Plants have a number of sophisticated uses for light that go far beyond their ability to photosynthesize low-
molecular-weight sugars using only carbon dioxide, light, and water. Photomorphogenesis is the growth
and development of plants in response to light. It allows plants to optimize their use of light and space.
Photoperiodism is the ability to use light to track time. Plants can tell the time of day and time of year by sensing
and using various wavelengths of sunlight. Phototropism is a directional response that allows plants to grow
towards, or even away from, light.
The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The
response of plants to light is mediated by different photoreceptors, which are comprised of a protein covalently
bonded to a light-absorbing pigment called a chromophore. Together, the two are called a chromoprotein.
The red/far-red and violet-blue regions of the visible light spectrum trigger structural development in plants.
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Chapter 30 | Plant Form and Physiology
Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the
quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in
the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths
are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted
to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight,
which is rich in blue-green emissions. Water absorbs red light, which makes the detection of blue light essential
for algae and aquatic plants.
The Phytochrome System and the Red/Far-Red Response
The phytochromes are a family of chromoproteins with a linear tetrapyrrole chromophore, similar to the ringed
tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-interconvertible forms: Pr
and Pfr. Pr absorbs red light (-667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (-730 nm)
and is quickly converted back to Pr. Absorption of red or far-red light causes a massive change to the shape of
the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the
physiologically active form of the protein; therefore, exposure to red light yields physiological activity. Exposure
to far-red light inhibits phytochrome activity. Together, the two forms represent the phytochrome system (Figure
30.38).
The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of
environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which
converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark, or break
down overtime. In all instances, the physiological response induced by red light is reversed. The active form of
phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus,
where it directly activates or represses specific gene expression.
Once the phytochrome system evolved, plants adapted it to serve a variety of needs. Unfiltered, full sunlight
contains much more red light than far-red light. Because chlorophyll absorbs strongly in the red region of the
visible spectrum, but not in the far-red region, any plant in the shade of another plant on the forest floor will be
exposed to red-depleted, far-red-enriched light. The preponderance of far-red light converts phytochrome in the
shaded leaves to the Pr (inactive) form, slowing growth. The nearest non-shaded (or even less-shaded) areas
on the forest floor have more red light; leaves exposed to these areas sense the red light, which activates the
Pfr form and induces growth. In short, plant shoots use the phytochrome system to grow away from shade and
towards light. Because competition for light is so fierce in a dense plant community, the evolutionary advantages
of the phytochrome system are obvious.
In seeds, the phytochrome system is not used to determine direction and quality of light (shaded versus
unshaded). Instead, is it used merely to determine if there is any light at all. This is especially important in
species with very small seeds, such as lettuce. Because of their size, lettuce seeds have few food reserves.
Their seedlings cannot grow for long before they run out of fuel. If they germinated even a centimeter under the
soil surface, the seedling would never make it into the sunlight and would die. In the dark, phytochrome is in the
Pr (inactive form) and the seed will not germinate; it will only germinate if exposed to light at the surface of the
soil. Upon exposure to light, Pr is converted to Pfr and germination proceeds.
Figure 30.38 The biologically inactive form of phytochrome (Pr) is converted to the biologically active form Pfr under
illumination with red light. Far-red light and darkness convert the molecule back to the inactive form.
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Plants also use the phytochrome system to sense the change of season. Photoperiodism is a biological
response to the timing and duration of day and night. It controls flowering, setting of winter buds, and vegetative
growth. Detection of seasonal changes is crucial to plant survival. Although temperature and light intensity
influence plant growth, they are not reliable indicators of season because they may vary from one year to the
next. Day length is a better indicator of the time of year.
As stated above, unfiltered sunlight is rich in red light but deficient in far-red light. Therefore, at dawn, all the
phytochrome molecules in a leaf quickly convert to the active Pfr form, and remain in that form until sunset. In
the dark, the Pfr form takes hours to slowly revert back to the Pr form. If the night is long (as in winter), all of
the Pfr form reverts. If the night is short (as in summer), a considerable amount of Pfr may remain at sunrise.
By sensing the Pr/Pfr ratio at dawn, a plant can determine the length of the day/night cycle. In addition, leaves
retain that information for several days, allowing a comparison between the length of the previous night and
the preceding several nights. Shorter nights indicate springtime to the plant; when the nights become longer,
autumn is approaching. This information, along with sensing temperature and water availability, allows plants
to determine the time of the year and adjust their physiology accordingly. Short-day (long-night) plants use this
information to flower in the late summer and early fall, when nights exceed a critical length (often eight or fewer
hours). Long-day (short-night) plants flower during the spring, when darkness is less than a critical length (often
eight to 15 hours). Not all plants use the phytochrome system in this way. Flowering in day-neutral plants is not
regulated by daylength.
ca eer connection
Horticulturalist
The word “horticulturist” comes from the Latin words for garden ( hortus ) and culture ( cultura ). This career
has been revolutionized by progress made in the understanding of plant responses to environmental stimuli.
Growers of crops, fruit, vegetables, and flowers were previously constrained by having to time their sowing
and harvesting according to the season. Now, horticulturists can manipulate plants to increase leaf, flower,
or fruit production by understanding how environmental factors affect plant growth and development.
Greenhouse management is an essential component of a horticulturist’s education. To lengthen the night,
plants are covered with a blackout shade cloth. Long-day plants are irradiated with red light in winter to
promote early flowering. For example, fluorescent (cool white) light high in blue wavelengths encourages
leafy growth and is excellent for starting seedlings. Incandescent lamps (standard light bulbs) are rich in
red light, and promote flowering in some plants. The timing of fruit ripening can be increased or delayed
by applying plant hormones. Recently, considerable progress has been made in the development of plant
breeds that are suited to different climates and resistant to pests and transportation damage. Both crop
yield and quality have increased as a result of practical applications of the knowledge of plant responses to
external stimuli and hormones.
Horticulturists find employment in private and governmental laboratories, greenhouses, botanical gardens,
and in the production or research fields. They improve crops by applying their knowledge of genetics and
plant physiology. To prepare for a horticulture career, students take classes in botany, plant physiology, plant
pathology, landscape design, and plant breeding. To complement these traditional courses, horticulture
majors add studies in economics, business, computer science, and communications.
The Blue Light Responses
Phototropism—the directional bending of a plant toward or away from a light source—is a response to blue
wavelengths of light. Positive phototropism is growth towards a light source (Figure 30.39), while negative
phototropism (also called skototropism) is growth away from light.
The aptly-named phototropins are protein-based receptors responsible for mediating the phototropic response.
Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the
chromophore. In phototropins, the chromophore is a covalently-bound molecule of flavin; hence, phototropins
belong to a class of proteins called flavo prate ins.
Other responses under the control of phototropins are leaf opening and closing, chloroplast movement, and the
opening of stomata. However, of all responses controlled by phototropins, phototropism has been studied the
longest and is the best understood.
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In their 1880 treatise The Power of Movements in Plants, Charles Darwin and his son Francis first described
phototropism as the bending of seedlings toward light. Darwin observed that light was perceived by the tip of
the plant (the apical meristem), but that the response (bending) took place in a different part of the plant. They
concluded that the signal had to travel from the apical meristem to the base of the plant.
Figure 30.39 Azure bluets (Houstonia caerulea) display a phototropic response by bending toward the light, (credit:
Cory Zanker)
In 1913, Peter Boysen-Jensen demonstrated that a chemical signal produced in the plant tip was responsible
for the bending at the base. He cut off the tip of a seedling, covered the cut section with a layer of gelatin, and
then replaced the tip. The seedling bent toward the light when illuminated. However, when impermeable mica
flakes were inserted between the tip and the cut base, the seedling did not bend. A refinement of the experiment
showed that the signal traveled on the shaded side of the seedling. When the mica plate was inserted on the
illuminated side, the plant did bend towards the light. Therefore, the chemical signal was a growth stimulant
because the phototropic response involved faster cell elongation on the shaded side than on the illuminated
side. We now know that as light passes through a plant stem, it is diffracted and generates phototropin activation
across the stem. Most activation occurs on the lit side, causing the plant hormone indole acetic acid (IAA) to
accumulate on the shaded side. Stem cells elongate under influence of IAA.
Cryptochromes are another class of blue-light absorbing photoreceptors that also contain a flavin-based
chromophore. Cryptochromes set the plants' 24-hour activity cycle, also know as its circadian rhythem, using
blue light cues. There is some evidence that cryptochromes work together with phototropins to mediate the
phototropic response.
LINK
T &
LEARNING
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view images of plants in motion.
Plant Responses to Gravity
Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, and
roots grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given
enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth
of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called
positive gravitropism.
Amyloplasts (also known as statoliths) are specialized plastids that contain starch granules and settle
downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When
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Chapter 30 | Plant Form and Physiology
939
a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show
growth in the new vertical direction.
The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the
bottom of the gravity-sensing cells in the root or shoot, they physically contact the endoplasmic reticulum (ER),
causing the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport
of the plant hormone IAA to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation.
The effect slows growth on the lower side of the root, while cells develop normally on the upper side. IAA has the
opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion,
causing the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their
normal position. Other hypotheses—involving the entire cell in the gravitropism effect—have been proposed to
explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response.
Growth Responses
A plant’s sensory response to external stimuli relies on chemical messengers (hormones). Plant hormones
affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall.
Potentially every cell in a plant can produce plant hormones. They can act in their cell of origin or be transported
to other portions of the plant body, with many plant responses involving the synergistic or antagonistic interaction
of two or more hormones. In contrast, animal hormones are produced in specific glands and transported to a
distant site for action, and they act alone.
Plant hormones are a group of unrelated chemical substances that affect plant morphogenesis. Five major plant
hormones are traditionally described: auxins (particularly IAA), cytokinins, gibberellins, ethylene, and abscisic
acid. In addition, other nutrients and environmental conditions can be characterized as growth factors.
Auxins
The term auxin is derived from the Greek word auxein, which means "to grow." Auxins are the main hormones
responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem
into vascular tissue, and promote leaf development and arrangement. While many synthetic auxins are used
as herbicides, IAA is the only naturally occurring auxin that shows physiological activity. Apical dominance—the
inhibition of lateral bud formation—is triggered by auxins produced in the apical meristem. Flowering, fruit setting
and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect
control of auxins. Auxins also act as a relay for the effects of the blue light and red/far-red responses.
Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting
hormone to promote growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins
to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes
synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless
cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.
Cytokinins
The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts
to developing plant embryos in culture stimulated their growth. The stimulating growth factor was found to
be cytokinin, a hormone that promotes cytokinesis (cell division). Almost 200 naturally occurring or synthetic
cytokinins are known to date. Cytokinins are most abundant in growing tissues, such as roots, embryos, and
fruits, where cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis,
and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under
the influence of cytokinins, either in conjunction with auxin or another hormone. For example, apical dominance
seems to result from a balance between auxins that inhibit lateral buds, and cytokinins that promote bushier
growth.
Gibberellins
Gibberellins (GAs) are a group of about 125 closely related plant hormones that stimulate shoot elongation,
seed germination, and fruit and flower maturation. GAs are synthesized in the root and stem apical meristems,
young leaves, and seed embryos. In urban areas, GA antagonists are sometimes applied to trees under power
lines to control growth and reduce the frequency of pruning.
GAs break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure
to cold or light to germinate. Abscisic acid is a strong antagonist of GA action. Other effects of GAs include
gender expression, seedless fruit development, and the delay of senescence in leaves and fruit. Seedless
grapes are obtained through standard breeding methods and contain inconspicuous seeds that fail to develop.
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Chapter 30 | Plant Form and Physiology
Because GAs are produced by the seeds, and because fruit development and stem elongation are under GA
control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are
routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces
the instance of mildew infection (Figure 30.40).
Figure 30.40 In grapes, application of gibberellic acid increases the size of fruit and loosens clustering, (credit: Bob
Nichols, US DA)
Abscisic Acid
The plant hormone abscisic acid (ABA) was first discovered as the agent that causes the abscission or dropping
of cotton bolls. However, more recent studies indicate that ABA plays only a minor role in the abscission process.
ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures,
or shortened day lengths. Its activity counters many of the growth-promoting effects of GAs and auxins. ABA
inhibits stem elongation and induces dormancy in lateral buds.
ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins.
Plants adapted to temperate climates require a long period of cold temperature before seeds germinate. This
mechanism protects young plants from sprouting too early during unseasonably warm weather in winter. As
the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when
conditions are favorable in spring. Another effect of ABA is to promote the development of winter buds; it
mediates the conversion of the apical meristem into a dormant bud. Low soil moisture causes an increase in
ABA, which causes stomata to close, reducing water loss in winter buds.
Ethylene
Ethylene is associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile
gas (C2H4). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to
lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were
caused by ethylene volatilizing from the lamps.
Aging tissues (especially senescing leaves) and nodes of stems produce ethylene. The best-known effect of the
hormone, however, is the promotion of fruit ripening. Ethylene stimulates the conversion of starch and acids to
sugars. Some people store unripe fruit, such as avocadoes, in a sealed paper bag to accelerate ripening; the
gas released by the first fruit to mature will speed up the maturation of the remaining fruit. Ethylene also triggers
leaf and fruit abscission, flower fading and dropping, and promotes germination in some cereals and sprouting
of bulbs and potatoes.
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Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with application
of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses
using fans and ventilation.
Nontraditional Hormones
Recent research has discovered a number of compounds that also influence plant development. Their roles are
less understood than the effects of the major hormones described so far.
Jasmonates play a major role in defense responses to herbivory. Their levels increase when a plant is wounded
by a predator, resulting in an increase in toxic secondary metabolites. They contribute to the production of volatile
compounds that attract natural enemies of predators. For example, chewing of tomato plants by caterpillars
leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract
predators of the pest.
Oligosaccharins also play a role in plant defense against bacterial and fungal infections. They act locally at the
site of injury, and can also be transported to other tissues. Strigolactones promote seed germination in some
species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the
establishment of mycorrhizae, a mutualistic association of plant roots and fungi. Brassinosteroids are important
to many developmental and physiological processes. Signals between these compounds and other hormones,
notably auxin and GAs, amplifies their physiological effect. Apical dominance, seed germination, gravitropism,
and resistance to freezing are all positively influenced by hormones. Root growth and fruit dropping are inhibited
by steroids.
Plant Responses to Wind and Touch
The shoot of a pea plant winds around a trellis, while a tree grows on an angle in response to strong prevailing
winds. These are examples of how plants respond to touch or wind.
The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek
words thigma meaning “touch,” and tropism implying “direction.” Tendrils are one example of this. The
meristematic region of tendrils is very touch sensitive; light touch will evoke a quick coiling response. Cells in
contact with a support surface contract, whereas cells on the opposite side of the support expand (Figure 30.14).
Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus.
A thigmonastic response is a touch response independent of the direction of stimulus Figure 30.24. in the
Venus flytrap, two modified leaves are joined at a hinge and lined with thin fork-like tines along the outer edges.
Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more
of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes
that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal.
Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous
mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens.
Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force. Researchers
hypothesize that mechanical strain induces growth and differentiation to strengthen the tissues. Ethylene and
jasmonate are likely involved in thigmomorphogenesis.
Use the menu at the left to navigate to three short movies: (http:// 0 penstaxc 0 llege. 0 rg/l/nastic_mvmt) a
Venus fly trap capturing prey, the progressive closing of sensitive plant leaflets, and the twining of tendrils.
Defense Responses against Herbivores and Pathogens
Plants face two types of enemies: herbivores and pathogens. Herbivores both large and small use plants as
food, and actively chew them. Pathogens are agents of disease. These infectious microorganisms, such as
fungi, bacteria, and nematodes, live off of the plant and damage its tissues. Plants have developed a variety of
strategies to discourage or kill attackers.
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The first line of defense in plants is an intact and impenetrable barrier. Bark and the waxy cuticle can protect
against predators. Other adaptations against herbivory include thorns, which are modified branches, and spines,
which are modified leaves. They discourage animals by causing physical damage and inducing rashes and
allergic reactions. A plant’s exterior protection can be compromised by mechanical damage, which may provide
an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of
defense mechanisms, such as toxins and enzymes.
Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary
for respiration or plant growth and development. Many metabolites are toxic, and can even be lethal to animals
that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors (such as the
volatile oils of mint and sage) or repellent tastes (like the bitterness of quinine). Other alkaloids affect herbivores
by causing either excessive stimulation (caffeine is one example) or the lethargy associated with opioids. Some
compounds become toxic after ingestion. For instance, glycol cyanide in the cassava root releases cyanide only
upon ingestion; the nearly 500 million humans who rely on cassava for nutrition must be certain to process the
root properly before eating.
Mechanical wounding and predator attacks activate defense and protection mechanisms both in the damaged
tissue and at sites farther from the injury location. Some defense reactions occur within minutes: others over
several hours. The infected and surrounding cells may die, thereby stopping the spread of infection.
Long-distance signaling elicits a systemic response aimed at deterring the predator. As tissue is damaged,
jasmonates may promote the synthesis of compounds that are toxic to predators. Jasmonates also elicit the
synthesis of volatile compounds that attract parasitoids, which are insects that spend their developing stages
in or on another insect, and eventually kill their host. The plant may activate abscission of injured tissue if it is
damaged beyond repair.
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Chapter 30 | Plant Form and Physiology
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KEY TERMS
abscisic acid (ABA) plant hormone that induces dormancy in seeds and other organs
abscission physiological process that leads to the fall of a plant organ (such as leaf or petal drop)
adventitious root aboveground root that arises from a plant part other than the radicle of the plant embryo
apical bud bud formed at the tip of the shoot
apical meristem meristematic tissue located at the tips of stems and roots; enables a plant to extend in length
auxin plant hormone that influences cell elongation (in phototropism), gravitropism, apical dominance, and root
growth
axillary bud bud located in the axil: the stem area where the petiole connects to the stem
bark tough, waterproof, outer epidermal layer of cork cells
bulb modified underground stem that consists of a large bud surrounded by numerous leaf scales
Casparian strip waxy coating that forces water to cross endodermal plasma membranes before entering the
vascular cylinder, instead of moving between endodermal cells
chromophore molecule that absorbs light
collenchyma cell elongated plant cell with unevenly thickened walls; provides structural support to the stem
and leaves
companion cell phloem cell that is connected to sieve-tube cells; has large amounts of ribosomes and
mitochondria
compound leaf leaf in which the leaf blade is subdivided to form leaflets, all attached to the midrib
corm rounded, fleshy underground stem that contains stored food
cortex ground tissue found between the vascular tissue and the epidermis in a stem or root
cryptochrome protein that absorbs light in the blue and ultraviolet regions of the light spectrum
cuticle waxy protective layer on the leaf surface
cuticle waxy covering on the outside of the leaf and stem that prevents the loss of water
cytokinin plant hormone that promotes cell division
dermal tissue protective plant tissue covering the outermost part of the plant; controls gas exchange
endodermis layer of cells in the root that forms a selective barrier between the ground tissue and the vascular
tissue, allowing water and minerals to enter the root while excluding toxins and pathogens
epidermis single layer of cells found in plant dermal tissue; covers and protects underlying tissue
ethylene volatile plant hormone that is associated with fruit ripening, flower wilting, and leaf fall
fibrous root system type of root system in which the roots arise from the base of the stem in a cluster, forming
a dense network of roots; found in monocots
gibberellin (GA) plant hormone that stimulates shoot elongation, seed germination, and the maturation and
dropping of fruit and flowers
ground tissue plant tissue involved in photosynthesis; provides support, and stores water and sugars
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guard cells paired cells on either side of a stoma that control stomatal opening and thereby regulate the
movement of gases and water vapor
intercalary meristem meristematic tissue located at nodes and the bases of leaf blades; found only in
monocots
internode region between nodes on the stem
jasmonates small family of compounds derived from the fatty acid linoleic acid
lamina leaf blade
lateral meristem meristematic tissue that enables a plant to increase in thickness or girth
lenticel opening on the surface of mature woody stems that facilitates gas exchange
megapascal (MPa) pressure units that measure water potential
meristem plant region of continuous growth
meristematic tissue tissue containing cells that constantly divide; contributes to plant growth
negative gravitropism growth away from Earth’s gravity
node point along the stem at which leaves, flowers, or aerial roots originate
oligosaccharin hormone important in plant defenses against bacterial and fungal infections
palmately compound leaf leaf type with leaflets that emerge from a point, resembling the palm of a hand
parenchyma cell most common type of plant cell; found in the stem, root, leaf, and in fruit pulp; site of
photosynthesis and starch storage
pericycle outer boundary of the stele from which lateral roots can arise
periderm outermost covering of woody stems; consists of the cork cambium, cork cells, and the phelloderm
permanent tissue plant tissue composed of cells that are no longer actively dividing
petiole stalk of the leaf
photomorphogenesis growth and development of plants in response to light
photoperiodism occurrence of plant processes, such as germination and flowering, according to the time of
year
phototropin blue-light receptor that promotes phototropism, stomatal opening and closing, and other responses
that promote photosynthesis
phototropism directional bending of a plant toward a light source
phyllotaxy arrangement of leaves on a stem
phytochrome plant pigment protein that exists in two reversible forms (Pr and Pfr) and mediates morphologic
changes in response to red light
pinnately compound leaf leaf type with a divided leaf blade consisting of leaflets arranged on both sides of the
midrib
pith ground tissue found towards the interior of the vascular tissue in a stem or root
positive gravitropism growth toward Earth’s gravitational center
primary growth growth resulting in an increase in length of the stem and the root; caused by cell division in the
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Chapter 30 | Plant Form and Physiology
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shoot or root apical meristem
rhizome modified underground stem that grows horizontally to the soil surface and has nodes and internodes
root cap protective cells covering the tip of the growing root
root hair hair-like structure that is an extension of epidermal cells; increases the root surface area and aids in
absorption of water and minerals
root system belowground portion of the plant that supports the plant and absorbs water and minerals
runner stolon that runs above the ground and produces new clone plants at nodes
sclerenchyma cell plant cell that has thick secondary walls and provides structural support; usually dead at
maturity
secondary growth growth resulting in an increase in thickness or girth; caused by the lateral meristem and cork
cambium
sessile leaf without a petiole that is attached directly to the plant stem
shoot system aboveground portion of the plant; consists of nonreproductive plant parts, such as leaves and
stems, and reproductive parts, such as flowers and fruits
sieve-tube cell phloem cell arranged end to end to form a sieve tube that transports organic substances such
as sugars and amino acids
simple leaf leaf type in which the lamina is completely undivided or merely lobed
sink growing parts of a plant, such as roots and young leaves, which require photosynthate
source organ that produces photosynthate for a plant
statolith (also, amyloplast) plant organelle that contains heavy starch granules
stele inner portion of the root containing the vascular tissue; surrounded by the endodermis
stipule small green structure found on either side of the leaf stalk or petiole
stolon modified stem that runs parallel to the ground and can give rise to new plants at the nodes
strigolactone hormone that promotes seed germination in some species and inhibits lateral apical development
in the absence of auxins
tap root system type of root system with a main root that grows vertically with few lateral roots; found in dicots
tendril modified stem consisting of slender, twining strands used for support or climbing
thigmomorphogenesis developmental response to touch
thigmonastic directional growth of a plant independent of the direction in which contact is applied
thigmotropism directional growth of a plant in response to constant contact
thorn modified stem branch appearing as a sharp outgrowth that protects the plant
tracheid xylem cell with thick secondary walls that helps transport water
translocation mass transport of photosynthates from source to sink in vascular plants
transpiration loss of water vapor to the atmosphere through stomata
trichome hair-like structure on the epidermal surface
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tuber modified underground stem adapted for starch storage; has many adventitious buds
vascular bundle strands of stem tissue made up of xylem and phloem
vascular stele strands of root tissue made up of xylem and phloem
vascular tissue tissue made up of xylem and phloem that transports food and water throughout the plant
venation pattern of veins in a leaf; may be parallel (as in monocots), reticulate (as in dicots), or dichotomous (as
in Cingko biloba)
vessel element xylem cell that is shorter than a tracheid and has thinner walls
water potential (^w) the potential energy of a water solution per unit volume in relation to pure water at
atmospheric pressure and ambient temperature
whorled pattern of leaf arrangement in which three or more leaves are connected at a node
CHAPTER SUMMARY
30.1 The Plant Body
A vascular plant consists of two organ systems: the shoot system and the root system. The shoot system
includes the aboveground vegetative portions (stems and leaves) and reproductive parts (flowers and fruits).
The root system supports the plant and is usually underground. A plant is composed of two main types of
tissue: meristematic tissue and permanent tissue. Meristematic tissue consists of actively dividing cells found in
root and shoot tips. As growth occurs, meristematic tissue differentiates into permanent tissue, which is
categorized as either simple or complex. Simple tissues are made up of similar cell types; examples include
dermal tissue and ground tissue. Dermal tissue provides the outer covering of the plant. Ground tissue is
responsible for photosynthesis; it also supports vascular tissue and may store water and sugars. Complex
tissues are made up of different cell types. Vascular tissue, for example, is made up of xylem and phloem cells.
30.2 Stems
The stem of a plant bears the leaves, flowers, and fruits. Stems are characterized by the presence of nodes
(the points of attachment for leaves or branches) and internodes (regions between nodes).
Plant organs are made up of simple and complex tissues. The stem has three tissue systems: dermal, vascular,
and ground tissue. Dermal tissue is the outer covering of the plant. It contains epidermal cells, stomata, guard
cells, and trichomes. Vascular tissue is made up of xylem and phloem tissues and conducts water, minerals,
and photosynthetic products. Ground tissue is responsible for photosynthesis and support and is composed of
parenchyma, collenchyma, and sclerenchyma cells.
Primary growth occurs at the tips of roots and shoots, causing an increase in length. Woody plants may also
exhibit secondary growth, or increase in thickness. In woody plants, especially trees, annual rings may form as
growth slows at the end of each season. Some plant species have modified stems that help to store food,
propagate new plants, or discourage predators. Rhizomes, corms, stolons, runners, tubers, bulbs, tendrils, and
thorns are examples of modified stems.
30.3 Roots
Roots help to anchor a plant, absorb water and minerals, and serve as storage sites for food. Taproots and
fibrous roots are the two main types of root systems. In a taproot system, a main root grows vertically
downward with a few lateral roots. Fibrous root systems arise at the base of the stem, where a cluster of roots
forms a dense network that is shallower than a taproot. The growing root tip is protected by a root cap. The root
tip has three main zones: a zone of cell division (cells are actively dividing), a zone of elongation (cells increase
in length), and a zone of maturation (cells differentiate to form different kinds of cells). Root vascular tissue
conducts water, minerals, and sugars. In some habitats, the roots of certain plants may be modified to form
aerial roots or epiphytic roots.
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30.4 Leaves
Leaves are the main site of photosynthesis. A typical leaf consists of a lamina (the broad part of the leaf, also
called the blade) and a petiole (the stalk that attaches the leaf to a stem). The arrangement of leaves on a
stem, known as phyllotaxy, enables maximum exposure to sunlight. Each plant species has a characteristic leaf
arrangement and form. The pattern of leaf arrangement may be alternate, opposite, or spiral, while leaf form
may be simple or compound. Leaf tissue consists of the epidermis, which forms the outermost cell layer, and
mesophyll and vascular tissue, which make up the inner portion of the leaf. In some plant species, leaf form is
modified to form structures such as tendrils, spines, bud scales, and needles.
30.5 Transport of Water and Solutes in Plants
Water potential 0+ 1 ) is a measure of the difference in potential energy between a water sample and pure water.
The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and matric
potential. Water potential and transpiration influence how water is transported through the xylem in plants.
These processes are regulated by stomatal opening and closing. Photosynthates (mainly sucrose) move from
sources to sinks through the plant’s phloem. Sucrose is actively loaded into the sieve-tube elements of the
phloem. The increased solute concentration causes water to move by osmosis from the xylem into the phloem.
The positive pressure that is produced pushes water and solutes down the pressure gradient. The sucrose is
unloaded into the sink, and the water returns to the xylem vessels.
30.6 Plant Sensory Systems and Responses
Plants respond to light by changes in morphology and activity. Irradiation by red light converts the
photoreceptor phytochrome to its far-red light-absorbing form—Pfr. This form controls germination and
flowering in response to length of day, as well as triggers photosynthesis in dormant plants or those that just
emerged from the soil. Blue-light receptors, cryptochromes, and phototropins are responsible for phototropism.
Amyloplasts, which contain heavy starch granules, sense gravity. Shoots exhibit negative gravitropism,
whereas roots exhibit positive gravitropism. Plant hormones—naturally occurring compounds synthesized in
small amounts—can act both in the cells that produce them and in distant tissues and organs. Auxins are
responsible for apical dominance, root growth, directional growth toward light, and many other growth
responses. Cytokinins stimulate cell division and counter apical dominance in shoots. Gibberellins inhibit
dormancy of seeds and promote stem growth. Abscisic acid induces dormancy in seeds and buds, and protects
plants from excessive water loss by promoting stomatal closure. Ethylene gas speeds up fruit ripening and
dropping of leaves. Plants respond to touch by rapid movements (thigmotropy and thigmonasty) and slow
differential growth (thigmomorphogenesis). Plants have evolved defense mechanisms against predators and
pathogens. Physical barriers like bark and spines protect tender tissues. Plants also have chemical defenses,
including toxic secondary metabolites and hormones, which elicit additional defense mechanisms.
VISUAL CONNECTION QUESTIONS
1. Figure 30.7 Which layers of the stem are made of
parenchyma cells?
A. cortex and pith
B. epidermis
C. sclerenchyma
D. epidermis and cortex
2. Figure 30.32 Positive water potential is placed on
the left side of the tube by increasing such that
the water level rises on the right side. Could you
equalize the water level on each side of the tube by
adding solute, and if so, how?
3. Figure 30.34 Which of the following statements is
false?
a. Negative water potential draws water into
the root hairs. Cohesion and adhesion draw
water up the xylem. Transpiration draws
water from the leaf.
b. Negative water potential draws water into
the root hairs. Cohesion and adhesion draw
water up the phloem. Transpiration draws
water from the leaf.
c. Water potential decreases from the roots to
the top of the plant.
d. Water enters the plants through root hairs
and exits through stoma.
REVIEW QUESTIONS
4. Plant regions of continuous growth are made up of
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Chapter 30 | Plant Form and Physiology
a. dermal tissue
b. vascular tissue
c. meristematic tissue
d. permanent tissue
5. Which of the following is the major site of
photosynthesis?
a. apical meristem
b. ground tissue
c. xylem cells
d. phloem cells
6 . Stem regions at which leaves are attached are
called_.
a. trichomes
b. lenticels
c. nodes
d. internodes
7. Which of the following cell types forms most of the
inside of a plant?
a. meristem cells
b. collenchyma cells
c. sclerenchyma cells
d. parenchyma cells
8. Tracheids, vessel elements, sieve-tube cells, and
companion cells are components of_.
a. vascular tissue
b. meristematic tissue
c. ground tissue
d. dermal tissue
9. The primary growth of a plant is due to the action
of the_.
a. lateral meristem
b. vascular cambium
c. apical meristem
d. cork cambium
10. Which of the following is an example of
secondary growth?
a. increase in length
b. increase in thickness or girth
c. increase in root hairs
d. increase in leaf number
11. Secondary growth in stems is usually seen in
a. monocots
b. dicots
c. both monocots and dicots
d. neither monocots nor dicots
12. Roots that enable a plant to grow on another
plant are called_.
a. epiphytic roots
b. prop roots
c. adventitious roots
d. aerial roots
13. The_forces selective uptake of
minerals in the root.
a. pericycle
b. epidermis
c. endodermis
d. root cap
14. Newly-formed root cells begin to form different
cell types in the_.
a. zone of elongation
b. zone of maturation
c. root meristem
d. zone of cell division
15. The stalk of a leaf is known as the_.
a. petiole
b. lamina
c. stipule
d. rachis
16. Leaflets are a characteristic of_leaves.
a. alternate
b. whorled
c. compound
d. opposite
17. Cells of the_contain chloroplasts.
a. epidermis
b. vascular tissue
c. stomata
d. mesophyll
18. Which of the following is most likely to be found in
a desert environment?
a. broad leaves to capture sunlight
b. spines instead of leaves
c. needle-like leaves
d. wide, flat leaves that can float
19. When stomata open, what occurs?
a. Water vapor is lost to the external
environment, increasing the rate of
transpiration.
b. Water vapor is lost to the external
environment, decreasing the rate of
transpiration.
c. Water vapor enters the spaces in the
mesophyll, increasing the rate of
transpiration.
d. Water vapor enters the spaces in the
mesophyll, decreasing the rate of
transpiration.
20. Which cells are responsible for the movement of
photosynthates through a plant?
a. tracheids, vessel elements
b. tracheids, companion cells
c. vessel elements, companion cells
d. sieve-tube elements, companion cells
21. The main photoreceptor that triggers
phototropism is a_.
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Chapter 30 | Plant Form and Physiology
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a. phytochrome
b. cryptochrome
c. phototropin
d. carotenoid
22. Phytochrome is a plant pigment protein that:
a. mediates plant infection
b. promotes plant growth
c. mediates morphological changes in
response to red and far-red light
d. inhibits plant growth
23. A mutant plant has roots that grow in all
directions. Which of the following organelles would
you expect to be missing in the cell?
a. mitochondria
b. amyloplast
c. chloroplast
d. nucleus
24. After buying green bananas or unripe avocadoes,
they can be kept in a brown bag to ripen. The
CRITICAL THINKING QUESTIONS
27. What type of meristem is found only in monocots,
such as lawn grasses? Explain how this type of
meristematic tissue is beneficial in lawn grasses that
are mowed each week.
28. Which plant part is responsible for transporting
water, minerals, and sugars to different parts of the
plant? Name the two types of tissue that make up
this overall tissue, and explain the role of each.
29. Describe the roles played by stomata and guard
cells. What would happen to a plant if these cells did
not function correctly?
30. Compare the structure and function of xylem to
that of phloem.
31. Explain the role of the cork cambium in woody
plants.
32. What is the function of lenticels?
33. Besides the age of a tree, what additional
information can annual rings reveal?
34. Give two examples of modified stems and explain
how each example benefits the plant.
35. Compare a tap root system with a fibrous root
system. For each type, name a plant that provides a
food in the human diet. Which type of root system is
hormone released by the fruit and trapped in the bag
is probably:
a. abscisic acid
b. cytokinin
c. ethylene
d. gibberellic acid
25. A decrease in the level of which hormone
releases seeds from dormancy?
a. abscisic acid
b. cytokinin
c. ethylene
d. gibberellic acid
26. A seedling germinating under a stone grows at an
angle away from the stone and upward. This
response to touch is called_.
a. gravitropism
b. thigmonasty
c. thigmotropism
d. skototropism
found in monocots? Which type of root system is
found in dicots?
36. What might happen to a root if the pericycle
disappeared?
37. How do dicots differ from monocots in terms of
leaf structure?
38. Describe an example of a plant with leaves that
are adapted to cold temperatures.
39. The process of bulk flow transports fluids in a
plant. Describe the two main bulk flow processes.
40. Owners and managers of plant nurseries have to
plan lighting schedules for a long-day plant that will
flower in February. What lighting periods will be most
effective? What color of light should be chosen?
41. What are the major benefits of gravitropism for a
germinating seedling?
42. Fruit and vegetable storage facilities are usually
refrigerated and well ventilated. Why are these
conditions advantageous?
43. Stomata close in response to bacterial infection.
Why is this response a mechanism of defense for the
plant? Which hormone is most likely to mediate this
response?
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Chapter 311 Soil and Plant Nutrition
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31 1 SOIL AND PLANT
NUTRITION
(a) (b)
Figure 31.1 For this (a) squash seedling (Cucurbita maxima) to develop into a mature plant bearing its (b) fruit,
numerous nutritional requirements must be met. (credit a: modification of work by Julian Colton; credit b: modification
of work by "Wildfeuer"/Wikimedia Commons)
Chapter Outline
31.1: Nutritional Requirements of Plants
31.2: The Soil
31.3: Nutritional Adaptations of Plants
Introduction
Cucurbitaceae is a family of plants first cultivated in Mesoamerica, although several species are native to North
America. The family includes many edible species, such as squash and pumpkin, as well as inedible gourds. In
order to grow and develop into mature, fruit-bearing plants, many requirements must be met and events must
be coordinated. Seeds must germinate under the right conditions in the soil; therefore, temperature, moisture,
and soil quality are important factors that play a role in germination and seedling development. Soil quality and
climate are significant to plant distribution and growth. The young seedling will eventually grow into a mature
plant, and the roots will absorb nutrients and water from the soil. At the same time, the aboveground parts
of the plant will absorb carbon dioxide from the atmosphere and use energy from sunlight to produce organic
compounds through photosynthesis. This chapter will explore the complex dynamics between plants and soils,
and the adaptations that plants have evolved to make better use of nutritional resources.
31.1 1 Nutritional Requirements of Plants
By the end of this section, you will be able to do the following:
• Describe how plants obtain nutrients
• List the elements and compounds required for proper plant nutrition
• Describe an essential nutrient
Plants are unique organisms that can absorb nutrients and water through their root system, as well as carbon
dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth.
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Chapter 311 Soil and Plant Nutrition
The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow.
The Chemical Composition of Plants
Since plants require nutrients in the form of elements such as carbon and potassium, it is important to
understand the chemical composition of plants. The majority of volume in a plant cell is water; it typically
comprises 80 to 90 percent of the plant’s total weight. Soil is the water source for land plants, and can be an
abundant source of water, even if it appears dry. Plant roots absorb water from the soil through root hairs and
transport it up to the leaves through the xylem. As water vapor is lost from the leaves, the process of transpiration
and the polarity of water molecules (which enables them to form hydrogen bonds) draws more water from the
roots up through the plant to the leaves (Figure 31.2). Plants need water to support cell structure, for metabolic
functions, to carry nutrients, and for photosynthesis.
Figure 31.2 Water is absorbed through the root hairs and moves up the xylem to the leaves.
Plant cells need essential substances, collectively called nutrients, to sustain life. Plant nutrients may be
composed of either organic or inorganic compounds. An organic compound is a chemical compound that
contains carbon, such as carbon dioxide obtained from the atmosphere. Carbon that was obtained from
atmospheric CO 2 composes the majority of the dry mass within most plants. An inorganic compound does
not contain carbon and is not part of, or produced by, a living organism. Inorganic substances, which form the
majority of the soil solution, are commonly called minerals: those required by plants include nitrogen (N) and
potassium (K) for structure and regulation.
Essential Nutrients
Plants require only light, water, and about 20 elements to support all their biochemical needs: these 20 elements
are called essential nutrients (Table 31.1). For an element to be regarded as essential, three criteria are
required: 1) a plant cannot complete its life cycle without the element; 2) no other element can perform the
function of the element; and 3) the element is directly involved in plant nutrition.
Essential Elements for Plant Growth
Macronutrients
Micronutrients
Carbon (C)
Iron (Fe)
Hydrogen (H)
Manganese (Mn)
Table 31.1
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Chapter 311 Soil and Plant Nutrition
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Essential Elements for Plant Growth
Macronutrients
Micronutrients
Oxygen (O)
Boron (B)
Nitrogen (N)
Molybdenum (Mo)
Phosphorus (P)
Copper (Cu)
Potassium (K)
Zinc (Zn)
Calcium (Ca)
Chlorine (Cl)
Magnesium (Mg)
Nickel (Ni)
Sulfur (S)
Cobalt (Co)
Sodium (Na)
Silicon (Si)
Table 31.1
Macronutrients and Micronutrients
The essential elements can be divided into two groups: macronutrients and micronutrients. Nutrients that plants
require in larger amounts are called macronutrients. About half of the essential elements are considered
macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulfur.
The first of these macronutrients, carbon (C), is required to form carbohydrates, proteins, nucleic acids, and
many other compounds; it is therefore present in all macromolecules. On average, the dry weight (excluding
water) of a cell is 50 percent carbon. As shown in Figure 31.3, carbon is a key part of plant biomolecules.
Cellulose fibers
Cellulose structure
Figure 31.3 Cellulose, the main structural component of the plant cell wall, makes up over thirty percent of plant matter.
It is the most abundant organic compound on earth.
The next most abundant element in plant cells is nitrogen (N); it is part of proteins and nucleic acids. Nitrogen
is also used in the synthesis of some vitamins. Hydrogen and oxygen are macronutrients that are part of many
organic compounds, and also form water. Oxygen is necessary for cellular respiration; plants use oxygen to
store energy in the form of ATP. Phosphorus (P), another macromolecule, is necessary to synthesize nucleic
acids and phospholipids. As part of ATP, phosphorus enables food energy to be converted into chemical
energy through oxidative phosphorylation. Likewise, light energy is converted into chemical energy during
photophosphorylation in photosynthesis, and into chemical energy to be extracted during respiration. Sulfur is
part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. Sulfur also
plays a role in photosynthesis as part of the electron transport chain, where hydrogen gradients play a key role
in the conversion of light energy into ATP. Potassium (K) is important because of its role in regulating stomatal
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Chapter 311 Soil and Plant Nutrition
opening and closing. As the openings for gas exchange, stomata help maintain a healthy water balance; a
potassium ion pump supports this process.
Magnesium (Mg) and calcium (Ca) are also important macronutrients. The role of calcium is twofold: to regulate
nutrient transport, and to support many enzyme functions. Magnesium is important to the photosynthetic
process. These minerals, along with the micronutrients, which are described below, also contribute to the plant’s
ionic balance.
in addition to macronutrients, organisms require various elements in small amounts. These micronutrients, or
trace elements, are present in very small quantities. They include boron (B), chlorine (Cl), manganese (Mn), iron
(Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), silicon (Si), and sodium (Na).
Deficiencies in any of these nutrients—particularly the macronutrients—can adversely affect plant growth
(Figure 31.4). Depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis
(yellowing of the leaves). Extreme deficiencies may result in leaves showing signs of cell death.
LINK
T a
LEARNING
Visit this website (http:// 0 penstaxc 0 llege. 0 rg/l/plant_mineral) to participate in an interactive experiment on
plant nutrient deficiencies. You can adjust the amounts of N, P, K, Ca, Mg, and Fe that plants receive . . . and
see what happens.
(a)
(0)
(a) («>
Figure 31.4 Nutrient deficiency is evident in the symptoms these plants show. This (a) grape tomato suffers from
blossom end rot caused by calcium deficiency. The yellowing in this (b) Frangula alnus results from magnesium
deficiency. Inadequate magnesium also leads to (c) intervenal chlorosis, seen here in a sweetgum leaf. This (d) palm
is affected by potassium deficiency, (credit c: modification of work by Jim Conrad; credit d: modification of work by
Malcolm Manners)
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Chapter 311 Soil and Plant Nutrition
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everyday CONNECTION
Figure 31.5 Plant physiologist Ray Wheeler checks onions being grown using hydroponic techniques. The other
plants are Bibb lettuce (left) and radishes (right). Credit: NASA
Hydroponics
Hydroponics is a method of growing plants in a water-nutrient solution instead of soil. Since its advent,
hydroponics has developed into a growing process that researchers often use. Scientists who are interested
in studying plant nutrient deficiencies can use hydroponics to study the effects of different nutrient
combinations under strictly controlled conditions. Hydroponics has also developed as a way to grow flowers,
vegetables, and other crops in greenhouse environments. You might find hydroponically grown produce
at your local grocery store. Today, many lettuces and tomatoes in your market have been hydroponically
grown.
31.2 | The Soil
By the end of this section, you will be able to do the following:
• Describe how soils are formed
• Explain soil composition
• Describe a soil profile
Plants obtain inorganic elements from the soil, which serves as a natural medium for land plants. Soil is the
outer loose layer that covers the surface of Earth. Soil quality is a major determinant, along with climate, of
plant distribution and growth. Soil quality depends not only on the chemical composition of the soil, but also the
topography (regional surface features) and the presence of living organisms. In agriculture, the history of the
soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.
Soil develops very slowly over long periods of time, and its formation results from natural and environmental
forces acting on mineral, rock, and organic compounds. Soils can be divided into two groups: organic soils are
those that are formed from sedimentation and primarily composed of organic matter, while those that are formed
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Chapter 311 Soil and Plant Nutrition
from the weathering of rocks and are primarily composed of inorganic material are called mineral soils. Mineral
soils are predominant in terrestrial ecosystems, where soils may be covered by water for part of the year or
exposed to the atmosphere.
Soil Composition
Soil consists of these major components (Figure 31.6):
• inorganic mineral matter, about 40 to 45 percent of the soil volume
• organic matter, about 5 percent of the soil volume
• water and air, about 50 percent of the soil volume
The amount of each of the four major components of soil depends on the amount of vegetation, soil compaction,
and water present in the soil. A good healthy soil has sufficient air, water, minerals, and organic material to
promote and sustain plant life.
visual
CONNECTION
Organic matter
(microorganisms
and macroorganisms)
5%
Inorganic
mineral matter
45%
Water
Air
25%
25%
1
1
50%
Figure 31.6 The four major components of soil are shown: inorganic minerals, organic matter, water, and air.
Soil compaction can result when soil is compressed by heavy machinery or even foot traffic. How might this
compaction change the soil composition?
The organic material of soil, called humus, is made up of microorganisms (dead and alive), and dead animals
and plants in varying stages of decay. Humus improves soil structure and provides plants with water and
minerals. The inorganic material of soil consists of rock, slowly broken down into smaller particles that vary in
size. Soil particles that are 0.1 to 2 mm in diameter are sand. Soil particles between 0.002 and 0.1 mm are called
silt, and even smaller particles, less than 0.002 mm in diameter, are called clay. Some soils have no dominant
particle size and contain a mixture of sand, silt, and humus; these soils are called loams.
LINK
T a
LEARNING
Explore this interactive map (http:// 0 penstaxc 0 llege. 0 rg/l/s 0 il_survey_map) from the USDA’s National
Cooperative Soil Survey to access soil data for almost any region in the United States.
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Chapter 311 Soil and Plant Nutrition
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Soil Formation
Soil formation is the consequence of a combination of biological, physical, and chemical processes. Soil should
ideally contain 50 percent solid material and 50 percent pore space. About one-half of the pore space should
contain water, and the other half should contain air. The organic component of soil serves as a cementing agent,
returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for
soil microorganisms. Most soil microorganisms—bacteria, algae, or fungi—are dormant in dry soil, but become
active once moisture is available.
Soil distribution is not homogenous because its formation results in the production of layers; together, the vertical
section of a soil is called the soil profile. Within the soil profile, soil scientists define zones called horizons. A
horizon is a soil layer with distinct physical and chemical properties that differ from those of other layers. Five
factors account for soil formation: parent material, climate, topography, biological factors, and time.
Parent Material
The organic and inorganic material in which soils form is the parent material. Mineral soils form directly from the
weathering of bedrock, the solid rock that lies beneath the soil, and therefore, they have a similar composition to
the original rock. Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials
located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in
rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-
moving river could have sediments of rocks and sand, whereas a slow-moving river could have fine-textured
material, such as clay.
Climate
Temperature, moisture, and wind cause different patterns of weathering and therefore affect soil characteristics.
The presence of moisture and nutrients from weathering will also promote biological activity: a key component
of a quality soil.
Topography
Regional surface features (familiarly called “the lay of the land”) can have a major influence on the characteristics
and fertility of a soil. Topography affects water runoff, which strips away parent material and affects plant growth.
Steeps soils are more prone to erosion and may be thinner than soils that are relatively flat or level.
Biological factors
The presence of living organisms greatly affects soil formation and structure. Animals and microorganisms can
produce pores and crevices, and plant roots can penetrate into crevices to produce more fragmentation. Plant
secretions promote the development of microorganisms around the root, in an area known as the rhizosphere.
Additionally, leaves and other material that fall from plants decompose and contribute to soil composition.
Time
Time is an important factor in soil formation because soils develop over long periods. Soil formation is a dynamic
process. Materials are deposited over time, decompose, and transform into other materials that can be used by
living organisms or deposited onto the surface of the soil.
Physical Properties of the Soil
Soils are named and classified based on their horizons. The soil profile has four distinct layers: 1) O horizon;
2) A horizon; 3) B horizon, or subsoil; and 4) C horizon, or soil base (Figure 31.7). The O horizon has freshly
decomposing organic matter—humus—at its surface, with decomposed vegetation at its base. Humus enriches
the soil with nutrients and enhances soil moisture retention. Topsoil—the top layer of soil—is usually two to
three inches deep, but this depth can vary considerably. For instance, river deltas like the Mississippi River
delta have deep layers of topsoil. Topsoil is rich in organic material; microbial processes occur there, and it
is the “workhorse" of plant production. The A horizon consists of a mixture of organic material with inorganic
products of weathering, and it is therefore the beginning of true mineral soil. This horizon is typically darkly
colored because of the presence of organic matter. In this area, rainwater percolates through the soil and carries
materials from the surface. The B horizon is an accumulation of mostly fine material that has moved downward,
resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium
carbonate. The C horizon, or soil base, includes the parent material, plus the organic and inorganic material
that is broken down to form soil. The parent material may be either created in its natural place, or transported
from elsewhere to its present location. Beneath the C horizon lies bedrock.
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Chapter 311 Soil and Plant Nutrition
visual
CONNECTION
Figure 31.7 This soil profile shows the different soil layers (O horizon, A horizon, B horizon, and C horizon) found
in typical soils, (credit: modification of work by USDA)
Which horizon is considered the topsoil, and which is considered the subsoil?
Some soils may have additional layers, or lack one of these layers. The thickness of the layers is also variable,
and depends on the factors that influence soil formation. In general, immature soils may have O, A, and C
horizons, whereas mature soils may display all of these, plus additional layers (Figure 31.8).
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Chapter 311 Soil and Plant Nutrition
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Ft.
-0 -
- 1 -
-2 -
- 3 -
- 4 -
Figure 31.8 The San Joaquin soil profile has an O horizon, A horizon, B horizon, and C horizon, (credit: modification
of work by USDA)
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Chapter 311 Soil and Plant Nutrition
ca eer connection
Soil Scientist
A soil scientist studies the biological components, physical and chemical properties, distribution, formation,
and morphology of soils. Soil scientists need to have a strong background in physical and life sciences,
plus a foundation in mathematics. They may work for federal or state agencies, academia, or the private
sector. Their work may involve collecting data, carrying out research, interpreting results, inspecting soils,
conducting soil surveys, and recommending soil management programs.
Figure 31.9 This soil scientist is studying the horizons and composition of soil at a research site, (credit: USDA)
Many soil scientists work both in an office and in the field. According to the United States Department
of Agriculture (USDA): “a soil scientist needs good observation skills to analyze and determine the
characteristics of different types of soils. Soil types are complex and the geographical areas a soil scientist
may survey are varied. Aerial photos or various satellite images are often used to research the areas.
Computer skills and geographic information systems (GIS) help the scientist to analyze the multiple facets
[i]
of geomorphology, topography, vegetation, and climate to discover the patterns left on the landscape.”
Soil scientists play a key role in understanding the soil’s past, analyzing present conditions, and making
recommendations for future soil-related practices.
31.3 | Nutritional Adaptations of Plants
By the end of this section, you will be able to do the following:
• Understand the nutritional adaptations of plants
• Describe mycorrhizae
• Explain nitrogen fixation
1 . National Resources Conservation Service / United States Department of Agriculture. “Careers in Soil Science.” http://soils.usda.gov/
education/facts/careers.html (http://0penstax.0rg/l/NRCS)
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Plants obtain food in two different ways. Autotrophic plants can make their own food from inorganic raw
materials, such as carbon dioxide and water, through photosynthesis in the presence of sunlight. Green plants
are included in this group. Some plants, however, are heterotrophic: they are totally parasitic and lacking in
chlorophyll. These plants, referred to as holo-parasitic plants, are unable to synthesize organic carbon and draw
all of their nutrients from the host plant.
Plants may also enlist the help of microbial partners in nutrient acquisition. Particular species of bacteria and
fungi have evolved along with certain plants to create a mutualistic symbiotic relationship with roots. This
improves the nutrition of both the plant and the microbe. The formation of nodules in legume plants and
mycorrhization can be considered among the nutritional adaptations of plants. However, these are not the only
type of adaptations that we may find; many plants have other adaptations that allow them to thrive under specific
conditions.
LINK TQ LEARNING
This video (http:// 0 penstaxc 0 llege. 0 rg/l/basic_ph 0 t 0 syn) reviews basic concepts about photosynthesis. In
the left panel, click each tab to select a topic for review.
Nitrogen Fixation: Root and Bacteria Interactions
Nitrogen is an important macronutrient because it is part of nucleic acids and proteins. Atmospheric nitrogen,
which is the diatomic molecule N 2 , or dinitrogen, is the largest pool of nitrogen in terrestrial ecosystems.
However, plants cannot take advantage of this nitrogen because they do not have the necessary enzymes to
convert it into biologically useful forms. However, nitrogen can be “fixed," which means that it can be converted
to ammonia (NH 3 ) through biological, physical, or chemical processes. As you have learned, biological nitrogen
fixation (BNF) is the conversion of atmospheric nitrogen (N 2 ) into ammonia (NH 3 ), exclusively carried out by
prokaryotes such as soil bacteria or cyanobacteria. Biological processes contribute 65 percent of the nitrogen
used in agriculture. The following equation represents the process:
N 2 + 16 ATP + 8 e“ + 8 H + -> 2NH 3 + 16 ADP + 16 Pi + H 2
The most important source of BNF is the symbiotic interaction between soil bacteria and legume plants, including
many crops important to humans (Figure 31.10). The NH 3 resulting from fixation can be transported into plant
tissue and incorporated into amino acids, which are then made into plant proteins. Some legume seeds, such as
soybeans and peanuts, contain high levels of protein, and serve among the most important agricultural sources
of protein in the world.
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Chapter 311 Soil and Plant Nutrition
visual
a CONNECTION
(a) (b) (c)
Figure 31.10 Some common edible legumes—like (a) peanuts, (b) beans, and (c) chickpeas—are able to
interact symbiotically with soil bacteria that fix nitrogen, (credit a: modification of work by Jules Clancy; credit b:
modification of work by USDA)
Farmers often rotate corn (a cereal crop) and soy beans (a legume), planting a field with each crop in
alternate seasons. What advantage might this crop rotation confer?
Soil bacteria, collectively called rhizobia, symbiotically interact with legume roots to form specialized structures
called nodules, in which nitrogen fixation takes place. This process entails the reduction of atmospheric
nitrogen to ammonia, by means of the enzyme nitrogenase. Therefore, using rhizobia is a natural and
environmentally friendly way to fertilize plants, as opposed to chemical fertilization that uses a nonrenewable
resource, such as natural gas. Through symbiotic nitrogen fixation, the plant benefits from using an endless
source of nitrogen from the atmosphere. The process simultaneously contributes to soil fertility because the
plant root system leaves behind some of the biologically available nitrogen. As in any symbiosis, both organisms
benefit from the interaction: the plant obtains ammonia, and bacteria obtain carbon compounds generated
through photosynthesis, as well as a protected niche in which to grow (Figure 31.11).
Rhizobia inside vesicles
Figure 31.11 Soybean roots contain (a) nitrogen-fixing nodules. Cells within the nodules are infected with
Bradyrhyzobium japonicum, a rhizobia or “root-loving” bacterium. The bacteria are encased in (b) vesicles inside
the cell, as can be seen in this transmission electron micrograph, (credit a: modification of work by USDA; credit b:
modification of work by Louisa Howard, Dartmouth Electron Microscope Facility; scale-bar data from Matt Russell)
Mycorrhizae: The Symbiotic Relationship between Fungi and Roots
A nutrient depletion zone can develop when there is rapid soil solution uptake, low nutrient concentration, low
diffusion rate, or low soil moisture. These conditions are very common; therefore, most plants rely on fungi to
facilitate the uptake of minerals from the soil. Fungi form symbiotic associations called mycorrhizae with plant
roots, in which the fungi actually are integrated into the physical structure of the root. The fungi colonize the living
root tissue during active plant growth.
Through mycorrhization, the plant obtains mainly phosphate and other minerals, such as zinc and copper, from
the soil. The fungus obtains nutrients, such as sugars, from the plant root (Figure 31.12). Mycorrhizae help
increase the surface area of the plant root system because hyphae, which are narrow, can spread beyond the
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nutrient depletion zone. Hyphae can grow into small soil pores that allow access to phosphorus that would
otherwise be unavailable to the plant. The beneficial effect on the plant is best observed in poor soils. The benefit
to fungi is that they can obtain up to 20 percent of the total carbon accessed by plants. Mycorrhizae functions
as a physical barrier to pathogens. It also provides an induction of generalized host defense mechanisms, and
sometimes involves production of antibiotic compounds by the fungi.
Figure 31.12 Root tips proliferate in the presence of mycorrhizal infection, which appears as off-white fuzz in this
image, (credit: modification of work by Nilsson et al., BMC Bioinformatics 2005)
There are two types of mycorrhizae: ectomycorrhizae and endomycorrhizae. Ectomycorrhizae form an extensive
dense sheath around the roots, called a mantle. Hyphae from the fungi extend from the mantle into the soil,
which increases the surface area for water and mineral absorption. This type of mycorrhizae is found in forest
trees, especially conifers, birches, and oaks. Endomycorrhizae, also called arbuscular mycorrhizae, do not form
a dense sheath over the root. Instead, the fungal mycelium is embedded within the root tissue. Endomycorrhizae
are found in the roots of more than 80 percent of terrestrial plants.
Nutrients from Other Sources
Some plants cannot produce their own food and must obtain their nutrition from outside sources. This may occur
with plants that are parasitic or saprophytic. Some plants are mutualistic symbionts, epiphytes, or insectivorous.
Plant Parasites
A parasitic plant depends on its host for survival. Some parasitic plants have no leaves. An example of this
is the dodder (Figure 31.13), which has a weak, cylindrical stem that coils around the host and forms suckers.
From these suckers, cells invade the host stem and grow to connect with the vascular bundles of the host.
The parasitic plant obtains water and nutrients through these connections. The plant is a total parasite (a
holoparasite) because it is completely dependent on its host. Other parasitic plants (hemiparasites) are fully
photosynthetic and only use the host for water and minerals. There are about 4,100 species of parasitic plants.
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Figure 31.13 The dodder is a holoparasite that penetrates the host’s vascular tissue and diverts nutrients for its own
growth. Note that the vines of the dodder, which has white flowers, are beige. The dodder has no chlorophyll and
cannot produce its own food, (credit: "Lalithamba"/Flickr)
Saprophytes
A saprophyte is a plant that does not have chlorophyll and gets its food from dead matter, similar to bacteria and
fungi (note that fungi are often called saprophytes, which is incorrect, because fungi are not plants). Plants like
these use enzymes to convert organic food materials into simpler forms from which they can absorb nutrients
(Figure 31.14). Most saprophytes do not directly digest dead matter: instead, they parasitize fungi that digest
dead matter, or are mycorrhizal, ultimately obtaining photosynthate from a fungus that derived photosynthate
from its host. Saprophytic plants are uncommon; only a few species are described.
Figure 31.14 Saprophytes, like this Dutchmen’s pipe (Monotropa hypopitys), obtain their food from dead matter and
do not have chlorophyll, (credit: modification of work by Iwona Erskine-Kellie)
Symbionts
A symbiont is a plant in a symbiotic relationship, with special adaptations such as mycorrhizae or nodule
formation. Fungi also form symbiotic associations with cyanobacteria and green algae (called lichens). Lichens
can sometimes be seen as colorful growths on the surface of rocks and trees (Figure 31.15). The algal partner
(phycobiont) makes food autotrophically, some of which it shares with the fungus; the fungal partner (mycobiont)
absorbs water and minerals from the environment, which are made available to the green alga. If one partner
was separated from the other, they would both die.
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Figure 31.15 Lichens, which often have symbiotic relationships with other plants, can sometimes be found growing on
trees, (credit: "benketaro'VFlickr)
Epiphytes
An epiphyte is a plant that grows on other plants, but is not dependent upon the other plant for nutrition
(Figure 31.16). Epiphytes have two types of roots: clinging aerial roots, which absorb nutrients from humus that
accumulates in the crevices of trees; and aerial roots, which absorb moisture from the atmosphere.
Figure 31.16 These epiphyte plants grow in the main greenhouse of the Jardin des Plantes in Paris.
Insectivorous Plants
An insectivorous plant has specialized leaves to attract and digest insects. The Venus flytrap is popularly
known for its insectivorous mode of nutrition, and has leaves that work as traps (Figure 31.17). The minerals it
obtains from prey compensate for those lacking in the boggy (low pH) soil of its native North Carolina coastal
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Chapter 311 Soil and Plant Nutrition
plains. There are three sensitive hairs in the center of each half of each leaf. The edges of each leaf are covered
with long spines. Nectar secreted by the plant attracts flies to the leaf. When a fly touches the sensory hairs, the
leaf immediately closes. Next, fluids and enzymes break down the prey and minerals are absorbed by the leaf.
Since this plant is popular in the horticultural trade, it is threatened in its original habitat.
Figure 31.17 A Venus flytrap has specialized leaves to trap insects, (credit: "Selena N. B. H.'VFlickr)
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KEY TERMS
A horizon consists of a mixture of organic material with inorganic products of weathering
B horizon soil layer that is an accumulation of mostly fine material that has moved downward
bedrock solid rock that lies beneath the soil
C horizon layer of soil that contains the parent material, and the organic and inorganic material that is broken
down to form soil; also known as the soil base
clay soil particles that are less than 0.002 mm in diameter
epiphyte plant that grows on other plants but is not dependent upon other plants for nutrition
horizon soil layer with distinct physical and chemical properties, which differs from other layers depending on
how and when it was formed
humus organic material of soil; made up of microorganisms, dead animals, and plants in varying stages of
decay
inorganic compound chemical compound that does not contain carbon; it is not part of or produced by a living
organism
insectivorous plant plant that has specialized leaves to attract and digest insects
loam soil that has no dominant particle size
macronutrient nutrient that is required in large amounts for plant growth; carbon, hydrogen, oxygen, nitrogen,
phosphorus, potassium, calcium, magnesium, and sulfur
micronutrient nutrient required in small amounts; also called trace element
mineral soil type of soil that is formed from the weathering of rocks and inorganic material; composed primarily
of sand, silt, and clay
nitrogenase enzyme that is responsible for the reduction of atmospheric nitrogen to ammonia
nodules specialized structures that contain Rhizobia bacteria where nitrogen fixation takes place
O horizon layer of soil with humus at the surface and decomposed vegetation at the base
organic compound chemical compound that contains carbon
organic soil type of soil that is formed from sedimentation; composed primarily of organic material
parasitic plant plant that is dependent on its host for survival
parent material organic and inorganic material in which soils form
rhizobia soil bacteria that symbiotically interact with legume roots to form nodules and fix nitrogen
rhizosphere area of soil affected by root secretions and microorganisms
sand soil particles between 0.1-2 mm in diameter
saprophyte plant that does not have chlorophyll and gets its food from dead matter
silt soil particles between 0.002 and 0.1 mm in diameter
soil outer loose layer that covers the surface of Earth
soil profile vertical section of a soil
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symbiont plant in a symbiotic relationship with bacteria or fungi
CHAPTER SUMMARY
31.1 Nutritional Requirements of Plants
Plants can absorb inorganic nutrients and water through their root system, and carbon dioxide from the
environment. The combination of organic compounds, along with water, carbon dioxide, and sunlight, produce
the energy that allows plants to grow. Inorganic compounds form the majority of the soil solution. Plants access
water though the soil. Water is absorbed by the plant root, transports nutrients throughout the plant, and
maintains the structure of the plant. Essential elements are indispensable elements for plant growth. They are
divided into macronutrients and micronutrients. The macronutrients plants require are carbon, nitrogen,
hydrogen, oxygen, phosphorus, potassium, calcium, magnesium, and sulfur. Important micronutrients include
iron, manganese, boron, molybdenum, copper, zinc, chlorine, nickel, cobalt, silicon, and sodium.
31.2 The Soil
Plants obtain mineral nutrients from the soil. Soil is the outer loose layer that covers the surface of Earth. Soil
quality depends on the chemical composition of the soil, the topography, the presence of living organisms, the
climate, and time. Agricultural practice and history may also modify the characteristics and fertility of soil. Soil
consists of four major components: 1) inorganic mineral matter, 2) organic matter, 3) water and air, and 4) living
matter. The organic material of soil is made of humus, which improves soil structure and provides water and
minerals. Soil inorganic material consists of rock slowly broken down into smaller particles that vary in size,
such as sand, silt, and loam.
Soil formation results from a combination of biological, physical, and chemical processes. Soil is not
homogenous because its formation results in the production of layers called a soil profile. Factors that affect
soil formation include: parent material, climate, topography, biological factors, and time. Soils are classified
based on their horizons, soil particle size, and proportions. Most soils have four distinct horizons: O, A, B, and
C.
31.3 Nutritional Adaptations of Plants
Atmospheric nitrogen is the largest pool of available nitrogen in terrestrial ecosystems. However, plants cannot
use this nitrogen because they do not have the necessary enzymes. Biological nitrogen fixation (BNF) is the
conversion of atmospheric nitrogen to ammonia. The most important source of BNF is the symbiotic interaction
between soil bacteria and legumes. The bacteria form nodules on the legume’s roots in which nitrogen fixation
takes place. Fungi form symbiotic associations (mycorrhizae) with plants, becoming integrated into the physical
structure of the root. Through mycorrhization, the plant obtains minerals from the soil and the fungus obtains
photosynthate from the plant root. Ectomycorrhizae form an extensive dense sheath around the root, while
endomycorrhizae are embedded within the root tissue. Some plants—parasites, saprophytes, symbionts,
epiphytes, and insectivores—have evolved adaptations to obtain their organic or mineral nutrition from various
sources.
VISUAL CONNECTION QUESTIONS
1. Figure 31.6 Soil compaction can result when soil
is compressed by heavy machinery or even foot
traffic. How might this compaction change the soil
composition?
2. Figure 31.7 Which horizon is considered the
REVIEW QUESTIONS
4. For an element to be regarded as essential, all of
the following criteria must be met, except:
topsoil, and which is considered the subsoil?
3. Figure 31.10 Farmers often rotate corn (a cereal
crop) and soy beans (a legume) planting a field with
each crop in alternate seasons. What advantage
might this crop rotation confer?
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a. No other element can perform the function.
b. The element is directly involved in plant
nutrition.
c. The element is inorganic.
d. The plant cannot complete its lifecycle
without the element.
5. The nutrient that is part of carbohydrates, proteins,
and nucleic acids, and that forms biomolecules, is
a. nitrogen
b. carbon
c. magnesium
d. iron
6. Most_are necessary for enzyme
function.
a. micronutrients
b. macronutrients
c. biomolecules
d. essential nutrients
7. What is the main water source for land plants?
a. rain
b. soil
c. biomolecules
d. essential nutrients
8. Which factors affect soil quality?
a. chemical composition
b. history of the soil
c. presence of living organisms and
topography
d. all of the above
9. Soil particles that are 0.1 to 2 mm in diameter are
called_.
a. sand
b. silt
c. clay
d. loam
CRITICAL THINKING QUESTIONS
16. What type of plant problems result from nitrogen
and calcium deficiencies?
17. Research the life of Jan Babtistavan Helmont.
What did the van Helmont experiment show?
18. List two essential macronutrients and two
essential micro nutrients.
19. Describe the main differences between a mineral
soil and an organic soil.
20. Name and briefly explain the factors that affect
soil formation.
10. A soil consists of layers called_that
taken together are called a_.
a. soil profiles : horizon
b. horizons : soil profile
c. horizons : humus
d. humus : soil profile
11. What is the term used to describe the solid rock
that lies beneath the soil?
a. sand
b. bedrock
c. clay
d. loam
12. Which process produces an inorganic compound
that plants can easily use?
a. photosynthesis
b. nitrogen fixation
c. mycorrhization
d. Calvin cycle
13. Through mycorrhization, a plant obtains important
nutrients such as_.
a. phosphorus, zinc, and copper
b. phosphorus, zinc, and calcium
c. nickel, calcium, and zinc
d. all of the above
14. What term describes a plant that requires
nutrition from a living host plant?
a. parasite
b. saprophyte
c. epiphyte
d. insectivorous
15. What is the term for the symbiotic association
between fungi and cyanobacteria?
a. lichen
b. mycorrhizae
c. epiphyte
d. nitrogen-fixing nodule
21. Describe how topography influences the
characteristics and fertility of a soil.
22. Why is biological nitrogen fixation an
environmentally friendly way of fertilizing plants?
23. What is the main difference, from an energy point
of view, between photosynthesis and biological
nitrogen fixation?
24. Why is a root nodule a nutritional adaptation of a
plant?
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Chapter 311 Soil and Plant Nutrition
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Chapter 32 | Plant Reproduction
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32 | PLANT
REPRODUCTION
(a) (b) (c)
Figure 32.1 Plants that reproduce sexually often achieve fertilization with the help of pollinators such as (a) bees, (b)
birds, and (c) butterflies, (credit a: modification of work by John Severns; credit b: modification of work by Charles J.
Sharp; credit c: modification of work by "Galawebdesign"/Flickr)
Chapter Outline
32.1: Reproductive Development and Structure
32.2: Pollination and Fertilization
32.3: Asexual Reproduction
Introduction
Plants have evolved different reproductive strategies for the continuation of their species. Some plants reproduce
sexually, and others asexually, in contrast to animal species, which rely almost exclusively on sexual
reproduction. Plant sexual reproduction usually depends on pollinating agents, while asexual reproduction is
independent of these agents. Flowers are often the showiest or most strongly scented part of plants. With their
bright colors, fragrances, and interesting shapes and sizes, flowers attract insects, birds, and animals to serve
their pollination needs. Other plants pollinate via wind or water; still others self-pollinate.
32.1 1 Reproductive Development and Structure
By the end of this section, you will be able to do the following:
• Describe the two stages of a plant’s lifecycle
• Compare and contrast male and female gametophytes and explain how they form in angiosperms
• Describe the reproductive structures of a plant
• Describe the components of a complete flower
• Describe the development of microsporangium and megasporangium in gymnosperms
Sexual reproduction takes place with slight variations in different groups of plants. Plants have two distinct
stages in their lifecycle: the gametophyte stage and the sporophyte stage. The haploid gametophyte produces
the male and female gametes by mitosis in distinct multicellular structures. Fusion of the male and females
gametes forms the diploid zygote, which develops into the sporophyte. After reaching maturity, the diploid
sporophyte produces spores by meiosis, which in turn divide by mitosis to produce the haploid gametophyte.
The new gametophyte produces gametes, and the cycle continues. This is the alternation of generations, and is
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Chapter 32 | Plant Reproduction
typical of plant reproduction (Figure 32.2).
Microsporophyte Megasporophyte
Figure 32.2 The alternation of generations in angiosperms is depicted in this diagram, (credit: modification of work by
Peter Coxhead)
The life cycle of higher plants is dominated by the sporophyte stage, with the gametophyte borne on the
sporophyte. in ferns, the gametophyte is free-living and very distinct in structure from the diploid sporophyte. In
bryophytes, such as mosses, the haploid gametophyte is more developed than the sporophyte.
During the vegetative phase of growth, plants increase in size and produce a shoot system and a root system.
As they enter the reproductive phase, some of the branches start to bear flowers. Many flowers are borne singly,
whereas some are borne in clusters. The flower is borne on a stalk known as a receptacle. Flower shape, color,
and size are unique to each species, and are often used by taxonomists to classify plants.
Sexual Reproduction in Angiosperms
The lifecycle of angiosperms follows the alternation of generations explained previously. The haploid
gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms.
Flowers contain the plant’s reproductive structures.
Flower Structure
A typical flower has four main parts—or whorls—known as the calyx, corolla, androecium, and gynoecium
(Figure 32.3). The outermost whorl of the flower has green, leafy structures known as sepals. The sepals,
collectively called the calyx, help to protect the unopened bud. The second whorl is comprised of petals—usually,
brightly colored—collectively called the corolla. The number of sepals and petals varies depending on whether
the plant is a monocot or dicot. In monocots, petals usually number three or multiples of three; in dicots, the
number of petals is four or five, or multiples of four and five. Together, the calyx and corolla are known as
the perianth. The third whorl contains the male reproductive structures and is known as the androecium. The
androecium has stamens with anthers that contain the microsporangia. The innermost group of structures in
the flower is the gynoecium, or the female reproductive component(s). The carpel is the individual unit of the
gynoecium and has a stigma, style, and ovary. A flower may have one or multiple carpels.
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Chapter 32 | Plant Reproduction
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visual
CONNECTION
Androecium
Figure 32.3 The four main parts of the flower are the calyx, corolla, androecium, and gynoecium. The androecium
is the sum of all the male reproductive organs, and the gynoecium is the sum of the female reproductive organs,
(credit: modification of work by Mariana Ruiz Villareal)
If the anther is missing, what type of reproductive structure will the flower be unable to produce? What term
is used to describe an incomplete flower lacking the androecium? What term describes an incomplete flower
lacking a gynoecium?
If all four whorls (the calyx, corolla, androecium, and gynoecium) are present, the flower is described as
complete. If any of the four parts is missing, the flower is known as incomplete. Flowers that contain both
an androecium and a gynoecium are called perfect, androgynous or hermaphrodites. There are two types
of incomplete flowers: staminate flowers contain only an androecium, and carpellate flowers have only a
gynoecium (Figure 32.4).
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Stem
and root kernels (stigma)
Figure 32.4 The corn plant has both staminate (male) and carpellate (female) flowers. Staminate flowers, which are
clustered in the tassel at the tip of the stem, produce pollen grains. Carpellate flowers are clustered in the immature
ears. Each strand of silk is a stigma. The corn kernels are seeds that develop on the ear after fertilization. Also shown
is the lower stem and root.
Staminate Carpellate
flowers flowers
If both male and female flowers are borne on the same plant, the species is called monoecious (meaning “one
home”): examples are com and pea. Species with male and female flowers borne on separate plants are termed
dioecious, or “two homes,” examples of which are C. papaya and Cannabis. The ovary, which may contain one
or multiple ovules, may be placed above other flower parts, which is referred to as superior; or, it may be placed
below the other flower parts, referred to as inferior (Figure 32.5).
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(a) Superior flower (b) Inferior flower
Figure 32.5 The (a) lily is a superior flower, which has the ovary above the other flower parts, (b) Fuchsia is an inferior
flower, which has the ovary beneath other flower parts, (credit a photo: modification of work by Benjamin Zwittnig;
credit b photo: modification of work by "Koshy Koshy'VFlickr)
Male Gametophyte (The Pollen Grain)
The male gametophyte develops and reaches maturity in an immature anther. In a plant’s male reproductive
organs, development of pollen takes place in a structure known as the microsporangium (Figure 32.6). The
microsporangia, which are usually bilobed, are pollen sacs in which the microspores develop into pollen grains.
These are found in the anther, which is at the end of the stamen—the long filament that supports the anther.
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Figure 32.6 Shown is (a) a cross section of an anther at two developmental stages. The immature anther (top) contains
four microsporangia, or pollen sacs. Each microsporangium contains hundreds of microspore mother cells that will
each give rise to four pollen grains. The tapetum supports the development and maturation of the pollen grains. Upon
maturation of the pollen (bottom), the pollen sac walls split open and the pollen grains (male gametophytes) are
released, as shown in the (b) micrograph of an immature lily anther. In these scanning electron micrographs, pollen
sacs are ready to burst, releasing their grains, (credit a: modification of work by LibreTexts; b: modification of work by
Robert R. Wise; scale-bar data from Matt Russell)
Within the microsporangium, the microspore mother cell divides by meiosis to give rise to four microspores, each
of which will ultimately form a pollen grain (Figure 32.7). An inner layer of cells, known as the tapetum, provides
nutrition to the developing microspores and contributes key components to the pollen wall. Mature pollen grains
contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen
tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to
enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes
(sperm cells). Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.
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Chapter 32 | Plant Reproduction
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Figure 32.7 Pollen develops from the microspore mother cells. The mature pollen grain is composed of two cells: the
pollen tube cell and the generative cell, which is inside the tube cell. The pollen grain has two coverings: an inner layer
(intine) and an outer layer (exine). The inset scanning electron micrograph shows Arabidopsis lyrata pollen grains,
(credit “pollen micrograph”: modification of work by Robert R. Wise; scale-bar data from Matt Russell)
Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine (Figure 32.7). The exine
contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. Sporopollenin allows
the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without
undergoing damage.
Female Gametophyte (The Embryo Sac)
While the details may vary between species, the overall development of the female gametophyte has two distinct
phases. First, in the process of megasporogenesis, a single cell in the diploid megasporangium —an area
of tissue in the ovules—undergoes meiosis to produce four megaspores, only one of which survives. During
the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis to produce an
eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac. Two of
the nuclei—the polar nuclei —move to the equator and fuse, forming a single, diploid central cell. This central
cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the
embryo sac opposite the micropyle and develop into the antipodal cells, which later degenerate. The nucleus
closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into
synergid cells (Figure 32.8). The synergids help guide the pollen tube for successful fertilization, after which
they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the
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Chapter 32 | Plant Reproduction
fertilized ovule forms the other tissues of the seed.
A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will
develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the
fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening
called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.
visual
CONNECTION
Embryo Sac
Micropylar end
Figure 32.8 As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments
and has an opening called a micropyle. Inside the embryo sac are three antipodal cells, two synergids, a central
cell, and the egg cell.
An embryo sac is missing the synergids. What specific impact would you expect this to have on fertilization?
a. The pollen tube will be unable to form.
b. The pollen tube will form but will not be guided toward the egg.
c. Fertilization will not occur because the synergid is the egg.
d. Fertilization will occur but the embryo will not be able to grow.
Sexual Reproduction in Gymnosperms
As with angiosperms, the lifecycle of a gymnosperm is also characterized by alternation of generations, in
conifers such as pines, the green leafy part of the plant is the sporophyte, and the cones contain the male
and female gametophytes (Figure 32.9). The female cones are larger than the male cones and are positioned
towards the top of the tree; the small, male cones are located in the lower region of the tree. Because the pollen
is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.
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Chapter 32 | Plant Reproduction
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Sporophyte (2n)
(mature tree)
Female cones grow in the
upper branches where they
may be fertilized by pollen
blown on the wind from the
male cones.
Male cones grow in the
lower branches.
Seeds are dispersed
and grow into
mature trees.
A pollen tube forms, allowing the pollen to
migrate toward the female gametophyte.
Upon fertilization, a diploid zygote forms.
Female cone
Male cone
Figure 32.9 This image shows the lifecycle of a conifer. Pollen from male cones blows up into upper branches, where
it fertilizes female cones. Examples are shown of female and male cones, (credit “female": modification of work by
“Geographer'VWikimedia Commons; credit “male”: modification of work by Roger Griffith)
Male Gametophyte
A male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts are known as
microsporophylls (Figure 32.10) and are the sites where microspores will develop. The microspores develop
inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis
to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative
nucleus, and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones
and is carried by the wind to land on the female cone.
LINK TQ LEARNING
Watch this video to see a cedar releasing its pollen in the wind. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66607/l.3/#eip-idll68023726478)
Female Gametophyte
The female cone also has a central axis on which bracts known as megasporophylls (Figure 32.10) are
present. In the female cone, megaspore mother cells are present in the megasporangium. The megaspore
mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form
the multicellular female gametophyte, while the others divide to form the rest of the structure. The female
gametophyte is contained within a structure called the archegonium.
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Chapter 32 | Plant Reproduction
(d) (e) (f)
Figure 32.10 This series of micrographs shows male and female gymnosperm gametophytes. (a) This male cone,
shown in cross section, has approximately 20 microsporophylls, each of which produces hundreds of male
gametophytes (pollen grains), (b) Pollen grains are visible in this single microsporophyll. (c) This micrograph shows an
individual pollen grain, (d) This cross section of a female cone shows portions of about 15 megasporophylls. (e) The
ovule can be seen in this single megasporophyll. (f) Within this single ovule are the megaspore mother cell (MMC),
micropyle, and a pollen grain, (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)
Reproductive Process
Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative
cell migrates towards the female gametophyte through the micropyle. It takes approximately one year for
the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte containing the
generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates. After
fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of
the cones are closed during development of the seed. The seed is covered by a seed coat, which is derived
from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be
dispersed, the bracts of the female cones open to allow the dispersal of seed; no fruit formation takes place
because gymnosperm seeds have no covering.
Angiosperms versus Gymnosperms
Gymnosperm reproduction differs from that of angiosperms in several ways (Figure 32.11). In angiosperms, the
female gametophyte exists in an enclosed structure—the ovule—which is within the ovary; in gymnosperms,
the female gametophyte is present on exposed bracts of the female cone. Double fertilization is a key event
in the lifecycle of angiosperms, but is completely absent in gymnosperms. The male and female gametophyte
structures are present on separate male and female cones in gymnosperms, whereas in angiosperms, they are
a part of the flower. Lastly, wind plays an important role in pollination in gymnosperms because pollen is blown by
the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination
is more common.
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Figure 32.11 (a) Angiosperms are flowering plants, and include grasses, herbs, shrubs and most deciduous trees,
while (b) gymnosperms are conifers. Both produce seeds but have different reproductive strategies, (credit a:
modification of work by Wendy Cutler; credit b: modification of work by Lews Castle UHI)
LINK
T &
LEARNING
View an animation of the double fertilization process of angiosperms. (This multimedia resource will open
in a browser.) (http://cnx.Org/content/m66607/l.3/#eip-id4418315)
32.2 | Pollination and Fertilization
By the end of this section, you will be able to do the following:
• Describe what must occur for plant fertilization
• Explain cross-pollination and the ways in which it takes place
• Describe the process that leads to the development of a seed
• Define double fertilization
in angiosperms, pollination is defined as the placement or transfer of pollen from the anther to the stigma of the
same flower or another flower. In gymnosperms, pollination involves pollen transfer from the male cone to the
female cone. Upon transfer, the pollen germinates to form the pollen tube and the sperm for fertilizing the egg.
Pollination has been well studied since the time of Gregor Mendel. Mendel successfully carried out self- as well
as cross-pollination in garden peas while studying how characteristics were passed on from one generation to
the next. Today’s crops are a result of plant breeding, which employs artificial selection to produce the present-
day cultivars. A case in point is today's corn, which is a result of years of breeding that started with its ancestor,
teosinte. The teosinte that the ancient Mayans originally began cultivating had tiny seeds—vastly different from
today’s relatively giant ears of corn. Interestingly, though these two plants appear to be entirely different, the
genetic difference between them is miniscule.
Pollination takes two forms: self-pollination and cross-pollination. Self-pollination occurs when the pollen from
the anther is deposited on the stigma of the same flower, or another flower on the same plant. Cross-pollination
is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of
the same species. Self-pollination occurs in flowers where the stamen and carpel mature at the same time, and
are positioned so that the pollen can land on the flower’s stigma. This method of pollination does not require an
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investment from the plant to provide nectar and pollen as food for pollinators.
LINK
T a
LEARNING
Explore this interactive website (http:// 0 penstaxc 0 llege. 0 rg/l/p 0 llinati 0 n) to review self-pollination and
cross-pollination.
Living species are designed to ensure survival of their progeny; those that fail become extinct. Genetic diversity
is therefore required so that in changing environmental or stress conditions, some of the progeny can survive.
Self-pollination leads to the production of plants with less genetic diversity, since genetic material from the same
plant is used to form gametes, and eventually, the zygote. In contrast, cross-pollination—or out-crossing—leads
to greater genetic diversity because the microgametophyte and megagametophyte are derived from different
plants.
Because cross-pollination allows for more genetic diversity, plants have developed many ways to avoid self-
pollination. In some species, the pollen and the ovary mature at different times. These flowers make self-
pollination nearly impossible. By the time pollen matures and has been shed, the stigma of this flower is mature
and can only be pollinated by pollen from another flower. Some flowers have developed physical features
that prevent self-pollination. The primrose is one such flower. Primroses have evolved two flower types with
differences in anther and stigma length: the pin-eyed flower has anthers positioned at the pollen tube’s halfway
point, and the thrum-eyed flower’s stigma is likewise located at the halfway point. Insects easily cross-pollinate
while seeking the nectar at the bottom of the pollen tube. This phenomenon is also known as heterostyly. Many
plants, such as cucumber, have male and female flowers located on different parts of the plant, thus making self-
pollination difficult. In yet other species, the male and female flowers are borne on different plants (dioecious). All
of these are barriers to self-pollination; therefore, the plants depend on pollinators to transfer pollen. The majority
of pollinators are biotic agents such as insects (like bees, flies, and butterflies), bats, birds, and other animals.
Other plant species are pollinated by abiotic agents, such as wind and water.
everyday CONNECTION
Incompatibility Genes in Flowers
In recent decades, incompatibility genes—which prevent pollen from germinating or growing into the stigma
of a flower—have been discovered in many angiosperm species. If plants do not have compatible genes,
the pollen tube stops growing. Self-incompatibility is controlled by the S (sterility) locus. Pollen tubes have
to grow through the tissue of the stigma and style before they can enter the ovule. The carpel is selective
in the type of pollen it allows to grow inside. The interaction is primarily between the pollen and the stigma
epidermal cells. In some plants, like cabbage, the pollen is rejected at the surface of the stigma, and the
unwanted pollen does not germinate. In other plants, pollen tube germination is arrested after growing
one-third the length of the style, leading to pollen tube death. Pollen tube death is due either to apoptosis
(programmed cell death) or to degradation of pollen tube RNA. The degradation results from the activity
of a ribonuclease encoded by the S locus. The ribonuclease is secreted from the cells of the style in the
extracellular matrix, which lies alongside the growing pollen tube.
In summary, self-incompatibility is a mechanism that prevents self-fertilization in many flowering plant
species. The working of this self-incompatibility mechanism has important consequences for plant breeders
because it inhibits the production of inbred and hybrid plants.
Pollination by Insects
Bees are perhaps the most important pollinator of many garden plants and most commercial fruit trees (Figure
32.12). The most common species of bees are bumblebees and honeybees. Since bees cannot see the color
red, bee-pollinated flowers usually have shades of blue, yellow, or other colors. Bees collect energy-rich pollen
or nectar for their survival and energy needs. They visit flowers that are open during the day, are brightly colored,
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have a strong aroma or scent, and have a tubular shape, typically with the presence of a nectar guide. A
nectar guide includes regions on the flower petals that are visible only to bees, and not to humans; it helps
to guide bees to the center of the flower, thus making the pollination process more efficient. The pollen sticks
to the bees’ fuzzy hair, and when the bee visits another flower, some of the pollen is transferred to the second
flower. Recently, there have been many reports about the declining population of honeybees. Many flowers will
remain unpollinated and not bear seed if honeybees disappear. The impact on commercial fruit growers could
be devastating.
Figure 32.12 Insects, such as bees, are important agents of pollination, (credit: modification of work by Jon Sullivan)
Many flies are attracted to flowers that have a decaying smell or an odor of rotting flesh. These flowers, which
produce nectar, usually have dull colors, such as brown or purple. They are found on the corpse flower or voodoo
lily ( Amorphophallus ), dragon arum ( Dracunculus ), and carrion flower ( Stapleia , Rafflesia). The nectar provides
energy, whereas the pollen provides protein. Wasps are also important insect pollinators, and pollinate many
species of figs.
Butterflies, such as the monarch, pollinate many garden flowers and wildflowers, which usually occur in clusters.
These flowers are brightly colored, have a strong fragrance, are open during the day, and have nectar guides to
make access to nectar easier. The pollen is picked up and carried on the butterfly’s limbs. Moths, on the other
hand, pollinate flowers during the late afternoon and night. The flowers pollinated by moths are pale or white
and are flat, enabling the moths to land. One well-studied example of a moth-pollinated plant is the yucca plant,
which is pollinated by the yucca moth. The shape of the flower and moth have adapted in such a way as to allow
successful pollination. The moth deposits pollen on the sticky stigma for fertilization to occur later. The female
moth also deposits eggs into the ovary. As the eggs develop into larvae, they obtain food from the flower and
developing seeds. Thus, both the insect and flower benefit from each other in this symbiotic relationship. The
corn earworm moth and Gaura plant have a similar relationship (Figure 32.13).
Figure 32.13 A corn earworm sips nectar from a night-blooming Gaura plant, (credit: Juan Lopez, USDA ARS)
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Pollination by Bats
in the tropics and deserts, bats are often the pollinators of nocturnal flowers such as agave, guava, and morning
glory. The flowers are usually large and white or pale-colored; thus, they can be distinguished from the dark
surroundings at night. The flowers have a strong, fruity, or musky fragrance and produce large amounts of nectar.
They are naturally large and wide-mouthed to accommodate the head of the bat. As the bats seek the nectar,
their faces and heads become covered with pollen, which is then transferred to the next flower.
Pollination by Birds
Many species of small birds, such as the hummingbird (Figure 32.14) and sun birds, are pollinators for plants
such as orchids and other wildflowers. Flowers visited by birds are usually sturdy and are oriented in such a way
as to allow the birds to stay near the flower without getting their wings entangled in the nearby flowers. The flower
typically has a curved, tubular shape, which allows access for the bird’s beak. Brightly colored, odorless flowers
that are open during the day are pollinated by birds. As a bird seeks energy-rich nectar, pollen is deposited
on the bird’s head and neck and is then transferred to the next flower it visits. Botanists have been known to
determine the range of extinct plants by collecting and identifying pollen from 200-year-old bird specimens from
the same site.
Figure 32.14 Hummingbirds have adaptations that allow them to reach the nectar of certain tubular flowers, (credit:
Lori Branham)
Pollination by Wind
Most species of conifers, and many angiosperms, such as grasses, maples and oaks, are pollinated by wind.
Pine cones are brown and unscented, while the flowers of wind-pollinated angiosperm species are usually green,
small, may have small or no petals, and produce large amounts of pollen. Unlike the typical insect-pollinated
flowers, flowers adapted to pollination by wind do not produce nectar or scent, in wind-pollinated species, the
microsporangia hang out of the flower, and, as the wind blows, the lightweight pollen is carried with it (Figure
32.15). The flowers usually emerge early in the spring, before the leaves, so that the leaves do not block the
movement of the wind. The pollen is deposited on the exposed feathery stigma of the flower (Figure 32.16).
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Figure 32.15 A person knocks pollen from a pine tree.
(a) (b)
Figure 32.16 These male (a) and female (b) catkins are from the goat willow tree (Salix caprea). Note how both
structures are light and feathery to better disperse and catch the wind-blown pollen.
Pollination by Water
Some weeds, such as Australian sea grass and pond weeds, are pollinated by water. The pollen floats on water,
and when it comes into contact with the flower, it is deposited inside the flower.
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V /
e olution CONNECTION
Pollination by Deception
Orchids are highly valued flowers, with many rare varieties (Figure 32.17). They grow in a range of specific
habitats, mainly in the tropics of Asia, South America, and Central America. At least 25,000 species of
orchids have been identified.
Figure 32.17 Certain orchids use food deception or sexual deception to attract pollinators. Shown here is a bee
orchid (Ophrys apifera). (credit: David Evans)
Flowers often attract pollinators with food rewards, in the form of nectar. However, some species of orchid
are an exception to this standard: they have evolved different ways to attract the desired pollinators.
They use a method known as food deception, in which bright colors and perfumes are offered, but no
food. Anacamptis morio, commonly known as the green-winged orchid, bears bright purple flowers and
emits a strong scent. The bumblebee, its main pollinator, is attracted to the flower because of the strong
scent—which usually indicates food for a bee—and in the process, picks up the pollen to be transported to
another flower.
Other orchids use sexual deception. Chiloglottis trapeziformis emits a compound that smells the same as
the pheromone emitted by a female wasp to attract male wasps. The male wasp is attracted to the scent,
lands on the orchid flower, and in the process, transfers pollen. Some orchids, like the Australian hammer
orchid, use scent as well as visual trickery in yet another sexual deception strategy to attract wasps. The
flower of this orchid mimics the appearance of a female wasp and emits a pheromone. The male wasp tries
to mate with what appears to be a female wasp, and in the process, picks up pollen, which it then transfers
to the next counterfeit mate.
Double Fertilization
After pollen is deposited on the stigma, it must germinate and grow through the style to reach the ovule. The
microspores, or the pollen, contain two cells: the pollen tube cell and the generative cell. The pollen tube cell
grows into a pollen tube through which the generative cell travels. The germination of the pollen tube requires
water, oxygen, and certain chemical signals. As it travels through the style to reach the embryo sac, the pollen
tube’s growth is supported by the tissues of the style. In the meantime, if the generative cell has not already split
into two cells, it now divides to form two sperm cells. The pollen tube is guided by the chemicals secreted by
the synergids present in the embryo sac, and it enters the ovule sac through the micropyle. Of the two sperm
cells, one sperm fertilizes the egg cell, forming a diploid zygote; the other sperm fuses with the two polar nuclei,
forming a triploid cell that develops into the endosperm. Together, these two fertilization events in angiosperms
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are known as double fertilization (Figure 32.18). After fertilization is complete, no other sperm can enter. The
fertilized ovule forms the seed, whereas the tissues of the ovary become the fruit, usually enveloping the seed.
Micropyle
Polar
nuclei
Antipod als
Embryo sac
Stigma
Style
Pollen grain
Sperm
Pollen tube
Ovules
The pollen grain adheres to
the stigma, which contains
two cells: a generative cell
and a tube cell.
The pollen tube cell grows
into the style. The generative
cell travels inside the pollen
tube. It divides to form two
sperm.
The pollen tube penetrates
an opening in the ovule
called a micropyle.
One of the sperm fertilizes
the egg to form the diploid
zygote. The other sperm
fertilizes two polar nuclei to
form the triploid endosperm,
which will become a food
source for the growing
embryo.
Figure 32.18 In angiosperms, one sperm fertilizes the egg to form the 2n zygote, and the other sperm fertilizes the
central cell to form the 3n endosperm. This is called a double fertilization.
After fertilization, the zygote divides to form two cells: the upper cell, or terminal cell, and the lower, or basal, cell.
The division of the basal cell gives rise to the suspensor, which eventually makes connection with the maternal
tissue. The suspensor provides a route for nutrition to be transported from the mother plant to the growing
embryo. The terminal cell also divides, giving rise to a globular-shaped proembryo (Figure 32.19a). In dicots
(eudicots), the developing embryo has a heart shape, due to the presence of the two rudimentary cotyledons
(Figure 32.19b). In non-endospermic dicots, such as Capsella bursa, the endosperm develops initially, but is
then digested, and the food reserves are moved into the two cotyledons. As the embryo and cotyledons enlarge,
they run out of room inside the developing seed, and are forced to bend (Figure 32.19c). Ultimately, the embryo
and cotyledons fill the seed (Figure 32.19d), and the seed is ready for dispersal. Embryonic development is
suspended after some time, and growth is resumed only when the seed germinates. The developing seedling
will rely on the food reserves stored in the cotyledons until the first set of leaves begin photosynthesis.
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50 pm
Proembryo
Suspensor
Basal cell
Endosperm
Cotyledons
100 pm
Cotyledons
Embryo
Suspensor
Basal cell
Short meristem
Seed coat
Cotyledons
Root meristem
Figure 32.19 Shown are the stages of embryo development in the ovule of a shepherd’s purse (Capsella bursa).
After fertilization, the zygote divides to form an upper terminal cell and a lower basal cell, (a) In the first stage of
development, the terminal cell divides, forming a globular pro-embryo. The basal cell also divides, giving rise to the
suspensor. (b) In the second stage, the developing embryo has a heart shape due to the presence of cotyledons, (c)
In the third stage, the growing embryo runs out of room and starts to bend, (d) Eventually, it completely fills the seed,
(credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)
Development of the Seed
The mature ovule develops into the seed. A typical seed contains a seed coat, cotyledons, endosperm, and a
single embryo (Figure 32.20).
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visual
CONNECTION
Bean seed Corn seed
(dicot) (monocot)
Figure 32.20 The structures of dicot and monocot seeds are shown. Dicots (left) have two cotyledons. Monocots,
such as corn (right), have one cotyledon, called the scutellum; it channels nutrition to the growing embryo. Both
monocot and dicot embryos have a plumule that forms the leaves, a hypocotyl that forms the stem, and a radicle
that forms the root. The embryonic axis comprises everything between the plumule and the radicle, not including
the cotyledon(s).
What of the following statements is true?
a. Both monocots and dicots have an endosperm.
b. The radicle develops into the root.
c. The plumule is part of the epicotyl.
d. The endosperm is part of the embryo.
The storage of food reserves in angiosperm seeds differs between monocots and dicots. In monocots, such as
corn and wheat, the single cotyledon is called a scutellum; the scutellum is connected directly to the embryo
via vascular tissue (xylem and phloem). Food reserves are stored in the large endosperm. Upon germination,
enzymes are secreted by the aleurone, a single layer of cells just inside the seed coat that surrounds the
endosperm and embryo. The enzymes degrade the stored carbohydrates, proteins and lipids, the products
of which are absorbed by the scutellum and transported via a vasculature strand to the developing embryo.
Therefore, the scutellum can be seen to be an absorptive organ, not a storage organ.
The two cotyledons in the dicot seed also have vascular connections to the embryo. In endospermic dicots, the
food reserves are stored in the endosperm. During germination, the two cotyledons therefore act as absorptive
organs to take up the enzymatically released food reserves, much like in monocots (monocots, by definition,
also have endospermic seeds). Tobacco ( Nicotiana tabaccum), tomato (Solarium lycopersicum), and pepper
(Capsicum annuum) are examples of endospermic dicots. In non-endospermic dicots, the triploid endosperm
develops normally following double fertilization, but the endosperm food reserves are quickly remobilized and
moved into the developing cotyledon for storage. The two halves of a peanut seed (Arachis hypogaea) and the
split peas (Pisum sativum) of split pea soup are individual cotyledons loaded with food reserves.
The seed, along with the ovule, is protected by a seed coat that is formed from the integuments of the ovule sac.
In dicots, the seed coat is further divided into an outer coat known as the testa and inner coat known as the
tegmen.
The embryonic axis consists of three parts: the plumule, the radicle, and the hypocotyl. The portion of the embryo
between the cotyledon attachment point and the radicle is known as the hypocotyl (hypocotyl means “below the
cotyledons"). The embryonic axis terminates in a radicle (the embryonic root), which is the region from which the
root will develop. In dicots, the hypocotyls extend above ground, giving rise to the stem of the plant. In monocots,
the hypocotyl does not show above ground because monocots do not exhibit stem elongation. The part of the
embryonic axis that projects above the cotyledons is known as the epicotyl. The plumule is composed of the
epicotyl, young leaves, and the shoot apical meristem.
Upon germination in dicot seeds, the epicotyl is shaped like a hook with the plumule pointing downwards. This
shape is called the plumule hook, and it persists as long as germination proceeds in the dark. Therefore, as the
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epicotyl pushes through the tough and abrasive soil, the plumule is protected from damage. Upon exposure to
light, the hypocotyl hook straightens out, the young foliage leaves face the sun and expand, and the epicotyl
continues to elongate. During this time, the radicle is also growing and producing the primary root. As it grows
downward to form the tap root, lateral roots branch off to all sides, producing the typical dicot tap root system.
in monocot seeds (Figure 32.21), the testa and tegmen of the seed coat are fused. As the seed germinates,
the primary root emerges, protected by the root-tip covering: the coleorhiza. Next, the primary shoot emerges,
protected by the coleoptile: the covering of the shoot tip. Upon exposure to light (i.e., when the plumule has
exited the soil and the protective coleoptile is no longer needed), elongation of the coleoptile ceases and the
leaves expand and unfold. At the other end of the embryonic axis, the primary root soon dies, while other,
adventitious roots (roots that do not arise from the usual place - i.e., the root) emerge from the base of the stem.
This gives the monocot a fibrous root system.
Adventitious
roots
Coleorhiza
Figure 32.21 As this monocot grass seed germinates, the primary root, or radicle, emerges first, followed by the
primary shoot, or coleoptile, and the adventitious roots.
Seed Germination
Many mature seeds enter a period of inactivity, or extremely low metabolic activity: a process known as
dormancy, which may last for months, years, or even centuries. Dormancy helps keep seeds viable during
unfavorable conditions. Upon a return to favorable conditions, seed germination takes place. Favorable
conditions could be as diverse as moisture, light, cold, fire, or chemical treatments. After heavy rains, many new
seedlings emerge. Forest fires also lead to the emergence of new seedlings. Some seeds require vernalization
(cold treatment) before they can germinate. This guarantees that seeds produced by plants in temperate
climates will not germinate until the spring. Plants growing in hot climates may have seeds that need a heat
treatment in order to germinate, to avoid germination in the hot, dry summers. In many seeds, the presence of a
thick seed coat retards the ability to germinate. Scarification, which includes mechanical or chemical processes
to soften the seed coat, is often employed before germination. Presoaking in hot water, or passing through an
acid environment, such as an animal’s digestive tract, may also be employed.
Depending on seed size, the time taken for a seedling to emerge may vary. Species with large seeds have
enough food reserves to germinate deep below ground, and still extend their epicotyl all the way to the soil
surface. Seeds of small-seeded species usually require light as a germination cue. This ensures the seeds only
germinate at or near the soil surface (where the light is greatest). If they were to germinate too far underneath
the surface, the developing seedling would not have enough food reserves to reach the sunlight.
Development of Fruit and Fruit Types
After fertilization, the ovary of the flower usually develops into the fruit. Fruits are usually associated with having
a sweet taste; however, not all fruits are sweet. Botanically, the term “fruit" is used for a ripened ovary. In most
cases, flowers in which fertilization has taken place will develop into fruits, and flowers in which fertilization has
not taken place will not. Some fruits develop from the ovary and are known as true fruits, whereas others develop
from other parts of the female gametophyte and are known as accessory fruits. The fruit encloses the seeds and
the developing embryo, thereby providing it with protection. Fruits are of many types, depending on their origin
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and texture. The sweet tissue of the blackberry, the red flesh of the tomato, the shell of the peanut, and the hull
of corn (the tough, thin part that gets stuck in your teeth when you eat popcorn) are all fruits. As the fruit matures,
the seeds also mature.
Fruits may be classified as simple, aggregate, multiple, or accessory, depending on their origin (Figure 32.22).
If the fruit develops from a single carpel or fused carpels of a single ovary, it is known as a simple fruit, as seen
in nuts and beans. An aggregate fruit is one that develops from more than one carpel, but all are in the same
flower: the mature carpels fuse together to form the entire fruit, as seen in the raspberry. Multiple fruit develops
from an inflorescence or a cluster of flowers. An example is the pineapple, where the flowers fuse together to
form the fruit. Accessory fruits (sometimes called false fruits) are not derived from the ovary, but from another
part of the flower, such as the receptacle (strawberry) or the hypanthium (apples and pears).
Multiple fruit
Figure 32.22 There are four main types of fruits. Simple fruits, such as these nuts, are derived from a single ovary.
Aggregate fruits, like raspberries, form from many carpels that fuse together. Multiple fruits, such as pineapple, form
from a cluster of flowers called an inflorescence. Accessory fruit, like the apple, are formed from a part of the plant
other than the ovary, (credit "nuts": modification of work by Petr Kratochvil; credit "raspberries": modification of work by
Cory Zanker; credit "pineapple": modification of work by Howie Le; credit "apple": modification of work by Paolo Neo)
Fruits generally have three parts: the exocarp (the outermost skin or covering), the mesocarp (middle part of
the fruit), and the endocarp (the inner part of the fruit). Together, all three are known as the pericarp. The
mesocarp is usually the fleshy, edible part of the fruit; however, in some fruits, such as the almond, the endocarp
is the edible part. In many fruits, two or all three of the layers are fused, and are indistinguishable at maturity.
Fruits can be dry or fleshy. Furthermore, fruits can be divided into dehiscent or indehiscent types. Dehiscent
fruits, such as peas, readily release their seeds, while indehiscent fruits, like peaches, rely on decay to release
their seeds.
Fruit and Seed Dispersal
The fruit has a single purpose: seed dispersal. Seeds contained within fruits need to be dispersed far from the
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mother plant, so they may find favorable and less competitive conditions in which to germinate and grow.
Some fruit have built-in mechanisms so they can disperse by themselves, whereas others require the help of
agents like wind, water, and animals (Figure 32.23). Modifications in seed structure, composition, and size help
in dispersal. Wind-dispersed fruit are lightweight and may have wing-like appendages that allow them to be
carried by the wind. Some have a parachute-like structure to keep them afloat. Some fruits—for example, the
dandelion—have hairy, weightless structures that are suited to dispersal by wind.
Seeds dispersed by water are contained in light and buoyant fruit, giving them the ability to float. Coconuts are
well known for their ability to float on water to reach land where they can germinate. Similarly, willow and silver
birches produce lightweight fruit that can float on water.
Animals and birds eat fruits, and the seeds that are not digested are excreted in their droppings some distance
away. Some animals, like squirrels, bury seed-containing fruits for later use; if the squirrel does not find its stash
of fruit, and if conditions are favorable, the seeds germinate. Some fruits, like the cocklebur, have hooks or sticky
structures that stick to an animal's coat and are then transported to another place. Humans also play a big role in
dispersing seeds when they carry fruits to new places and throw away the inedible part that contains the seeds.
All of the above mechanisms allow for seeds to be dispersed through space, much like an animal’s offspring can
move to a new location. Seed dormancy, which was described earlier, allows plants to disperse their progeny
through time: something animals cannot do. Dormant seeds can wait months, years, or even decades for the
proper conditions for germination and propagation of the species.
(a) (b) (c)
Figure 32.23 Fruits and seeds are dispersed by various means, (a) Dandelion seeds are dispersed by wind, the (b)
coconut seed is dispersed by water, and the (c) acorn is dispersed by animals that cache and then forget it. (credit a:
modification of work by "Rosendahl'VFlickr; credit b: modification of work by Shine Oa; credit c: modification of work by
Paolo Neo)
32.3 | Asexual Reproduction
By the end of this section, you will be able to do the following:
• Compare the mechanisms and methods of natural and artificial asexual reproduction
• Describe the advantages and disadvantages of natural and artificial asexual reproduction
• Discuss plant life spans
Many plants are able to propagate themselves using asexual reproduction. This method does not require
the investment required to produce a flower, attract pollinators, or find a means of seed dispersal. Asexual
reproduction produces plants that are genetically identical to the parent plant because no mixing of male and
female gametes takes place. Traditionally, these plants survive well under stable environmental conditions when
compared with plants produced from sexual reproduction because they carry genes identical to those of their
parents.
Many different types of roots exhibit asexual reproduction (Figure 32.24). The corm is used by gladiolus and
garlic. Bulbs, such as a scaly bulb in lilies and a tunicate bulb in daffodils, are other common examples. A potato
is a stem tuber, while parsnip propagates from a taproot. Ginger and iris produce rhizomes, while ivy uses an
adventitious root (a root arising from a plant part other than the main or primary root), and the strawberry plant
has a stolon, which is also called a runner.
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Figure 32.24 Different types of stems allow for asexual reproduction, (a) The corm of a garlic plant looks similar to (b) a
tulip bulb, but the corm is solid tissue, while the bulb consists of layers of modified leaves that surround an underground
stem. Both corms and bulbs can self-propagate, giving rise to new plants, (c) Ginger forms masses of stems called
rhizomes that can give rise to multiple plants, (d) Potato plants form fleshy stem tubers. Each eye in the stem tuber can
give rise to a new plant, (e) Strawberry plants form stolons: stems that grow at the soil surface or just below ground
and can give rise to new plants, (credit a: modification of work by Dwight Sipler; credit c: modification of work by Albert
Cahalan, USDA ARS; credit d: modification of work by Richard North; credit e: modification of work by Julie Magro)
Some plants can produce seeds without fertilization. Either the ovule or part of the ovary, which is diploid in
nature, gives rise to a new seed. This method of reproduction is known as apomixis.
An advantage of asexual reproduction is that the resulting plant will reach maturity faster. Since the new plant is
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arising from an adult plant or plant parts, it will also be sturdier than a seedling. Asexual reproduction can take
place by natural or artificial (assisted by humans) means.
Natural Methods of Asexual Reproduction
Natural methods of asexual reproduction include strategies that plants have developed to self-propagate. Many
plants—like ginger, onion, gladioli, and dahlia—continue to grow from buds that are present on the surface of
the stem. In some plants, such as the sweet potato, adventitious roots or runners can give rise to new plants
(Figure 32.25). In Bryophyllum and kalanchoe, the leaves have small buds on their margins. When these are
detached from the plant, they grow into independent plants; or, they may start growing into independent plants if
the leaf touches the soil. Some plants can be propagated through cuttings alone.
Figure 32.25 A stolon, or runner, is a stem that runs along the ground. At the nodes, it forms adventitious roots and
buds that grow into a new plant.
Artificial Methods of Asexual Reproduction
These methods are frequently employed to give rise to new, and sometimes novel, plants. They include grafting,
cutting, layering, and micropropagation.
Grafting
Grafting has long been used to produce novel varieties of roses, citrus species, and other plants. In grafting,
two plant species are used; part of the stem of the desirable plant is grafted onto a rooted plant called the stock.
The part that is grafted or attached is called the scion. Both are cut at an oblique angle (any angle other than
a right angle), placed in close contact with each other, and are then held together (Figure 32.26). Matching up
these two surfaces as closely as possible is extremely important because these will be holding the plant together.
The vascular systems of the two plants grow and fuse, forming a graft. After a period of time, the scion starts
producing shoots, and eventually starts bearing flowers and fruits. Grafting is widely used in viticulture (grape
growing) and the citrus industry. Scions capable of producing a particular fruit variety are grafted onto root stock
with specific resistance to disease.
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Stock
U
Figure 32.26 Grafting is an artificial method of asexual reproduction used to produce plants combining favorable stem
characteristics with favorable root characteristics. The stem of the plant to be grafted is known as the scion, and the
root is called the stock.
Cutting
Plants such as coleus and money plant are propagated through stem cuttings, where a portion of the stem
containing nodes and internodes is placed in moist soil and allowed to root. In some species, stems can start
producing a root even when placed only in water. For example, leaves of the African violet will root if kept in
water undisturbed for several weeks.
Layering
Layering is a method in which a stem attached to the plant is bent and covered with soil. Young stems that can
be bent easily without any injury are preferred. Jasmine and bougainvillea (paper flower) can be propagated this
way (Figure 32.27). In some plants, a modified form of layering known as air layering is employed. A portion
of the bark or outermost covering of the stem is removed and covered with moss, which is then taped. Some
gardeners also apply rooting hormone. After some time, roots will appear, and this portion of the plant can be
removed and transplanted into a separate pot.
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Figure 32.27 In layering, a part of the stem is buried so that it forms a new plant, (credit: modification of work by
Pearson Scott Foresman, donated to the Wikimedia Foundation)
Micropropagation
Micropropagation (also called plant tissue culture) is a method of propagating a large number of plants from a
single plant in a short time under laboratory conditions (Figure 32.28). This method allows propagation of rare,
endangered species that may be difficult to grow under natural conditions, are economically important, or are in
demand as disease-free plants.
Figure 32.28 Micropropagation is used to propagate plants in sterile conditions, (credit: Nikhilesh Sanyal)
To start plant tissue culture, a part of the plant such as a stem, leaf, embryo, anther, or seed can be used.
The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that
species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all
the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated
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mass known as callus, from which individual plantlets begin to grow after a period of time. These can be
separated and are first grown under greenhouse conditions before they are moved to field conditions.
Plant Life Spans
The length of time from the beginning of development to the death of a plant is called its life span. The life cycle,
on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of
the mature plant. Some plants, such as annuals, only need a few weeks to grow, produce seeds and die. Other
plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented
age of 4,500 years (Figure 32.29). Even as some parts of a plant, such as regions containing meristematic
tissue—the area of active plant growth consisting of undifferentiated cells capable of cell division—continue
to grow, some parts undergo programmed cell death (apoptosis). The cork found on stems, and the water¬
conducting tissue of the xylem, for example, are composed of dead cells.
Figure 32.29 The bristlecone pine, shown here in the Ancient Bristlecone Pine Forest in the White Mountains of
eastern California, has been known to live for 4,500 years, (credit: Rick Goldwaser)
Plant species that complete their lifecycle in one season are known as annuals, an example of which is
Arabidopsis, or mouse-ear cress. Biennials such as carrots complete their lifecycle in two seasons. In a
biennial’s first season, the plant has a vegetative phase, whereas in the next season, it completes its
reproductive phase. Commercial growers harvest the carrot roots after the first year of growth, and do not allow
the plants to flower. Perennials, such as the magnolia, complete their lifecycle in two years or more.
In another classification based on flowering frequency, monocarpic plants flower only once in their lifetime;
examples include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as
120 years in some bamboo species), these plants may reproduce asexually and accumulate a great deal of food
material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon
after flowering, these plants die. Polycarpic plants form flowers many times during their lifetime. Fruit trees, such
as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials,
flower several times during their life span, but not each year. By this means, the plant does not require all its
nutrients to be channelled towards flowering each year.
As is the case with all living organisms, genetics and environmental conditions have a role to play in determining
how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and
competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow,
despite the presence of dead tissue such as cork. Individual parts of plants, such as flowers and leaves, have
different rates of survival. In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is
triggered by factors such as a decrease in photosynthetic efficiency, due to shading by upper leaves, or oxidative
damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled
by the plant for use in other processes, such as development of seed and storage. This process is known as
nutrient recycling.
The aging of a plant and all the associated processes is known as senescence, which is marked by several
complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which
is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery
such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are
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broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of
other plant tissues.
The complex pathways of nutrient recycling within a plant are not well understood. Hormones are known to play
a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic
acid causes premature onset of senescence.
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KEY TERMS
accessory fruit fruit derived from tissues other than the ovary
aggregate fruit fruit that develops from multiple carpels in the same flower
aleurone single layer of cells just inside the seed coat that secretes enzymes upon germination
androecium sum of all the stamens in a flower
antipodals the three cells away from the micropyle
apomixis process by which seeds are produced without fertilization of sperm and egg
coleoptile covering of the shoot tip, found in germinating monocot seeds
coleorhiza covering of the root tip, found in germinating monocot seeds
cotyledon fleshy part of seed that provides nutrition to the seed
cross-pollination transfer of pollen from the anther of one flower to the stigma of a different flower
cutting method of asexual reproduction where a portion of the stem contains nodes and internodes is placed in
moist soil and allowed to root
dormancy period of no growth and very slow metabolic processes
double fertilization two fertilization events in angiosperms; one sperm fuses with the egg, forming the zygote,
whereas the other sperm fuses with the polar nuclei, forming endosperm
endocarp innermost part of fruit
endosperm triploid structure resulting from fusion of a sperm with polar nuclei, which serves as a nutritive tissue
for embryo
endospermic dicot dicot that stores food reserves in the endosperm
epicotyl embryonic shoot above the cotyledons
exine outermost covering of pollen
exocarp outermost covering of a fruit
gametophyte multicellular stage of the plant that gives rise to haploid gametes or spores
grafting method of asexual reproduction where the stem from one plant species is spliced to a different plant
gravitropism response of a plant growth in the same direction as gravity
gynoecium the sum of all the carpels in a flower
hypocotyl embryonic axis above the cotyledons
intine inner lining of the pollen
layering method of propagating plants by bending a stem under the soil
megagametogenesis second phase of female gametophyte development, during which the surviving haploid
megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known
as the megagametophyte or embryo sac.
megasporangium tissue found in the ovary that gives rise to the female gamete or egg
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megasporogenesis first phase of female gametophyte development, during which a single cell in the diploid
megasporangium undergoes meiosis to produce four megaspores, only one of which survives
megasporophyll bract (a type of modified leaf) on the central axis of a female gametophyte
mesocarp middle part of a fruit
micropropagation propagation of desirable plants from a plant part; carried out in a laboratory
micropyle opening on the ovule sac through which the pollen tube can gain entry
microsporangium tissue that gives rise to the microspores or the pollen grain
microsporophyll central axis of a male cone on which bracts (a type of modified leaf) are attached
monocarpic plants that flower once in their lifetime
multiple fruit fruit that develops from multiple flowers on an inflorescence
nectar guide pigment pattern on a flower that guides an insect to the nectaries
non-endospermic dicot dicot that stores food reserves in the developing cotyledon
perianth (also, petal or sepal) part of the flower consisting of the calyx and/or corolla; forms the outer envelope
of the flower
pericarp collective term describing the exocarp, mesocarp, and endocarp; the structure that encloses the seed
and is a part of the fruit
plumule shoot that develops from the germinating seed
polar nuclei found in the ovule sac; fusion with one sperm cell forms the endosperm
pollination transfer of pollen to the stigma
polycarpic plants that flower several times in their lifetime
radicle original root that develops from the germinating seed
scarification mechanical or chemical processes to soften the seed coat
scion the part of a plant that is grafted onto the root stock of another plant
scutellum type of cotyledon found in monocots, as in grass seeds
self-pollination transfer of pollen from the anther to the stigma of the same flower
senescence process that describes aging in plant tissues
simple fruit fruit that develops from a single carpel or fused carpels
sporophyte multicellular diploid stage in plants that is formed after the fusion of male and female gametes
suspensor part of the growing embryo that makes connection with the maternal tissues
synergid type of cell found in the ovule sac that secretes chemicals to guide the pollen tube towards the egg
tegmen inner layer of the seed coat
testa outer layer of the seed coat
vernalization exposure to cold required by some seeds before they can germinate
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CHAPTER SUMMARY
32.1 Reproductive Development and Structure
The flower contains the reproductive structures of a plant. All complete flowers contain four whorls: the calyx,
corolla, androecium, and gynoecium. The stamens are made up of anthers, in which pollen grains are
produced, and a supportive strand called the filament. The pollen contains two cells— a generative cell and a
tube cell—and is covered by two layers called the intine and the exine. The carpels, which are the female
reproductive structures, consist of the stigma, style, and ovary. The female gametophyte is formed from mitotic
divisions of the megaspore, forming an eight-nuclei ovule sac. This is covered by a layer known as the
integument. The integument contains an opening called the micropyle, through which the pollen tube enters the
embryo sac.
The diploid sporophyte of angiosperms and gymnosperms is the conspicuous and long-lived stage of the life
cycle. The sporophytes differentiate specialized reproductive structures called sporangia, which are dedicated
to the production of spores. The microsporangium contains microspore mother cells, which divide by meiosis to
produce haploid microspores. The microspores develop into male gametophytes that are released as pollen.
The megasporangium contains megaspore mother cells, which divide by meiosis to produce haploid
megaspores. A megaspore develops into a female gametophyte containing a haploid egg. A new diploid
sporophyte is formed when a male gamete from a pollen grain enters the ovule sac and fertilizes this egg.
32.2 Pollination and Fertilization
For fertilization to occur in angiosperms, pollen has to be transferred to the stigma of a flower: a process known
as pollination. Gymnosperm pollination involves the transfer of pollen from a male cone to a female cone.
When the pollen of the flower is transferred to the stigma of the same flower, it is called self-pollination. Cross¬
pollination occurs when pollen is transferred from one flower to another flower on the same plant, or another
plant. Cross-pollination requires pollinating agents such as water, wind, or animals, and increases genetic
diversity. After the pollen lands on the stigma, the tube cell gives rise to the pollen tube, through which the
generative nucleus migrates. The pollen tube gains entry through the micropyle on the ovule sac. The
generative cell divides to form two sperm cells: one fuses with the egg to form the diploid zygote, and the other
fuses with the polar nuclei to form the endosperm, which is triploid in nature. This is known as double
fertilization. After fertilization, the zygote divides to form the embryo and the fertilized ovule forms the seed. The
walls of the ovary form the fruit in which the seeds develop. The seed, when mature, will germinate under
favorable conditions and give rise to the diploid sporophyte.
32.3 Asexual Reproduction
Many plants reproduce asexually as well as sexually. In asexual reproduction, part of the parent plant is used to
generate a new plant. Grafting, layering, and micropropagation are some methods used for artificial asexual
reproduction. The new plant is genetically identical to the parent plant from which the stock has been taken.
Asexually reproducing plants thrive well in stable environments.
Plants have different life spans, dependent on species, genotype, and environmental conditions. Parts of the
plant, such as regions containing meristematic tissue, continue to grow, while other parts experience
programmed cell death. Leaves that are no longer photosynthetically active are shed from the plant as part of
senescence, and the nutrients from these leaves are recycled by the plant. Other factors, including the
presence of hormones, are known to play a role in delaying senescence.
VISUAL CONNECTION QUESTIONS
1. Figure 32.3 If the anther is missing, what type of
reproductive structure will the flower be unable to
produce? What term is used to describe a flower that
is normally lacking the androecium? What term
describes a flower lacking a gynoecium?
2. Figure 32.8 An embryo sac is missing the
synergids. What specific impact would you expect
this to have on fertilization?
a. The pollen tube will be unable to form.
b. The pollen tube will form but will not be
guided toward the egg.
c. Fertilization will not occur because the
synergid is the egg.
d. Fertilization will occur but the embryo will
not be able to grow.
3. Figure 32.20 What is the function of the
cotyledon?
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Chapter 32 | Plant Reproduction
a. It develops into the root.
b. It provides nutrition for the embryo.
c. It forms the embryo.
d. It protects the embryo.
REVIEW QUESTIONS
4. In a plant’s male reproductive organs,
development of pollen takes place in a structure
known as the_.
a. stamen
b. microsporangium
c. anther
d. tapetum
5. The stamen consists of a long stalk called the
filament that supports the_.
a. stigma
b. sepal
c. style
d. anther
6. The_are collectively called the calyx.
a. sepals
b. petals
c. tepals
d. stamens
7. The pollen lands on which part of the flower?
a. stigma
b. style
c. ovule
d. integument
8. After double fertilization, a zygote and_
form.
a. an ovule
b. endosperm
c. a cotyledon
d. asuspensor
9. The fertilized ovule gives rise to the_.
a. fruit
b. seed
c. endosperm
d. embryo
10. What is the term for a fruit that develops from
tissues other than the ovary?
a.
simple fruit
b.
aggregate fruit
c.
multiple fruit
d.
accessory fruit
11. The
is the outermost coverina of a
fruit.
a.
endocarp
b.
pericarp
c.
exocarp
d.
mesocarp
12.
is a useful method of asexual
reproduction for propagating hard-to-root plants.
a.
grafting
b.
layering
c.
cuttings
d.
budding
13. Which of the following is an advantage of asexual
reproduction?
a.
Cuttings taken from an adult plant show
increased resistance to diseases.
b.
Grafted plants can more successfully
endure drought.
c.
When cuttings or buds are taken from an
adult plant or plant parts, the resulting plant
will grow into an adult faster than a seedling.
d.
Asexual reproduction takes advantage of a
more diverse gene pool.
14. Plants that flower once in their lifetime are known
as
a.
monoecious
b.
dioecious
c.
polycarpic
d.
monocarpic
15. Plant species that complete their lifecycle in one
season are known as
a.
biennials
b.
perennials
c.
annuals
d.
polycarpic
CRITICAL THINKING QUESTIONS
16. Describe the reproductive organs inside a flower.
17. Describe the two-stage lifecycle of plants: the
gametophyte stage and the sporophyte stage.
18. Describe the four main parts, or whorls, of a
flower.
19. Discuss the differences between a complete
flower and an incomplete flower.
20. Why do some seeds undergo a period of
dormancy, and how do they break dormancy?
21. Discuss some ways in which fruit seeds are
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dispersed.
22. What are some advantages of asexual
reproduction in plants?
23. Describe natural and artificial methods of asexual
reproduction in plants.
24. Discuss the life cycles of various plants.
25. How are plants classified on the basis of
flowering frequency?
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Chapter 33 | The Animal Body: Basic Form and Function
1005
33 | THE ANIMAL BODY:
BASIC FORM AND
FUNCTION
Figure 33.1 An arctic fox is a complex animal, well adapted to its environment. It changes coat color with the seasons,
and has longer fur in winter to trap heat, (credit: modification of work by Keith Morehouse, USFWS)
Chapter Outline
33.1: Animal Form and Function
33.2: Animal Primary Tissues
33.3: Homeostasis
Introduction
The arctic fox is an example of a complex animal that has adapted to its environment and illustrates the
relationships between an animal’s form and function. The structures of animals consist of primary tissues that
make up more complex organs and organ systems. Homeostasis allows an animal to maintain a balance
between its internal and external environments.
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33.1 1 Animal Form and Function
By the end of this section, you will be able to do the following:
• Describe the various types of body plans that occur in animals
• Describe limits on animal size and shape
• Relate bioenergetics to body size, levels of activity, and the environment
Animals vary in form and function. From a sponge to a worm to a goat, an organism has a distinct body plan
that limits its size and shape. Animals’ bodies are also designed to interact with their environments, whether in
the deep sea, a rainforest canopy, or the desert. Therefore, a large amount of information about the structure of
an organism's body (anatomy) and the function of its cells, tissues and organs (physiology) can be learned by
studying that organism's environment.
Body Plans
Asymmetry
Radial symmetry
Dorsal
Figure 33.2 Animals exhibit different types of body symmetry. The sponge is asymmetrical, the sea anemone has
radial symmetry, and the goat has bilateral symmetry.
Animal body plans follow set patterns related to symmetry. They are asymmetrical, radial, or bilateral in form
as illustrated in Figure 33.2. Asymmetrical animals are animals with no pattern or symmetry; an example of
an asymmetrical animal is a sponge. Radial symmetry, as illustrated in Figure 33.2, describes when an animal
has an up-and-down orientation: any plane cut along its longitudinal axis through the organism produces equal
halves, but not a definite right or left side. This plan is found mostly in aquatic animals, especially organisms that
attach themselves to a base, like a rock or a boat, and extract their food from the surrounding water as it flows
around the organism. Bilateral symmetry is illustrated in the same figure by a goat. The goat also has an upper
and lower component to it, but a plane cut from front to back separates the animal into definite right and left
sides. Additional terms used when describing positions in the body are anterior (front), posterior (rear), dorsal
(toward the back), and ventral (toward the stomach). Bilateral symmetry is found in both land-based and aquatic
animals; it enables a high level of mobility.
Limits on Animal Size and Shape
Animals with bilateral symmetry that live in water tend to have a fusiform shape: this is a tubular shaped body
that is tapered at both ends. This shape decreases the drag on the body as it moves through water and allows
the animal to swim at high speeds. Table 33.1 lists the maximum speed of various animals. Certain types of
sharks can swim at fifty kilometers per hour and some dolphins at 32 to 40 kilometers per hour. Land animals
frequently travel faster, although the tortoise and snail are significantly slower than cheetahs. Another difference
in the adaptations of aquatic and land-dwelling organisms is that aquatic organisms are constrained in shape
by the forces of drag in the water since water has higher viscosity than air. On the other hand, land-dwelling
organisms are constrained mainly by gravity, and drag is relatively unimportant. For example, most adaptations
in birds are for gravity not for drag.
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Maximum Speed of Assorted Land & Marine Animals
Animal Speed (kmh) Speed (mph)
Cheetah
113
70
Quarter horse
77
48
Fox
68
42
Shortfin mako shark
50
31
Domestic house cat
48
30
Human
45
28
Dolphin
32-40
20-25
Mouse
13
8
Snail
0.05
0.03
Table 33.1
Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and
crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The
exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage
from predators and from water loss (for land animals); it also provides for the attachments of muscles.
As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer
such as chitin and is often biomineralized with materials such as calcium carbonate. This is fused to the animal’s
epidermis. Ingrowths of the exoskeleton, called apodemes, function as attachment sites for muscles, similar
to tendons in more advanced animals (Figure 33.3). In order to grow, the animal must first synthesize a new
exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability
to grow continually, and may limit the individual’s ability to mature if molting does not occur at the proper time.
The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is
estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of
the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively small size.
The same principles apply to endoskeletons, but they are more efficient because muscles are attached on the
outside, making it easier to compensate for increased mass.
Apodemes
Figure 33.3 Apodemes are ingrowths on arthropod exoskeletons to which muscles attach. The apodemes on
leg are located above and below the fulcrum of the claw. Contraction of muscles attached to the apodemes
claw closed.
An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in
support the other tissues and the amount of muscle it needs for movement. As the body size increases, both
bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and
the bone and muscle that provide support and movement.
Limiting Effects of Diffusion on Size and Development
The exchange of nutrients and wastes between a cell and its watery environment occurs through the process
this crab
pulls the
order to
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Chapter 33 | The Animal Body: Basic Form and Function
of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular
one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell
is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through
diffusion. If the cell is too large, then diffusion is ineffective and the center of the cell does not receive adequate
nutrients nor is it able to effectively dispel its waste.
An important concept in understanding how efficient diffusion is as a means of transport is the surface to volume
ratio. Recall that any three-dimensional object has a surface area and volume; the ratio of these two quantities
is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4nr 2 , and a
volume of (4/3)nr 3 . The surface-to-volume ratio of a sphere is 3/r; as the cell gets bigger, its surface to volume
ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area
for diffusion it possesses.
The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex
organisms, allowing cells to become more efficient at doing fewer tasks. For example, circulatory systems
bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon
dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently
control body functions. Moreover, surface-to-volume ratio applies to other areas of animal development, such
as the relationship between muscle mass and cross-sectional surface area in supporting skeletons, and in the
relationship between muscle mass and the generation of dissipation of heat.
Visit this interactive site (http:// 0 penstaxc 0 llege. 0 rg/l/nan 0 sc 0 py) to see an entire animal (a zebrafish
embryo) at the cellular and sub-cellular level. Use the zoom and navigation functions for a virtual nanoscopy
exploration.
Animal Bioenergetics
All animals must obtain their energy from food they ingest or absorb. These nutrients are converted to adenosine
triphosphate (ATP) for short-term storage and use by all cells. Some animals store energy for slightly longer
times as glycogen, and others store energy for much longer times in the form of triglycerides housed in
specialized adipose tissues. No energy system is one hundred percent efficient, and an animal’s metabolism
produces waste energy in the form of heat. If an animal can conserve that heat and maintain a relatively constant
body temperature, it is classified as a warm-blooded animal and called an endotherm. The insulation used to
conserve the body heat comes in the forms of fur, fat, or feathers. The absence of insulation in ectothermic
animals increases their dependence on the environment for body heat.
The amount of energy expended by an animal over a specific time is called its metabolic rate. The rate is
measured variously in joules, calories, or kilocalories (1000 calories). Carbohydrates and proteins contain about
4.5 to 5 kcal/g, and fat contains about 9 kcal/g. Metabolic rate is estimated as the basal metabolic rate
(BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Human males
have a BMR of 1600 to 1800 kcal/day, and human females have a BMR of 1300 to 1500 kcal/day. Even with
insulation, endothermal animals require extensive amounts of energy to maintain a constant body temperature.
An ectotherm such as an alligator has an SMR of 60 kcal/day.
Energy Requirements Related to Body Size
Smaller endothermic animals have a greater surface area for their mass than larger ones (Figure 33.4).
Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain
a constant internal temperature. This results in a smaller endothermic animal having a higher BMR, per body
weight, than a larger endothermic animal.
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Chapter 33 | The Animal Body: Basic Form and Function
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Species
Mass
35 g
4,500,000 g
Metabolic rate
890 mm 3 0 2 /g body mass/hr
75 mm 3 0 2 /g body mass/hr
Figure 33.4 The mouse has a much higher metabolic rate than the elephant, (credit “mouse”: modification of work by
Magnus Kjaergaard; credit “elephant”: modification of work by “TheLizardQueen’VFlickr)
Energy Requirements Related to Levels of Activity
The more active an animal is, the more energy is needed to maintain that activity, and the higher its BMR
or SMR. The average daily rate of energy consumption is about two to four times an animal’s BMR or SMR.
Humans are more sedentary than most animals and have an average daily rate of only 1.5 times the BMR. The
diet of an endothermic animal is determined by its BMR. For example: the type of grasses, leaves, or shrubs that
an herbivore eats affects the number of calories that it takes in. The relative caloric content of herbivore foods,
in descending order, is tall grasses > legumes > short grasses > forbs (any broad-leaved plant, not a grass) >
subshrubs > annuals/biennials.
Energy Requirements Related to Environment
Animals adapt to extremes of temperature or food availability through torpor. Torpor is a process that leads to
a decrease in activity and metabolism and allows animals to survive adverse conditions. Torpor can be used
by animals for long periods, such as entering a state of hibernation during the winter months, in which case
it enables them to maintain a reduced body temperature. During hibernation, ground squirrels can achieve an
abdominal temperature of 0° C (32° F), while a bear’s internal temperature is maintained higher at about 37° C
(99° F).
If torpor occurs during the summer months with high temperatures and little water, it is called estivation. Some
desert animals use this to survive the harshest months of the year. Torpor can occur on a daily basis; this is
seen in bats and hummingbirds. While endothermy is limited in smaller animals by surface to volume ratio, some
organisms can be smaller and still be endotherms because they employ daily torpor during the part of the day
that is coldest. This allows them to conserve energy during the colder parts of the day, when they consume more
energy to maintain their body temperature.
Animal Body Planes and Cavities
A standing vertebrate animal can be divided by several planes. A sagittal plane divides the body into right
and left portions. A midsagittal plane divides the body exactly in the middle, making two equal right and left
halves. A frontal plane (also called a coronal plane) separates the front from the back. A transverse plane (or,
horizontal plane) divides the animal into upper and lower portions. This is sometimes called a cross section, and,
if the transverse cut is at an angle, it is called an oblique plane. Figure 33.5 illustrates these planes on a goat (a
four-legged animal) and a human being.
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Chapter 33 | The Animal Body: Basic Form and Function
Midline
Midsagittal Frontal
plane plane
Figure 33.5 Shown are the planes of a quadrupedal goat and a bipedal human. The midsagittal plane divides the body
exactly in half, into right and left portions. The frontal plane divides the front and back, and the transverse plane divides
the body into upper and lower portions.
Vertebrate animals have a number of defined body cavities, as illustrated in Figure 33.6. Two of these are
major cavities that contain smaller cavities within them. The dorsal cavity contains the cranial and the vertebral
(or spinal) cavities. The ventral cavity contains the thoracic cavity, which in turn contains the pleural cavity
around the lungs and the pericardial cavity, which surrounds the heart. The ventral cavity also contains the
abdominopelvic cavity, which can be separated into the abdominal and the pelvic cavities.
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Cranial cavity
Dorsal
cavity
Ventral
cavity
Figure 33.6 Vertebrate animals have two major body cavities. The dorsal cavity, indicated in green, contains the cranial
and the spinal cavity. The ventral cavity, indicated in yellow, contains the thoracic cavity and the abdominopelvic cavity.
The thoracic cavity is separated from the abdominopelvic cavity by the diaphragm. The thoracic cavity is separated
into the abdominal cavity and the pelvic cavity by an imaginary line parallel to the pelvis bones, (credit: modification of
work by NCI)
career connection
Physical Anthropologist
Physical anthropologists study the adaption, variability, and evolution of human beings, plus their living and
fossil relatives. They can work in a variety of settings, although most will have an academic appointment at
a university, usually in an anthropology department or a biology, genetics, or zoology department.
Nonacademic positions are available in the automotive and aerospace industries where the focus is
on human size, shape, and anatomy. Research by these professionals might range from studies of
how the human body reacts to car crashes to exploring how to make seats more comfortable. Other
nonacademic positions can be obtained in museums of natural history, anthropology, archaeology, or
science and technology. These positions involve educating students from grade school through graduate
school. Physical anthropologists serve as education coordinators, collection managers, writers for museum
publications, and as administrators. Zoos employ these professionals, especially if they have an expertise
in primate biology; they work in collection management and captive breeding programs for endangered
species. Forensic science utilizes physical anthropology expertise in identifying human and animal remains,
assisting in determining the cause of death, and for expert testimony in trials.
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Chapter 33 | The Animal Body: Basic Form and Function
33.2 | Animal Primary Tissues
By the end of this section, you will be able to do the following:
• Describe epithelial tissues
• Discuss the different types of connective tissues in animals
• Describe three types of muscle tissues
• Describe nervous tissue
The tissues of multicellular, complex animals are four primary types: epithelial, connective, muscle, and nervous.
Recall that tissues are groups of similar cells (cells carrying out related functions). These tissues combine to form
organs—like the skin or kidney—that have specific, specialized functions within the body. Organs are organized
into organ systems to perform functions; examples include the circulatory system, which consists of the heart
and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines,
liver, and pancreas. Organ systems come together to create an entire organism.
Epithelial Tissues
Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a
single layer or multiple layers of cells. The types of epithelia are classified by the shapes of cells present and
the number of layers of cells. Epithelia composed of a single layer of cells is called simple epithelia; epithelial
tissue composed of multiple layers is called stratified epithelia. Table 33.2 summarizes the different types of
epithelial tissues.
Different Types of Epithelial Tissues
Cell
shape
Description
Location
squamous
flat, irregular round shape
simple: lung alveoli, capillaries; stratified: skin,
mouth, vagina
cuboidal
cube shaped, central nucleus
glands, renal tubules
columnar
tall, narrow, nucleus toward base; tall, narrow,
nucleus along cell
simple: digestive tract; pseudostratified:
respiratory tract
transitional
round, simple but appear stratified
urinary bladder
Table 33.2
Squamous Epithelia
Squamous epithelial cells are generally round, flat, and have a small, centrally located nucleus. The cell outline
is slightly irregular, and cells fit together to form a covering or lining. When the cells are arranged in a single
layer (simple epithelia), they facilitate diffusion in tissues, such as the areas of gas exchange in the lungs and
the exchange of nutrients and waste at blood capillaries.
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Chapter 33 | The Animal Body: Basic Form and Function
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(a) (b)
Figure 33.7 Squamous epithelia cells (a) have a slightly irregular shape, and a small, centrally located nucleus. These
cells can be stratified into layers, as in (b) this human cervix specimen, (credit b: modification of work by Ed Uthman;
scale-bar data from Matt Russell)
Figure 33.7a illustrates a layer of squamous cells with their membranes joined together to form an epithelium,
image Figure 33.7b illustrates squamous epithelial cells arranged in stratified layers, where protection is needed
on the body from outside abrasion and damage. This is called a stratified squamous epithelium and occurs in
the skin and in tissues lining the mouth and vagina.
Cuboidal Epithelia
Cuboidal epithelial cells, shown in Figure 33.8, are cube-shaped with a single, central nucleus. They are most
commonly found in a single layer representing a simple epithelia in glandular tissues throughout the body where
they prepare and secrete glandular material. They are also found in the walls of tubules and in the ducts of the
kidney and liver.
Figure 33.8 Simple cuboidal epithelial cells line tubules in the mammalian kidney, where they are involved in filtering
the blood.
Columnar Epithelia
Columnar epithelial cells are taller than they are wide: they resemble a stack of columns in an epithelial layer,
and are most commonly found in a single-layer arrangement. The nuclei of columnar epithelial cells in the
digestive tract appear to be lined up at the base of the cells, as illustrated in Figure 33.9. These cells absorb
material from the lumen of the digestive tract and prepare it for entry into the body through the circulatory and
lymphatic systems.
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Chapter 33 | The Animal Body: Basic Form and Function
Goblet cells
Figure 33.9 Simple columnar epithelial cells absorb material from the digestive tract. Goblet cells secret mucous into
the digestive tract lumen.
Columnar epithelial cells lining the respiratory tract appear to be stratified. However, each cell is attached to the
base membrane of the tissue and, therefore, they are simple tissues. The nuclei are arranged at different levels
in the layer of cells, making it appear as though there is more than one layer, as seen in Figure 33.10. This
is called pseudostratified, columnar epithelia. This cellular covering has cilia at the apical, or free, surface of
the cells. The cilia enhance the movement of mucous and trapped particles out of the respiratory tract, helping
to protect the system from invasive microorganisms and harmful material that has been breathed into the body.
Goblet cells are interspersed in some tissues (such as the lining of the trachea). The goblet cells contain mucous
that traps irritants, which in the case of the trachea keep these irritants from getting into the lungs.
Goblet cells
_ Pseudostratified
‘ epithelial cells
Figure 33.10 Pseudostratified columnar epithelia line the respiratory tract. They exist in one layer, but the arrangement
of nuclei at different levels makes it appear that there is more than one layer. Goblet cells interspersed between the
columnar epithelial cells secrete mucous into the respiratory tract.
Transitional Epithelia
Transitional or uroepithelial cells appear only in the urinary system, primarily in the bladder and ureter. These
cells are arranged in a stratified layer, but they have the capability of appearing to pile up on top of each other in
a relaxed, empty bladder, as illustrated in Figure 33.11. As the urinary bladder fills, the epithelial layer unfolds
and expands to hold the volume of urine introduced into it. As the bladder fills, it expands and the lining becomes
thinner. In other words, the tissue transitions from thick to thin.
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visual
CONNECTION
t«H I
Figure 33.11 Transitional epithelia of the urinary bladder undergo changes in thickness depending on how full the
bladder is.
Which of the following statements about types of epithelial cells is false?
a. Simple columnar epithelial cells line the tissue of the lung.
b. Simple cuboidal epithelial cells are involved in the filtering of blood in the kidney.
c. Pseudostratisfied columnar epithilia occur in a single layer, but the arrangement of nuclei makes it
appear that more than one layer is present.
d. Transitional epithelia change in thickness depending on how full the bladder is.
Connective Tissues
Connective tissues are made up of a matrix consisting of living cells and a nonliving substance, called the
ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic
substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes
the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can
synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes
can be found in some of the tissues. Some tissues have specialized cells that are not found in the others. The
matrix in connective tissues gives the tissue its density. When a connective tissue has a high concentration of
cells or fibers, it has proportionally a less dense matrix.
The organic portion or protein fibers found in connective tissues are either collagen, elastic, or reticular fibers.
Collagen fibers provide strength to the tissue, preventing it from being torn or separated from the surrounding
tissues. Elastic fibers are made of the protein elastin; this fiber can stretch to one and one half of its length and
return to its original size and shape. Elastic fibers provide flexibility to the tissues. Reticular fibers are the third
type of protein fiber found in connective tissues. This fiber consists of thin strands of collagen that form a network
of fibers to support the tissue and other organs to which it is connected. The various types of connective tissues,
the types of cells and fibers they are made of, and sample locations of the tissues is summarized in Table 33.3.
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Chapter 33 | The Animal Body: Basic Form and Function
Connective Tissues
Tissue
Cells
Fibers
Location
loose/areolar
fibroblasts, macrophages,
some lymphocytes, some
neutrophils
few: collagen, elastic,
reticular
around blood vessels;
anchors epithelia
dense, fibrous
connective
tissue
fibroblasts, macrophages
mostly collagen
irregular: skin; regular:
tendons, ligaments
hyaline: few: collagen
shark skeleton, fetal bones,
cartilage
chondrocytes, chondroblasts
fibrocartilage: large amount of
human ears, intervertebral
collagen
discs
bone
osteoblasts, osteocytes,
osteoclasts
some: collagen, elastic
vertebrate skeletons
adipose
adipocytes
few
adipose (fat)
blood
red blood cells, white blood
cells
none
blood
Table 33.3
Loose/Areolar Connective Tissue
Loose connective tissue, also called areolar connective tissue, has a sampling of all of the components of a
connective tissue. As illustrated in Figure 33.12, loose connective tissue has some fibroblasts; macrophages
are present as well. Collagen fibers are relatively wide and stain a light pink, while elastic fibers are thin and stain
dark blue to black. The space between the formed elements of the tissue is filled with the matrix. The material
in the connective tissue gives it a loose consistency similar to a cotton ball that has been pulled apart. Loose
connective tissue is found around every blood vessel and helps to keep the vessel in place. The tissue is also
found around and between most body organs. In summary, areolar tissue is tough, yet flexible, and comprises
membranes.
Elastin fiber Fibroblasts Collagen fiber
Figure 33.12 Loose connective tissue is composed of loosely woven collagen and elastic fibers. The fibers and other
components of the connective tissue matrix are secreted by fibroblasts.
Fibrous Connective Tissue
Fibrous connective tissues contain large amounts of collagen fibers and few cells or matrix material. The
fibers can be arranged irregularly or regularly with the strands lined up in parallel. Irregularly arranged fibrous
connective tissues are found in areas of the body where stress occurs from all directions, such as the dermis of
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Chapter 33 | The Animal Body: Basic Form and Function
1017
the skin. Regular fibrous connective tissue, shown in Figure 33.13, is found in tendons (which connect muscles
to bones) and ligaments (which connect bones to bones).
Fibroblast nuclei
Fibroblasts
Collagen fibers
Figure 33.13 Fibrous connective tissue from the tendon has strands of collagen fibers lined up in parallel.
Cartilage
Cartilage is a connective tissue with a large amount of the matrix and variable amounts of fibers. The cells,
called chondrocytes, make the matrix and fibers of the tissue. Chondrocytes are found in spaces within the
tissue called lacunae.
A cartilage with few collagen and elastic fibers is hyaline cartilage, illustrated in Figure 33.14. The lacunae
are randomly scattered throughout the tissue and the matrix takes on a milky or scrubbed appearance with
routine histological stains. Sharks have cartilaginous skeletons, as does nearly the entire human skeleton during
a specific pre-birth developmental stage. A remnant of this cartilage persists in the outer portion of the human
nose. Hyaline cartilage is also found at the ends of long bones, reducing friction and cushioning the articulations
of these bones.
Figure 33.14 Hyaline cartilage consists of a matrix with cells called chondrocytes embedded in it. The chondrocytes
exist in cavities in the matrix called lacunae.
Elastic cartilage has a large amount of elastic fibers, giving it tremendous flexibility. The ears of most vertebrate
animals contain this cartilage as do portions of the larynx, or voice box. Fibrocartilage contains a large amount
of collagen fibers, giving the tissue tremendous strength. Fibrocartilage comprises the intervertebral discs in
vertebrate animals. Hyaline cartilage found in movable joints such as the knee and shoulder becomes damaged
as a result of age or trauma. Damaged hyaline cartilage is replaced by fibrocartilage and results in the joints
becoming “stiff.”
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Chapter 33 | The Animal Body: Basic Form and Function
Bone
Bone, or osseous tissue, is a connective tissue that has a large amount of two different types of matrix material.
The organic matrix is similar to the matrix material found in other connective tissues, including some amount
of collagen and elastic fibers. This gives strength and flexibility to the tissue. The inorganic matrix consists of
mineral salts—mostly calcium salts—that give the tissue hardness. Without adequate organic material in the
matrix, the tissue breaks; without adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and osteoclasts. Osteoblasts are active in
making bone for growth and remodeling. Osteoblasts deposit bone material into the matrix and, after the matrix
surrounds them, they continue to live, but in a reduced metabolic state as osteocytes. Osteocytes are found
in lacunae of the bone. Osteoclasts are active in breaking down bone for bone remodeling, and they provide
access to calcium stored in tissues. Osteoclasts are usually found on the surface of the tissue.
Bone can be divided into two types: compact and spongy. Compact bone is found in the shaft (or diaphysis)
of a long bone and the surface of the flat bones, while spongy bone is found in the end (or epiphysis) of a
long bone. Compact bone is organized into subunits called osteons, as illustrated in Figure 33.15. A blood
vessel and a nerve are found in the center of the structure within the Haversian canal, with radiating circles
of lacunae around it known as lamellae. The wavy lines seen between the lacunae are microchannels called
canaliculi; they connect the lacunae to aid diffusion between the cells. Spongy bone is made of tiny plates
called trabeculae; these plates serve as struts to give the spongy bone strength. Over time, these plates can
break causing the bone to become less resilient. Bone tissue forms the internal skeleton of vertebrate animals,
providing structure to the animal and points of attachment for tendons.
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\ roV
Lacunae (contains osteocytes)
(b) (c)
Figure 33.15 (a) Compact bone is a dense matrix on the outer surface of bone. Spongy bone, inside the compact
bone, is porous with web-like trabeculae, (b) Compact bone is organized into rings called osteons. Blood vessels,
nerves, and lymphatic vessels are found in the central Haversian canal. Rings of lamellae surround the Haversian
canal. Between the lamellae are cavities called lacunae. Canaliculi are microchannels connecting the lacunae together,
(c) Osteoblasts surround the exterior of the bone. Osteoclasts bore tunnels into the bone and osteocytes are found in
the lacunae.
Adipose Tissue
Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a
real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and
store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to
help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against
damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat
during the processing of the material for viewing, as seen in Figure 33.16. The thin lines in the image are the
cell membranes, and the nuclei are the small, black dots at the edges of the cells.
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Chapter 33 | The Animal Body: Basic Form and Function
w I
/ /
\ A/
y —
3-^ / L
A
Figure 33.16 Adipose is a connective tissue is made up of cells called adipocytes. Adipocytes have small nuclei
localized at the cell edge.
Blood
Blood is considered a connective tissue because it has a matrix, as shown in Figure 33.17. The living cell types
are red blood cells (RBC), also called erythrocytes, and white blood cells (WBC), also called leukocytes. The
fluid portion of whole blood, its matrix, is commonly called plasma.
Macrophage
Neutrophil
6
- Monocyte
Lymphocyte -
©©
• •
Erythrocyte
(red blood cell)J^^^
® ^
- Basophil
&
©
Figure 33.17 Blood is a connective tissue that has a fluid matrix, called plasma, and no fibers. Erythrocytes (red blood
cells), the predominant cell type, are involved in the transport of oxygen and carbon dioxide. Also present are various
leukocytes (white blood cells) involved in immune response.
The cell found in greatest abundance in blood is the erythrocyte. Erythrocytes are counted in millions in a blood
sample: the average number of red blood cells in primates is 4.7 to 5.5 million cells per microliter. Erythrocytes
are consistently the same size in a species, but vary in size between species. For example, the average diameter
of a primate red blood cell is 7.5 pi, a dog is close at 7.0 pi, but a cat’s RBC diameter is 5.9 pi. Sheep erythrocytes
are even smaller at 4.6 pi. Mammalian erythrocytes lose their nuclei and mitochondria when they are released
from the bone marrow where they are made. Fish, amphibian, and avian red blood cells maintain their nuclei
and mitochondria throughout the cell’s life. The principal job of an erythrocyte is to carry and deliver oxygen to
the tissues.
Leukocytes are the predominant white blood cells found in the peripheral blood. Leukocytes are counted in the
thousands in the blood with measurements expressed as ranges: primate counts range from 4,800 to 10,800
cells per pi, dogs from 5,600 to 19,200 cells per pi, cats from 8,000 to 25,000 cells per pi, cattle from 4,000 to
12,000 cells per pi, and pigs from 11,000 to 22,000 cells per pi.
Lymphocytes function primarily in the immune response to foreign antigens or material. Different types of
lymphocytes make antibodies tailored to the foreign antigens and control the production of those antibodies.
Neutrophils are phagocytic cells and they participate in one of the early lines of defense against microbial
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Chapter 33 | The Animal Body: Basic Form and Function
1021
invaders, aiding in the removal of bacteria that has entered the body. Another leukocyte that is found in the
peripheral blood is the monocyte. Monocytes give rise to phagocytic macrophages that clean up dead and
damaged cells in the body, whether they are foreign or from the host animal. Two additional leukocytes in the
blood are eosinophils and basophils—both help to facilitate the inflammatory response.
The slightly granular material among the cells is a cytoplasmic fragment of a cell in the bone marrow. This is
called a platelet or thrombocyte. Platelets participate in the stages leading up to coagulation of the blood to stop
bleeding through damaged blood vessels. Blood has a number of functions, but primarily it transports material
through the body to bring nutrients to cells and remove waste material from them.
Muscle Tissues
There are three types of muscle in animal bodies: smooth, skeletal, and cardiac. They differ by the presence or
absence of striations or bands, the number and location of nuclei, whether they are voluntarily or involuntarily
controlled, and their location within the body. Table 33.4 summarizes these differences.
Types of Muscles
Type of Muscle
Striations
Nuclei
Control
Location
smooth
no
single, in center
involuntary
visceral organs
skeletal
yes
many, at periphery
voluntary
skeletal muscles
cardiac
yes
single, in center
involuntary
heart
Table 33.4
Smooth Muscle
Smooth muscle does not have striations in its cells. It has a single, centrally located nucleus, as shown in Figure
33.18. Constriction of smooth muscle occurs under involuntary, autonomic nervous control and in response
to local conditions in the tissues. Smooth muscle tissue is also called non-striated as it lacks the banded
appearance of skeletal and cardiac muscle. The walls of blood vessels, the tubes of the digestive system, and
the tubes of the reproductive systems are composed of mostly smooth muscle.
Smooth muscle cells Skeletal muscle cells Cardiac muscle cells
Figure 33.18 Smooth muscle cells do not have striations, while skeletal muscle cells do. Cardiac muscle cells have
striations, but, unlike the multinucleate skeletal cells, they have only one nucleus. Cardiac muscle tissue also has
intercalated discs, specialized regions running along the plasma membrane that join adjacent cardiac muscle cells and
assist in passing an electrical impulse from cell to cell.
Skeletal Muscle
Skeletal muscle has striations across its cells caused by the arrangement of the contractile proteins actin and
myosin. These muscle cells are relatively long and have multiple nuclei along the edge of the cell. Skeletal
muscle is under voluntary, somatic nervous system control and is found in the muscles that move bones. Figure
33.18 illustrates the histology of skeletal muscle.
Cardiac Muscle
Cardiac muscle, shown in Figure 33.18, is found only in the heart. Like skeletal muscle, it has cross striations
in its cells, but cardiac muscle has a single, centrally located nucleus. Cardiac muscle is not under voluntary
control but can be influenced by the autonomic nervous system to speed up or slow down. An added feature to
cardiac muscle cells is a line than extends along the end of the cell as it abuts the next cardiac cell in the row.
This line is called an intercalated disc: it assists in passing electrical impulse efficiently from one cell to the next
and maintains the strong connection between neighboring cardiac cells.
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Chapter 33 | The Animal Body: Basic Form and Function
Nervous Tissues
Nervous tissues are made of cells specialized to receive and transmit electrical impulses from specific areas of
the body and to send them to specific locations in the body. The main cell of the nervous system is the neuron,
illustrated in Figure 33.19. The large structure with a central nucleus is the cell body of the neuron. Projections
from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting
impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and
oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently. Other glial cells
that are not shown support the nutritional and waste requirements of the neuron. Some of the glial cells are
phagocytic and remove debris or damaged cells from the tissue. A nerve consists of neurons and glial cells.
Figure 33.19 The neuron has projections called dendrites that receive signals and projections called axons that send
signals. Also shown are two types of glial cells: astrocytes regulate the chemical environment of the nerve cell, and
oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently.
LINK
T a
LEARNING
Click through the interactive review (http:// 0 penstaxc 0 llege. 0 rg/l/tissues) to learn more about epithelial
tissues.
ca eer connection
Pathologist
A pathologist is a medical doctor or veterinarian who has specialized in the laboratory detection of disease
in animals, including humans. These professionals complete medical school education and follow it with an
extensive post-graduate residency at a medical center. A pathologist may oversee clinical laboratories for
the evaluation of body tissue and blood samples for the detection of disease or infection. They examine
tissue specimens through a microscope to identify cancers and other diseases. Some pathologists perform
autopsies to determine the cause of death and the progression of disease.
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Chapter 33 | The Animal Body: Basic Form and Function
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33.3 | Homeostasis
By the end of this section, you will be able to do the following:
• Define homeostasis
• Describe the factors affecting homeostasis
• Discuss positive and negative feedback mechanisms used in homeostasis
• Describe thermoregulation of endothermic and ectothermic animals
Animal organs and organ systems constantly adjust to internal and external changes through a process called
homeostasis (“steady state"). These changes might be in the level of glucose or calcium in blood or in external
temperatures. Homeostasis means to maintain dynamic equilibrium in the body. It is dynamic because it is
constantly adjusting to the changes that the body’s systems encounter. It is equilibrium because body functions
are kept within specific ranges. Even an animal that is apparently inactive is maintaining this homeostatic
equilibrium.
Homeostatic Process
The goal of homeostasis is the maintenance of equilibrium around a point or value called a set point. While
there are normal fluctuations from the set point, the body’s systems will usually attempt to go back to this point.
A change in the internal or external environment is called a stimulus and is detected by a receptor; the response
of the system is to adjust the deviation parameter toward the set point. For instance, if the body becomes too
warm, adjustments are made to cool the animal. If the blood’s glucose rises after a meal, adjustments are made
to lower the blood glucose level by getting the nutrient into tissues that need it or to store it for later use.
Control of Homeostasis
When a change occurs in an animal’s environment, an adjustment must be made. The receptor senses the
change in the environment, then sends a signal to the control center (in most cases, the brain) which in turn
generates a response that is signaled to an effector. The effector is a muscle (that contracts or relaxes) or a
gland that secretes. Homeostatsis is maintained by negative feedback loops. Positive feedback loops actually
push the organism further out of homeostasis, but may be necessary for life to occur. Homeostasis is controlled
by the nervous and endocrine system of mammals.
Negative Feedback Mechanisms
Any homeostatic process that changes the direction of the stimulus is a negative feedback loop. It may either
increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed
it. In other words, if a level is too high, the body does something to bring it down, and conversely, if a level is
too low, the body does something to make it go up. Hence the term negative feedback. An example is animal
maintenance of blood glucose levels. When an animal has eaten, blood glucose levels rise. This is sensed
by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by
the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative
feedback system, as illustrated in Figure 33.20. However, if an animal has not eaten and blood glucose levels
decrease, this is sensed in another group of cells in the pancreas, and the hormone glucagon is released causing
glucose levels to increase. This is still a negative feedback loop, but not in the direction expected by the use
of the term “negative." Another example of an increase as a result of the feedback loop is the control of blood
calcium. If calcium levels decrease, specialized cells in the parathyroid gland sense this and release parathyroid
hormone (PTH), causing an increased absorption of calcium through the intestines and kidneys and, possibly,
the breakdown of bone in order to liberate calcium. The effects of PTH are to raise blood levels of the element.
Negative feedback loops are the predominant mechanism used in homeostasis.
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Chapter 33 | The Animal Body: Basic Form and Function
In response to the lower
concentration of glucose, the
pancreas stops secreting insulin.
Food is consumed and digested,
causing blood level glucose to rise.
In response to higher insulin levels,
glucose is transported into cells
and liver cells store glucose as
glycogen. As a result, glucose
levels drop.
In response to higher glucose
levels, the pancreas secretes
insulin into the blood.
Figure 33.20 Blood sugar levels are controlled by a negative feedback loop, (credit: modification of work by Jon
Sullivan)
Positive Feedback Loop
A positive feedback loop maintains the direction of the stimulus, possibly accelerating it. Few examples of
positive feedback loops exist in animal bodies, but one is found in the cascade of chemical reactions that result
in blood clotting, or coagulation. As one clotting factor is activated, it activates the next factor in sequence until a
fibrin clot is achieved. The direction is maintained, not changed, so this is positive feedback. Another example of
positive feedback is uterine contractions during childbirth, as illustrated in Figure 33.21. The hormone oxytocin,
made by the endocrine system, stimulates the contraction of the uterus. This produces pain sensed by the
nervous system. Instead of lowering the oxytocin and causing the pain to subside, more oxytocin is produced
until the contractions are powerful enough to produce childbirth.
visual
CONNECTION
The baby pushes
against the cervix,
causing it to stretch.
Uterus
Oxytocin causes
the uterus to
contract.
Stretching of the
cervix causes
nerve impulses
to be sent to
the brain.
Cervi>
The brain stimulates
the pituitary to release
oxytocin.
Figure 33.21 The birth of a human infant is the result of positive feedback.
State whether each of the following processes is regulated by a positive feedback loop or a negative
feedback loop.
a. A person feels satiated after eating a large meal.
b. The blood has plenty of red blood cells. As a result, erythropoietin, a hormone that stimulates the
production of new red blood cells, is no longer released from the kidney.
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Set Point
It is possible to adjust a system’s set point. When this happens, the feedback loop works to maintain the new
setting. An example of this is blood pressure: over time, the normal or set point for blood pressure can increase
as a result of continued increases in blood pressure. The body no longer recognizes the elevation as abnormal
and no attempt is made to return to the lower set point. The result is the maintenance of an elevated blood
pressure that can have harmful effects on the body. Medication can lower blood pressure and lower the set point
in the system to a more healthy level. This is called a process of alteration of the set point in a feedback loop.
Changes can be made in a group of body organ systems in order to maintain a set point in another system. This
is called acclimatization. This occurs, for instance, when an animal migrates to a higher altitude than that to
which it is accustomed. In order to adjust to the lower oxygen levels at the new altitude, the body increases the
number of red blood cells circulating in the blood to ensure adequate oxygen delivery to the tissues. Another
example of acclimatization is animals that have seasonal changes in their coats: a heavier coat in the winter
ensures adequate heat retention, and a light coat in summer assists in keeping body temperature from rising to
harmful levels.
Feedback mechanisms can be understood in terms of driving a race car along a track: watch a short video
lesson on positive and negative feedback loops. (This multimedia resource will open in a browser.)
(http://cnx.Org/content/m66613/l.3/#eip-id2699660)
Homeostasis: Thermoregulation
Body temperature affects body activities. Generally, as body temperature rises, enzyme activity rises as well. For
every ten degree centigrade rise in temperature, enzyme activity doubles, up to a point. Body proteins, including
enzymes, begin to denature and lose their function with high heat (around 50°C for mammals). Enzyme activity
will decrease by half for every ten degree centigrade drop in temperature, to the point of freezing, with a few
exceptions. Some fish can withstand freezing solid and return to normal with thawing.
Watch this Discovery Channel video on thermoregulation to see illustrations of this process in a variety
of animals. (This multimedia resource will open in a browser.) (http://cnx.org/content/m66613/1.3/#eip-
idll68127177818)
Endotherms and Ectotherms
Animals can be divided into two groups: some maintain a constant body temperature in the face of differing
environmental temperatures, while others have a body temperature that is the same as their environment and
thus varies with the environment. Animals that do not control their body temperature are ectotherms. This group
has been called cold-blooded, but the term may not apply to an animal in the desert with a very warm body
temperature. In contrast to ectotherms, which rely on external temperatures to set their body temperatures,
poikilotherms are animals with constantly varying internal temperatures. An animal that maintains a constant
body temperature in the face of environmental changes is called a homeotherm. Endotherms are animals that
rely on internal sources for body temperature but which can exhibit extremes in temperature. These animals are
able to maintain a level of activity at cooler temperature, which an ectotherm cannot due to differing enzyme
levels of activity.
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Heat can be exchanged between an animal and its environment through four mechanisms: radiation,
evaporation, convection, and conduction (Figure 33.22). Radiation is the emission of electromagnetic “heat"
waves. Heat comes from the sun in this manner and radiates from dry skin the same way. Heat can be removed
with liquid from a surface during evaporation. This occurs when a mammal sweats. Convection currents of air
remove heat from the surface of dry skin as the air passes over it. Heat will be conducted from one surface to
another during direct contact with the surfaces, such as an animal resting on a warm rock.
(a) Radiation (b) Evaporation
(c) Convection (d) Conduction
Figure 33.22 Heat can be exchanged by four mechanisms: (a) radiation, (b) evaporation, (c) convection, or (d)
conduction, (credit b: modification of work by “Kullez’VFlickr; credit c: modification of work by Chad Rosenthal; credit d:
modification of work by “stacey.d'VFlickr)
Heat Conservation and Dissipation
Animals conserve or dissipate heat in a variety of ways. In certain climates, endothermic animals have some
form of insulation, such as fur, fat, feathers, or some combination thereof. Animals with thick fur or feathers
create an insulating layer of air between their skin and internal organs. Polar bears and seals live and swim in
a subfreezing environment and yet maintain a constant, warm, body temperature. The arctic fox, for example,
uses its fluffy tail as extra insulation when it curls up to sleep in cold weather. Mammals have a residual effect
from shivering and increased muscle activity: arrector pili muscles cause “goose bumps," causing small hairs to
stand up when the individual is cold; this has the intended effect of increasing body temperature. Mammals use
layers of fat to achieve the same end. Loss of significant amounts of body fat will compromise an individual’s
ability to conserve heat.
Endotherms use their circulatory systems to help maintain body temperature. Vasodilation brings more blood
and heat to the body surface, facilitating radiation and evaporative heat loss, which helps to cool the body.
Vasoconstriction reduces blood flow in peripheral blood vessels, forcing blood toward the core and the vital
organs found there, and conserving heat. Some animals have adaptions to their circulatory system that enable
them to transfer heat from arteries to veins, warming blood returning to the heart. This is called a countercurrent
heat exchange; it prevents the cold venous blood from cooling the heart and other internal organs. This adaption
can be shut down in some animals to prevent overheating the internal organs. The countercurrent adaption
is found in many animals, including dolphins, sharks, bony fish, bees, and hummingbirds. In contrast, similar
adaptations can help cool endotherms when needed, such as dolphin flukes and elephant ears.
Some ectothermic animals use changes in their behavior to help regulate body temperature. For example, a
desert ectothermic animal may simply seek cooler areas during the hottest part of the day in the desert to keep
from getting too warm. The same animals may climb onto rocks to capture heat during a cold desert night. Some
animals seek water to aid evaporation in cooling them, as seen with reptiles. Other ectotherms use group activity
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Chapter 33 | The Animal Body: Basic Form and Function
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such as the activity of bees to warm a hive to survive winter.
Many animals, especially mammals, use metabolic waste heat as a heat source. When muscles are contracted,
most of the energy from the ATP used in muscle actions is wasted energy that translates into heat. Severe cold
elicits a shivering reflex that generates heat for the body. Many species also have a type of adipose tissue called
brown fat that specializes in generating heat.
Neural Control of Thermoregulation
The nervous system is important to thermoregulation, as illustrated in Figure 33.22. The processes of
homeostasis and temperature control are centered in the hypothalamus of the advanced animal brain.
visual
CONNECTION
Body temperature falls
Blood vessels constrict
so that heat is conserved.
Sweat glands do not
secrete fluid. Shivering
(involuntary contraction of
muscles) generates heat,
which warms the body.
Body temperature rises
Heat is retained
Normal body
temperature
Blood vessels dilate,
resulting in heat loss to the
environment. Sweat glands
secrete fluid. As the fluid
evaporates, heat is lost
from the body.
Heat is lost to
the environment
Figure 33.23 The body is able to regulate temperature in response to signals from the nervous system.
When bacteria are destroyed by leuckocytes, pyrogens are released into the blood. Pyrogens reset
the body’s thermostat to a higher temperature, resulting in fever. How might pyrogens cause the body
temperature to rise?
The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation
and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. It
responds to chemicals from the body. When a bacterium is destroyed by phagocytic leukocytes, chemicals called
endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the
thermostat. This allows the body’s temperature to increase in what is commonly called a fever. An increase in
body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in
body heat also increases the activity of the animal’s enzymes and protective cells while inhibiting the enzymes
and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen. A fever that was once
thought to be a complication of an infection is now understood to be a normal defense mechanism.
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KEY TERMS
acclimatization alteration in a body system in response to environmental change
alteration change of the set point in a homeostatic system
apodeme ingrowth of an animal’s exoskeleton that functions as an attachment site for muscles
asymmetrical describes animals with no axis of symmetry in their body pattern
basal metabolic rate (BMR) metabolic rate at rest in endothermic animals
canaliculus microchannel that connects the lacunae and aids diffusion between cells
cartilage type of connective tissue with a large amount of ground substance matrix, cells called chondrocytes,
and some amount of fibers
chondrocyte cell found in cartilage
columnar epithelia epithelia made of cells taller than they are wide, specialized in absorption
connective tissue type of tissue made of cells, ground substance matrix, and fibers
cuboidal epithelia epithelia made of cube-shaped cells, specialized in glandular functions
dorsal cavity body cavity on the posterior or back portion of an animal; includes the cranial and vertebral
cavities
ectotherm animal incapable of maintaining a relatively constant internal body temperature
endotherm animal capable of maintaining a relatively constant internal body temperature
epithelial tissue tissue that either lines or covers organs or other tissues
estivation torpor in response to extremely high temperatures and low water availability
fibrous connective tissue type of connective tissue with a high concentration of fibers
frontal (coronal) plane plane cutting through an animal separating the individual into front and back portions
fusiform animal body shape that is tubular and tapered at both ends
hibernation torpor over a long period of time, such as a winter
homeostasis dynamic equilibrium maintaining appropriate body functions
lacuna space in cartilage and bone that contains living cells
loose (areolar) connective tissue type of connective tissue with small amounts of cells, matrix, and fibers;
found around blood vessels
matrix component of connective tissue made of both living and nonliving (ground substances) cells
midsagittal plane plane cutting through an animal separating the individual into even right and left sides
negative feedback loop feedback to a control mechanism that increases or decreases a stimulus instead of
maintaining it
osteon subunit of compact bone
positive feedback loop feedback to a control mechanism that continues the direction of a stimulus
pseudostratified layer of epithelia that appears multilayered, but is a simple covering
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Chapter 33 | The Animal Body: Basic Form and Function
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sagittal plane plane cutting through an animal separating the individual into right and left sides
set point midpoint or target point in homeostasis
simple epithelia single layer of epithelial cells
squamous epithelia type of epithelia made of flat cells, specialized in aiding diffusion or preventing abrasion
standard metabolic rate (SMR) metabolic rate at rest in ectothermic animals
stratified epithelia multiple layers of epithelial cells
thermoregulation regulation of body temperature
torpor decrease in activity and metabolism that allows an animal to survive adverse conditions
trabecula tiny plate that makes up spongy bone and gives it strength
transitional epithelia epithelia that can transition for appearing multilayered to simple; also called uroepithelial
transverse (horizontal) plane plane cutting through an animal separating the individual into upper and lower
portions
ventral cavity body cavity on the anterior or front portion of an animal that includes the thoracic cavities and the
abdominopelvic cavities
CHAPTER SUMMARY
33.1 Animal Form and Function
Animal bodies come in a variety of sizes and shapes. Limits on animal size and shape include impacts to their
movement. Diffusion affects their size and development. Bioenergetics describes how animals use and obtain
energy in relation to their body size, activity level, and environment.
33.2 Animal Primary Tissues
The basic building blocks of complex animals are four primary tissues. These are combined to form organs,
which have a specific, specialized function within the body, such as the skin or kidney. Organs are organized
together to perform common functions in the form of systems. The four primary tissues are epithelia,
connective tissues, muscle tissues, and nervous tissues.
33.3 Homeostasis
Homeostasis is a dynamic equilibrium that is maintained in body tissues and organs. It is dynamic because it is
constantly adjusting to the changes that the systems encounter. It is in equilibrium because body functions are
kept within a normal range, with some fluctuations around a set point for the processes.
VISUAL CONNECTION QUESTIONS
1. Figure 33.11 Which of the following statements
about types of epithelial cells is false?
a. Simple columnar epithelial cells line the
tissue of the lung.
b. Simple cuboidal epithelial cells are involved
in the filtering of blood in the kidney.
c. Pseudostratisfied columnar epithilia occur in
a single layer, but the arrangement of nuclei
makes it appear that more than one layer is
present.
d. Transitional epithelia change in thickness
depending on how full the bladder is.
2. Figure 33.21 State whether each of the following
processes are regulated by a positive feedback loop
or a negative feedback loop.
a. A person feels satiated after eating a large
meal.
b. The blood has plenty of red blood cells. As a
result, erythropoietin, a hormone that
stimulates the production of new red blood
cells, is no longer released from the kidney.
3. Figure 33.23 When bacteria are destroyed by
leuckocytes, pyrogens are released into the blood.
Pyrogens reset the body’s thermostat to a higher
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Chapter 33 | The Animal Body: Basic Form and Function
temperature, resulting in fever. How might pyrogens cause the body temperature to rise?
REVIEW QUESTIONS
4. Which type of animal maintains a constant internal
body temperature?
a. endotherm
b. ectotherm
c. coelomate
d. mesoderm
5. The symmetry found in animals that move swiftly is
a. radial
b. bilateral
c. sequential
d. interrupted
6. What term describes the condition of a desert
mouse that lowers its metabolic rate and “sleeps”
during the hot day?
a. turgid
b. hibernation
c. estivation
d. normal sleep pattern
7. A plane that divides an animal into equal right and
left portions is_.
a. diagonal
b. midsagittal
c. coronal
d. transverse
8. A plane that divides an animal into dorsal and
ventral portions is_.
a. sagittal
b. midsagittal
c. coronal
d. transverse
9. The pleural cavity is a part of which cavity?
a. dorsal cavity
b. thoracic cavity
c. abdominal cavity
d. pericardial cavity
10. How could the increasing global temperature
associated with climate change impact ectotherms?
a. Ectotherm diversity will decrease in cool
regions.
b. Ectotherms will be able to be active all day
in the tropics.
c. Ectotherms will have to expend more
energy to cool their body temperatures.
d. Ectotherms will be able to expand into new
habitats.
11. Although most animals are bilaterally
symmetrical, a few exhibit radial symmetry. What is
an advantage of radial symmetry?
a. It confuses predators.
b. It allows the animal to gather food from all
sides.
c. It allows the animal to undergo rapid,
purposeful movement in any direction.
d. It lets an animal use its dorsal surface to
sense its environment.
12. Which type of epithelial cell is best adapted to aid
diffusion?
a. squamous
b. cuboidal
c. columnar
d. transitional
13. Which type of epithelial cell is found in glands?
a. squamous
b. cuboidal
c. columnar
d. transitional
14. Which type of epithelial cell is found in the urinary
bladder?
a. squamous
b. cuboidal
c. columnar
d. transitional
15. Which type of connective tissue has the most
fibers?
a. loose connective tissue
b. fibrous connective tissue
c. cartilage
d. bone
16. Which type of connective tissue has a
mineralized different matrix?
a. loose connective tissue
b. fibrous connective tissue
c. cartilage
d. bone
17. The cell found in bone that breaks it down is
called an_.
a. osteoblast
b. osteocyte
c. osteoclast
d. osteon
18. The cell found in bone that makes the bone is
called an_.
a. osteoblast
b. osteocyte
c. osteoclast
d. osteon
19. Plasma is the_.
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Chapter 33 | The Animal Body: Basic Form and Function
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a. fibers in blood
b. matrix of blood
c. cell that phagocytizes bacteria
d. cell fragment found in the tissue
20. The type of muscle cell under voluntary control is
the_.
a. smooth muscle
b. skeletal muscle
c. cardiac muscle
d. visceral muscle
21. The part of a neuron that contains the nucleus is
the
a. cell body
b. dendrite
c. axon
d. glial
22. Why are intercalated discs essential to the
function of cardiac muscle?
a. The discs maintain the barriers between the
cells.
b. The discs pass nutrients between cells.
c. The discs ensure that all the cardiac muscle
cells beat as a single unit.
d. The discs control the heart rate.
23. When faced with a sudden drop in environmental
temperature, an endothermic animal will:
a. experience a drop in its body temperature
b. wait to see if it goes lower
c. increase muscle activity to generate heat
d. add fur or fat to increase insulation
24. Which is an example of negative feedback?
CRITICAL THINKING QUESTIONS
29. How does diffusion limit the size of an organism?
How is this counteracted?
30. What is the relationship between BMR and body
size? Why?
31. Explain how using an open circulatory system
constrains the size of animals.
32. Describe one key environmental constraint for
ectotherms and one for endotherms. Why are they
limited by different factors?
33. How can squamous epithelia both facilitate
diffusion and prevent damage from abrasion?
34. What are the similarities between cartilage and
bone?
35. Multiple sclerosis is a debilitating autoimmune
disease that results in the loss of the insulation
around neuron axons. What cell type is the immune
a. lowering of blood glucose after a meal
b. blood clotting after an injury
c. lactation during nursing
d. uterine contractions during labor
25. Which method of heat exchange occurs during
direct contact between the source and animal?
a. radiation
b. evaporation
c. convection
d. conduction
26. The body’s thermostat is located in the
a. homeostatic receptor
b. hypothalamus
c. medulla
d. vasodilation center
27. Which of the following is not true about
acclimatization?
a. Acclimatization allows animals to
compensate for changes in their
environment.
b. Acclimatization improves function in a new
environment.
c. Acclimatization occurs when an animal tries
to reestablish a homeostatic set point.
d. Acclimatization is passed on to offspring of
acclimated individuals.
28. Which of the following is not a way that
ectotherms can change their body temperatures?
a. Sweating for evaporative cooling.
b. Adjusting the timing of their daily activities.
c. Seek out or avoid direct sunlight.
d. Huddle in a group.
system attacking, and how does this disrupt the
transfer of messages by the nervous system?
36. When a person leads a sedentary life his skeletal
muscles atrophy, but his smooth muscles do not.
Why?
37. Why are negative feedback loops used to control
body homeostasis?
38. Why is a fever a “good thing" during a bacterial
infection?
39. How is a condition such as diabetes a good
example of the failure of a set point in humans?
40. On a molecular level, how can endotherms
produce their own heat by adjusting processes
associated with cellular respiration? If needed, review
Ch. 7 for details on respiration.
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Chapter 33 | The Animal Body: Basic Form and Function
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Chapter 34 | Animal Nutrition and the Digestive System
1033
34 | ANIMAL NUTRITION
AND THE DIGESTIVE
SYSTEM
Figure 34.1 For humans, fruits and vegetables are important in maintaining a balanced diet, (credit: modification of
work by Julie Rybarczyk)
Chapter Outline
34.1: Digestive Systems
34.2: Nutrition and Energy Production
34.3: Digestive System Processes
34.4: Digestive System Regulation
Introduction
All living organisms need nutrients to survive. While plants can obtain the molecules required for cellular
function through the process of photosynthesis, most animals obtain their nutrients by the consumption of
other organisms. At the cellular level, the biological molecules necessary for animal function are amino acids,
lipid molecules, nucleotides, and simple sugars. However, the food consumed consists of protein, fat, and
complex carbohydrates. Animals must convert these macromolecules into the simple molecules required for
maintaining cellular functions, such as assembling new molecules, cells, and tissues. The conversion of the food
consumed to the nutrients required is a multistep process involving digestion and absorption. During digestion,
food particles are broken down to smaller components, and later, they are absorbed by the body.
One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy
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Chapter 34 | Animal Nutrition and the Digestive System
expenditure. Imbalances can have serious health consequences. For example, eating too much food while
not expending much energy leads to obesity, which in turn will increase the risk of developing illnesses
such as type-2 diabetes and cardiovascular disease. The recent rise in obesity and related diseases makes
understanding the role of diet and nutrition in maintaining good health all the more important.
34.1 1 Digestive Systems
By the end of this section, you will be able to do the following:
• Explain the processes of digestion and absorption
• Compare and contrast different types of digestive systems
• Explain the specialized functions of the organs involved in processing food in the body
• Describe the ways in which organs work together to digest food and absorb nutrients
Animals obtain their nutrition from the consumption of other organisms. Depending on their diet, animals can be
classified into the following categories: plant eaters (herbivores), meat eaters (carnivores), and those that eat
both plants and animals (omnivores). The nutrients and macromolecules present in food are not immediately
accessible to the cells. There are a number of processes that modify food within the animal body in order to
make the nutrients and organic molecules accessible for cellular function. As animals evolved in complexity of
form and function, their digestive systems have also evolved to accommodate their various dietary needs.
Herbivores, Omnivores, and Carnivores
Herbivores are animals whose primary food source is plant-based. Examples of herbivores, as shown in Figure
34.2 include vertebrates like deer, koalas, and some bird species, as well as invertebrates such as crickets and
caterpillars. These animals have evolved digestive systems capable of handling large amounts of plant material.
Herbivores can be further classified into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar
feeders), and folivores (leaf eaters).
Figure 34.2 Herbivores, like this (a) mule deer and (b) monarch caterpillar, eat primarily plant material, (credit a:
modification of work by Bill Ebbesen; credit b: modification of work by Doug Bowman)
Carnivores are animals that eat other animals. The word carnivore is derived from Latin and literally means
“meat eater.” Wild cats such as lions, shown in Figure 34.3a and tigers are examples of vertebrate carnivores,
as are snakes and sharks, while invertebrate carnivores include sea stars, spiders, and ladybugs, shown in
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Chapter 34 | Animal Nutrition and the Digestive System
1035
Figure 34.3b. Obligate carnivores are those that rely entirely on animal flesh to obtain their nutrients; examples
of obligate carnivores are members of the cat family, such as lions and cheetahs. Facultative carnivores are
those that also eat non-animal food in addition to animal food. Note that there is no clear line that differentiates
facultative carnivores from omnivores; dogs would be considered facultative carnivores.
Figure 34.3 Carnivores like the (a) lion eat primarily meat. The (b) ladybug is also a carnivore that consumes small
insects called aphids, (credit a: modification of work by Kevin Pluck; credit b: modification of work by Jon Sullivan)
Omnivores are animals that eat both plant- and animal-derived food. In Latin, omnivore means to eat
everything. Humans, bears (shown in Figure 34.4a), and chickens are example of vertebrate omnivores;
invertebrate omnivores include cockroaches and crayfish (shown in Figure 34.4b).
Figure 34.4 Omnivores like the (a) bear and (b) crayfish eat both plant and animal based food, (credit a: modification
of work by Dave Menke; credit b: modification of work by Jon Sullivan)
Invertebrate Digestive Systems
Animals have evolved different types of digestive systems to aid in the digestion of the different foods they
consume. The simplest example is that of a gastrovascular cavity and is found in organisms with only one
opening for digestion. Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and
sea anemones) use this type of digestion. Gastrovascular cavities, as shown in Figure 34.5a, are typically a
blind tube or cavity with only one opening, the “mouth", which also serves as an “anus”. Ingested material enters
the mouth and passes through a hollow, tubular cavity. Cells within the cavity secrete digestive enzymes that
breakdown the food. The food particles are engulfed by the cells lining the gastrovascular cavity.
The alimentary canal, shown in Figure 34.5b, is a more advanced system: it consists of one tube with a mouth
at one end and an anus at the other. Earthworms are an example of an animal with an alimentary canal. Once
the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the
crop; then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through
the intestine, the nutrients are absorbed, and the waste is eliminated as feces, called castings, through the anus.
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Chapter 34 | Animal Nutrition and the Digestive System
Mouth
Mouth
(a) A gastrovascular cavity has one opening.
Figure 34.5 (a) A gastrovascular cavity has a single opening through which food is ingested and waste is excreted, as
shown in this hydra and in this jellyfish medusa, (b) An alimentary canal has two openings: a mouth for ingesting food,
and an anus for eliminating waste, as shown in this nematode.
Vertebrate Digestive Systems
Vertebrates have evolved more complex digestive systems to adapt to their dietary needs. Some animals have a
single stomach, while others have multi-chambered stomachs. Birds have developed a digestive system adapted
to eating unmasticated food.
Monogastric: Single-chambered Stomach
As the word monogastric suggests, this type of digestive system consists of one (“mono") stomach chamber
(“gastric”). Humans and many animals have a monogastric digestive system as illustrated in Figure 34.6ab.
The process of digestion begins with the mouth and the intake of food. The teeth play an important role in
masticating (chewing) or physically breaking down food into smaller particles. The enzymes present in saliva
also begin to chemically breakdown food. The esophagus is a long tube that connects the mouth to the stomach.
Using peristalsis, or wave-like smooth muscle contractions, the muscles of the esophagus push the food towards
the stomach. In order to speed up the actions of enzymes in the stomach, the stomach is an extremely acidic
environment, with a pH between 1.5 and 2.5. The gastric juices, which include enzymes in the stomach, act on
the food particles and continue the process of digestion. Further breakdown of food takes place in the small
intestine where enzymes produced by the liver, the small intestine, and the pancreas continue the process of
digestion. The nutrients are absorbed into the bloodstream across the epithelial cells lining the walls of the small
intestines. The waste material travels on to the large intestine where water is absorbed and the drier waste
material is compacted into feces; it is stored until it is excreted through the rectum.
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Chapter 34 | Animal Nutrition and the Digestive System
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Stomach
Small intestine
Cecum
Large intestine
(a) Human digestive system (b) Rabbit digestive system
Figure 34.6 (a) Humans and herbivores, such as the (b) rabbit, have a monogastric digestive system. However, in
the rabbit the small intestine and cecum are enlarged to allow more time to digest plant material. The enlarged organ
provides more surface area for absorption of nutrients. Rabbits digest their food twice: the first time food passes
through the digestive system, it collects in the cecum, and then it passes as soft feces called cecotrophes. The rabbit
re-ingests these cecotrophes to further digest them.
Avian
Birds face special challenges when it comes to obtaining nutrition from food. They do not have teeth and
so their digestive system, shown in Figure 34.7, must be able to process un-masticated food. Birds have
evolved a variety of beak types that reflect the vast variety in their diet, ranging from seeds and insects to fruits
and nuts. Because most birds fly, their metabolic rates are high in order to efficiently process food and keep
their body weight low. The stomach of birds has two chambers: the proventriculus, where gastric juices are
produced to digest the food before it enters the stomach, and the gizzard, where the food is stored, soaked, and
mechanically ground. The undigested material forms food pellets that are sometimes regurgitated. Most of the
chemical digestion and absorption happens in the intestine and the waste is excreted through the cloaca.
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Chapter 34 | Animal Nutrition and the Digestive System
Esophagus
Crop
Proventriculus
Gizzard
Small intestine
Large intestine
Cloaca
Figure 34.7 The avian esophagus has a pouch, called a crop, which stores food. Food passes from the crop to
the first of two stomachs, called the proventriculus, which contains digestive juices that breakdown food. From the
proventriculus, the food enters the second stomach, called the gizzard, which grinds food. Some birds swallow stones
or grit, which are stored in the gizzard, to aid the grinding process. Birds do not have separate openings to excrete
urine and feces. Instead, uric acid from the kidneys is secreted into the large intestine and combined with waste from
the digestive process. This waste is excreted through an opening called the cloaca.
e olution CONNECTION
Avian Adaptations
Birds have a highly efficient, simplified digestive system. Recent fossil evidence has shown that the
evolutionary divergence of birds from other land animals was characterized by streamlining and simplifying
the digestive system. Unlike many other animals, birds do not have teeth to chew their food. In place of
lips, they have sharp pointy beaks. The horny beak, lack of jaws, and the smaller tongue of the birds can
be traced back to their dinosaur ancestors. The emergence of these changes seems to coincide with the
inclusion of seeds in the bird diet. Seed-eating birds have beaks that are shaped for grabbing seeds and
the two-compartment stomach allows for delegation of tasks. Since birds need to remain light in order to fly,
their metabolic rates are very high, which means they digest their food very quickly and need to eat often.
Contrast this with the ruminants, where the digestion of plant matter takes a very long time.
Ruminants
Ruminants are mainly herbivores like cows, sheep, and goats, whose entire diet consists of eating large
amounts of roughage or fiber. They have evolved digestive systems that help them digest vast amounts of
cellulose. An interesting feature of the ruminants’ mouth is that they do not have upper incisor teeth. They use
their lower teeth, tongue and lips to tear and chew their food. From the mouth, the food travels to the esophagus
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Chapter 34 | Animal Nutrition and the Digestive System
1039
and on to the stomach.
To help digest the large amount of plant material, the stomach of the ruminants is a multi-chambered organ, as
illustrated in Figure 34.8. The four compartments of the stomach are called the rumen, reticulum, omasum, and
abomasum. These chambers contain many microbes that breakdown cellulose and ferment ingested food. The
abomasum is the “true” stomach and is the equivalent of the monogastric stomach chamber where gastric juices
are secreted. The four-compartment gastric chamber provides larger space and the microbial support necessary
to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach
chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient
absorption, and the large intestine helps in the elimination of waste.
Figure 34.8 Ruminant animals, such as goats and cows, have four stomachs. The first two stomachs, the rumen and
the reticulum, contain prokaryotes and protists that are able to digest cellulose fiber. The ruminant regurgitates cud
from the reticulum, chews it, and swallows it into a third stomach, the omasum, which removes water. The cud then
passes onto the fourth stomach, the abomasum, where it is digested by enzymes produced by the ruminant.
Pseudo-ruminants
Some animals, such as camels and alpacas, are pseudo-ruminants. They eat a lot of plant material and
roughage. Digesting plant material is not easy because plant cell walls contain the polymeric sugar molecule
cellulose. The digestive enzymes of these animals cannot breakdown cellulose, but microorganisms present in
the digestive system can. Therefore, the digestive system must be able to handle large amounts of roughage and
breakdown the cellulose. Pseudo-ruminants have a three-chamber stomach in the digestive system. However,
their cecum—a pouched organ at the beginning of the large intestine containing many microorganisms that are
necessary for the digestion of plant materials—is large and is the site where the roughage is fermented and
digested. These animals do not have a rumen but have an omasum, abomasum, and reticulum.
Parts of the Digestive System
The vertebrate digestive system is designed to facilitate the transformation of food matter into the nutrient
components that sustain organisms.
Oral Cavity
The oral cavity, or mouth, is the point of entry of food into the digestive system, illustrated in Figure 34.9. The
food consumed is broken into smaller particles by mastication, the chewing action of the teeth. All mammals
have teeth and can chew their food.
The extensive chemical process of digestion begins in the mouth. As food is being chewed, saliva, produced by
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Chapter 34 | Animal Nutrition and the Digestive System
the salivary glands, mixes with the food. Saliva is a watery substance produced in the mouths of many animals.
There are three major glands that secrete saliva—the parotid, the submandibular, and the sublingual. Saliva
contains mucus that moistens food and buffers the pH of the food. Saliva also contains immunoglobulins and
lysozymes, which have antibacterial action to reduce tooth decay by inhibiting growth of some bacteria. Saliva
also contains an enzyme called salivary amylase that begins the process of converting starches in the food into
a disaccharide called maltose. Another enzyme called lipase is produced by the cells in the tongue. Lipases are
a class of enzymes that can breakdown triglycerides. The lingual lipase begins the breakdown of fat components
in the food. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called
the bolus for swallowing. The tongue helps in swallowing—moving the bolus from the mouth into the pharynx.
The pharynx opens to two passageways: the trachea, which leads to the lungs, and the esophagus, which leads
to the stomach. The trachea has an opening called the glottis, which is covered by a cartilaginous flap called
the epiglottis. When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the
trachea. This arrangement allows food to be kept out of the trachea.
Nasal cavitv
(a)
(b)
Figure 34.9 Digestion of food begins in the (a) oral cavity. Food is masticated by teeth and moistened by saliva
secreted from the (b) salivary glands. Enzymes in the saliva begin to digest starches and fats. With the help of the
tongue, the resulting bolus is moved into the esophagus by swallowing, (credit: modification of work by the National
Cancer Institute)
Esophagus
The esophagus is a tubular organ that connects the mouth to the stomach. The chewed and softened food
passes through the esophagus after being swallowed. The smooth muscles of the esophagus undergo a series
of wave like movements called peristalsis that push the food toward the stomach, as illustrated in Figure 34.10.
The peristalsis wave is unidirectional—it moves food from the mouth to the stomach, and reverse movement is
not possible. The peristaltic movement of the esophagus is an involuntary reflex; it takes place in response to
the act of swallowing.
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Chapter 34 | Animal Nutrition and the Digestive System
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Direction
of food
Figure 34.10 The esophagus transfers food from the mouth to the stomach through peristaltic movements.
A ring-like muscle called a sphincter forms valves in the digestive system. The gastro-esophageal sphincter
is located at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the
bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this
sphincter is shut and prevents the contents of the stomach from traveling up the esophagus. Many animals
have a true sphincter; however, in humans, there is no true sphincter, but the esophagus remains closed when
there is no swallowing action. Acid reflux or “heartburn” occurs when the acidic digestive juices escape into the
esophagus.
Stomach
A large part of digestion occurs in the stomach, shown in Figure 34.11. The stomach is a saclike organ that
secretes gastric digestive juices. The pH in the stomach is between 1.5 and 2.5. This highly acidic environment is
required for the chemical breakdown of food and the extraction of nutrients. When empty, the stomach is a rather
small organ; however, it can expand to up to 20 times its resting size when filled with food. This characteristic is
particularly useful for animals that need to eat when food is available.
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Chapter 34 | Animal Nutrition and the Digestive System
visual
CONNECTION
Mouth -
Tongue
Esophagus
Salivary glands:
- Parotid gland
- Sublingual gland
- Submandibular gland
Pharynx
\
Liver
Gallbladder .
Small intestine:
Duodenum-
Jejunum ^
Ileum.
Anus
Stomach
Spleen
Pancreas
Large intestine:
- Transverse colon
Ascending colon
Descending colon
Cecum
Sigmoid colon
Appendix
Rectum
Anal canal
Figure 34.11 The human stomach has an extremely acidic environment where most of the protein gets digested,
(credit: modification of work by Mariana Ruiz Villareal)
Which of the following statements about the digestive system is false?
a. Chyme is a mixture of food and digestive juices that is produced in the stomach.
b. Food enters the large intestine before the small intestine.
c. in the small intestine, chyme mixes with bile, which emulsifies fats.
d. The stomach is separated from the small intestine by the pyloric sphincter.
The stomach is also the major site for protein digestion in animals other than ruminants. Protein digestion is
mediated by an enzyme called pepsin in the stomach chamber. Pepsin is secreted by the chief cells in the
stomach in an inactive form called pepsinogen. Pepsin breaks peptide bonds and cleaves proteins into smaller
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Chapter 34 | Animal Nutrition and the Digestive System
1043
polypeptides; it also helps activate more pepsinogen, starting a positive feedback mechanism that generates
more pepsin. Another cell type—parietal cells—secrete hydrogen and chloride ions, which combine in the lumen
to form hydrochloric acid, the primary acidic component of the stomach juices. Hydrochloric acid helps to convert
the inactive pepsinogen to pepsin. The highly acidic environment also kills many microorganisms in the food
and, combined with the action of the enzyme pepsin, results in the hydrolysis of protein in the food. Chemical
digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles
mixes the stomach contents about every 20 minutes. The partially digested food and gastric juice mixture is
called chyme. Chyme passes from the stomach to the small intestine. Further protein digestion takes place in
the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme
is released into the small intestine at a time. The movement of chyme from the stomach into the small intestine
is regulated by the pyloric sphincter.
When digesting protein and some fats, the stomach lining must be protected from getting digested by pepsin.
There are two points to consider when describing how the stomach lining is protected. First, as previously
mentioned, the enzyme pepsin is synthesized in the inactive form. This protects the chief cells, because
pepsinogen does not have the same enzyme functionality of pepsin. Second, the stomach has a thick mucus
lining that protects the underlying tissue from the action of the digestive juices. When this mucus lining is
ruptured, ulcers can form in the stomach. Ulcers are open wounds in or on an organ caused by bacteria
(,Helicobacter pylori ) when the mucus lining is ruptured and fails to reform.
Small Intestine
Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of
protein, fats, and carbohydrates is completed. The small intestine is a long tube-like organ with a highly folded
surface containing finger-like projections called the villi. The apical surface of each villus has many microscopic
projections called microvilli. These structures, illustrated in Figure 34.12, are lined with epithelial cells on the
luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the bloodstream
on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and
increase absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal
vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and
removes toxic substances, including drugs, alcohol, and some pathogens.
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Chapter 34 | Animal Nutrition and the Digestive System
CONNECTION
Ml
Lymphatic vessel
Figure 34.12 Villi are folds on the small intestine lining that increase the surface area to facilitate the absorption
of nutrients.
Which of the following statements about the small intestine is false?
a. Absorptive cells that line the small intestine have microvilli, small projections that increase surface area
and aid in the absorption of food.
b. The inside of the small intestine has many folds, called villi.
c. Microvilli are lined with blood vessels as well as lymphatic vessels.
d. The inside of the small intestine is called the lumen.
The human small intestine is over 6m long and is divided into three parts: the duodenum, the jejunum, and the
ileum. The “C-shaped,” fixed part of the small intestine is called the duodenum and is shown in Figure 34.11.
The duodenum is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from
the stomach to the duodenum. In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution
rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer. Pancreatic juices also contain
several digestive enzymes. Digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells
of the intestinal wall itself, enter the duodenum. Bile is produced in the liver and stored and concentrated in the
gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize
starches, disaccharides, proteins, and fats. These digestive juices breakdown the food particles in the chyme
into glucose, triglycerides, and amino acids. Some chemical digestion of food takes place in the duodenum.
Absorption of fatty acids also takes place in the duodenum.
The second part of the small intestine is called the jejunum, shown in Figure 34.11. Here, hydrolysis of nutrients
is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. The
bulk of chemical digestion and nutrient absorption occurs in the jejunum.
The ileum, also illustrated in Figure 34.11 is the last part of the small intestine and here the bile salts and
vitamins are absorbed into the bloodstream. The undigested food is sent to the colon from the ileum via
peristaltic movements of the muscle. The ileum ends and the large intestine begins at the ileocecal valve. The
vermiform, “worm-like,” appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes
and has an insignificant role in immunity.
Large Intestine
The large intestine, illustrated in Figure 34.13, reabsorbs the water from the undigested food material and
processes the waste material. The human large intestine is much smaller in length compared to the small
intestine but larger in diameter. It has three parts: the cecum, the colon, and the rectum. The cecum joins the
ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or
“intestinal flora" that aid in the digestive processes. The colon can be divided into four regions, the ascending
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Chapter 34 | Animal Nutrition and the Digestive System
1045
colon, the transverse colon, the descending colon, and the sigmoid colon. The main functions of the colon are
to extract the water and mineral salts from undigested food, and to store waste material. Carnivorous mammals
have a shorter large intestine compared to herbivorous mammals due to their diet.
Transverse
colon
Ascending
colon
Cecum
Vermiform
appendix
Descending
colon
Sigmoid
colon
Rectum
Anus
Figure 34.13 The large intestine reabsorbs water from undigested food and stores waste material until it is eliminated.
Rectum and Anus
The rectum is the terminal end of the large intestine, as shown in Figure 34.13. The primary role of the rectum
is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The
anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. Two sphincters
between the rectum and anus control elimination: the inner sphincter is involuntary and the outer sphincter is
voluntary.
Accessory Organs
The organs discussed above are the organs of the digestive tract through which food passes. Accessory organs
are organs that add secretions (enzymes) that catabolize food into nutrients. Accessory organs include salivary
glands, the liver, the pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by
hormones in response to the food consumed.
The liver is the largest internal organ in humans and it plays a very important role in digestion of fats and
detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fatty components
of the food in the duodenum. The liver also processes the vitamins and fats and synthesizes many plasma
proteins.
The pancreas is another important gland that secretes digestive juices. The chyme produced from the stomach
is highly acidic in nature; the pancreatic juices contain high levels of bicarbonate, an alkali that neutralizes the
acidic chyme. Additionally, the pancreatic juices contain a large variety of enzymes that are required for the
digestion of protein and carbohydrates.
The gallbladder is a small organ that aids the liver by storing bile and concentrating bile salts. When chyme
containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum.
34.2 | Nutrition and Energy Production
By the end of this section, you will be able to do the following:
• Explain why an animal’s diet should be balanced and meet the needs of the body
• Define the primary components of food
• Describe the essential nutrients required for cellular function that cannot be synthesized by the animal
body
• Explain how energy is produced through diet and digestion
• Describe how excess carbohydrates and energy are stored in the body
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Chapter 34 | Animal Nutrition and the Digestive System
Given the diversity of animal life on our planet, it is not surprising that the animal diet would also vary
substantially. The animal diet is the source of materials needed for building DNA and other complex molecules
needed for growth, maintenance, and reproduction; collectively these processes are called biosynthesis. The
diet is also the source of materials for ATP production in the cells. The diet must be balanced to provide the
minerals and vitamins that are required for cellular function.
Food Requirements
What are the fundamental requirements of the animal diet? The animal diet should be well balanced and
provide nutrients required for bodily function and the minerals and vitamins required for maintaining structure and
regulation necessary for good health and reproductive capability. These requirements for a human are illustrated
graphically in Figure 34.14
Figure 34.14 For humans, a balanced diet includes fruits, vegetables, grains, and protein, (credit: USDA)
LINK
T &
LEARNING
The first step in ensuring that you are meeting the food requirements of your body is an awareness of the
food groups and the nutrients they provide. To learn more about each food group and the recommended
daily amounts, explore this interactive site (http:// 0 penstaxc 0 llege. 0 rg/l/f 00 d_gmups) by the United States
Department of Agriculture.
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Chapter 34 | Animal Nutrition and the Digestive System
1047
everyday CONNECTION
Let’s Move! Campaign
Obesity is a growing epidemic and the rate of obesity among children is rapidly rising in the United States.
To combat childhood obesity and ensure that children get a healthy start in life, first lady Michelle Obama
has launched the Let’s Move! campaign. The goal of this campaign is to educate parents and caregivers
on providing healthy nutrition and encouraging active lifestyles to future generations. This program aims to
involve the entire community, including parents, teachers, and healthcare providers to ensure that children
have access to healthy foods—more fruits, vegetables, and whole grains—and consume fewer calories from
processed foods. Another goal is to ensure that children get physical activity. With the increase in television
viewing and stationary pursuits such as video games, sedentary lifestyles have become the norm. Learn
more at https://letsmove.obamawhitehouse.archives.gov (http:// 0 penstax. 0 rg/l/Letsm 0 ve) .
Organic Precursors
The organic molecules required for building cellular material and tissues must come from food. Carbohydrates or
sugars are the primary source of organic carbons in the animal body. During digestion, digestible carbohydrates
are ultimately broken down into glucose and used to provide energy through metabolic pathways. Complex
carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification;
however, humans do not produce the enzyme cellulase and lack the ability to derive glucose from the
polysaccharide cellulose. In humans, these molecules provide the fiber required for moving waste through the
large intestine and a healthy colon. The intestinal flora in the human gut are able to extract some nutrition from
these plant fibers. The excess sugars in the body are converted into glycogen and stored in the liver and muscles
for later use. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide
energy during food shortage. Excess glycogen can be converted to fats, which are stored in the lower layer of
the skin of mammals for insulation and energy storage. Excess digestible carbohydrates are stored by mammals
in order to survive famine and aid in mobility.
Another important requirement is that of nitrogen. Protein catabolism provides a source of organic nitrogen.
Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for
cellular function. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic
acids, proteins, cells, and tissues. Excess nitrogen must be excreted as it is toxic. Fats add flavor to food and
promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram
of fat contains nine calories. Fats are required in the diet to aid the absorption of fat-soluble vitamins and the
production of fat-soluble hormones.
Essential Nutrients
While the animal body can synthesize many of the molecules required for function from the organic precursors,
there are some nutrients that need to be consumed from food. These nutrients are termed essential nutrients,
meaning they must be eaten, and the body cannot produce them.
The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to make some
membrane phospholipids. Vitamins are another class of essential organic molecules that are required in small
quantities for many enzymes to function and, for this reason, are considered to be coenzymes. Absence or low
levels of vitamins can have a dramatic effect on health, as outlined in Table 34.1 and Table 34.2. Both fat-soluble
and water-soluble vitamins must be obtained from food. Minerals, listed in Table 34.3, are inorganic essential
nutrients that must be obtained from food. Among their many functions, minerals help in structure and regulation
and are considered cofactors. Certain amino acids also must be procured from food and cannot be synthesized
by the body. These amino acids are the “essential" amino acids. The human body can synthesize only 11 of the
20 required amino acids; the rest must be obtained from food. The essential amino acids are listed in Table 34.4.
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Chapter 34 | Animal Nutrition and the Digestive System
Water-soluble Essential Vitamins
Vitamin
Function
Deficiencies Can Lead To
Sources
Vitamin Bi
(Thiamine)
Needed by the body to process lipids,
proteins, and carbohydrates; coenzyme
removes CO 2 from organic compounds
Muscle weakness, Beriberi: reduced
heart function, CNS problems
Milk, meat,
dried beans,
whole grains
Vitamin B 2
(Riboflavin)
Takes an active role in metabolism,
aiding in the conversion of food to energy
(FAD and FMN)
Cracks or sores on the outer surface
of the lips (cheliosis); inflammation
and redness of the tongue; moist,
scaly skin inflammation (seborrheic
dermatitis)
Meat, eggs,
enriched
grains,
vegetables
Vitamin B3
(Niacin)
Used by the body to release energy from
carbohydrates and to process alcohol;
required for the synthesis of sex
hormones; component of coenzyme
NAD + and NADP +
Pellagra, which can result in
dermatitis, diarrhea, dementia, and
death
Meat, eggs,
grains, nuts,
potatoes
Vitamin B5
(Pantothenic
acid)
Assists in producing energy from foods
(lipids, in particular); component of
coenzyme A
Fatigue, poor coordination, retarded
growth, numbness, tingling of hands
and feet
Meat, whole
grains, milk,
fruits,
vegetables
Vitamin B6
(Pyridoxine)
The principal vitamin for processing
amino acids and lipids; also helps
convert nutrients into energy
Irritability, depression, confusion,
mouth sores or ulcers, anemia,
muscular twitching
Meat, dairy
products,
whole grains,
orange juice
Vitamin B7
(Biotin)
Used in energy and amino acid
metabolism, fat synthesis, and fat
breakdown; helps the body use blood
sugar
Hair loss, dermatitis, depression,
numbness and tingling in the
extremities; neuromuscular disorders
Meat, eggs,
legumes and
other
vegetables
Vitamin Bg
(Folic acid)
Assists the normal development of cells,
especially during fetal development;
helps metabolize nucleic and amino
acids
Deficiency during pregnancy is
associated with birth defects, such as
neural tube defects and anemia
Leafy green
vegetables,
whole wheat,
fruits, nuts,
legumes
Vitamin B 12
(Cobalamin)
Maintains healthy nervous system and
assists with blood cell formation;
coenzyme in nucleic acid metabolism
Anemia, neurological disorders,
numbness, loss of balance
Meat, eggs,
animal
products
Vitamin C
(Ascorbic
acid)
Helps maintain connective tissue: bone,
cartilage, and dentin; boosts the immune
system
Scurvy, which results in bleeding, hair
and tooth loss; joint pain and
swelling; delayed wound healing
Citrus fruits,
broccoli,
tomatoes, red
sweet bell
peppers
Table 34.1
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Chapter 34 | Animal Nutrition and the Digestive System
1049
Fat-soluble Essential Vitamins
Deficiencies
Vitamin Function Can Lead Sources
To
Vitamin A
(Retinol)
Critical to the development of bones, teeth, and
skin; helps maintain eyesight, enhances the
immune system, fetal development, gene
expression
Night-blindness,
skin disorders,
impaired
immunity
Dark green leafy
vegetables, yellow-
orange vegetables,
fruits, milk, butter
Vitamin D
Critical for calcium absorption for bone
development and strength; maintains a stable
nervous system; maintains a normal and strong
heartbeat; helps in blood clotting
Rickets,
osteomalacia,
immunity
Cod liver oil, milk, egg
yolk
Vitamin E
(Tocopherol)
Lessens oxidative damage of cells and prevents
lung damage from pollutants; vital to the immune
system
Deficiency is
rare; anemia,
nervous system
degeneration
Wheat germ oil,
unrefined vegetable
oils, nuts, seeds,
grains
Vitamin K
(Phylloquinone)
Essential to blood clotting
Bleeding and
easy bruising
Leafy green
vegetables, tea
Table 34.2
Figure 34.15 A healthy diet should include a variety of foods to ensure that needs for essential nutrients are met.
(credit: Keith Weller, USDA ARS)
Minerals and Their Function in the Human Body
Mineral
Function
Deficiencies Can
Lead To
Sources
*Calcium
Needed for muscle and neuron function;
heart health; builds bone and supports
synthesis and function of blood cells;
nerve function
Osteoporosis, rickets,
muscle spasms,
impaired growth
Milk, yogurt, fish, green leafy
vegetables, legumes
Table 34.3
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Chapter 34 | Animal Nutrition and the Digestive System
Minerals and Their Function in the Human Body
Mineral
Function
Deficiencies Can
Lead To
Sources
*Chlorine
Needed for production of hydrochloric
acid (HCI) in the stomach and nerve
function; osmotic balance
Muscle cramps, mood
disturbances, reduced
appetite
Table salt
Copper
(trace
amounts)
Required component of many redox
enzymes, including cytochrome c
oxidase; cofactor for hemoglobin
synthesis
Copper deficiency is
rare
Liver, oysters, cocoa,
chocolate, sesame, nuts
Iodine
Required for the synthesis of thyroid
hormones
Goiter
Seafood, iodized salt, dairy
products
Iron
Required for many proteins and
enzymes, notably hemoglobin, to
prevent anemia
Anemia, which causes
poor concentration,
fatigue, and poor
immune function
Red meat, leafy green
vegetables, fish (tuna,
salmon), eggs, dried fruits,
beans, whole grains
*Magnesium
Required cofactor for ATP formation;
bone formation; normal membrane
functions; muscle function
Mood disturbances,
muscle spasms
Whole grains, leafy green
vegetables
Manganese
(trace
amounts)
A cofactor in enzyme functions; trace
amounts are required
Manganese deficiency
is rare
Common in most foods
Molybdenum
(trace
amounts)
Acts as a cofactor for three essential
enzymes in humans: sulfite oxidase,
xanthine oxidase, and aldehyde oxidase
Molybdenum deficiency
is rare
*Phosphorus
A component of bones and teeth; helps
regulate acid-base balance; nucleotide
synthesis
Weakness, bone
abnormalities, calcium
loss
Milk, hard cheese, whole
grains, meats
*Potassium
Vital for muscles, heart, and nerve
function
Cardiac rhythm
disturbance, muscle
weakness
Legumes, potato skin,
tomatoes, bananas
Selenium
(trace
amounts)
A cofactor essential to activity of
antioxidant enzymes like glutathione
peroxidase; trace amounts are required
Selenium deficiency is
rare
Common in most foods
*Sodium
Systemic electrolyte required for many
functions; acid-base balance; water
balance; nerve function
Muscle cramps,
fatigue, reduced
appetite
Table salt
Zinc (trace
amounts)
Required for several enzymes such as
carboxypeptidase, liver alcohol
dehydrogenase, and carbonic
anhydrase
Anemia, poor wound
healing, can lead to
short stature
Common in most foods
*Greater than 200mg/day required
Table 34.3
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Chapter 34 | Animal Nutrition and the Digestive System
1051
Essential Amino Acids
Amino acids that must be consumed
Amino acids anabolized by the body
isoleucine
alanine
leucine
selenocysteine
lysine
aspartate
methionine
cysteine
phenylalanine
glutamate
tryptophan
glycine
valine
proline
histidine*
serine
threonine
tyrosine
arginine*
asparagine
*The human body can synthesize histidine and arginine
growing children.
, but not in the quantities required, especially for
Table 34.4
Food Energy and ATP
Animals need food to obtain energy and maintain homeostasis. Homeostasis is the ability of a system to maintain
a stable internal environment even in the face of external changes to the environment. For example, the normal
body temperature of humans is 37°C (98.6°F). Humans maintain this temperature even when the external
temperature is hot or cold. It takes energy to maintain this body temperature, and animals obtain this energy
from food.
The primary source of energy for animals is carbohydrates, mainly glucose. Glucose is called the body’s fuel.
The digestible carbohydrates in an animal’s diet are converted to glucose molecules through a series of catabolic
chemical reactions.
Adenosine triphosphate, or ATP, is the primary energy currency in cells; ATP stores energy in phosphate ester
bonds. ATP releases energy when the phosphodiester bonds are broken and ATP is converted to ADP and
a phosphate group. ATP is produced by the oxidative reactions in the cytoplasm and mitochondrion of the
cell, where carbohydrates, proteins, and fats undergo a series of metabolic reactions collectively called cellular
respiration. For example, glycolysis is a series of reactions in which glucose is converted to pyruvic acid and
some of its chemical potential energy is transferred to NADH and ATP.
ATP is required for all cellular functions. It is used to build the organic molecules that are required for cells and
tissues; it provides energy for muscle contraction and for the transmission of electrical signals in the nervous
system. When the amount of ATP is available in excess of the body’s requirements, the liver uses the excess
ATP and excess glucose to produce molecules called glycogen. Glycogen is a polymeric form of glucose and
is stored in the liver and skeletal muscle cells. When blood sugar drops, the liver releases glucose from stores
of glycogen. Skeletal muscle converts glycogen to glucose during intense exercise. The process of converting
glucose and excess ATP to glycogen and the storage of excess energy is an evolutionarily important step in
helping animals deal with mobility, food shortages, and famine.
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Chapter 34 | Animal Nutrition and the Digestive System
everyday CONNECTION
Obesity
Obesity is a major health concern in the United States, and there is a growing focus on reducing obesity and
the diseases it may lead to, such as type-2 diabetes, cancers of the colon and breast, and cardiovascular
disease. How does the food consumed contribute to obesity?
Fatty foods are calorie-dense, meaning that they have more calories per unit mass than carbohydrates or
proteins. One gram of carbohydrates has four calories, one gram of protein has four calories, and one gram
of fat has nine calories. Animals tend to seek lipid-rich food for their higher energy content.
The signals of hunger (“time to eat”) and satiety (“time to stop eating”) are controlled in the hypothalamus
region of the brain. Foods that are rich in fatty acids tend to promote satiety more than foods that are rich
only in carbohydrates.
Excess carbohydrate and ATP are used by the liver to synthesize glycogen. The pyruvate produced during
glycolysis is used to synthesize fatty acids. When there is more glucose in the body than required, the
resulting excess pyruvate is converted into molecules that eventually result in the synthesis of fatty acids
within the body. These fatty acids are stored in adipose cells—the fat cells in the mammalian body whose
primary role is to store fat for later use.
It is important to note that some animals benefit from obesity. Polar bears and seals need body fat for
insulation and to keep them from losing body heat during Arctic winters. When food is scarce, stored body
fat provides energy for maintaining homeostasis. Fats prevent famine in mammals, allowing them to access
energy when food is not available on a daily basis; fats are stored when a large kill is made or lots of food
is available.
34.3 | Digestive System Processes
By the end of this section, you will be able to do the following:
• Describe the process of digestion
• Detail the steps involved in digestion and absorption
• Define elimination
• Explain the role of both the small and large intestines in absorption
Obtaining nutrition and energy from food is a multistep process. For true animals, the first step is ingestion, the
act of taking in food. This is followed by digestion, absorption, and elimination. In the following sections, each of
these steps will be discussed in detail.
Ingestion
The large molecules found in intact food cannot pass through the cell membranes. Food needs to be broken into
smaller particles so that animals can harness the nutrients and organic molecules. The first step in this process
is ingestion. Ingestion is the process of taking in food through the mouth. In vertebrates, the teeth, saliva, and
tongue play important roles in mastication (preparing the food into bolus). While the food is being mechanically
broken down, the enzymes in saliva begin to chemically process the food as well. The combined action of these
processes modifies the food from large particles to a soft mass that can be swallowed and can travel the length
of the esophagus.
Digestion and Absorption
Digestion is the mechanical and chemical breakdown of food into small organic fragments. It is important to
breakdown macromolecules into smaller fragments that are of suitable size for absorption across the digestive
epithelium. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler
particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play
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Chapter 34 | Animal Nutrition and the Digestive System
1053
specific roles in the digestive process. The animal diet needs carbohydrates, protein, and fat, as well as vitamins
and inorganic components for nutritional balance. How each of these components is digested is discussed in the
following sections.
Carbohydrates
The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of
food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach,
no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does
produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.
The next step of carbohydrate digestion takes place in the duodenum. Recall that the chyme from the stomach
enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic
juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a
disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases,
sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Maltase breaks
down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase
and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and
lactase breaks down lactose (or “milk sugar”) into glucose and galactose. The monosaccharides (glucose) thus
produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides
are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the
body. The steps in carbohydrate digestion are summarized in Figure 34.16 and Table 34.5.
Polysaccharides
Disaccharides
Monosaccharides
Figure 34.16 Digestion of carbohydrates is performed by several enzymes. Starch and glycogen are broken down into
glucose by amylase and maltase. Sucrose (table sugar) and lactose (milk sugar) are broken down by sucrase and
lactase, respectively.
Digestion of Carbohydrates
Enzyme
Produced By
Site of
Action
Substrate
Acting On
End Products
Salivary amylase
Salivary glands
Mouth
Polysaccharides
(Starch)
Disaccharides (maltose),
oligosaccharides
Pancreatic amylase
Pancreas
Small
intestine
Polysaccharides
(starch)
Disaccharides (maltose),
monosaccharides
Oligosaccharidases
Lining of the intestine;
brush border membrane
Small
intestine
Disaccharides
Monosaccharides (e.g.,
glucose, fructose, galactose)
Table 34.5
Protein
A large part of protein digestion takes place in the stomach. The enzyme pepsin plays an important role in the
digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino
acids. In the duodenum, other enzymes— trypsin, elastase, and chymotrypsin —act on the peptides reducing
them to smaller peptides. Trypsin elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas
and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino
acids is aided by enzymes called peptidases (those that breakdown peptides). Specifically, carboxypeptidase,
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Chapter 34 | Animal Nutrition and the Digestive System
dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The
amino acids are absorbed into the bloodstream through the small intestines. The steps in protein digestion are
summarized in Figure 34.17 and Table 34.6.
The liver regulates
distribution of
amino acids to the
rest of the body.
Protein-digesting
enzymes are
secreted from
the pancreas
into the small
intestine.
A small amount —
of dietary protein
is lost in the feces.
In the small
intestine, a variety
of enzymes break
large peptides
into smaller
peptides, and then
into individual
amino acids.
In the stomach,
pepsin breaks
down proteins
into fragments,
called peptides
Amino acids are
absorbed from the
small intestine into
the bloodstream.
Figure 34.17 Protein digestion is a multistep process that begins in the stomach and continues through the intestines.
Digestion of Protein
Enzyme
Produced
By
Site of
Action
Substrate Acting
On
End Products
Pepsin
Stomach chief
cells
Stomach
Proteins
Peptides
Table 34.6
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Chapter 34 | Animal Nutrition and the Digestive System
1055
Digestion of Protein
Enzyme
Produced
By
Site of
Action
Substrate Acting
On
End Products
Trypsin
Elastase
Chymotrypsin
Pancreas
Small intestine
Proteins
Peptides
Carboxypeptidase
Pancreas
Small intestine
Peptides
Amino acids and
peptides
Aminopeptidase
Dipeptidase
Lining of intestine
Small intestine
Peptides
Amino acids
Table 34.6
Lipids
Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the bulk of lipid
digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal
responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in
the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid
globules are broken down into several small lipid globules. These small globules are more widely distributed
in the chyme rather than forming large aggregates. Lipids are hydrophobic substances: in the presence of
water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are
amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can
interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile
salts emulsify large lipid globules into small lipid globules.
Why is emulsification important for digestion of lipids? Pancreatic juices contain enzymes called lipases
(enzymes that breakdown lipids). If the lipid in the chyme aggregates into large globules, very little surface area
of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion,
bile salts increase the available surface area of the lipids many fold. The pancreatic lipases can then act on the
lipids more efficiently and digest them, as detailed in Figure 34.18. Lipases breakdown the lipids into fatty acids
and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial
cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny
spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where
the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the
micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells
to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are
called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their
surface. The surface is also composed of the hydrophilic phosphate "heads" of phospholipids. Together, they
enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons
leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in
the subclavian vein.
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Chapter 34 | Animal Nutrition and the Digestive System
1. Lipids are emulsified
by the bile.
Lumen of V Emu | s | on \
., . Micelles
Absorptive
epithelial cell
Lymphatic
capillary
-2. Lipases break
down triglycerides
into fatty acids and
monoglycerides.
3. Fatty acids and
monoglycerides
are packaged into
micelles that are
absorbed by
microvilli.
4. Fatty acids and
monoglycerides
are converted
back into
triglycerides.
The triglycerides
aggregate with
cholesterol,
proteins, and
phospholipids to
form chylomicrons.
-5. The chylomicrons
move into a lymph
capillary, which
transports them to
the rest of the body.
(a)
Triglyceride (fat)
(b)
Figure 34.18 Lipids are digested and absorbed in the small intestine.
Vitamins
Vitamins can be either water-soluble or lipid-soluble. Fat soluble vitamins are absorbed in the same manner as
lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins.
Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.
LINK TO LEARNING
This website (http:// 0 penstaxc 0 llege. 0 rg/l/digest_enzymes) has an overview of the digestion of protein, fat,
and carbohydrates.
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Chapter 34 | Animal Nutrition and the Digestive System
1057
visual
CONNECTION
Mechanical digestion (chewing
and swallowing)
Chemical digestion of
carbohydrates
Mechanical digestion (peristaltic
mixing and propulsion)
Chemical digestion of proteins
Absorption of lipid-soluble
substances, such as aspirin
Mechanical digestion (mixing
and propulsion, primarily
by segmentation)
Chemical digestion of
carbohydrates, lipids, proteins,
and nucleic acids
Absorption of peptides, amino
acids, glucose, fructose, lipids,
water, minerals, and vitamins
Mechanical digestion
(segmental mixing, mass
movement for propulsion)
No chemical digestion except
by bacteria
Absorption of ions, water,
minerals, vitamins, and small
organic molecules produced
by bacteria
Figure 34.19 Mechanical and chemical digestion of food takes place in many steps, beginning in the mouth and
ending in the rectum.
Which of the following statements about digestive processes is true?
a. Amylase, maltase, and lactase in the mouth digest carbohydrates.
b. Trypsin and lipase in the stomach digest protein.
c. Bile emulsifies lipids in the small intestine.
d. No food is absorbed until the small intestine.
Elimination
The final step in digestion is the elimination of undigested food content and waste products. The undigested
food material enters the colon, where most of the water is reabsorbed. Recall that the colon is also home to the
microflora called “intestinal flora” that aid in the digestion process. The semi-solid waste is moved through the
colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to
storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is
eliminated through the anus using peristaltic movements of the rectum.
Common Problems with Elimination
Diarrhea and constipation are some of the most common health concerns that affect digestion. Constipation is
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Chapter 34 | Animal Nutrition and the Digestive System
a condition where the feces are hardened because of excess water removal in the colon. In contrast, if enough
water is not removed from the feces, it results in diarrhea. Many bacteria, including the ones that cause cholera,
affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.
Emesis
Emesis, or vomiting, is elimination of food by forceful expulsion through the mouth. It is often in response to an
irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food
poisoning. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles.
The process of emesis is regulated by the medulla.
34.4 | Digestive System Regulation
By the end of this section, you will be able to do the following:
• Discuss the role of neural regulation in digestive processes
• Explain how hormones regulate digestion
The brain is the control center for the sensation of hunger and satiety. The functions of the digestive system are
regulated through neural and hormonal responses.
Neural Responses to Food
In reaction to the smell, sight, or thought of food, like that shown in Figure 34.20, the first response is that of
salivation. The salivary glands secrete more saliva in response to stimulation by the autonomic nervous system
triggered by food in preparation for digestion. Simultaneously, the stomach begins to produce hydrochloric acid
to digest the food. Recall that the peristaltic movements of the esophagus and other organs of the digestive tract
are under the control of the brain. The brain prepares these muscles for movement as well. When the stomach
is full, the part of the brain that detects satiety signals fullness. There are three overlapping phases of gastric
control—the cephalic phase, the gastric phase, and the intestinal phase—each requires many enzymes and is
under neural control as well.
Figure 34.20 Seeing a plate of food triggers the secretion of saliva in the mouth and the production of HCL in the
stomach, (credit: Kelly Bailey)
Digestive Phases
The response to food begins even before food enters the mouth. The first phase of ingestion, called the cephalic
phase, is controlled by the neural response to the stimulus provided by food. All aspects—such as sight, sense,
and smell—trigger the neural responses resulting in salivation and secretion of gastric juices. The gastric and
salivary secretion in the cephalic phase can also take place due to the thought of food. Right now, if you think
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Chapter 34 | Animal Nutrition and the Digestive System
1059
about a piece of chocolate or a crispy potato chip, the increase in salivation is a cephalic phase response to the
thought. The central nervous system prepares the stomach to receive food.
The gastric phase begins once the food arrives in the stomach. It builds on the stimulation provided during
the cephalic phase. Gastric acids and enzymes process the ingested materials. The gastric phase is stimulated
by (1) distension of the stomach, (2) a decrease in the pH of the gastric contents, and (3) the presence of
undigested material. This phase consists of local, hormonal, and neural responses. These responses stimulate
secretions and powerful contractions.
The intestinal phase begins when chyme enters the small intestine triggering digestive secretions. This phase
controls the rate of gastric emptying. In addition to gastrin emptying, when chyme enters the small intestine, it
triggers other hormonal and neural events that coordinate the activities of the intestinal tract, pancreas, liver, and
gallbladder.
Hormonal Responses to Food
The endocrine system controls the response of the various glands in the body and the release of hormones at
the appropriate times.
One of the important factors under hormonal control is the stomach acid environment. During the gastric phase,
the hormone gastrin is secreted by G cells in the stomach in response to the presence of proteins. Gastrin
stimulates the release of stomach acid, or hydrochloric acid (HCI) which aids in the digestion of the proteins.
However, when the stomach is emptied, the acidic environment need not be maintained and a hormone called
somatostatin stops the release of hydrochloric acid. This is controlled by a negative feedback mechanism.
In the duodenum, digestive secretions from the liver, pancreas, and gallbladder play an important role in
digesting chyme during the intestinal phase. In order to neutralize the acidic chyme, a hormone called secretin
stimulates the pancreas to produce alkaline bicarbonate solution and deliver it to the duodenum. Secretin acts
in tandem with another hormone called cholecystokinin (CCK). Not only does CCK stimulate the pancreas to
produce the requisite pancreatic juices, it also stimulates the gallbladder to release bile into the duodenum.
LINK TQ LEARNING
Visit this website (http:// 0 penstaxc 0 llege. 0 rg/l/enteric_end 0 ) to learn more about the endocrine system.
Review the text and watch the animation of how control is implemented in the endocrine system.
Another level of hormonal control occurs in response to the composition of food. Foods high in lipids take a long
time to digest. A hormone called gastric inhibitory peptide is secreted by the small intestine to slow down the
peristaltic movements of the intestine to allow fatty foods more time to be digested and absorbed.
Understanding the hormonal control of the digestive system is an important area of ongoing research. Scientists
are exploring the role of each hormone in the digestive process and developing ways to target these hormones.
Advances could lead to knowledge that may help to battle the obesity epidemic.
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Chapter 34 | Animal Nutrition and the Digestive System
KEY TERMS
alimentary canal tubular digestive system with a mouth and anus
aminopeptidase protease that breaks down peptides to single amino acids; secreted by the brush border of
small intestine
anus exit point for waste material
bile digestive juice produced by the liver; important for digestion of lipids
bolus mass of food resulting from chewing action and wetting by saliva
carboxypeptidase protease that breaks down peptides to single amino acids; secreted by the brush border of
the small intestine
carnivore animal that consumes animal flesh
cephalic phase first phase of digestion, controlled by the neural response to the stimulus provided by food
cholecystokinin hormone that stimulates the contraction of the gallbladder to release bile
chylomicron small lipid globule
chyme mixture of partially digested food and stomach juices
chymotrypsin pancreatic protease
digestion mechanical and chemical breakdown of food into small organic fragments
dipeptidase protease that breaks down peptides to single amino acids; secreted by the brush border of small
intestine
duodenum first part of the small intestine where a large part of digestion of carbohydrates and fats occurs
elastase pancreatic protease
endocrine system system that controls the response of the various glands in the body and the release of
hormones at the appropriate times
esophagus tubular organ that connects the mouth to the stomach
essential nutrient nutrient that cannot be synthesized by the body; it must be obtained from food
gallbladder organ that stores and concentrates bile
gastric inhibitory peptide hormone secreted by the small intestine in the presence of fatty acids and sugars; it
also inhibits acid production and peristalsis in order to slow down the rate at which food enters the small
intestine
gastric phase digestive phase beginning once food enters the stomach; gastric acids and enzymes process the
ingested materials
gastrin hormone which stimulates hydrochloric acid secretion in the stomach
gastrovascular cavity digestive system consisting of a single opening
gizzard muscular organ that grinds food
herbivore animal that consumes a strictly plant diet
ileum last part of the small intestine; connects the small intestine to the large intestine; important for absorption
of B-12
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Chapter 34 | Animal Nutrition and the Digestive System
1061
ingestion act of taking in food
intestinal phase third digestive phase; begins when chyme enters the small intestine triggering digestive
secretions and controlling the rate of gastric emptying
jejunum second part of the small intestine
lactase enzyme that breaks down lactose into glucose and galactose
large intestine digestive system organ that reabsorbs water from undigested material and processes waste
matter
lipase enzyme that chemically breaks down lipids
liver organ that produces bile for digestion and processes vitamins and lipids
maltase enzyme that breaks down maltose into glucose
mineral inorganic, elemental molecule that carries out important roles in the body
monogastric digestive system that consists of a single-chambered stomach
omnivore animal that consumes both plants and animals
pancreas gland that secretes digestive juices
pepsin enzyme found in the stomach whose main role is protein digestion
pepsinogen inactive form of pepsin
peristalsis wave-like movements of muscle tissue
proventriculus glandular part of a bird’s stomach
rectum area of the body where feces is stored until elimination
roughage component of food that is low in energy and high in fiber
ruminant animal with a stomach divided into four compartments
salivary amylase enzyme found in saliva, which converts carbohydrates to maltose
secretin hormone which stimulates sodium bicarbonate secretion in the small intestine
small intestine organ where digestion of protein, fats, and carbohydrates is completed
somatostatin hormone released to stop acid secretion when the stomach is empty
sphincter band of muscle that controls movement of materials throughout the digestive tract
stomach saclike organ containing acidic digestive juices
sucrase enzyme that breaks down sucrose into glucose and fructose
trypsin pancreatic protease that breaks down protein
villi folds on the inner surface of the small intestine whose role is to increase absorption area
vitamin organic substance necessary in small amounts to sustain life
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Chapter 34 | Animal Nutrition and the Digestive System
CHAPTER SUMMARY
34.1 Digestive Systems
Different animals have evolved different types of digestive systems specialized to meet their dietary needs.
Humans and many other animals have monogastric digestive systems with a single-chambered stomach. Birds
have evolved a digestive system that includes a gizzard where the food is crushed into smaller pieces. This
compensates for their inability to masticate. Ruminants that consume large amounts of plant material have a
multi-chambered stomach that digests roughage. Pseudo-ruminants have similar digestive processes as
ruminants but do not have the four-compartment stomach. Processing food involves ingestion (eating),
digestion (mechanical and enzymatic breakdown of large molecules), absorption (cellular uptake of nutrients),
and elimination (removal of undigested waste as feces).
Many organs work together to digest food and absorb nutrients. The mouth is the point of ingestion and the
location where both mechanical and chemical breakdown of food begins. Saliva contains an enzyme called
amylase that breaks down carbohydrates. The food bolus travels through the esophagus by peristaltic
movements to the stomach. The stomach has an extremely acidic environment. An enzyme called pepsin
digests protein in the stomach. Further digestion and absorption take place in the small intestine. The large
intestine reabsorbs water from the undigested food and stores waste until elimination.
34.2 Nutrition and Energy Production
Animal diet should be balanced and meet the needs of the body. Carbohydrates, proteins, and fats are the
primary components of food. Some essential nutrients are required for cellular function but cannot be produced
by the animal body. These include vitamins, minerals, some fatty acids, and some amino acids. Food intake in
more than necessary amounts is stored as glycogen in the liver and muscle cells, and in fat cells. Excess
adipose storage can lead to obesity and serious health problems. ATP is the energy currency of the cell and is
obtained from the metabolic pathways. Excess carbohydrates and energy are stored as glycogen in the body.
34.3 Digestive System Processes
Digestion begins with ingestion, where the food is taken in the mouth. Digestion and absorption take place in a
series of steps with special enzymes playing important roles in digesting carbohydrates, proteins, and lipids.
Elimination describes removal of undigested food contents and waste products from the body. While most
absorption occurs in the small intestines, the large intestine is responsible for the final removal of water that
remains after the absorptive process of the small intestines. The cells that line the large intestine absorb some
vitamins as well as any leftover salts and water. The large intestine (colon) is also where feces is formed.
34.4 Digestive System Regulation
The brain and the endocrine system control digestive processes. The brain controls the responses of hunger
and satiety. The endocrine system controls the release of hormones and enzymes required for digestion of food
in the digestive tract.
VISUAL CONNECTION QUESTIONS
1. Figure 34.11 Which of the following statements
about the digestive system is false?
a. Chyme is a mixture of food and digestive
juices that is produced in the stomach.
b. Food enters the large intestine before the
small intestine.
c. In the small intestine, chyme mixes with bile,
which emulsifies fats.
d. The stomach is separated from the small
intestine by the pyloric sphincter.
2. Figure 34.12 Which of the following statements
about the small intestine is false?
a. Absorptive cells that line the small intestine
have microvilli, small projections that
increase surface area and aid in the
absorption of food.
b. The inside of the small intestine has many
folds, called villi.
c. Microvilli are lined with blood vessels as well
as lymphatic vessels.
d. The inside of the small intestine is called the
lumen.
3. Figure 34.19 Which of the following statements
about digestive processes is true?
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Chapter 34 | Animal Nutrition and the Digestive System
1063
a. Amylase, maltase, and lactase in the mouth
digest carbohydrates.
b. Trypsin and lipase in the stomach digest
protein.
c. Bile emulsifies lipids in the small intestine.
d. No food is absorbed until the small intestine.
REVIEW QUESTIONS
4. Which of the following is a pseudo-ruminant?
a. cow
b. pig
c. crow
d. horse
5. Which of the following statements is untrue?
a. Roughage takes a long time to digest.
b. Birds eat large quantities at one time so that
they can fly long distances.
c. Cows do not have upper teeth.
d. In pseudo-ruminants, roughage is digested
in the cecum.
6. The acidic nature of chyme is neutralized by
a. potassium hydroxide
b. sodium hydroxide
c. bicarbonates
d. vinegar
7. The digestive juices from the liver are delivered to
the_.
a. stomach
b. liver
c. duodenum
d. colon
8. A scientist dissects a new species of animal. If the
animal’s digestive system has a single stomach with
an extended small intestine, to which animal could
the dissected specimen be closely related?
a. lion
b. snowshoe hare
c. earthworm
d. eagle
9. Which of the following statements is not true?
a. Essential nutrients can be synthesized by
the body.
b. Vitamins are required in small quantities for
bodily function.
c. Some amino acids can be synthesized by
the body, while others need to be obtained
from diet.
d. Vitamins come in two categories: fat-soluble
and water-soluble.
10. Which of the following is a water-soluble vitamin?
a. vitamin A
b. vitamin E
c. vitamin K
d. vitamin C
11. What is the primary fuel for the body?
a. carbohydrates
b. lipids
c. protein
d. glycogen
12. Excess qlucose is stored as
a. fat
b. glucagon
c. glycogen
d. it is not stored in the body
13. Many distance runners “carb load” the day before
a big race. How does this eating strategy provide an
advantage to the runner?
a. The carbohydrates cause the release of
insulin.
b. The excess carbohydrates are converted to
fats, which have a higher calorie density.
c. The glucose from the carbohydrates lets the
muscles make excess ATP overnight.
d. The excess carbohydrates can be stored in
the muscles as glycogen.
14. Where does the majority of protein digestion take
place?
a. stomach
b. duodenum
c. mouth
d. jejunum
15. Lipases are enzymes that breakdown_.
a. disaccharides
b. lipids
c. proteins
d. cellulose
16. Which of the following conditions is most likely to
cause constipation?
a. bacterial infection
b. dehydration
c. ulcer
d. excessive cellulose consumption
17. Which hormone controls the release of bile from
the gallbladder
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Chapter 34 | Animal Nutrition and the Digestive System
a. pepsin
b. amylase
c. CCK
d. gastrin
18. Which hormone stops acid secretion in the
stomach?
a. gastrin
b. somatostatin
c. gastric inhibitory peptide
d. CCK
CRITICAL THINKING QUESTIONS
20. How does the polygastric digestive system aid in
digesting roughage?
21. How do birds digest their food in the absence of
teeth?
22. What is the role of the accessory organs in
digestion?
23. Explain how the villi and microvilli aid in
absorption.
24. Name two components of the digestive system
that perform mechanical digestion. Describe how
mechanical digestion contributes to acquiring
nutrients from food.
25. What are essential nutrients?
26. What is the role of minerals in maintaining good
health?
27. Discuss why obesity is a growing epidemic.
28. There are several nations where malnourishment
is a common occurrence. What may be some of the
health challenges posed by malnutrition?
29. Generally describe how a piece of bread can
power your legs as you walk up a flight of stairs.
30. In the 1990s fat-free foods became popular
19. In the famous conditioning experiment, Pavlov
demonstrated that his dogs started drooling in
response to a bell sounding. What part of the
digestive process did he stimulate?
a. cephalic phase
b. gastric phase
c. intestinal phase
d. elimination phase
among people trying to lose weight. However, many
dieticians now conclude that the fat-free trend made
people less healthy and heavier. Describe how this
could occur.
31. Explain why some dietary lipid is a necessary
part of a balanced diet.
32. The gut microbiome (the bacterial colonies in the
intestines) have become a popular area of study in
biomedical research. How could varying gut
microbiomes impact a person’s nutrition?
33. Many mammals become ill if they drink milk as
adults even though they could consume it as babies.
What causes this digestive issue?
34. Describe how hormones regulate digestion.
35. Describe one or more scenarios where loss of
hormonal regulation of digestion can lead to
diseases.
36. A scientist is studying a model that has a
mutation in the receptor for somatostatin that
prevents hormone binding. How would this mutation
affect the structure and function of the digestive
system?
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Chapter 35 | The Nervous System
1065
35 | THE NERVOUS
SYSTEM
Figure 35.1 An athlete’s nervous system is hard at work during the planning and execution of a movement as precise
as a high jump. Parts of the nervous system are involved in determining how hard to push off and when to turn, as well
as controlling the muscles throughout the body that make this complicated movement possible without knocking the
bar down—all in just a few seconds, (credit: modification of work by Shane T. McCoy, U.S. Navy)
Chapter Outline
35.1: Neurons and Glial Cells
35.2: How Neurons Communicate
35.3: The Central Nervous System
35.4: The Peripheral Nervous System
35.5: Nervous System Disorders
Introduction
When you’re reading this book, your nervous system is performing several functions simultaneously. The visual
system is processing what is seen on the page; the motor system controls the turn of the pages (or click of the
mouse); the prefrontal cortex maintains attention. Even fundamental functions, like breathing and regulation of
body temperature, are controlled by the nervous system. A nervous system is an organism’s control center: it
processes sensory information from outside (and inside) the body and controls all behaviors—from eating to
sleeping to finding a mate.
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Chapter 35 | The Nervous System
35.1 1 Neurons and Glial Cells
By the end of this section, you will be able to do the following:
• List and describe the functions of the structural components of a neuron
• List and describe the four main types of neurons
• Compare the functions of different types of glial cells
Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety
of animals shown in Figure 35.2. Some organisms, like sea sponges, lack a true nervous system. Others, like
jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called
a “nerve net." Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves.
Flatworms of the phylum Platyhelminthes have both a central nervous system (CNS), made up of a small
“brain” and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend
throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a
brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements
and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous
systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to
vertebrate species.
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Chapter 35 | The Nervous System
1067
(a) Cnidarian
(hydra)
Radial
nerves
(b) Echinoderm
(sea star)
Eyespot
(c) Planarian
(flatworm)
Central ganglia
(brain)
Segmental ganglia
(d) Arthropod
(bee)
Central nervous system
(e) Mollusk (f) Vertebrate
(octopus) (human)
Figure 35.2 Nervous systems vary in structure and complexity. In (a) cnidarians, nerve cells form a decentralized nerve
net. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry
such as (c) planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, (d)
arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks
such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. In (f)
vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of
the body comprise the peripheral nervous system, (credit e: modification of work by Michael Vecchione, Clyde F.E.
Roper, and Michael J. Sweeney, NOAA; credit f: modification of work by NIH)
Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While
there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that
contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves. One interesting
difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many
invertebrates are located ventrally whereas the vertebrate spinal cords are located dorsally. There is debate
among evolutionary biologists as to whether these different nervous system plans evolved separately or whether
the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.
LINK TQ LEARNING
Watch this video of biologist Mark Kirschner discussing the “flipping” phenomenon of vertebrate
evolution. (This multimedia resource will open in a browser.) (http://cnx.org/content/m66620/l-3/
#eip-id 1170503155069)
The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical
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Chapter 35 | The Nervous System
signals, and glia, cells that provide support functions for the neurons by playing an information processing role
that is complementary to neurons. A neuron can be compared to an electrical wire—it transmits a signal from
one place to another. Glia can be compared to the workers at the electric company who make sure wires go to
the right places, maintain the wires, and take down wires that are broken. Although glia have been compared to
workers, recent evidence suggests that they also usurp some of the signaling functions of neurons.
There is great diversity in the types of neurons and glia that are present in different parts of the nervous system.
There are four major types of neurons, and they share several important cellular components.
Neurons
The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons,
the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus.
A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems
of these animals control many of the same behaviors—from basic reflexes to more complicated behaviors like
finding food and courting mates. The ability of neurons to communicate with each other as well as with other
types of cells underlies all of these behaviors.
Most neurons share the same cellular components. But neurons are also highly specialized—different types of
neurons have different sizes and shapes that relate to their functional roles.
Parts of a Neuron
Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic
reticulum, Golgi apparatus, mitochondria, and other cellular components. Neurons also contain unique
structures, illustrated in Figure 35.3 for receiving and sending the electrical signals that make neuronal
communication possible. Dendrites are tree-like structures that extend away from the cell body to receive
messages from other neurons at specialized junctions called synapses. Although some neurons do not have
any dendrites, some types of neurons have multiple dendrites. Dendrites can have small protrusions called
dendritic spines, which further increase surface area for possible synaptic connections.
Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a
specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction
between the cell body and an axon. An axon is a tube-like structure that propagates the integrated signal to
specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target
organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons
usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons.
Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal
as it travels down the axon, greatly increasing the speed of conduction. This insulation is important as the axon
from a human motor neuron can be as long as a meter—from the base of the spine to the toes. The myelin
sheath is not actually part of the neuron. Myelin is produced by glial cells. Along the axon there are periodic gaps
in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is “recharged" as
it travels along the axon.
It is important to note that a single neuron does not act alone—neuronal communication depends on the
connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from
a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje
cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.
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Chapter 35 | The Nervous System
1069
visual
CONNECTION
Figure 35.3 Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They
also have more specialized structures, including dendrites and axons.
Which of the following statements is false?
a. The soma is the cell body of a nerve cell.
b. Myelin sheath provides an insulating layer to the dendrites.
c. Axons carry the signal from the soma to the target.
d. Dendrites carry the signal to the soma.
Types of Neurons
There are different types of neurons, and the functional role of a given neuron is intimately dependent on its
structure. There is an amazing diversity of neuron shapes and sizes found in different parts of the nervous
system (and across species), as illustrated by the neurons shown in Figure 35.4.
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Chapter 35 | The Nervous System
(a) Pyramidal cell of the
cerebral cortex
(b) Purkinje cell of the
cerebellar cortex
(c) Olfactory neurons
Figure 35.4 There is great diversity in the size and shape of neurons throughout the nervous system. Examples include
(a) a pyramidal cell from the cerebral cortex, (b) a Purkinje cell from the cerebellar cortex, and (c) olfactory cells from
the olfactory epithelium and olfactory bulb.
While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar,
bipolar, multipolar, and pseudounipolar. Figure 35.5 illustrates these four basic neuron types. Unipolar neurons
have only one structure that extends away from the soma. These neurons are not found in vertebrates but
are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite
extending from the soma. An example of a bipolar neuron is a retinal bipolar cell, which receives signals from
photoreceptor cells that are sensitive to light and transmits these signals to ganglion cells that carry the signal to
the brain. Multipolar neurons are the most common type of neuron. Each multipolar neuron contains one axon
and multiple dendrites. Multipolar neurons can be found in the central nervous system (brain and spinal cord).
An example of a multipolar neuron is a Purkinje cell in the cerebellum, which has many branching dendrites but
only one axon. Pseudounipolar cells share characteristics with both unipolar and bipolar cells. A pseudounipolar
cell has a single process that extends from the soma, like a unipolar cell, but this process later branches into two
distinct structures, like a bipolar cell. Most sensory neurons are pseudounipolar and have an axon that branches
into two extensions: one connected to dendrites that receive sensory information and another that transmits this
information to the spinal cord.
Pseudounipolar neuron
Figure 35.5 Neurons are broadly divided into four main types based on the number and placement of axons: (1)
unipolar, (2) bipolar, (3) multipolar, and (4) pseudounipolar.
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Chapter 35 | The Nervous System
1071
everyday CONNECTION
Neurogenesis
At one time, scientists believed that people were bom with all the neurons they would ever have. Research
performed during the last few decades indicates that neurogenesis, the birth of new neurons, continues
into adulthood. Neurogenesis was first discovered in songbirds that produce new neurons while learning
songs. For mammals, new neurons also play an important role in learning: about 1000 new neurons
develop in the hippocampus (a brain structure involved in learning and memory) each day. While most of
the new neurons will die, researchers found that an increase in the number of surviving new neurons in
the hippocampus correlated with how well rats learned a new task. Interestingly, both exercise and some
antidepressant medications also promote neurogenesis in the hippocampus. Stress has the opposite effect.
While neurogenesis is quite limited compared to regeneration in other tissues, research in this area may
lead to new treatments for disorders such as Alzheimer’s, stroke, and epilepsy.
How do scientists identify new neurons? A researcher can inject a compound called bromodeoxyuridine
(BrdU) into the brain of an animal. While all cells will be exposed to BrdU, BrdU will only be incorporated
into the DNA of newly generated cells that are in S phase. A technique called immunohistochemistry can
be used to attach a fluorescent label to the incorporated BrdU, and a researcher can use fluorescent
microscopy to visualize the presence of BrdU, and thus new neurons, in brain tissue. Figure 35.6 is a
micrograph which shows fluorescently labeled neurons in the hippocampus of a rat.
BrdU/Nestin
A
__Neuron
‘ f '
&
Astrocyte
1
25 um
Figure 35.6 This micrograph shows fluorescently labeled new neurons in a rat hippocampus. Cells that are
actively dividing have bromodoxyuridine (BrdU) incorporated into their DNA and are labeled in red. Cells that
express glial fibrillary acidic protein (GFAP) are labeled in green. Astrocytes, but not neurons, express GFAP.
Thus, cells that are labeled both red and green are actively dividing astrocytes, whereas cells labeled red only
are actively dividing neurons, (credit: modification of work by Dr. Maryam Faiz, et. al., University of Barcelona;
scale-bar data from Matt Russell)
LINK
%
LEARNING
This site (http:// 0 penstaxc 0 llege. 0 rg/l/neur 0 genesis) contains more information about neurogenesis,
including an interactive laboratory simulation and a video that explains how BrdU labels new cells.
Glia
While glia are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain
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Chapter 35 | The Nervous System
actually outnumbers the number of neurons by a factor of ten. Neurons would be unable to function without the
vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions
and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have
recently discovered that they also play a role in responding to nerve activity and modulating communication
between nerve cells. When glia do not function properly, the result can be disastrous—most brain tumors are
caused by mutations in glia.
Types of Glia
There are several different types of glia with different functions, two of which are shown in Figure 35.7.
Astrocytes, shown in Figure 35.8a make contact with both capillaries and neurons in the CNS. They provide
nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular
fluid, and provide structural support for synapses. Astrocytes also form the blood-brain barrier—a structure
that blocks entrance of toxic substances into the brain. Astrocytes, in particular, have been shown through
calcium imaging experiments to become active in response to nerve activity, transmit calcium waves between
astrocytes, and modulate the activity of surrounding synapses. Satellite glia provide nutrients and structural
support for neurons in the PNS. Microglia scavenge and degrade dead cells and protect the brain from invading
microorganisms. Oligodendrocytes, shown in Figure 35.8b form myelin sheaths around axons in the CNS. One
axon can be myelinated by several oligodendrocytes, and one oligodendrocyte can provide myelin for multiple
neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the
entire Schwann cell surrounds the axon. Radial glia serve as scaffolds for developing neurons as they migrate
to their end destinations. Ependymal cells line fluid-filled ventricles of the brain and the central canal of the
spinal cord. They are involved in the production of cerebrospinal fluid, which serves as a cushion for the brain,
moves the fluid between the spinal cord and the brain, and is a component for the choroid plexus.
Oligodendrocyte
Microglial
cell
Pseudounipolar
neuron
Schwann
cells
Axon
Satellite
cells
(a) Central nervous system (b) Peripheral nervous system
Figure 35.7 Glial cells support neurons and maintain their environment. Glial cells of the (a) central nervous system
include oligodendrocytes, astrocytes, ependymal cells, and microglial cells. Oligodendrocytes form the myelin sheath
around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, and provide structural
support. Microglia scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions the
neurons. Glial cells of the (b) peripheral nervous system include Schwann cells, which form the myelin sheath, and
satellite cells, which provide nutrients and structural support to neurons.
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Chapter 35 | The Nervous System
1073
(a) Astrocyte (b) Oligodendrocyte
Figure 35.8 (a) Astrocytes and (b) oligodendrocytes are glial cells of the central nervous system, (credit a: modification
of work by Uniformed Services University; credit b: modification of work by Jurjen Broeke; scale-bar data from Matt
Russell)
35.2 | How Neurons Communicate
By the end of this section, you will be able to do the following:
• Describe the basis of the resting membrane potential
• Explain the stages of an action potential and how action potentials are propagated
• Explain the similarities and differences between chemical and electrical synapses
• Describe long-term potentiation and long-term depression
All functions performed by the nervous system—from a simple motor reflex to more advanced functions like
making a memory or a decision—require neurons to communicate with one another. While humans use words
and body language to communicate, neurons use electrical and chemical signals. Just like a person in a
committee, one neuron usually receives and synthesizes messages from multiple other neurons before “making
the decision" to send the message on to other neurons.
Nerve Impulse Transmission within a Neuron
For the nervous system to function, neurons must be able to send and receive signals. These signals are
possible because each neuron has a charged cellular membrane (a voltage difference between the inside
and the outside), and the charge of this membrane can change in response to neurotransmitter molecules
released from other neurons and environmental stimuli. To understand how neurons communicate, one must
first understand the basis of the baseline or ‘resting’ membrane charge.
Neuronal Charged Membranes
The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions. To enter
or exit the neuron, ions must pass through special proteins called ion channels that span the membrane.
Ion channels have different configurations: open, closed, and inactive, as illustrated in Figure 35.9. Some
ion channels need to be activated in order to open and allow ions to pass into or out of the cell. These ion
channels are sensitive to the environment and can change their shape accordingly. Ion channels that change
their structure in response to voltage changes are called voltage-gated ion channels. Voltage-gated ion channels
regulate the relative concentrations of different ions inside and outside the cell. The difference in total charge
between the inside and outside of the cell is called the membrane potential.
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Chapter 35 | The Nervous System
Voltage-gated Na + Channels
Closed At the resting potential, the
channel is closed.
Open In response to a nerve impulse, Inactivated For a brief period following
the gate opens and Na + enters the cell. activation, the channel does not open
in response to a new signal.
Figure 35.9 Voltage-gated ion channels open in response to changes in membrane voltage. After activation, they
become inactivated for a brief period and will no longer open in response to a signal.
This video discusses the basis of the resting membrane potential. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66621/l.3/#eip-idll72233931654)
Resting Membrane Potential
A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than
the outside (-70 mV, note that this number varies by neuron type and by species). This voltage is called the
resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell.
If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the
system would reach equilibrium. Because ions cannot simply cross the membrane at will, there are different
concentrations of several ions inside and outside the cell, as shown in Table 35.1. The difference in the number
of positively charged potassium ions (K + ) inside and outside the cell dominates the resting membrane potential
(Figure 35.10). When the membrane is at rest, K + ions accumulate inside the cell due to a net movement with
the concentration gradient. The negative resting membrane potential is created and maintained by increasing
the concentration of cations outside the cell (in the extracellular fluid) relative to inside the cell (in the cytoplasm).
The negative charge within the cell is created by the cell membrane being more permeable to potassium
ion movement than sodium ion movement. In neurons, potassium ions are maintained at high concentrations
within the cell while sodium ions are maintained at high concentrations outside of the cell. The cell possesses
potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient.
However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore,
potassium diffuses out of the cell at a much faster rate than sodium leaks in. Because more cations are leaving
the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of
the cell. The actions of the sodium potassium pump help to maintain the resting potential, once established.
Recall that sodium potassium pumps brings two K + ions into the cell while removing three Na + ions per ATP
consumed. As more cations are expelled from the cell than taken in, the inside of the cell remains negatively
charged relative to the extracellular fluid. It should be noted that chloride ions (Cl - ) tend to accumulate outside
of the cell because they are repelled by negatively-charged proteins within the cytoplasm.
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Chapter 35 | The Nervous System
1075
Ion Concentration Inside and Outside Neurons
Ion
Extracellular
concentration (mM)
Intracellular concentration
(mM)
Ratio outside/
inside
Na +
145
12
12
K+
4
155
0.026
cr
120
4
30
Organic anions
(A-)
—
100
Table 35.1 The resting membrane potential is a result of different concentrations inside and outside the
cell.
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Chapter 35 | The Nervous System
(a) Resting potential
At the resting potential, all voltage-gated Na + channels and most voltage-gated K + channels are closed. The Na + /K + transporter
pumps K + ions into the cell and Na + ions out.
(b) Depolarization
(the charge across the membrane lessens). If the threshold of excitation is reached, all the Na + channels open,
(c) Hyperpolarization
At the peak action potential, Na + channels close while K + channels open. K + leaves the cell, and the membrane eventually
becomes hyperpolarized.
Figure 35.10 The (a) resting membrane potential is a result of different concentrations of Na + and K + ions inside
and outside the cell. A nerve impulse causes Na + to enter the cell, resulting in (b) depolarization. At the peak action
potential, K + channels open and the cell becomes (c) hyperpolarized.
Action Potential
A neuron can receive input from other neurons and, if this input is strong enough, send the signal to downstream
neurons. Transmission of a signal between neurons is generally carried by a chemical called a neurotransmitter.
Transmission of a signal within a neuron (from dendrite to axon terminal) is carried by a brief reversal of the
resting membrane potential called an action potential. When neurotransmitter molecules bind to receptors
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located on a neuron’s dendrites, ion channels open. At excitatory synapses, this opening allows positive ions
to enter the neuron and results in depolarization of the membrane—a decrease in the difference in voltage
between the inside and outside of the neuron. A stimulus from a sensory cell or another neuron depolarizes
the target neuron to its threshold potential (-55 mV). Na + channels in the axon hillock open, allowing positive
ions to enter the cell (Figure 35.10 and Figure 35.11). Once the sodium channels open, the neuron completely
depolarizes to a membrane potential of about +40 mV. Action potentials are considered an "all-or nothing" event,
in that, once the threshold potential is reached, the neuron always completely depolarizes. Once depolarization
is complete, the cell must now "reset" its membrane voltage back to the resting potential. To accomplish this,
the Na + channels close and cannot be opened. This begins the neuron's refractory period, in which it cannot
produce another action potential because its sodium channels will not open. At the same time, voltage-gated
K + channels open, allowing K + to leave the cell. As K + ions leave the cell, the membrane potential once again
becomes negative. The diffusion of K + out of the cell actually hyperpolarizes the cell, in that the membrane
potential becomes more negative than the cell's normal resting potential. At this point, the sodium channels will
return to their resting state, meaning they are ready to open again if the membrane potential again exceeds the
threshold potential. Eventually the extra K + ions diffuse out of the cell through the potassium leakage channels,
bringing the cell from its hyperpolarized state, back to its resting membrane potential.
visual
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Figure 35.11 The formation of an action potential can be divided into five steps: (1) A stimulus from a sensory
cell or another neuron causes the target cell to depolarize toward the threshold potential. (2) If the threshold of
excitation is reached, all Na + channels open and the membrane depolarizes. (3) At the peak action potential,
K + channels open and K + begins to leave the cell. At the same time, Na + channels close. (4) The membrane
becomes hyperpolarized as K + ions continue to leave the cell. The hyperpolarized membrane is in a refractory
period and cannot fire. (5) The K + channels close and the Na + /K + transporter restores the resting potential.
Potassium channel blockers, such as amiodarone and procainamide, which are used to treat abnormal
electrical activity in the heart, called cardiac dysrhythmia, impede the movement of K + through voltage-gated
K + channels. Which part of the action potential would you expect potassium channels to affect?
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a. In response to a signal, the
soma end of the axon becomes
depolarized.
b. The depolarization spreads down
the axon. Meanwhile, the first part of
the membrane repolarizes. Because
Na + channels are inactivated and
additional K + channels have opened,
the membrane cannot depolarize again
- + + + + + + ^r + +^+
Repolarizing Depolarized
Resting
+ + + -
■ + + + -
++++++++
c. The action potential continues to
travel down the axon.
Resting
Repolarizing
_ Depolarized
Figure 35.12 The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes.
LINK TQ LEARNING
This video (http:// 0 penstaxc 0 llege. 0 rg/l/acti 0 np 0 tential) presents an overview of action potential.
Myelin and the Propagation of the Action Potential
For an action potential to communicate information to another neuron, it must travel along the axon and reach
the axon terminals where it can initiate neurotransmitter release. The speed of conduction of an action potential
along an axon is influenced by both the diameter of the axon and the axon’s resistance to current leak. Myelin
acts as an insulator that prevents current from leaving the axon; this increases the speed of action potential
conduction. In demyelinating diseases like multiple sclerosis, action potential conduction slows because current
leaks from previously insulated axon areas. The nodes of Ranvier, illustrated in Figure 35.13 are gaps in the
myelin sheath along the axon. These unmyelinated spaces are about one micrometer long and contain voltage¬
gated Na + and K + channels. Flow of ions through these channels, particularly the Na + channels, regenerates the
action potential over and over again along the axon. This ‘jumping’ of the action potential from one node to the
next is called saltatory conduction. If nodes of Ranvier were not present along an axon, the action potential
would propagate very slowly since Na + and K + channels would have to continuously regenerate action potentials
at every point along the axon instead of at specific points. Nodes of Ranvier also save energy for the neuron
since the channels only need to be present at the nodes and not along the entire axon.
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Axon Nodes of Ranvier Myelin sheath
Figure 35.13 Nodes of Ranvier are gaps in myelin coverage along axons. Nodes contain voltage-gated K + and Na +
channels. Action potentials travel down the axon by jumping from one node to the next.
Synaptic Transmission
The synapse or “gap” is the place where information is transmitted from one neuron to another. Synapses
usually form between axon terminals and dendritic spines, but this is not universally true. There are also axon-
to-axon, dendrite-to-dendrite, and axon-to-cell body synapses. The neuron transmitting the signal is called the
presynaptic neuron, and the neuron receiving the signal is called the postsynaptic neuron. Note that these
designations are relative to a particular synapse—most neurons are both presynaptic and postsynaptic. There
are two types of synapses: chemical and electrical.
Chemical Synapse
When an action potential reaches the axon terminal it depolarizes the membrane and opens voltage-gated Na +
channels. Na + ions enter the cell, further depolarizing the presynaptic membrane. This depolarization causes
voltage-gated Ca 2+ channels to open. Calcium ions entering the cell initiate a signaling cascade that causes
small membrane-bound vesicles, called synaptic vesicles, containing neurotransmitter molecules to fuse with
the presynaptic membrane. Synaptic vesicles are shown in Figure 35.14, which is an image from a scanning
electron microscope.
Figure 35.14 This pseudocolored image taken with a scanning electron microscope shows an axon terminal that was
broken open to reveal synaptic vesicles (blue and orange) inside the neuron, (credit: modification of work by Tina
Carvalho, NIH-NIGMS; scale-bar data from Matt Russell)
Fusion of a vesicle with the presynaptic membrane causes neurotransmitter to be released into the synaptic
cleft, the extracellular space between the presynaptic and postsynaptic membranes, as illustrated in Figure
35.15. The neurotransmitter diffuses across the synaptic cleft and binds to receptor proteins on the postsynaptic
membrane.
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Action potential
arrives at axon
terminal.
Axon terminal
Synaptic
vesicles
©
Synaptic cleft
( 3 ) Ca 2+ entry causes
neurotransmitter-containing
synaptic vesicles to release
their contents by exocytosis.
Voltage-gated Ca 2+
channels open and
Ca 2+ enters the
axon terminal.
©
Neurotransmitter diffuses across
the synaptic cleft and binds to
ligand-gated ion channels on the
postsynaptic membrane.
Neurotransmitter
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Open ligand-gated ion channel
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Reuptake by degradation^ ,rom s V na P se
presynaptic w f j
nai irnn v —'
( 5 ) Binding of neurotransmitter opens ligand-gated (6) Reuptake by the presynapic neuron, enzymatic degradation,
ion channels, resulting in graded potentials. and diffusion reduce neurotransmitter levels, terminating the
signal.
Figure 35.15 Communication at chemical synapses requires release of neurotransmitters. When the presynaptic
membrane is depolarized, voltage-gated Ca 2+ channels open and allow Ca 2+ to enter the cell. The calcium entry
causes synaptic vesicles to fuse with the membrane and release neurotransmitter molecules into the synaptic cleft.
The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic
membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron.
The binding of a specific neurotransmitter causes particular ion channels, in this case ligand-gated channels,
on the postsynaptic membrane to open. Neurotransmitters can either have excitatory or inhibitory effects on the
postsynaptic membrane, as detailed in Table 35.1. For example, when acetylcholine is released at the synapse
between a nerve and muscle (called the neuromuscular junction) by a presynaptic neuron, it causes postsynaptic
Na + channels to open. Na + enters the postsynaptic cell and causes the postsynaptic membrane to depolarize.
This depolarization is called an excitatory postsynaptic potential (EPSP) and makes the postsynaptic neuron
more likely to fire an action potential. Release of neurotransmitter at inhibitory synapses causes inhibitory
postsynaptic potentials (IPSPs), a hyperpolarization of the presynaptic membrane. For example, when the
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Chapter 35 | The Nervous System
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neurotransmitter GABA (gamma-aminobutyric acid) is released from a presynaptic neuron, it binds to and opens
Cl" channels. Cl" ions enter the cell and hyperpolarizes the membrane, making the neuron less likely to fire an
action potential.
Once neurotransmission has occurred, the neurotransmitter must be removed from the synaptic cleft so the
postsynaptic membrane can “reset" and be ready to receive another signal. This can be accomplished in three
ways: the neurotransmitter can diffuse away from the synaptic cleft, it can be degraded by enzymes in the
synaptic cleft, or it can be recycled (sometimes called reuptake) by the presynaptic neuron. Several drugs act
at this step of neurotransmission. For example, some drugs that are given to Alzheimer’s patients work by
inhibiting acetylcholinesterase, the enzyme that degrades acetylcholine. This inhibition of the enzyme essentially
increases neurotransmission at synapses that release acetylcholine. Once released, the acetylcholine stays in
the cleft and can continually bind and unbind to postsynaptic receptors.
Neurotransmitter Function and Location
Neurotransmitter
Example
Location
Acetylcholine
—
CNS and/or PNS
Biogenic amine
Dopamine, serotonin, norepinephrine
CNS and/or PNS
Amino acid
Glycine, glutamate, aspartate, gamma aminobutyric acid
CNS
Neuropeptide
Substance P, endorphins
CNS and/or PNS
Table 35.2
Electrical Synapse
While electrical synapses are fewer in number than chemical synapses, they are found in all nervous systems
and play important and unique roles. The mode of neurotransmission in electrical synapses is quite different from
that in chemical synapses. In an electrical synapse, the presynaptic and postsynaptic membranes are very close
together and are actually physically connected by channel proteins forming gap junctions. Gap junctions allow
current to pass directly from one cell to the next. In addition to the ions that carry this current, other molecules,
such as ATP, can diffuse through the large gap junction pores.
There are key differences between chemical and electrical synapses. Because chemical synapses depend
on the release of neurotransmitter molecules from synaptic vesicles to pass on their signal, there is an
approximately one millisecond delay between when the axon potential reaches the presynaptic terminal and
when the neurotransmitter leads to opening of postsynaptic ion channels. Additionally, this signaling is
unidirectional. Signaling in electrical synapses, in contrast, is virtually instantaneous (which is important for
synapses involved in key reflexes), and some electrical synapses are bidirectional. Electrical synapses are also
more reliable as they are less likely to be blocked, and they are important for synchronizing the electrical activity
of a group of neurons. For example, electrical synapses in the thalamus are thought to regulate slow-wave sleep,
and disruption of these synapses can cause seizures.
Signal Summation
Sometimes a single EPSP is strong enough to induce an action potential in the postsynaptic neuron, but
often multiple presynaptic inputs must create EPSPs around the same time for the postsynaptic neuron to
be sufficiently depolarized to fire an action potential. This process is called summation and occurs at the
axon hillock, as illustrated in Figure 35.16. Additionally, one neuron often has inputs from many presynaptic
neurons—some excitatory and some inhibitory—so IPSPs can cancel out EPSPs and vice versa. It is the
net change in postsynaptic membrane voltage that determines whether the postsynaptic cell has reached its
threshold of excitation needed to fire an action potential. Together, synaptic summation and the threshold for
excitation act as a filter so that random “noise” in the system is not transmitted as important information.
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Time
Figure 35.16 A single neuron can receive both excitatory and inhibitory inputs from multiple neurons, resulting in local
membrane depolarization (EPSP input) and hyperpolarization (IPSP input). All these inputs are added together at the
axon hillock. If the EPSPs are strong enough to overcome the IPSPs and reach the threshold of excitation, the neuron
will fire.
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Brain-computer interface
Amyotrophic lateral sclerosis (ALS, also called Lou Gehrig’s Disease) is a neurological disease
characterized by the degeneration of the motor neurons that control voluntary movements. The disease
begins with muscle weakening and lack of coordination and eventually destroys the neurons that control
speech, breathing, and swallowing; in the end, the disease can lead to paralysis. At that point, patients
require assistance from machines to be able to breathe and to communicate. Several special technologies
have been developed to allow “locked-in” patients to communicate with the rest of the world. One
technology, for example, allows patients to type out sentences by twitching their cheek. These sentences
can then be read aloud by a computer.
A relatively new line of research for helping paralyzed patients, including those with ALS, to communicate
and retain a degree of self-sufficiency is called brain-computer interface (BCI) technology and is illustrated
in Figure 35.17. This technology sounds like something out of science fiction: it allows paralyzed patients
to control a computer using only their thoughts. There are several forms of BCI. Some forms use EEG
recordings from electrodes taped onto the skull. These recordings contain information from large
populations of neurons that can be decoded by a computer. Other forms of BCI require the implantation of
an array of electrodes smaller than a postage stamp in the arm and hand area of the motor cortex. This
form of BCI, while more invasive, is very powerful as each electrode can record actual action potentials from
one or more neurons. These signals are then sent to a computer, which has been trained to decode the
signal and feed it to a tool—such as a cursor on a computer screen. This means that a patient with ALS can
use e-mail, read the Internet, and communicate with others by thinking of moving his or her hand or arm
(even though the paralyzed patient cannot make that bodily movement). Recent advances have allowed a
paralyzed locked-in patient who suffered a stroke 15 years ago to control a robotic arm and even to feed
herself coffee using BCI technology.
Despite the amazing advancements in BCI technology, it also has limitations. The technology can require
many hours of training and long periods of intense concentration for the patient; it can also require brain
surgery to implant the devices.
Neural signals travel to computer
Figure 35.17 With brain-computer interface technology, neural signals from a paralyzed patient are collected,
decoded, and then fed to a tool, such as a computer, a wheelchair, or a robotic arm.
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LINK TQ LEARNING
Watch this video (http:// 0 penstaxc 0 llege. 0 rg/l/paralyzati 0 n) in which a paralyzed woman uses a brain-
controlled robotic arm to bring a drink to her mouth, among other images of brain-computer interface
technology in action. (This multimedia resource will open in a browser.) (http://cnx.org/content/
m66621/1.3/#eip-id2102965)
Synaptic Plasticity
Synapses are not static structures. They can be weakened or strengthened. They can be broken, and new
synapses can be made. Synaptic plasticity allows for these changes, which are all needed for a functioning
nervous system. In fact, synaptic plasticity is the basis of learning and memory. Two processes in particular,
long-term potentiation (LTP) and long-term depression (LTD) are important forms of synaptic plasticity that occur
in synapses in the hippocampus, a brain region that is involved in storing memories.
Long-term Potentiation (LTP)
Long-term potentiation (LTP) is a persistent strengthening of a synaptic connection. LTP is based on the
Hebbian principle: cells that fire together wire together. There are various mechanisms, none fully understood,
behind the synaptic strengthening seen with LTP. One known mechanism involves a type of postsynaptic
glutamate receptor, called NMDA (N-Methyl-D-aspartate) receptors, shown in Figure 35.18. These receptors
are normally blocked by magnesium ions; however, when the postsynaptic neuron is depolarized by multiple
presynaptic inputs in quick succession (either from one neuron or multiple neurons), the magnesium ions
are forced out allowing Ca ions to pass into the postsynaptic cell. Next, Ca 2+ ions entering the cell initiate
a signaling cascade that causes a different type of glutamate receptor, called AMPA (a-
amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, to be inserted into the postsynaptic membrane,
since activated AMPA receptors allow positive ions to enter the cell. So, the next time glutamate is released
from the presynaptic membrane, it will have a larger excitatory effect (EPSP) on the postsynaptic cell because
the binding of glutamate to these AMPA receptors will allow more positive ions into the cell. The insertion of
additional AMPA receptors strengthens the synapse and means that the postsynaptic neuron is more likely to
fire in response to presynaptic neurotransmitter release. Some drugs of abuse co-opt the LTP pathway, and this
synaptic strengthening can lead to addiction.
Long-term Depression (LTD)
Long-term depression (LTD) is essentially the reverse of LTP: it is a long-term weakening of a synaptic
connection. One mechanism known to cause LTD also involves AMPA receptors. In this situation, calcium that
enters through NMDA receptors initiates a different signaling cascade, which results in the removal of AMPA
receptors from the postsynaptic membrane, as illustrated in Figure 35.18. The decrease in AMPA receptors
in the membrane makes the postsynaptic neuron less responsive to glutamate released from the presynaptic
neuron. While it may seem counterintuitive, LTD may be just as important for learning and memory as LTP.
The weakening and pruning of unused synapses allows for unimportant connections to be lost and makes the
synapses that have undergone LTP that much stronger by comparison.
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The NMDA receptor is
activated by glutamate
binding, but only after
depolarization removes
inhibitory Mg 2+ . Once the
Mg 2 * is removed, Ca 2+ can
enter the cell.
Postsynaptic
terminal
Presynaptic
terminal
Postsynaptic
terminal
Presynaptic
terminal
Some AMPA receptors are
present in the membrane
initially. In response to an
increase in intracellular Ca2*.
more are inserted.
Low-frequency stimulation
results in a different Ca 2+
-signaling cascade. AMPA
receptor is removed from
the membrane, and as a
result, the nerve cell
becomes less responsive
to glutamate.
Figure 35.18 Calcium entry through postsynaptic NMDA receptors can initiate two different forms of synaptic plasticity:
long-term potentiation (LTP) and long-term depression (LTD). LTP arises when a single synapse is repeatedly
stimulated. This stimulation causes a calcium- and CaMKII-dependent cellular cascade, which results in the insertion of
more AMPA receptors into the postsynaptic membrane. The next time glutamate is released from the presynaptic cell,
it will bind to both NMDA and the newly inserted AMPA receptors, thus depolarizing the membrane more efficiently. LTD
occurs when few glutamate molecules bind to NMDA receptors at a synapse (due to a low firing rate of the presynaptic
neuron). The calcium that does flow through NMDA receptors initiates a different calcineurin and protein phosphatase
1-dependent cascade, which results in the endocytosis of AMPA receptors. This makes the postsynaptic neuron less
responsive to glutamate released from the presynaptic neuron.
35.3 | The Central Nervous System
By the end of this section, you will be able to do the following:
• identify the spinal cord, cerebral lobes, and other brain areas on a diagram of the brain
• Describe the basic functions of the spinal cord, cerebral lobes, and other brain areas
The central nervous system (CNS) is made up of the brain, a part of which is shown in Figure 35.19 and
spinal cord and is covered with three layers of protective coverings called meninges (from the Greek word
for membrane). The outermost layer is the dura mater (Latin for “hard mother”). As the Latin suggests, the
primary function for this thick layer is to protect the brain and spinal cord. The dura mater also contains vein-like
structures that carry blood from the brain back to the heart. The middle layer is the web-like arachnoid mater.
The last layer is the pia mater (Latin for “soft mother”), which directly contacts and covers the brain and spinal
cord like plastic wrap. The space between the arachnoid and pia maters is filled with cerebrospinal fluid (CSF).
CSF is produced by a tissue called choroid plexus in fluid-filled compartments in the CNS called ventricles.
The brain floats in CSF, which acts as a cushion and shock absorber and makes the brain neutrally buoyant.
CSF also functions to circulate chemical substances throughout the brain and into the spinal cord.
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The entire brain contains only about 8.5 tablespoons of CSF, but CSF is constantly produced in the ventricles.
This creates a problem when a ventricle is blocked—the CSF builds up and creates swelling and the brain is
pushed against the skull. This swelling condition is called hydrocephalus (“water head") and can cause seizures,
cognitive problems, and even death if a shunt is not inserted to remove the fluid and pressure.
Skin Veins
Figure 35.19 The cerebral cortex is covered by three layers of meninges: the dura, arachnoid, and pia maters, (credit:
modification of work by Gray’s Anatomy)
Brain
The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. It includes
the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, and cerebellum. There are three
different ways that a brain can be sectioned in order to view internal structures: a sagittal section cuts the brain
left to right, as shown in Figure 35.21b, a coronal section cuts the brain front to back, as shown in Figure 35.20a,
and a horizontal section cuts the brain top to bottom.
Cerebral Cortex
The outermost part of the brain is a thick piece of nervous system tissue called the cerebral cortex, which is
folded into hills called gyri (singular: gyrus) and valleys called sulci (singular: sulcus). The cortex is made up of
two hemispheres—right and left—which are separated by a large sulcus. A thick fiber bundle called the corpus
callosum (Latin: “tough body") connects the two hemispheres and allows information to be passed from one
side to the other. Although there are some brain functions that are localized more to one hemisphere than the
other, the functions of the two hemispheres are largely redundant. In fact, sometimes (very rarely) an entire
hemisphere is removed to treat severe epilepsy. While patients do suffer some deficits following the surgery,
they can have surprisingly few problems, especially when the surgery is performed on children who have very
immature nervous systems.
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Right
hemisphere
of the
cerebral
cortex
Corpus callosum
Left hemisphere of
the cerebral cortex
Brainstem
Cerebellum
Corpus
Cerebral
cortex
Basal
ganglia
Thalamus
Cerebellum
Brainstem
(a) Coronal section
(b) Sagittal section
(a) (b)
Figure 35.20 These illustrations show the (a) coronal and (b) sagittal sections of the human brain.
In other surgeries to treat severe epilepsy, the corpus callosum is cut instead of removing an entire hemisphere.
This causes a condition called split-brain, which gives insights into unique functions of the two hemispheres.
For example, when an object is presented to patients’ left visual field, they may be unable to verbally name the
object (and may claim to not have seen an object at all). This is because the visual input from the left visual field
crosses and enters the right hemisphere and cannot then signal to the speech center, which generally is found
in the left side of the brain. Remarkably, if a split-brain patient is asked to pick up a specific object out of a group
of objects with the left hand, the patient will be able to do so but will still be unable to vocally identify it.
LINK TQ LEARNING
See this website (http:// 0 penstaxc 0 llege. 0 rg/l/split-brain) to learn more about split-brain patients and to
play a game where you can model the split-brain experiments yourself.
Each cortical hemisphere contains regions called lobes that are involved in different functions. Scientists use
various techniques to determine what brain areas are involved in different functions: they examine patients
who have had injuries or diseases that affect specific areas and see how those areas are related to functional
deficits. They also conduct animal studies where they stimulate brain areas and see if there are any behavioral
changes. They use a technique called transmagnetic stimulation (TMS) to temporarily deactivate specific parts
of the cortex using strong magnets placed outside the head; and they use functional magnetic resonance
imaging (fMRI) to look at changes in oxygenated blood flow in particular brain regions that correlate with specific
behavioral tasks. These techniques, and others, have given great insight into the functions of different brain
regions but have also showed that any given brain area can be involved in more than one behavior or process,
and any given behavior or process generally involves neurons in multiple brain areas. That being said, each
hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined
lobes: frontal, parietal, temporal, and occipital. Figure 35.21 illustrates these four lobes of the human cerebral
cortex.
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Figure 35.21 The human cerebral cortex includes the frontal, parietal, temporal, and occipital lobes.
The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb,
which processes smells. The frontal lobe also contains the motor cortex, which is important for planning and
implementing movement. Areas within the motor cortex map to different muscle groups, and there is some
organization to this map, as shown in Figure 35.22. For example, the neurons that control movement of the
fingers are next to the neurons that control movement of the hand. Neurons in the frontal lobe also control
cognitive functions like maintaining attention, speech, and decision-making. Studies of humans who have
damaged their frontal lobes show that parts of this area are involved in personality, socialization, and assessing
risk.
Wrists
Figure 35.22 Different parts of the motor cortex control different muscle groups. Muscle groups that are neighbors in
the body are generally controlled by neighboring regions of the motor cortex as well. For example, the neurons that
control finger movement are near the neurons that control hand movement.
The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and
also reading. Two of the parietal lobe’s main functions are processing somatosensation —touch sensations like
pressure, pain, heat, cold—and processing proprioception —the sense of how parts of the body are oriented in
space. The parietal lobe contains a somatosensory map of the body similar to the motor cortex.
The occipital lobe is located at the back of the brain. It is primarily involved in vision—seeing, recognizing, and
identifying the visual world.
The temporal lobe is located at the base of the brain by your ears and is primarily involved in processing
and interpreting sounds. It also contains the hippocampus (Greek for “seahorse")—a structure that processes
memory formation. The hippocampus is illustrated in Figure 35.24. The role of the hippocampus in memory
was partially determined by studying one famous epileptic patient, HM, who had both sides of his hippocampus
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Chapter 35 | The Nervous System
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removed in an attempt to cure his epilepsy. His seizures went away, but he could no longer form new memories
(although he could remember some facts from before his surgery and could learn new motor tasks).
\ / _
e olution CONNECTION
Cerebral Cortex
Compared to other vertebrates, mammals have exceptionally large brains for their body size. An entire
alligator’s brain, for example, would fill about one and a half teaspoons. This increase in brain to body
size ratio is especially pronounced in apes, whales, and dolphins. While this increase in overall brain size
doubtlessly played a role in the evolution of complex behaviors unique to mammals, it does not tell the
whole story. Scientists have found a relationship between the relatively high surface area of the cortex and
the intelligence and complex social behaviors exhibited by some mammals. This increased surface area is
due, in part, to increased folding of the cortical sheet (more sulci and gyri). For example, a rat cortex is very
smooth with very few sulci and gyri. Cat and sheep cortices have more sulci and gyri. Chimps, humans, and
dolphins have even more.
Figure 35.23 Mammals have larger brain-to-body ratios than other vertebrates. Within mammals, increased
cortical folding and surface area is correlated with complex behavior.
Basal Ganglia
Interconnected brain areas called the basal ganglia (or basal nuclei), shown in Figure 35.20b, play important
roles in movement control and posture. Damage to the basal ganglia, as in Parkinson’s disease, leads to motor
impairments like a shuffling gait when walking. The basal ganglia also regulate motivation. For example, when a
wasp sting led to bilateral basal ganglia damage in a 25-year-old businessman, he began to spend all his days in
bed and showed no interest in anything or anybody. But when he was externally stimulated—as when someone
asked to play a card game with him—he was able to function normally. Interestingly, he and other similar patients
do not report feeling bored or frustrated by their state.
Thalamus
The thalamus (Greek for “inner chamber”), illustrated in Figure 35.24, acts as a gateway to and from the cortex.
It receives sensory and motor inputs from the body and also receives feedback from the cortex. This feedback
mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and
arousal state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states. A rare
genetic disorder called fatal familial insomnia causes the degeneration of thalamic neurons and glia. This
disorder prevents affected patients from being able to sleep, among other symptoms, and is eventually fatal.
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Chapter 35 | The Nervous System
Figure 35.24 The limbic system regulates emotion and other behaviors. It includes parts of the cerebral cortex located
near the center of the brain, including the cingulate gyrus and the hippocampus as well as the thalamus, hypothalamus,
and amygdala.
Hypothalamus
Below the thalamus is the hypothalamus, shown in Figure 35.24. The hypothalamus controls the endocrine
system by sending signals to the pituitary gland, a pea-sized endocrine gland that releases several different
hormones that affect other glands as well as other cells. This relationship means that the hypothalamus
regulates important behaviors that are controlled by these hormones. The hypothalamus is the body’s
thermostat—it makes sure key functions like food and water intake, energy expenditure, and body temperature
are kept at appropriate levels. Neurons within the hypothalamus also regulate circadian rhythms, sometimes
called sleep cycles.
Limbic System
The limbic system is a connected set of structures that regulates emotion, as well as behaviors related to fear
and motivation. It plays a role in memory formation and includes parts of the thalamus and hypothalamus as
well as the hippocampus. One important structure within the limbic system is a temporal lobe structure called
the amygdala (Greek for “almond"), illustrated in Figure 35.24. The two amygdala are important both for the
sensation of fear and for recognizing fearful faces. The cingulate gyrus helps regulate emotions and pain.
Cerebellum
The cerebellum (Latin for “little brain"), shown in Figure 35.21, sits at the base of the brain on top of the
brainstem. The cerebellum controls balance and aids in coordinating movement and learning new motor tasks.
Brainstem
The brainstem, illustrated in Figure 35.21, connects the rest of the brain with the spinal cord. It consists of the
midbrain, medulla oblongata, and the pons. Motor and sensory neurons extend through the brainstem allowing
for the relay of signals between the brain and spinal cord. Ascending neural pathways cross in this section of
the brain allowing the left hemisphere of the cerebrum to control the right side of the body and vice versa. The
brainstem coordinates motor control signals sent from the brain to the body. The brainstem controls several
important functions of the body including alertness, arousal, breathing, blood pressure, digestion, heart rate,
swallowing, walking, and sensory and motor information integration.
Spinal Cord
Connecting to the brainstem and extending down the body through the spinal column is the spinal cord, shown
in Figure 35.21. The spinal cord is a thick bundle of nerve tissue that carries information about the body to the
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brain and from the brain to the body. The spinal cord is contained within the bones of the vertebrate column
but is able to communicate signals to and from the body through its connections with spinal nerves (part of the
peripheral nervous system). A cross-section of the spinal cord looks like a white oval containing a gray butterfly-
shape, as illustrated in Figure 35.25. Myelinated axons make up the “white matter” and neuron and glial cell
bodies make up the “gray matter.” Gray matter is also composed of interneurons, which connect two neurons
each located in different parts of the body. Axons and cell bodies in the dorsal (facing the back of the animal)
spinal cord convey mostly sensory information from the body to the brain. Axons and cell bodies in the ventral
(facing the front of the animal) spinal cord primarily transmit signals controlling movement from the brain to the
body.
The spinal cord also controls motor reflexes. These reflexes are quick, unconscious movements—like
automatically removing a hand from a hot object. Reflexes are so fast because they involve local synaptic
connections. For example, the knee reflex that a doctor tests during a routine physical is controlled by a single
synapse between a sensory neuron and a motor neuron. While a reflex may only require the involvement of one
or two synapses, synapses with interneurons in the spinal column transmit information to the brain to convey
what happened (the knee jerked, or the hand was hot).
In the United States, there around 10,000 spinal cord injuries each year. Because the spinal cord is the
information superhighway connecting the brain with the body, damage to the spinal cord can lead to paralysis.
The extent of the paralysis depends on the location of the injury along the spinal cord and whether the spinal
cord was completely severed. For example, if the spinal cord is damaged at the level of the neck, it can cause
paralysis from the neck down, whereas damage to the spinal column further down may limit paralysis to the legs.
Spinal cord injuries are notoriously difficult to treat because spinal nerves do not regenerate, although ongoing
research suggests that stem cell transplants may be able to act as a bridge to reconnect severed nerves.
Researchers are also looking at ways to prevent the inflammation that worsens nerve damage after injury. One
such treatment is to pump the body with cold saline to induce hypothermia. This cooling can prevent swelling
and other processes that are thought to worsen spinal cord injuries.
Figure 35.25 A cross-section of the spinal cord shows gray matter (containing cell bodies and interneurons) and white
matter (containing axons).
35.4 | The Peripheral Nervous System
By the end of this section, you will be able to do the following:
• Describe the organization and functions of the sympathetic and parasympathetic nervous systems
• Describe the organization and function of the sensory-somatic nervous system
The peripheral nervous system (PNS) is the connection between the central nervous system and the rest of the
body. The CNS is like the power plant of the nervous system. It creates the signals that control the functions
of the body. The PNS is like the wires that go to individual houses. Without those “wires,” the signals produced
by the CNS could not control the body (and the CNS would not be able to receive sensory information from the
body either).
The PNS can be broken down into the autonomic nervous system, which controls bodily functions without
conscious control, and the sensory-somatic nervous system, which transmits sensory information from the
skin, muscles, and sensory organs to the CNS and sends motor commands from the CNS to the muscles.
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Autonomic Nervous System
visual
CONNECTION
Autonomic Nervous System
Parasympathetic
Sympathetic
Preganglionic
neuron: soma is
usually in the brain¬
stem or sacral
(toward the bottom)
spinal cord
Preganglionic
neuron: soma is
usually in the spine
Neurotransmitter
released from the
preganglionic
synapse:
acetylcholine
Postganglionic
neuron: soma is
usually in a
ganglion near the
target organ
Neurotransmitters
released from
postganglionic
synapse:
acetylcholine or nitric
oxide
"Rest and digest”
response is activated
Postganglionic
neuron: soma is in a
sympathetic
ganglion, located next
to the spinal cord
Neurotransmitters
released from
postganglionic
synapse:
norepinephrine
“Fight or flight”
response is activated
Figure 35.26 In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a
postganglionic neuron of the PNS. The postganglionic neuron, in turn, acts on a target organ. Autonomic
responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one
another. The sympathetic system activates the “fight or flight" response, while the parasympathetic system
activates the “rest and digest” response.
Which of the following statements is false?
a. The parasympathetic pathway is responsible for resting the body, while the sympathetic pathway is
responsible for preparing for an emergency.
b. Most preganglionic neurons in the sympathetic pathway originate in the spinal cord.
c. Slowing of the heartbeat is a parasympathetic response.
d. Parasympathetic neurons are responsible for releasing norepinephrine on the target organ, while
sympathetic neurons are responsible for releasing acetylcholine.
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The autonomic nervous system serves as the relay between the CNS and the internal organs. It controls the
lungs, the heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls
these organs largely without conscious control; it can continuously monitor the conditions of these different
systems and implement changes as needed. Signaling to the target tissue usually involves two synapses: a
preganglionic neuron (originating in the CNS) synapses to a neuron in a ganglion that, in turn, synapses on the
target organ, as illustrated in Figure 35.26. There are two divisions of the autonomic nervous system that often
have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.
Sympathetic Nervous System
The sympathetic nervous system is responsible for the “fight or flight” response that occurs when an animal
encounters a dangerous situation. One way to remember this is to think of the surprise a person feels when
encountering a snake (“snake" and “sympathetic" both begin with “s"). Examples of functions controlled by the
sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help
prepare an organism’s body for the physical strain required to escape a potentially dangerous situation or to fend
off a predator.
Figure 35.27 The sympathetic and parasympathetic nervous systems often have opposing effects on target organs.
Most preganglionic neurons in the sympathetic nervous system originate in the spinal cord, as illustrated in
Figure 35.27. The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic
ganglia (the sympathetic ganglia form a chain that extends alongside the spinal cord). The acetylcholine
activates the postganglionic neurons. Postganglionic neurons then release norepinephrine onto target organs.
As anyone who has ever felt a rush before a big test, speech, or athletic event can attest, the effects of the
sympathetic nervous system are quite pervasive. This is both because one preganglionic neuron synapses
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on multiple postganglionic neurons, amplifying the effect of the original synapse, and because the adrenal
gland also releases norepinephrine (and the closely related hormone epinephrine) into the bloodstream. The
physiological effects of this norepinephrine release include dilating the trachea and bronchi (making it easier for
the animal to breathe), increasing heart rate, and moving blood from the skin to the heart, muscles, and brain
(so the animal can think and run). The strength and speed of the sympathetic response helps an organism avoid
danger, and scientists have found evidence that it may also increase LTP—allowing the animal to remember the
dangerous situation and avoid it in the future.
Parasympathetic Nervous System
While the sympathetic nervous system is activated in stressful situations, the parasympathetic nervous
system allows an animal to “rest and digest." One way to remember this is to think that during a restful situation
like a picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with
“p”). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward
the bottom) spinal cord, as shown in Figure 35.27. The axons of the preganglionic neurons release acetylcholine
on the postganglionic neurons, which are generally located very near the target organs. Most postganglionic
neurons release acetylcholine onto target organs, although some release nitric oxide.
The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated
(the common adrenaline dump you feel after a ‘fight-or-flight’ event). Effects of acetylcholine release on target
organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion.
Sensory-Somatic Nervous System
The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and
motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory
organs to the CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles
to make them contract. Without its sensory-somatic nervous system, an animal would be unable to process any
information about its environment (what it sees, feels, hears, and so on) and could not control motor movements.
Unlike the autonomic nervous system, which has two synapses between the CNS and the target organ, sensory
and motor neurons have only one synapse—one ending of the neuron is at the organ and the other directly
contacts a CNS neuron. Acetylcholine is the main neurotransmitter released at these synapses.
Humans have 12 cranial nerves, nerves that emerge from or enter the skull (cranium), as opposed to the spinal
nerves, which emerge from the vertebral column. Each cranial nerve is accorded a name, which are detailed in
Figure 35.28. Some cranial nerves transmit only sensory information. For example, the olfactory nerve transmits
information about smells from the nose to the brainstem. Other cranial nerves transmit almost solely motor
information. For example, the oculomotor nerve controls the opening and closing of the eyelid and some eye
movements. Other cranial nerves contain a mix of sensory and motor fibers. For example, the glossopharyngeal
nerve has a role in both taste (sensory) and swallowing (motor).
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Optic
Trochlear
Abducens
Vestibulocochlear
Hypoglossal
Accessory -
Olfactory
Oculomotor
Trigeminal
Facial
Glossopharyngeal
Vagus
Figure 35.28 The human brain contains 12 cranial nerves that receive sensory input and control motor output for the
head and neck.
Spinal nerves transmit sensory and motor information between the spinal cord and the rest of the body. Each
of the 31 spinal nerves (in humans) contains both sensory and motor axons. The sensory neuron cell bodies are
grouped in structures called dorsal root ganglia and are shown in Figure 35.29. Each sensory neuron has one
projection—with a sensory receptor ending in skin, muscle, or sensory organs—and another that synapses with
a neuron in the dorsal spinal cord. Motor neurons have cell bodies in the ventral gray matter of the spinal cord
that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the
spinal cord but are sometimes directly stimulated by sensory neurons.
Cross Section of Spinal Cord
Figure 35.29 Spinal nerves contain both sensory and motor axons. The somas of sensory neurons are located in
dorsal root ganglia. The somas of motor neurons are found in the ventral portion of the gray matter of the spinal cord.
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35.5 | Nervous System Disorders
By the end of this section, you will be able to do the following:
• Describe the symptoms, potential causes, and treatment of several examples of nervous system
disorders
A nervous system that functions correctly is a fantastically complex, well-oiled machine—synapses fire
appropriately, muscles move when needed, memories are formed and stored, and emotions are well regulated.
Unfortunately, each year millions of people in the United States deal with some sort of nervous system disorder.
While scientists have discovered potential causes of many of these diseases, and viable treatments for some,
ongoing research seeks to find ways to better prevent and treat all of these disorders.
Neurodegenerative Disorders
Neurodegenerative disorders are illnesses characterized by a loss of nervous system functioning that are
usually caused by neuronal death. These diseases generally worsen over time as more and more neurons
die. The symptoms of a particular neurodegenerative disease are related to where in the nervous system
the death of neurons occurs. Spinocerebellar ataxia, for example, leads to neuronal death in the cerebellum.
The death of these neurons causes problems in balance and walking. Neurodegenerative disorders include
Huntington’s disease, amyotrophic lateral sclerosis, Alzheimer’s disease and other types of dementia disorders,
and Parkinson’s disease. Here, Alzheimer’s and Parkinson’s disease will be discussed in more depth.
Alzheimer’s Disease
Alzheimer’s disease is the most common cause of dementia in the elderly. In 2012, an estimated 5.4 million
Americans suffered from Alzheimer’s disease, and payments for their care are estimated at $200 billion. Roughly
one in every eight people age 65 or older has the disease. Due to the aging of the baby-boomer generation,
there are projected to be as many as 13 million Alzheimer’s patients in the United States in the year 2050.
Symptoms of Alzheimer’s disease include disruptive memory loss, confusion about time or place, difficulty
planning or executing tasks, poor judgment, and personality changes. Problems smelling certain scents can also
be indicative of Alzheimer’s disease and may serve as an early warning sign. Many of these symptoms are also
common in people who are aging normally, so it is the severity and longevity of the symptoms that determine
whether a person is suffering from Alzheimer’s.
Alzheimer’s disease was named for Alois Alzheimer, a German psychiatrist who published a report in 1911
about a woman who showed severe dementia symptoms. Along with his colleagues, he examined the woman’s
brain following her death and reported the presence of abnormal clumps, which are now called amyloid plaques,
along with tangled brain fibers called neurofibrillary tangles. Amyloid plaques, neurofibrillary tangles, and an
overall shrinking of brain volume are commonly seen in the brains of Alzheimer’s patients. Loss of neurons in
the hippocampus is especially severe in advanced Alzheimer’s patients. Figure 35.30 compares a normal brain
to the brain of an Alzheimer’s patient. Many research groups are examining the causes of these hallmarks of the
disease.
One form of the disease is usually caused by mutations in one of three known genes. This rare form of early
onset Alzheimer’s disease affects fewer than five percent of patients with the disease and causes dementia
beginning between the ages of 30 and 60. The more prevalent, late-onset form of the disease likely also has a
genetic component. One particular gene, apolipoprotein E (APOE) has a variant (E4) that increases a carrier’s
likelihood of getting the disease. Many other genes have been identified that might be involved in the pathology.
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Visit this website (http:// 0 penstaxc 0 llege. 0 rg/l/alzheimers) for video links discussing genetics and
Alzheimer’s disease.
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Unfortunately, there is no cure for Alzheimer’s disease. Current treatments focus on managing the symptoms
of the disease. Because decrease in the activity of cholinergic neurons (neurons that use the neurotransmitter
acetylcholine) is common in Alzheimer’s disease, several drugs used to treat the disease work by increasing
acetylcholine neurotransmission, often by inhibiting the enzyme that breaks down acetylcholine in the synaptic
cleft. Other clinical interventions focus on behavioral therapies like psychotherapy, sensory therapy, and
cognitive exercises. Since Alzheimer’s disease appears to hijack the normal aging process, research into
prevention is prevalent. Smoking, obesity, and cardiovascular problems may be risk factors for the disease, so
treatments for those may also help to prevent Alzheimer’s disease. Some studies have shown that people who
remain intellectually active by playing games, reading, playing musical instruments, and being socially active in
later life have a reduced risk of developing the disease.
Figure 35.30 Compared to a normal brain (left), the brain from a patient with Alzheimer's disease (right) shows
a dramatic neurodegeneration, particularly within the ventricles and hippocampus, (credit: modification of work by
“Garrando’VWikimedia Commons based on original images by ADEAR: "Alzheimer's Disease Education and Referral
Center, a service of the National Institute on Aging”)
Parkinson’s Disease
Like Alzheimer’s disease, Parkinson’s disease is a neurodegenerative disease. It was first characterized by
James Parkinson in 1817. Each year, 50,000-60,000 people in the United States are diagnosed with the disease.
Parkinson’s disease causes the loss of dopamine neurons in the substantia nigra, a midbrain structure that
regulates movement. Loss of these neurons causes many symptoms including tremor (shaking of fingers or a
limb), slowed movement, speech changes, balance and posture problems, and rigid muscles. The combination
of these symptoms often causes a characteristic slow hunched shuffling walk, illustrated in Figure 35.31.
Patients with Parkinson’s disease can also exhibit psychological symptoms, such as dementia or emotional
problems.
Although some patients have a form of the disease known to be caused by a single mutation, for most
patients the exact causes of Parkinson’s disease remain unknown: the disease likely results from a combination
of genetic and environmental factors (similar to Alzheimer’s disease). Post-mortem analysis of brains from
Parkinson’s patients shows the presence of Lewy bodies—abnormal protein clumps—in dopaminergic neurons.
The prevalence of these Lewy bodies often correlates with the severity of the disease.
There is no cure for Parkinson’s disease, and treatment is focused on easing symptoms. One of the most
commonly prescribed drugs for Parkinson’s is L-DOPA, which is a chemical that is converted into dopamine by
neurons in the brain. This conversion increases the overall level of dopamine neurotransmission and can help
compensate for the loss of dopaminergic neurons in the substantia nigra. Other drugs work by inhibiting the
enzyme that breaks down dopamine.
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Figure 35.31 Parkinson’s patients often have a characteristic hunched walk.
Neurodevelopmental Disorders
Neurodevelopmental disorders occur when the development of the nervous system is disturbed. There are
several different classes of neurodevelopmental disorders. Some, like Down Syndrome, cause intellectual
deficits. Others specifically affect communication, learning, or the motor system. Some disorders like autism
spectrum disorder and attention deficit/hyperactivity disorder have complex symptoms.
Autism
Autism spectrum disorder (ASD) is a neurodevelopmental disorder. Its severity differs from person to person.
Estimates for the prevalence of the disorder have changed rapidly in the past few decades. Current estimates
suggest that one in 88 children will develop the disorder. ASD is four times more prevalent in males than females.
LINK
T a
LEARNING
This video (http:// 0 penstaxc 0 llege. 0 rg/l/autism) discusses possible reasons why there has been a recent
increase in the number of people diagnosed with autism.
A characteristic symptom of ASD is impaired social skills. Children with autism may have difficulty making and
maintaining eye contact and reading social cues. They also may have problems feeling empathy for others.
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Other symptoms of ASD include repetitive motor behaviors (such as rocking back and forth), preoccupation with
specific subjects, strict adherence to certain rituals, and unusual language use. Up to 30 percent of patients
with ASD develop epilepsy, and patients with some forms of the disorder (like Fragile X) also have intellectual
disability. Because it is a spectrum disorder, other ASD patients are very functional and have good-to-excellent
language skills. Many of these patients do not feel that they suffer from a disorder and instead think that their
brains just process information differently.
Except for some well-characterized, clearly genetic forms of autism (like Fragile X and Rett’s Syndrome), the
causes of ASD are largely unknown. Variants of several genes correlate with the presence of ASD, but for
any given patient, many different mutations in different genes may be required for the disease to develop. At
a general level, ASD is thought to be a disease of “incorrect" wiring. Accordingly, brains of some ASD patients
lack the same level of synaptic pruning that occurs in non-affected people. In the 1990s, a research paper linked
autism to a common vaccine given to children. This paper was retracted when it was discovered that the author
falsified data, and follow-up studies showed no connection between vaccines and autism.
Treatment for autism usually combines behavioral therapies and interventions, along with medications to treat
other disorders common to people with autism (depression, anxiety, obsessive compulsive disorder). Although
early interventions can help mitigate the effects of the disease, there is currently no cure for ASD.
Attention Deficit Hyperactivity Disorder (ADHD)
Approximately three to five percent of children and adults are affected by attention deficit/hyperactivity
disorder (ADHD). Like ASD, ADHD is more prevalent in males than females. Symptoms of the disorder
include inattention (lack of focus), executive functioning difficulties, impulsivity, and hyperactivity beyond what
is characteristic of the normal developmental stage. Some patients do not have the hyperactive component
of symptoms and are diagnosed with a subtype of ADHD: attention deficit disorder (ADD). Many people
with ADHD also show comorbitity, in that they develop secondary disorders in addition to ADHD. Examples
include depression or obsessive compulsive disorder (OCD). Figure 35.32 provides some statistics concerning
comorbidity with ADHD.
The cause of ADHD is unknown, although research points to a delay and dysfunction in the development of the
prefrontal cortex and disturbances in neurotransmission. According to studies of twins, the disorder has a strong
genetic component. There are several candidate genes that may contribute to the disorder, but no definitive links
have been discovered. Environmental factors, including exposure to certain pesticides, may also contribute to
the development of ADHD in some patients. Treatment for ADHD often involves behavioral therapies and the
prescription of stimulant medications, which paradoxically cause a calming effect in these patients.
ADHD Alone
49 %
ADHD +Conduct
disorder
7 %
ADHD + Depression
11 %
ADHD + Anxiety disorder
11 %
ADHD + Combined disorders (depression, anxiety and conduct)
24 %
Figure 35.32 Many people with ADHD have one or more other neurological disorders, (credit “chart design and
illustration”: modification of work by Leigh Coriale; credit “data”: Drs. Biederman and Faraone, Massachusetts General
Hospital).
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ca eer connection
Neurologist
Neurologists are physicians who specialize in disorders of the nervous system. They diagnose and treat
disorders such as epilepsy, stroke, dementia, nervous system injuries, Parkinson’s disease, sleep disorders,
and multiple sclerosis. Neurologists are medical doctors who have attended college, medical school, and
completed three to four years of neurology residency.
When examining a new patient, a neurologist takes a full medical history and performs a complete physical
exam. The physical exam contains specific tasks that are used to determine what areas of the brain, spinal
cord, or peripheral nervous system may be damaged. For example, to check whether the hypoglossal nerve
is functioning correctly, the neurologist will ask the patient to move his or her tongue in different ways. If the
patient does not have full control over tongue movements, then the hypoglossal nerve may be damaged
or there may be a lesion in the brainstem where the cell bodies of these neurons reside (or there could be
damage to the tongue muscle itself).
Neurologists have other tools besides a physical exam they can use to diagnose particular problems
in the nervous system. If the patient has had a seizure, for example, the neurologist can use
electroencephalography (EEG), which involves taping electrodes to the scalp to record brain activity, to try
to determine which brain regions are involved in the seizure. In suspected stroke patients, a neurologist can
use a computerized tomography (CT) scan, which is a type of X-ray, to look for bleeding in the brain or a
possible brain tumor. To treat patients with neurological problems, neurologists can prescribe medications
or refer the patient to a neurosurgeon for surgery.
LINK
T a
LEARNING
This website (http:// 0 penstaxc 0 llege. 0 rg/l/neur 0 l 0 gic_exam) allows you to see the different tests a
neurologist might use to see what regions of the nervous system may be damaged in a patient.
Mental Illnesses
Mental illnesses are nervous system disorders that result in problems with thinking, mood, or relating with other
people. These disorders are severe enough to affect a person’s quality of life and often make it difficult for
people to perform the routine tasks of daily living. Debilitating mental disorders plague approximately 12.5 million
Americans (about 1 in 17 people) at an annual cost of more than $300 billion. There are several types of mental
disorders including schizophrenia, major depression, bipolar disorder, anxiety disorders and phobias, post-
traumatic stress disorders, and obsessive-compulsive disorder (OCD), among others. The American Psychiatric
Association publishes the Diagnostic and Statistical Manual of Mental Disorders (or DSM), which describes the
symptoms required for a patient to be diagnosed with a particular mental disorder. Each newly released version
of the DSM contains different symptoms and classifications as scientists learn more about these disorders, their
causes, and how they relate to each other. A more detailed discussion of two mental illnesses—schizophrenia
and major depression—is given below.
Schizophrenia
Schizophrenia is a serious and often debilitating mental illness affecting one percent of people in the United
States. Symptoms of the disease include the inability to differentiate between reality and imagination,
inappropriate and unregulated emotional responses, difficulty thinking, and problems with social situations.
People with schizophrenia can suffer from hallucinations and hear voices; they may also suffer from delusions.
Patients also have so-called “negative" symptoms like a flattened emotional state, loss of pleasure, and loss of
basic drives. Many schizophrenic patients are diagnosed in their late adolescence or early 20s. The development
of schizophrenia is thought to involve malfunctioning dopaminergic neurons and may also involve problems with
glutamate signaling. Treatment for the disease usually requires antipsychotic medications that work by blocking
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dopamine receptors and decreasing dopamine neurotransmission in the brain. This decrease in dopamine can
cause Parkinson’s disease-like symptoms in some patients. While some classes of antipsychotics can be quite
effective at treating the disease, they are not a cure, and most patients must remain medicated for the rest of
their lives.
Depression
Major depression affects approximately 6.7 percent of the adults in the United States each year and is one
of the most common mental disorders. To be diagnosed with major depressive disorder, a person must have
experienced a severely depressed mood lasting longer than two weeks along with other symptoms including
a loss of enjoyment in activities that were previously enjoyed, changes in appetite and sleep schedules,
difficulty concentrating, feelings of worthlessness, and suicidal thoughts. The exact causes of major depression
are unknown and likely include both genetic and environmental risk factors. Some research supports the
“classic monoamine hypothesis,” which suggests that depression is caused by a decrease in norepinephrine
and serotonin neurotransmission. One argument against this hypothesis is the fact that some antidepressant
medications cause an increase in norepinephrine and serotonin release within a few hours of beginning
treatment—but clinical results of these medications are not seen until weeks later. This has led to alternative
hypotheses: for example, dopamine may also be decreased in depressed patients, or it may actually be
an increase in norepinephrine and serotonin that causes the disease, and antidepressants force a feedback
loop that decreases this release. Treatments for depression include psychotherapy, electroconvulsive therapy,
deep-brain stimulation, and prescription medications. There are several classes of antidepressant medications
that work through different mechanisms. For example, monoamine oxidase inhibitors (MAO inhibitors) block
the enzyme that degrades many neurotransmitters (including dopamine, serotonin, norepinephrine), resulting
in increased neurotransmitter in the synaptic cleft. Selective serotonin reuptake inhibitors (SSRIs) block the
reuptake of serotonin into the presynaptic neuron. This blockage results in an increase in serotonin in the
synaptic cleft. Other types of drugs such as norepinephrine-dopamine reuptake inhibitors and norepinephrine-
serotonin reuptake inhibitors are also used to treat depression.
Other Neurological Disorders
There are several other neurological disorders that cannot be easily placed in the above categories. These
include chronic pain conditions, cancers of the nervous system, epilepsy disorders, and stroke. Epilepsy and
stroke are discussed below.
Epilepsy
Estimates suggest that up to three percent of people in the United States will be diagnosed with epilepsy in their
lifetime. While there are several different types of epilepsy, all are characterized by recurrent seizures. Epilepsy
itself can be a symptom of a brain injury, disease, or other illness. For example, people who have intellectual
disability or ASD can experience seizures, presumably because the developmental wiring malfunctions that
caused their disorders also put them at risk for epilepsy. For many patients, however, the cause of their epilepsy
is never identified and is likely to be a combination of genetic and environmental factors. Often, seizures can be
controlled with anticonvulsant medications. However, for very severe cases, patients may undergo brain surgery
to remove the brain area where seizures originate.
Stroke
A stroke results when blood fails to reach a portion of the brain for a long enough time to cause damage. Without
the oxygen supplied by blood flow, neurons in this brain region die. This neuronal death can cause many different
symptoms—depending on the brain area affected— including headache, muscle weakness or paralysis, speech
disturbances, sensory problems, memory loss, and confusion. Stroke is often caused by blood clots and can
also be caused by the bursting of a weak blood vessel. Strokes are extremely common and are the third most
common cause of death in the United States. On average one person experiences a stroke every 40 seconds
in the United States. Approximately 75 percent of strokes occur in people older than 65. Risk factors for stroke
include high blood pressure, diabetes, high cholesterol, and a family history of stroke. Smoking doubles the risk
of stroke. Because a stroke is a medical emergency, patients with symptoms of a stroke should immediately go
to the emergency room, where they can receive drugs that will dissolve any clot that may have formed. These
drugs will not work if the stroke was caused by a burst blood vessel or if the stroke occurred more than three
hours before arriving at the hospital. Treatment following a stroke can include blood pressure medication (to
prevent future strokes) and (sometimes intense) physical therapy.
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KEY TERMS
acetylcholine neurotransmitter released by neurons in the central nervous system and peripheral nervous
system
action potential self-propagating momentary change in the electrical potential of a neuron (or muscle)
membrane
Alzheimer’s disease neurodegenerative disorder characterized by problems with memory and thinking
amygdala structure within the limbic system that processes fear
arachnoid mater spiderweb-like middle layer of the meninges that cover the central nervous system
astrocyte glial cell in the central nervous system that provide nutrients, extracellular buffering, and structural
support for neurons; also makes up the blood-brain barrier
attention deficit hyperactivity disorder (ADHD) neurodevelopmental disorder characterized by difficulty
maintaining attention and controlling impulses
autism spectrum disorder (ASD) neurodevelopmental disorder characterized by impaired social interaction
and communication abilities
autonomic nervous system part of the peripheral nervous system that controls bodily functions
axon tube-like structure that propagates a signal from a neuron’s cell body to axon terminals
axon hillock electrically sensitive structure on the cell body of a neuron that integrates signals from multiple
neuronal connections
axon terminal structure on the end of an axon that can form a synapse with another neuron
basal ganglia interconnected collections of cells in the brain that are involved in movement and motivation; also
known as basal nuclei
basal nuclei see basal ganglia
brainstem portion of the brain that connects with the spinal cord; controls basic nervous system functions like
breathing, heart rate, and swallowing
cerebellum brain structure involved in posture, motor coordination, and learning new motor actions
cerebral cortex outermost sheet of brain tissue; involved in many higher-order functions
cerebrospinal fluid (CSF) clear liquid that surrounds the brain and spinal cord and fills the ventricles and
central canal; acts as a shock absorber and circulates material throughout the brain and spinal cord
choroid plexus spongy tissue within ventricles that produces cerebrospinal fluid
cingulate gyrus helps regulate emotions and pain; thought to directly drive the body’s conscious response to
unpleasant experiences
corpus callosum thick fiber bundle that connects the cerebral hemispheres
cranial nerve sensory and/or motor nerve that emanates from the brain
dendrite structure that extends away from the cell body to receive messages from other neurons
depolarization change in the membrane potential to a less negative value
dura mater tough outermost layer that covers the central nervous system
ependymal cell that lines fluid-filled ventricles of the brain and the central canal of the spinal cord; involved in
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production of cerebrospinal fluid
epilepsy neurological disorder characterized by recurrent seizures
excitatory postsynaptic potential (EPSP) depolarization of a postsynaptic membrane caused by
neurotransmitter molecules released from a presynaptic cell
frontal lobe part of the cerebral cortex that contains the motor cortex and areas involved in planning, attention,
and language
glia (also, glial cells) cells that provide support functions for neurons
gyrus (plural: gyri) ridged protrusions in the cortex
hippocampus brain structure in the temporal lobe involved in processing memories
hyperpolarization change in the membrane potential to a more negative value
hypothalamus brain structure that controls hormone release and body homeostasis
inhibitory postsynaptic potential (IPSP) hyperpolarization of a postsynaptic membrane caused by
neurotransmitter molecules released from a presynaptic cell
limbic system connected brain areas that process emotion and motivation
long-term depression (LTD) prolonged decrease in synaptic coupling between a pre- and postsynaptic cell
long-term potentiation (LTP) prolonged increase in synaptic coupling between a pre-and postsynaptic cell
major depression mental illness characterized by prolonged periods of sadness
membrane potential difference in electrical potential between the inside and outside of a cell
meninge membrane that covers and protects the central nervous system
microglia glia that scavenge and degrade dead cells and protect the brain from invading microorganisms
myelin fatty substance produced by glia that insulates axons
neurodegenerative disorder nervous system disorder characterized by the progressive loss of neurological
functioning, usually caused by neuron death
neuron specialized cell that can receive and transmit electrical and chemical signals
nodes of Ranvier gaps in the myelin sheath where the signal is recharged
norepinephrine neurotransmitter and hormone released by activation of the sympathetic nervous system
occipital lobe part of the cerebral cortex that contains visual cortex and processes visual stimuli
oligodendrocyte glial cell that myelinates central nervous system neuron axons
parasympathetic nervous system division of autonomic nervous system that regulates visceral functions
during rest and digestion
parietal lobe part of the cerebral cortex involved in processing touch and the sense of the body in space
Parkinson’s disease neurodegenerative disorder that affects the control of movement
pia mater thin membrane layer directly covering the brain and spinal cord
proprioception sense about how parts of the body are oriented in space
radial glia glia that serve as scaffolds for developing neurons as they migrate to their final destinations
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refractory period period after an action potential when it is more difficult or impossible for an action potential to
be fired; caused by inactivation of sodium channels and activation of additional potassium channels of the
membrane
saltatory conduction “jumping” of an action potential along an axon from one node of Ranvier to the next
satellite glia glial cell that provides nutrients and structural support for neurons in the peripheral nervous
system
schizophrenia mental disorder characterized by the inability to accurately perceive reality; patients often have
difficulty thinking clearly and can suffer from delusions
Schwann cell glial cell that creates myelin sheath around a peripheral nervous system neuron axon
sensory-somatic nervous system system of sensory and motor nerves
somatosensation sense of touch
spinal cord thick fiber bundle that connects the brain with peripheral nerves; transmits sensory and motor
information; contains neurons that control motor reflexes
spinal nerve nerve projecting between skin or muscle and spinal cord
sulcus (plural: sulci) indents or “valleys" in the cortex
summation process of multiple presynaptic inputs creating EPSPs around the same time for the postsynaptic
neuron to be sufficiently depolarized to fire an action potential
sympathetic nervous system division of autonomic nervous system activated during stressful “fight or flight”
situations
synapse junction between two neurons where neuronal signals are communicated
synaptic cleft space between the presynaptic and postsynaptic membranes
synaptic vesicle spherical structure that contains a neurotransmitter
temporal lobe part of the cerebral cortex that processes auditory input; parts of the temporal lobe are involved
in speech, memory, and emotion processing
thalamus brain area that relays sensory information to the cortex
threshold of excitation level of depolarization needed for an action potential to fire
ventricle cavity within brain that contains cerebrospinal fluid
CHAPTER SUMMARY
35.1 Neurons and Glial Cells
The nervous system is made up of neurons and glia. Neurons are specialized cells that are capable of sending
electrical as well as chemical signals. Most neurons contain dendrites, which receive these signals, and axons
that send signals to other neurons or tissues. There are four main types of neurons: unipolar, bipolar,
multipolar, and pseudounipolar neurons. Glia are non-neuronal cells in the nervous system that support
neuronal development and signaling. There are several types of glia that serve different functions.
35.2 How Neurons Communicate
Neurons have charged membranes because there are different concentrations of ions inside and outside of the
cell. Voltage-gated ion channels control the movement of ions into and out of a neuron. When a neuronal
membrane is depolarized to at least the threshold of excitation, an action potential is fired. The action potential
is then propagated along a myelinated axon to the axon terminals. In a chemical synapse, the action potential
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causes release of neurotransmitter molecules into the synaptic cleft. Through binding to postsynaptic receptors,
the neurotransmitter can cause excitatory or inhibitory postsynaptic potentials by depolarizing or
hyperpolarizing, respectively, the postsynaptic membrane, in electrical synapses, the action potential is directly
communicated to the postsynaptic cell through gap junctions—large channel proteins that connect the pre-and
postsynaptic membranes. Synapses are not static structures and can be strengthened and weakened. Two
mechanisms of synaptic plasticity are long-term potentiation and long-term depression.
35.3 The Central Nervous System
The vertebrate central nervous system contains the brain and the spinal cord, which are covered and protected
by three meninges. The brain contains structurally and functionally defined regions. In mammals, these include
the cortex (which can be broken down into four primary functional lobes: frontal, temporal, occipital, and
parietal), basal ganglia, thalamus, hypothalamus, limbic system, cerebellum, and brainstem—although
structures in some of these designations overlap. While functions may be primarily localized to one structure in
the brain, most complex functions, like language and sleep, involve neurons in multiple brain regions. The
spinal cord is the information superhighway that connects the brain with the rest of the body through its
connections with peripheral nerves. It transmits sensory and motor input and also controls motor reflexes.
35.4 The Peripheral Nervous System
The peripheral nervous system contains both the autonomic and sensory-somatic nervous systems. The
autonomic nervous system provides unconscious control over visceral functions and has two divisions: the
sympathetic and parasympathetic nervous systems. The sympathetic nervous system is activated in stressful
situations to prepare the animal for a “fight or flight” response. The parasympathetic nervous system is active
during restful periods. The sensory-somatic nervous system is made of cranial and spinal nerves that transmit
sensory information from skin and muscle to the CNS and motor commands from the CNS to the muscles.
35.5 Nervous System Disorders
Some general themes emerge from the sampling of nervous system disorders presented above. The causes
for most disorders are not fully understood—at least not for all patients—and likely involve a combination of
nature (genetic mutations that become risk factors) and nurture (emotional trauma, stress, hazardous chemical
exposure). Because the causes have yet to be fully determined, treatment options are often lacking and only
address symptoms.
VISUAL CONNECTION QUESTIONS
3. Figure 35.26 Which of the following statements is
false?
a. The parasympathetic pathway is
responsible for relaxing the body, while the
sympathetic pathway is responsible for
preparing for an emergency.
b. Most preganglionic neurons in the
sympathetic pathway originate in the spinal
cord.
c. Slowing of the heartbeat is a
parasympathetic response.
d. Parasympathetic neurons are responsible
for releasing norepinephrine on the target
organ, while sympathetic neurons are
responsible for releasing acetylcholine.
REVIEW QUESTIONS
4. Neurons contain , which can receive
a.
axons
signals from other neurons.
b.
mitochondria
c.
dendrites
d.
Golgi bodies
1. Figure 35.3 Which of the following statements is
false?
a. The soma is the cell body of a nerve cell.
b. Myelin sheath provides an insulating layer to
the dendrites.
c. Axons carry the signal from the soma to the
target.
d. Dendrites carry the signal to the soma.
2. Figure 35.11 Potassium channel blockers, such as
amiodarone and procainamide, which are used to
treat abnormal electrical activity in the heart, called
cardiac dysrhythmia, impede the movement of K+
through voltage-gated K+ channels. Which part of the
action potential would you expect potassium
channels to affect?
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5. A(n)_neuron has one axon and one
dendrite extending directly from the cell body.
a. unipolar
b. bipolar
c. multipolar
d. pseudounipolar
6. Glia that provide myelin for neurons in the brain
are called_.
a. Schwann cells
b. oligodendrocytes
c. microglia
d. astrocytes
7. Meningitis is a viral or bacterial infection of the
brain. Which cell type is the first to have its function
disrupted during meningitis?
a. astrocytes
b. microglia
c. neurons
d. satellite glia
8. For a neuron to fire an action potential, its
membrane must reach_.
a. hyperpolarization
b. the threshold of excitation
c. the refractory period
d. inhibitory postsynaptic potential
9. After an action potential, the opening of additional
voltage-gated_channels and the
inactivation of sodium channels, cause the
membrane to return to its resting membrane
potential.
a. sodium
b. potassium
c. calcium
d. chloride
10. What is the term for protein channels that
connect two neurons at an electrical synapse?
a. synaptic vesicles
b. voltage-gated ion channels
c. gap junction protein
d. sodium-potassium exchange pumps
11. Which of the following molecules is not involved
in the maintenance of the resting membrane
potential?
a.
potassium cations
b.
ATP
c.
voltage-gated ion channels
d.
calcium cations
12. The
lobe contains the visual cortex.
a.
frontal
b.
parietal
c.
temporal
d.
occipital
13. The
connects the two cerebral
hemispheres.
a. limbic system
b. corpus callosum
c. cerebellum
d. pituitary
14. Neurons in the_control motor reflexes.
a. thalamus
b. spinal cord
c. parietal lobe
d. hippocampus
15. Phineas Gage was a 19 th century railroad worker
who survived an accident that drove a large iron rod
through his head. If the injury resulted in him
becoming temperamental and capricious what part of
his brain was damaged?
a. frontal lobe
b. hippocampus
c. parietal lobe
d. temporal lobe
16. Activation of the sympathetic nervous system
causes:
a. increased blood flow into the skin
b. a decreased heart rate
c. an increased heart rate
d. increased digestion
17. Where are parasympathetic preganglionic cell
bodies located?
a. cerebellum
b. brainstem
c. dorsal root ganglia
d. skin
18. _is released by motor nerve endings
onto muscle.
a. Acetylcholine
b. Norepinephrine
c. Dopamine
d. Serotonin
19. Parkinson’s disease is a caused by the
degeneration of neurons that release_.
a. serotonin
b. dopamine
c. glutamate
d. norepinephrine
20._medications are often used to treat
patients with ADHD.
a. Tranquilizer
b. Antibiotic
c. Stimulant
d. Anti-seizure
21. Strokes are often caused by
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a. neurodegeneration
b. blood clots or burst blood vessels
c. seizures
d. viruses
22. Why is it difficult to identify the cause of many
nervous system disorders?
a. The genes associated with the diseases are
not known.
b. There are no obvious defects in brain
structure.
c. The onset and display of symptoms varies
between patients.
d. all of the above
CRITICAL THINKING QUESTIONS
24. How are neurons similar to other cells? How are
they unique?
25. Multiple sclerosis causes demyelination of axons
in the brain and spinal cord. Why is this problematic?
26. Many neurons have only a single axon, but many
terminals at the end of the axon. How does this end
structure of the axon support its function?
27. How does myelin aid propagation of an action
potential along an axon? How do the nodes of
Ranvier help this process?
28. What are the main steps in chemical
neurotransmission?
29. Describe how long-term potentiation can lead to a
nicotine addiction.
30. What methods can be used to determine the
function of a particular brain region?
31. What are the main functions of the spinal cord?
32. Alzheimer’s disease involves three of the four
lobes of the brain. Identify one of the involved lobes
and describe the lobe’s symptoms associated with
23. Why do many patients with neurodevelopmental
disorders develop secondary disorders?
a. Their genes predispose them to
schizophrenia.
b. Stimulant medications cause new
behavioral disorders.
c. Behavioral therapies only improve
neurodevelopmental disorders.
d. Dysfunction in the brain can affect many
aspects of the body.
the disease.
33. What are the main differences between the
sympathetic and parasympathetic branches of the
autonomic nervous system?
34. What are the main functions of the sensory-
somatic nervous system?
35. Describe how the sensory-somatic nervous
system reacts by reflex to a person touching
something hot. How does this allow for rapid
responses in potentially dangerous situations?
36. Scientists have suggested that the autonomic
nervous system is not well-adapted to modern
human life. How is the sympathetic nervous system
an ineffective response to the everyday challenges
faced by modern humans?
37. What are the main symptoms of Alzheimer’s
disease?
38. What are possible treatments for patients with
major depression?
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36 | SENSORY SYSTEMS
Figure 36.1 This shark uses its senses of sight, vibration (lateral-line system), and smell to hunt, but it also relies on
its ability to sense the electric fields of prey, a sense not present in most land animals, (credit: modification of work by
Hermanus Backpackers Hostel, South Africa)
Chapter Outline
36.1: Sensory Processes
36.2: Somatosensation
36.3: Taste and Smell
36.4: Hearing and Vestibular Sensation
36.5: Vision
Introduction
In more advanced animals, the senses are constantly at work, making the animal aware of stimuli—such as light,
or sound, or the presence of a chemical substance in the external environment—and monitoring information
about the organism’s internal environment. All bilaterally symmetric animals have a sensory system, and the
development of any species’ sensory system has been driven by natural selection; thus, sensory systems
differ among species according to the demands of their environments. The shark, unlike most fish predators, is
electrosensitive—that is, sensitive to electrical fields produced by other animals in its environment. While it is
helpful to this underwater predator, electrosensitivity is a sense not found in most land animals.
36.1 1 Sensory Processes
By the end of this section, you will be able to do the following:
• Identify the general and special senses in humans
• Describe three important steps in sensory perception
• Explain the concept of just-noticeable difference in sensory perception
Senses provide information about the body and its environment. Humans have five special senses: olfaction
(smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess
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general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and
vibration. Vestibular sensation, which is an organism’s sense of spatial orientation and balance,
proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track
kinesthesia (limb movement) are part of somatosensation. Although the sensory systems associated with
these senses are very different, all share a common function: to convert a stimulus (such as light, or sound,
or the position of the body) into an electrical signal in the nervous system. This process is called sensory
transduction.
There are two broad types of cellular systems that perform sensory transduction. In one, a neuron works with
a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus.
Stimulation of the sensory receptor activates the associated afferent neuron, which carries information about
the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending
responds to a stimulus in the internal or external environment: this neuron constitutes the sensory receptor. Free
nerve endings can be stimulated by several different stimuli, thus showing little receptor specificity. For example,
pain receptors in your gums and teeth may be stimulated by temperature changes, chemical stimulation, or
pressure.
Reception
The first step in sensation is reception, which is the activation of sensory receptors by stimuli such as
mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then
respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it
far away or in contact with the body, is that receptor’s receptive field. Think for a moment about the differences
in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with the
body. For the sense of hearing, a stimulus can be a moderate distance away (some baleen whale sounds can
propagate for many kilometers). For vision, a stimulus can be very far away; for example, the visual system
perceives light from stars at enormous distances.
Transduction
The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal
in the nervous system. This takes place at the sensory receptor, and the change in electrical potential that is
produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a
receptor potential? In this example, a type of receptor called a mechanoreceptor (as shown in Figure 36.2)
possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing
them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing
its electrical potential. Recall that in the nervous system, a positive change of a neuron’s electrical potential
(also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the
magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of
depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action
potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a
positive direction, although for some receptors, such as those in the visual system, this is not always the case.
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Tectorial membrane
Tether
Cytoplasm
Gated ion channel
Cytoskeleton
Tectorial membrane
Cytoskeleton
(a)
Figure 36.2 (a) Mechanosensitive ion channels are gated ion channels that respond to mechanical deformation of the
plasma membrane. A mechanosensitive channel is connected to the plasma membrane and the cytoskeleton by hair¬
like tethers. When pressure causes the extracellular matrix to move, the channel opens, allowing ions to enter or exit
the cell, (b) Stereocilia in the human ear are connected to mechanosensitive ion channels. When a sound causes the
stereocilia to move, mechanosensitive ion channels transduce the signal to the cochlear nerve.
Sensory receptors for different senses are very different from each other, and they are specialized according
to the type of stimulus they sense: they have receptor specificity. For example, touch receptors, light receptors,
and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound;
they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as
happens with olfaction, contributing to our sense of taste.
Encoding and Transmission of Sensory Information
Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the
stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action
potentials transmitted over a sensory receptor’s afferent axons encode one type of stimulus, and this segregation
of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their
own dedicated system, and electrical activity in the axons of the auditory receptors will be interpreted by the
brain as an auditory stimulus—a sound.
The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor.
Thus, an intense stimulus will produce a more rapid train of action potentials, and reducing the stimulus will
likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the
number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent
receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information
begins as soon as the information is received in the CNS, and the brain will further process incoming signals.
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Perception
Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory
receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system,
in the brain. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory
receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that
particular stimulus and synapse with particular neurons in the brain or spinal cord.
All sensory signals, except those from the olfactory system, are transmitted though the central nervous system
and are routed to the thalamus and to the appropriate region of the cortex. Recall that the thalamus is a structure
in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When
the sensory signal exits the thalamus, it is conducted to the specific area of the cortex (Figure 36.3) dedicated
to processing that particular sense.
How are neural signals interpreted? Interpretation of sensory signals between individuals of the same species
is largely similar, owing to the inherited similarity of their nervous systems; however, there are some individual
differences. A good example of this is individual tolerances to a painful stimulus, such as dental pain, which
certainly differ.
Figure 36.3 In humans, with the exception of olfaction, all sensory signals are routed from the (a) thalamus to (b) final
processing regions in the cortex of the brain, (credit b: modification of work by Polina Tishina)
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scientific methf d CONNECTION
Just-Noticeable Difference
It is easy to differentiate between a one-pound bag of rice and a two-pound bag of rice. There is a one-
pound difference, and one bag is twice as heavy as the other. However, would it be as easy to differentiate
between a 20- and a 21-pound bag?
Question: What is the smallest detectible weight difference between a one-pound bag of rice and a larger
bag? What is the smallest detectible difference between a 20-pound bag and a larger bag? In both cases,
at what weights are the differences detected? This smallest detectible difference in stimuli is known as the
just-noticeable difference (JND).
Background: Research background literature on JND and on Weber’s Law, a description of a proposed
mathematical relationship between the overall magnitude of the stimulus and the JND. You will be testing
JND of different weights of rice in bags. Choose a convenient increment that is to be stepped through while
testing. For example, you could choose 10 percent increments between one and two pounds (1.1, 1.2, 1.3,
1.4, and so on) or 20 percent increments (1.2, 1.4, 1.6, and 1.8).
Hypothesis: Develop a hypothesis about JND in terms of percentage of the whole weight being tested
(such as “the JND between the two small bags and between the two large bags is proportionally the same,”
or . . is not proportionally the same.”) So, for the first hypothesis, if the JND between the one-pound
bag and a larger bag is 0.2 pounds (that is, 20 percent; 1.0 pound feels the same as 1.1 pounds, but 1.0
pound feels less than 1.2 pounds), then the JND between the 20-pound bag and a larger bag will also be
20 percent. (So, 20 pounds feels the same as 22 pounds or 23 pounds, but 20 pounds feels less than 24
pounds.)
Test the hypothesis: Enlist 24 participants, and split them into two groups of 12. To set up the
demonstration, assuming a 10 percent increment was selected, have the first group be the one-pound
group. As a counter-balancing measure against a systematic error, however, six of the first group will
compare one pound to two pounds, and step down in weight (1.0 to 2.0, 1.0 to 1.9, and so on), while the
other six will step up (1.0 to 1.1, 1.0 to 1.2, and so on). Apply the same principle to the 20-pound group (20
to 40, 20 to 38, and so on, and 20 to 22, 20 to 24, and so on). Given the large difference between 20 and 40
pounds, you may wish to use 30 pounds as your larger weight. In any case, use two weights that are easily
detectable as different.
Record the observations: Record the data in a table similar to the table below. For the one-pound and
20-pound groups (base weights) record a plus sign (+) for each participant that detects a difference between
the base weight and the step weight. Record a minus sign (-) for each participant that finds no difference. If
one-tenth steps were not used, then replace the steps in the “Step Weight” columns with the step you are
using.
Results of JND Testing (+ = difference; - = no difference)
Step Weight
One pound
20 pounds
Step Weight
1.1
22
1.2
24
1.3
26
1.4
28
1.5
30
1.6
32
1.7
34
1.8
36
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Chapter 36 | Sensory Systems
Results of JND Testing (+ = difference; - = no difference)
Step Weight
One pound
20 pounds
Step Weight
1.9
38
2.0
40
Table 36.1
Analyze the data/report the results: What step weight did all participants find to be equal with one-pound
base weight? What about the 20-pound group?
Draw a conclusion: Did the data support the hypothesis? Are the final weights proportionally the same?
If not, why not? Do the findings adhere to Weber’s Law? Weber’s Law states that the concept that a just-
noticeable difference in a stimulus is proportional to the magnitude of the original stimulus.
36.2 | Somatosensation
By the end of this section, you will be able to do the following:
• Describe four important mechanoreceptors in human skin
• Describe the topographical distribution of somatosensory receptors between glabrous and hairy skin
• Explain why the perception of pain is subjective
Somatosensation is a mixed sensory category and includes all sensation received from the skin and mucous
membranes, as well from as the limbs and joints. Somatosensation is also known as tactile sense, or more
familiarly, as the sense of touch. Somatosensation occurs all over the exterior of the body and at some interior
locations as well. A variety of receptor types—embedded in the skin, mucous membranes, muscles, joints,
internal organs, and cardiovascular system—play a role.
Recall that the epidermis is the outermost layer of skin in mammals. It is relatively thin, is composed of keratin-
filled cells, and has no blood supply. The epidermis serves as a barrier to water and to invasion by pathogens.
Below this, the much thicker dermis contains blood vessels, sweat glands, hair follicles, lymph vessels, and
lipid-secreting sebaceous glands (Figure 36.4). Below the epidermis and dermis is the subcutaneous tissue, or
hypodermis, the fatty layer that contains blood vessels, connective tissue, and the axons of sensory neurons.
The hypodermis, which holds about 50 percent of the body’s fat, attaches the dermis to the bone and muscle,
and supplies nerves and blood vessels to the dermis.
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vessels follicle Oil gland vessel
Figure 36.4 Mammalian skin has three layers: an epidermis, a dermis, and a hypodermis. (credit: modification of work
by Don Bliss, National Cancer Institute)
Somatosensory Receptors
Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, pain
receptors, and chemoreceptors. These categories are based on the nature of stimuli each receptor class
transduces. What is commonly referred to as “touch” involves more than one kind of stimulus and more than one
kind of receptor. Mechanoreceptors in the skin are described as encapsulated (that is, surrounded by a capsule)
or unencapsulated (a group that includes free nerve endings). A free nerve ending, as its name implies, is an
unencapsulated dendrite of a sensory neuron. Free nerve endings are the most common nerve endings in skin,
and they extend into the middle of the epidermis. Free nerve endings are sensitive to painful stimuli, to hot and
cold, and to light touch. They are slow to adjust to a stimulus and so are less sensitive to abrupt changes in
stimulation.
There are three classes of mechanoreceptors: tactile, proprioceptors, and baroreceptors. Mechanoreceptors
sense stimuli due to physical deformation of their plasma membranes. They contain mechanically gated ion
channels whose gates open or close in response to pressure, touch, stretching, and sound." There are four
primary tactile mechanoreceptors in human skin: Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and
Pacinian corpuscles; two are located toward the surface of the skin and two are located deeper. A fifth type of
mechanoreceptor, Krause end bulbs, are found only in specialized regions. Merkel’s disks (shown in Figure
36.5) are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on
glabrous skin, that is, the hairless skin found on the palms and fingers, the soles of the feet, and the lips of
humans and other primates. Merkel’s disks are densely distributed in the fingertips and lips. They are slow-
adapting, encapsulated nerve endings, and they respond to light touch. Light touch, also known as discriminative
touch, is a light pressure that allows the location of a stimulus to be pinpointed. The receptive fields of Merkel’s
disks are small with well-defined borders. That makes them finely sensitive to edges and they come into use in
tasks such as typing on a keyboard.
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visual
CONNECTION
Merkel’s disk
Meissner's corpuscle
Ruffini
ending
-Epidermis
-Dermis
Pacinian •
corpuscle Nerve Krause end bulb
Figure 36.5 Four of the primary mechanoreceptors in human skin are shown. Merkel’s disks, which are
unencapsulated, respond to light touch. Meissner’s corpuscles, Ruffini endings, Pacinian corpuscles, and Krause
end bulbs are all encapsulated. Meissner’s corpuscles respond to touch and low-frequency vibration. Ruffini
endings detect stretch, deformation within joints, and warmth. Pacinian corpuscles detect transient pressure and
high-frequency vibration. Krause end bulbs detect cold.
Which of the following statements about mechanoreceptors is false?
a. Pacinian corpuscles are found in both glabrous and hairy skin.
b. Merkel’s disks are abundant on the fingertips and lips.
c. Ruffini endings are encapsulated mechanoreceptors.
d. Meissner’s corpuscles extend into the lower dermis.
Meissner’s corpuscles, (shown in Figure 36.6) also known as tactile corpuscles, are found in the upper dermis,
but they project into the epidermis. They, too, are found primarily in the glabrous skin on the fingertips and
eyelids. They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter.
They are rapidly adapting, fluid-filled, encapsulated neurons with small, well-defined borders and are responsive
to fine details. Like Merkel’s disks, Meissner’s corpuscles are not as plentiful in the palms as they are in the
fingertips.
Figure 36.6 Meissner corpuscles in the fingertips, such as the one viewed here using bright field light microscopy,
allow for touch discrimination of fine detail, (credit: modification of work by "Wbensmith'VWikimedia Commons; scale-
bar data from Matt Russell)
Deeper in the epidermis, near the base, are Ruffini endings, which are also known as bulbous corpuscles. They
are found in both glabrous and hairy skin. These are slow-adapting, encapsulated mechanoreceptors that detect
skin stretch and deformations within joints, so they provide valuable feedback for gripping objects and controlling
finger position and movement. Thus, they also contribute to proprioception and kinesthesia. Ruffini endings also
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detect warmth. Note that these warmth detectors are situated deeper in the skin than are the cold detectors. It is
not surprising, then, that humans detect cold stimuli before they detect warm stimuli.
Pacinian corpuscles (seen in Figure 36.7) are located deep in the dermis of both glabrous and hairy skin and
are structurally similar to Meissner’s corpuscles; they are found in the bone periosteum, joint capsules, pancreas
and other viscera, breast, and genitals. They are rapidly adapting mechanoreceptors that sense deep transient
(but not prolonged) pressure and high-frequency vibration. Pacinian receptors detect pressure and vibration by
being compressed, stimulating their internal dendrites. There are fewer Pacinian corpuscles and Ruffini endings
in skin than there are Merkel’s disks and Meissner’s corpuscles.
200 )im
Figure 36.7 Pacinian corpuscles, such as these visualized using bright field light microscopy, detect pressure (touch)
and high-frequency vibration, (credit: modification of work by Ed Uthman; scale-bar data from Matt Russell)
In proprioception, proprioceptive and kinesthetic signals travel through myelinated afferent neurons running from
the spinal cord to the medulla. Neurons are not physically connected, but communicate via neurotransmitters
secreted into synapses or “gaps” between communicating neurons. Once in the medulla, the neurons continue
carrying the signals to the thalamus.
Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. Related
to these are Golgi tendon organs, which are tension receptors that detect the force of muscle contraction.
Proprioceptive and kinesthetic signals come from limbs. Unconscious proprioceptive signals run from the spinal
cord to the cerebellum, the brain region that coordinates muscle contraction, rather than to the thalamus, like
most other sensory information.
Baroreceptors detect pressure changes in an organ. They are found in the walls of the carotid artery and the
aorta where they monitor blood pressure, and in the lungs where they detect the degree of lung expansion.
Stretch receptors are found at various sites in the digestive and urinary systems.
In addition to these two types of deeper receptors, there are also rapidly adapting hair receptors, which are found
on nerve endings that wrap around the base of hair follicles. There are a few types of hair receptors that detect
slow and rapid hair movement, and they differ in their sensitivity to movement. Some hair receptors also detect
skin deflection, and certain rapidly adapting hair receptors allow detection of stimuli that have not yet touched
the skin.
Integration of Signals from Mechanoreceptors
The configuration of the different types of receptors working in concert in human skin results in a very refined
sense of touch. The nociceptive receptors—those that detect pain—are located near the surface. Small, finely
calibrated mechanoreceptors—Merkel’s disks and Meissner’s corpuscles—are located in the upper layers
and can precisely localize even gentle touch. The large mechanoreceptors—Pacinian corpuscles and Ruffini
endings—are located in the lower layers and respond to deeper touch. (Consider that the deep pressure
that reaches those deeper receptors would not need to be finely localized.) Both the upper and lower layers
of the skin hold rapidly and slowly adapting receptors. Both primary somatosensory cortex and secondary
cortical areas are responsible for processing the complex picture of stimuli transmitted from the interplay of
mechanoreceptors.
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Density of Mechanoreceptors
The distribution of touch receptors in human skin is not consistent over the body. In humans, touch receptors
are less dense in skin covered with any type of hair, such as the arms, legs, torso, and face. Touch receptors
are denser in glabrous skin (the type found on human fingertips and lips, for example), which is typically more
sensitive and is thicker than hairy skin (4 to 5 mm versus 2 to 3 mm).
How is receptor density estimated in a human subject? The relative density of pressure receptors in different
locations on the body can be demonstrated experimentally using a two-point discrimination test. In this
demonstration, two sharp points, such as two thumbtacks, are brought into contact with the subject’s skin
(though not hard enough to cause pain or break the skin). The subject reports if he or she feels one point or two
points. If the two points are felt as one point, it can be inferred that the two points are both in the receptive field
of a single sensory receptor. If two points are felt as two separate points, each is in the receptive field of two
separate sensory receptors. The points could then be moved closer and retested until the subject reports feeling
only one point, and the size of the receptive field of a single receptor could be estimated from that distance.
Thermoreception
In addition to Krause end bulbs that detect cold and Ruffini endings that detect warmth, there are different types
of cold receptors on some free nerve endings: thermoreceptors, located in the dermis, skeletal muscles, liver,
and hypothalamus, that are activated by different temperatures. Their pathways into the brain run from the spinal
cord through the thalamus to the primary somatosensory cortex. Warmth and cold information from the face
travels through one of the cranial nerves to the brain. You know from experience that a tolerably cold or hot
stimulus can quickly progress to a much more intense stimulus that is no longer tolerable. Any stimulus that is
too intense can be perceived as pain because temperature sensations are conducted along the same pathways
that carry pain sensations.
Pain
Pain is the name given to nociception, which is the neural processing of injurious stimuli in response to tissue
damage. Pain is caused by true sources of injury, such as contact with a heat source that causes a thermal burn
or contact with a corrosive chemical. But pain also can be caused by harmless stimuli that mimic the action of
damaging stimuli, such as contact with capsaicins, the compounds that cause peppers to taste hot and which
are used in self-defense pepper sprays and certain topical medications. Peppers taste “hot" because the protein
receptors that bind capsaicin open the same calcium channels that are activated by warm receptors.
Nociception starts at the sensory receptors, but pain, inasmuch as it is the perception of nociception, does not
start until it is communicated to the brain. There are several nociceptive pathways to and through the brain.
Most axons carrying nociceptive information into the brain from the spinal cord project to the thalamus (as do
other sensory neurons) and the neural signal undergoes final processing in the primary somatosensory cortex.
Interestingly, one nociceptive pathway projects not to the thalamus but directly to the hypothalamus in the
forebrain, which modulates the cardiovascular and neuroendocrine functions of the autonomic nervous system.
Recall that threatening—or painful—stimuli stimulate the sympathetic branch of the visceral sensory system,
readying a fight-or-flight response.
View this video (http:// 0 penstaxc 0 llege. 0 rg/l/n 0 ciceptive) that animates the five phases of nociceptive
pain. (This multimedia resource will open in a browser.) (http://cnx.org/content/m66404/1.3/#eip-
id8192636)
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36.3 | Taste and Smell
By the end of this section, you will be able to do the following:
• Explain in what way smell and taste stimuli differ from other sensory stimuli
• Identify the five primary tastes that can be distinguished by humans
• Explain in anatomical terms why a dog’s sense of smell is more acute than a human’s
Taste, also called gustation, and smell, also called olfaction, are the most interconnected senses in that both
involve molecules of the stimulus entering the body and bonding to receptors. Smell lets an animal sense the
presence of food or other animals—whether potential mates, predators, or prey—or other chemicals in the
environment that can impact their survival. Similarly, the sense of taste allows animals to discriminate between
types of foods. While the value of a sense of smell is obvious, what is the value of a sense of taste? Different
tasting foods have different attributes, both helpful and harmful. For example, sweet-tasting substances tend to
be highly caloric, which could be necessary for survival in lean times. Bitterness is associated with toxicity, and
sourness is associated with spoiled food. Salty foods are valuable in maintaining homeostasis by helping the
body retain water and by providing ions necessary for cells to function.
Tastes and Odors
Both taste and odor stimuli are molecules taken in from the environment. The primary tastes detected by humans
are sweet, sour, bitter, salty, and umami. The first four tastes need little explanation. The identification of umami
as a fundamental taste occurred fairly recently—it was identified in 1908 by Japanese scientist Kikunae Ikeda
while he worked with seaweed broth, but it was not widely accepted as a taste that could be physiologically
distinguished until many years later. The taste of umami, also known as savoriness, is attributable to the taste
of the amino acid L-glutamate. In fact, monosodium glutamate, or MSG, is often used in cooking to enhance the
savory taste of certain foods. What is the adaptive value of being able to distinguish umami? Savory substances
tend to be high in protein.
All odors that we perceive are molecules in the air we breathe. If a substance does not release molecules
into the air from its surface, it has no smell. And if a human or other animal does not have a receptor that
recognizes a specific molecule, then that molecule has no smell. Humans have about 350 olfactory receptor
subtypes that work in various combinations to allow us to sense about 10,000 different odors. Compare that to
mice, for example, which have about 1,300 olfactory receptor types, and therefore probably sense more odors.
Both odors and tastes involve molecules that stimulate specific chemoreceptors. Although humans commonly
distinguish taste as one sense and smell as another, they work together to create the perception of flavor. A
person’s perception of flavor is reduced if he or she has congested nasal passages.
Reception and Transduction
Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of
the nasal cavity (as illustrated in Figure 36.8). The olfactory epithelium is a collection of specialized olfactory
receptors in the back of the nasal cavity that spans an area about 5 cm 2 in humans. Recall that sensory cells
are neurons. An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds certain
molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans
have about 12 million olfactory receptors, distributed among hundreds of different receptor types that respond to
different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits
have about 100 million, most dogs have about 1 billion, and bloodhounds—dogs selectively bred for their sense
of smell—have about 4 billion. The overall size of the olfactory epithelium also differs between species, with that
of bloodhounds, for example, being many times larger than that of humans.
Olfactory neurons are bipolar neurons (neurons with two processes from the cell body). Each neuron has a
single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden,
hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins, and it is the variations
in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron
has only one type of receptor on its cilia, and the receptors are specialized to detect specific odorants, so
the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the
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sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information
that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus.
Bipolar neuron
Olfactory
bulb
Olfactory
epithelium
Nerve
endings
Nasal
cavity
(a)
(b)
Figure 36.8 In the human olfactory system, (a) bipolar olfactory neurons extend from (b) the olfactory epithelium,
where olfactory receptors are located, to the olfactory bulb, (credit: modification of work by Patrick J. Lynch, medical
illustrator; C. Carl Jaffe, MD, cardiologist)
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Pheromones
A pheromone is a chemical released by an animal that affects the behavior or physiology of animals
of the same species. Pheromonal signals can have profound effects on animals that inhale them, but
pheromones apparently are not consciously perceived in the same way as other odors. There are several
different types of pheromones, which are released in urine or as glandular secretions. Certain pheromones
are attractants to potential mates, others are repellants to potential competitors of the same sex, and still
others play roles in mother-infant attachment. Some pheromones can also influence the timing of puberty,
modify reproductive cycles, and even prevent embryonic implantation. While the roles of pheromones in
many nonhuman species are important, pheromones have become less important in human behavior over
evolutionary time compared to their importance to organisms with more limited behavioral repertoires.
The vomeronasal organ (VNO, or Jacobson’s organ) is a tubular, fluid-filled, olfactory organ present in many
vertebrate animals that sits adjacent to the nasal cavity. It is very sensitive to pheromones and is connected
to the nasal cavity by a duct. When molecules dissolve in the mucosa of the nasal cavity, they then enter
the VNO where the pheromone molecules among them bind with specialized pheromone receptors. Upon
exposure to pheromones from their own species or others, many animals, including cats, may display the
flehmen response (shown in Figure 36.9), a curling of the upper lip that helps pheromone molecules enter
the VNO.
Pheromonal signals are sent, not to the main olfactory bulb, but to a different neural structure that projects
directly to the amygdala (recall that the amygdala is a brain center important in emotional reactions, such
as fear). The pheromonal signal then continues to areas of the hypothalamus that are key to reproductive
physiology and behavior. While some scientists assert that the VNO is apparently functionally vestigial in
humans, even though there is a similar structure located near human nasal cavities, others are researching
it as a possible functional system that may, for example, contribute to synchronization of menstrual cycles
in women living in close proximity.
Figure 36.9 The flehmen response in this tiger results in the curling of the upper lip and helps airborne pheromone
molecules enter the vomeronasal organ, (credit: modification of work by "chadh'VFlickr)
Taste
Detecting a taste (gustation) is fairly similar to detecting an odor (olfaction), given that both taste and smell
rely on chemical receptors being stimulated by certain molecules. The primary organ of taste is the taste bud.
A taste bud is a cluster of gustatory receptors (taste cells) that are located within the bumps on the tongue
called papillae (singular: papilla) (illustrated in Figure 36.11). There are several structurally distinct papillae.
Filiform papillae, which are located across the tongue, are tactile, providing friction that helps the tongue move
substances, and contain no taste cells. In contrast, fungiform papillae, which are located mainly on the anterior
two-thirds of the tongue, each contain one to eight taste buds and also have receptors for pressure and
temperature. The large circumvallate papillae contain up to 100 taste buds and form a V near the posterior
margin of the tongue.
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Figure 36.10 (a) Foliate, circumvallate, and fungiform papillae are located on different regions of the tongue, (b) Foliate
papillae are prominent protrusions on this light micrograph, (credit a: modification of work by NCI; scale-bar data from
Matt Russell)
in addition to those two types of chemically and mechanically sensitive papillae are foliate papillae—leaf-like
papillae located in parallel folds along the edges and toward the back of the tongue, as seen in the Figure 36.10
micrograph. Foliate papillae contain about 1,300 taste buds within their folds. Finally, there are circumvallate
papillae, which are wall-like papillae in the shape of an inverted “V" at the back of the tongue. Each of these
papillae is surrounded by a groove and contains about 250 taste buds.
Each taste bud’s taste cells are replaced every 10 to 14 days. These are elongated cells with hair-like processes
called microvilli at the tips that extend into the taste bud pore (illustrated in Figure 36.11). Food molecules (
tastants) are dissolved in saliva, and they bind with and stimulate the receptors on the microvilli. The receptors
for tastants are located across the outer portion and front of the tongue, outside of the middle area where the
filiform papillae are most prominent.
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Circumvallate papilla
Filiform papilla
Foliate papilla
Taste hairs . /Taste pore
Taste buds
Basal cell / Transitional cell
Gustatory cell'
Figure 36.11 Pores in the tongue allow tastants to enter taste pores in the tongue, (credit: modification of work by
Vincenzo Rizzo)
In humans, there are five primary tastes, and each taste has only one corresponding type of receptor. Thus,
like olfaction, each receptor is specific to its stimulus (tastant). Transduction of the five tastes happens through
different mechanisms that reflect the molecular composition of the tastant. A salty tastant (containing NaCI)
provides the sodium ions (Na + ) that enter the taste neurons and excite them directly. Sour tastants are acids
and belong to the thermoreceptor protein family. Binding of an acid or other sour-tasting molecule triggers a
change in the ion channel and these increase hydrogen ion (H + ) concentrations in the taste neurons, thus
depolarizing them. Sweet, bitter, and umami tastants require a G-protein coupled receptor. These tastants bind
to their respective receptors, thereby exciting the specialized neurons associated with them.
Both tasting abilities and sense of smell change with age. In humans, the senses decline dramatically by age 50
and continue to decline. A child may find a food to be too spicy, whereas an elderly person may find the same
food to be bland and unappetizing.
LINK TQ LEARNING
View this animation (http:// 0 penstaxc 0 llege. 0 rg/l/taste) that shows how the sense of taste works.
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Smell and Taste in the Brain
Olfactory neurons project from the olfactory epithelium to the olfactory bulb as thin, unmyelinated axons. The
olfactory bulb is composed of neural clusters called glomeruli, and each glomerulus receives signals from
one type of olfactory receptor, so each glomerulus is specific to one odorant. From glomeruli, olfactory signals
travel directly to the olfactory cortex and then to the frontal cortex and the thalamus. Recall that this is a
different path from most other sensory information, which is sent directly to the thalamus before ending up in the
cortex. Olfactory signals also travel directly to the amygdala, thereafter reaching the hypothalamus, thalamus,
and frontal cortex. The last structure that olfactory signals directly travel to is a cortical center in the temporal
lobe structure important in spatial, autobiographical, declarative, and episodic memories. Olfaction is finally
processed by areas of the brain that deal with memory, emotions, reproduction, and thought.
Taste neurons project from taste cells in the tongue, esophagus, and palate to the medulla, in the brainstem.
From the medulla, taste signals travel to the thalamus and then to the primary gustatory cortex. Information from
different regions of the tongue is segregated in the medulla, thalamus, and cortex.
36.4 | Hearing and Vestibular Sensation
By the end of this section, you will be able to do the following:
• Describe the relationship of amplitude and frequency of a sound wave to attributes of sound
• Trace the path of sound through the auditory system to the site of transduction of sound
• Identify the structures of the vestibular system that respond to gravity
Audition, or hearing, is important to humans and to other animals for many different interactions. It enables an
organism to detect and receive information about danger, such as an approaching predator, and to participate
in communal exchanges like those concerning territories or mating. On the other hand, although it is physically
linked to the auditory system, the vestibular system is not involved in hearing. Instead, an animal’s vestibular
system detects its own movement, both linear and angular acceleration and deceleration, and balance.
Sound
Auditory stimuli are sound waves, which are mechanical, pressure waves that move through a medium, such as
air or water. There are no sound waves in a vacuum since there are no air molecules to move in waves. The
speed of sound waves differs, based on altitude, temperature, and medium, but at sea level and a temperature
of 20° C (68° F), sound waves travel in the air at about 343 meters per second.
As is true for all waves, there are four main characteristics of a sound wave: frequency, wavelength, period, and
amplitude. Frequency is the number of waves per unit of time, and in sound is heard as pitch. High-frequency
(>15.000Hz) sounds are higher-pitched (short wavelength) than low-frequency (long wavelengths; <100Hz)
sounds. Frequency is measured in cycles per second, and for sound, the most commonly used unit is hertz (Hz),
or cycles per second. Most humans can perceive sounds with frequencies between 30 and 20,000 Hz. Women
are typically better at hearing high frequencies, but everyone’s ability to hear high frequencies decreases with
age. Dogs detect up to about 40,000 Hz; cats, 60,000 Hz; bats, 100,000 Hz; and dolphins 150,000 Hz, and
American shad (Alosa sapidissima), a fish, can hear 180,000 Hz. Those frequencies above the human range
are called ultrasound.
Amplitude, or the dimension of a wave from peak to trough, in sound is heard as volume and is illustrated
in Figure 36.12. The sound waves of louder sounds have greater amplitude than those of softer sounds. For
sound, volume is measured in decibels (dB). The softest sound that a human can hear is the zero point. Humans
speak normally at 60 decibels.
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Figure 36.12 For sound waves, wavelength corresponds to pitch. Amplitude of the wave corresponds to volume. The
sound wave shown with a dashed line is softer in volume than the sound wave shown with a solid line, (credit: NIH)
Reception of Sound
In mammals, sound waves are collected by the external, cartilaginous part of the ear called the pinna, then
travel through the auditory canal and cause vibration of the thin diaphragm called the tympanum or ear drum,
the innermost part of the outer ear (illustrated in Figure 36.13). Interior to the tympanum is the middle ear. The
middle ear holds three small bones called the ossicles, which transfer energy from the moving tympanum to the
inner ear. The three ossicles are the malleus (also known as the hammer), the incus (the anvil), and stapes
(the stirrup). The aptly named stapes looks very much like a stirrup. The three ossicles are unique to mammals,
and each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic
membrane. The incus attaches the malleus to the stapes. In humans, the stapes is not long enough to reach the
tympanum. If we did not have the malleus and the incus, then the vibrations of the tympanum would never reach
the inner ear. These bones also function to collect force and amplify sounds. The ear ossicles are homologous
to bones in a fish mouth: the bones that support gills in fish are thought to be adapted for use in the vertebrate
ear over evolutionary time. Many animals (frogs, reptiles, and birds, for example) use the stapes of the middle
ear to transmit vibrations to the middle ear.
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Auricle
Ear canal
Tympanic
membrane
_ I |_
Tympanic
cavity
__I L
Stapes (attached to
oval window)
Vestibule
Vestibular nerve
Cochlear nerve
Round window
Cochlea
Eustachian tube
External ear Middle ear Inner ear
Figure 36.13 Sound travels through the outer ear to the middle ear, which is bounded on its exterior by the tympanic
membrane. The middle ear contains three bones called ossicles that transfer the sound wave to the oval window, the
exterior boundary of the inner ear. The organ of Corti, which is the organ of sound transduction, lies inside the cochlea.
Transduction of Sound
Vibrating objects, such as vocal cords, create sound waves or pressure waves in the air. When these pressure
waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse
(electrical signal) that the brain perceives as sound. The pressure waves strike the tympanum, causing it to
vibrate. The mechanical energy from the moving tympanum transmits the vibrations to the three bones of the
middle ear. The stapes transmits the vibrations to a thin diaphragm called the oval window, which is the
outermost structure of the inner ear. The structures of the inner ear are found in the labyrinth, a bony, hollow
structure that is the most interior portion of the ear. Here, the energy from the sound wave is transferred from the
stapes through the flexible oval window and to the fluid of the cochlea. The vibrations of the oval window create
pressure waves in the fluid (perilymph) inside the cochlea. The cochlea is a whorled structure, like the shell of
a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal (as illustrated
in Figure 36.14). Inside the cochlea, the basilar membrane is a mechanical analyzer that runs the length of the
cochlea, curling toward the cochlea’s center.
The mechanical properties of the basilar membrane change along its length, such that it is thicker, tauter, and
narrower at the outside of the whorl (where the cochlea is largest), and thinner, floppier, and broader toward the
apex, or center, of the whorl (where the cochlea is smallest). Different regions of the basilar membrane vibrate
according to the frequency of the sound wave conducted through the fluid in the cochlea. For these reasons, the
fluid-filled cochlea detects different wave frequencies (pitches) at different regions of the membrane. When the
sound waves in the cochlear fluid contact the basilar membrane, it flexes back and forth in a wave-like fashion.
Above the basilar membrane is the tectorial membrane.
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CONNECTION
(D Tympanic membrane
vibrates in response
to sound wave.
(T) Sound wave represents alternating
areas of high and low pressure.
))))))))))
i i
Wavelength
J
AAAAA/
Frequency of sound wave
measured in Hz (cycles
per second)
(3) Vibrations are
amplified across
ossicles.
(4) Vibrations against oval window set up standing
wave in fluid of vestibuli.
Organ of Corti -
■ ^CjC jCCCCCZi
/ V
■ Scala
vestibuli
■ Cochlear
duct
(5) Pressure bends the membrane of the '
cochlear duct at a point of maximum
vibration for a given frequency, causing
hair cells in the basilar membrane to
vibrate.
Scala tympam
l\AAAAAA
Frequency of standing
wave is the same as
sound wave
Figure 36.14 A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves
across the malleus, incus, and stapes. The amplified vibration is picked up by the oval window causing pressure
waves in the fluid of the scala vestibuli and scala tympani. The complexity of the pressure waves is determined
by the changes in amplitude and frequency of the sound waves entering the ear.
Cochlear implants can restore hearing in people who have a nonfunctional cochlea. The implant consists of
a microphone that picks up sound. A speech processor selects sounds in the range of human speech, and
a transmitter converts these sounds to electrical impulses, which are then sent to the auditory nerve. Which
of the following types of hearing loss would not be restored by a cochlear implant?
a. Hearing loss resulting from absence or loss of hair cells in the organ of Corti.
b. Hearing loss resulting from an abnormal auditory nerve.
c. Hearing loss resulting from fracture of the cochlea.
d. Hearing loss resulting from damage to bones of the middle ear.
The site of transduction is in the organ of Corti (spiral organ). It is composed of hair cells held in place above the
basilar membrane like flowers projecting up from soil, with their exposed short, hair-like stereocilia contacting
or embedded in the tectorial membrane above them. The inner hair cells are the primary auditory receptors
and exist in a single row, numbering approximately 3,500. The stereocilia from inner hair cells extend into
small dimples on the tectorial membrane’s lower surface. The outer hair cells are arranged in three or four
rows. They number approximately 12,000, and they function to fine tune incoming sound waves. The longer
stereocilia that project from the outer hair cells actually attach to the tectorial membrane. All of the stereocilia are
mechanoreceptors, and when bent by vibrations they respond by opening a gated ion channel (refer to Figure
36.15). As a result, the hair cell membrane is depolarized, and a signal is transmitted to the chochlear nerve.
Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated.
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Tectorial
membrane
Tether
Stereocilia
Hair cell
Figure 36.15 The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface. The
stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member
of their array, and closed when the array is bent toward the shortest member of their array.
The hair cells are arranged on the basilar membrane in an orderly way. The basilar membrane vibrates in
different regions, according to the frequency of the sound waves impinging on it. Likewise, the hair cells that
lay above it are most sensitive to a specific frequency of sound waves. Hair cells can respond to a small range
of similar frequencies, but they require stimulation of greater intensity to fire at frequencies outside of their
optimal range. The difference in response frequency between adjacent inner hair cells is about 0.2 percent.
Compare that to adjacent piano strings, which are about six percent different. Place theory, which is the model
for how biologists think pitch detection works in the human ear, states that high frequency sounds selectively
vibrate the basilar membrane of the inner ear near the entrance port (the oval window). Lower frequencies travel
farther along the membrane before causing appreciable excitation of the membrane. The basic pitch-determining
mechanism is based on the location along the membrane where the hair cells are stimulated. The place theory is
the first step toward an understanding of pitch perception. Considering the extreme pitch sensitivity of the human
ear, it is thought that there must be some auditory “sharpening” mechanism to enhance the pitch resolution.
When sound waves produce fluid waves inside the cochlea, the basilar membrane flexes, bending the stereocilia
that attach to the tectorial membrane. Their bending results in action potentials in the hair cells, and auditory
information travels along the neural endings of the bipolar neurons of the hair cells (collectively, the auditory
nerve) to the brain. When the hairs bend, they release an excitatory neurotransmitter at a synapse with a
sensory neuron, which then conducts action potentials to the central nervous system. The cochlear branch of the
vestibulocochlear cranial nerve sends information on hearing. The auditory system is very refined, and there is
some modulation or “sharpening” built in. The brain can send signals back to the cochlea, resulting in a change
of length in the outer hair cells, sharpening or dampening the hair cells’ response to certain frequencies.
Watch an animation (http:// 0 penstaxc 0 llege. 0 rg/l/hearing) of sound entering the outer ear, moving through
the ear structure, stimulating cochlear nerve impulses, and eventually sending signals to the temporal lobe.
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Higher Processing
The inner hair cells are most important for conveying auditory information to the brain. About 90 percent of the
afferent neurons carry information from inner hair cells, with each hair cell synapsing with 10 or so neurons.
Outer hair cells connect to only 10 percent of the afferent neurons, and each afferent neuron innervates many
hair cells. The afferent, bipolar neurons that convey auditory information travel from the cochlea to the medulla,
through the pons and midbrain in the brainstem, finally reaching the primary auditory cortex in the temporal lobe.
Vestibular Information
The stimuli associated with the vestibular system are linear acceleration (gravity) and angular acceleration and
deceleration. Gravity, acceleration, and deceleration are detected by evaluating the inertia on receptive cells
in the vestibular system. Gravity is detected through head position. Angular acceleration and deceleration are
expressed through turning or tilting of the head.
The vestibular system has some similarities with the auditory system. It utilizes hair cells just like the auditory
system, but it excites them in different ways. There are five vestibular receptor organs in the inner ear: the utricle,
the saccule, and three semicircular canals. Together, they make up what’s known as the vestibular labyrinth that
is shown in Figure 36.16. The utricle and saccule respond to acceleration in a straight line, such as gravity. The
roughly 30,000 hair cells in the utricle and 16,000 hair cells in the saccule lie below a gelatinous layer, with their
stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate crystals—like tiny rocks.
When the head is tilted, the crystals continue to be pulled straight down by gravity, but the new angle of the head
causes the gelatin to shift, thereby bending the stereocilia. The bending of the stereocilia stimulates the neurons,
and they signal to the brain that the head is tilted, allowing the maintenance of balance. It is the vestibular branch
of the vestibulocochlear cranial nerve that deals with balance.
Posterior Canal
Horizontal
Canal
Figure 36.16 The structure of the vestibular labyrinth is shown, (credit: modification of work by NIH)
The fluid-filled semicircular canals are tubular loops set at oblique angles. They are arranged in three spatial
planes. The base of each canal has a swelling that contains a cluster of hair cells. The hairs project into a
gelatinous cap called the cupula and monitor angular acceleration and deceleration from rotation. They would be
stimulated by driving your car around a corner, turning your head, or falling forward. One canal lies horizontally,
while the other two lie at about 45 degree angles to the horizontal axis, as illustrated in Figure 36.16. When the
brain processes input from all three canals together, it can detect angular acceleration or deceleration in three
dimensions. When the head turns, the fluid in the canals shifts, thereby bending stereocilia and sending signals
to the brain. Upon cessation accelerating or decelerating—or just moving—the movement of the fluid within the
canals slows or stops. For example, imagine holding a glass of water. When moving forward, water may splash
backwards onto the hand, and when motion has stopped, water may splash forward onto the fingers. While in
motion, the water settles in the glass and does not splash. Note that the canals are not sensitive to velocity itself,
but to changes in velocity, so moving forward at 60mph with your eyes closed would not give the sensation of
movement, but suddenly accelerating or braking would stimulate the receptors.
Higher Processing
Hair cells from the utricle, saccule, and semicircular canals also communicate through bipolar neurons to
the cochlear nucleus in the medulla. Cochlear neurons send descending projections to the spinal cord and
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ascending projections to the pons, thalamus, and cerebellum. Connections to the cerebellum are important
for coordinated movements. There are also projections to the temporal cortex, which account for feelings of
dizziness; projections to autonomic nervous system areas in the brainstem, which account for motion sickness;
and projections to the primary somatosensory cortex, which monitors subjective measurements of the external
world and self-movement. People with lesions in the vestibular area of the somatosensory cortex see vertical
objects in the world as being tilted. Finally, the vestibular signals project to certain optic muscles to coordinate
eye and head movements.
Click through this interactive tutorial (http:// 0 penstaxc 0 llege. 0 rg/l/ear_anat 0 my) to review the parts of the
ear and how they function to process sound.
36.5 | Vision
By the end of this section, you will be able to do the following:
• Explain how electromagnetic waves differ from sound waves
• Trace the path of light through the eye to the point of the optic nerve
• Explain tonic activity as it is manifested in photoreceptors in the retina
Vision is the ability to detect light patterns from the outside environment and interpret them into images. Animals
are bombarded with sensory information, and the sheer volume of visual information can be problematic.
Fortunately, the visual systems of species have evolved to attend to the most-important stimuli. The importance
of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is
dedicated to analyzing and perceiving visual information.
Light
As with auditory stimuli, light travels in waves. The compression waves that compose sound must travel in a
medium—a gas, a liquid, or a solid. In contrast, light is composed of electromagnetic waves and needs no
medium; light can travel in a vacuum (Figure 36.17). The behavior of light can be discussed in terms of the
behavior of waves and also in terms of the behavior of the fundamental unit of light—a packet of electromagnetic
radiation called a photon. A glance at the electromagnetic spectrum shows that visible light for humans is just a
small slice of the entire spectrum, which includes radiation that we cannot see as light because it is below the
frequency of visible red light and above the frequency of visible violet light.
Certain variables are important when discussing perception of light. Wavelength (which varies inversely with
frequency) manifests itself as hue. Light at the red end of the visible spectrum has longer wavelengths (and is
lower frequency), while light at the violet end has shorter wavelengths (and is higher frequency). The wavelength
of light is expressed in nanometers (nm); one nanometer is one billionth of a meter. Humans perceive light
that ranges between approximately 380 nm and 740 nm. Some other animals, though, can detect wavelengths
outside of the human range. For example, bees see near-ultraviolet light in order to locate nectar guides on
flowers, and some non-avian reptiles sense infrared light (heat that prey gives off).
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Wavelength
(meters)
10 3
10 2
10 5
.5 x 10 6
10 8
10-io
Buildings Humans Honeybee Pinpoint Protozoans Molecules Atoms Atomic nuclei
Frequency
(Hz)
10“ 10 s 10 12 10“ 10“ 10“ 10 20
Figure 36.17 In the electromagnetic spectrum, visible light lies between 380 nm and 740 nm. (credit: modification of
work by NASA)
Wave amplitude is perceived as luminous intensity, or brightness. The standard unit of intensity of light is the
candela, which is approximately the luminous intensity of one common candle.
Light waves travel 299,792 km per second in a vacuum, (and somewhat slower in various media such as air
and water), and those waves arrive at the eye as long (red), medium (green), and short (blue) waves. What is
termed “white light” is light that is perceived as white by the human eye. This effect is produced by light that
stimulates equally the color receptors in the human eye. The apparent color of an object is the color (or colors)
that the object reflects. Thus a red object reflects the red wavelengths in mixed (white) light and absorbs all other
wavelengths of light.
Anatomy of the Eye
The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the
retina (shown in Figure 36.18) on the inner surface of the back of the eye. But light does not impinge on
the retina unaltered. It passes through other layers that process it so that it can be interpreted by the retina
(Figure 36.18b). The cornea, the front transparent layer of the eye, and the crystalline lens, a transparent
convex structure behind the cornea, both refract (bend) light to focus the image on the retina. The iris, which is
conspicuous as the colored part of the eye, is a circular muscular ring lying between the lens and cornea that
regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the
size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges.
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visual
CONNECTION
nerve
Ganglion
cells
Amacrine
cells
Bipolar cells
Horizontal
cells
Light
Retinal
blood vessels
Optic
Rod
Figure 36.18 (a) The human eye is shown in cross section, (b) A blowup shows the layers of the retina.
Which of the following statements about the human eye is false?
a. Rods detect color, while cones detect only shades of gray.
b. When light enters the retina, it passes the ganglion cells and bipolar cells before reaching
photoreceptors at the rear of the eye.
c. The iris adjusts the amount of light coming into the eye.
d. The cornea is a protective layer on the front of the eye.
The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing
and re-focusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles
that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on
the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called presbyopia
results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a
different type of farsightedness called hyperopia caused by an eyeball that is too short. For both defects, images
in the distance are clear but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is
elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but
images nearby are clear.
There are two types of photoreceptors in the retina: rods and cones, named for their general appearance as
illustrated in Figure 36.19. Rods are strongly photosensitive and are located in the outer edges of the retina.
They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive
and are located near the center of the retina. They respond to bright light, and their primary role is in daytime,
color vision.
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Rod outer
segment
Outer segment
contains
rhodopsin
Outer segment
contains
photopigments
Oil droplet
Nucleus
Rod
Cone
Figure 36.19 Rods and cones are photoreceptors in the retina. Rods respond in low light and can detect only shades
of gray. Cones respond in intense light and are responsible for color vision, (credit: modification of work by Piotr Sliwa)
The fovea is the region in the center back of the eye that is responsible for acute vision. The fovea has a high
density of cones. When you bring your gaze to an object to examine it intently in bright light, the eyes orient so
that the object’s image falls on the fovea. However, when looking at a star in the night sky or other object in dim
light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina,
rather than the cones at the center, that operate better in low light. In humans, cones far outnumber rods in the
fovea.
LINK TQ LEARNING
Review the anatomical structure (http:// 0 penstaxc 0 llege. 0 rg/l/eye_diagram) of the eye, clicking on each
part to practice identification.
Transduction of Light
The rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain
photopigments. Invertebrates, the main photopigment, rhodopsin, has two main parts (Figure 36.20): an opsin,
which is a membrane protein (in the form of a cluster of a-helices that span the membrane), and retinal—a
molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its
structure from a bent (c/s) form of the molecule to its linear (trans) isomer. This isomerization of retinal activates
the rhodopsin, starting a cascade of events that ends with the closing of Na + channels in the membrane of the
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photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus)
visual receptors become hyperpolarized and thus driven away from threshold (Figure 36.21).
trans retinal
Figure 36.20 (a) Rhodopsin, the photoreceptor in vertebrates, has two parts: the trans-membrane protein opsin, and
retinal. When light strikes retinal, it changes shape from (b) a c/s to a trans form. The signal is passed to a G-protein
called transducin, triggering a series of downstream events.
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Bipolar cell
Dark: depolarized Light: hyperpolarized
Figure 36.21 When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates
phosphodiesterase. Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels. As a result, the
membrane becomes hyperpolarized. The hyperpolarized membrane does not release glutamate to the bipolar cell.
Trichromatic Coding
There are three types of cones (with different photopsins), and they differ in the wavelength to which they are
most responsive, as shown in Figure 36.22. Some cones are maximally responsive to short light waves of 420
nm, so they are called S cones (“S” for “short”); others respond maximally to waves of 530 nm (M cones, for
“medium”); a third group responds maximally to light of longer wavelengths, at 560 nm (L, or “long" cones). With
only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has limitations.
Primates use a three-cone (trichromatic) system, resulting in full color vision.
The color we perceive is a result of the ratio of activity of our three types of cones. The colors of the visual
spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm),
green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have very sensitive perception of
color and can distinguish about 500 levels of brightness, 200 different hues, and 20 steps of saturation, or about
2 million distinct colors.
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Wavelength (nm)
Figure 36.22 Human rod cells and the different types of cone cells each have an optimal wavelength. However, there
is considerable overlap in the wavelengths of light detected.
Retinal Processing
Visual signals leave the cones and rods, travel to the bipolar cells, and then to ganglion cells. A large degree of
processing of visual information occurs in the retina itself, before visual information is sent to the brain.
Photoreceptors in the retina continuously undergo tonic activity. That is, they are always slightly active even
when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at
a baseline; while some stimuli increase firing rate from the baseline, and other stimuli decrease firing rate. In the
absence of light, the bipolar neurons that connect rods and cones to ganglion cells are continuously and actively
inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones and removes
their inhibition of bipolar cells. The now active bipolar cells in turn stimulate the ganglion cells, which send action
potentials along their axons (which leave the eye as the optic nerve). Thus, the visual system relies on change in
retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes
horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a
rod or cone stimulates a horizontal cell, the horizontal cell inhibits more distant photoreceptors and bipolar cells,
creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions
receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information
from one bipolar cell to many ganglion cells.
You can demonstrate this using an easy demonstration to “trick" your retina and brain about the colors you are
observing in your visual field. Look fixedly at Figure 36.23 for about 45 seconds. Then quickly shift your gaze to
a sheet of blank white paper or a white wall. You should see an afterimage of the Norwegian flag in its correct
colors. At this point, close your eyes for a moment, then reopen them, looking again at the white paper or wall;
the afterimage of the flag should continue to appear as red, white, and blue. What causes this? According to
an explanation called opponent process theory, as you gazed fixedly at the green, black, and yellow flag, your
retinal ganglion cells that respond positively to green, black, and yellow increased their firing dramatically. When
you shifted your gaze to the neutral white ground, these ganglion cells abruptly decreased their activity and the
brain interpreted this abrupt downshift as if the ganglion cells were responding now to their “opponent” colors:
red, white, and blue, respectively, in the visual field. Once the ganglion cells return to their baseline activity state,
the false perception of color will disappear.
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Figure 36.23 View this flag to understand how retinal processing works. Stare at the center of the flag (indicated by
the white dot) for 45 seconds, and then quickly look at a white background, noticing how colors appear.
Higher Processing
The myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry
different qualities of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries
information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular
(small cell) pathway, which carries information on color and fine detail. Some visual information projects directly
back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical
pathways produces the distinctive optic chiasma (Greek, for “crossing”) found at the base of the brain and allows
us to coordinate information from both eyes.
Once in the brain, visual information is processed in several places, and its routes reflect the complexity and
importance of visual information to humans and other animals. One route takes the signals to the thalamus,
which serves as the routing station for all incoming sensory impulses except olfaction. In the thalamus,
the magnocellular and parvocellular distinctions remain intact, and there are different layers of the thalamus
dedicated to each. When visual signals leave the thalamus, they travel to the primary visual cortex at the rear
of the brain. From the visual cortex, the visual signals travel in two directions. One stream that projects to the
parietal lobe, in the side of the brain, carries magnocellular (“where”) information. A second stream projects to
the temporal lobe and carries both magnocellular (“where”) and parvocellular (“what”) information.
Another important visual route is a pathway from the retina to the superior colliculus in the midbrain, where eye
movements are coordinated and integrated with auditory information. Finally, there is the pathway from the retina
to the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is a cluster of cells that is considered
to be the body’s internal clock, which controls our circadian (day-long) cycle. The SCN sends information to the
pineal gland, which is important in sleep/wake patterns and annual cycles.
View this interactive presentation (http:// 0 penstaxc 0 llege. 0 rg/l/sense_ 0 f sight) to review what you have
learned about how vision functions.
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KEY TERMS
audition sense of hearing
basilar membrane stiff structure in the cochlea that indirectly anchors auditory receptors
bipolar neuron neuron with two processes from the cell body, typically in opposite directions
candela (cd) unit of measurement of luminous intensity (brightness)
circadian describes a time cycle about one day in length
cochlea whorled structure that contains receptors for transduction of the mechanical wave into an electrical
signal
cone weakly photosensitive, chromatic, cone-shaped neuron in the fovea of the retina that detects bright light
and is used in daytime color vision
cornea transparent layer over the front of the eye that helps focus light waves
fovea region in the center of the retina with a high density of photoreceptors and which is responsible for acute
vision
free nerve ending ending of an afferent neuron that lacks a specialized structure for detection of sensory
stimuli; some respond to touch, pain, or temperature
glabrous describes the non-hairy skin found on palms and fingers, soles of feet, and lips of humans and other
primates
glomerulus in the olfactory bulb, one of the two neural clusters that receives signals from one type of olfactory
receptor
Golgi tendon organ muscular proprioceptive tension receptor that provides the sensory component of the
Golgi tendon reflex
gustation sense of taste
hyperopia (also, farsightedness) visual defect in which the image focus falls behind the retina, thereby making
images in the distance clear, but close-up images blurry
incus (also, anvil) second of the three bones of the middle ear
inner ear innermost part of the ear; consists of the cochlea and the vestibular system
iris pigmented, circular muscle at the front of the eye that regulates the amount of light entering the eye
kinesthesia sense of body movement
labyrinth bony, hollow structure that is the most internal part of the ear; contains the sites of transduction of
auditory and vestibular information
lens transparent, convex structure behind the cornea that helps focus lightwaves on the retina
malleus (also, hammer) first of the three bones of the middle ear
mechanoreceptor sensory receptor modified to respond to mechanical disturbance such as being bent, touch,
pressure, motion, and sound
Meissner’s corpuscle (also, tactile corpuscle) encapsulated, rapidly-adapting mechanoreceptor in the skin that
responds to light touch
Merkel's disk unencapsulated, slowly-adapting mechanoreceptor in the skin that responds to touch
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Chapter 36 | Sensory Systems
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middle ear part of the hearing apparatus that functions to transfer energy from the tympanum to the oval
window of the inner ear
muscle spindle proprioceptive stretch receptor that lies within a muscle and that shortens the muscle to an
optimal length for efficient contraction
myopia (also, nearsightedness) visual defect in which the image focus falls in front of the retina, thereby making
images in the distance blurry, but close-up images clear
nociception neural processing of noxious (such as damaging) stimuli
odorant airborne molecule that stimulates an olfactory receptor
olfaction sense of smell
olfactory bulb neural structure in the vertebrate brain that receives signals from olfactory receptors
olfactory epithelium specialized tissue in the nasal cavity where olfactory receptors are located
olfactory receptor dendrite of a specialized neuron
organ of Corti in the basilar membrane, the site of the transduction of sound, a mechanical wave, to a neural
signal
ossicle one of the three bones of the middle ear
outer ear part of the ear that consists of the pinna, ear canal, and tympanum and which conducts sound waves
into the middle ear
oval window thin diaphragm between the middle and inner ears that receives sound waves from contact with
the stapes bone of the middle ear
Pacinian corpuscle encapsulated mechanoreceptor in the skin that responds to deep pressure and vibration
papilla one of the small bump-like projections from the tongue
perception individual interpretation of a sensation; a brain function
pheromone substance released by an animal that can affect the physiology or behavior of other animals
pinna cartilaginous outer ear
presbyopia visual defect in which the image focus falls behind the retina, thereby making images in the
distance clear, but close-up images blurry; caused by age-based changes in the lens
proprioception sense of limb position; used to track kinesthesia
pupil small opening though which light enters
reception receipt of a signal (such as light or sound) by sensory receptors
receptive field region in space in which a stimulus can activate a given sensory receptor
receptor potential membrane potential in a sensory receptor in response to detection of a stimulus
retina layer of photoreceptive and supporting cells on the inner surface of the back of the eye
rhodopsin main photopigment in vertebrates
rod strongly photosensitive, achromatic, cylindrical neuron in the outer edges of the retina that detects dim light
and is used in peripheral and nighttime vision
Ruffini ending (also, bulbous corpuscle) slowly-adapting mechanoreceptor in the skin that responds to skin
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stretch and joint position
semicircular canal one of three half-circular, fluid-filled tubes in the vestibular labyrinth that monitors angular
acceleration and deceleration
sensory receptor specialized neuron or other cells associated with a neuron that is modified to receive specific
sensory input
sensory transduction conversion of a sensory stimulus into electrical energy in the nervous system by a
change in the membrane potential
stapes (also, stirrup) third of the three bones of the middle ear
stereocilia in the auditory system, hair-like projections from hair cells that help detect sound waves
superior colliculus paired structure in the top of the midbrain, which manages eye movements and auditory
integration
suprachiasmatic nucleus cluster of cells in the hypothalamus that plays a role in the circadian cycle
tastant food molecule that stimulates gustatory receptors
taste bud clusters of taste cells
tectorial membrane cochlear structure that lies above the hair cells and participates in the transduction of
sound at the hair cells
tonic activity in a neuron, slight continuous activity while at rest
tympanum (also, tympanic membrane or ear drum) thin diaphragm between the outer and middle ears
ultrasound sound frequencies above the human detectable ceiling of approximately 20,000 Hz
umami one of the five basic tastes, which is described as “savory" and which may be largely the taste of L-
glutamate
vestibular sense sense of spatial orientation and balance
vision sense of sight
CHAPTER SUMMARY
36.1 Sensory Processes
A sensory activation occurs when a physical or chemical stimulus is processed into a neural signal (sensory
transduction) by a sensory receptor. Perception is an individual interpretation of a sensation and is a brain
function. Humans have special senses: olfaction, gustation, equilibrium, and hearing, plus the general senses
of somatosensation.
Sensory receptors are either specialized cells associated with sensory neurons or the specialized ends of
sensory neurons that are a part of the peripheral nervous system, and they are used to receive information
about the environment (internal or external). Each sensory receptor is modified for the type of stimulus it
detects. For example, neither gustatory receptors nor auditory receptors are sensitive to light. Each sensory
receptor is responsive to stimuli within a specific region in space, which is known as that receptor’s receptive
field. The most fundamental function of a sensory system is the translation of a sensory signal to an electrical
signal in the nervous system.
All sensory signals, except those from the olfactory system, enter the central nervous system and are routed to
the thalamus. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex
dedicated to processing that particular sense.
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36.2 Somatosensation
Somatosensation includes all sensation received from the skin and mucous membranes, as well as from the
limbs and joints. Somatosensation occurs all over the exterior of the body and at some interior locations as
well, and a variety of receptor types, embedded in the skin and mucous membranes, play a role.
There are several types of specialized sensory receptors. Rapidly adapting free nerve endings detect
nociception, hot and cold, and light touch. Slowly adapting, encapsulated Merkel’s disks are found in fingertips
and lips, and respond to light touch. Meissner’s corpuscles, found in glabrous skin, are rapidly adapting,
encapsulated receptors that detect touch, low-frequency vibration, and flutter. Ruffini endings are slowly
adapting, encapsulated receptors that detect skin stretch, joint activity, and warmth. Hair receptors are rapidly
adapting nerve endings wrapped around the base of hair follicles that detect hair movement and skin
deflection. Finally, Pacinian corpuscles are encapsulated, rapidly adapting receptors that detect transient
pressure and high-frequency vibration.
36.3 Taste and Smell
There are five primary tastes in humans: sweet, sour, bitter, salty, and umami. Each taste has its own receptor
type that responds only to that taste. Tastants enter the body and are dissolved in saliva. Taste cells are located
within taste buds, which are found on three of the four types of papillae in the mouth.
Regarding olfaction, there are many thousands of odorants, but humans detect only about 10,000. Like taste
receptors, olfactory receptors are each responsive to only one odorant. Odorants dissolve in nasal mucosa,
where they excite their corresponding olfactory sensory cells. When these cells detect an odorant, they send
their signals to the main olfactory bulb and then to other locations in the brain, including the olfactory cortex.
36.4 Hearing and Vestibular Sensation
Audition is important for territory defense, predation, predator defense, and communal exchanges. The
vestibular system, which is not auditory, detects linear acceleration and angular acceleration and deceleration.
Both the auditory system and vestibular system use hair cells as their receptors.
Auditory stimuli are sound waves. The sound wave energy reaches the outer ear (pinna, canal, tympanum),
and vibrations of the tympanum send the energy to the middle ear. The middle ear bones shift and the stapes
transfers mechanical energy to the oval window of the fluid-filled inner ear cochlea. Once in the cochlea, the
energy causes the basilar membrane to flex, thereby bending the stereocilia on receptor hair cells. This
activates the receptors, which send their auditory neural signals to the brain.
The vestibular system has five parts that work together to provide the sense of direction, thus helping to
maintain balance. The utricle and saccule measure head orientation: their calcium carbonate crystals shift
when the head is tilted, thereby activating hair cells. The semicircular canals work similarly, such that when the
head is turned, the fluid in the canals bends stereocilia on hair cells. The vestibular hair cells also send signals
to the thalamus and to the somatosensory cortex, but also to the cerebellum, the structure above the brainstem
that plays a large role in timing and coordination of movement.
36.5 Vision
Vision is the only photo responsive sense. Visible light travels in waves and is a very small slice of the
electromagnetic radiation spectrum. Light waves differ based on their frequency (wavelength = hue) and
amplitude (intensity = brightness).
In the vertebrate retina, there are two types of light receptors (photoreceptors): cones and rods. Cones, which
are the source of color vision, exist in three forms—L, M, and S—and they are differentially sensitive to different
wavelengths. Cones are located in the retina, along with the dim-light, achromatic receptors (rods). Cones are
found in the fovea, the central region of the retina, whereas rods are found in the peripheral regions of the
retina.
Visual signals travel from the eye over the axons of retinal ganglion cells, which make up the optic nerves.
Ganglion cells come in several versions. Some ganglion cell axons carry information on form, movement,
depth, and brightness, while other axons carry information on color and fine detail. Visual information is sent to
the superior colliculi in the midbrain, where coordination of eye movements and integration of auditory
information takes place. Visual information is also sent to the suprachiasmatic nucleus (SCN) of the
hypothalamus, which plays a role in the circadian cycle.
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Chapter 36 | Sensory Systems
VISUAL CONNECTION QUESTIONS
1. Figure 36.5 Which of the following statements
about mechanoreceptors is false?
a. Pacini corpuscles are found in both glabrous
and hairy skin.
b. Merkel’s disks are abundant on the
fingertips and lips.
c. Ruffini endings are encapsulated
mechanoreceptors.
d. Meissner’s corpuscles extend into the lower
dermis.
2. Figure 36.14 Cochlear implants can restore
hearing in people who have a nonfunctional cochlea.
The implant consists of a microphone that picks up
sound. A speech processor selects sounds in the
range of human speech, and a transmitter converts
these sounds to electrical impulses, which are then
sent to the auditory nerve. Which of the following
types of hearing loss would not be restored by a
cochlear implant?
REVIEW QUESTIONS
4. Where does perception occur?
a. spinal cord
b. cerebral cortex
c. receptors
d. thalamus
5. If a person’s cold receptors no longer convert cold
stimuli into sensory signals, that person has a
problem with the process of_.
a. reception
b. transmission
c. perception
d. transduction
6. After somatosensory transduction, the sensory
signal travels through the brain as a(n)_signal.
a. electrical
b. pressure
c. optical
d. thermal
7. Many people experience motion sickness while
traveling in a car. This sensation results from
contradictory inputs arising from which senses?
a. Proprioception and Kinesthesia
b. Somatosensation and Equilibrium
c. Gustation and Vibration
d. Vision and Vestibular System
8. _are found only in_skin, and detect
skin deflection.
a. Hearing loss resulting from absence or loss
of hair cells in the organ of Corti.
b. Hearing loss resulting from an abnormal
auditory nerve.
c. Hearing loss resulting from fracture of the
cochlea.
d. Hearing loss resulting from damage to
bones of the middle ear.
3. Figure 36.18 Which of the following statements
about the human eye is false?
a. Rods detect color, while cones detect only
shades of gray.
b. When light enters the retina, it passes the
ganglion cells and bipolar cells before
reaching photoreceptors at the rear of the
eye.
c. The iris adjusts the amount of light coming
into the eye.
d. The cornea is a protective layer on the front
of the eye.
a. Meissner’s corpuscles; hairy
b. Merkel’s disks; glabrous
c. hair receptors; hairy
d. Krause end bulbs; hairy
9. If you were to burn your epidermis, what receptor
type would you most likely burn?
a. free nerve endings
b. Ruffini endings
c. Pacinian corpuscle
d. hair receptors
10. Many diabetic patients are warned by their
doctors to test their glucose levels by pricking the
sides of their fingers rather than the pads. Pricking
the sides avoids stimulating which receptor?
a. Krause end bulbs
b. Meissner’s corpuscles
c. Ruffini ending
d. Nociceptors
11. Which of the following has the fewest taste
receptors?
a. fungiform papillae
b. circumvallate papillae
c. foliate papillae
d. filiform papillae
12. How many different taste molecules do taste cells
each detect?
a.
one
b.
five
c.
ten
d. It depends on the spot on the tongue.
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Chapter 36 | Sensory Systems
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13. Salty foods activate the taste cells by_.
a. exciting the taste cell directly
b. causing hydrogen ions to enter the cell
c. causing sodium channels to close
d. binding directly to the receptors
14. All sensory signals except_travel to the
_in the brain before the cerebral cortex.
a. vision; thalamus
b. olfaction; thalamus
c. vision; cranial nerves
d. olfaction; cranial nerves
15. How is the ability to recognize the umami taste an
evolutionary advantage?
a. Umami identifies healthy foods that are low
in salt and sugar.
b. Umami enhances the flavor of bland foods.
c. Umami identifies foods that might contain
essential amino acids.
d. Umami identifies foods that help maintain
electrolyte balance.
16. In sound, pitch is measured in_, and
volume is measured in_.
a. nanometers (nm); decibels (dB)
b. decibels (dB); nanometers (nm)
c. decibels (dB); hertz (Hz)
d. hertz (Hz); decibels (dB)
17. Auditory hair cells are indirectly anchored to the
a. basilar membrane
b. oval window
c. tectorial membrane
d. ossicles
18. Which of the following are found both in the
auditory system and the vestibular system?
a. basilar membrane
b. hair cells
c. semicircular canals
d. ossicles
19. Benign Paroxysmal Positional Vertigo is a
disorder where some of the calcium carbonate
crystals in the utricle migrate into the semicircular
canals. Why does this condition cause periods of
dizziness?
CRITICAL THINKING QUESTIONS
24. If a person sustains damage to axons leading
from sensory receptors to the central nervous
system, which step or steps of sensory perception
will be affected?
25. In what way does the overall magnitude of a
a. The hair cells in the semicircular canals will
be constantly activated.
b. The hair cells in the semicircular canals will
now be stimulated by gravity.
c. The utricle will no longer recognize
acceleration.
d. There will be too much volume in the
semicircular canals for them to detect
motion.
20. Why do people over 55 often need reading
glasses?
a. Their cornea no longer focuses correctly.
b. Their lens no longer focuses correctly.
c. Their eyeball has elongated with age,
causing images to focus in front of their
retina.
d. Their retina has thinned with age, making
vision more difficult.
21. Why is it easier to see images at night using
peripheral, rather than the central, vision?
a. Cones are denser in the periphery of the
retina.
b. Bipolar cells are denser in the periphery of
the retina.
c. Rods are denser in the periphery of the
retina.
d. The optic nerve exits at the periphery of the
retina.
22. A person catching a ball must coordinate her
head and eyes. What part of the brain is helping to
do this?
a. hypothalamus
b. pineal gland
c. thalamus
d. superior colliculus
23. A satellite is launched into space, but explodes
after exiting the Earth’s atmosphere. Which
statement accurately reflects the observations made
by an astronaut on a space walk outside the
International Space Station during the explosion?
a. The astronaut would see the explosion, but
would not hear a boom.
b. The astronaut will not sense the explosion.
c. The astronaut will see the explosion, and
then hear the boom.
d. The astronaut will feel the concussive force
of the explosion, but will not see it.
stimulus affect the just-noticeable difference in the
perception of that stimulus?
26. Describe the difference in the localization of the
sensory receptors for general and special senses in
humans.
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Chapter 36 | Sensory Systems
27. What can be inferred about the relative sizes of
the areas of cortex that process signals from skin not
densely innervated with sensory receptors and skin
that is densely innervated with sensory receptors?
28. Many studies have demonstrated that women are
able to tolerate the same painful stimuli for longer
than men. Why don’t all people experience pain the
same way?
29. From the perspective of the recipient of the
signal, in what ways do pheromones differ from other
odorants?
30. What might be the effect on an animal of not
being able to perceive taste?
31. A few recent cancer detection studies have used
trained dogs to detect lung cancer in urine samples.
What is the hypothesis behind this study? Why are
dogs a better choice of detectors in this study than
humans?
32. How would a rise in altitude likely affect the
speed of a sound transmitted through air? Why?
33. How might being in a place with less gravity than
Earth has (such as Earth’s moon) affect vestibular
sensation, and why?
34. How does the structure of the ear allow a person
to determine where a sound originates?
35. How could the pineal gland, the brain structure
that plays a role in annual cycles, use visual
information from the suprachiasmatic nucleus of the
hypothalamus?
36. How is the relationship between photoreceptors
and bipolar cells different from other sensory
receptors and adjacent cells?
37. Cataracts, the medical condition where the lens
of the eye becomes cloudy, are a leading cause of
blindness. Describe how developing a cataract would
change the path of light through the eye.
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Chapter 37 | The Endocrine System
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37 | THE ENDOCRINE
SYSTEM
Figure 37.1 The process of amphibian metamorphosis, as seen in the tadpole-to-frog stages shown here, is driven by
hormones, (credit "tadpole": modification of work by Brian Gratwicke)
Chapter Outline
37.1: Types of Hormones
37.2: How Hormones Work
37.3: Regulation of Body Processes
37.4: Regulation of Hormone Production
37.5: Endocrine Glands
Introduction
An animal’s endocrine system controls body processes through the production, secretion, and regulation of
hormones, which serve as chemical “messengers” functioning in cellular and organ activity and, ultimately,
maintaining the body’s homeostasis. The endocrine system plays a role in growth, metabolism, and sexual
development. In humans, common endocrine system diseases include thyroid disease and diabetes mellitus. In
organisms that undergo metamorphosis, the process is controlled by the endocrine system. The transformation
from tadpole to frog, for example, is complex and nuanced to adapt to specific environments and ecological
circumstances.
37.1 1 Types of Hormones
By the end of this section, you will be able to do the following:
• List the different types of hormones
• Explain their role in maintaining homeostasis
Maintaining homeostasis within the body requires the coordination of many different systems and organs.
Communication between neighboring cells, and between cells and tissues in distant parts of the body, occurs
through the release of chemicals called hormones. Hormones are released into body fluids (usually blood) that
carry these chemicals to their target cells. At the target cells, which are cells that have a receptor for a signal or
ligand from a signal cell, the hormones elicit a response. The cells, tissues, and organs that secrete hormones
make up the endocrine system. Examples of glands of the endocrine system include the adrenal glands, which
produce hormones such as epinephrine and norepinephrine that regulate responses to stress, and the thyroid
gland, which produces thyroid hormones that regulate metabolic rates.
Although there are many different hormones in the human body, they can be divided into three classes based on
their chemical structure: lipid-derived, amino acid-derived, and peptide (peptide and proteins) hormones. One
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Chapter 37 | The Endocrine System
of the key distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes
whereas the amino acid-derived and peptide hormones cannot.
Lipid-Derived Hormones (or Lipid-soluble Hormones)
Most lipid hormones are derived from cholesterol and thus are structurally similar to it, as illustrated in Figure
37.2. The primary class of lipid hormones in humans is the steroid hormones. Chemically, these hormones are
usually ketones or alcohols; their chemical names will end in “-ol" for alcohols or “-one" for ketones. Examples of
steroid hormones include estradiol, which is an estrogen, or female sex hormone, and testosterone, which is an
androgen, or male sex hormone. These two hormones are released by the female and male reproductive organs,
respectively. Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands
along with some other types of androgens. Steroid hormones are insoluble in water, and they are transported by
transport proteins in blood. As a result, they remain in circulation longer than peptide hormones. For example,
cortisol has a half-life of 60 to 90 minutes, while epinephrine, an amino acid derived-hormone, has a half-life of
approximately one minute.
Figure 37.2 The structures shown here represent (a) cholesterol, plus the steroid hormones (b) testosterone and (c)
estradiol.
Amino Acid-Derived Hormones
The amino acid-derived hormones are relatively small molecules that are derived from the amino acids
tyrosine and tryptophan, shown in Figure 37.3. If a hormone is amino acid-derived, its chemical name will
end in “-ine”. Examples of amino acid-derived hormones include epinephrine and norepinephrine, which are
synthesized in the medulla of the adrenal glands, and thyroxine, which is produced by the thyroid gland. The
pineal gland in the brain makes and secretes melatonin which regulates sleep cycles.
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Chapter 37 | The Endocrine System
1147
o
OH H
(a)
Figure 37.3 (a) The hormone epinephrine, which triggers the fight-or-flight response, is derived from the amino acid
tyrosine, (b) The hormone melatonin, which regulates circadian rhythms, is derived from the amino acid tryptophan.
Peptide Hormones
The structure of peptide hormones is that of a polypeptide chain (chain of amino acids). The peptide hormones
include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in
the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins,
like growth hormones produced by the pituitary, and large glycoproteins such as follicle-stimulating hormone
produced by the pituitary. Figure 37.4 illustrates these peptide hormones.
Secreted peptides like insulin are stored within vesicles in the cells that synthesize them. They are then
released in response to stimuli such as high blood glucose levels in the case of insulin. Amino acid-derived and
polypeptide hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma
membranes of cells; therefore, their receptors are found on the surface of the target cells.
(a) (b) (c)
Figure 37.4 The structures of peptide hormones (a) oxytocin, (b) growth hormone, and (c) follicle-stimulating hormone
are shown. These peptide hormones are much larger than those derived from cholesterol or amino acids.
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Chapter 37 | The Endocrine System
ca eer connection
Endocrinologist
An endocrinologist is a medical doctor who specializes in treating disorders of the endocrine glands,
hormone systems, and glucose and lipid metabolic pathways. An endocrine surgeon specializes in the
surgical treatment of endocrine diseases and glands. Some of the diseases that are managed by
endocrinologists: disorders of the pancreas (diabetes mellitus), disorders of the pituitary (gigantism,
acromegaly, and pituitary dwarfism), disorders of the thyroid gland (goiter and Graves’ disease), and
disorders of the adrenal glands (Cushing’s disease and Addison’s disease).
Endocrinologists are required to assess patients and diagnose endocrine disorders through extensive use of
laboratory tests. Many endocrine diseases are diagnosed using tests that stimulate or suppress endocrine
organ functioning. Blood samples are then drawn to determine the effect of stimulating or suppressing an
endocrine organ on the production of hormones. For example, to diagnose diabetes mellitus, patients are
required to fast for 12 to 24 hours. They are then given a sugary drink, which stimulates the pancreas to
produce insulin to decrease blood glucose levels. A blood sample is taken one to two hours after the sugar
drink is consumed. If the pancreas is functioning properly, the blood glucose level will be within a normal
range. Another example is the A1C test, which can be performed during blood screening. The A1C test
measures average blood glucose levels over the past two to three months by examining how well the blood
glucose is being managed over a long time.
Once a disease has been diagnosed, endocrinologists can prescribe lifestyle changes and/or medications
to treat the disease. Some cases of diabetes mellitus can be managed by exercise, weight loss, and a
healthy diet; in other cases, medications may be required to enhance insulin release. If the disease cannot
be controlled by these means, the endocrinologist may prescribe insulin injections.
in addition to clinical practice, endocrinologists may also be involved in primary research and development
activities. For example, ongoing islet transplant research is investigating how healthy pancreas islet cells
may be transplanted into diabetic patients. Successful islet transplants may allow patients to stop taking
insulin injections.
37.2 | How Hormones Work
By the end of this section, you will be able to do the following:
• Explain how hormones work
• Discuss the role of different types of hormone receptors
Fiormones mediate changes in target cells by binding to specific hormone receptors. In this way, even though
hormones circulate throughout the body and come into contact with many different cell types, they only affect
cells that possess the necessary receptors. Receptors for a specific hormone may be found on many different
cells or may be limited to a small number of specialized cells. For example, thyroid hormones act on many
different tissue types, stimulating metabolic activity throughout the body. Cells can have many receptors for the
same hormone but often also possess receptors for different types of hormones. The number of receptors that
respond to a hormone determines the cell’s sensitivity to that hormone, and the resulting cellular response.
Additionally, the number of receptors that respond to a hormone can change over time, resulting in increased or
decreased cell sensitivity. In up-regulation, the number of receptors increases in response to rising hormone
levels, making the cell more sensitive to the hormone and allowing for more cellular activity. When the number of
receptors decreases in response to rising hormone levels, called down-regulation, cellular activity is reduced.
Receptor binding alters cellular activity and results in an increase or decrease in normal body processes.
Depending on the location of the protein receptor on the target cell and the chemical structure of the hormone,
hormones can mediate changes directly by binding to intracellular hormone receptors and modulating gene
transcription, or indirectly by binding to cell surface receptors and stimulating signaling pathways.
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Chapter 37 | The Endocrine System
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Intracellular Hormone Receptors
Lipid-derived (soluble) hormones such as steroid hormones diffuse across the membranes of the endocrine
cell. Once outside the cell, they bind to transport proteins that keep them soluble in the bloodstream. At the
target cell, the hormones are released from the carrier protein and diffuse across the lipid bilayer of the plasma
membrane of cells. The steroid hormones pass through the plasma membrane of a target cell and adhere
to intracellular receptors residing in the cytoplasm or in the nucleus. The cell signaling pathways induced by
the steroid hormones regulate specific genes on the cell's DNA. The hormones and receptor complex act as
transcription regulators by increasing or decreasing the synthesis of mRNA molecules of specific genes. This,
in turn, determines the amount of corresponding protein that is synthesized by altering gene expression. This
protein can be used either to change the structure of the cell or to produce enzymes that catalyze chemical
reactions. In this way, the steroid hormone regulates specific cell processes as illustrated in Figure 37.5.
visual
CONNECTION
Hormone
Heat
stock protein
(HSP) .
Changed
cell function
NR dimer
Protein
NR/hormone
complex
Nuclear
pore
Nuclear •
receptor
(NR)
mRNA
NR/HSP
complex
Ribosome
cytoplasm
Coactivator
Nuclear
envelope
mRNA
RNA polymerase
NR dimer
Nuclear DNA
Cell
membrane
Figure 37.5 An intracellular nuclear receptor (NR) is located in the cytoplasm bound to a heat shock protein
(HSP). Upon hormone binding, the receptor dissociates from the heat shock protein and translocates to the
nucleus. In the nucleus, the hormone-receptor complex binds to a DNA sequence called a hormone response
element (HRE), which triggers gene transcription and translation. The corresponding protein product can then
mediate changes in cell function.
Heat shock proteins (HSP) are so named because they help refold misfolded proteins. In response to
increased temperature (a “heat shock”), heat shock proteins are activated by release from the NR/HSP
complex. At the same time, transcription of HSP genes is activated. Why do you think the cell responds to
a heat shock by increasing the activity of proteins that help refold misfolded proteins?
Other lipid-soluble hormones that are not steroid hormones, such as vitamin D and thyroxine, have receptors
located in the nucleus. The hormones diffuse across both the plasma membrane and the nuclear envelope, then
bind to receptors in the nucleus. The hormone-receptor complex stimulates transcription of specific genes.
Plasma Membrane Hormone Receptors
Amino acid derived hormones and polypeptide hormones are not lipid-derived (lipid-soluble) and therefore
cannot diffuse through the plasma membrane of cells. Lipid insoluble hormones bind to receptors on the outer
surface of the plasma membrane, via plasma membrane hormone receptors. Unlike steroid hormones, lipid
insoluble hormones do not directly affect the target cell because they cannot enter the cell and act directly on
DNA. Binding of these hormones to a cell surface receptor results in activation of a signaling pathway; this
triggers intracellular activity and carries out the specific effects associated with the hormone. In this way, nothing
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Chapter 37 | The Endocrine System
passes through the cell membrane; the hormone that binds at the surface remains at the surface of the cell while
the intracellular product remains inside the cell. The hormone that initiates the signaling pathway is called a first
messenger, which activates a second messenger in the cytoplasm, as illustrated in Figure 37.6.
Plasma
membrane
Figure 37.6 The amino acid-derived hormones epinephrine and norepinephrine bind to beta-adrenergic receptors on
the plasma membrane of cells. Hormone binding to receptor activates a G-protein, which in turn activates adenylyl
cyclase, converting ATP to cAMP. cAMP is a second messenger that mediates a cell-specific response. An enzyme
called phosphodiesterase breaks down cAMP, terminating the signal.
One very important second messenger is cyclic AMP (cAMP). When a hormone binds to its membrane receptor,
a G-protein that is associated with the receptor is activated; G-proteins are proteins separate from receptors
that are found in the cell membrane. When a hormone is not bound to the receptor, the G-protein is inactive and
is bound to guanosine diphosphate, or GDP. When a hormone binds to the receptor, the G-protein is activated
by binding guanosine triphosphate, or GTP, in place of GDP. After binding, GTP is hydrolysed by the G-protein
into GDP and becomes inactive.
The activated G-protein in turn activates a membrane-bound enzyme called adenylyl cyclase. Adenylyl cyclase
catalyzes the conversion of ATP to cAMP. cAMP, in turn, activates a group of proteins called protein kinases,
which transfer a phosphate group from ATP to a substrate molecule in a process called phosphorylation. The
phosphorylation of a substrate molecule changes its structural orientation, thereby activating it. These activated
molecules can then mediate changes in cellular processes.
The effect of a hormone is amplified as the signaling pathway progresses. The binding of a hormone at a single
receptor causes the activation of many G-proteins, which activates adenylyl cyclase. Each molecule of adenylyl
cyclase then triggers the formation of many molecules of cAMP. Further amplification occurs as protein kinases,
once activated by cAMP, can catalyze many reactions. In this way, a small amount of hormone can trigger the
formation of a large amount of cellular product. To stop hormone activity, cAMP is deactivated by the cytoplasmic
enzyme phosphodiesterase, or PDE. PDE is always present in the cell and breaks down cAMP to control
hormone activity, preventing overproduction of cellular products.
The specific response of a cell to a lipid insoluble hormone depends on the type of receptors that are present
on the cell membrane and the substrate molecules present in the cell cytoplasm. Cellular responses to hormone
binding of a receptor include altering membrane permeability and metabolic pathways, stimulating synthesis of
proteins and enzymes, and activating hormone release.
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Chapter 37 | The Endocrine System
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37.3 | Regulation of Body Processes
By the end of this section, you will be able to do the following:
• Explain how hormones regulate the excretory system
• Discuss the role of hormones in the reproductive system
• Describe how hormones regulate metabolism
• Explain the role of hormones in different diseases
Hormones have a wide range of effects and modulate many different body processes. The key regulatory
processes that will be examined here are those affecting the excretory system, the reproductive system,
metabolism, blood calcium concentrations, growth, and the stress response.
Hormonal Regulation of the Excretory System
Maintaining a proper water balance in the body is important to avoid dehydration or over-hydration
(hyponatremia). The water concentration of the body is monitored by osmoreceptors in the hypothalamus,
which detect the concentration of electrolytes in the extracellular fluid. The concentration of electrolytes in the
blood rises when there is water loss caused by excessive perspiration, inadequate water intake, or low blood
volume due to blood loss. An increase in blood electrolyte levels results in a neuronal signal being sent from
the osmoreceptors in hypothalamic nuclei. The pituitary gland has two components: anterior and posterior. The
anterior pituitary is composed of glandular cells that secrete protein hormones. The posterior pituitary is an
extension of the hypothalamus. It is composed largely of neurons that are continuous with the hypothalamus.
The hypothalamus produces a polypeptide hormone known as antidiuretic hormone (ADH), which is
transported to and released from the posterior pituitary gland. The principal action of ADH is to regulate the
amount of water excreted by the kidneys. As ADH (which is also known as vasopressin) causes direct water
reabsorption from the kidney tubules, salts and wastes are concentrated in what will eventually be excreted as
urine. The hypothalamus controls the mechanisms of ADH secretion, either by regulating blood volume or the
concentration of water in the blood. Dehydration or physiological stress can cause an increase of osmolarity
above 300 mOsm/L, which in turn, raises ADH secretion and water will be retained, causing an increase in blood
pressure. ADH travels in the bloodstream to the kidneys. Once at the kidneys, ADH changes the kidneys to
become more permeable to water by temporarily inserting water channels, aquaporins, into the kidney tubules.
Water moves out of the kidney tubules through the aquaporins, reducing urine volume. The water is reabsorbed
into the capillaries lowering blood osmolarity back toward normal. As blood osmolarity decreases, a negative
feedback mechanism reduces osmoreceptor activity in the hypothalamus, and ADH secretion is reduced. ADH
release can be reduced by certain substances, including alcohol, which can cause increased urine production
and dehydration.
Chronic underproduction of ADH or a mutation in the ADH receptor results in diabetes insipidus. If the posterior
pituitary does not release enough ADH, water cannot be retained by the kidneys and is lost as urine. This causes
increased thirst, but water taken in is lost again and must be continually consumed. If the condition is not severe,
dehydration may not occur, but severe cases can lead to electrolyte imbalances due to dehydration.
Another hormone responsible for maintaining electrolyte concentrations in extracellular fluids is aldosterone, a
steroid hormone that is produced by the adrenal cortex. In contrast to ADH, which promotes the reabsorption
of water to maintain proper water balance, aldosterone maintains proper water balance by enhancing Na +
reabsorption and K + secretion from extracellular fluid of the cells in kidney tubules. Because it is produced in the
cortex of the adrenal gland and affects the concentrations of minerals Na + and K + , aldosterone is referred to as
a mineralocorticoid, a corticosteroid that affects ion and water balance. Aldosterone release is stimulated by a
decrease in blood sodium levels, blood volume, or blood pressure, or an increase in blood potassium levels. It
also prevents the loss of Na + from sweat, saliva, and gastric juice. The reabsorption of Na + also results in the
osmotic reabsorption of water, which alters blood volume and blood pressure.
Aldosterone production can be stimulated by low blood pressure, which triggers a sequence of chemical release,
as illustrated in Figure 37.7. When blood pressure drops, the renin-angiotensin-aldosterone system (RAAS)
is activated. Cells in the juxtaglomerular apparatus, which regulates the functions of the nephrons of the
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Chapter 37 | The Endocrine System
kidney, detect this and release renin. Renin, an enzyme, circulates in the blood and reacts with a plasma
protein produced by the liver called angiotensinogen. When angiotensinogen is cleaved by renin, it produces
angiotensin I, which is then converted into angiotensin II in the lungs. Angiotensin II functions as a hormone
and then causes the release of the hormone aldosterone by the adrenal cortex, resulting in increased Na +
reabsorption, water retention, and an increase in blood pressure. Angiotensin II in addition to being a potent
vasoconstrictor also causes an increase in ADH and increased thirst, both of which help to raise blood pressure.
Angiotensinogen
The renin-angiotensin-aldosterone system increases blood volume and pressure
Renin
Angiotensin I
Triggers release of
other hormones
Angiotensin II
/ V
Direct effects:
• Causes arteries to constrict resulting
in an increase in blood pressure.
• Decreases glomerular filtration rate
resulting in water retention.
• Increases thirst.
Angiotensinogen is
made by the liver
ADH is made in the
hypothalamus and
released by the
posterior pituitary
Aldosterone
/
Causes nephron distal
tubules to reabsorb more
Na + and water, which
increases blood volume.
Renin is
produced
by the kidney.
ADH
V
Mediates insertion of aquaporins
into nephron collecting duct cells.
As a result, more water is reabsorbed
into the blood.
Causes arteries to constrict.
Figure 37.7 ADH and aldosterone increase blood pressure and volume. Angiotensin II stimulates release of these
hormones. Angiotensin II, in turn, is formed when renin cleaves angiotensinogen. (credit: modification of work by
Mikael Haggstrdm)
Hormonal Regulation of the Reproductive System
Regulation of the reproductive system is a process that requires the action of hormones from the pituitary gland,
the adrenal cortex, and the gonads. During puberty in both males and females, the hypothalamus produces
gonadotropin-releasing hormone (GnRH), which stimulates the production and release of follicle-stimulating
hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. These hormones regulate the
gonads (testes in males and ovaries in females) and therefore are called gonadotropins, in both males and
females, FSH stimulates gamete production and LH stimulates production of hormones by the gonads. An
increase in gonad hormone levels inhibits GnRH production through a negative feedback loop.
Regulation of the Male Reproductive System
In males, FSH stimulates the maturation of sperm cells. FSH production is inhibited by the hormone inhibin,
which is released by the testes. LH stimulates production of the sex hormones ( androgens) by the interstitial
cells of the testes and therefore is also called interstitial cell-stimulating hormone.
The most widely known androgen in males is testosterone. Testosterone promotes the production of sperm and
masculine characteristics. The adrenal cortex also produces small amounts of testosterone precursor, although
the role of this additional hormone production is not fully understood.
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Chapter 37 | The Endocrine System
1153
everyday CONNECTION
The Dangers of Synthetic Hormones
Figure 37.8 Professional baseball player Jason Giambi publically admitted to, and apologized for, his use of
anabolic steroids supplied by a trainer, (credit: Bryce Edwards)
Some athletes attempt to boost their performance by using artificial hormones that enhance muscle
performance. Anabolic steroids, a form of the male sex hormone testosterone, are one of the most widely
known performance-enhancing drugs. Steroids are used to help build muscle mass. Other hormones that
are used to enhance athletic performance include erythropoietin, which triggers the production of red blood
cells, and human growth hormone, which can help in building muscle mass. Most performance enhancing
drugs are illegal for nonmedical purposes. They are also banned by national and international governing
bodies including the International Olympic Committee, the U.S. Olympic Committee, the National Collegiate
Athletic Association, the Major League Baseball, and the National Football League.
The side effects of synthetic hormones are often significant and nonreversible, and in some cases, fatal.
Androgens produce several complications such as liver dysfunctions and liver tumors, prostate gland
enlargement, difficulty urinating, premature closure of epiphyseal cartilages, testicular atrophy, infertility, and
immune system depression. The physiological strain caused by these substances is often greater than what
the body can handle, leading to unpredictable and dangerous effects and linking their use to heart attacks,
strokes, and impaired cardiac function.
Regulation of the Female Reproductive System
In females, FSH stimulates development of egg cells, called ova, which develop in structures called follicles.
Follicle cells produce the hormone inhibin, which inhibits FSH production. LH also plays a role in the
development of ova, induction of ovulation, and stimulation of estradiol and progesterone production by the
ovaries, as illustrated in Figure 37.9. Estradiol and progesterone are steroid hormones that prepare the body for
pregnancy. Estradiol produces secondary sex characteristics in females, while both estradiol and progesterone
regulate the menstrual cycle.
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Chapter 37 | The Endocrine System
GnRH secreted from
the hypothalmus
stimulates FSH
and LH production
in the pituitary.
FSH and LH
stimulate follicle
growth in
the ovaries.
A surge in
LH triggers
ovulation.
Hypothalamus
Pituitary
Estradiol,
progesterone
and inhibin are
secreted from
the ovaries.
Estradiol and
progesterone
regulate
female sex
characteristics
and the female
cycle. Inhibin
inhibits FSH
production by
the pituitary.
Uterus
Ovary
Figure 37.9 Hormonal regulation of the female reproductive system involves hormones from the hypothalamus,
pituitary, and ovaries.
In addition to producing FSH and LH, the anterior portion of the pituitary gland also produces the hormone
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Chapter 37 | The Endocrine System
1155
prolactin (PRL) in females. Prolactin stimulates the production of milk by the mammary glands following
childbirth. Prolactin levels are regulated by the hypothalamic hormones prolactin-releasing hormone (PRH)
and prolactin-inhibiting hormone (PIH), which is now known to be dopamine. PRH stimulates the release of
prolactin and PIH inhibits it.
The posterior pituitary releases the hormone oxytocin, which stimulates uterine contractions during childbirth.
The uterine smooth muscles are not very sensitive to oxytocin until late in pregnancy when the number of
oxytocin receptors in the uterus peaks. Stretching of tissues in the uterus and cervix stimulates oxytocin release
during childbirth. Contractions increase in intensity as blood levels of oxytocin rise via a positive feedback
mechanism until the birth is complete. Oxytocin also stimulates the contraction of myoepithelial cells around the
milk-producing mammary glands. As these cells contract, milk is forced from the secretory alveoli into milk ducts
and is ejected from the breasts in milk ejection (“let-down") reflex. Oxytocin release is stimulated by the suckling
of an infant, which triggers the synthesis of oxytocin in the hypothalamus and its release into circulation at the
posterior pituitary.
Hormonal Regulation of Metabolism
Blood glucose levels vary widely over the course of a day as periods of food consumption alternate with periods
of fasting. Insulin and glucagon are the two hormones primarily responsible for maintaining homeostasis of blood
glucose levels. Additional regulation is mediated by the thyroid hormones.
Regulation of Blood Glucose Levels by Insulin and Glucagon
Cells of the body require nutrients in order to function, and these nutrients are obtained through feeding, in order
to manage nutrient intake, storing excess intake and utilizing reserves when necessary, the body uses hormones
to moderate energy stores. Insulin is produced by the beta cells of the pancreas, which are stimulated to release
insulin as blood glucose levels rise (for example, after a meal is consumed). Insulin lowers blood glucose levels
by enhancing the rate of glucose uptake and utilization by target cells, which use glucose for ATP production. It
also stimulates the liver to convert glucose to glycogen, which is then stored by cells for later use. Insulin also
increases glucose transport into certain cells, such as muscle cells and the liver. This results from an insulin-
mediated increase in the number of glucose transporter proteins in cell membranes, which remove glucose from
circulation by facilitated diffusion. As insulin binds to its target cell via insulin receptors and signal transduction, it
triggers the cell to incorporate glucose transport proteins into its membrane. This allows glucose to enter the cell,
where it can be used as an energy source. However, this does not occur in all cells: some cells, including those
in the kidneys and brain, can access glucose without the use of insulin. Insulin also stimulates the conversion
of glucose to fat in adipocytes and the synthesis of proteins. These actions mediated by insulin cause blood
glucose concentrations to fall, called a hypoglycemic “low sugar" effect, which inhibits further insulin release from
beta cells through a negative feedback loop.
This animation describes the role of insulin and the pancreas in diabetes. (This multimedia resource
will open in a browser.) (http://cnx.Org/content/m66629/l.3/#eip-idll71750017253)
Impaired insulin function can lead to a condition called diabetes mellitus, the main symptoms of which are
illustrated in Figure 37.10. This can be caused by low levels of insulin production by the beta cells of the
pancreas, or by reduced sensitivity of tissue cells to insulin. This prevents glucose from being absorbed by cells,
causing high levels of blood glucose, or hyperglycemia (high sugar). High blood glucose levels make it difficult
for the kidneys to recover all the glucose from nascent urine, resulting in glucose being lost in urine. High glucose
levels also result in less water being reabsorbed by the kidneys, causing high amounts of urine to be produced;
this may result in dehydration. Over time, high blood glucose levels can cause nerve damage to the eyes and
peripheral body tissues, as well as damage to the kidneys and cardiovascular system. Oversecretion of insulin
can cause hypoglycemia, low blood glucose levels. This causes insufficient glucose availability to cells, often
leading to muscle weakness, and can sometimes cause unconsciousness or death if left untreated.
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Chapter 37 | The Endocrine System
Central nervous —
system
• Lethargy
• Stupor
• Excessive thirst
• Excessive hunger
Systemic
• Weight loss
. J-
Eyes
• Blurred vision
• Breath
• Smell of acetone
Gastric
• Nausea
• Vomiting
• Abdominal
pain
Urinary
• Frequent
urination
• Glucose
in urine
Figure 37.10 The main symptoms of diabetes are shown, (credit: modification of work by Mikael Haggstrom)
When blood glucose levels decline below normal levels, for example between meals or when glucose is utilized
rapidly during exercise, the hormone glucagon is released from the alpha cells of the pancreas. Glucagon raises
blood glucose levels, eliciting what is called a hyperglycemic effect, by stimulating the breakdown of glycogen to
glucose in skeletal muscle cells and liver cells in a process called glycogenolysis. Glucose can then be utilized
as energy by muscle cells and released into circulation by the liver cells. Glucagon also stimulates absorption of
amino acids from the blood by the liver, which then converts them to glucose. This process of glucose synthesis
is called gluconeogenesis. Glucagon also stimulates adipose cells to release fatty acids into the blood. These
actions mediated by glucagon result in an increase in blood glucose levels to normal homeostatic levels. Rising
blood glucose levels inhibit further glucagon release by the pancreas via a negative feedback mechanism. In
this way, insulin and glucagon work together to maintain homeostatic glucose levels, as shown in Figure 37.11.
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Chapter 37 | The Endocrine System
1157
visual
CONNECTION
Blood glucose level rises.
/
In response to glycogen
the liver breaks down
glycogen and releases
glucose into the blood.
The pancreas releases
insulin.
The pancreas releases
glucagon.
In response to insulin
target cells take up
glucose and the liver
converts glucose to
glycogen.
✓
Blood glucose level falls.
Figure 37.11 Insulin and glucagon regulate blood glucose levels.
Pancreatic tumors may cause excess secretion of glucagon. Type I diabetes results from the failure of the
pancreas to produce insulin. Which of the following statement about these two conditions is true?
a. A pancreatic tumor and type I diabetes will have the opposite effects on blood sugar levels.
b. A pancreatic tumor and type I diabetes will both cause hyperglycemia.
c. A pancreatic tumor and type I diabetes will both cause hypoglycemia.
d. Both pancreatic tumors and type I diabetes result in the inability of cells to take up glucose.
Regulation of Blood Glucose Levels by Thyroid Hormones
The basal metabolic rate, which is the amount of calories required by the body at rest, is determined by
two hormones produced by the thyroid gland: thyroxine, also known as tetraiodothyronine or T 4 , and
triiodothyronine, also known as T 3 . These hormones affect nearly every cell in the body except for the adult
brain, uterus, testes, blood cells, and spleen. They are transported across the plasma membrane of target
cells and bind to receptors on the mitochondria resulting in increased ATP production. In the nucleus, T 3
and T 4 activate genes involved in energy production and glucose oxidation. This results in increased rates of
metabolism and body heat production, which is known as the hormone’s calorigenic effect.
T 3 and T 4 release from the thyroid gland is stimulated by thyroid-stimulating hormone (TSH), which is
produced by the anterior pituitary. TSH binding at the receptors of the follicle of the thyroid triggers the production
of T 3 and T 4 from a glycoprotein called thyroglobulin. Thyroglobulin is present in the follicles of the thyroid,
and is converted into thyroid hormones with the addition of iodine. Iodine is formed from iodide ions that are
actively transported into the thyroid follicle from the bloodstream. A peroxidase enzyme then attaches the iodine
to the tyrosine amino acid found in thyroglobulin. T 3 has three iodine ions attached, while T 4 has four iodine ions
attached. T 3 and T 4 are then released into the bloodstream, with T 4 being released in much greater amounts
than T 3 . As T 3 is more active than T 4 and is responsible for most of the effects of thyroid hormones, tissues of
the body convert T 4 to T 3 by the removal of an iodine ion. Most of the released T 3 and T 4 becomes attached to
transport proteins in the bloodstream and is unable to cross the plasma membrane of cells. These protein-bound
molecules are only released when blood levels of the unattached hormone begin to decline. In this way, a week’s
worth of reserve hormone is maintained in the blood. Increased T 3 and T 4 levels in the blood inhibit the release
of TSH, which results in lower T 3 and T 4 release from the thyroid.
The follicular cells of the thyroid require iodides (anions of iodine) in order to synthesize T 3 and T 4 . Iodides
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Chapter 37 | The Endocrine System
obtained from the diet are actively transported into follicle cells resulting in a concentration that is approximately
30 times higher than in blood. The typical diet in North America provides more iodine than required due to the
addition of iodide to table salt. Inadequate iodine intake, which occurs in many developing countries, results in
an inability to synthesize T 3 and T 4 hormones. The thyroid gland enlarges in a condition called goiter, which is
caused by overproduction of TSH without the formation of thyroid hormone. Thyroglobulin is contained in a fluid
called colloid, and TSH stimulation results in higher levels of colloid accumulation in the thyroid. In the absence
of iodine, this is not converted to thyroid hormone, and colloid begins to accumulate more and more in the thyroid
gland, leading to goiter.
Disorders can arise from both the underproduction and overproduction of thyroid hormones. Hypothyroidism,
underproduction of the thyroid hormones, can cause a low metabolic rate leading to weight gain, sensitivity
to cold, and reduced mental activity, among other symptoms. In children, hypothyroidism can cause cretinism,
which can lead to mental retardation and growth defects. Hyperthyroidism, the overproduction of thyroid
hormones, can lead to an increased metabolic rate and its effects: weight loss, excess heat production,
sweating, and an increased heart rate. Graves’ disease is one example of a hyperthyroid condition.
Hormonal Control of Blood Calcium Levels
Regulation of blood calcium concentrations is important for generation of muscle contractions and nerve
impulses, which are electrically stimulated. If calcium levels get too high, membrane permeability to sodium
decreases and membranes become less responsive. If calcium levels get too low, membrane permeability to
sodium increases and convulsions or muscle spasms can result.
Blood calcium levels are regulated by parathyroid hormone (PTH), which is produced by the parathyroid
glands, as illustrated in Figure 37.12. PTH is released in response to low blood Ca 2+ levels. PTH increases
Ca 2+ levels by targeting the skeleton, the kidneys, and the intestine. In the skeleton, PTH stimulates osteoclasts,
which causes bone to be reabsorbed, releasing Ca 2+ from bone into the blood. PTH also inhibits osteoblasts,
reducing Ca 2+ deposition in bone. In the intestines, PTH increases dietary Ca 2+ absorption, and in the kidneys,
PTH stimulates reabsorption of the CA 2+ . While PTH acts directly on the kidneys to increase Ca 2+ reabsorption,
its effects on the intestine are indirect. PTH triggers the formation of calcitriol, an active form of vitamin D, which
acts on the intestines to increase absorption of dietary calcium. PTH release is inhibited by rising blood calcium
levels.
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Chapter 37 | The Endocrine System
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+
Parathyroid glands
Increased f
calcium inipiooal
parathyroid hormone
+ Calcium reabsorption
Calciummi
absorption]
[Galcitrjioll
Intestines
-f- Calcium absorption
Bones
+ Calcitriol [formation from
vitamin D
W
Virlnowc
Figure 37.12 Parathyroid hormone (PTH) is released in response to low blood calcium levels. It increases blood
calcium levels by targeting the skeleton, the kidneys, and the intestine, (credit: modification of work by Mikael
Haggstrom)
Hyperparathyroidism results from an overproduction of parathyroid hormone. This results in excessive calcium
being removed from bones and introduced into blood circulation, producing structural weakness of the bones,
which can lead to deformation and fractures, plus nervous system impairment due to high blood calcium levels.
Hypoparathyroidism, the underproduction of PTH, results in extremely low levels of blood calcium, which causes
impaired muscle function and may result in tetany (severe sustained muscle contraction).
The hormone calcitonin, which is produced by the parafollicular or C cells of the thyroid, has the opposite
effect on blood calcium levels as does PTH. Calcitonin decreases blood calcium levels by inhibiting osteoclasts,
stimulating osteoblasts, and stimulating calcium excretion by the kidneys. This results in calcium being added
to the bones to promote structural integrity. Calcitonin is most important in children (when it stimulates bone
growth), during pregnancy (when it reduces maternal bone loss), and during prolonged starvation (because it
reduces bone mass loss). In healthy nonpregnant, unstarved adults, the role of calcitonin is unclear.
Hormonal Regulation of Growth
Hormonal regulation is required for the growth and replication of most cells in the body. Growth hormone (GH),
produced by the anterior portion of the pituitary gland, accelerates the rate of protein synthesis, particularly in
skeletal muscle and bones. Growth hormone has direct and indirect mechanisms of action. The first direct action
of GH is stimulation of triglyceride breakdown (lipolysis) and release into the blood by adipocytes. This results in
a switch by most tissues from utilizing glucose as an energy source to utilizing fatty acids. This process is called
a glucose-sparing effect. In another direct mechanism, GH stimulates glycogen breakdown in the liver; the
glycogen is then released into the blood as glucose. Blood glucose levels increase as most tissues are utilizing
fatty acids instead of glucose for their energy needs. The GH mediated increase in blood glucose levels is called
a diabetogenic effect because it is similar to the high blood glucose levels seen in diabetes mellitus.
The indirect mechanism of GH action is mediated by insulin-like growth factors (IGFs) or somatomedins,
which are a family of growth-promoting proteins produced by the liver, which stimulates tissue growth. IGFs
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Chapter 37 | The Endocrine System
stimulate the uptake of amino acids from the blood, allowing the formation of new proteins, particularly in skeletal
muscle cells, cartilage cells, and other target cells, as shown in Figure 37.13. This is especially important after
a meal, when glucose and amino acid concentration levels are high in the blood. GH levels are regulated by two
hormones produced by the hypothalamus. GH release is stimulated by growth hormone-releasing hormone
(GHRH) and is inhibited by growth hormone-inhibiting hormone (GHIH), also called somatostatin.
Pituitary gland ■
Muscle growth
Bone
growth
Growth
hormone
(GH)
GHRH (GH-releasing
hormone) stimulates
the release of GH.
GHIN (GH-inhibiting
hormone) inhibits
the release of GH.
Adipocytes break down
triglycerides.
The liver breaks down glycogen.
Insulin-like growth factors (IGFs) stimulate
amino acid uptake by target cells, promoting
protein synthesis.
Figure 37.13 Growth hormone directly accelerates the rate of protein synthesis in skeletal muscle and bones. Insulin¬
like growth factor 1 (IGF-1) is activated by growth hormone and also allows formation of new proteins in muscle cells
and bone, (credit: modification of work by Mikael Haggstrom)
A balanced production of growth hormone is critical for proper development. Underproduction of GH in adults
does not appear to cause any abnormalities, but in children it can result in pituitary dwarfism, in which growth is
reduced. Pituitary dwarfism is characterized by symmetric body formation. In some cases, individuals are under
30 inches in height. Oversecretion of growth hormone can lead to gigantism in children, causing excessive
growth. In some documented cases, individuals can reach heights of over eight feet. In adults, excessive GH
can lead to acromegaly, a condition in which there is enlargement of bones in the face, hands, and feet that are
still capable of growth.
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Chapter 37 | The Endocrine System
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Hormonal Regulation of Stress
When a threat or danger is perceived, the body responds by releasing hormones that will ready it for the “fight-
or-flight" response. The effects of this response are familiar to anyone who has been in a stressful situation:
increased heart rate, dry mouth, and hair standing up.
V / _
e olution CONNECTION
Fight-or-Flight Response
Interactions of the endocrine hormones have evolved to ensure the body’s internal environment remains
stable. Stressors are stimuli that disrupt homeostasis. The sympathetic division of the vertebrate autonomic
nervous system has evolved the fight-or-flight response to counter stress-induced disruptions of
homeostasis. In the initial alarm phase, the sympathetic nervous system stimulates an increase in energy
levels through increased blood glucose levels. This prepares the body for physical activity that may be
required to respond to stress: to either fight for survival or to flee from danger.
However, some stresses, such as illness or injury, can last for a long time. Glycogen reserves, which provide
energy in the short-term response to stress, are exhausted after several hours and cannot meet long-term
energy needs. If glycogen reserves were the only energy source available, neural functioning could not be
maintained once the reserves became depleted due to the nervous system’s high requirement for glucose.
In this situation, the body has evolved a response to counter long-term stress through the actions of the
glucocorticoids, which ensure that long-term energy requirements can be met. The glucocorticoids mobilize
lipid and protein reserves, stimulate gluconeogenesis, conserve glucose for use by neural tissue, and
stimulate the conservation of salts and water. The mechanisms to maintain homeostasis that are described
here are those observed in the human body. However, the fight-or-flight response exists in some form in all
vertebrates.
The sympathetic nervous system regulates the stress response via the hypothalamus. Stressful stimuli cause
the hypothalamus to signal the adrenal medulla (which mediates short-term stress responses) via nerve
impulses, and the adrenal cortex, which mediates long-term stress responses, via the hormone
adrenocorticotropic hormone (ACTH), which is produced by the anterior pituitary.
Short-term Stress Response
When presented with a stressful situation, the body responds by calling for the release of hormones that provide
a burst of energy. The hormones epinephrine (also known as adrenaline) and norepinephrine (also known
as noradrenaline) are released by the adrenal medulla. How do these hormones provide a burst of energy?
Epinephrine and norepinephrine increase blood glucose levels by stimulating the liver and skeletal muscles to
break down glycogen and by stimulating glucose release by liver cells. Additionally, these hormones increase
oxygen availability to cells by increasing the heart rate and dilating the bronchioles. The hormones also prioritize
body function by increasing blood supply to essential organs such as the heart, brain, and skeletal muscles,
while restricting blood flow to organs not in immediate need, such as the skin, digestive system, and kidneys.
Epinephrine and norepinephrine are collectively called catecholamines.
LINK
T a
LEARNING
Watch this Discovery Channel animation (http:// 0 penstaxc 0 llege. 0 rg/l/adrenaline) describing the flight-or-
flight response.
Long-term Stress Response
Long-term stress response differs from short-term stress response. The body cannot sustain the bursts of
energy mediated by epinephrine and norepinephrine for long times, instead, other hormones come into play. In
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Chapter 37 | The Endocrine System
a long-term stress response, the hypothalamus triggers the release of ACTH from the anterior pituitary gland.
The adrenal cortex is stimulated by ACTH to release steroid hormones called corticosteroids. Corticosteroids
turn on transcription of certain genes in the nuclei of target cells. They change enzyme concentrations in the
cytoplasm and affect cellular metabolism. There are two main corticosteroids: glucocorticoids such as cortisol,
and mineralocorticoids such as aldosterone. These hormones target the breakdown of fat into fatty acids in the
adipose tissue. The fatty acids are released into the bloodstream for other tissues to use for ATP production.
The glucocorticoids primarily affect glucose metabolism by stimulating glucose synthesis. Glucocorticoids also
have anti-inflammatory properties through inhibition of the immune system. For example, cortisone is used as
an anti-inflammatory medication; however, it cannot be used long term as it increases susceptibility to disease
due to its immune-suppressing effects.
Mineralocorticoids function to regulate ion and water balance of the body. The hormone aldosterone stimulates
the reabsorption of water and sodium ions in the kidney, which results in increased blood pressure and volume.
Hypersecretion of glucocorticoids can cause a condition known as Cushing’s disease, characterized by a
shifting of fat storage areas of the body. This can cause the accumulation of adipose tissue in the face and neck,
and excessive glucose in the blood. Hyposecretion of the corticosteroids can cause Addison’s disease, which
may result in bronzing of the skin, hypoglycemia, and low electrolyte levels in the blood.
37.4 | Regulation of Hormone Production
By the end of this section, you will be able to do the following:
• Explain how hormone production is regulated
• Discuss the different stimuli that control hormone levels in the body
Hormone production and release are primarily controlled by negative feedback. In negative feedback systems,
a stimulus elicits the release of a substance; once the substance reaches a certain level, it sends a signal that
stops further release of the substance. In this way, the concentration of hormones in blood is maintained within
a narrow range. For example, the anterior pituitary signals the thyroid to release thyroid hormones. Increasing
levels of these hormones in the blood then give feedback to the hypothalamus and anterior pituitary to inhibit
further signaling to the thyroid gland, as illustrated in Figure 37.14. There are three mechanisms by which
endocrine glands are stimulated to synthesize and release hormones: humoral stimuli, hormonal stimuli, and
neural stimuli.
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Chapter 37 | The Endocrine System
1163
visual
CONNECTION
Thyroid System
Hypothalamus
Anterior pituit4darJ J^ , e P |^^ asinfl
Negative feedback
I
Thyroid-stimulating
hormone (TSH)
Thyroid gland
Thyroid hormones
(T3 and T4)
Increased metabolism
Growth an'd'd.evelopment
Figure 37.14 The anterior pituitary stimulates the thyroid gland to release thyroid hormones T 3 and T 4 . Increasing
levels of these hormones in the blood results in feedback to the hypothalamus and anterior pituitary to inhibit
further signaling to the thyroid gland, (credit: modification of work by Mikael Haggstrom)
Hyperthyroidism is a condition in which the thyroid gland is overactive. Hypothyroidism is a condition in
which the thyroid gland is underactive. Which of the conditions are the following two patients most likely to
have?
Patient A has symptoms including weight gain, cold sensitivity, low heart rate, and fatigue.
Patient B has symptoms including weight loss, profuse sweating, increased heart rate, and difficulty
sleeping.
Humoral Stimuli
The term “humoral" is derived from the term “humor,” which refers to bodily fluids such as blood. A humoral
stimulus refers to the control of hormone release in response to changes in extracellular fluids such as blood
or the ion concentration in the blood. For example, a rise in blood glucose levels triggers the pancreatic release
of insulin. Insulin causes blood glucose levels to drop, which signals the pancreas to stop producing insulin in a
negative feedback loop.
Hormonal Stimuli
Hormonal stimuli refers to the release of a hormone in response to another hormone. A number of endocrine
glands release hormones when stimulated by hormones released by other endocrine glands. For example, the
hypothalamus produces hormones that stimulate the anterior portion of the pituitary gland. The anterior pituitary
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Chapter 37 | The Endocrine System
in turn releases hormones that regulate hormone production by other endocrine glands. The anterior pituitary
releases the thyroid-stimulating hormone, which then stimulates the thyroid gland to produce the hormones T 3
and T 4 . As blood concentrations of T 3 and T 4 rise, they inhibit both the pituitary and the hypothalamus in a
negative feedback loop.
Neural Stimuli
In some cases, the nervous system directly stimulates endocrine glands to release hormones, which is referred
to as neural stimuli. Recall that in a short-term stress response, the hormones epinephrine and norepinephrine
are important for providing the bursts of energy required for the body to respond. Here, neuronal signaling from
the sympathetic nervous system directly stimulates the adrenal medulla to release the hormones epinephrine
and norepinephrine in response to stress.
37.5 | Endocrine Glands
By the end of this section, you will be able to do the following:
• Describe the role of different glands in the endocrine system
• Explain how the different glands work together to maintain homeostasis
Both the endocrine and nervous systems use chemical signals to communicate and regulate the body's
physiology. The endocrine system releases hormones that act on target cells to regulate development, growth,
energy metabolism, reproduction, and many behaviors. The nervous system releases neurotransmitters or
neurohormones that regulate neurons, muscle cells, and endocrine cells. Because the neurons can regulate the
release of hormones, the nervous and endocrine systems work in a coordinated manner to regulate the body's
physiology.
Hypothalamic-Pituitary Axis
The hypothalamus in vertebrates integrates the endocrine and nervous systems. The hypothalamus is an
endocrine organ located in the diencephalon of the brain. It receives input from the body and other brain areas
and initiates endocrine responses to environmental changes. The hypothalamus acts as an endocrine organ,
synthesizing hormones and transporting them along axons to the posterior pituitary gland. It synthesizes and
secretes regulatory hormones that control the endocrine cells in the anterior pituitary gland. The hypothalamus
contains autonomic centers that control endocrine cells in the adrenal medulla via neuronal control.
The pituitary gland, sometimes called the hypophysis or “master gland” is located at the base of the brain in
the sella turcica, a groove of the sphenoid bone of the skull, illustrated in Figure 37.15. It is attached to the
hypothalamus via a stalk called the pituitary stalk (or infundibulum). The anterior portion of the pituitary gland is
regulated by releasing or release-inhibiting hormones produced by the hypothalamus, and the posterior pituitary
receives signals via neurosecretory cells to release hormones produced by the hypothalamus. The pituitary has
two distinct regions—the anterior pituitary and the posterior pituitary—which between them secrete nine different
peptide or protein hormones. The posterior lobe of the pituitary gland contains axons of the hypothalamic
neurons.
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Chapter 37 | The Endocrine System
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Figure 37.15 The pituitary gland is located at (a) the base of the brain and (b) connected to the hypothalamus by the
pituitary stalk, (credit a: modification of work by NCI; credit b: modification of work by Gray’s Anatomy)
Anterior Pituitary
The anterior pituitary gland, or adenohypophysis, is surrounded by a capillary network that extends from the
hypothalamus, down along the infundibulum, and to the anterior pituitary. This capillary network is a part of
the hypophyseal portal system that carries substances from the hypothalamus to the anterior pituitary and
hormones from the anterior pituitary into the circulatory system. A portal system carries blood from one capillary
network to another; therefore, the hypophyseal portal system allows hormones produced by the hypothalamus
to be carried directly to the anterior pituitary without first entering the circulatory system.
The anterior pituitary produces seven hormones: growth hormone (GH), prolactin (PRL), thyroid-stimulating
hormone (TSH), melanin-stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), follicle-stimulating
hormone (FSH), and luteinizing hormone (LH). Anterior pituitary hormones are sometimes referred to as tropic
hormones, because they control the functioning of other organs. While these hormones are produced by the
anterior pituitary, their production is controlled by regulatory hormones produced by the hypothalamus. These
regulatory hormones can be releasing hormones or inhibiting hormones, causing more or less of the anterior
pituitary hormones to be secreted. These travel from the hypothalamus through the hypophyseal portal system
to the anterior pituitary where they exert their effect. Negative feedback then regulates how much of these
regulatory hormones are released and how much anterior pituitary hormone is secreted.
Posterior Pituitary
The posterior pituitary is significantly different in structure from the anterior pituitary. It is a part of the brain,
extending down from the hypothalamus, and contains mostly nerve fibers and neuroglial cells, which support
axons that extend from the hypothalamus to the posterior pituitary. The posterior pituitary and the infundibulum
together are referred to as the neurohypophysis.
The hormones antidiuretic hormone (ADH), also known as vasopressin, and oxytocin are produced by neurons
in the hypothalamus and transported within these axons along the infundibulum to the posterior pituitary. They
are released into the circulatory system via neural signaling from the hypothalamus. These hormones are
considered to be posterior pituitary hormones, even though they are produced by the hypothalamus, because
that is where they are released into the circulatory system. The posterior pituitary itself does not produce
hormones, but instead stores hormones produced by the hypothalamus and releases them into the bloodstream.
Thyroid Gland
The thyroid gland is located in the neck, just below the larynx and in front of the trachea, as shown in Figure
37.16. It is a butterfly-shaped gland with two lobes that are connected by the isthmus. It has a dark red color
due to its extensive vascular system. When the thyroid swells due to dysfunction, it can be felt under the skin of
the neck.
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Figure 37.16 This illustration shows the location of the thyroid gland.
The thyroid gland is made up of many spherical thyroid follicles, which are lined with a simple cuboidal
epithelium. These follicles contain a viscous fluid, called colloid, which stores the glycoprotein thyroglobulin, the
precursor to the thyroid hormones. The follicles produce hormones that can be stored in the colloid or released
into the surrounding capillary network for transport to the rest of the body via the circulatory system.
Thyroid follicle cells synthesize the hormone thyroxine, which is also known as T 4 because it contains four atoms
of iodine, and triiodothyronine, also known as T 3 because it contains three atoms of iodine. Follicle cells are
stimulated to release stored T 3 and T 4 by thyroid stimulating hormone (TSH), which is produced by the anterior
pituitary. These thyroid hormones increase the rates of mitochondrial ATP production.
A third hormone, calcitonin, is produced by parafollicular cells of the thyroid either releasing hormones or
inhibiting hormones. Calcitonin release is not controlled by TSH, but instead is released when calcium ion
concentrations in the blood rise. Calcitonin functions to help regulate calcium concentrations in body fluids.
It acts in the bones to inhibit osteoclast activity and in the kidneys to stimulate excretion of calcium. The
combination of these two events lowers body fluid levels of calcium.
Parathyroid Glands
Most people have four parathyroid glands; however, the number can vary from two to six. These glands are
located on the posterior surface of the thyroid gland, as shown in Figure 37.17. Normally, there is a superior
gland and an inferior gland associated with each of the thyroid’s two lobes. Each parathyroid gland is covered
by connective tissue and contains many secretory cells that are associated with a capillary network.
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Thyroid gland
Parathyroid gland
Figure 37.17 The parathyroid glands are located on the posterior of the thyroid gland, (credit: modification of work by
NCI)
The parathyroid glands produce parathyroid hormone (PTH). PTH increases blood calcium concentrations when
calcium ion levels fall below normal. PTH (1) enhances reabsorption of Ca 2+ by the kidneys, (2) stimulates
osteoclast activity and inhibits osteoblast activity, and (3) it stimulates synthesis and secretion of calcitriol by
the kidneys, which enhances Ca 2+ absorption by the digestive system. PTH is produced by chief cells of the
parathyroid. PTH and calcitonin work in opposition to one another to maintain homeostatic Ca 2+ levels in body
fluids. Another type of cells, oxyphil cells, exist in the parathyroid but their function is not known. These hormones
encourage bone growth, muscle mass, and blood cell formation in children and women.
Adrenal Glands
The adrenal glands are associated with the kidneys; one gland is located on top of each kidney as illustrated
in Figure 37.18. The adrenal glands consist of an outer adrenal cortex and an inner adrenal medulla. These
regions secrete different hormones.
Adrenal gland
Kidney
Figure 37.18 The location of the adrenal glands on top of the kidneys is shown, (credit: modification of work by NCI)
Adrenal Cortex
The adrenal cortex is made up of layers of epithelial cells and associated capillary networks. These layers form
three distinct regions: an outer zona glomerulosa that produces mineralocorticoids, a middle zona fasciculata
that produces glucocorticoids, and an inner zona reticularis that produces androgens.
The main mineralocorticoid is aldosterone, which regulates the concentration of Na + ions in urine, sweat,
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pancreas, and saliva. Aldosterone release from the adrenal cortex is stimulated by a decrease in blood
concentrations of sodium ions, blood volume, or blood pressure, or by an increase in blood potassium levels.
The three main glucocorticoids are cortisol, corticosterone, and cortisone. The glucocorticoids stimulate the
synthesis of glucose and gluconeogenesis (converting a non-carbohydrate to glucose) by liver cells and they
promote the release of fatty acids from adipose tissue. These hormones increase blood glucose levels to
maintain levels within a normal range between meals. These hormones are secreted in response to ACTH and
levels are regulated by negative feedback.
Androgens are sex hormones that promote masculinity. They are produced in small amounts by the adrenal
cortex in both males and females. They do not affect sexual characteristics and may supplement sex hormones
released from the gonads.
Adrenal Medulla
The adrenal medulla contains large, irregularly shaped cells that are closely associated with blood vessels.
These cells are innervated by preganglionic autonomic nerve fibers from the central nervous system.
The adrenal medulla contains two types of secretory cells: one that produces epinephrine (adrenaline) and
another that produces norepinephrine (noradrenaline). Epinephrine is the primary adrenal medulla hormone
accounting for 75 to 80 percent of its secretions. Epinephrine and norepinephrine increase heart rate, breathing
rate, cardiac muscle contractions, blood pressure, and blood glucose levels. They also accelerate the
breakdown of glucose in skeletal muscles and stored fats in adipose tissue.
The release of epinephrine and norepinephrine is stimulated by neural impulses from the sympathetic nervous
system. Secretion of these hormones is stimulated by acetylcholine release from preganglionic sympathetic
fibers innervating the adrenal medulla. These neural impulses originate from the hypothalamus in response to
stress to prepare the body for the fight-or-flight response.
Pancreas
The pancreas, illustrated in Figure 37.19, is an elongated organ that is located between the stomach and
the proximal portion of the small intestine. It contains both exocrine cells that excrete digestive enzymes and
endocrine cells that release hormones. It is sometimes referred to as a heterocrine gland because it has both
endocrine and exocrine functions.
Stomach
Gall bladder
Common bile duct
Duodenum Pancreas
Figure 37.19 The pancreas is found underneath the stomach and points toward the spleen, (credit: modification of
work by NCI)
The endocrine cells of the pancreas form clusters called pancreatic islets or the islets of Langerhans, as visible
in the micrograph shown in Figure 37.20. The pancreatic islets contain two primary cell types: alpha cells,
which produce the hormone glucagon, and beta cells, which produce the hormone insulin. These hormones
regulate blood glucose levels. As blood glucose levels decline, alpha cells release glucagon to raise the blood
glucose levels by increasing rates of glycogen breakdown and glucose release by the liver. When blood glucose
levels rise, such as after a meal, beta cells release insulin to lower blood glucose levels by increasing the rate
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of glucose uptake in most body cells, and by increasing glycogen synthesis in skeletal muscles and the liver.
Together, glucagon and insulin regulate blood glucose levels.
Figure 37.20 The islets of Langerhans are clusters of endocrine cells found in the pancreas; they stain lighter than
surrounding cells, (credit: modification of work by Muhammad T. Tabiin, Christopher P. White, Grant Morahan, and
Bernard E. Tuch; scale-bar data from Matt Russell)
Pineal Gland
The pineal gland produces melatonin. The rate of melatonin production is affected by the photoperiod.
Collaterals from the visual pathways innervate the pineal gland. During the day photoperiod, little melatonin
is produced; however, melatonin production increases during the dark photoperiod (night), in some mammals,
melatonin has an inhibitory affect on reproductive functions by decreasing production and maturation of sperm,
oocytes, and reproductive organs. Melatonin is an effective antioxidant, protecting the CNS from free radicals
such as nitric oxide and hydrogen peroxide. Lastly, melatonin is involved in biological rhythms, particularly
circadian rhythms such as the sleep-wake cycle and eating habits.
Gonads
The gonads—the male testes and female ovaries—produce steroid hormones. The testes produce androgens,
testosterone being the most prominent, which allow for the development of secondary sex characteristics and
the production of sperm cells. The ovaries produce estradiol and progesterone, which cause secondary sex
characteristics and prepare the body for childbirth.
Endocrine Glands and their Associated Hormones
Endocrine
Gland
Associated
Hormones
Effect
Hypothalamus
releasing and
inhibiting hormones
regulate hormone release from pituitary gland; produce oxytocin;
produce uterine contractions and milk secretion in females
antidiuretic hormone
(ADH)
water reabsorption from kidneys; vasoconstriction to increase blood
pressure
growth hormone
(GH)
promotes growth of body tissues, protein synthesis; metabolic functions
prolactin (PRL)
promotes milk production
Pituitary
(Anterior)
thyroid stimulating
hormone (TSH)
stimulates thyroid hormone release
adrenocorticotropic
hormone (ACTH)
stimulates hormone release by adrenal cortex, glucocorticoids
follicle-stimulating
hormone (FSH)
stimulates gamete production (both ova and sperm); secretion of
estradiol
Table 37.1
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Endocrine Glands and their Associated Hormones
Endocrine Associated
Gland Hormones
luteinizing hormone
(LH)
stimulates androgen production by gonads; ovulation, secretion of
progesterone
melanocyte-
stimulating hormone
(MSH)
stimulates melanocytes of the skin increasing melanin pigment
production.
Pituitary
(Posterior)
antidiuretic hormone
(ADH)
stimulates water reabsorption by kidneys
oxytocin
stimulates uterine contractions during childbirth; milk ejection;
stimulates ductus deferens and prostate gland contraction during
emission
Thyroid
thyroxine,
triiodothyronine
stimulate and maintain metabolism; growth and development
calcitonin
reduces blood Ca 2+ levels
Parathyroid
parathyroid
hormone (PTH)
increases blood Ca 2+ levels
Adrenal
(Cortex)
aldosterone
increases blood Na + levels; increases K + secretion
cortisol,
corticosterone,
cortisone
increase blood glucose levels; anti-inflammatory effects
Adrenal
(Medulla)
epinephrine,
norepinephrine
stimulate fight-or-flight response; increase blood gluclose levels;
increase metabolic activities
Pancreas
insulin
reduces blood glucose levels
glucagon
increases blood glucose levels
Pineal gland
melatonin
regulates some biological rhythms and protects CNS from free radicals
Testes
androgens
regulate, promote, increase or maintain sperm production; male
secondary sexual characteristics
Ovaries
estrogen
promotes uterine lining growth; female secondary sexual characteristics
progestins
promote and maintain uterine lining growth
Table 37.1
Organs with Secondary Endocrine Functions
There are several organs whose primary functions are non-endocrine but that also possess endocrine functions.
These include the heart, kidneys, intestines, thymus, gonads, and adipose tissue.
The heart possesses endocrine cells in the walls of the atria that are specialized cardiac muscle cells. These
cells release the hormone atrial natriuretic peptide (ANP) in response to increased blood volume. High blood
volume causes the cells to be stretched, resulting in hormone release. ANP acts on the kidneys to reduce the
reabsorption of Na + , causing Na + and water to be excreted in the urine. ANP also reduces the amounts of renin
released by the kidneys and aldosterone released by the adrenal cortex, further preventing the retention of water.
In this way, ANP causes a reduction in blood volume and blood pressure, and reduces the concentration of Na +
in the blood.
The gastrointestinal tract produces several hormones that aid in digestion. The endocrine cells are located in
the mucosa of the Gl tract throughout the stomach and small intestine. Some of the hormones produced include
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gastrin, secretin, and cholecystokinin, which are secreted in the presence of food, and some of which act on
other organs such as the pancreas, gallbladder, and liver. They trigger the release of gastric juices, which help
to break down and digest food in the Gl tract.
While the adrenal glands associated with the kidneys are major endocrine glands, the kidneys themselves
also possess endocrine function. Renin is released in response to decreased blood volume or pressure and
is part of the renin-angiotensin-aldosterone system that leads to the release of aldosterone. Aldosterone then
causes the retention of Na + and water, raising blood volume. The kidneys also release calcitriol, which aids in the
absorption of Ca 2+ and phosphate ions. Erythropoietin (EPO) is a protein hormone that triggers the formation
of red blood cells in the bone marrow. EPO is released in response to low oxygen levels. Because red blood
cells are oxygen carriers, increased production results in greater oxygen delivery throughout the body. EPO has
been used by athletes to improve performance, as greater oxygen delivery to muscle cells allows for greater
endurance. Because red blood cells increase the viscosity of blood, artificially high levels of EPO can cause
severe health risks.
The thymus is found behind the sternum; it is most prominent in infants, becoming smaller in size through
adulthood. The thymus produces hormones referred to as thymosins, which contribute to the development of the
immune response.
Adipose tissue is a connective tissue found throughout the body. It produces the hormone leptin in response
to food intake. Leptin increases the activity of anorexigenic neurons and decreases that of orexigenic neurons,
producing a feeling of satiety after eating, thus affecting appetite and reducing the urge for further eating.
Leptin is also associated with reproduction. It must be present for GnRH and gonadotropin synthesis to occur.
Extremely thin females may enter puberty late; however, if adipose levels increase, more leptin will be produced,
improving fertility.
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KEY TERMS
acromegaly condition caused by overproduction of GH in adults
Addison’s disease disorder caused by the hyposecretion of corticosteroids
adenylate cyclase an enzyme that catalyzes the conversion of ATP to cyclic AMP
adrenal cortex outer portion of adrenal glands that produces corticosteroids
adrenal gland endocrine glands associated with the kidneys
adrenal medulla inner portion of adrenal glands that produces epinephrine and norepinephrine
adrenocorticotropic hormone (ACTH) hormone released by the anterior pituitary, which stimulates the
adrenal cortex to release corticosteroids during the long-term stress response
aldosterone steroid hormone produced by the adrenal cortex that stimulates the reabsorption of Na + from
extracellular fluids and secretion of K +
alpha cell endocrine cell of the pancreatic islets that produces the hormone glucagon
amino acid-derived hormone hormone derived from amino acids
androgen male sex hormone such as testosterone
anterior pituitary portion of the pituitary gland that produces six hormones; also called adenohypophysis
antidiuretic hormone (ADH) hormone produced by the hypothalamus and released by the posterior pituitary
that increases water reabsorption by the kidneys
atrial natriuretic peptide (ANP) hormone produced by the heart to reduce blood volume, pressure, and Na +
concentration
beta cell endocrine cell of the pancreatic islets that produces the hormone insulin
calcitonin hormone produced by the parafollicular cells of the thyroid gland that functions to lower blood Ca 2+
levels and promote bone growth
colloid fluid inside the thyroid gland that contains the glycoprotein thyroglobulin
corticosteroid hormone released by the adrenal cortex in response to long-term stress
cortisol glucocorticoid produced in response to stress
Cushing’s disease disorder caused by the hypersecretion of glucocorticoids
diabetes insipidus disorder caused by underproduction of ADH
diabetes mellitus disorder caused by low levels of insulin activity
diabetogenic effect effect of GH that causes blood glucose levels to rise similar to diabetes mellitus
down-regulation a decrease in the number of hormone receptors in response to increased hormone levels
endocrine gland gland that secretes hormones into the surrounding interstitial fluid, which then diffuse into
blood and are carried to various organs and tissues within the body
epinephrine hormone released by the adrenal medulla in response to a short term stress
erythropoietin (EPO) hormone produced by the kidneys to stimulate red blood cell production in the bone
marrow
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estrogens a group of steroid hormones, including estradiol and several others, that are produced by the ovaries
and elicit secondary sex characteristics in females as well as control the maturation of the ova
first messenger the hormone that binds to a plasma membrane hormone receptor to trigger a signal
transduction pathway
follicle-stimulating hormone (FSH) hormone produced by the anterior pituitary that stimulates gamete
production
G-protein a membrane protein activated by the hormone first messenger to activate formation of cyclic AMP
gigantism condition caused by overproduction of GH in children
glucagon hormone produced by the alpha cells of the pancreas in response to low blood sugar; functions to
raise blood sugar levels
glucocorticoid corticosteroid that affects glucose metabolism
gluconeogenesis synthesis of glucose from amino acids
glucose-sparing effect effect of GH that causes tissues to use fatty acids instead of glucose as an energy
source
glycogenolysis breakdown of glycogen into glucose
goiter enlargement of the thyroid gland caused by insufficient dietary iodine levels
gonadotropin hormone that regulates the gonads, including FSH and LH
growth hormone (GH) hormone produced by the anterior pituitary that promotes protein synthesis and body
growth
growth hormone-inhibiting hormone (GHIH) hormone produced by the hypothalamus that inhibits growth
hormone production, also called somatostatin
growth hormone-releasing hormone (GHRH) hormone released by the hypothalamus that triggers the
release of GH
hormonal stimuli release of a hormone in response to another hormone
hormone receptor the cellular protein that binds to a hormone
humoral stimuli control of hormone release in response to changes in extracellular fluids such as blood or the
ion concentration in the blood
hyperglycemia high blood sugar level
hyperthyroidism overactivity of the thyroid gland
hypoglycemia low blood sugar level
hypophyseal portal system system of blood vessels that carries hormones from the hypothalamus to the
anterior pituitary
hypothyroidism underactivity of the thyroid gland
insulin hormone produced by the beta cells of the pancreas in response to high blood glucose levels; functions
to lower blood glucose levels
insulin-like growth factor (IGF) growth-promoting protein produced by the liver
intracellular hormone receptor a hormone receptor in the cytoplasm or nucleus of a cell
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islets of Langerhans (pancreatic islets) endocrine cells of the pancreas
isthmus tissue mass that connects the two lobes of the thyroid gland
leptin hormone produced by adipose tissue that promotes feelings of satiety and reduces hunger
lipid-derived hormone hormone derived mostly from cholesterol
mineralocorticoid corticosteroid that affects ion and water balance
neural stimuli stimulation of endocrine glands by the nervous system
norepinephrine hormone released by the adrenal medulla in response to a short-term stress hormone
production by the gonads
osmoreceptor receptor in the hypothalamus that monitors the concentration of electrolytes in the blood
oxytocin hormone released by the posterior pituitary to stimulate uterine contractions during childbirth and milk
let-down in the mammary glands
pancreas organ located between the stomach and the small intestine that contains exocrine and endocrine cells
parafollicular cell thyroid cell that produces the hormone calcitonin
parathyroid gland gland located on the surface of the thyroid that produces parathyroid hormone
parathyroid hormone (PTH) hormone produced by the parathyroid glands in response to low blood Ca 2+
levels; functions to raise blood Ca 2+ levels
peptide hormone hormone composed of a polypeptide chain
phosphodiesterase (PDE) enzyme that deactivates cAMP, stopping hormone activity
pituitary dwarfism condition caused by underproduction of GH in children
pituitary gland endocrine gland located at the base of the brain composed of an anterior and posterior region;
also called hypophysis
pituitary stalk (also, infundibulum) stalk that connects the pituitary gland to the hypothalamus
plasma membrane hormone receptor a hormone receptor on the surface of the plasma membrane of a cell
posterior pituitary extension of the brain that releases hormones produced by the hypothalamus; along with
the infundibulum, it is also referred to as the neurohypophysis
prolactin (PRL) hormone produced by the anterior pituitary that stimulates milk production
prolactin-inhibiting hormone hormone produced by the hypothalamus that inhibits the release of prolactin
prolactin-releasing hormone hormone produced by the hypothalamus that stimulates the release of prolactin
renin enzyme produced by the juxtaglomerular apparatus of the kidneys that reacts with angiotensinogen to
cause the release of aldosterone
thymus gland located behind the sternum that produces thymosin hormones that contribute to the development
of the immune system
thyroglobulin glycoprotein found in the thyroid that is converted into thyroid hormone
thyroid gland endocrine gland located in the neck that produces thyroid hormones thyroxine and
triiodothyronine
thyroid-stimulating hormone (TSH) hormone produced by the anterior pituitary that controls the release of T 3
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and T 4 from the thyroid gland
thyroxine (tetraiodothyronine, T 4 ) thyroid hormone containing 4 iodines that controls the basal metabolic rate
triiodothyronine (T 3 ) thyroid hormone containing 3 iodines that controls the basal metabolic rate
up-regulation an increase in the number of hormone receptors in response to increased hormone levels
CHAPTER SUMMARY
37.1 Types of Hormones
There are three basic types of hormones: lipid-derived, amino acid-derived, and peptide. Lipid-derived
hormones are structurally similar to cholesterol and include steroid hormones such as estradiol and
testosterone. Amino acid-derived hormones are relatively small molecules and include the adrenal hormones
epinephrine and norepinephrine. Peptide hormones are polypeptide chains or proteins and include the pituitary
hormones, antidiuretic hormone (vasopressin), and oxytocin.
37.2 How Hormones Work
Hormones cause cellular changes by binding to receptors on target cells. The number of receptors on a target
cell can increase or decrease in response to hormone activity. Hormones can affect cells directly through
intracellular hormone receptors or indirectly through plasma membrane hormone receptors.
Lipid-derived (soluble) hormones can enter the cell by diffusing across the plasma membrane and binding to
DNA to regulate gene transcription and to change the cell’s activities by inducing production of proteins that
affect, in general, the long-term structure and function of the cell. Lipid insoluble hormones bind to receptors on
the plasma membrane surface and trigger a signaling pathway to change the cell’s activities by inducing
production of various cell products that affect the cell in the short-term. The hormone is called a first messenger
and the cellular component is called a second messenger. G-proteins activate the second messenger (cyclic
AMP), triggering the cellular response. Response to hormone binding is amplified as the signaling pathway
progresses. Cellular responses to hormones include the production of proteins and enzymes and altered
membrane permeability.
37.3 Regulation of Body Processes
Water levels in the body are controlled by antidiuretic hormone (ADH), which is produced in the hypothalamus
and triggers the reabsorption of water by the kidneys. Underproduction of ADH can cause diabetes insipidus.
Aldosterone, a hormone produced by the adrenal cortex of the kidneys, enhances Na + reabsorption from the
extracellular fluids and subsequent water reabsorption by diffusion. The renin-angiotensin-aldosterone system
is one way that aldosterone release is controlled.
The reproductive system is controlled by the gonadotropins follicle-stimulating hormone (FSH) and luteinizing
hormone (LH), which are produced by the pituitary gland. Gonadotropin release is controlled by the
hypothalamic hormone gonadotropin-releasing hormone (GnRH). FSH stimulates the maturation of sperm cells
in males and is inhibited by the hormone inhibin, while LH stimulates the production of the androgen
testosterone. FSH stimulates egg maturation in females, while LH stimulates the production of estrogens and
progesterone. Estrogens are a group of steroid hormones produced by the ovaries that trigger the development
of secondary sex characteristics in females as well as control the maturation of the ova. In females, the pituitary
also produces prolactin, which stimulates milk production after childbirth, and oxytocin, which stimulates uterine
contraction during childbirth and milk let-down during suckling.
Insulin is produced by the pancreas in response to rising blood glucose levels and allows cells to utilize blood
glucose and store excess glucose for later use. Diabetes mellitus is caused by reduced insulin activity and
causes high blood glucose levels, or hyperglycemia. Glucagon is released by the pancreas in response to low
blood glucose levels and stimulates the breakdown of glycogen into glucose, which can be used by the body.
The body’s basal metabolic rate is controlled by the thyroid hormones thyroxine (T 4 ) and triiodothyronine (T 3 ).
The anterior pituitary produces thyroid stimulating hormone (TSH), which controls the release of T 3 and T 4 from
the thyroid gland. Iodine is necessary in the production of thyroid hormone, and the lack of iodine can lead to a
condition called goiter.
Parathyroid hormone (PTH) is produced by the parathyroid glands in response to low blood Ca 2+ levels. The
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parafollicular cells of the thyroid produce calcitonin, which reduces blood Ca 2+ levels. Growth hormone (GH) is
produced by the anterior pituitary and controls the growth rate of muscle and bone. GH action is indirectly
mediated by insulin-like growth factors (IGFs). Short-term stress causes the hypothalamus to trigger the
adrenal medulla to release epinephrine and norepinephrine, which trigger the fight or flight response. Long¬
term stress causes the hypothalamus to trigger the anterior pituitary to release adrenocorticotropic hormone
(ACTH), which causes the release of corticosteroids, glucocorticoids, and mineralocorticoids, from the adrenal
cortex.
37.4 Regulation of Hormone Production
Hormone levels are primarily controlled through negative feedback, in which rising levels of a hormone inhibit
its further release. The three mechanisms of hormonal release are humoral stimuli, hormonal stimuli, and
neural stimuli. Humoral stimuli refers to the control of hormonal release in response to changes in extracellular
fluid levels or ion levels. Hormonal stimuli refers to the release of hormones in response to hormones released
by other endocrine glands. Neural stimuli refers to the release of hormones in response to neural stimulation.
37.5 Endocrine Glands
The pituitary gland is located at the base of the brain and is attached to the hypothalamus by the infundibulum.
The anterior pituitary receives products from the hypothalamus by the hypophyseal portal system and produces
six hormones. The posterior pituitary is an extension of the brain and releases hormones (antidiuretic hormone
and oxytocin) produced by the hypothalamus.
The thyroid gland is located in the neck and is composed of two lobes connected by the isthmus. The thyroid is
made up of follicle cells that produce the hormones thyroxine and triiodothyronine. Parafollicular cells of the
thyroid produce calcitonin. The parathyroid glands lie on the posterior surface of the thyroid gland and produce
parathyroid hormone.
The adrenal glands are located on top of the kidneys and consist of the renal cortex and renal medulla. The
adrenal cortex is the outer part of the adrenal gland and produces the corticosteroids, glucocorticoids, and
mineralocorticoids. The adrenal medulla is the inner part of the adrenal gland and produces the catecholamines
epinephrine and norepinephrine.
The pancreas lies in the abdomen between the stomach and the small intestine. Clusters of endocrine cells in
the pancreas form the islets of Langerhans, which are composed of alpha cells that release glucagon and beta
cells that release insulin.
Some organs possess endocrine activity as a secondary function but have another primary function. The heart
produces the hormone atrial natriuretic peptide, which functions to reduce blood volume, pressure, and Na +
concentration. The gastrointestinal tract produces various hormones that aid in digestion. The kidneys produce
renin, calcitriol, and erythropoietin. Adipose tissue produces leptin, which promotes satiety signals in the brain.
VISUAL CONNECTION QUESTIONS
1. Figure 37.5 Heat shock proteins (HSP) are so
named because they help refold misfolded proteins.
In response to increased temperature (a “heat
shock"), heat shock proteins are activated by release
from the NR/HSP complex. At the same time,
transcription of HSP genes is activated. Why do you
think the cell responds to a heat shock by increasing
the activity of proteins that help refold misfolded
proteins?
2. Figure 37.11 Pancreatic tumors may cause
excess secretion of glucagon. Type I diabetes results
from the failure of the pancreas to produce insulin.
Which of the following statement about these two
conditions is true?
a. A pancreatic tumor and type i diabetes will
have the opposite effects on blood sugar
levels.
b. A pancreatic tumor and type I diabetes will
both cause hyperglycemia.
c. A pancreatic tumor and type I diabetes will
both cause hypoglycemia.
d. Both pancreatic tumors and type I diabetes
result in the inability of cells to take up
glucose.
3. Figure 37.14 Hyperthyroidism is a condition in
which the thyroid gland is overactive. Hypothyroidism
is a condition in which the thyroid gland is
underactive. Which of the conditions are the following
two patients most likely to have?
Patient A has symptoms including weight gain, cold
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sensitivity, low heart rate, and fatigue.
Patient B has symptoms including weight loss,
REVIEW QUESTIONS
4. A newly discovered hormone contains four amino
acids linked together. Under which chemical class
would this hormone be classified?
a. lipid-derived hormone
b. amino acid-derived hormone
c. peptide hormone
d. glycoprotein
5. Which class of hormones can diffuse through
plasma membranes?
a. lipid-derived hormones
b. amino acid-derived hormones
c. peptide hormones
d. glycoprotein hormones
6. Why are steroids able to diffuse across the plasma
membrane?
a. Their transport protein moves them through
the membrane.
b. They are amphipathic, allowing them to
interact with the entire phospholipid.
c. Cells express channels that let hormones
flow down their concentration gradient into
the cells.
d. They are non-polar molecules.
7. A new antagonist molecule has been discovered
that binds to and blocks plasma membrane
receptors. What effect will this antagonist have on
testosterone, a steroid hormone?
a. It will block testosterone from binding to its
receptor.
b. It will block testosterone from activating
cAMP signaling.
c. It will increase testosterone-mediated
signaling.
d. It will not affect testosterone-mediated
signaling.
8. What effect will a cAMP inhibitor have on a peptide
hormone-mediated signaling pathway?
a. It will prevent the hormone from binding its
receptor.
b. It will prevent activation of a G-protein.
c. It will prevent activation of adenylate
cyclase.
d. It will prevent activation of protein kinases.
9. When insulin binds to its receptor, the complex is
endocytosed into the cell. This is an example of
_in response to hormone signaling.
a. cAMP activation
b. generating an intracellular receptor
c. activation of a hormone response element
d. receptor down-regulation
10. Drinking alcoholic beverages causes an increase
profuse sweating, increased heart rate, and difficulty
sleeping.
in urine output. This most likely occurs because
alcohol:
a. inhibits ADH release.
b. stimulates ADH release.
c. inhibits TSH release.
d. stimulates TSH release.
11. FSH and LH release from the anterior pituitary is
stimulated by_.
a. TSH
b. GnRH
c. T 3
d. PTH
12. What hormone is produced by beta cells of the
pancreas?
a. T 3
b. glucagon
c. insulin
d. T 4
13. When blood calcium levels are low, PTH
stimulates:
a. excretion of calcium from the kidneys.
b. excretion of calcium from the intestines.
c. osteoblasts.
d. osteoclasts.
14. How would mutations that completely ablate the
function of the androgen receptor impact the
phenotypic development of humans with XY
chromosomes?
a. Patients would appear phenotypically
female.
b. Patients would appear phenotypically male
with underdeveloped secondary sex
characteristics.
c. Patients would appear phenotypically male,
but cannot produce sperm.
d. Patients would express both male and
female secondary sex characteristics.
15. A rise in blood glucose levels triggers release of
insulin from the pancreas. This mechanism of
hormone production is stimulated by:
a. humoral stimuli
b. hormonal stimuli
c. neural stimuli
d. negative stimuli
16. Which mechanism of hormonal stimulation would
be affected if signaling and hormone release from the
hypothalamus was blocked?
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Chapter 37 | The Endocrine System
a. humoral and hormonal stimuli
b. hormonal and neural stimuli
c. neural and humoral stimuli
d. hormonal and negative stimuli
17. A scientist hypothesizes that the pancreas’s
hormone production is controlled by neural stimuli.
Which observation would support this hypothesis?
a. Insulin is produced in response to sudden
stress without a rise in blood glucose.
b. Insulin is produced in response to a rise in
glucagon levels.
c. Beta cells express epinephrine receptors.
d. Insulin is produced in response to a rise in
blood glucose in the brain.
18. Which endocrine glands are associated with the
kidneys?
CRITICAL THINKING QUESTIONS
21. Although there are many different hormones in
the human body, they can be divided into three
classes based on their chemical structure. What are
these classes and what is one factor that
distinguishes them?
22. Where is insulin stored, and why would it be
released?
23. Glucagon is the peptide hormone that signals for
the body to release glucose into the bloodstream.
How does glucagon contribute to maintaining
homeostasis throughout the body? What other
hormones are involved in regulating the blood
glucose cycle?
24. Name two important functions of hormone
receptors.
25. How can hormones mediate changes?
26. Why is cAMP-mediated signal amplification not
required in steroid hormone signaling? Describe how
steroid signaling is amplified instead.
27. Name and describe a function of one hormone
produced by the anterior pituitary and one hormone
produced by the posterior pituitary.
28. Describe one direct action of growth hormone
a. thyroid glands
b. pituitary glands
c. adrenal glands
d. gonads
19. Which of the following hormones is not produced
by the anterior pituitary?
a. oxytocin
b. growth hormone
c. prolactin
d. thyroid-stimulating hormone
20. Recent studies suggest that blue light exposure
can impact human circadian rhythms. This suggests
that blue light disrupts the function of the_
gland(s).
a. adrenal
b. pituitary
c. pineal
d. thyroid
(GH).
29. Researchers have recently demonstrated that
stressed people are more susceptible to contracting
the common cold than people who are not stressed.
What kind of stress must the infected patients be
experiencing, and why does it make them more
susceptible to the virus?
30. How is hormone production and release primarily
controlled?
31. Compare and contrast hormonal and humoral
stimuli.
32. Oral contraceptive pills work by delivering
synthetic progestins to a woman every day. Describe
why this is an effective method of birth control.
33. What does aldosterone regulate, and how is it
stimulated?
34. The adrenal medulla contains two types of
secretory cells, what are they and what are their
functions?
35. How would damage to the posterior pituitary
gland affect the production and release of ADH and
inhibiting hormones?
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Chapter 38 | The Musculoskeletal System
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38 | THE
MUSCULOSKELETAL
SYSTEM
Figure 38.1 Improvements in the design of prostheses have allowed for a wider range of activities in recipients, (credit:
modification of work by Stuart Grout)
Chapter Outline
38.1: Types of Skeletal Systems
38.2: Bone
38.3: Joints and Skeletal Movement
38.4: Muscle Contraction and Locomotion
Introduction
The muscular and skeletal systems provide support to the body and allow for a wide range of movement. The
bones of the skeletal system protect the body’s internal organs and support the weight of the body. The muscles
of the muscular system contract and pull on the bones, allowing for movements as diverse as standing, walking,
running, and grasping items.
Injury or disease affecting the musculoskeletal system can be very debilitating. In humans, the most common
musculoskeletal diseases worldwide are caused by malnutrition. Ailments that affect the joints are also
widespread, such as arthritis, which can make movement difficult and—in advanced cases—completely impair
mobility. In severe cases in which the joint has suffered extensive damage, joint replacement surgery may be
needed.
Progress in the science of prosthesis design has resulted in the development of artificial joints, with joint
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Chapter 38 | The Musculoskeletal System
replacement surgery in the hips and knees being the most common. Replacement joints for shoulders, elbows,
and fingers are also available. Even with this progress, there is still room for improvement in the design of
prostheses. The state-of-the-art prostheses have limited durability and therefore wear out quickly, particularly in
young or active individuals. Current research is focused on the use of new materials, such as carbon fiber, that
may make prostheses more durable.
38.1 1 Types of Skeletal Systems
By the end of this section, you will be able to do the following:
• Discuss the different types of skeletal systems
• Explain the role of the human skeletal system
• Compare and contrast different skeletal systems
A skeletal system is necessary to support the body, protect internal organs, and allow for the movement
of an organism. There are three different skeleton designs that fulfill these functions: hydrostatic skeleton,
exoskeleton, and endoskeleton.
Hydrostatic Skeleton
A hydrostatic skeleton is a skeleton formed by a fluid-filled compartment within the body, called the coelom.
The organs of the coelom are supported by the aqueous fluid, which also resists external compression. This
compartment is under hydrostatic pressure because of the fluid and supports the other organs of the organism.
This type of skeletal system is found in soft-bodied animals such as sea anemones, earthworms, Cnidaria, and
other invertebrates (Figure 38.2).
Figure 38.2 The skeleton of the red-knobbed sea star (Protoreaster linckii) is an example of a hydrostatic skeleton,
(credit: “Amada447Wikimedia Commons)
Movement in a hydrostatic skeleton is provided by muscles that surround the coelom. The muscles in a
hydrostatic skeleton contract to change the shape of the coelom; the pressure of the fluid in the coelom produces
movement. For example, earthworms move by waves of muscular contractions of the skeletal muscle of the
body wall hydrostatic skeleton, called peristalsis, which alternately shorten and lengthen the body. Lengthening
the body extends the anterior end of the organism. Most organisms have a mechanism to fix themselves in the
substrate. Shortening the muscles then draws the posterior portion of the body forward. Although a hydrostatic
skeleton is well-suited to invertebrate organisms such as earthworms and some aquatic organisms, it is not an
efficient skeleton for terrestrial animals.
Exoskeleton
An exoskeleton is an external skeleton that consists of a hard encasement on the surface of an organism. For
example, the shells of crabs and insects are exoskeletons (Figure 38.3). This skeleton type provides defence
against predators, supports the body, and allows for movement through the contraction of attached muscles.
As with vertebrates, muscles must cross a joint inside the exoskeleton. Shortening of the muscle changes the
relationship of the two segments of the exoskeleton. Arthropods such as crabs and lobsters have exoskeletons
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that consist of 30-50 percent chitin, a polysaccharide derivative of glucose that is a strong but flexible material.
Chitin is secreted by the epidermal cells. The exoskeleton is further strengthened by the addition of calcium
carbonate in organisms such as the lobster. Because the exoskeleton is acellular, arthropods must periodically
shed their exoskeletons because the exoskeleton does not grow as the organism grows.
Figure 38.3 Muscles attached to the exoskeleton of the Halloween crab (Gecarcinus quadratus) allow it to move.
Endoskeleton
An endoskeleton is a skeleton that consists of hard, mineralized structures located within the soft tissue of
organisms. An example of a primitive endoskeletal structure is the spicules of sponges. The bones of vertebrates
are composed of tissues, whereas sponges have no true tissues (Figure 38.4). Endoskeletons provide support
for the body, protect internal organs, and allow for movement through contraction of muscles attached to the
skeleton.
Figure 38.4 The skeletons of humans and horses are examples of endoskeletons. (credit: Ross Murphy)
The human skeleton is an endoskeleton that consists of 206 bones in the adult. It has five main functions:
providing support to the body, storing minerals and lipids, producing blood cells, protecting internal organs, and
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allowing for movement. The skeletal system in vertebrates is divided into the axial skeleton (which consists of
the skull, vertebral column, and rib cage), and the appendicular skeleton (which consists of the shoulders, limb
bones, the pectoral girdle, and the pelvic girdle).
LINK TQ LEARNING
Visit the interactive body (http:// 0 penstaxc 0 llege. 0 rg/l/virt_skelet 0 n) site to build a virtual skeleton: select
"skeleton" and click through the activity to place each bone.
Human Axial Skeleton
The axial skeleton forms the central axis of the body and includes the bones of the skull, ossicles of the middle
ear, hyoid bone of the throat, vertebral column, and the thoracic cage (ribcage) (Figure 38.5). The function of
the axial skeleton is to provide support and protection for the brain, the spinal cord, and the organs in the ventral
body cavity. It provides a surface for the attachment of muscles that move the head, neck, and trunk, performs
respiratory movements, and stabilizes parts of the appendicular skeleton.
Vertebral
column
Figure 38.5 The axial skeleton consists of the bones of the skull, ossicles of the middle ear, hyoid bone, vertebral
column, and rib cage, (credit: modification of work by Mariana Ruiz Villareal)
The Skull
The bones of the skull support the structures of the face and protect the brain. The skull consists of 22 bones,
which are divided into two categories: cranial bones and facial bones. The cranial bones are eight bones that
form the cranial cavity, which encloses the brain and serves as an attachment site for the muscles of the head
and neck. The eight cranial bones are the frontal bone, two parietal bones, two temporal bones, occipital bone,
sphenoid bone, and the ethmoid bone. Although the bones developed separately in the embryo and fetus, in the
adult, they are tightly fused with connective tissue and adjoining bones do not move (Figure 38.6).
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Chapter 38 | The Musculoskeletal System
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Figure 38.6 The bones of the skull support the structures of the face and protect the brain, (credit: modification of work
by Mariana Ruiz Villareal)
The auditory ossicles of the middle ear transmit sounds from the air as vibrations to the fluid-filled cochlea. The
auditory ossicles consist of three bones each: the malleus, incus, and stapes. These are the smallest bones in
the body and are unique to mammals.
Fourteen facial bones form the face, provide cavities for the sense organs (eyes, mouth, and nose), protect
the entrances to the digestive and respiratory tracts, and serve as attachment points for facial muscles. The
14 facial bones are the nasal bones, the maxillary bones, zygomatic bones, palatine, vomer, lacrimal bones,
the inferior nasal conchae, and the mandible. All of these bones occur in pairs except for the mandible and the
vomer (Figure 38.7).
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Chapter 38 | The Musculoskeletal System
Cranial bones —
Orbit
Temporal bone
Lacrimal bone
Palatine bone
Zygomatic bone
Nasal bone
Vomer
Maxilla
Mandible
— Facial bones
Anterior view
Figure 38.7 The cranial bones, including the frontal, parietal, and sphenoid bones, cover the top of the head. The facial
bones of the skull form the face and provide cavities for the eyes, nose, and mouth.
Although it is not found in the skull, the hyoid bone is considered a component of the axial skeleton. The hyoid
bone lies below the mandible in the front of the neck. It acts as a movable base for the tongue and is connected
to muscles of the jaw, larynx, and tongue. The mandible articulates with the base of the skull. The mandible
controls the opening to the airway and gut. In animals with teeth, the mandible brings the surfaces of the teeth
in contact with the maxillary teeth.
The Vertebral Column
The vertebral column, or spinal column, surrounds and protects the spinal cord, supports the head, and acts
as an attachment point for the ribs and muscles of the back and neck. The adult vertebral column comprises 26
bones: the 24 vertebrae, the sacrum, and the coccyx bones, in the adult, the sacrum is typically composed of
five vertebrae that fuse into one. The coccyx is typically 3-4 vertebrae that fuse into one. Around the age of 70,
the sacrum and the coccyx may fuse together. We begin life with approximately 33 vertebrae, but as we grow,
several vertebrae fuse together. The adult vertebrae are further divided into the 7 cervical vertebrae, 12 thoracic
vertebrae, and 5 lumbar vertebrae (Figure 38.8).
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Chapter 38 | The Musculoskeletal System
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Vertebral Column
Cervical vertebrae
Thoracic vertebrae
Lumbar vertebrae
Sacrum
Coccygeal vertebrae
Cervical curve
Thoracic curve
Lumbar curve
Sacral curve
(a) (b)
Figure 38.8 (a) The vertebral column consists of seven cervical vertebrae (Cl-7) twelve thoracic vertebrae (Thl-12),
five lumbar vertebrae (Ll-5), the os sacrum, and the coccyx, (b) Spinal curves increase the strength and flexibility of
the spine, (credit a: modification of work by Uwe Gille based on original work by Gray's Anatomy; credit b: modification
of work by NCI, NIH)
Each vertebral body has a large hole in the center through which the nerves of the spinal cord pass. There is
also a notch on each side through which the spinal nerves, which serve the body at that level, can exit from the
spinal cord. The vertebral column is approximately 71 cm (28 inches) in adult male humans and is curved, which
can be seen from a side view. The names of the spinal curves correspond to the region of the spine in which
they occur. The thoracic and sacral curves are concave (curve inwards relative to the front of the body) and the
cervical and lumbar curves are convex (curve outwards relative to the front of the body). The arched curvature
of the vertebral column increases its strength and flexibility, allowing it to absorb shocks like a spring (Figure
38.8).
Intervertebral discs composed of fibrous cartilage lie between adjacent vertebral bodies from the second
cervical vertebra to the sacrum. Each disc is part of a joint that allows for some movement of the spine and acts
as a cushion to absorb shocks from movements such as walking and running. Intervertebral discs also act as
ligaments to bind vertebrae together. The inner part of discs, the nucleus pulposus, hardens as people age and
becomes less elastic. This loss of elasticity diminishes its ability to absorb shocks.
The Thoracic Cage
The thoracic cage, also known as the ribcage, is the skeleton of the chest, and consists of the ribs, sternum,
thoracic vertebrae, and costal cartilages (Figure 38.9). The thoracic cage encloses and protects the organs of
the thoracic cavity, including the heart and lungs. It also provides support for the shoulder girdles and upper
limbs, and serves as the attachment point for the diaphragm, muscles of the back, chest, neck, and shoulders.
Changes in the volume of the thorax enable breathing.
The sternum, or breastbone, is a long, flat bone located at the anterior of the chest. It is formed from three
bones that fuse in the adult. The ribs are 12 pairs of long, curved bones that attach to the thoracic vertebrae and
curve toward the front of the body, forming the ribcage. Costal cartilages connect the anterior ends of the ribs to
the sternum, with the exception of rib pairs 11 and 12, which are free-floating ribs.
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Chapter 38 | The Musculoskeletal System
Costal Cartilage
Sternum
Ribs
Thoracic Cage
Figure 38.9 The thoracic cage, or rib cage, protects the heart and the lungs, (credit: modification of work by NCI, NIH)
Human Appendicular Skeleton
The appendicular skeleton is composed of the bones of the upper limbs (which function to grasp and
manipulate objects) and the lower limbs (which permit locomotion). It also includes the pectoral girdle, or
shoulder girdle, that attaches the upper limbs to the body, and the pelvic girdle that attaches the lower limbs to
the body (Figure 38.10).
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Shoulder girdle
Arm —
Hand -
Figure 38.10 The appendicular skeleton is composed of the bones of the pectoral limbs (arm, forearm, hand), the
pelvic limbs (thigh, leg, foot), the pectoral girdle, and the pelvic girdle, (credit: modification of work by Mariana Ruiz
Villareal)
The Pectoral Girdle
The pectoral girdle bones provide the points of attachment of the upper limbs to the axial skeleton. The human
pectoral girdle consists of the clavicle (or collarbone) in the anterior, and the scapula (or shoulder blades) in the
posterior (Figure 38.11).
Spine of
scapula
Scapula
(b)
Figure 38.11 (a) The pectoral girdle in primates consists of the clavicles and scapulae, (b) The posterior view reveals
the spine of the scapula to which muscle attaches.
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The clavicles are S-shaped bones that position the arms on the body. The clavicles lie horizontally across the
front of the thorax (chest) just above the first rib. These bones are fairly fragile and are susceptible to fractures.
For example, a fall with the arms outstretched causes the force to be transmitted to the clavicles, which can
break if the force is excessive. The clavicle articulates with the sternum and the scapula.
The scapulae are flat, triangular bones that are located at the back of the pectoral girdle. They support the
muscles crossing the shoulder joint. A ridge, called the spine, runs across the back of the scapula and can easily
be felt through the skin (Figure 38.11). The spine of the scapula is a good example of a bony protrusion that
facilitates a broad area of attachment for muscles to bone.
The Upper Limb
The upper limb contains 30 bones in three regions: the arm (shoulder to elbow), the forearm (ulna and radius),
and the wrist and hand (Figure 38.12).
Figure 38.12 The upper limb consists of the humerus of the upper arm, the radius and ulna of the forearm, eight bones
of the carpus, five bones of the metacarpus, and 14 bones of the phalanges.
An articulation is any place at which two bones are joined. The humerus is the largest and longest bone of the
upper limb and the only bone of the arm. it articulates with the scapula at the shoulder and with the forearm at
the elbow. The forearm extends from the elbow to the wrist and consists of two bones: the ulna and the radius.
The radius is located along the lateral (thumb) side of the forearm and articulates with the humerus at the elbow.
The ulna is located on the medial aspect (pinky-finger side) of the forearm. It is longer than the radius. The ulna
articulates with the humerus at the elbow. The radius and ulna also articulate with the carpal bones and with
each other, which in vertebrates enables a variable degree of rotation of the carpus with respect to the long axis
of the limb. The hand includes the eight bones of the carpus (wrist), the five bones of the metacarpus (palm),
and the 14 bones of the phalanges (digits). Each digit consists of three phalanges, except for the thumb, when
present, which has only two.
The Pelvic Girdle
The pelvic girdle attaches to the lower limbs of the axial skeleton. Because it is responsible for bearing the
weight of the body and for locomotion, the pelvic girdle is securely attached to the axial skeleton by strong
ligaments. It also has deep sockets with robust ligaments to securely attach the femur to the body. The pelvic
girdle is further strengthened by two large hip bones. In adults, the hip bones, or coxal bones, are formed by
the fusion of three pairs of bones: the ilium, ischium, and pubis. The pelvis joins together in the anterior of the
body at a joint called the pubic symphysis and with the bones of the sacrum at the posterior of the body.
The female pelvis is slightly different from the male pelvis. Over generations of evolution, females with a wider
pubic angle and larger diameter pelvic canal reproduced more successfully. Therefore, their offspring also had
pelvic anatomy that enabled successful childbirth (Figure 38.13).
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Chapter 38 | The Musculoskeletal System
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Figure 38.13 To adapt to reproductive fitness, the (a) female pelvis is lighter, wider, shallower, and has a broader angle
between the pubic bones than (b) the male pelvis.
The Lower Limb
The lower limb consists of the thigh, the leg, and the foot. The bones of the lower limb are the femur (thigh
bone), patella (kneecap), tibia and fibula (bones of the leg), tarsals (bones of the ankle), and metatarsals and
phalanges (bones of the foot) (Figure 38.14). The bones of the lower limbs are thicker and stronger than the
bones of the upper limbs because of the need to support the entire weight of the body and the resulting forces
from locomotion, in addition to evolutionary fitness, the bones of an individual will respond to forces exerted
upon them.
Femur
Patella
Tibia
Fibula
Tarsals
Metatarsals
Phalanges
Figure 38.14 The lower limb consists of the thigh (femur), kneecap (patella), leg (tibia and fibula), ankle (tarsals), and
foot (metatarsals and phalanges) bones.
The femur, or thighbone, is the longest, heaviest, and strongest bone in the body. The femur and pelvis form
the hip joint at the proximal end. At the distal end, the femur, tibia, and patella form the knee joint. The patella,
or kneecap, is a triangular bone that lies anterior to the knee joint. The patella is embedded in the tendon of
the femoral extensors (quadriceps). It improves knee extension by reducing friction. The tibia, or shinbone, is a
large bone of the leg that is located directly below the knee. The tibia articulates with the femur at its proximal
end, with the fibula and the tarsal bones at its distal end. It is the second largest bone in the human body and is
responsible for transmitting the weight of the body from the femur to the foot. The fibula, or calf bone, parallels
and articulates with the tibia. It does not articulate with the femur and does not bear weight. The fibula acts as a
site for muscle attachment and forms the lateral part of the ankle joint.
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The tarsals are the seven bones of the ankle. The ankle transmits the weight of the body from the tibia and the
fibula to the foot. The metatarsals are the five bones of the foot. The phalanges are the 14 bones of the toes.
Each toe consists of three phalanges, except for the big toe that has only two (Figure 38.15). Variations exist
in other species; for example, the horse’s metacarpals and metatarsals are oriented vertically and do not make
contact with the substrate.
Figure 38.15 This drawing shows the bones of the human foot and ankle, including the metatarsals and the phalanges.
e olution CONNECTION
Evolution of Body Design for Locomotion on Land
The transition of vertebrates onto land required a number of changes in body design, as movement on
land presents a number of challenges for animals that are adapted to movement in water. The buoyancy
of water provides a certain amount of lift, and a common form of movement by fish is lateral undulations
of the entire body. This back and forth movement pushes the body against the water, creating forward
movement. In most fish, the muscles of paired fins attach to girdles within the body, allowing for some
control of locomotion. As certain fish began moving onto land, they retained their lateral undulation form of
locomotion (anguilliform). However, instead of pushing against water, their fins or flippers became points of
contact with the ground, around which they rotated their bodies.
The effect of gravity and the lack of buoyancy on land meant that body weight was suspended on the limbs,
leading to increased strengthening and ossification of the limbs. The effect of gravity also required changes
to the axial skeleton. Lateral undulations of land animal vertebral columns cause torsional strain. A firmer,
more ossified vertebral column became common in terrestrial tetrapods because it reduces strain while
providing the strength needed to support the body’s weight. In later tetrapods, the vertebrae began allowing
for vertical motion rather than lateral flexion. Another change in the axial skeleton was the loss of a direct
attachment between the pectoral girdle and the head. This reduced the jarring to the head caused by the
impact of the limbs on the ground. The vertebrae of the neck also evolved to allow movement of the head
independently of the body.
The appendicular skeleton of land animals is also different from aquatic animals. The shoulders attach to the
pectoral girdle through muscles and connective tissue, thus reducing the jarring of the skull. Because of a
lateral undulating vertebral column, in early tetrapods, the limbs were splayed out to the side and movement
occurred by performing “push-ups.” The vertebrae of these animals had to move side-to-side in a similar
manner to fish and reptiles. This type of motion requires large muscles to move the limbs toward the midline;
it was almost like walking while doing push-ups, and it is not an efficient use of energy. Later tetrapods have
their limbs placed under their bodies, so that each stride requires less force to move forward. This resulted
in decreased adductor muscle size and an increased range of motion of the scapulae. This also restricts
movement primarily to one plane, creating forward motion rather than moving the limbs upward as well as
forward. The femur and humerus were also rotated, so that the ends of the limbs and digits were pointed
forward, in the direction of motion, rather than out to the side. By placement underneath the body, limbs can
swing forward like a pendulum to produce a stride that is more efficient for moving over land.
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Chapter 38 | The Musculoskeletal System
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38.2 | Bone
By the end of this section, you will be able to do the following:
• Classify the different types of bones in the skeleton
• Explain the role of the different cell types in bone
• Explain how bone forms during development
Bone, or osseous tissue, is a connective tissue that constitutes the endoskeleton. It contains specialized cells
and a matrix of mineral salts and collagen fibers.
The mineral salts primarily include hydroxyapatite, a mineral formed from calcium phosphate. Calcification is
the process of deposition of mineral salts on the collagen fiber matrix that crystallizes and hardens the tissue.
The process of calcification only occurs in the presence of collagen fibers.
The bones of the human skeleton are classified by their shape: long bones, short bones, flat bones, sutural
bones, sesamoid bones, and irregular bones (Figure 38.16).
Flat bone Irregular bone
Figure 38.16 Shown are different types of bones: flat, irregular, long, short, and sesamoid.
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Long bones are longer than they are wide and have a shaft and two ends. The diaphysis, or central shaft,
contains bone marrow in a marrow cavity. The rounded ends, the epiphyses, are covered with articular cartilage
and are filled with red bone marrow, which produces blood cells (Figure 38.17). Most of the limb bones are long
bones—for example, the femur, tibia, ulna, and radius. Exceptions to this include the patella and the bones of
the wrist and ankle.
Long Bone
Articular cartilage
Spongy bone
Medullary cavity
Articular cartilage
Figure 38.17 The long bone is covered by articular cartilage at either end and contains bone marrow (shown in yellow
in this illustration) in the marrow cavity.
Short bones, or cuboidal bones, are bones that are the same width and length, giving them a cube-like shape.
For example, the bones of the wrist (carpals) and ankle (tarsals) are short bones (Figure 38.16).
Flat bones are thin and relatively broad bones that are found where extensive protection of organs is required or
where broad surfaces of muscle attachment are required. Examples of flat bones are the sternum (breast bone),
ribs, scapulae (shoulder blades), and the roof of the skull (Figure 38.16).
Irregular bones are bones with complex shapes. These bones may have short, flat, notched, or ridged surfaces.
Examples of irregular bones are the vertebrae, hip bones, and several skull bones.
Sesamoid bones are small, flat bones and are shaped similarly to a sesame seed. The patellae are sesamoid
bones (Figure 38.18). Sesamoid bones develop inside tendons and may be found near joints at the knees,
hands, and feet.
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Figure 38.18 The patella of the knee is an example of a sesamoid bone.
Sutural bones are small, flat, irregularly shaped bones. They may be found between the flat bones of the skull.
They vary in number, shape, size, and position.
Bone Tissue
Bones are considered organs because they contain various types of tissue, such as blood, connective tissue,
nerves, and bone tissue. Osteocytes, the living cells of bone tissue, form the mineral matrix of bones. There are
two types of bone tissue: compact and spongy.
Compact Bone Tissue
Compact bone (or cortical bone) forms the hard external layer of all bones and surrounds the medullary
cavity, or bone marrow. It provides protection and strength to bones. Compact bone tissue consists of units
called osteons or Haversian systems. Osteons are cylindrical structures that contain a mineral matrix and living
osteocytes connected by canaliculi, which transport blood. They are aligned parallel to the long axis of the bone.
Each osteon consists of lamellae, which are layers of compact matrix that surround a central canal called the
Haversian canal. The Haversian canal (osteonic canal) contains the bone’s blood vessels and nerve fibers
(Figure 38.19). Osteons in compact bone tissue are aligned in the same direction along lines of stress and help
the bone resist bending or fracturing. Therefore, compact bone tissue is prominent in areas of bone at which
stresses are applied in only a few directions.
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CONNECTION
Lamellae
Osteon of
Trabeculae of
' bone
Oste
Haversian
canal
Figure 38.19 Compact bone tissue consists of osteons that are aligned parallel to the long axis of the bone, and
the Haversian canal that contains the bone’s blood vessels and nerve fibers. The inner layer of bones consists
of spongy bone tissue. The small dark ovals in the osteon represent the living osteocytes. (credit: modification of
work by NCI, NIH)
Which of the following statements about bone tissue is false?
a. Compact bone tissue is made of cylindrical osteons that are aligned such that they travel the length of
the bone.
b. Haversian canals contain blood vessels only.
c. Haversian canals contain blood vessels and nerve fibers.
d. Spongy tissue is found on the interior of the bone, and compact bone tissue is found on the exterior.
Spongy Bone Tissue
Whereas compact bone tissue forms the outer layer of all bones, spongy bone or cancellous bone forms the
inner layer of all bones. Spongy bone tissue does not contain osteons that constitute compact bone tissue.
Instead, it consists of trabeculae, which are lamellae that are arranged as rods or plates. Red bone marrow is
found between the trabuculae. Blood vessels within this tissue deliver nutrients to osteocytes and remove waste.
The red bone marrow of the femur and the interior of other large bones, such as the ileum, forms blood cells.
Spongy bone reduces the density of bone and allows the ends of long bones to compress as the result of
stresses applied to the bone. Spongy bone is prominent in areas of bones that are not heavily stressed or where
stresses arrive from many directions. The epiphyses of bones, such as the neck of the femur, are subject to
stress from many directions, imagine laying a heavy framed picture flat on the floor. You could hold up one side
of the picture with a toothpick if the toothpick was perpendicular to the floor and the picture. Now drill a hole and
stick the toothpick into the wall to hang up the picture. In this case, the function of the toothpick is to transmit
the downward pressure of the picture to the wall. The force on the picture is straight down to the floor, but the
force on the toothpick is both the picture wire pulling down and the bottom of the hole in the wall pushing up. The
toothpick will break off right at the wall.
The neck of the femur is horizontal like the toothpick in the wall. The weight of the body pushes it down near the
joint, but the vertical diaphysis of the femur pushes it up at the other end. The neck of the femur must be strong
enough to transfer the downward force of the body weight horizontally to the vertical shaft of the femur (Figure
38.20).
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Figure 38.20 Trabeculae in spongy bone are arranged such that one side of the bone bears tension and the other
withstands compression.
View micrographs (http:// 0 penstaxc 0 llege. 0 rg/l/muscle_tissue) of musculoskeletal tissues as you review
the anatomy.
Cell Types in Bones
Bone consists of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells.
Osteoblasts are bone cells that are responsible for bone formation. Osteoblasts synthesize and secrete the
organic part and inorganic part of the extracellular matrix of bone tissue, and collagen fibers. Osteoblasts
become trapped in these secretions and differentiate into less active osteocytes. Osteoclasts are large bone
cells with up to 50 nuclei. They remove bone structure by releasing lysosomal enzymes and acids that dissolve
the bony matrix. These minerals, released from bones into the blood, help regulate calcium concentrations in
body fluids. Bone may also be resorbed for remodeling, if the applied stresses have changed. Osteocytes
are mature bone cells and are the main cells in bony connective tissue; these cells cannot divide. Osteocytes
maintain normal bone structure by recycling the mineral salts in the bony matrix. Osteoprogenitor cells are
squamous stem cells that divide to produce daughter cells that differentiate into osteoblasts. Osteoprogenitor
cells are important in the repair of fractures.
Development of Bone
Ossification, or osteogenesis, is the process of bone formation by osteoblasts. Ossification is distinct from
the process of calcification; whereas calcification takes place during the ossification of bones, it can also occur
in other tissues. Ossification begins approximately six weeks after fertilization in an embryo. Before this time,
the embryonic skeleton consists entirely of fibrous membranes and hyaline cartilage. The development of bone
from fibrous membranes is called intramembranous ossification; development from hyaline cartilage is called
endochondral ossification. Bone growth continues until approximately age 25. Bones can grow in thickness
throughout life, but after age 25, ossification functions primarily in bone remodeling and repair.
Intramembranous Ossification
Intramembranous ossification is the process of bone development from fibrous membranes. It is involved in
the formation of the flat bones of the skull, the mandible, and the clavicles. Ossification begins as mesenchymal
cells form a template of the future bone. They then differentiate into osteoblasts at the ossification center.
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Osteoblasts secrete the extracellular matrix and deposit calcium, which hardens the matrix. The non-mineralized
portion of the bone or osteoid continues to form around blood vessels, forming spongy bone. Connective tissue
in the matrix differentiates into red bone marrow in the fetus. The spongy bone is remodeled into a thin layer of
compact bone on the surface of the spongy bone.
Endochondral Ossification
Endochondral ossification is the process of bone development from hyaline cartilage. All of the bones of
the body, except for the flat bones of the skull, mandible, and clavicles, are formed through endochondral
ossification.
In long bones, chondrocytes form a template of the hyaline cartilage diaphysis. Responding to complex
developmental signals, the matrix begins to calcify. This calcification prevents diffusion of nutrients into the
matrix, resulting in chondrocytes dying and the opening up of cavities in the diaphysis cartilage. Blood vessels
invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone.
Osteoclasts then break down some of the spongy bone to create a marrow, or medullary, cavity in the center of
the diaphysis. Dense, irregular connective tissue forms a sheath (periosteum) around the bones. The periosteum
assists in attaching the bone to surrounding tissues, tendons, and ligaments. The bone continues to grow and
elongate as the cartilage cells at the epiphyses divide.
In the last stage of prenatal bone development, the centers of the epiphyses begin to calcify. Secondary
ossification centers form in the epiphyses as blood vessels and osteoblasts enter these areas and convert
hyaline cartilage into spongy bone. Until adolescence, hyaline cartilage persists at the epiphyseal plate (growth
plate), which is the region between the diaphysis and epiphysis that is responsible for the lengthwise growth of
long bones (Figure 38.21).
Primary
Hyaline ossification
Periosteum
(covers
compact
bone)
Secondary
ossification
Medullary
cavity
Artery and vein
(provide nutrients
to bone)
Artery and vein
(provide nutrients
to bone)
Figure 38.21 Endochondral ossification is the process of bone development from hyaline cartilage. The periosteum
is the connective tissue on the outside of bone that acts as the interface between bone, blood vessels, tendons, and
ligaments.
Growth of Bone
Long bones continue to lengthen, potentially until adolescence, through the addition of bone tissue at the
epiphyseal plate. They also increase in width through appositional growth.
Lengthening of Long Bones
Chondrocytes on the epiphyseal side of the epiphyseal plate divide; one cell remains undifferentiated near the
epiphysis, and one cell moves toward the diaphysis. The cells, which are pushed from the epiphysis, mature
and are destroyed by calcification. This process replaces cartilage with bone on the diaphyseal side of the plate,
resulting in a lengthening of the bone.
Long bones stop growing at around the age of 18 in females and the age of 21 in males in a process called
epiphyseal plate closure. During this process, cartilage cells stop dividing and all of the cartilage is replaced by
bone. The epiphyseal plate fades, leaving a structure called the epiphyseal line or epiphyseal remnant, and the
epiphysis and diaphysis fuse.
Thickening of Long Bones
Appositional growth is the increase in the diameter of bones by the addition of bony tissue at the surface of
bones. Osteoblasts at the bone surface secrete bone matrix, and osteoclasts on the inner surface break down
bone. The osteoblasts differentiate into osteocytes. A balance between these two processes allows the bone to
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thicken without becoming too heavy.
Bone Remodeling and Repair
Bone renewal continues after birth into adulthood. Bone remodeling is the replacement of old bone tissue
by new bone tissue. It involves the processes of bone deposition by osteoblasts and bone resorption by
osteoclasts. Normal bone growth requires vitamins D, C, and A, plus minerals such as calcium, phosphorous,
and magnesium. Hormones such as parathyroid hormone, growth hormone, and calcitonin are also required for
proper bone growth and maintenance.
Bone turnover rates are quite high, with five to seven percent of bone mass being recycled every week.
Differences in turnover rate exist in different areas of the skeleton and in different areas of a bone. For example,
the bone in the head of the femur may be fully replaced every six months, whereas the bone along the shaft is
altered much more slowly.
Bone remodeling allows bones to adapt to stresses by becoming thicker and stronger when subjected to stress.
Bones that are not subject to normal stress, for example when a limb is in a cast, will begin to lose mass. A
fractured or broken bone undergoes repair through four stages:
1. Blood vessels in the broken bone tear and hemorrhage, resulting in the formation of clotted blood, or a
hematoma, at the site of the break. The severed blood vessels at the broken ends of the bone are sealed
by the clotting process, and bone cells that are deprived of nutrients begin to die.
2. Within days of the fracture, capillaries grow into the hematoma, and phagocytic cells begin to clear away
the dead cells. Though fragments of the blood clot may remain, fibroblasts and osteoblasts enter the area
and begin to reform bone. Fibroblasts produce collagen fibers that connect the broken bone ends, and
osteoblasts start to form spongy bone. The repair tissue between the broken bone ends is called the
fibrocartilaginous callus, as it is composed of both hyaline and fibrocartilage (Figure 38.22). Some bone
spicules may also appear at this point.
3. The fibrocartilaginous callus is converted into a bony callus of spongy bone. It takes about two months for
the broken bone ends to be firmly joined together after the fracture. This is similar to the endochondral
formation of bone, as cartilage becomes ossified; osteoblasts, osteoclasts, and bone matrix are present.
4. The bony callus is then remodelled by osteoclasts and osteoblasts, with excess material on the exterior of
the bone and within the medullary cavity being removed. Compact bone is added to create bone tissue that
is similar to the original, unbroken bone. This remodeling can take many months, and the bone may remain
uneven for years.
Figure 38.22 After this bone is set, a callus will knit the two ends together, (credit: Bill Rhodes)
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scientific methf d CONNECTION
Decalcification of Bones
Question: What effect does the removal of calcium and collagen have on bone structure?
Background: Conduct a literature search on the role of calcium and collagen in maintaining bone structure.
Conduct a literature search on diseases in which bone structure is compromised.
Hypothesis: Develop a hypothesis that states predictions of the flexibility, strength, and mass of bones that
have had the calcium and collagen components removed. Develop a hypothesis regarding the attempt to
add calcium back to decalcified bones.
Test the hypothesis: Test the prediction by removing calcium from chicken bones by placing them in ajar
of vinegar for seven days. Test the hypothesis regarding adding calcium back to decalcified bone by placing
the decalcified chicken bones into a jar of water with calcium supplements added. Test the prediction by
denaturing the collagen from the bones by baking them at 250°C for three hours.
Analyze the data: Create a table showing the changes in bone flexibility, strength, and mass in the three
different environments.
Report the results: Under which conditions was the bone most flexible? Under which conditions was the
bone the strongest?
Draw a conclusion: Did the results support or refute the hypothesis? How do the results observed in this
experiment correspond to diseases that destroy bone tissue?
38.3 | Joints and Skeletal Movement
By the end of this section, you will be able to do the following:
• Classify the different types of joints on the basis of structure
• Explain the role of joints in skeletal movement
The point at which two or more bones meet is called a joint, or articulation. Joints are responsible for
movement, such as the movement of limbs, and stability, such as the stability found in the bones of the skull.
Classification of Joints on the Basis of Structure
There are two ways to classify joints: on the basis of their structure or on the basis of their function. The
structural classification divides joints into bony, fibrous, cartilaginous, and synovial joints depending on the
material composing the joint and the presence or absence of a cavity in the joint.
Fibrous Joints
The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity, or space, present
between the bones and so most fibrous joints do not move at all, or are only capable of minor movements. There
are three types of fibrous joints: sutures, syndesmoses, and gomphoses. Sutures are found only in the skull and
possess short fibers of connective tissue that hold the skull bones tightly in place (Figure 38.23).
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Figure 38.23 Sutures are fibrous joints found only in the skull.
Syndesmoses are joints in which the bones are connected by a band of connective tissue, allowing for more
movement than in a suture. An example of a syndesmosis is the joint of the tibia and fibula in the ankle.
The amount of movement in these types of joints is determined by the length of the connective tissue fibers.
Gomphoses occur between teeth and their sockets; the term refers to the way the tooth fits into the socket like
a peg (Figure 38.24). The tooth is connected to the socket by a connective tissue referred to as the periodontal
ligament.
Figure 38.24 Gomphoses are fibrous joints between the teeth and their sockets, (credit: modification of work by Gray's
Anatomy)
Cartilaginous Joints
Cartilaginous joints are joints in which the bones are connected by cartilage. There are two types of
cartilaginous joints: synchondroses and symphyses. In a synchondrosis, the bones are joined by hyaline
cartilage. Synchondroses are found in the epiphyseal plates of growing bones in children. In symphyses,
hyaline cartilage covers the end of the bone but the connection between bones occurs through fibrocartilage.
Symphyses are found at the joints between vertebrae. Either type of cartilaginous joint allows for very little
movement.
Synovial Joints
Synovial joints are the only joints that have a space between the adjoining bones (Figure 38.25). This space
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Chapter 38 | The Musculoskeletal System
is referred to as the synovial (or joint) cavity and is filled with synovial fluid. Synovial fluid lubricates the joint,
reducing friction between the bones and allowing for greater movement. The ends of the bones are covered
with articular cartilage, a hyaline cartilage, and the entire joint is surrounded by an articular capsule composed
of connective tissue that allows movement of the joint while resisting dislocation. Articular capsules may also
possess ligaments that hold the bones together. Synovial joints are capable of the greatest movement of the
three structural joint types; however, the more mobile a joint, the weaker the joint. Knees, elbows, and shoulders
are examples of synovial joints.
Bone-
'rSt*
n r s
Figure 38.25 Synovial joints are the only joints that have a space or “synovial cavity” in the joint.
Classification of Joints on the Basis of Function
The functional classification divides joints into three categories: synarthroses, amphiarthroses, and diarthroses.
A synarthrosis is a joint that is immovable. This includes sutures, gomphoses, and synchondroses.
Amphiarthroses are joints that allow slight movement, including syndesmoses and symphyses. Diarthroses
are joints that allow for free movement of the joint, as in synovial joints.
Movement at Synovial Joints
The wide range of movement allowed by synovial joints produces different types of movements. The movement
of synovial joints can be classified as one of four different types: gliding, angular, rotational, or special movement.
Gliding Movement
Gliding movements occur as relatively flat bone surfaces move past each other. Gliding movements produce
very little rotation or angular movement of the bones. The joints of the carpal and tarsal bones are examples of
joints that produce gliding movements.
Angular Movement
Angular movements are produced when the angle between the bones of a joint changes. There are several
different types of angular movements, including flexion, extension, hyperextension, abduction, adduction, and
circumduction. Flexion, or bending, occurs when the angle between the bones decreases. Moving the forearm
upward at the elbow or moving the wrist to move the hand toward the forearm are examples of flexion.
Extension is the opposite of flexion in that the angle between the bones of a joint increases. Straightening a
limb after flexion is an example of extension. Extension past the regular anatomical position is referred to as
hyperextension. This includes moving the neck back to look upward, or bending the wrist so that the hand
moves away from the forearm.
Abduction occurs when a bone moves away from the midline of the body. Examples of abduction are moving
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the arms or legs laterally to lift them straight out to the side. Adduction is the movement of a bone toward the
midline of the body. Movement of the limbs inward after abduction is an example of adduction. Circumduction
is the movement of a limb in a circular motion, as in moving the arm in a circular motion.
Rotational Movement
Rotational movement is the movement of a bone as it rotates around its longitudinal axis. Rotation can be
toward the midline of the body, which is referred to as medial rotation, or away from the midline of the body,
which is referred to as lateral rotation. Movement of the head from side to side is an example of rotation.
Special Movements
Some movements that cannot be classified as gliding, angular, or rotational are called special movements.
Inversion involves the soles of the feet moving inward, toward the midline of the body. Eversion is the opposite
of inversion, movement of the sole of the foot outward, away from the midline of the body. Protraction is the
anterior movement of a bone in the horizontal plane. Retraction occurs as a joint moves back into position
after protraction. Protraction and retraction can be seen in the movement of the mandible as the jaw is thrust
outwards and then back inwards. Elevation is the movement of a bone upward, such as when the shoulders are
shrugged, lifting the scapulae. Depression is the opposite of elevation—movement downward of a bone, such
as after the shoulders are shrugged and the scapulae return to their normal position from an elevated position.
Dorsiflexion is a bending at the ankle such that the toes are lifted toward the knee. Plantar flexion is a bending
at the ankle when the heel is lifted, such as when standing on the toes. Supination is the movement of the
radius and ulna bones of the forearm so that the palm faces forward. Pronation is the opposite movement, in
which the palm faces backward. Opposition is the movement of the thumb toward the fingers of the same hand,
making it possible to grasp and hold objects.
Types of Synovial Joints
Synovial joints are further classified into six different categories on the basis of the shape and structure of the
joint. The shape of the joint affects the type of movement permitted by the joint (Figure 38.26). These joints can
be described as planar, hinge, pivot, condyloid, saddle, or ball-and-socket joints.
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Condyloid joint
(between radius and
carpal bones of wrist)
(c) Saddle joint
(between trapezium
carpal bone and 1st
metacarpal bone)
(d) Plane joint
(between tarsal bones)
(a) Pivot joint
(between Cl and
C2 vertebrae)
(f) Ball-and-socket joint
(hip joint)
(b) Hinge joint
(elbow)
Figure 38.26 Different types of joints allow different types of movement. Planar, hinge, pivot, condyloid, saddle, and
ball-and-socket are all types of synovial joints.
Planar Joints
Planar joints have bones with articulating surfaces that are flat or slightly curved faces. These joints allow for
gliding movements, and so the joints are sometimes referred to as gliding joints. The range of motion is limited in
these joints and does not involve rotation. Planar joints are found in the carpal bones in the hand and the tarsal
bones of the foot, as well as between vertebrae (Figure 38.27).
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Figure 38.27 The joints of the carpal bones in the wrist are examples of planar joints, (credit: modification of work by
Brian C. Goss)
Hinge Joints
In hinge joints, the slightly rounded end of one bone fits into the slightly hollow end of the other bone. In this
way, one bone moves while the other remains stationary, like the hinge of a door. The elbow is an example of a
hinge joint. The knee is sometimes classified as a modified hinge joint (Figure 38.28).
Ulna Elbow joint
Radius Humerus
Figure 38.28 The elbow joint, where the radius articulates with the humerus, is an example of a hinge joint, (credit:
modification of work by Brian C. Goss)
Pivot Joints
Pivot joints consist of the rounded end of one bone fitting into a ring formed by the other bone. This structure
allows rotational movement, as the rounded bone moves around its own axis. An example of a pivot joint is the
joint of the first and second vertebrae of the neck that allows the head to move back and forth (Figure 38.29).
The joint of the wrist that allows the palm of the hand to be turned up and down is also a pivot joint.
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Figure 38.29 The joint in the neck that allows the head to move back and forth is an example of a pivot joint.
Condyloid Joints
Condyloid joints consist of an oval-shaped end of one bone fitting into a similarly oval-shaped hollow of
another bone (Figure 38.30). This is also sometimes called an ellipsoidal joint. This type of joint allows angular
movement along two axes, as seen in the joints of the wrist and fingers, which can move both side to side and
up and down.
Metacarpal
Metacarpophalangeal
joint
Proximal phalange
Intermediate phalange
Distal phalanges
Figure 38.30 The metacarpophalangeal joints in the finger are examples of condyloid joints, (credit: modification of
work by Gray's Anatomy)
Saddle Joints
Saddle joints are so named because the ends of each bone resemble a saddle, with concave and convex
portions that fit together. Saddle joints allow angular movements similar to condyloid joints but with a greater
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Chapter 38 | The Musculoskeletal System
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range of motion. An example of a saddle joint is the thumb joint, which can move back and forth and up and
down, but more freely than the wrist or fingers (Figure 38.31).
Figure 38.31 The carpometacarpal joints in the thumb are examples of saddle joints, (credit: modification of work by
Brian C. Goss)
Ball-and-Socket Joints
Ball-and-socket joints possess a rounded, ball-like end of one bone fitting into a cuplike socket of another
bone. This organization allows the greatest range of motion, as all movement types are possible in all directions.
Examples of ball-and-socket joints are the shoulder and hip joints (Figure 38.32).
Clavicle
Scapula
Humerus
Figure 38.32 The shoulder joint is an example of a ball-and-socket joint.
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LINK
T &
LEARNING
Watch this animation showing the six types of synovial joints. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66639/l.3/#eip-idl511301)
ca eer connection
Rheumatologist
Rheumatologists are medical doctors who specialize in the diagnosis and treatment of disorders of the
joints, muscles, and bones. They diagnose and treat diseases such as arthritis, musculoskeletal disorders,
osteoporosis, and autoimmune diseases such as ankylosing spondylitis and rheumatoid arthritis.
Rheumatoid arthritis (RA) is an inflammatory disorder that primarily affects the synovial joints of the hands,
feet, and cervical spine. Affected joints become swollen, stiff, and painful. Although it is known that RA is an
autoimmune disease in which the body’s immune system mistakenly attacks healthy tissue, the cause of RA
remains unknown. Immune cells from the blood enter joints and the synovium causing cartilage breakdown,
swelling, and inflammation of the joint lining. Breakdown of cartilage causes bones to rub against each other
causing pain. RA is more common in women than men and the age of onset is usually 40-50 years of age.
Rheumatologists can diagnose RA on the basis of symptoms such as joint inflammation and pain, X-ray and
MRI imaging, and blood tests. Arthrography is a type of medical imaging of joints that uses a contrast agent,
such as a dye, that is opaque to X-rays. This allows the soft tissue structures of joints—such as cartilage,
tendons, and ligaments—to be visualized. An arthrogram differs from a regular X-ray by showing the surface
of soft tissues lining the joint in addition to joint bones. An arthrogram allows early degenerative changes in
joint cartilage to be detected before bones become affected.
There is currently no cure for RA; however, rheumatologists have a number of treatment options available.
Early stages can be treated with rest of the affected joints by using a cane or by using joint splints that
minimize inflammation. When inflammation has decreased, exercise can be used to strengthen the muscles
that surround the joint and to maintain joint flexibility. If joint damage is more extensive, medications can
be used to relieve pain and decrease inflammation. Anti-inflammatory drugs such as aspirin, topical pain
relievers, and corticosteroid injections may be used. Surgery may be required in cases in which joint
damage is severe.
38.4 | Muscle Contraction and Locomotion
By the end of this section, you will be able to do the following:
• Classify the different types of muscle tissue
• Explain the role of muscles in locomotion
Muscle cells are specialized for contraction. Muscles allow for motions such as walking, and they also facilitate
bodily processes such as respiration and digestion. The body contains three types of muscle tissue: skeletal
muscle, cardiac muscle, and smooth muscle (Figure 38.33).
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Skeletal muscle Smooth muscle Cardiac muscle
Figure 38.33 The body contains three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle,
visualized here using light microscopy. Smooth muscle cells are short, tapered at each end, and have only one plump
nucleus in each. Cardiac muscle cells are branched and striated, but short. The cytoplasm may branch, and they have
one nucleus in the center of the cell, (credit: modification of work by NCI, NIH; scale-bar data from Matt Russell)
Skeletal muscle tissue forms skeletal muscles, which attach to bones or skin and control locomotion and
any movement that can be consciously controlled. Because it can be controlled by thought, skeletal muscle is
also called voluntary muscle. Skeletal muscles are long and cylindrical in appearance; when viewed under a
microscope, skeletal muscle tissue has a striped or striated appearance. The striations are caused by the regular
arrangement of contractile proteins (actin and myosin). Actin is a globular contractile protein that interacts with
myosin for muscle contraction. Skeletal muscle also has multiple nuclei present in a single cell.
Smooth muscle tissue occurs in the walls of hollow organs such as the intestines, stomach, and urinary
bladder, and around passages such as the respiratory tract and blood vessels. Smooth muscle has no striations,
is not under voluntary control, has only one nucleus per cell, is tapered at both ends, and is called involuntary
muscle.
Cardiac muscle tissue is only found in the heart, and cardiac contractions pump blood throughout the body
and maintain blood pressure. Like skeletal muscle, cardiac muscle is striated, but unlike skeletal muscle, cardiac
muscle cannot be consciously controlled and is called involuntary muscle. It has one nucleus per cell, is
branched, and is distinguished by the presence of intercalated disks.
Skeletal Muscle Fiber Structure
Each skeletal muscle fiber is a skeletal muscle cell. These cells are incredibly large, with diameters of up to 100
pm and lengths of up to 30 cm. The plasma membrane of a skeletal muscle fiber is called the sarcolemma.
The sarcolemma is the site of action potential conduction, which triggers muscle contraction. Within each muscle
fiber are myofibrils —long cylindrical structures that lie parallel to the muscle fiber. Myofibrils run the entire
length of the muscle fiber, and because they are only approximately 1.2 pm in diameter, hundreds to thousands
can be found inside one muscle fiber. They attach to the sarcolemma at their ends, so that as myofibrils shorten,
the entire muscle cell contracts (Figure 38.34).
Figure 38.34 A skeletal muscle cell is surrounded by a plasma membrane called the sarcolemma with a cytoplasm
called the sarcoplasm. A muscle fiber is composed of many fibrils, packaged into orderly units.
The striated appearance of skeletal muscle tissue is a result of repeating bands of the proteins actin and myosin
that are present along the length of myofibrils. Dark A bands and light I bands repeat along myofibrils, and the
alignment of myofibrils in the cell causes the entire cell to appear striated or banded.
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Each I band has a dense line running vertically through the middle called a Z disc or Z line. The Z discs mark
the border of units called sarcomeres, which are the functional units of skeletal muscle. One sarcomere is the
space between two consecutive Z discs and contains one entire A band and two halves of an 1 band, one on
either side of the A band. A myofibril is composed of many sarcomeres running along its length, and as the
sarcomeres individually contract, the myofibrils and muscle cells shorten (Figure 38.35).
Z line Sarcomere
Thin filament Thick filament
< - >
Myofibril
Figure 38.35 A sarcomere is the region from one Z line to the next Z line. Many sarcomeres are present in a myofibril,
resulting in the striation pattern characteristic of skeletal muscle.
Myofibrils are composed of smaller structures called myofilaments. There are two main types of filaments: thick
filaments and thin filaments; each has different compositions and locations. Thick filaments occur only in the
A band of a myofibril. Thin filaments attach to a protein in the Z disc called alpha-actinin and occur across the
entire length of the I band and partway into the A band. The region at which thick and thin filaments overlap has
a dense appearance, as there is little space between the filaments. Thin filaments do not extend all the way into
the A bands, leaving a central region of the A band that only contains thick filaments. This central region of the
A band looks slightly lighter than the rest of the A band and is called the H zone. The middle of the H zone has a
vertical line called the M line, at which accessory proteins hold together thick filaments. Both the Z disc and the
M line hold myofilaments in place to maintain the structural arrangement and layering of the myofibril. Myofibrils
are connected to each other by intermediate, or desmin, filaments that attach to the Z disc.
Thick and thin filaments are themselves composed of proteins. Thick filaments are composed of the protein
myosin. The tail of a myosin molecule connects with other myosin molecules to form the central region of a thick
filament near the M line, whereas the heads align on either side of the thick filament where the thin filaments
overlap. The primary component of thin filaments is the actin protein. Two other components of the thin filament
are tropomyosin and troponin. Actin has binding sites for myosin attachment. Strands of tropomyosin block the
binding sites and prevent actin-myosin interactions when the muscles are at rest. Troponin consists of three
globular subunits. One subunit binds to tropomyosin, one subunit binds to actin, and one subunit binds Ca 2+
ions.
LINK TQ LEARNING
View this animation showing the organization of muscle fibers. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66643/l.3/#eip-id992518)
Sliding Filament Model of Contraction
For a muscle cell to contract, the sarcomere must shorten. However, thick and thin filaments—the components
of sarcomeres—do not shorten. Instead, they slide by one another, causing the sarcomere to shorten while
the filaments remain the same length. The sliding filament theory of muscle contraction was developed to fit
the differences observed in the named bands on the sarcomere at different degrees of muscle contraction and
relaxation. The mechanism of contraction is the binding of myosin to actin, forming cross-bridges that generate
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Chapter 38 | The Musculoskeletal System
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filament movement (Figure 38.36).
(a)
Thin filament
(actin)
Thick filaments
(myosin)
I band
A band
I band
S* ..............
itininiitint^ ,n> 111,1111 .
... t P> imm .
Ul
1 band A b
ILJ
and 1 band
Figure 38.36 When (a) a sarcomere (b) contracts, the Z lines move closer together and the I band gets smaller. The A
band stays the same width and, at full contraction, the thin filaments overlap.
When a sarcomere shortens, some regions shorten whereas others stay the same length. A sarcomere is
defined as the distance between two consecutive Z discs or Z lines; when a muscle contracts, the distance
between the Z discs is reduced. The H zone—the central region of the A zone—contains only thick filaments
and is shortened during contraction. The I band contains only thin filaments and also shortens. The A band
does not shorten—it remains the same length—but A bands of different sarcomeres move closer together during
contraction, eventually disappearing. Thin filaments are pulled by the thick filaments toward the center of the
sarcomere until the Z discs approach the thick filaments. The zone of overlap, in which thin filaments and thick
filaments occupy the same area, increases as the thin filaments move inward.
ATP and Muscle Contraction
The motion of muscle shortening occurs as myosin heads bind to actin and pull the actin inwards. This action
requires energy, which is provided by ATP. Myosin binds to actin at a binding site on the globular actin protein.
Myosin has another binding site for ATP at which enzymatic activity hydrolyzes ATP to ADP, releasing an
inorganic phosphate molecule and energy.
ATP binding causes myosin to release actin, allowing actin and myosin to detach from each other. After this
happens, the newly bound ATP is converted to ADP and inorganic phosphate, P;. The enzyme at the binding
site on myosin is called ATPase. The energy released during ATP hydrolysis changes the angle of the myosin
head into a “cocked” position. The myosin head is then in a position for further movement, possessing potential
energy, but ADP and P; are still attached. If actin binding sites are covered and unavailable, the myosin will
remain in the high energy configuration with ATP hydrolyzed, but still attached.
If the actin binding sites are uncovered, a cross-bridge will form; that is, the myosin head spans the distance
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between the actin and myosin molecules. Pi is then released, allowing myosin to expend the stored energy as
a conformational change. The myosin head moves toward the M line, pulling the actin along with it. As the actin
is pulled, the filaments move approximately 10 nm toward the M line. This movement is called the power stroke,
as it is the step at which force is produced. As the actin is pulled toward the M line, the sarcomere shortens and
the muscle contracts.
When the myosin head is “cocked,” it contains energy and is in a high-energy configuration. This energy is
expended as the myosin head moves through the power stroke; at the end of the power stroke, the myosin head
is in a low-energy position. After the power stroke, ADP is released; however, the cross-bridge formed is still in
place, and actin and myosin are bound together. ATP can then attach to myosin, which allows the cross-bridge
cycle to start again and further muscle contraction can occur (Figure 38.37).
Watch this video explaining how a muscle contraction is signaled. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66643/l.3/#eip-id4815428)
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visual
CONNECTION
The active site on actin is
(T) exposed as Ca 2+ binds
troponin.
©
The myosin head forms
a cross-bridge with actin.
During the power stroke,
® the myosin head bends,
and ADP and phosphate
are released.
©
A new molecule of ATP
attaches to the myosin
head, causing the
cross-bridge to detach.
©
ATP hydrolyzes to ADP
and phosphate, which
returns the myosin to the
“cocked” position.
Figure 38.37 The cross-bridge muscle contraction cycle, which is triggered by Ca 2+ binding to the actin active
site, is shown. With each contraction cycle, actin moves relative to myosin.
Which of the following statements about muscle contraction is true?
a. The power stroke occurs when ATP is hydrolyzed to ADP and phosphate.
b. The power stroke occurs when ADP and phosphate dissociate from the myosin head.
c. The power stroke occurs when ADP and phosphate dissociate from the actin active site.
d. The power stroke occurs when Ca 2+ binds the calcium head.
LINK
T &
LEARNING
View this animation (http:// 0 penstaxc 0 llege. 0 rg/l/muscle_c 0 ntract) of the cross-bridge muscle contraction.
Regulatory Proteins
When a muscle is in a resting state, actin and myosin are separated. To keep actin from binding to the active
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Chapter 38 | The Musculoskeletal System
site on myosin, regulatory proteins block the molecular binding sites. Tropomyosin blocks myosin binding sites
on actin molecules, preventing cross-bridge formation and preventing contraction in a muscle without nervous
input. Troponin binds to tropomyosin and helps to position it on the actin molecule; it also binds calcium ions.
To enable a muscle contraction, tropomyosin must change conformation, uncovering the myosin-binding site on
an actin molecule and allowing cross-bridge formation. This can only happen in the presence of calcium, which
is kept at extremely low concentrations in the sarcoplasm. If present, calcium ions bind to troponin, causing
conformational changes in troponin that allow tropomyosin to move away from the myosin binding sites on actin.
Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction.
Cross-bridge cycling continues until Ca 2+ ions and ATP are no longer available and tropomyosin again covers
the binding sites on actin.
Excitation-Contraction Coupling
Excitation-contraction coupling is the link (transduction) between the action potential generated in the
sarcolemma and the start of a muscle contraction. The trigger for calcium release from the sarcoplasmic
reticulum into the sarcoplasm is a neural signal. Each skeletal muscle fiber is controlled by a motor neuron,
which conducts signals from the brain or spinal cord to the muscle. The area of the sarcolemma on the muscle
fiber that interacts with the neuron is called the motor end plate. The end of the neuron’s axon is called the
synaptic terminal, and it does not actually contact the motor end plate. A small space called the synaptic cleft
separates the synaptic terminal from the motor end plate. Electrical signals travel along the neuron’s axon, which
branches through the muscle and connects to individual muscle fibers at a neuromuscular junction.
The ability of cells to communicate electrically requires that the cells expend energy to create an electrical
gradient across their cell membranes. This charge gradient is carried by ions, which are differentially distributed
across the membrane. Each ion exerts an electrical influence and a concentration influence. Just as milk will
eventually mix with coffee without the need to stir, ions also distribute themselves evenly, if they are permitted to
do so. In this case, they are not permitted to return to an evenly mixed state.
The sodium-potassium ATPase uses cellular energy to move K + ions inside the cell and Na + ions outside. This
alone accumulates a small electrical charge, but a big concentration gradient. There is lots of K + in the cell and
lots of Na + outside the cell. Potassium is able to leave the cell through K + channels that are open 90% of the
time, and it does. However, Na + channels are rarely open, so Na + remains outside the cell. When K + leaves
the cell, obeying its concentration gradient, that effectively leaves a negative charge behind. So at rest, there
is a large concentration gradient for Na + to enter the cell, and there is an accumulation of negative charges
left behind in the cell. This is the resting membrane potential. Potential in this context means a separation
of electrical charge that is capable of doing work. It is measured in volts, just like a battery. However, the
transmembrane potential is considerably smaller (0.07 V); therefore, the small value is expressed as millivolts
(mV) or 70 mV. Because the inside of a cell is negative compared with the outside, a minus sign signifies the
excess of negative charges inside the cell, -70 mV.
If an event changes the permeability of the membrane to Na + ions, they will enter the cell. That will change
the voltage. This is an electrical event, called an action potential, that can be used as a cellular signal.
Communication occurs between nerves and muscles through neurotransmitters. Neuron action potentials cause
the release of neurotransmitters from the synaptic terminal into the synaptic cleft, where they can then diffuse
across the synaptic cleft and bind to a receptor molecule on the motor end plate. The motor end plate possesses
junctional folds—folds in the sarcolemma that create a large surface area for the neurotransmitter to bind to
receptors. The receptors are actually sodium channels that open to allow the passage of Na + into the cell when
they receive a neurotransmitter signal.
Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor end
plate. Neurotransmitter release occurs when an action potential travels down the motor neuron’s axon, resulting
in altered permeability of the synaptic terminal membrane and an influx of calcium. The Ca 2+ ions allow synaptic
vesicles to move to and bind with the presynaptic membrane (on the neuron), and release neurotransmitter from
the vesicles into the synaptic cleft. Once released by the synaptic terminal, ACh diffuses across the synaptic
cleft to the motor end plate, where it binds with ACh receptors. As a neurotransmitter binds, these ion channels
open, and Na + ions cross the membrane into the muscle cell. This reduces the voltage difference between
the inside and outside of the cell, which is called depolarization. As ACh binds at the motor end plate, this
depolarization is called an end-plate potential. The depolarization then spreads along the sarcolemma, creating
an action potential as sodium channels adjacent to the initial depolarization site sense the change in voltage and
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Chapter 38 | The Musculoskeletal System
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open. The action potential moves across the entire cell, creating a wave of depolarization.
ACh is broken down by the enzyme acetylcholinesterase (AChE) into acetyl and choline. AChE resides in
the synaptic cleft, breaking down ACh so that it does not remain bound to ACh receptors, which would cause
unwanted extended muscle contraction (Figure 38.38).
visual
CONNECTION
Sarcolemma
(muscle cell
plasma
membrane)
Sarcoplasmic
reticulum
(muscle cell
endoplasmic
reticulum)
Axon terminal
Synaptic vesicles
Acetylcholine
Synaptic cleft
Acetylcholinesterase
1. Acetylcholine released from the axon 5. Acetylcholinesterase removes acetylcholine
terminal binds to receptors on the from the synaptic cleft.
sarcolemma. 6. Ca z+ is transported back into the
2. An action potential is generated and travels sarcoplasmic reticulum.
down the T tubule. 7. Tropomyosin binds active sites on actin
3. Ca 2+ is released from the sarcoplasmic causing the cross-bridge to detach,
reticulum in response to the change in
voltage.
4. Ca 2+ binds troponin; Cross-bridges form
between actin and myosin.
Figure 38.38 This diagram shows excitation-contraction coupling in a skeletal muscle contraction. The
sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells.
The deadly nerve gas Sarin irreversibly inhibits acetycholinesterase. What effect would Sarin have on
muscle contraction?
After depolarization, the membrane returns to its resting state. This is called repolarization, during which
voltage-gated sodium channels close. Potassium channels continue at 90% conductance. Because the plasma
membrane sodium-potassium ATPase always transports ions, the resting state (negatively charged inside
relative to the outside) is restored. The period immediately following the transmission of an impulse in a nerve
or muscle, in which a neuron or muscle cell regains its ability to transmit another impulse, is called the refractory
period. During the refractory period, the membrane cannot generate another action potential. The refractory
period allows the voltage-sensitive ion channels to return to their resting configurations. The sodium potassium
ATPase continually moves Na + back out of the cell and K + back into the cell, and the K + leaks out leaving
negative charge behind. Very quickly, the membrane repolarizes, so that it can again be depolarized.
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Control of Muscle Tension
Neural control initiates the formation of actin-myosin cross-bridges, leading to the sarcomere shortening
involved in muscle contraction. These contractions extend from the muscle fiber through connective tissue to
pull on bones, causing skeletal movement. The pull exerted by a muscle is called tension, and the amount of
force created by this tension can vary. This enables the same muscles to move very light objects and very heavy
objects. In individual muscle fibers, the amount of tension produced depends on the cross-sectional area of the
muscle fiber and the frequency of neural stimulation.
The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle
fiber can produce. Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind
to actin. If more cross-bridges are formed, more myosin will pull on actin, and more tension will be produced.
The ideal length of a sarcomere during production of maximal tension occurs when thick and thin filaments
overlap to the greatest degree. If a sarcomere at rest is stretched past an ideal resting length, thick and thin
filaments do not overlap to the greatest degree, and fewer cross-bridges can form. This results in fewer myosin
heads pulling on actin, and less tension is produced. As a sarcomere is shortened, the zone of overlap is reduced
as the thin filaments reach the H zone, which is composed of myosin tails. Because it is myosin heads that
form cross-bridges, actin will not bind to myosin in this zone, reducing the tension produced by this myofiber. If
the sarcomere is shortened even more, thin filaments begin to overlap with each other—reducing cross-bridge
formation even further, and producing even less tension. Conversely, if the sarcomere is stretched to the point
at which thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced.
This amount of stretching does not usually occur because accessory proteins, internal sensory nerves, and
connective tissue oppose extreme stretching.
The primary variable determining force production is the number of myofibers within the muscle that receive an
action potential from the neuron that controls that fiber. When using the biceps to pick up a pencil, the motor
cortex of the brain only signals a few neurons of the biceps, and only a few myofibers respond, in vertebrates,
each myofiber responds fully if stimulated. When picking up a piano, the motor cortex signals all of the neurons
in the biceps and every myofiber participates. This is close to the maximum force the muscle can produce. As
mentioned above, increasing the frequency of action potentials (the number of signals per second) can increase
the force a bit more, because the tropomyosin is flooded with calcium.
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KEY TERMS
abduction when a bone moves away from the midline of the body
acetylcholinesterase (AChE) enzyme that breaks down ACh into acetyl and choline
actin globular contractile protein that interacts with myosin for muscle contraction
adduction movement of the limbs inward after abduction
amphiarthrosis joint that allows slight movement; includes syndesmoses and symphyses
angular movement produced when the angle between the bones of a joint changes
appendicular skeleton composed of the bones of the upper limbs, which function to grasp and manipulate
objects, and the lower limbs, which permit locomotion
appositional growth increase in the diameter of bones by the addition of bone tissue at the surface of bones
articulation any place where two bones are joined
auditory ossicle (also, middle ear) transduces sounds from the air into vibrations in the fluid-filled cochlea
axial skeleton forms the central axis of the body and includes the bones of the skull, the ossicles of the middle
ear, the hyoid bone of the throat, the vertebral column, and the thoracic cage (ribcage)
ball-and-socket joint joint with a rounded, ball-like end of one bone fitting into a cuplike socket of another bone
bone (also, osseous tissue) connective tissue that constitutes the endoskeleton
bone remodeling replacement of old bone tissue by new bone tissue
calcification process of deposition of mineral salts in the collagen fiber matrix that crystallizes and hardens the
tissue
cardiac muscle tissue muscle tissue found only in the heart; cardiac contractions pump blood throughout the
body and maintain blood pressure
carpus eight bones that comprise the wrist
cartilaginous joint joint in which the bones are connected by cartilage
circumduction movement of a limb in a circular motion
clavicle S-shaped bone that positions the arms laterally
compact bone forms the hard external layer of all bones
condyloid joint oval-shaped end of one bone fitting into a similarly oval-shaped hollow of another bone
coxal bone hip bone
cranial bone one of eight bones that form the cranial cavity that encloses the brain and serves as an attachment
site for the muscles of the head and neck
depression movement downward of a bone, such as after the shoulders are shrugged and the scapulae return
to their normal position from an elevated position; opposite of elevation
diaphysis central shaft of bone, contains bone marrow in a marrow cavity
diarthrosis joint that allows for free movement of the joint; found in synovial joints
dorsiflexion bending at the ankle such that the toes are lifted toward the knee
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elevation movement of a bone upward, such as when the shoulders are shrugged, lifting the scapulae
endochondral ossification process of bone development from hyaline cartilage
endoskeleton skeleton of living cells that produces a hard, mineralized tissue located within the soft tissue of
organisms
epiphyseal plate region between the diaphysis and epiphysis that is responsible for the lengthwise growth of
long bones
epiphysis rounded end of bone, covered with articular cartilage and filled with red bone marrow, which
produces blood cells
eversion movement of the sole of the foot outward, away from the midline of the body; opposite of inversion
exoskeleton a secreted cellular product external skeleton that consists of a hard encasement on the surface of
an organism
extension movement in which the angle between the bones of a joint increases; opposite of flexion
facial bone one of the 14 bones that form the face; provides cavities for the sense organs (eyes, mouth, and
nose) and attachment points for facial muscles
femur (also, thighbone) longest, heaviest, and strongest bone in the body
fibrous joint joint held together by fibrous connective tissue
fibula (also, calf bone) parallels and articulates with the tibia
flat bone thin and relatively broad bone found where extensive protection of organs is required or where broad
surfaces of muscle attachment are required
flexion movement in which the angle between the bones decreases; opposite of extension
forearm extends from the elbow to the wrist and consists of two bones: the ulna and the radius
gliding movement when relatively flat bone surfaces move past each other
gomphosis the joint in which the tooth fits into the socket like a peg
Haversian canal contains the bone’s blood vessels and nerve fibers
hinge joint slightly rounded end of one bone fits into the slightly hollow end of the other bone
humerus only bone of the arm
hydrostatic skeleton skeleton that consists of aqueous fluid held under pressure in a closed body
compartment
hyoid bone lies below the mandible in the front of the neck
hyperextension extension past the regular anatomical position
intervertebral disc composed of fibrous cartilage; lies between adjacent vertebrae from the second cervical
vertebra to the sacrum
intramembranous ossification process of bone development from fibrous membranes
inversion soles of the feet moving inward, toward the midline of the body
irregular bone bone with complex shapes; examples include vertebrae and hip bones
joint point at which two or more bones meet
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lamella layer of compact tissue that surrounds a central canal called the Haversian canal
lateral rotation rotation away from the midline of the body
long bone bone that is longer than wide, and has a shaft and two ends
lower limb consists of the thigh, the leg, and the foot
medial rotation rotation toward the midline of the body
metacarpus five bones that comprise the palm
metatarsal one of the five bones of the foot
motor end plate sarcolemma of the muscle fiber that interacts with the neuron
myofibril long cylindrical structures that lie parallel to the muscle fiber
myofilament small structures that make up myofibrils
myosin contractile protein that interacts with actin for muscle contraction
opposition movement of the thumb toward the fingers of the same hand, making it possible to grasp and hold
objects
osseous tissue connective tissue that constitutes the endoskeleton
ossification (also, osteogenesis) process of bone formation by osteoblasts
osteoblast bone cell responsible for bone formation
osteoclast large bone cells with up to 50 nuclei, responsible for bone remodeling
osteocyte mature bone cells and the main cell in bone tissue
osteon cylindrical structure aligned parallel to the long axis of the bone
patella (also, kneecap) triangular bone that lies anterior to the knee joint
pectoral girdle bones that transmit the force generated by the upper limbs to the axial skeleton
pelvic girdle bones that transmit the force generated by the lower limbs to the axial skeleton
phalange one of the bones of the fingers or toes
pivot joint joint with the rounded end of one bone fitting into a ring formed by the other bone
planar joint joint with bones whose articulating surfaces are flat
plantar flexion bending at the ankle such that the heel is lifted, such as when standing on the toes
pronation movement in which the palm faces backward
protraction anterior movement of a bone in the horizontal plane
radius bone located along the lateral (thumb) side of the forearm; articulates with the humerus at the elbow
resorption process by which osteoclasts release minerals stored in bones
retraction movement in which a joint moves back into position after protraction
rib one of 12 pairs of long, curved bones that attach to the thoracic vertebrae and curve toward the front of the
body to form the ribcage
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Chapter 38 | The Musculoskeletal System
rotational movement movement of a bone as it rotates around its own longitudinal axis
saddle joint joint with concave and convex portions that fit together; named because the ends of each bone
resemble a saddle
sarcolemma plasma membrane of a skeletal muscle fiber
sarcomere functional unit of skeletal muscle
scapula flat, triangular bone located at the posterior pectoral girdle
sesamoid bone small, flat bone shaped like a sesame seed; develops inside tendons
short bone bone that has the same width and length, giving it a cube-like shape
skeletal muscle tissue forms skeletal muscles, which attach to bones and control locomotion and any
movement that can be consciously controlled
skull bone that supports the structures of the face and protects the brain
smooth muscle tissue occurs in the walls of hollow organs such as the intestines, stomach, and urinary
bladder, and around passages such as the respiratory tract and blood vessels
spongy bone tissue forms the inner layer of all bones
sternum (also, breastbone) long, flat bone located at the front of the chest
supination movement of the radius and ulna bones of the forearm so that the palm faces forward
sutural bone small, flat, irregularly shaped bone that forms between the flat bones of the cranium
suture short fiber of connective tissue that holds the skull bones tightly in place; found only in the skull
symphysis hyaline cartilage covers the end of the bone, but the connection between bones occurs through
fibrocartilage; symphyses are found at the joints between vertebrae
synarthrosis joint that is immovable
synchondrosis bones joined by hyaline cartilage; synchondroses are found in the epiphyseal plates of growing
bones in children
syndesmosis joint in which the bones are connected by a band of connective tissue, allowing for more
movement than in a suture
synovial joint only joint that has a space between the adjoining bones
tarsal one of the seven bones of the ankle
thick filament a group of myosin molecules
thin filament two polymers of actin wound together along with tropomyosin and troponin
thoracic cage (also, ribcage) skeleton of the chest, which consists of the ribs, thoracic vertebrae, sternum, and
costal cartilages
tibia (also, shinbone) large bone of the leg that is located directly below the knee
trabeculae lamellae that are arranged as rods or plates
tropomyosin acts to block myosin binding sites on actin molecules, preventing cross-bridge formation and
preventing contraction until a muscle receives a neuron signal
troponin binds to tropomyosin and helps to position it on the actin molecule, and also binds calcium ions
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Chapter 38 | The Musculoskeletal System
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ulna bone located on the medial aspect (pinky-finger side) of the forearm
vertebral column (also, spine) surrounds and protects the spinal cord, supports the head, and acts as an
attachment point for ribs and muscles of the back and neck
CHAPTER SUMMARY
38.1 Types of Skeletal Systems
The three types of skeleton designs are hydrostatic skeletons, exoskeletons, and endoskeletons. A hydrostatic
skeleton is formed by a fluid-filled compartment held under hydrostatic pressure; movement is created by the
muscles producing pressure on the fluid. An exoskeleton is a hard external skeleton that protects the outer
surface of an organism and enables movement through muscles attached on the inside. An endoskeleton is an
internal skeleton composed of hard, mineralized tissue that also enables movement by attachment to muscles.
The human skeleton is an endoskeleton that is composed of the axial and appendicular skeleton. The axial
skeleton is composed of the bones of the skull, ossicles of the ear, hyoid bone, vertebral column, and ribcage.
The skull consists of eight cranial bones and 14 facial bones. Six bones make up the ossicles of the middle ear,
while the hyoid bone is located in the neck under the mandible. The vertebral column contains 26 bones, and it
surrounds and protects the spinal cord. The thoracic cage consists of the sternum, ribs, thoracic vertebrae, and
costal cartilages. The appendicular skeleton is made up of the limbs of the upper and lower limbs. The pectoral
girdle is composed of the clavicles and the scapulae. The upper limb contains 30 bones in the arm, the
forearm, and the hand. The pelvic girdle attaches the lower limbs to the axial skeleton. The lower limb includes
the bones of the thigh, the leg, and the foot.
38.2 Bone
Bone, or osseous tissue, is connective tissue that includes specialized cells, mineral salts, and collagen fibers.
The human skeleton can be divided into long bones, short bones, flat bones, and irregular bones. Compact
bone tissue is composed of osteons and forms the external layer of all bones. Spongy bone tissue is composed
of trabeculae and forms the inner part of all bones. Four types of cells compose bony tissue: osteocytes,
osteoclasts, osteoprogenitor cells, and osteoblasts. Ossification is the process of bone formation by
osteoblasts. Intramembranous ossification is the process of bone development from fibrous membranes.
Endochondral ossification is the process of bone development from hyaline cartilage. Long bones lengthen as
chondrocytes divide and secrete hyaline cartilage. Osteoblasts replace cartilage with bone. Appositional growth
is the increase in the diameter of bones by the addition of bone tissue at the surface of bones. Bone
remodeling involves the processes of bone deposition by osteoblasts and bone resorption by osteoclasts. Bone
repair occurs in four stages and can take several months.
38.3 Joints and Skeletal Movement
The structural classification of joints divides them into bony, fibrous, cartilaginous, and synovial joints. The
bones of fibrous joints are held together by fibrous connective tissue; the three types of fibrous joints are
sutures, syndesomes, and gomphoses. Cartilaginous joints are joints in which the bones are connected by
cartilage; the two types of cartilaginous joints are synchondroses and symphyses. Synovial joints are joints that
have a space between the adjoining bones. The functional classification divides joints into three categories:
synarthroses, amphiarthroses, and diarthroses. The movement of synovial joints can be classified as one of
four different types: gliding, angular, rotational, or special movement. Gliding movements occur as relatively flat
bone surfaces move past each other. Angular movements are produced when the angle between the bones of
a joint changes. Rotational movement is the movement of a bone as it rotates around its own longitudinal axis.
Special movements include inversion, eversion, protraction, retraction, elevation, depression, dorsiflexion,
plantar flexion, supination, pronation, and opposition. Synovial joints are also classified into six different
categories on the basis of the shape and structure of the joint: planar, hinge, pivot, condyloid, saddle, and ball-
and-socket.
38.4 Muscle Contraction and Locomotion
The body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle. Skeleton
muscle tissue is composed of sarcomeres, the functional units of muscle tissue. Muscle contraction occurs
when sarcomeres shorten, as thick and thin filaments slide past each other, which is called the sliding filament
model of muscle contraction. ATP provides the energy for cross-bridge formation and filament sliding.
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Chapter 38 | The Musculoskeletal System
Regulatory proteins, such as troponin and tropomyosin, control cross-bridge formation. Excitation-contraction
coupling transduces the electrical signal of the neuron, via acetylcholine, to an electrical signal on the muscle
membrane, which initiates force production. The number of muscle fibers contracting determines how much
force the whole muscle produces.
VISUAL CONNECTION QUESTIONS
1. Figure 38.19 Which of the following statements
about bone tissue is false?
a. Compact bone tissue is made of cylindrical
osteons that are aligned such that they
travel the length of the bone.
b. Haversian canals contain blood vessels
only.
c. Haversian canals contain blood vessels and
nerve fibers.
d. Spongy tissue is found on the interior of the
bone, and compact bone tissue is found on
the exterior.
2. Figure 38.37 Which of the following statements
about muscle contraction is true?
REVIEW QUESTIONS
4. The forearm consists of the:
a. radius and ulna
b. radius and humerus
c. ulna and humerus
d. humerus and carpus
5. The pectoral girdle consists of the:
a. clavicle and sternum
b. sternum and scapula
c. clavicle and scapula
d. clavicle and coccyx
6. All of the following are groups of vertebrae except
_, which is a curvature.
a. thoracic
b. cervical
c. lumbar
d. pelvic
7. Which of these is a facial bone?
a. frontal
b. occipital
c. lacrimal
d. temporal
8. Which of the following is not a true statement
comparing exoskeletons and endoskeletons?
a. Endoskeletons can support larger
organisms.
b. Only endoskeletons can grow as an
organism grows.
c. Exoskeletons provide greater protection of
the internal organs.
d. Exoskeletons provide less mechanical
leverage.
a. The power stroke occurs when ATP is
hydrolyzed to ADP and phosphate.
b. The power stroke occurs when ADP and
phosphate dissociate from the myosin head.
c. The power stroke occurs when ADP and
phosphate dissociate from the actin active
site.
d. The power stroke occurs when Ca 2+ binds
the calcium head.
3. Figure 38.38 The deadly nerve gas Sarin
irreversibly inhibits acetycholinesterase. What effect
would Sarin have on muscle contraction?
9. The Haversian canal:
a. is arranged as rods or plates
b. contains the bone’s blood vessels and nerve
fibers
c. is responsible for the lengthwise growth of
long bones
d. synthesizes and secretes matrix
10. The epiphyseal plate:
a. is arranged as rods or plates
b. contains the bone’s blood vessels and nerve
fibers
c. is responsible for the lengthwise growth of
long bones
d. synthesizes and secretes bone matrix
11. The cells responsible for bone resorption are
a. osteoclasts
b. osteoblasts
c. fibroblasts
d. osteocytes
12. Compact bone is composed of_.
a. trabeculae
b. compacted collagen
c. osteons
d. calcium phosphate only
13. Osteoporosis is a condition where bones become
weak and brittle. It is caused by an imbalance in the
activity of which cells?
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Chapter 38 | The Musculoskeletal System
1221
a. osteoclasts and osteoblasts
b. osteoclasts and osteocytes
c. osteoblasts and chondrocytes
d. osteocytes and chondrocytes
14. While assembling a skeleton of a new species, a
scientist points to one of the bones and observes that
it looks like the most likely site of leg muscle
attachment. What kind of bone did she indicate?
a. sesamoid bone
b. long bone
c. trabecular bone
d. flat bone
15. Synchondroses and symphyses are:
a. synovial joints
b. cartilaginous joints
c. fibrous joints
d. condyloid joints
16. The movement of bone away from the midline of
the body is called_.
a. circumduction
b. extension
c. adduction
d. abduction
17. Which of the following is not a characteristic of
the synovial fluid?
a. lubrication
b. shock absorption
c. regulation of water balance in the joint
d. protection of articular cartilage
18. The elbow is an example of which type of joint?
a. hinge
b. pivot
c. saddle
d. gliding
19. A high ankle sprain is an injury caused by over¬
stretching the ligaments connecting the tibia and
fibula. What type of joint is involved in this sprain?
CRITICAL THINKING QUESTIONS
25. What are the major differences between the male
pelvis and female pelvis that permit childbirth in
females?
26. What are the major differences between the
pelvic girdle and the pectoral girdle that allow the
pelvic girdle to bear the weight of the body?
27. Both hydrostatic and exoskeletons can protect
internal organs from harm. Contrast the ways the
skeletons perform these functions.
a. ball and socket
b. gomphosis
c. syndesmosis
d. symphysis
20. In relaxed muscle, the myosin-binding site on
actin is blocked by_.
a. titin
b. troponin
c. myoglobin
d. tropomyosin
21. The cell membrane of a muscle fiber is called a
a. myofibril
b. sarcolemma
c. sarcoplasm
d. myofilament
22. The muscle relaxes if no new nerve signal
arrives. However the neurotransmitter from the
previous stimulation is still present in the synapse.
The activity of_helps to remove this
neurotransmitter.
a. myosin
b. action potential
c. tropomyosin
d. acetylcholinesterase
23. The ability of a muscle to generate tension
immediately after stimulation is dependent on:
a. myosin interaction with the M line
b. overlap of myosin and actin
c. actin attachments to the Z line
d. none of the above
24. Botulinum toxin causes flaccid paralysis of the
muscles, and is used for cosmetic purposes under
the name Botox. Which of the following is the most
likely mechanism of action of Botox?
a. Botox decreases the production of
acetylcholinesterase.
b. Botox increases calcium release from the
sarcoplasmic reticulum.
c. Botox blocks the ATP binding site in actin.
d. Botox decreases the release of
acetylcholine from motor neurons.
28. Scoliosis is a medical condition where the spine
develops a sideways curvature. How would this
change interfere with the normal function of the
spine?
29. What are the major differences between spongy
bone and compact bone?
30. What are the roles of osteoblasts, osteocytes,
and osteoclasts?
31. Thalidomide was a morning sickness drug given
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Chapter 38 | The Musculoskeletal System
to women that caused babies to be bom without arm
bones. If recent studies have shown that thalidomide
prevents the formation of new blood vessels,
describe the type of bone development inhibited by
the drug and what stage of ossification was affected.
32. What movements occur at the hip joint and knees
as you bend down to touch your toes?
33. What movement(s) occur(s) at the scapulae
when you shrug your shoulders?
34. Describe the joints and motions involved in taking
a step forward if a person is initially standing still.
Assume the person holds his foot at the same angle
throughout the motion.
35. How would muscle contractions be affected if
ATP was completely depleted in a muscle fiber?
36. What factors contribute to the amount of tension
produced in an individual muscle fiber?
37. What effect will low blood calcium have on
neurons? What effect will low blood calcium have on
skeletal muscles?
38. Skeletal muscles can only produce a mechanical
force as they are contracted, but a leg flexes and
extends while walking. How can muscles perform this
task?
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Chapter 39 | The Respiratory System
1223
39 | THE RESPIRATORY
SYSTEM
Figure 39.1 Lungs, which appear as nearly transparent tissue surrounding the heart in this X-ray of a dog (left), are
the central organs of the respiratory system. The left lung is smaller than the right lung to accommodate space for the
heart. A dog’s nose (right) has a slit on the side of each nostril. When tracking a scent, the slits open, blocking the
front of the nostrils. This allows the dog to exhale though the now-open area on the side of the nostrils without losing
the scent that is being followed, (credit a: modification of work by Geoff Stearns; credit b: modification of work by Cory
Zanker)
Chapter Outline
39.1: Systems of Gas Exchange
39.2: Gas Exchange across Respiratory Surfaces
39.3: Breathing
39.4: Transport of Gases in Human Bodily Fluids
Introduction
Breathing is an involuntary event. How often a breath is taken and how much air is inhaled or exhaled are
tightly regulated by the respiratory center in the brain. Humans, when they aren’t exerting themselves, breathe
approximately 15 times per minute on average. Canines, like the dog in Figure 39.1, have a respiratory rate of
about 15-30 breaths per minute. With every inhalation, air fills the lungs, and with every exhalation, air rushes
back out. That air is doing more than just inflating and deflating the lungs in the chest cavity. The air contains
oxygen that crosses the lung tissue, enters the bloodstream, and travels to organs and tissues. Oxygen (O 2 )
enters the cells where it is used for metabolic reactions that produce ATP, a high-energy compound. At the same
time, these reactions release carbon dioxide (CO 2 ) as a by-product. CO 2 is toxic and must be eliminated. Carbon
dioxide exits the cells, enters the bloodstream, travels back to the lungs, and is expired out of the body during
exhalation.
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Chapter 39 | The Respiratory System
39.1 1 Systems of Gas Exchange
By the end of this section, you will be able to do the following:
• Describe the passage of air from the outside environment to the lungs
• Explain how the lungs are protected from particulate matter
The primary function of the respiratory system is to deliver oxygen to the cells of the body’s tissues and remove
carbon dioxide, a cell waste product. The main structures of the human respiratory system are the nasal cavity,
the trachea, and lungs.
All aerobic organisms require oxygen to carry out their metabolic functions. Along the evolutionary tree,
different organisms have devised different means of obtaining oxygen from the surrounding atmosphere. The
environment in which the animal lives greatly determines how an animal respires. The complexity of the
respiratory system is correlated with the size of the organism. As animal size increases, diffusion distances
increase and the ratio of surface area to volume drops. In unicellular organisms, diffusion across the cell
membrane is sufficient for supplying oxygen to the cell (Figure 39.2). Diffusion is a slow, passive transport
process. In order for diffusion to be a feasible means of providing oxygen to the cell, the rate of oxygen uptake
must match the rate of diffusion across the membrane. In other words, if the cell were very large or thick,
diffusion would not be able to provide oxygen quickly enough to the inside of the cell. Therefore, dependence
on diffusion as a means of obtaining oxygen and removing carbon dioxide remains feasible only for small
organisms or those with highly-flattened bodies, such as many flatworms (Platyhelminthes). Larger organisms
had to evolve specialized respiratory tissues, such as gills, lungs, and respiratory passages accompanied by
complex circulatory systems, to transport oxygen throughout their entire body.
Figure 39.2 The cell of the unicellular alga Ventricaria ventricosa is one of the largest known, reaching one to five
centimeters in diameter. Like all single-celled organisms, V. ventricosa exchanges gases across the cell membrane.
Direct Diffusion
For small multicellular organisms, diffusion across the outer membrane is sufficient to meet their oxygen needs.
Gas exchange by direct diffusion across surface membranes is efficient for organisms less than 1 mm in
diameter. In simple organisms, such as cnidarians and flatworms, every cell in the body is close to the external
environment. Their cells are kept moist and gases diffuse quickly via direct diffusion. Flatworms are small,
literally flat worms, which ‘breathe’ through diffusion across the outer membrane (Figure 39.3). The flat shape of
these organisms increases the surface area for diffusion, ensuring that each cell within the body is close to the
outer membrane surface and has access to oxygen. If the flatworm had a cylindrical body, then the cells in the
center would not be able to get oxygen.
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Chapter 39 | The Respiratory System
1225
Figure 39.3 This flatworm’s process of respiration works by diffusion across the outer membrane, (credit: Stephen
Childs)
Skin and Gills
Earthworms and amphibians use their skin (integument) as a respiratory organ. A dense network of capillaries
lies just below the skin and facilitates gas exchange between the external environment and the circulatory
system. The respiratory surface must be kept moist in order for the gases to dissolve and diffuse across cell
membranes.
Organisms that live in water need to obtain oxygen from the water. Oxygen dissolves in water but at a lower
concentration than in the atmosphere. The atmosphere has roughly 21 percent oxygen. In water, the oxygen
concentration is much lower than that. Fish and many other aquatic organisms have evolved gills to take up
the dissolved oxygen from water (Figure 39.4). Gills are thin tissue filaments that are highly branched and
folded. When water passes over the gills, the dissolved oxygen in water rapidly diffuses across the gills into
the bloodstream. The circulatory system can then carry the oxygenated blood to the other parts of the body. In
animals that contain coelomic fluid instead of blood, oxygen diffuses across the gill surfaces into the coelomic
fluid. Gills are found in mollusks, annelids, and crustaceans.
Figure 39.4 This common carp, like many other aquatic organisms, has gills that allow it to obtain oxygen from water,
(credit: "Guitardude012"/Wikimedia Commons)
The folded surfaces of the gills provide a large surface area to ensure that the fish gets sufficient oxygen.
Diffusion is a process in which material travels from regions of high concentration to low concentration until
equilibrium is reached. In this case, blood with a low concentration of oxygen molecules circulates through the
gills. The concentration of oxygen molecules in water is higher than the concentration of oxygen molecules
in gills. As a result, oxygen molecules diffuse from water (high concentration) to blood (low concentration),
as shown in Figure 39.5. Similarly, carbon dioxide molecules in the blood diffuse from the blood (high
concentration) to water (low concentration).
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Chapter 39 | The Respiratory System
Oxygi
blood
Blood
vessels
Oxygen-poor
blood
Gill filaments ‘ pr
Figure 39.5 As water flows over the gills, oxygen is transferred to blood via the veins, (credit "fish": modification of
work by Duane Raver, NOAA)
Tracheal Systems
Insect respiration is independent of its circulatory system; therefore, the blood does not play a direct role in
oxygen transport. Insects have a highly specialized type of respiratory system called the tracheal system, which
consists of a network of small tubes that carries oxygen to the entire body. The tracheal system is the most direct
and efficient respiratory system in active animals. The tubes in the tracheal system are made of a polymeric
material called chitin.
Insect bodies have openings, called spiracles, along the thorax and abdomen. These openings connect to the
tubular network, allowing oxygen to pass into the body (Figure 39.6) and regulating the diffusion of CO 2 and
water vapor. Air enters and leaves the tracheal system through the spiracles. Some insects can ventilate the
tracheal system with body movements.
Figure 39.6 Insects perform respiration via a tracheal system.
Mammalian Systems
In mammals, pulmonary ventilation occurs via inhalation (breathing). During inhalation, air enters the body
through the nasal cavity located just inside the nose (Figure 39.7). As air passes through the nasal cavity, the
air is warmed to body temperature and humidified. The respiratory tract is coated with mucus to seal the tissues
from direct contact with air. Mucus is high in water. As air crosses these surfaces of the mucous membranes,
it picks up water. These processes help equilibrate the air to the body conditions, reducing any damage that
cold, dry air can cause. Particulate matter that is floating in the air is removed in the nasal passages via mucus
and cilia. The processes of warming, humidifying, and removing particles are important protective mechanisms
that prevent damage to the trachea and lungs. Thus, inhalation serves several purposes in addition to bringing
oxygen into the respiratory system.
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Chapter 39 | The Respiratory System
1227
CONNECTION
Alveolar sac
Figure 39.7 Air enters the respiratory system through the nasal cavity and pharynx, and then passes through the
trachea and into the bronchi, which bring air into the lungs, (credit: modification of work by NCI)
Which of the following statements about the mammalian respiratory system is false?
a. When we breathe in, air travels from the pharynx to the trachea.
b. The bronchioles branch into bronchi.
c. Alveolar ducts connect to alveolar sacs.
d. Gas exchange between the lung and blood takes place in the alveolus.
From the nasal cavity, air passes through the pharynx (throat) and the larynx (voice box), as it makes its way
to the trachea (Figure 39.7). The main function of the trachea is to funnel the inhaled air to the lungs and the
exhaled air back out of the body. The human trachea is a cylinder about 10 to 12 cm long and 2 cm in diameter
that sits in front of the esophagus and extends from the larynx into the chest cavity where it divides into the
two primary bronchi at the midthorax. It is made of incomplete rings of hyaline cartilage and smooth muscle
(Figure 39.8). The trachea is lined with mucus-producing goblet cells and ciliated epithelia. The cilia propel
foreign particles trapped in the mucus toward the pharynx. The cartilage provides strength and support to the
trachea to keep the passage open. The smooth muscle can contract, decreasing the trachea’s diameter, which
causes expired air to rush upwards from the lungs at a great force. The forced exhalation helps expel mucus
when we cough. Smooth muscle can contract or relax, depending on stimuli from the external environment or
the body’s nervous system.
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Chapter 39 | The Respiratory System
Figure 39.8 The trachea and bronchi are made of incomplete rings of cartilage, (credit: modification of work by Gray's
Anatomy)
Lungs: Bronchi and Alveoli
The end of the trachea bifurcates (divides) to the right and left lungs. The lungs are not identical. The right lung
is larger and contains three lobes, whereas the smaller left lung contains two lobes (Figure 39.9). The muscular
diaphragm, which facilitates breathing, is inferior to (below) the lungs and marks the end of the thoracic cavity.
Figure 39.9 The trachea bifurcates into the right and left bronchi in the lungs. The right lung is made of three lobes
and is larger. To accommodate the heart, the left lung is smaller and has only two lobes.
In the lungs, air is diverted into smaller and smaller passages, or bronchi. Air enters the lungs through the
two primary (main) bronchi (singular: bronchus). Each bronchus divides into secondary bronchi, then into
tertiary bronchi, which in turn divide, creating smaller and smaller diameter bronchioles as they split and spread
through the lung. Like the trachea, the bronchi are made of cartilage and smooth muscle. At the bronchioles,
the cartilage is replaced with elastic fibers. Bronchi are innervated by nerves of both the parasympathetic and
sympathetic nervous systems that control muscle contraction (parasympathetic) or relaxation (sympathetic) in
the bronchi and bronchioles, depending on the nervous system’s cues. In humans, bronchioles with a diameter
smaller than 0.5 mm are the respiratory bronchioles. They lack cartilage and therefore rely on inhaled air to
support their shape. As the passageways decrease in diameter, the relative amount of smooth muscle increases.
The terminal bronchioles subdivide into microscopic branches called respiratory bronchioles. The respiratory
bronchioles subdivide into several alveolar ducts. Numerous alveoli and alveolar sacs surround the alveolar
ducts. The alveolar sacs resemble bunches of grapes tethered to the end of the bronchioles (Figure 39.10). In
the acinar region, the alveolar ducts are attached to the end of each bronchiole. At the end of each duct are
approximately 100 alveolar sacs, each containing 20 to 30 alveoli that are 200 to 300 microns in diameter. Gas
exchange occurs only in alveoli. Alveoli are made of thin-walled parenchymal cells, typically one-cell thick, that
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Chapter 39 | The Respiratory System
1229
look like tiny bubbles within the sacs. Alveoli are in direct contact with capillaries (one-cell thick) of the circulatory
system. Such intimate contact ensures that oxygen will diffuse from alveoli into the blood and be distributed to
the cells of the body. In addition, the carbon dioxide that was produced by cells as a waste product will diffuse
from the blood into alveoli to be exhaled. The anatomical arrangement of capillaries and alveoli emphasizes
the structural and functional relationship of the respiratory and circulatory systems. Because there are so many
alveoli (-300 million per lung) within each alveolar sac and so many sacs at the end of each alveolar duct, the
lungs have a sponge-like consistency. This organization produces a very large surface area that is available
for gas exchange. The surface area of alveoli in the lungs is approximately 75 m 2 This large surface area,
combined with the thin-walled nature of the alveolar parenchymal cells, allows gases to easily diffuse across the
cells.
Alveolar duct
Mucous gland
Respiratory bronchiole
Pulmonary artery
Atrium
Alveolus
Capillaries
Alveolar sac
Figure 39.10 Terminal bronchioles are connected by respiratory bronchioles to alveolar ducts and alveolar sacs. Each
alveolar sac contains 20 to 30 spherical alveoli and has the appearance of a bunch of grapes. Air flows into the atrium
of the alveolar sac, then circulates into alveoli where gas exchange occurs with the capillaries. Mucous glands secrete
mucous into the airways, keeping them moist and flexible, (credit: modification of work by Mariana Ruiz Villareal)
LINK
T a
LEARNING
Watch the following video to review the respiratory system. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66645/l.3/#eip-idll71744721747)
Protective Mechanisms
The air that organisms breathe contains particulate matter such as dust, dirt, viral particles, and bacteria that
can damage the lungs or trigger allergic immune responses. The respiratory system contains several protective
mechanisms to avoid problems or tissue damage. In the nasal cavity, hairs and mucus trap small particles,
viruses, bacteria, dust, and dirt to prevent their entry.
If particulates do make it beyond the nose, or enter through the mouth, the bronchi and bronchioles of the
lungs also contain several protective devices. The lungs produce mucus —a sticky substance made of mucin,
a complex glycoprotein, as well as salts and water—that traps particulates. The bronchi and bronchioles contain
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Chapter 39 | The Respiratory System
cilia, small hair-like projections that line the walls of the bronchi and bronchioles (Figure 39.11). These cilia beat
in unison and move mucus and particles out of the bronchi and bronchioles back up to the throat where it is
swallowed and eliminated via the esophagus.
In humans, for example, tar and other substances in cigarette smoke destroy or paralyze the cilia, making the
removal of particles more difficult. In addition, smoking causes the lungs to produce more mucus, which the
damaged cilia are not able to move. This causes a persistent cough, as the lungs try to rid themselves of
particulate matter, and makes smokers more susceptible to respiratory ailments.
Figure 39.11 The bronchi and bronchioles contain cilia that help move mucus and other particles out of the lungs,
(credit: Louisa Howard, modification of work by Dartmouth Electron Microscope Facility)
39.2 | Gas Exchange across Respiratory Surfaces
By the end of this section, you will be able to do the following:
• Name and describe lung volumes and capacities
• Understand how gas pressure influences how gases move into and out of the body
The structure of the lung maximizes its surface area to increase gas diffusion. Because of the enormous number
of alveoli (approximately 300 million in each human lung), the surface area of the lung is very large (75 m 2 ).
Having such a large surface area increases the amount of gas that can diffuse into and out of the lungs.
Basic Principles of Gas Exchange
Gas exchange during respiration occurs primarily through diffusion. Diffusion is a process in which transport is
driven by a concentration gradient. Gas molecules move from a region of high concentration to a region of low
concentration. Blood that is low in oxygen concentration and high in carbon dioxide concentration undergoes
gas exchange with air in the lungs. The air in the lungs has a higher concentration of oxygen than that of oxygen-
depleted blood and a lower concentration of carbon dioxide. This concentration gradient allows for gas exchange
during respiration.
Partial pressure is a measure of the concentration of the individual components in a mixture of gases. The total
pressure exerted by the mixture is the sum of the partial pressures of the components in the mixture. The rate
of diffusion of a gas is proportional to its partial pressure within the total gas mixture. This concept is discussed
further in detail below.
Lung Volumes and Capacities
Different animals have different lung capacities based on their activities. Cheetahs have evolved a much higher
lung capacity than humans; it helps provide oxygen to all the muscles in the body and allows them to run very
fast. Elephants also have a high lung capacity. In this case, it is not because they run fast but because they have
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Chapter 39 | The Respiratory System
1231
a large body and must be able to take up oxygen in accordance with their body size.
Human lung size is determined by genetics, sex, and height. At maximal capacity, an average lung can hold
almost six liters of air, but lungs do not usually operate at maximal capacity. Air in the lungs is measured in terms
of lung volumes and lung capacities (Figure 39.12 and Table 39.1). Volume measures the amount of air for
one function (such as inhalation or exhalation). Capacity is any two or more volumes (for example, how much
can be inhaled from the end of a maximal exhalation).
t
IF
* j
V
r ! V
'
i
■
: 1
Inspiratory
Capacity
(1C)
Vital
Capacity
(VC)
Inspiratory
Reserve
Volume
(IRV)
i\ f\ f\ f\ n \
1 \WWW
_ W _ U V U _ U _
A A
T 1 1 i i
TV 11 1
1 J, ,
▼ 1 V V V u v L U
Tidal
Volume
(TV)
J
FF
k
*C >
<
, t
ERV
\! \
Expiratory
Reserve
Volume
(ERV)
Functional
Residual
Capacity
(FRC)
>
r
RV
1
Residual
Volume
(RV)
Residual
Volume
(RV)
Total Lung
Capacity
(TLC)
Figure 39.12 Human lung volumes and capacities are shown. The total lung capacity of the adult male is six liters.
Tidal volume is the volume of air inhaled in a single, normal breath. Inspiratory capacity is the amount of air taken in
during a deep breath, and residual volume is the amount of air left in the lungs after forceful respiration.
Lung Volumes and Capacities (Avg Adult Male)
Volume/
Capacity
Definition
Volume
(liters)
Equations
Tidal volume (TV)
Amount of air inhaled during a normal breath
0.5
-
Expiratory reserve
volume (ERV)
Amount of air that can be exhaled after a normal
exhalation
1.2
-
Inspiratory reserve
volume (IRV)
Amount of air that can be further inhaled after a normal
inhalation
3.1
-
Residual volume
(RV)
Air left in the lungs after a forced exhalation
1.2
-
Vital capacity (VC)
Maximum amount of air that can be moved in or out of
the lungs in a single respiratory cycle
4.8
ERV+TV+IRV
Inspiratory capacity
(1C)
Volume of air that can be inhaled in addition to a
normal exhalation
3.6
TV+IRV
Functional residual
capacity (FRC)
Volume of air remaining after a normal exhalation
2.4
ERV+RV
Total lung capacity
(TLC)
Total volume of air in the lungs after a maximal
inspiration
6.0
RV+ERV+TV+IRV
Forced expiratory
volume (FEV1)
How much air can be forced out of the lungs over a
specific time period, usually one second
-4.1 to 5.5
-
Table 39.1
The volume in the lung can be divided into four units: tidal volume, expiratory reserve volume, inspiratory reserve
1232
Chapter 39 | The Respiratory System
volume, and residual volume. Tidal volume (TV) measures the amount of air that is inspired and expired during
a normal breath. On average, this volume is around one-half liter, which is a little less than the capacity of a
20-ounce drink bottle. The expiratory reserve volume (ERV) is the additional amount of air that can be exhaled
after a normal exhalation. It is the reserve amount that can be exhaled beyond what is normal. Conversely, the
inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled after a normal inhalation.
The residual volume (RV) is the amount of air that is left after expiratory reserve volume is exhaled. The lungs
are never completely empty: There is always some air left in the lungs after a maximal exhalation. If this residual
volume did not exist and the lungs emptied completely, the lung tissues would stick together and the energy
necessary to reinflate the lung could be too great to overcome. Therefore, there is always some air remaining
in the lungs. Residual volume is also important for preventing large fluctuations in respiratory gases (O 2 and
CO 2 ). The residual volume is the only lung volume that cannot be measured directly because it is impossible to
completely empty the lung of air. This volume can only be calculated rather than measured.
Capacities are measurements of two or more volumes. The vital capacity (VC) measures the maximum amount
of air that can be inhaled or exhaled during a respiratory cycle. It is the sum of the expiratory reserve volume,
tidal volume, and inspiratory reserve volume. The inspiratory capacity (1C) is the amount of air that can be
inhaled after the end of a normal expiration. It is, therefore, the sum of the tidal volume and inspiratory reserve
volume. The functional residual capacity (FRC) includes the expiratory reserve volume and the residual
volume. The FRC measures the amount of additional air that can be exhaled after a normal exhalation. Lastly,
the total lung capacity (TLC) is a measurement of the total amount of air that the lung can hold. It is the sum of
the residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume.
Lung volumes are measured by a technique called spirometry. An important measurement taken during
spirometry is the forced expiratory volume (FEV), which measures how much air can be forced out of the lung
over a specific period, usually one second (FEV1). In addition, the forced vital capacity (FVC), which is the total
amount of air that can be forcibly exhaled, is measured. The ratio of these values ( FEV1/FVC ratio) is used to
diagnose lung diseases including asthma, emphysema, and fibrosis. If the FEV1/FVC ratio is high, the lungs are
not compliant (meaning they are stiff and unable to bend properly), and the patient most likely has lung fibrosis.
Patients exhale most of the lung volume very quickly. Conversely, when the FEV1/FVC ratio is low, there is
resistance in the lung that is characteristic of asthma. In this instance, it is hard for the patient to get the air out
of his or her lungs, and it takes a long time to reach the maximal exhalation volume. In either case, breathing is
difficult and complications arise.
ca eer connection
Respiratory Therapist
Respiratory therapists or respiratory practitioners evaluate and treat patients with lung and cardiovascular
diseases. They work as part of a medical team to develop treatment plans for patients. Respiratory
therapists may treat premature babies with underdeveloped lungs, patients with chronic conditions such
as asthma, or older patients suffering from lung disease such as emphysema and chronic obstructive
pulmonary disease (COPD). They may operate advanced equipment such as compressed gas delivery
systems, ventilators, blood gas analyzers, and resuscitators. Specialized programs to become a respiratory
therapist generally lead to a bachelor’s degree with a respiratory therapist specialty. Because of a growing
aging population, career opportunities as a respiratory therapist are expected to remain strong.
Gas Pressure and Respiration
The respiratory process can be better understood by examining the properties of gases. Gases move freely, but
gas particles are constantly hitting the walls of their vessel, thereby producing gas pressure.
Air is a mixture of gases, primarily nitrogen (N 2 ; 78.6 percent), oxygen (O 2 ; 20.9 percent), water vapor (H 2 O;
0.5 percent), and carbon dioxide (CO 2 ; 0.04 percent). Each gas component of that mixture exerts a pressure.
The pressure for an individual gas in the mixture is the partial pressure of that gas. Approximately 21 percent
of atmospheric gas is oxygen. Carbon dioxide, however, is found in relatively small amounts, 0.04 percent. The
partial pressure for oxygen is much greater than that of carbon dioxide. The partial pressure of any gas can be
calculated by:
P = (P atm ) X (percent content in mixture).
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Chapter 39 | The Respiratory System
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Patm, the atmospheric pressure, is the sum of all of the partial pressures of the atmospheric gases added
together,
P atm = P Nt + P 0 2 + P H 9 O + P COt = 7 ^0 mm Hg
x (percent content in mixture).
The pressure of the atmosphere at sea level is 760 mm Hg. Therefore, the partial pressure of oxygen is:
P 0l = (760 mm Hg) (0.21) = 160 mm Hg
and for carbon dioxide:
P c0 ^ = (760 mm Hg) (0.0004) = 0.3 mm Hg.
At high altitudes, Patm decreases but concentration does not change; the partial pressure decrease is due to the
reduction in Patm-
When the air mixture reaches the lung, it has been humidified. The pressure of the water vapor in the lung does
not change the pressure of the air, but it must be included in the partial pressure equation. For this calculation,
the water pressure (47 mm Hg) is subtracted from the atmospheric pressure:
760mmHg - 47 mm Hg = 713mmHg
and the partial pressure of oxygen is:
(760 mm Hg - 47 mm Hg) X 0.21 = 150 mm Hg.
These pressures determine the gas exchange, or the flow of gas, in the system. Oxygen and carbon dioxide will
flow according to their pressure gradient from high to low. Therefore, understanding the partial pressure of each
gas will aid in understanding how gases move in the respiratory system.
Gas Exchange across the Alveoli
in the body, oxygen is used by cells of the body’s tissues and carbon dioxide is produced as a waste product. The
ratio of carbon dioxide production to oxygen consumption is the respiratory quotient (RQ). RQ varies between
0.7 and 1.0. If just glucose were used to fuel the body, the RQ would equal one. One mole of carbon dioxide
would be produced for every mole of oxygen consumed. Glucose, however, is not the only fuel for the body.
Protein and fat are also used as fuels for the body. Because of this, less carbon dioxide is produced than oxygen
is consumed and the RQ is, on average, about 0.7 for fat and about 0.8 for protein.
The RQ is used to calculate the partial pressure of oxygen in the alveolar spaces within the lung, the alveolar
P 0l . Above, the partial pressure of oxygen in the lungs was calculated to be 150 mm Hg. However, lungs
never fully deflate with an exhalation; therefore, the inspired air mixes with this residual air and lowers the partial
pressure of oxygen within the alveoli. This means that there is a lower concentration of oxygen in the lungs
than is found in the air outside the body. Knowing the RQ, the partial pressure of oxygen in the alveoli can be
calculated:
alveolar P Q
alveolar Pq 2 = inspired P ()n - (-^-=■)
With an RQ of 0.8 and a P cc , 2 ' n the alveoli of 40 mm Hg, the alveolar P Qi is equal to:
alveolar P 0o = 150 mm Hg - (^OmmHg) _ jqq pjg
Notice that this pressure is less than the external air. Therefore, the oxygen will flow from the inspired air in the
lung (P 0 , = 150 mm Hg) into the bloodstream (P 0i = 100 mm Hg) (Figure 39.13).
In the lungs, oxygen diffuses out of the alveoli and into the capillaries surrounding the alveoli. Oxygen (about
98 percent) binds reversibly to the respiratory pigment hemoglobin found in red blood cells (RBCs). RBCs carry
oxygen to the tissues where oxygen dissociates from the hemoglobin and diffuses into the cells of the tissues.
More specifically, alveolar P 0 ^ is higher in the alveoli (P ALV o 1 = 100 mm Hg) than blood P Qi (40 mm Hg) in
the capillaries. Because this pressure gradient exists, oxygen diffuses down its pressure gradient, moving out of
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Chapter 39 | The Respiratory System
the alveoli and entering the blood of the capillaries where O 2 binds to hemoglobin. At the same time, alveolar
P COo is lower PalvOt = 40 mm Hg than blood P CC)9 = (45 mm Hg). c °2 diffuses down its pressure gradient,
moving out of the capillaries and entering the alveoli.
Oxygen and carbon dioxide move independently of each other; they diffuse down their own pressure gradients.
As blood leaves the lungs through the pulmonary veins, the venous Po 2 = !00 mm Hg, whereas the venous
P C02 = 40 mm Hg. As blood enters the systemic capillaries, the blood will lose oxygen and gain carbon dioxide
because of the pressure difference of the tissues and blood. In systemic capillaries, P 09 = 100 mm Hg, but in
the tissue cells, P 09 = 40 mm Hg. This pressure gradient drives the diffusion of oxygen out of the capillaries
and into the tissue cells. At the same time, blood P C q 9 = 40 mm Hg and systemic tissue P C09 = 45 mm Hg.
The pressure gradient drives CO 2 out of tissue cells and into the capillaries. The blood returning to the lungs
through the pulmonary arteries has a venous P 0o = 40 mm Hg and a P C09 = 45 mm Hg. The blood enters the
lung capillaries where the process of exchanging gases between the capillaries and alveoli begins again (Figure
39.13).
Figure 39.13 The partial pressures of oxygen and carbon dioxide change as blood moves through the body.
Which of the following statements is false?
a. In the tissues, Pq 9 drops as blood passes from the arteries to the veins, while P C09 increases.
b. Blood travels from the lungs to the heart to body tissues, then back to the heart, then the lungs.
c. Blood travels from the lungs to the heart to body tissues, then back to the lungs, then the heart.
d. Pq 9 is higher in air than in the lungs.
In short, the change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and
the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences
in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs, and oxygen
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Chapter 39 | The Respiratory System
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into the blood.
LINK TQ LEARNING
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39.3 | Breathing
By the end of this section, you will be able to do the following:
• Describe how the structures of the lungs and thoracic cavity control the mechanics of breathing
• Explain the importance of compliance and resistance in the lungs
• Discuss problems that may arise due to a V/Q mismatch
Mammalian lungs are located in the thoracic cavity where they are surrounded and protected by the rib cage,
intercostal muscles, and bound by the chest wall. The bottom of the lungs is contained by the diaphragm, a
skeletal muscle that facilitates breathing. Breathing requires the coordination of the lungs, the chest wall, and
most importantly, the diaphragm.
Types of Breathing
Amphibians have evolved multiple ways of breathing. Young amphibians, like tadpoles, use gills to breathe, and
they don’t leave the water. Some amphibians retain gills for life. As the tadpole grows, the gills disappear and
lungs grow. These lungs are primitive and not as evolved as mammalian lungs. Adult amphibians are lacking
or have a reduced diaphragm, so breathing via lungs is forced. The other means of breathing for amphibians is
diffusion across the skin. To aid this diffusion, amphibian skin must remain moist.
Birds face a unique challenge with respect to breathing: They fly. Flying consumes a great amount of energy;
therefore, birds require a lot of oxygen to aid their metabolic processes. Birds have evolved a respiratory system
that supplies them with the oxygen needed to enable flying. Similar to mammals, birds have lungs, which are
organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of
the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs and expelled during
exhalation. The details of breathing between birds and mammals differ substantially.
In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs
to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, and gas
exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen,
even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles
of air intake and exhalation to completely get the air out of the lungs.
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V /
Avian Respiration
Birds have evolved a respiratory system that enables them to fly. Flying is a high-energy process and
requires a lot of oxygen. Furthermore, many birds fly in high altitudes where the concentration of oxygen is
low. Flow did birds evolve a respiratory system that is so unique?
Decades of research by paleontologists have shown that birds evolved from therapods, meat-eating
dinosaurs (Figure 39.14). In fact, fossil evidence shows that meat-eating dinosaurs that lived more than 100
million years ago had a similar flow-through respiratory system with lungs and air sacs. Archaeopteryx and
Xiaotingia, for example, were flying dinosaurs and are believed to be early precursors of birds.
Figure 39.14 Dinosaurs, from which birds descended, have similar hollow bones and are believed to have had a
similar respiratory system, (credit b: modification of work by Zina Deretsky, National Science Foundation)
Most of us consider that dinosaurs are extinct. However, modern birds are descendants of avian dinosaurs.
The respiratory system of modern birds has been evolving for hundreds of millions of years.
All mammals have lungs that are the main organs for breathing. Lung capacity has evolved to support the
animal’s activities. During inhalation, the lungs expand with air, and oxygen diffuses across the lung’s surface
and enters the bloodstream. During exhalation, the lungs expel air and lung volume decreases, in the next few
sections, the process of human breathing will be explained.
The Mechanics of Human Breathing
Boyle’s Law is the gas law that states that in a closed space, pressure and volume are inversely related. As
volume decreases, pressure increases and vice versa (Figure 39.15). The relationship between gas pressure
and volume helps to explain the mechanics of breathing.
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Chapter 39 | The Respiratory System
1237
Figure 39.15 This graph shows data from Boyle’s original 1662 experiment, which shows that pressure and volume
are inversely related. No units are given as Boyle used arbitrary units in his experiments.
There is always a slightly negative pressure within the thoracic cavity, which aids in keeping the airways
of the lungs open. During inhalation, volume increases as a result of contraction of the diaphragm, and
pressure decreases (according to Boyle’s Law). This decrease of pressure in the thoracic cavity relative to the
environment makes the cavity less than the atmosphere (Figure 39.16a). Because of this drop in pressure, air
rushes into the respiratory passages. To increase the volume of the lungs, the chest wall expands. This results
from the contraction of the intercostal muscles, the muscles that are connected to the rib cage. Lung volume
expands because the diaphragm contracts and the intercostal muscles contract, thus expanding the thoracic
cavity. This increase in the volume of the thoracic cavity lowers pressure compared to the atmosphere, so air
rushes into the lungs, thus increasing its volume. The resulting increase in volume is largely attributed to an
increase in alveolar space, because the bronchioles and bronchi are stiff structures that do not change in size.
Inhalation Expiration
(a) (b)
Figure 39.16 The lungs, chest wall, and diaphragm are all involved in respiration, both (a) inhalation and (b) expiration,
(credit: modification of work by Mariana Ruiz Villareal)
The chest wall expands out and away from the lungs. The lungs are elastic; therefore, when air fills the lungs, the
elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs. These outward
and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil
to force the air out of the lungs, and the intercostal muscles relax, returning the chest wall back to its original
position (Figure 39.16b). The diaphragm also relaxes and moves higher into the thoracic cavity. This increases
the pressure within the thoracic cavity relative to the environment, and air rushes out of the lungs. The movement
of air out of the lungs is a passive event. No muscles are contracting to expel the air.
Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and dips into spaces
is called the visceral pleura. A second layer of parietal pleura lines the interior of the thorax (Figure 39.17).
The space between these layers, the intrapleural space, contains a small amount of fluid that protects the
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Chapter 39 | The Respiratory System
tissue and reduces the friction generated from rubbing the tissue layers together as the lungs contract and relax.
Pleurisy results when these layers of tissue become inflamed; it is painful because the inflammation increases
the pressure within the thoracic cavity and reduces the volume of the lung.
Parietal
pleura
Visceral
pleura
Intrapleural
space
Figure 39.17 A tissue layer called pleura surrounds the lung and interior of the thoracic cavity, (credit: modification of
work by NCI)
LINK
T &
LEARNING
View how Boyle’s Law is related to breathing and watch a video (https://www. 0 penstaxc 0 llege. 0 rg/l/
boylesjaw) on Boyle’s Law. (This multimedia resource will open in a browser.) (http://cnx.org/content/
m66647/1.3/#eip-id 1169422250553)
The Work of Breathing
The number of breaths per minute is the respiratory rate. On average, under non-exertion conditions, the
human respiratory rate is 12-15 breaths/minute. The respiratory rate contributes to the alveolar ventilation, or
how much air moves into and out of the alveoli. Alveolar ventilation prevents carbon dioxide buildup in the alveoli.
There are two ways to keep the alveolar ventilation constant: increase the respiratory rate while decreasing
the tidal volume of air per breath (shallow breathing), or decrease the respiratory rate while increasing the tidal
volume per breath. In either case, the ventilation remains the same, but the work done and type of work needed
are quite different. Both tidal volume and respiratory rate are closely regulated when oxygen demand increases.
There are two types of work conducted during respiration, flow-resistive and elastic work. Flow-resistive refers
to the work of the alveoli and tissues in the lung, whereas elastic work refers to the work of the intercostal
muscles, chest wall, and diaphragm. Increasing the respiration rate increases the flow-resistive work of the
airways and decreases the elastic work of the muscles. Decreasing the respiratory rate reverses the type of work
required.
Surfactant
The air-tissue/water interface of the alveoli has a high surface tension. This surface tension is similar to the
surface tension of water at the liquid-air interface of a water droplet that results in the bonding of the water
molecules together. Surfactant is a complex mixture of phospholipids and lipoproteins that works to reduce the
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Chapter 39 | The Respiratory System
1239
surface tension that exists between the alveoli tissue and the air found within the alveoli. By lowering the surface
tension of the alveolar fluid, it reduces the tendency of alveoli to collapse.
Surfactant works like a detergent to reduce the surface tension and allows for easier inflation of the airways.
When a balloon is first inflated, it takes a large amount of effort to stretch the plastic and start to inflate the
balloon. If a little bit of detergent was applied to the interior of the balloon, then the amount of effort or work
needed to begin to inflate the balloon would decrease, and it would become much easier to start blowing up the
balloon. This same principle applies to the airways. A small amount of surfactant to the airway tissues reduces
the effort or work needed to inflate those airways. Babies born prematurely sometimes do not produce enough
surfactant. As a result, they suffer from respiratory distress syndrome, because it requires more effort to inflate
their lungs. Surfactant is also important for preventing collapse of small alveoli relative to large alveoli.
Lung Resistance and Compliance
Pulmonary diseases reduce the rate of gas exchange into and out of the lungs. Two main causes of decreased
gas exchange are compliance (how elastic the lung is) and resistance (how much obstruction exists in the
airways). A change in either can dramatically alter breathing and the ability to take in oxygen and release carbon
dioxide.
Examples of restrictive diseases are respiratory distress syndrome and pulmonary fibrosis. In both diseases,
the airways are less compliant and they are stiff or fibrotic. There is a decrease in compliance because the lung
tissue cannot bend and move. In these types of restrictive diseases, the intrapleural pressure is more positive
and the airways collapse upon exhalation, which traps air in the lungs. Forced or functional vital capacity
(FVC), which is the amount of air that can be forcibly exhaled after taking the deepest breath possible, is much
lower than in normal patients, and the time it takes to exhale most of the air is greatly prolonged (Figure 39.18).
A patient suffering from these diseases cannot exhale the normal amount of air.
Obstructive diseases and conditions include emphysema, asthma, and pulmonary edema. In emphysema,
which mostly arises from smoking tobacco, the walls of the alveoli are destroyed, decreasing the surface area
for gas exchange. The overall compliance of the lungs is increased, because as the alveolar walls are damaged,
lung elastic recoil decreases due to a loss of elastic fibers, and more air is trapped in the lungs at the end
of exhalation. Asthma is a disease in which inflammation is triggered by environmental factors. Inflammation
obstructs the airways. The obstruction may be due to edema (fluid accumulation), smooth muscle spasms in the
walls of the bronchioles, increased mucus secretion, damage to the epithelia of the airways, or a combination of
these events. Those with asthma or edema experience increased occlusion from increased inflammation of the
airways. This tends to block the airways, preventing the proper movement of gases (Figure 39.18). Those with
obstructive diseases have large volumes of air trapped after exhalation and breathe at a very high lung volume
to compensate for the lack of airway recruitment.
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Chapter 39 | The Respiratory System
FEV1/FVC Ratio
Time (seconds)
Figure 39.18 The ratio of FEV1 (the amount of air that can be forcibly exhaled in one second after taking a deep
breath) to FVC (the total amount of air that can be forcibly exhaled) can be used to diagnose whether a person has
restrictive or obstructive lung disease. In restrictive lung disease, FVC is reduced but airways are not obstructed, so
the person is able to expel air reasonably fast. In obstructive lung disease, airway obstruction results in slow exhalation
as well as reduced FVC. Thus, the FEV1/FVC ratio is lower in persons with obstructive lung disease (less than 69
percent) than in persons with restrictive disease (88 to 90 percent).
Dead Space: V/Q Mismatch
Pulmonary circulation pressure is very low compared to that of the systemic circulation. It is also independent
of cardiac output. This is because of a phenomenon called recruitment, which is the process of opening
airways that normally remain closed when cardiac output increases. As cardiac output increases, the number
of capillaries and arteries that are perfused (filled with blood) increases. These capillaries and arteries are not
always in use but are ready if needed. At times, however, there is a mismatch between the amount of air
(ventilation, V) and the amount of blood (perfusion, Q) in the lungs. This is referred to as ventilation/perfusion
(V/Q) mismatch.
There are two types of V/Q mismatch. Both produce dead space, regions of broken down or blocked lung
tissue. Dead spaces can severely impact breathing, because they reduce the surface area available for gas
diffusion. As a result, the amount of oxygen in the blood decreases, whereas the carbon dioxide level increases.
Dead space is created when no ventilation and/or perfusion takes place. Anatomical dead space or anatomical
shunt, arises from an anatomical failure, while physiological dead space or physiological shunt, arises from a
functional impairment of the lung or arteries.
An example of an anatomical shunt is the effect of gravity on the lungs. The lung is particularly susceptible to
changes in the magnitude and direction of gravitational forces. When someone is standing or sitting upright, the
pleural pressure gradient leads to increased ventilation further down in the lung. As a result, the intrapleural
pressure is more negative at the base of the lung than at the top, and more air fills the bottom of the lung
than the top. Likewise, it takes less energy to pump blood to the bottom of the lung than to the top when in
a prone position. Perfusion of the lung is not uniform while standing or sitting. This is a result of hydrostatic
forces combined with the effect of airway pressure. An anatomical shunt develops because the ventilation of
the airways does not match the perfusion of the arteries surrounding those airways. As a result, the rate of gas
exchange is reduced. Note that this does not occur when lying down, because in this position, gravity does not
preferentially pull the bottom of the lung down.
A physiological shunt can develop if there is infection or edema in the lung that obstructs an area. This will
decrease ventilation but not affect perfusion; therefore, the V/Q ratio changes and gas exchange is affected.
The lung can compensate for these mismatches in ventilation and perfusion. If ventilation is greater than
perfusion, the arterioles dilate and the bronchioles constrict. This increases perfusion and reduces ventilation.
Likewise, if ventilation is less than perfusion, the arterioles constrict and the bronchioles dilate to correct the
imbalance.
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Chapter 39 | The Respiratory System
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LINK TQ LEARNING
View the mechanics of breathing.
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idll66285291346)
39.4 | Transport of Gases in Human Bodily Fluids
By the end of this section, you will be able to do the following:
• Describe how oxygen is bound to hemoglobin and transported to body tissues
• Explain how carbon dioxide is transported from body tissues to the lungs
Once the oxygen diffuses across the alveoli, it enters the bloodstream and is transported to the tissues where
it is unloaded, and carbon dioxide diffuses out of the blood and into the alveoli to be expelled from the body.
Although gas exchange is a continuous process, the oxygen and carbon dioxide are transported by different
mechanisms.
Transport of Oxygen in the Blood
Although oxygen dissolves in blood, only a small amount of oxygen is transported this way. Only 1.5 percent of
oxygen in the blood is dissolved directly into the blood itself. Most oxygen—98.5 percent—is bound to a protein
called hemoglobin and carried to the tissues.
Hemoglobin
Hemoglobin, or Hb, is a protein molecule found in red blood cells (erythrocytes) made of four subunits:
two alpha subunits and two beta subunits (Figure 39.19). Each subunit surrounds a central heme group
that contains iron and binds one oxygen molecule, allowing each hemoglobin molecule to bind four oxygen
molecules. Molecules with more oxygen bound to the heme groups are brighter red. As a result, oxygenated
arterial blood where the Hb is carrying four oxygen molecules is bright red, while venous blood that is
deoxygenated is darker red.
(a) Red blood cells (b) Hemoglobin
Figure 39.19 The protein inside (a) red blood cells that carries oxygen to cells and carbon dioxide to the lungs is (b)
hemoglobin. Hemoglobin is made up of four symmetrical subunits and four heme groups. Iron associated with the
heme binds oxygen. It is the iron in hemoglobin that gives blood its red color.
It is easier to bind a second and third oxygen molecule to Hb than the first molecule. This is because the
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Chapter 39 | The Respiratory System
hemoglobin molecule changes its shape, or conformation, as oxygen binds. The fourth oxygen is then more
difficult to bind. The binding of oxygen to hemoglobin can be plotted as a function of the partial pressure of
oxygen in the blood (x-axis) versus the relative Hb-oxygen saturation (y-axis). The resulting graph—an oxygen
dissociation curve —is sigmoidal, or S-shaped (Figure 39.20). As the partial pressure of oxygen increases, the
hemoglobin becomes increasingly saturated with oxygen.
Figure 39.20 The oxygen dissociation curve demonstrates that, as the partial pressure of oxygen increases,
more oxygen binds hemoglobin. However, the affinity of hemoglobin for oxygen may shift to the left or the right
depending on environmental conditions.
The kidneys are responsible for removing excess H+ ions from the blood. If the kidneys fail, what would
happen to blood pH and to hemoglobin affinity for oxygen?
Factors That Affect Oxygen Binding
The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition
to p o 2 , other environmental factors and diseases can affect oxygen carrying capacity and delivery.
Carbon dioxide levels, blood pH, and body temperature affect oxygen-carrying capacity (Figure 39.20). When
carbon dioxide is in the blood, it reacts with water to form bicarbonate (HCOJ ) and hydrogen ions (H + ). As
the level of carbon dioxide in the blood increases, more H + is produced and the pH decreases. This increase
in carbon dioxide and subsequent decrease in pH reduce the affinity of hemoglobin for oxygen. The oxygen
dissociates from the Hb molecule, shifting the oxygen dissociation curve to the right. Therefore, more oxygen is
needed to reach the same hemoglobin saturation level as when the pH was higher. A similar shift in the curve
also results from an increase in body temperature. Increased temperature, such as from increased activity of
skeletal muscle, causes the affinity of hemoglobin for oxygen to be reduced.
Diseases like sickle cell anemia and thalassemia decrease the blood’s ability to deliver oxygen to tissues and its
oxygen-carrying capacity. In sickle cell anemia, the shape of the red blood cell is crescent-shaped, elongated,
and stiffened, reducing its ability to deliver oxygen (Figure 39.21). In this form, red blood cells cannot pass
through the capillaries. This is painful when it occurs. Thalassemia is a rare genetic disease caused by a defect
in either the alpha or the beta subunit of Hb. Patients with thalassemia produce a high number of red blood
cells, but these cells have lower-than-normal levels of hemoglobin. Therefore, the oxygen-carrying capacity is
diminished.
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Chapter 39 | The Respiratory System
1243
Figure 39.21 Individuals with sickle cell anemia have crescent-shaped red blood cells, (credit: modification of work by
Ed Uthman; scale-bar data from Matt Russell)
Transport of Carbon Dioxide in the Blood
Carbon dioxide molecules are transported in the blood from body tissues to the lungs by one of three methods:
dissolution directly into the blood, binding to hemoglobin, or carried as a bicarbonate ion. Several properties
of carbon dioxide in the blood affect its transport. First, carbon dioxide is more soluble in blood than oxygen.
About 5 to 7 percent of all carbon dioxide is dissolved in the plasma. Second, carbon dioxide can bind to plasma
proteins or can enter red blood cells and bind to hemoglobin. This form transports about 10 percent of the
carbon dioxide. When carbon dioxide binds to hemoglobin, a molecule called carbaminohemoglobin is formed.
Binding of carbon dioxide to hemoglobin is reversible. Therefore, when it reaches the lungs, the carbon dioxide
can freely dissociate from the hemoglobin and be expelled from the body.
Third, the majority of carbon dioxide molecules (85 percent) are carried as part of the bicarbonate buffer
system. In this system, carbon dioxide diffuses into the red blood cells. Carbonic anhydrase (CA) within the
red blood cells quickly converts the carbon dioxide into carbonic acid (H 2 CO 3 ). Carbonic acid is an unstable
intermediate molecule that immediately dissociates into bicarbonate ions (HCO^ ) and hydrogen (H + ) ions.
Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of
carbon dioxide into the blood down its concentration gradient. It also results in the production of H + ions. If too
much H + is produced, it can alter blood pH. However, hemoglobin binds to the free H + ions and thus limits shifts
in pH. The newly synthesized bicarbonate ion is transported out of the red blood cell into the liquid component
of the blood in exchange for a chloride ion (Cl'); this is called the chloride shift. When the blood reaches the
lungs, the bicarbonate ion is transported back into the red blood cell in exchange for the chloride ion. The H + ion
dissociates from the hemoglobin and binds to the bicarbonate ion. This produces the carbonic acid intermediate,
which is converted back into carbon dioxide through the enzymatic action of CA. The carbon dioxide produced
is expelled through the lungs during exhalation.
co 2 +h 2 o ^
h 2 co 3 hco 3 +h +
(carbonic acid) (bicarbonate)
The benefit of the bicarbonate buffer system is that carbon dioxide is “soaked up” into the blood with little change
to the pH of the system. This is important because it takes only a small change in the overall pH of the body for
severe injury or death to result. The presence of this bicarbonate buffer system also allows for people to travel
and live at high altitudes: When the partial pressure of oxygen and carbon dioxide change at high altitudes, the
bicarbonate buffer system adjusts to regulate carbon dioxide while maintaining the correct pH in the body.
Carbon Monoxide Poisoning
While carbon dioxide can readily associate and dissociate from hemoglobin, other molecules such as carbon
monoxide (CO) cannot. Carbon monoxide has a greater affinity for hemoglobin than oxygen. Therefore, when
carbon monoxide is present, it binds to hemoglobin preferentially over oxygen. As a result, oxygen cannot bind
to hemoglobin, so very little oxygen is transported through the body (Figure 39.22). Carbon monoxide is a
colorless, odorless gas and is therefore difficult to detect. It is produced by gas-powered vehicles and tools.
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Chapter 39 | The Respiratory System
Carbon monoxide can cause headaches, confusion, and nausea; long-term exposure can cause brain damage
or death. Administering 100 percent (pure) oxygen is the usual treatment for carbon monoxide poisoning.
Administration of pure oxygen speeds up the separation of carbon monoxide from hemoglobin.
Figure 39.22 As percent CO increases, the oxygen saturation of hemoglobin decreases.
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Chapter 39 | The Respiratory System
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KEY TERMS
alveolar P 0i partial pressure of oxygen in the alveoli (usually around 100 mmHg)
alveolar duct duct that extends from the terminal bronchiole to the alveolar sac
alveolar sac structure consisting of two or more alveoli that share a common opening
alveolar ventilation how much air is in the alveoli
alveolus (plural: alveoli) (also, air sac) terminal region of the lung where gas exchange occurs
anatomical dead space (also, anatomical shunt) region of the lung that lacks proper ventilation/perfusion due
to an anatomical block
bicarbonate (HC0 3 ) ion j on cre ated when carbonic acid dissociates into H + and (HCO 3 )
bicarbonate buffer system system in the blood that absorbs carbon dioxide and regulates pH levels
bronchiole airway that extends from the main tertiary bronchi to the alveolar sac
bronchus (plural: bronchi) smaller branch of cartilaginous tissue that stems off of the trachea; air is tunneled
through the bronchi to the region where gas exchange occurs in alveoli
carbaminohemoglobin molecule that forms when carbon dioxide binds to hemoglobin
carbonic anhydrase (CA) enzyme that catalyzes carbon dioxide and water into carbonic acid
chloride shift exchange of chloride for bicarbonate into or out of the red blood cell
compliance measurement of the elasticity of the lung
dead space area in the lung that lacks proper ventilation or perfusion
diaphragm domed-shaped skeletal muscle located under lungs that separates the thoracic cavity from the
abdominal cavity
elastic recoil property of the lung that drives the lung tissue inward
elastic work work conducted by the intercostal muscles, chest wall, and diaphragm
expiratory reserve volume (ERV) amount of additional air that can be exhaled after a normal exhalation
FEV1/FVC ratio ratio of how much air can be forced out of the lung in one second to the total amount that is
forced out of the lung; a measurement of lung function that can be used to detect disease states
flow-resistive work of breathing performed by the alveoli and tissues in the lung
forced expiratory volume (FEV) (also, forced vital capacity) measure of how much air can be forced out of the
lung from maximal inspiration over a specific amount of time
functional residual capacity (FRC) expiratory reserve volume plus residual volume
functional vital capacity (FVC) amount of air that can be forcibly exhaled after taking the deepest breath
possible
heme group centralized iron-containing group that is surrounded by the alpha and beta subunits of hemoglobin
hemoglobin molecule in red blood cells that can bind oxygen, carbon dioxide, and carbon monoxide
inspiratory capacity (1C) tidal volume plus inspiratory reserve volume
inspiratory reserve volume (IRV) amount of additional air that can be inspired after a normal inhalation
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Chapter 39 | The Respiratory System
intercostal muscle muscle connected to the rib cage that contracts upon inspiration
intrapleural space space between the layers of pleura
larynx voice box, a short passageway connecting the pharynx and the trachea
lung capacity measurement of two or more lung volumes (how much air can be inhaled from the end of an
expiration to maximal capacity)
lung volume measurement of air for one lung function (normal inhalation or exhalation)
mucin complex glycoprotein found in mucus
mucus sticky protein-containing fluid secretion in the lung that traps particulate matter to be expelled from the
body
nasal cavity opening of the respiratory system to the outside environment
obstructive disease disease (such as emphysema and asthma) that arises from obstruction of the airways;
compliance increases in these diseases
oxygen dissociation curve curve depicting the affinity of oxygen for hemoglobin
oxygen-carrying capacity amount of oxygen that can be transported in the blood
partial pressure amount of pressure exerted by one gas within a mixture of gases
particulate matter small particle such as dust, dirt, viral particles, and bacteria that are in the air
pharynx throat; a tube that starts in the internal nares and runs partway down the neck, where it opens into the
esophagus and the larynx
physiological dead space (also, physiological shunt) region of the lung that lacks proper ventilation/perfusion
due to a physiological change in the lung (like inflammation or edema)
pleura tissue layer that surrounds the lungs and lines the interior of the thoracic cavity
pleurisy painful inflammation of the pleural tissue layers
primary bronchus (also, main bronchus) region of the airway within the lung that attaches to the trachea and
bifurcates to each lung where it branches into secondary bronchi
recruitment process of opening airways that normally remain closed when the cardiac output increases
residual volume (RV) amount of air remaining in the lung after a maximal expiration
resistance measurement of lung obstruction
respiratory bronchiole terminal portion of the bronchiole tree that is attached to the terminal bronchioles and
alveoli ducts, alveolar sacs, and alveoli
respiratory distress syndrome disease that arises from a deficient amount of surfactant
respiratory quotient (RQ) ratio of carbon dioxide production to each oxygen molecule consumed
respiratory rate number of breaths per minute
restrictive disease disease that results from a restriction and decreased compliance of the alveoli; respiratory
distress syndrome and pulmonary fibrosis are examples
sickle cell anemia genetic disorder that affects the shape of red blood cells, and their ability to transport oxygen
and move through capillaries
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Chapter 39 | The Respiratory System
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spirometry method to measure lung volumes and to diagnose lung diseases
surfactant detergent-like liquid in the airways that lowers the surface tension of the alveoli to allow for
expansion
terminal bronchiole region of bronchiole that attaches to the respiratory bronchioles
thalassemia rare genetic disorder that results in mutation of the alpha or beta subunits of hemoglobin, creating
smaller red blood cells with less hemoglobin
tidal volume (TV) amount of air that is inspired and expired during normal breathing
total lung capacity (TLC) sum of the residual volume, expiratory reserve volume, tidal volume, and inspiratory
reserve volume
trachea cartilaginous tube that transports air from the larynx to the primary bronchi
venous Pqo 2 P art ' a l pressure of carbon dioxide in the veins (40 mm Hg in the pulmonary veins)
venous Pq 2 partial pressure of oxygen in the veins (100 mm Hg in the pulmonary veins)
ventilation/perfusion (V/Q) mismatch region of the lung that lacks proper alveolar ventilation (V) and/or
arterial perfusion (Q)
vital capacity (VC) sum of the expiratory reserve volume, tidal volume, and inspiratory reserve volume
CHAPTER SUMMARY
39.1 Systems of Gas Exchange
Animal respiratory systems are designed to facilitate gas exchange. In mammals, air is warmed and humidified
in the nasal cavity. Air then travels down the pharynx, through the trachea, and into the lungs. In the lungs, air
passes through the branching bronchi, reaching the respiratory bronchioles, which house the first site of gas
exchange. The respiratory bronchioles open into the alveolar ducts, alveolar sacs, and alveoli. Because there
are so many alveoli and alveolar sacs in the lung, the surface area for gas exchange is very large. Several
protective mechanisms are in place to prevent damage or infection. These include the hair and mucus in the
nasal cavity that trap dust, dirt, and other particulate matter before they can enter the system. In the lungs,
particles are trapped in a mucus layer and transported via cilia up to the esophageal opening at the top of the
trachea to be swallowed.
39.2 Gas Exchange across Respiratory Surfaces
The lungs can hold a large volume of air, but they are not usually filled to maximal capacity. Lung volume
measurements include tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual
volume. The sum of these equals the total lung capacity. Gas movement into or out of the lungs is dependent
on the pressure of the gas. Air is a mixture of gases; therefore, the partial pressure of each gas can be
calculated to determine how the gas will flow in the lung. The difference between the partial pressure of the gas
in the air drives oxygen into the tissues and carbon dioxide out of the body.
39.3 Breathing
The structure of the lungs and thoracic cavity control the mechanics of breathing. Upon inspiration, the
diaphragm contracts and lowers. The intercostal muscles contract and expand the chest wall outward. The
intrapleural pressure drops, the lungs expand, and air is drawn into the airways. When exhaling, the intercostal
muscles and diaphragm relax, returning the intrapleural pressure back to the resting state. The lungs recoil and
airways close. The air passively exits the lung. There is high surface tension at the air-airway interface in the
lung. Surfactant, a mixture of phospholipids and lipoproteins, acts like a detergent in the airways to reduce
surface tension and allow for opening of the alveoli.
Breathing and gas exchange are both altered by changes in the compliance and resistance of the lung. If the
compliance of the lung decreases, as occurs in restrictive diseases like fibrosis, the airways stiffen and collapse
upon exhalation. Air becomes trapped in the lungs, making breathing more difficult. If resistance increases, as
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Chapter 39 | The Respiratory System
happens with asthma or emphysema, the airways become obstructed, trapping air in the lungs and causing
breathing to become difficult. Alterations in the ventilation of the airways or perfusion of the arteries can affect
gas exchange. These changes in ventilation and perfusion, called V/Q mismatch, can arise from anatomical or
physiological changes.
39.4 Transport of Gases in Human Bodily Fluids
Hemoglobin is a protein found in red blood cells that is comprised of two alpha and two beta subunits that
surround an iron-containing heme group. Oxygen readily binds this heme group. The ability of oxygen to bind
increases as more oxygen molecules are bound to heme. Disease states and altered conditions in the body
can affect the binding ability of oxygen, and increase or decrease its ability to dissociate from hemoglobin.
Carbon dioxide can be transported through the blood via three methods. It is dissolved directly in the blood,
bound to plasma proteins or hemoglobin, or converted into bicarbonate. The majority of carbon dioxide is
transported as part of the bicarbonate system. Carbon dioxide diffuses into red blood cells. Inside, carbonic
anhydrase converts carbon dioxide into carbonic acid (H 2 CO 3 ), which is subsequently hydrolyzed into
bicarbonate (HCOJ ) and H + . The H + ion binds to hemoglobin in red blood cells, and bicarbonate is
transported out of the red blood cells in exchange for a chloride ion. This is called the chloride shift.
Bicarbonate leaves the red blood cells and enters the blood plasma. In the lungs, bicarbonate is transported
back into the red blood cells in exchange for chloride. The H + dissociates from hemoglobin and combines with
bicarbonate to form carbonic acid with the help of carbonic anhydrase, which further catalyzes the reaction to
convert carbonic acid back into carbon dioxide and water. The carbon dioxide is then expelled from the lungs.
VISUAL CONNECTION QUESTIONS
1. Figure 39.7 Which of the following statements
about the mammalian respiratory system is false?
a. When we breathe in, air travels from the
pharynx to the trachea.
b. The bronchioles branch into bronchi.
c. Alveolar ducts connect to alveolar sacs.
d. Gas exchange between the lung and blood
takes place in the alveolus.
2. Figure 39.13 Which of the following statements is
false?
REVIEW QUESTIONS
4. The respiratory system_.
a. provides body tissues with oxygen
b. provides body tissues with oxygen and
carbon dioxide
c. establishes how many breaths are taken per
minute
d. provides the body with carbon dioxide
5. Air is warmed and humidified in the nasal
passages. This helps to_.
a. ward off infection
b. decrease sensitivity during breathing
c. prevent damage to the lungs
d. all of the above
a. in the tissues, P Qi drops as blood passes
from the arteries to the veins, while Pco 9
increases.
b. Blood travels from the lungs to the heart to
body tissues, then back to the heart, then
the lungs.
c. Blood travels from the lungs to the heart to
body tissues, then back to the lungs, then
the heart.
d. P Qi is higher in air than in the lungs.
3. Figure 39.20 The kidneys are responsible for
removing excess H+ ions from the blood. If the
kidneys fail, what would happen to blood pH and to
hemoglobin affinity for oxygen?
6. Which is the order of airflow during inhalation?
a. nasal cavity, trachea, larynx, bronchi,
bronchioles, alveoli
b. nasal cavity, larynx, trachea, bronchi,
bronchioles, alveoli
c. nasal cavity, larynx, trachea, bronchioles,
bronchi, alveoli
d. nasal cavity, trachea, larynx, bronchioles,
bronchi, alveoli
7. The inspiratory reserve volume measures the
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Chapter 39 | The Respiratory System
1249
a. amount of air remaining in the lung after a
maximal exhalation
b. amount of air that the lung holds
c. amount of air that can be further exhaled
after a normal breath
d. amount of air that can be further inhaled
after a normal breath
8. Of the following, which does not explain why the
partial pressure of oxygen is lower in the lung than in
the external air?
a. Air in the lung is humidified; therefore, water
vapor pressure alters the pressure.
b. Carbon dioxide mixes with oxygen.
c. Oxygen is moved into the blood and is
headed to the tissues.
d. Lungs exert a pressure on the air to reduce
the oxygen pressure.
9. The total lung capacity is calculated using which of
the following formulas?
a. residual volume + tidal volume + inspiratory
reserve volume
b. residual volume + expiratory reserve volume
+ inspiratory reserve volume
c. expiratory reserve volume + tidal volume +
inspiratory reserve volume
d. residual volume + expiratory reserve volume
+ tidal volume + inspiratory reserve volume
10. How would paralysis of the diaphragm alter
inspiration?
a. It would prevent contraction of the
intercostal muscles.
b. It would prevent inhalation because the
intrapleural pressure would not change.
c. It would decrease the intrapleural pressure
and allow more air to enter the lungs.
d. It would slow expiration because the lung
would not relax.
CRITICAL THINKING QUESTIONS
16. Describe the function of these terms and describe
where they are located: main bronchus, trachea,
alveoli, and acinus.
17. How does the structure of alveoli maximize gas
exchange?
18. What does FEV1/FVC measure? What factors
may affect FEV1/FVC?
19. What is the reason for having residual volume in
the lung?
20. How can a decrease in the percent of oxygen in
the air affect the movement of oxygen in the body?
21. If a patient has increased resistance in his or her
lungs, how can this be detected by a doctor? What
does this mean?
22. How would increased airway resistance affect
11. Restrictive airway diseases_.
a. increase the compliance of the lung
b. decrease the compliance of the lung
c. increase the lung volume
d. decrease the work of breathing
12. Alveolar ventilation remains constant when
a. the respiratory rate is increased while the
volume of air per breath is decreased
b. the respiratory rate and the volume of air
per breath are increased
c. the respiratory rate is decreased while
increasing the volume per breath
d. both a and c
13. Which of the following will NOT facilitate the
transfer of oxygen to tissues?
a. decreased body temperature
b. decreased pH of the blood
c. increased carbon dioxide
d. increased exercise
14. The majority of carbon dioxide in the blood is
transported by_.
a. binding to hemoglobin
b. dissolution in the blood
c. conversion to bicarbonate
d. binding to plasma proteins
15. The majority of oxygen in the blood is transported
by_■
a. dissolution in the blood
b. being carried as bicarbonate ions
c. binding to blood plasma
d. binding to hemoglobin
intrapleural pressure during inhalation?
23. Explain how a puncture to the thoracic cavity
(from a knife wound, for instance) could alter the
ability to inhale.
24. When someone is standing, gravity stretches the
bottom of the lung down toward the floor to a greater
extent than the top of the lung. What implication
could this have on the flow of air in the lungs? Where
does gas exchange occur in the lungs?
25. What would happen if no carbonic anhydrase
were present in red blood cells?
26. How does the administration of 100 percent
oxygen save a patient from carbon monoxide
poisoning? Why wouldn’t giving carbon dioxide work?
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Chapter 39 | The Respiratory System
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Chapter 40 | The Circulatory System
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40 | THE CIRCULATORY
SYSTEM
Figure 40.1 Just as highway systems transport people and goods through a complex network, the circulatory system
transports nutrients, gases, and wastes throughout the animal body, (credit: modification of work by Andrey Belenko)
Chapter Outline
40.1: Overview of the Circulatory System
40.2: Components of the Blood
40.3: Mammalian Heart and Blood Vessels
40.4: Blood Flow and Blood Pressure Regulation
Introduction
Most animals are complex multicellular organisms that require a mechanism for transporting nutrients throughout
their bodies and removing waste products. The circulatory system has evolved over time from simple diffusion
through cells in the early evolution of animals to a complex network of blood vessels that reach all parts of
the human body. This extensive network supplies the cells, tissues, and organs with oxygen and nutrients, and
removes carbon dioxide and waste, which are byproducts of respiration.
At the core of the human circulatory system is the heart. The size of a clenched fist, the human heart is protected
beneath the rib cage. Made of specialized and unique cardiac muscle, it pumps blood throughout the body and
to the heart itself. Heart contractions are driven by intrinsic electrical impulses that the brain and endocrine
hormones help to regulate. Understanding the heart’s basic anatomy and function is important to understanding
the body’s circulatory and respiratory systems.
Gas exchange is one essential function of the circulatory system. A circulatory system is not needed in
organisms with no specialized respiratory organs because oxygen and carbon dioxide diffuse directly between
their body tissues and the external environment. However, in organisms that possess lungs and gills, oxygen
must be transported from these specialized respiratory organs to the body tissues via a circulatory system.
Therefore, circulatory systems have had to evolve to accommodate the great diversity of body sizes and body
types present among animals.
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Chapter 40 | The Circulatory System
40.1 1 Overview of the Circulatory System
By the end of this section, you will be able to do the following:
• Describe an open and closed circulatory system
• Describe interstitial fluid and hemolymph
• Compare and contrast the organization and evolution of the vertebrate circulatory system
In all animals, except a few simple types, the circulatory system is used to transport nutrients and gases through
the body. Simple diffusion allows some water, nutrient, waste, and gas exchange into primitive animals that
are only a few cell layers thick; however, bulk flow is the only method by which the entire body of larger more
complex organisms is accessed.
Circulatory System Architecture
The circulatory system is effectively a network of cylindrical vessels: the arteries, veins, and capillaries that
emanate from a pump, the heart. In all vertebrate organisms, as well as some invertebrates, this is a closed-
loop system, in which the blood is not free in a cavity. In a closed circulatory system, blood is contained
inside blood vessels and circulates unidirectionally from the heart around the systemic circulatory route, then
returns to the heart again, as illustrated in Figure 40.2a. As opposed to a closed system, arthropods—including
insects, crustaceans, and most mollusks—have an open circulatory system, as illustrated in Figure 40.2b. In
an open circulatory system, the blood is not enclosed in the blood vessels but is pumped into a cavity called
a hemocoel and is called hemolymph because the blood mixes with the interstitial fluid. As the heart beats
and the animal moves, the hemolymph circulates around the organs within the body cavity and then reenters the
hearts through openings called ostia. This movement allows for gas and nutrient exchange. An open circulatory
system does not use as much energy as a closed system to operate or to maintain; however, there is a trade-off
with the amount of blood that can be moved to metabolically active organs and tissues that require high levels
of oxygen. In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today
is probably because they were outcompeted by the arrival of birds 150 million years ago. Birds, having a closed
circulatory system, are thought to have moved more agilely, allowing them to get food faster and possibly to prey
on the insects.
Ostia (openings in heart)
Dorsal blood
vessel
Ventral blood vessel
Dorsal blood vessel
(main
Hearts
(a) Closed circulatory system (b) Open circulatory system
Figure 40.2 In (a) closed circulatory systems, the heart pumps blood through vessels that are separate from the
interstitial fluid of the body. Most vertebrates and some invertebrates, like this annelid earthworm, have a closed
circulatory system. In (b) open circulatory systems, a fluid called hemolymph is pumped through a blood vessel that
empties into the body cavity. Hemolymph returns to the blood vessel through openings called ostia. Arthropods like
this bee and most mollusks have open circulatory systems.
Circulatory System Variation in Animals
The circulatory system varies from simple systems in invertebrates to more complex systems in vertebrates.
The simplest animals, such as the sponges (Porifera) and rotifers (Rotifera), do not need a circulatory system
because diffusion allows adequate exchange of water, nutrients, and waste, as well as dissolved gases, as
shown in Figure 40.3a. Organisms that are more complex but still only have two layers of cells in their body
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Chapter 40 | The Circulatory System
1253
plan, such as jellies (Cnidaria) and comb jellies (Ctenophora) also use diffusion through their epidermis and
internally through the gastrovascular compartment. Both their internal and external tissues are bathed in an
aqueous environment and exchange fluids by diffusion on both sides, as illustrated in Figure 40.3b. Exchange
of fluids is assisted by the pulsing of the jellyfish body.
(a) Sponge
(b) Jellyfish
Figure 40.3 Simple animals consisting of a single cell layer such as the (a) sponge or only a few cell layers such as
the (b) jellyfish do not have a circulatory system. Instead, gases, nutrients, and wastes are exchanged by diffusion.
For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through
the body; therefore, more complex circulatory systems evolved. Most arthropods and many mollusks have
open circulatory systems. In an open system, an elongated beating heart pushes the hemolymph through the
body and muscle contractions help to move fluids. The larger more complex crustaceans, including lobsters,
have developed arterial-like vessels to push blood through their bodies, and the most active mollusks, such as
squids, have evolved a closed circulatory system and are able to move rapidly to catch prey. Closed circulatory
systems are a characteristic of vertebrates; however, there are significant differences in the structure of the
heart and the circulation of blood between the different vertebrate groups due to adaptation during evolution and
associated differences in anatomy. Figure 40.4 illustrates the basic circulatory systems of some vertebrates:
fish, amphibians, reptiles, and mammals.
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Chapter 40 | The Circulatory System
Gill circulation
Systemic circulation
(a) Fish
Gill
capillaries
Ventricle
Artery
Body
capillaries
Pulmonary and
skin circulation
Lung and
skin capillaries
Left atrium
Ventricle
Systemic circulation
(b) Amphibians
Pulmonary circulation
Body
capillaries
Systemic circulation
Right
atrium
Left
atrium
Septum
Right
ventricle
Left
ventricle
Pulmonary circulation
Lung
capillaries
Systemic circulation
Right atrium
Left atrium
Right
ventricle
Left
ventricle
(c) Reptiles (d) Mammals
Figure 40.4 (a) Fish have the simplest circulatory systems of the vertebrates: blood flows unidirectionally from the two-
chambered heart through the gills and then the rest of the body, (b) Amphibians have two circulatory routes: one for
oxygenation of the blood through the lungs and skin, and the other to take oxygen to the rest of the body. The blood
is pumped from a three-chambered heart with two atria and a single ventricle, (c) Reptiles also have two circulatory
routes; however, blood is only oxygenated through the lungs. The heart is three chambered, but the ventricles are
partially separated so some mixing of oxygenated and deoxygenated blood occurs except in crocodilians and birds,
(d) Mammals and birds have the most efficient heart with four chambers that completely separate the oxygenated and
deoxygenated blood; it pumps only oxygenated blood through the body and deoxygenated blood to the lungs.
As illustrated in Figure 40.4a. Fish have a single circuit for blood flow and a two-chambered heart that has
only a single atrium and a single ventricle. The atrium collects blood that has returned from the body and
the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated; this
is called gill circulation. The blood then continues through the rest of the body before arriving back at the
atrium; this is called systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated
to deoxygenated blood around the fish’s systemic circuit. The result is a limit in the amount of oxygen that can
reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish.
In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and
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Chapter 40 | The Circulatory System
1255
back to the heart, which is called pulmonary circulation, and the other throughout the rest of the body and
its organs including the brain (systemic circulation). In amphibians, gas exchange also occurs through the skin
during pulmonary circulation and is referred to as pulmocutaneous circulation.
As shown in Figure 40.4b, amphibians have a three-chambered heart that has two atria and one ventricle
rather than the two-chambered heart of fish. The two atria (superior heart chambers) receive blood from the two
different circuits (the lungs and the systems), and then there is some mixing of the blood in the heart’s ventricle
(inferior heart chamber), which reduces the efficiency of oxygenation. The advantage to this arrangement is that
high pressure in the vessels pushes blood to the lungs and body. The mixing is mitigated by a ridge within the
ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the
pulmocutaneous circuit. For this reason, amphibians are often described as having double circulation.
Most reptiles also have a three-chambered heart similar to the amphibian heart that directs blood to the
pulmonary and systemic circuits, as shown in Figure 40.4c. The ventricle is divided more effectively by a
partial septum, which results in less mixing of oxygenated and deoxygenated blood. Some reptiles (alligators
and crocodiles) are the most primitive animals to exhibit a four-chambered heart. Crocodilians have a unique
circulatory mechanism where the heart shunts blood from the lungs toward the stomach and other organs during
long periods of submergence, for instance, while the animal waits for prey or stays underwater waiting for prey to
rot. One adaptation includes two main arteries that leave the same part of the heart: one takes blood to the lungs
and the other provides an alternate route to the stomach and other parts of the body. Two other adaptations
include a hole in the heart between the two ventricles, called the foramen of Panizza, which allows blood to
move from one side of the heart to the other, and specialized connective tissue that slows the blood flow to the
lungs. Together these adaptations have made crocodiles and alligators one of the most evolutionarily successful
animal groups on earth.
In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles, as illustrated in
Figure 40.4d. The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency
of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. The four-
chambered heart of birds and mammals evolved independently from a three-chambered heart. The independent
evolution of the same or a similar biological trait is referred to as convergent evolution.
40.2 | Components of the Blood
By the end of this section, you will be able to do the following:
• List the basic components of the blood
• Compare red and white blood cells
• Describe blood plasma and serum
Hemoglobin is responsible for distributing oxygen, and to a lesser extent, carbon dioxide, throughout the
circulatory systems of humans, vertebrates, and many invertebrates. The blood is more than the proteins,
though. Blood is actually a term used to describe the liquid that moves through the vessels and includes plasma
(the liquid portion, which contains water, proteins, salts, lipids, and glucose) and the cells (red and white cells)
and cell fragments called platelets. Blood plasma is actually the dominant component of blood and contains
the water, proteins, electrolytes, lipids, and glucose. The cells are responsible for carrying the gases (red cells)
and the immune response (white). The platelets are responsible for blood clotting. Interstitial fluid that surrounds
cells is separate from the blood, but in hemolymph, they are combined. In humans, cellular components make
up approximately 45 percent of the blood and the liquid plasma 55 percent. Blood is 20 percent of a person’s
extracellular fluid and eight percent of weight.
The Role of Blood in the Body
Blood, like the human blood illustrated in Figure 40.5 is important for regulation of the body’s systems and
homeostasis. Blood helps maintain homeostasis by stabilizing pH, temperature, osmotic pressure, and by
eliminating excess heat. Blood supports growth by distributing nutrients and hormones, and by removing waste.
Blood plays a protective role by transporting clotting factors and platelets to prevent blood loss and transporting
the disease-fighting agents or white blood cells to sites of infection.
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Chapter 40 | The Circulatory System
Figure 40.5 The cells and cellular components of human blood are shown. Red blood cells deliver oxygen to the
cells and remove carbon dioxide. White blood cells—including neutrophils, monocytes, lymphocytes, eosinophils, and
basophils—are involved in the immune response. Platelets form clots that prevent blood loss after injury.
Red Blood Cells
Red blood cells, or erythrocytes (erythro- = “red”; -cyte = “cell”), are specialized cells that circulate through the
body delivering oxygen to cells; they are formed from stem cells in the bone marrow. In mammals, red blood
cells are small biconcave cells that at maturity do not contain a nucleus or mitochondria and are only 7-8 pm in
size. In birds and non-avian reptiles, a nucleus is still maintained in red blood cells.
The red coloring of blood comes from the iron-containing protein hemoglobin, illustrated in Figure 40.6a. The
principle job of this protein is to carry oxygen, but it also transports carbon dioxide as well. Hemoglobin is packed
into red blood cells at a rate of about 250 million molecules of hemoglobin per cell. Each hemoglobin molecule
binds four oxygen molecules so that each red blood cell carries one billion molecules of oxygen. There are
approximately 25 trillion red blood cells in the five liters of blood in the human body, which could carry up to
25 sextillion (25 x io 21 ) molecules of oxygen in the body at any time. In mammals, the lack of organelles in
erythrocytes leaves more room for the hemoglobin molecules, and the lack of mitochondria also prevents use
of the oxygen for metabolic respiration. Only mammals have anucleated red blood cells, and some mammals
(camels, for instance) even have nucleated red blood cells. The advantage of nucleated red blood cells is that
these cells can undergo mitosis. Anucleated red blood cells metabolize anaerobically (without oxygen), making
use of a primitive metabolic pathway to produce ATP and increase the efficiency of oxygen transport.
Not all organisms use hemoglobin as the method of oxygen transport. Invertebrates that utilize hemolymph
rather than blood use different pigments to bind to the oxygen. These pigments use copper or iron to the oxygen.
Invertebrates have a variety of other respiratory pigments. Hemocyanin, a blue-green, copper-containing protein,
illustrated in Figure 40.6b is found in mollusks, crustaceans, and some of the arthropods. Chlorocruorin, a
green-colored, iron-containing pigment is found in four families of polychaete tubeworms. Hemerythrin, a red,
iron-containing protein is found in some polychaete worms and annelids and is illustrated in Figure 40.6c.
Despite the name, hemerythrin does not contain a heme group and its oxygen-carrying capacity is poor
compared to hemoglobin.
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Chapter 40 | The Circulatory System
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Oxygen
Heme
(a) Hemoglobin
(b) Hemocyanin
(c) Hemerythrin
Figure 40.6 In most vertebrates, (a) hemoglobin delivers oxygen to the body and removes some carbon dioxide.
Hemoglobin is composed of four protein subunits, two alpha chains and two beta chains, and a heme group that has
iron associated with it. The iron reversibly associates with oxygen, and in so doing is oxidized from Fe 2+ to Fe 3+ . In
most mollusks and some arthropods, (b) hemocyanin delivers oxygen. Unlike hemoglobin, hemolymph is not carried in
blood cells, but floats free in the hemolymph. Copper instead of iron binds the oxygen, giving the hemolymph a blue-
green color. In annelids, such as the earthworm, and some other invertebrates, (c) hemerythrin carries oxygen. Like
hemoglobin, hemerythrin is carried in blood cells and has iron associated with it, but despite its name, hemerythrin
does not contain heme.
The small size and large surface area of red blood cells allows for rapid diffusion of oxygen and carbon dioxide
across the plasma membrane. In the lungs, carbon dioxide is released and oxygen is taken in by the blood. In the
tissues, oxygen is released from the blood and carbon dioxide is bound for transport back to the lungs. Studies
have found that hemoglobin also binds nitrous oxide (NO). NO is a vasodilator that relaxes the blood vessels
and capillaries and may help with gas exchange and the passage of red blood cells through narrow vessels.
Nitroglycerin, a heart medication for angina and heart attacks, is converted to NO to help relax the blood vessels
and increase oxygen flow through the body.
A characteristic of red blood cells is their glycolipid and glycoprotein coating; these are lipids and proteins that
have carbohydrate molecules attached. In humans, the surface glycoproteins and glycolipids on red blood cells
vary between individuals, producing the different blood types, such as A, B, and O. Red blood cells have an
average life span of 120 days, at which time they are broken down and recycled in the liver and spleen by
phagocytic macrophages, a type of white blood cell.
White Blood Cells
White blood cells, also called leukocytes (leuko = white), make up approximately one percent by volume of the
cells in blood. The role of white blood cells is very different than that of red blood cells: they are primarily involved
in the immune response to identify and target pathogens, such as invading bacteria, viruses, and other foreign
organisms. White blood cells are formed continually; some only live for hours or days, but some live for years.
The morphology of white blood cells differs significantly from red blood cells. They have nuclei and do not
contain hemoglobin. The different types of white blood cells are identified by their microscopic appearance after
histologic staining, and each has a different specialized function. The two main groups, both illustrated in Figure
40.7 are the granulocytes, which include the neutrophils, eosinophils, and basophils, and the agranulocytes,
which include the monocytes and lymphocytes.
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Chapter 40 | The Circulatory System
Neutrophil Eosinophil
Basophil
(a) Granulocytes
Monocyte Lymphocyte
(b) Agranulocytes
Figure 40.7 (a) Granulocytes—including neutrophils, eosinophils and basophils—are characterized by a lobed nucleus
and granular inclusions in the cytoplasm. Granulocytes are typically first-responders during injury or infection, (b)
Agranulocytes include lymphocytes and monocytes. Lymphocytes, including B and T cells, are responsible for adaptive
immune response. Monocytes differentiate into macrophages and dendritic cells, which in turn respond to infection or
injury.
Granulocytes contain granules in their cytoplasm; the agranulocytes are so named because of the lack of
granules in their cytoplasm. Some leukocytes become macrophages that either stay at the same site or move
through the bloodstream and gather at sites of infection or inflammation where they are attracted by chemical
signals from foreign particles and damaged cells. Lymphocytes are the primary cells of the immune system and
include B cells, T cells, and natural killer cells. B cells destroy bacteria and inactivate their toxins. They also
produce antibodies. T cells attack viruses, fungi, some bacteria, transplanted cells, and cancer cells. T cells
attack viruses by releasing toxins that kill the viruses. Natural killer cells attack a variety of infectious microbes
and certain tumor cells.
One reason that HIV poses significant management challenges is because the virus directly targets T cells
by gaining entry through a receptor. Once inside the cell, HIV then multiplies using the T cell’s own genetic
machinery. After the HIV virus replicates, it is transmitted directly from the infected T cell to macrophages. The
presence of HIV can remain unrecognized for an extensive period of time before full disease symptoms develop.
Platelets and Coagulation Factors
Blood must clot to heal wounds and prevent excess blood loss. Small cell fragments called platelets
(thrombocytes) are attracted to the wound site where they adhere by extending many projections and releasing
their contents. These contents activate other platelets and also interact with other coagulation factors, which
convert fibrinogen, a water-soluble protein present in blood serum into fibrin (a non-water soluble protein),
causing the blood to clot. Many of the clotting factors require vitamin K to work, and vitamin K deficiency can lead
to problems with blood clotting. Many platelets converge and stick together at the wound site forming a platelet
plug (also called a fibrin clot), as illustrated in Figure 40.8b. The plug or clot lasts for a number of days and
stops the loss of blood. Platelets are formed from the disintegration of larger cells called megakaryocytes, like
that shown in Figure 40.8a. For each megakaryocyte, 2000-3000 platelets are formed with 150,000 to 400,000
platelets present in each cubic millimeter of blood. Each platelet is disc shaped and 2-4 pm in diameter. They
contain many small vesicles but do not contain a nucleus.
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Chapter 40 | The Circulatory System
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Figure 40.8 (a) Platelets are formed from large cells called megakaryocytes. The megakaryocyte breaks up into
thousands of fragments that become platelets, (b) Platelets are required for clotting of the blood. The platelets collect
at a wound site in conjunction with other clotting factors, such as fibrinogen, to form a fibrin clot that prevents blood
loss and allows the wound to heal.
Plasma and Serum
The liquid component of blood is called plasma, and it is separated by spinning or centrifuging the blood at high
rotations (3000 rpm or higher). The blood cells and platelets are separated by centrifugal forces to the bottom of
a specimen tube. The upper liquid layer, the plasma, consists of 90 percent water along with various substances
required for maintaining the body’s pH, osmotic load, and for protecting the body. The plasma also contains the
coagulation factors and antibodies.
The plasma component of blood without the coagulation factors is called the serum. Serum is similar to
interstitial fluid in which the correct composition of key ions acting as electrolytes is essential for normal
functioning of muscles and nerves. Other components in the serum include proteins that assist with maintaining
pH and osmotic balance while giving viscosity to the blood. The serum also contains antibodies, specialized
proteins that are important for defense against viruses and bacteria. Lipids, including cholesterol, are also
transported in the serum, along with various other substances including nutrients, hormones, metabolic waste,
plus external substances, such as, drugs, viruses, and bacteria.
Human serum albumin is the most abundant protein in human blood plasma and is synthesized in the liver.
Albumin, which constitutes about half of the blood serum protein, transports hormones and fatty acids, buffers
pH, and maintains osmotic pressures. Immunoglobin is a protein antibody produced in the mucosal lining and
plays an important role in antibody mediated immunity.
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Chapter 40 | The Circulatory System
V /
e olution CONNECTION
Blood Types Related to Proteins on the Surface of the Red Blood
Cells
Red blood cells are coated in antigens made of glycolipids and glycoproteins. The composition of these
molecules is determined by genetics, which have evolved over time. In humans, the different surface
antigens are grouped into 24 different blood groups with more than 100 different antigens on each red
blood cell. The two most well known blood groups are the ABO, shown in Figure 40.9, and Rh systems.
The surface antigens in the ABO blood group are glycolipids, called antigen A and antigen B. People with
blood type A have antigen A, those with blood type B have antigen B, those with blood type AB have both
antigens, and people with blood type O have neither antigen. Antibodies called agglutinougens are found in
the blood plasma and react with the A or B antigens, if the two are mixed. When type A and type B blood are
combined, agglutination (clumping) of the blood occurs because of antibodies in the plasma that bind with
the opposing antigen; this causes clots that coagulate in the kidney causing kidney failure. Type O blood
has neither A or B antigens, and therefore, type O blood can be given to all blood types. Type O negative
blood is the universal donor. Type AB positive blood is the universal acceptor because it has both A and B
antigen. The ABO blood groups were discovered in 1900 and 1901 by Karl Landsteiner at the University of
Vienna.
The Rh blood group was first discovered in Rhesus monkeys. Most people have the Rh antigen (Rh+) and
do not have anti-Rh antibodies in their blood. The few people who do not have the Rh antigen and are
Rh- can develop anti-Rh antibodies if exposed to Rh+ blood. This can happen after a blood transfusion
or after an Rh- woman has an Rh+ baby. The first exposure does not usually cause a reaction; however,
at the second exposure, enough antibodies have built up in the blood to produce a reaction that causes
agglutination and breakdown of red blood cells. An injection can prevent this reaction.
© ©
(O) (A) (B) (AB)
Figure 40.9 Human red blood cells may have either type A or B glycoproteins on their surface, both glycoproteins
combined (AB), or neither (O). The glycoproteins serve as antigens and can elicit an immune response in a person
who receives a transfusion containing unfamiliar antigens. Type O blood, which has no A or B antigens, does not
elicit an immune response when injected into a person of any blood type. Thus, O is considered the universal
donor. Persons with type AB blood can accept blood from any blood type, and type AB is considered the universal
acceptor.
LINK
T a
LEARNING
Play a blood typing game on the Nobel Prize website (http:// 0 penstaxc 0 llege. 0 rg/l/bl 00 d_typing) to
solidify your understanding of blood types.
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Chapter 40 | The Circulatory System
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40.3 | Mammalian Heart and Blood Vessels
By the end of this section, you will be able to do the following:
• Describe the structure of the heart and explain how cardiac muscle is different from other muscles
• Describe the cardiac cycle
• Explain the structure of arteries, veins, and capillaries, and how blood flows through the body
The heart is a complex muscle that pumps blood through the three divisions of the circulatory system: the
coronary (vessels that serve the heart), pulmonary (heart and lungs), and systemic (systems of the body), as
shown in Figure 40.10. Coronary circulation intrinsic to the heart takes blood directly from the main artery (aorta)
coming from the heart. For pulmonary and systemic circulation, the heart has to pump blood to the lungs or the
rest of the body, respectively. In vertebrates, the lungs are relatively close to the heart in the thoracic cavity. The
shorter distance to pump means that the muscle wall on the right side of the heart is not as thick as the left side
which must have enough pressure to pump blood all the way to your big toe.
visual
CONNECTION
Figure 40.10 The mammalian circulatory system is divided into three circuits: the systemic circuit, the pulmonary
circuit, and the coronary circuit. Blood is pumped from veins of the systemic circuit into the right atrium of the
heart, then into the right ventricle. Blood then enters the pulmonary circuit, and is oxygenated by the lungs. From
the pulmonary circuit, blood reenters the heart through the left atrium. From the left ventricle, blood reenters the
systemic circuit through the aorta and is distributed to the rest of the body. The coronary circuit, which provides
blood to the heart, is not shown.
Which of the following statements about the circulatory system is false?
a. Blood in the pulmonary vein is deoxygenated.
b. Blood in the inferior vena cava is deoxygenated.
c. Blood in the pulmonary artery is deoxygenated.
d. Blood in the aorta is oxygenated.
Structure of the Heart
The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic
circuits. Since the right side of the heart sends blood to the pulmonary circuit it is smaller than the left side which
must send blood out to the whole body in the systemic circuit, as shown in Figure 40.11. In humans, the heart is
about the size of a clenched fist; it is divided into four chambers: two atria and two ventricles. There is one atrium
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Chapter 40 | The Circulatory System
and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers
that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated
blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from
the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins
that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary
sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right
ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in
only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the
heart valve is called the biscuspid or mitral valve. After it is filled, the right ventricle pumps the blood through
the pulmonary arteries, bypassing the semilunar valve (or pulmonic valve) to the lungs for re-oxygenation. After
blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing
backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the
pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on
the left side of the heart) to the left ventricle where the blood is pumped out through the aorta, the major artery
of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the
left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing
backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all
mammals.
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Chapter 40 | The Circulatory System
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visual
CONNECTION
Aorta
Pulmonary artery
Left atrium
Parietal
pericardium
Endocardium
Pulmonary artery
Superior vena cava
Right atrium
Pulmonary valve
(semilunar)
Tricuspid valve
(atrioventricular)
Right ventricle —
Inferior vena cava
Coronary arteries
Cardiac veins
Aortic valve
(semilunar)
Fibrous
pericardium
Myocardium Epicardium
Figure 40.11 (a) The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by
membranes. One-way valves separate the four chambers, (b) Blood vessels of the coronary system, including the
coronary arteries and veins, keep the heart musculature oxygenated.
Which of the following statements about the heart is false?
a. The mitral valve separates the left ventricle from the left atrium.
b. Blood travels through the bicuspid valve to the left atrium.
c. Both the aortic and the pulmonary valves are semilunar valves.
d. The mitral valve is an atrioventricular valve.
The heart is composed of three layers; the epicardium, the myocardium, and the endocardium, illustrated in
Figure 40.11. The inner wall of the heart has a lining called the endocardium. The myocardium consists of
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Chapter 40 | The Circulatory System
the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is
called the epicardium, of which the second layer is a membranous layered structure called the pericardium
that surrounds and protects the heart; it allows enough room for vigorous pumping but also keeps the heart in
place to reduce friction between the heart and other structures.
The heart has its own blood vessels that supply the heart muscle with blood. The coronary arteries branch
from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the
heart muscle is supplied with oxygen before converging again into the coronary veins to take the deoxygenated
blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart
muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of
fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself,
atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation
that results from atherosclerosis causes severe pain, known as angina, and complete blockage of the arteries
will cause myocardial infarction: the death of cardiac muscle tissue, commonly known as a heart attack.
The Cardiac Cycle
The main purpose of the heart is to pump blood through the body; it does so in a repeating sequence called the
cardiac cycle. The cardiac cycle is the coordination of the filling and emptying of the heart of blood by electrical
signals that cause the heart muscles to contract and relax. The human heart beats over 100,000 times per day.
In each cardiac cycle, the heart contracts ( systole), pushing out the blood and pumping it through the body; this
is followed by a relaxation phase ( diastole), where the heart fills with blood, as illustrated in Figure 40.12. The
atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles. Closing of
the atrioventricular valves produces a monosyllabic “lup" sound. Following a brief delay, the ventricles contract
at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to
the lungs (via the pulmonary artery). Closing of the semilunar valves produces a monosyllabic “dup” sound.
(a) Cardiac diastole: all chambers (b) Atrial systole, ventricular (c) Atrial diastole, ventricular
are relaxed, and blood flows into diastole: atria contract, pushing systole: after the atria relax, the
the heart. blood into the ventricles. ventricles contract, pushing blood
out of the heart.
Figure 40.12 During (a) cardiac diastole, the heart muscle is relaxed and blood flows into the heart. During (b) atrial
systole, the atria contract, pushing blood into the ventricles. During (c) atrial diastole, the ventricles contract, forcing
blood out of the heart.
The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart
muscle. Cardiomyocytes, shown in Figure 40.13, are distinctive muscle cells that are striated like skeletal
muscle but pump rhythmically and involuntarily like smooth muscle; they are connected by intercalated disks
exclusive to cardiac muscle. They are self-stimulated for a period of time and isolated cardiomyocytes will beat
if given the correct balance of nutrients and electrolytes.
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Chapter 40 | The Circulatory System
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Figure 40.13 Cardiomyocytes are striated muscle cells found in cardiac tissue, (credit: modification of work by Dr. S.
Girod, Anton Becker; scale-bar data from Matt Russell)
The autonomous beating of cardiac muscle cells is regulated by the heart’s internal pacemaker that uses
electrical signals to time the beating of the heart. The electrical signals and mechanical actions, illustrated in
Figure 40.14, are intimately intertwined. The internal pacemaker starts at the sinoatrial (SA) node, which is
located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node causing the
two atria to contract in unison. The pulse reaches a second node, called the atrioventricular (AV) node, between
the right atrium and right ventricle where it pauses for approximately 0.1 second before spreading to the walls of
the ventricles. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle
branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the
apex of the heart up the ventricular myocardium, and then the ventricles contract. This pause allows the atria
to empty completely into the ventricles before the ventricles pump out the blood. The electrical impulses in the
heart produce electrical currents that flow through the body and can be measured on the skin using electrodes.
This information can be observed as an electrocardiogram (ECG) —a recording of the electrical impulses of the
cardiac muscle.
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Chapter 40 | The Circulatory System
Purkinje fiber
Sinoatrial
Atrioventricular
node
Heart apex
(a) An electrical impulse
travels from the sinoatrial
node to the walls of the
atria, causing them to
contract.
(b) The impulse reaches
the atrioventricular
node, which delays it by
about 0.1 second.
(c) Bundle branches
carry signals from the
atrioventricular node
to the heart apex.
(d) The signal spreads
through the ventricle
walls, causing them to
contract.
Figure 40.14 The beating of the heart is regulated by an electrical impulse that causes the characteristic reading of an
ECG. The signal is initiated at the sinoatrial valve. The signal then (a) spreads to the atria, causing them to contract.
The signal is (b) delayed at the atrioventricular node before it is passed on to the (c) heart apex. The delay allows the
atria to relax before the (d) ventricles contract. The final part of the ECG cycle prepares the heart for the next beat.
LINK TQ LEARNING
Visit this site (http:// 0 penstaxc 0 llege. 0 rg/l/electric_heart) to see the heart’s “pacemaker” in action.
Arteries, Veins, and Capillaries
The blood from the heart is carried through the body by a complex network of blood vessels (Figure 40.15).
Arteries take blood away from the heart. The main artery is the aorta that branches into major arteries that
take blood to different limbs and organs. These major arteries include the carotid artery that takes blood to the
brain, the brachial arteries that take blood to the arms, and the thoracic artery that takes blood to the thorax and
then into the hepatic, renal, and gastric arteries for the liver, kidney, and stomach, respectively. The iliac artery
takes blood to the lower limbs. The major arteries diverge into minor arteries, and then smaller vessels called
arterioles, to reach more deeply into the muscles and organs of the body.
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Chapter 40 | The Circulatory System
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Jugular
veins
Carotid
artery
Pulmonary
arteries
Pulmonary
veins
Heart
Thoracic
aorta
Hepatic
artery
Superior
mesenteric
artery
Renal
artery
Abdominal
aorta
Figure 40.15 The major human arteries and veins are shown, (credit: modification of work by Mariana Ruiz Villareal)
Arterioles diverge into capillary beds. Capillary beds contain a large number (10 to 100) of capillaries that
branch among the cells and tissues of the body. Capillaries are narrow-diameter tubes that can fit red blood
cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the
cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into
venules that connect to minor veins that finally connect to major veins that take blood high in carbon dioxide
back to the heart. Veins are blood vessels that bring blood back to the heart. The major veins drain blood from
the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic
system.
The structure of the different types of blood vessels reflects their function or layers. There are three distinct
layers, or tunics, that form the walls of blood vessels (Figure 40.16). The first tunic is a smooth, inner lining
of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the
endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon
dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and
exocytosis. The movement of materials at the site of capillaries is regulated by vasoconstriction, narrowing of
the blood vessels, and vasodilation, widening of the blood vessels; this is important in the overall regulation of
blood pressure.
Veins and arteries both have two further tunics that surround the endothelium: the middle tunic is composed of
smooth muscle and the outermost layer is connective tissue (collagen and elastic fibers). The elastic connective
tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by
altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle
and connective tissue than the veins to accommodate the higher pressure and speed of freshly pumped blood.
The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally
different than arteries in that veins have valves to prevent the backflow of blood. Because veins have to work
against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back
to the heart.
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Chapter 40 | The Circulatory System
Lumen
Valve
Tunica intima
(endothelium)
Tunica media
(smooth muscle
and
elastic fibers)
Tunica externa
(connective tissue and
elastic fibers)
(a) Artery (b) Vein
Figure 40.16 Arteries and veins consist of three layers: an outer tunica externa, a middle tunica media, and an inner
tunica intima. Capillaries consist of a single layer of epithelial cells, the tunica intima. (credit: modification of work by
NCI, NIH)
40.4 | Blood Flow and Blood Pressure Regulation
By the end of this section, you will be able to do the following:
• Describe the system of blood flow through the body
• Describe how blood pressure is regulated
Blood pressure (BP) is the pressure exerted by blood on the walls of a blood vessel that helps to push blood
through the body. Systolic blood pressure measures the amount of pressure that blood exerts on vessels while
the heart is beating. The optimal systolic blood pressure is 120 mmHg. Diastolic blood pressure measures the
pressure in the vessels between heartbeats. The optimal diastolic blood pressure is 80 mmHg. Many factors can
affect blood pressure, such as hormones, stress, exercise, eating, sitting, and standing. Blood flow through the
body is regulated by the size of blood vessels, by the action of smooth muscle, by one-way valves, and by the
fluid pressure of the blood itself.
How Blood Flows Through the Body
Blood is pushed through the body by the action of the pumping heart. With each rhythmic pump, blood is
pushed under high pressure and velocity away from the heart, initially along the main artery, the aorta. In the
aorta, the blood travels at 30 cm/sec. As blood moves into the arteries, arterioles, and ultimately to the capillary
beds, the rate of movement slows dramatically to about 0.026 cm/sec, one-thousand times slower than the
rate of movement in the aorta. While the diameter of each individual arteriole and capillary is far narrower than
the diameter of the aorta, and according to the law of continuity, fluid should travel faster through a narrower
diameter tube, the rate is actually slower due to the overall diameter of all the combined capillaries being far
greater than the diameter of the individual aorta.
The slow rate of travel through the capillary beds, which reach almost every cell in the body, assists with gas and
nutrient exchange and also promotes the diffusion of fluid into the interstitial space. After the blood has passed
through the capillary beds to the venules, veins, and finally to the main venae cavae, the rate of flow increases
again but is still much slower than the initial rate in the aorta. Blood primarily moves in the veins by the rhythmic
movement of smooth muscle in the vessel wall and by the action of the skeletal muscle as the body moves.
Because most veins must move blood against the pull of gravity, blood is prevented from flowing backward in
the veins by one-way valves. Because skeletal muscle contraction aids in venous blood flow, it is important to
get up and move frequently after long periods of sitting so that blood will not pool in the extremities.
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Chapter 40 | The Circulatory System
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Blood flow through the capillary beds is regulated depending on the body’s needs and is directed by nerve and
hormone signals. For example, after a large meal, most of the blood is diverted to the stomach by vasodilation
of vessels of the digestive system and vasoconstriction of other vessels. During exercise, blood is diverted
to the skeletal muscles through vasodilation while blood to the digestive system would be lessened through
vasoconstriction. The blood entering some capillary beds is controlled by small muscles, called precapillary
sphincters, illustrated in Figure 40.17. If the sphincters are open, the blood will flow into the associated branches
of the capillary blood. If all of the sphincters are closed, then the blood will flow directly from the arteriole to the
venule through the thoroughfare channel (see Figure 40.17). These muscles allow the body to precisely control
when capillary beds receive blood flow. At any given moment only about 5-10% of our capillary beds actually
have blood flowing through them.
visual
CONNECTION
Arteriole
Tissue cells Vein
Precapillary Thoroughfare
Venule
(a)
o
x
Figure 40.17 (a) Precapillary sphincters are rings of smooth muscle that regulate the flow of blood through
capillaries; they help control the location of blood flow to where it is needed, (b) Valves in the veins prevent blood
from moving backward, (credit a: modification of work by NCI)
Varicose veins are veins that become enlarged because the valves no longer close properly, allowing blood
to flow backward. Varicose veins are often most prominent on the legs. Why do you think this is the case?
LINK
T &
LEARNING
See the circulatory system’s blood flow. (This multimedia resource will open in a browser.) (http://
cnx.org/content/m66654/1.3/#eip-idll71733853207)
Proteins and other large solutes cannot leave the capillaries. The loss of the watery plasma creates a
hyperosmotic solution within the capillaries, especially near the venules. This causes about 85% of the plasma
that leaves the capillaries to eventually diffuse back into the capillaries near the venules. The remaining 15%
of blood plasma drains out from the interstitial fluid into nearby lymphatic vessels (Figure 40.18). The fluid in
the lymph is similar in composition to the interstitial fluid. The lymph fluid passes through lymph nodes before it
returns to the heart via the vena cava. Lymph nodes are specialized organs that filter the lymph by percolation
through a maze of connective tissue filled with white blood cells. The white blood cells remove infectious agents,
such as bacteria and viruses, to clean the lymph before it returns to the bloodstream. After it is cleaned, the
lymph returns to the heart by the action of smooth muscle pumping, skeletal muscle action, and one-way valves
joining the returning blood near the junction of the venae cavae entering the right atrium of the heart.
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Lymph Capillaries in the Tissue Spaces
vessel
Figure 40.18 Fluid from the capillaries moves into the interstitial space and lymph capillaries by diffusion down a
pressure gradient and also by osmosis. Out of 7,200 liters of fluid pumped by the average heart in a day, over 1,500
liters is filtered, (credit: modification of work by NCI, NIH)
V / _
e olution CONNECTION
Vertebrate Diversity in Blood Circulation
Blood circulation has evolved differently in vertebrates and may show variation in different animals for the
required amount of pressure, organ and vessel location, and organ size. Animals with longs necks and those
that live in cold environments have distinct blood pressure adaptations.
Long necked animals, such as giraffes, need to pump blood upward from the heart against gravity. The
blood pressure required from the pumping of the left ventricle would be equivalent to 250 mm Hg (mm Hg
= millimeters of mercury, a unit of pressure) to reach the height of a giraffe’s head, which is 2.5 meters
higher than the heart. However, if checks and balances were not in place, this blood pressure would damage
the giraffe’s brain, particularly if it was bending down to drink. These checks and balances include valves
and feedback mechanisms that reduce the rate of cardiac output. Long-necked dinosaurs such as the
sauropods had to pump blood even higher, up to ten meters above the heart. This would have required
a blood pressure of more than 600 mm Hg, which could only have been achieved by an enormous heart.
Evidence for such an enormous heart does not exist and mechanisms to reduce the blood pressure required
include the slowing of metabolism as these animals grew larger. It is likely that they did not routinely feed
on tree tops but grazed on the ground.
Living in cold water, whales need to maintain the temperature in their blood. This is achieved by the
veins and arteries being close together so that heat exchange can occur. This mechanism is called a
countercurrent heat exchanger. The blood vessels and the whole body are also protected by thick layers
of blubber to prevent heat loss. In land animals that live in cold environments, thick fur and hibernation are
used to retain heat and slow metabolism.
Blood Pressure
The pressure of the blood flow in the body is produced by the hydrostatic pressure of the fluid (blood) against
the walls of the blood vessels. Fluid will move from areas of high to low hydrostatic pressures. In the arteries,
the hydrostatic pressure near the heart is very high and blood flows to the arterioles where the rate of flow is
slowed by the narrow openings of the arterioles. During systole, when new blood is entering the arteries, the
artery walls stretch to accommodate the increase of pressure of the extra blood; during diastole, the walls return
to normal because of their elastic properties. The blood pressure of the systole phase and the diastole phase,
graphed in Figure 40.19, gives the two pressure readings for blood pressure. For example, 120/80 indicates a
reading of 120 mm Hg during the systole and 80 mm Hg during diastole. Throughout the cardiac cycle, the blood
continues to empty into the arterioles at a relatively even rate. This resistance to blood flow is called peripheral
resistance.
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Chapter 40 | The Circulatory System
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Arteries/arterioles Capillaries Veins/venules
Figure 40.19 Blood pressure is related to the blood velocity in the arteries and arterioles. In the capillaries and veins,
the blood pressure continues to decrease but velocity increases.
Blood Pressure Regulation
Cardiac output is the volume of blood pumped by the heart in one minute. It is calculated by multiplying the
number of heart contractions that occur per minute (heart rate) times the stroke volume (the volume of blood
pumped into the aorta per contraction of the left ventricle). Therefore, cardiac output can be increased by
increasing heart rate, as when exercising. However, cardiac output can also be increased by increasing stroke
volume, such as if the heart contracts with greater strength. Stroke volume can also be increased by speeding
blood circulation through the body so that more blood enters the heart between contractions. During heavy
exertion, the blood vessels relax and increase in diameter, offsetting the increased heart rate and ensuring
adequate oxygenated blood gets to the muscles. Stress triggers a decrease in the diameter of the blood vessels,
consequently increasing blood pressure. These changes can also be caused by nerve signals or hormones, and
even standing up or lying down can have a great effect on blood pressure.
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KEY TERMS
angina pain caused by partial blockage of the coronary arteries by the buildup of plaque and lack of oxygen to
the heart muscle
aorta major artery of the body that takes blood away from the heart
arteriole small vessel that connects an artery to a capillary bed
artery blood vessel that takes blood away from the heart
atherosclerosis buildup of fatty plaques in the coronary arteries in the heart
atrioventricular valve one-way membranous flap of connective tissue between the atrium and the ventricle in
the right side of the heart; also known as tricuspid valve
atrium (plural: atria) chamber of the heart that receives blood from the veins and sends blood to the ventricles
bicuspid valve (also, mitral valve; left atrioventricular valve) one-way membranous flap between the atrium and
the ventricle in the left side of the heart
blood pressure (BP) pressure of blood in the arteries that helps to push blood through the body
capillary smallest blood vessel that allows the passage of individual blood cells and the site of diffusion of
oxygen and nutrient exchange
capillary bed large number of capillaries that converge to take blood to a particular organ or tissue
cardiac cycle filling and emptying the heart of blood by electrical signals that cause the heart muscles to
contract and relax
cardiac output the volume of blood pumped by the heart in one minute as a product of heart rate multiplied by
stroke volume
cardiomyocyte specialized heart muscle cell that is striated but contracts involuntarily like smooth muscle
closed circulatory system system in which the blood is separated from the bodily interstitial fluid and
contained in blood vessels
coronary artery vessel that supplies the heart tissue with blood
coronary vein vessel that takes blood away from the heart tissue back to the chambers in the heart
diastole relaxation phase of the cardiac cycle when the heart is relaxed and the ventricles are filling with blood
double circulation flow of blood in two circuits: the pulmonary circuit through the lungs and the systemic circuit
through the organs and body
electrocardiogram (ECG) recording of the electrical impulses of the cardiac muscle
endocardium innermost layer of tissue in the heart
epicardium outermost tissue layer of the heart
gill circulation circulatory system that is specific to animals with gills for gas exchange; the blood flows through
the gills for oxygenation
hemocoel cavity into which blood is pumped in an open circulatory system
hemolymph mixture of blood and interstitial fluid that is found in insects and other arthropods as well as most
mollusks
inferior vena cava drains blood from the veins that come from the lower organs and the legs
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Chapter 40 | The Circulatory System
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interstitial fluid fluid between cells
lymph node specialized organ that contains a large number of macrophages that clean the lymph before the
fluid is returned to the heart
myocardial infarction (also, heart attack) complete blockage of the coronary arteries and death of the cardiac
muscle tissue
myocardium heart muscle cells that make up the middle layer and the bulk of the heart wall
open circulatory system system in which the blood is mixed with interstitial fluid and directly covers the organs
ostium (plural: ostia) holes between blood vessels that allow the movement of hemolymph through the body of
insects, arthropods, and mollusks with open circulatory systems
pericardium membrane layer protecting the heart; also part of the epicardium
peripheral resistance resistance of the artery and blood vessel walls to the pressure placed on them by the
force of the heart pumping
plasma liquid component of blood that is left after the cells are removed
platelet (also, thrombocyte) small cellular fragment that collects at wounds, cross-reacts with clotting factors,
and forms a plug to prevent blood loss
precapillary sphincter small muscle that controls blood circulation in the capillary beds
pulmocutaneous circulation circulatory system in amphibians; the flow of blood to the lungs and the moist skin
for gas exchange
pulmonary circulation flow of blood away from the heart through the lungs where oxygenation occurs and then
returns to the heart again
red blood cell small (7-8 pm) biconcave cell without mitochondria (and in mammals without nuclei) that is
packed with hemoglobin, giving the cell its red color; transports oxygen through the body
semilunar valve membranous flap of connective tissue between the aorta and a ventricle of the heart (the aortic
or pulmonary semilunar valves)
serum plasma without the coagulation factors
sinoatrial (SA) node the heart’s internal pacemaker; located near the wall of the right atrium
stroke volume the volume of blood pumped into the aorta per contraction of the left ventricle
superior vena cava drains blood from the jugular vein that comes from the brain and from the veins that come
from the arms
systemic circulation flow of blood away from the heart to the brain, liver, kidneys, stomach, and other organs,
the limbs, and the muscles of the body, and then the return of this blood to the heart
systole contraction phase of cardiac cycle when the ventricles are pumping blood into the arteries
tricuspid valve one-way membranous flap of connective tissue between the atrium and the ventricle in the right
side of the heart; also known as atrioventricular valve
unidirectional circulation flow of blood in a single circuit; occurs in fish where the blood flows through the gills,
then past the organs and the rest of the body, before returning to the heart
vasoconstriction narrowing of a blood vessel
vasodilation widening of a blood vessel
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Chapter 40 | The Circulatory System
vein blood vessel that brings blood back to the heart
vena cava major vein of the body returning blood from the upper and lower parts of the body; see the superior
vena cava and inferior vena cava
ventricle (heart) large inferior chamber of the heart that pumps blood into arteries
venule blood vessel that connects a capillary bed to a vein
white blood cell large (30 pm) cell with nuclei of which there are many types with different roles including the
protection of the body from viruses and bacteria, and cleaning up dead cells and other waste
CHAPTER SUMMARY
40.1 Overview of the Circulatory System
In most animals, the circulatory system is used to transport blood through the body. Some primitive animals use
diffusion for the exchange of water, nutrients, and gases. However, complex organisms use the circulatory
system to carry gases, nutrients, and waste through the body. Circulatory systems may be open (mixed with
the interstitial fluid) or closed (separated from the interstitial fluid). Closed circulatory systems are a
characteristic of vertebrates; however, there are significant differences in the structure of the heart and the
circulation of blood between the different vertebrate groups due to adaptions during evolution and associated
differences in anatomy. Fish have a two-chambered heart with unidirectional circulation. Amphibians have a
three-chambered heart, which has some mixing of the blood, and they have double circulation. Most non-avian
reptiles have a three-chambered heart, but have little mixing of the blood; they have double circulation.
Mammals and birds have a four-chambered heart with no mixing of the blood and double circulation.
40.2 Components of the Blood
Specific components of the blood include red blood cells, white blood cells, platelets, and the plasma, which
contains coagulation factors and serum. Blood is important for regulation of the body’s pH, temperature,
osmotic pressure, the circulation of nutrients and removal of waste, the distribution of hormones from endocrine
glands, and the elimination of excess heat; it also contains components for blood clotting. Red blood cells are
specialized cells that contain hemoglobin and circulate through the body delivering oxygen to cells. White blood
cells are involved in the immune response to identify and target invading bacteria, viruses, and other foreign
organisms; they also recycle waste components, such as old red blood cells. Platelets and blood clotting
factors cause the change of the soluble protein fibrinogen to the insoluble protein fibrin at a wound site forming
a plug. Plasma consists of 90 percent water along with various substances, such as coagulation factors and
antibodies. The serum is the plasma component of the blood without the coagulation factors.
40.3 Mammalian Heart and Blood Vessels
The heart muscle pumps blood through three divisions of the circulatory system: coronary, pulmonary, and
systemic. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left
side. The pumping of the heart is a function of cardiomyocytes, distinctive muscle cells that are striated like
skeletal muscle but pump rhythmically and involuntarily like smooth muscle. The internal pacemaker starts at
the sinoatrial node, which is located near the wall of the right atrium. Electrical charges pulse from the SA node
causing the two atria to contract in unison; then the pulse reaches the atrioventricular node between the right
atrium and right ventricle. A pause in the electric signal allows the atria to empty completely into the ventricles
before the ventricles pump out the blood. The blood from the heart is carried through the body by a complex
network of blood vessels; arteries take blood away from the heart, and veins bring blood back to the heart.
40.4 Blood Flow and Blood Pressure Regulation
Blood primarily moves through the body by the rhythmic movement of smooth muscle in the vessel wall and by
the action of the skeletal muscle as the body moves. Blood is prevented from flowing backward in the veins by
one-way valves. Blood flow through the capillary beds is controlled by precapillary sphincters to increase and
decrease flow depending on the body’s needs and is directed by nerve and hormone signals. Lymph vessels
take fluid that has leaked out of the blood to the lymph nodes where it is cleaned before returning to the heart.
During systole, blood enters the arteries, and the artery walls stretch to accommodate the extra blood. During
diastole, the artery walls return to normal. The blood pressure of the systole phase and the diastole phase
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Chapter 40 | The Circulatory System
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gives the two pressure readings for blood pressure.
VISUAL CONNECTION QUESTIONS
1. Figure 40.10 Which of the following statements
about the circulatory system is false?
a. Blood in the pulmonary vein is
deoxygenated.
b. Blood in the inferior vena cava is
deoxygenated.
c. Blood in the pulmonary artery is
deoxygenated.
d. Blood in the aorta is oxygenated.
2. Figure 40.11 Which of the following statements
about the heart is false?
REVIEW QUESTIONS
4. Why are open circulatory systems advantageous
to some animals?
a. They use less metabolic energy.
b. They help the animal move faster.
c. They do not need a heart.
d. They help large insects develop.
5. Some animals use diffusion instead of a circulatory
system. Examples include:
a. birds and jellyfish
b. flatworms and arthropods
c. mollusks and jellyfish
d. none of the above
6. Blood flow that is directed through the lungs and
back to the heart is called_.
a. unidirectional circulation
b. gill circulation
c. pulmonary circulation
d. pulmocutaneous circulation
7. White blood cells:
a. can be classified as granulocytes or
agranulocytes
b. defend the body against bacteria and
viruses
c. are also called leucocytes
d. all of the above
8. Platelet plug formation occurs at which point?
a. when large megakaryocytes break up into
thousands of smaller fragments
b. when platelets are dispersed through the
bloodstream
c. when platelets are attracted to a site of
blood vessel damage
d. none of the above
9. In humans, the plasma comprises what
percentage of the blood?
a. The mitral valve separates the left ventricle
from the left atrium.
b. Blood travels through the bicuspid valve to
the left atrium.
c. Both the aortic and the pulmonary valves
are semilunar valves.
d. The mitral valve is an atrioventricular valve.
3. Figure 40.17 Varicose veins are veins that
become enlarged because the valves no longer close
properly, allowing blood to flow backward. Varicose
veins are often most prominent on the legs. Why do
you think this is the case?
a.
45 percent
b.
55 percent
c.
25 percent
d.
90 percent
10. The red blood cells of birds differ from
mammalian red blood cells because:
a.
they are white and have nuclei
b.
they do not have nuclei
c.
they have nuclei
d.
they fight disease
11. The
heart’s internal pacemaker beats by:
a.
an internal implant that sends an electrical
impulse through the heart
b.
the excitation of cardiac muscle cells at the
sinoatrial node followed by the
atrioventricular node
c.
the excitation of cardiac muscle cells at the
atrioventricular node followed by the
sinoatrial node
d.
the action of the sinus
12. During the systolic phase of the cardiac cycle, the
heart is
a.
contracting
b.
relaxing
c.
contracting and relaxing
d.
filling with blood
13. Cardiomyocytes are similar to skeletal muscle
because:
a.
they beat involuntarily
b.
they are used for weight lifting
c.
they pulse rhythmically
d.
they are striated
14. How do arteries differ from veins?
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Chapter 40 | The Circulatory System
a. Arteries have thicker smooth muscle layers
to accommodate the changes in pressure
from the heart.
b. Arteries carry blood.
c. Arteries have thinner smooth muscle layers
and valves and move blood by the action of
skeletal muscle.
d. Arteries are thin walled and are used for gas
exchange.
15. High blood pressure would be a result of
CRITICAL THINKING QUESTIONS
16. Describe a closed circulatory system.
17. Describe systemic circulation.
18. Describe the cause of different blood type
groups.
19. List some of the functions of blood in the body.
20. How does the lymphatic system work with blood
a. a high cardiac output and high peripheral
resistance
b. a high cardiac output and low peripheral
resistance
c. a low cardiac output and high peripheral
resistance
d. a low cardiac output and low peripheral
resistance
flow?
21. Describe the cardiac cycle.
22. What happens in capillaries?
23. How does blood pressure change during heavy
exercise?
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Chapter 411 Osmotic Regulation and Excretion
1277
41 1 OSMOTIC
REGULATION AND
EXCRETION
Figure 41.1 Just as humans recycle what we can and dump the remains into landfills, our bodies use and recycle what
they can and excrete the remaining waste products. Our bodies’ complex systems have developed ways to treat waste
and maintain a balanced internal environment, (credit: modification of work by Redwin Law)
Chapter Outline
41.1: Osmoregulation and Osmotic Balance
41.2: The Kidneys and Osmoregulatory Organs
41.3: Excretion Systems
41.4: Nitrogenous Wastes
41.5: Hormonal Control of Osmoregulatory Functions
Introduction
The daily intake recommendation for human water consumption is eight to ten glasses of water. In order to
achieve a healthy balance, the human body should excrete the eight to ten glasses of water every day. This
occurs via the processes of urination, defecation, sweating and, to a small extent, respiration. The organs
and tissues of the human body are soaked in fluids that are maintained at constant temperature, pH, and
solute concentration, all crucial elements of homeostasis. The solutes in body fluids are mainly mineral salts
and sugars, and osmotic regulation is the process by which the mineral salts and water are kept in balance.
Osmotic homeostasis is maintained despite the influence of external factors like temperature, diet, and weather
conditions.
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Chapter 411 Osmotic Regulation and Excretion
41.1 1 Osmoregulation and Osmotic Balance
By the end of this section, you will be able to do the following:
• Define osmosis and explain its role within molecules
• Explain why osmoregulation and osmotic balance are important body functions
• Describe active transport mechanisms
• Explain osmolarity and the way in which it is measured
• Describe osmoregulators or osmoconformers and how these tools allow animals to adapt to different
environments
Osmosis is the diffusion of water across a membrane in response to osmotic pressure caused by an imbalance
of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water
balance ( osmotic balance) across membranes within the body’s fluids, which are composed of water, plus
electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved in water.
A non-electrolyte, in contrast, doesn’t dissociate into ions during water dissolution. Both electrolytes and non¬
electrolytes contribute to the osmotic balance. The body’s fluids include blood plasma, the cytosol within cells,
and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of
the body (such as the pleural, serous, and cell membranes) are semi-permeable membranes. Semi-permeable
membranes are permeable (or permissive) to certain types of solutes and water. Solutions on two sides of a
semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across
the membrane. As seen in Figure 41.2, a cell placed in water tends to swell due to gain of water from the
hypotonic or “low salt" environment. A cell placed in a solution with higher salt concentration, on the other hand,
tends to make the membrane shrivel up due to loss of water into the hypertonic or “high salt” environment.
Isotonic cells have an equal concentration of solutes inside and outside the cell; this equalizes the osmotic
pressure on either side of the cell membrane which is a semi-permeable membrane.
Hypertonic Isotonic Hypotonic
solution solution solution
Figure 41.2 Cells placed in a hypertonic environment tend to shrink due to loss of water. In a hypotonic environment,
cells tend to swell due to intake of water. The blood maintains an isotonic environment so that cells neither shrink nor
swell, (credit: Mariana Ruiz Villareal)
The body does not exist in isolation. There is a constant input of water and electrolytes into the system. While
osmoregulation is achieved across membranes within the body, excess electrolytes and wastes are transported
to the kidneys and excreted, helping to maintain osmotic balance.
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Chapter 411 Osmotic Regulation and Excretion
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Need for Osmoregulation
Biological systems constantly interact and exchange water and nutrients with the environment by way of
consumption of food and water and through excretion in the form of sweat, urine, and feces. Without a
mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to
accumulate toxic waste and water, which can have dire consequences.
Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but
also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma,
extracellular fluid, and intracellular fluid. Since osmotic pressure is regulated by the movement of water across
membranes, the volume of the fluid compartments can also change temporarily. Because blood plasma is one
of the fluid components, osmotic pressures have a direct bearing on blood pressure.
Transport of Electrolytes across Cell Membranes
Electrolytes, such as sodium chloride, ionize in water, meaning that they dissociate into their component
ions. In water, sodium chloride (NaCI), dissociates into the sodium ion (Na + ) and the chloride ion (CP). The
most important ions, whose concentrations are very closely regulated in body fluids, are the cations sodium
(Na + ), potassium (K + ), calcium (Ca +2 ), magnesium (Mg +2 ), and the anions chloride (Cl'), carbonate (CO 3 ’ 2 ),
bicarbonate (HCO 3 ’), and phosphate(PC> 3 "). Electrolytes are lost from the body during urination and perspiration.
For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity
and perspiration.
Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to
the number of solute atoms or molecules and not dependent on the size of the solute molecules. Because
electrolytes dissociate into their component ions, they, in essence, add more solute particles into the solution
and have a greater effect on osmotic pressure, per mass than compounds that do not dissociate in water, such
as glucose.
Water can pass through membranes by passive diffusion. If electrolyte ions could passively diffuse across
membranes, it would be impossible to maintain specific concentrations of ions in each fluid compartment
therefore they require special mechanisms to cross the semi-permeable membranes in the body. This movement
can be accomplished by facilitated diffusion and active transport. Facilitated diffusion requires protein-based
channels for moving the solute. Active transport requires energy in the form of ATP conversion, carrier proteins,
or pumps in order to move ions against the concentration gradient.
Concept of Osmolality and Milliequivalent
In order to calculate osmotic pressure, it is necessary to understand how solute concentrations are measured.
The unit for measuring solutes is the mole. One mole is defined as the gram molecular weight of the solute.
For example, the molecular weight of sodium chloride is 58.44. Thus, one mole of sodium chloride weighs 58.44
grams. The molarity of a solution is the number of moles of solute per liter of solution. The molality of a solution
is the number of moles of solute per kilogram of solvent. If the solvent is water, one kilogram of water is equal
to one liter of water. While molarity and molality are used to express the concentration of solutions, electrolyte
concentrations are usually expressed in terms of milliequivalents per liter (mEq/L): the mEq/L is equal to the ion
concentration (in millimoles) multiplied by the number of electrical charges on the ion. The unit of milliequivalent
takes into consideration the ions present in the solution (since electrolytes form ions in aqueous solutions) and
the charge on the ions.
Thus, for ions that have a charge of one, one milliequivalent is equal to one millimole. For ions that have
a charge of two (like calcium), one milliequivalent is equal to 0.5 millimoles. Another unit for the expression
of electrolyte concentration is the milliosmole (mOsm), which is the number of milliequivalents of solute per
kilogram of solvent. Body fluids are usually maintained within the range of 280 to 300 mOsm.
Osmoregulators and Osmoconformers
Persons lost at sea without any freshwater to drink are at risk of severe dehydration because the human
body cannot adapt to drinking seawater, which is hypertonic in comparison to body fluids. Organisms such as
goldfish that can tolerate only a relatively narrow range of salinity are referred to as stenohaline. About 90
percent of all bony fish are restricted to either freshwater or seawater. They are incapable of osmotic regulation
in the opposite environment. It is possible, however, for a few fishes like salmon to spend part of their life
in freshwater and part in seawater. Organisms like the salmon and molly that can tolerate a relatively wide
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Chapter 411 Osmotic Regulation and Excretion
range of salinity are referred to as euryhaline organisms. This is possible because some fish have evolved
osmoregulatory mechanisms to survive in all kinds of aquatic environments. When they live in freshwater,
their bodies tend to take up water because the environment is relatively hypotonic, as illustrated in Figure
41.3a. In such hypotonic environments, these fish do not drink much water. Instead, they pass a lot of very
dilute urine, and they achieve electrolyte balance by active transport of salts through the gills. When they move
to a hypertonic marine environment, these fish start drinking seawater; they excrete the excess salts through
their gills and their urine, as illustrated in Figure 41.3b. Most marine invertebrates, on the other hand, may be
isotonic with seawater ( osmoconformers). Their body fluid concentrations conform to changes in seawater
concentration. Cartilaginous fishes’ salt composition of the blood is similar to bony fishes; however, the blood
of sharks contains the organic compounds urea and trimethylamine oxide (TMAO). This does not mean that
their electrolyte composition is similar to that of seawater. They achieve isotonicity with the sea by storing large
concentrations of urea. These animals that secrete urea are called ureotelic animals. TMAO stabilizes proteins
in the presence of high urea levels, preventing the disruption of peptide bonds that would occur in other animals
exposed to similar levels of urea. Sharks are cartilaginous fish with a rectal gland to secrete salt and assist in
osmoregulation.
Absorbs water through skin
(a) Osmoregulation in a freshwater environment
Loses water through skin
u Excretes concentrated urine
(b) Osmoregulation in a saltwater environment
Figure 41.3 Fish are osmoregulators, but must use different mechanisms to survive in (a) freshwater or (b) saltwater
environments, (credit: modification of work by Duane Raver, NOAA)
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Chapter 411 Osmotic Regulation and Excretion
1281
ca eer connection
Dialysis Technician
Dialysis is a medical process of removing wastes and excess water from the blood by diffusion and
ultrafiltration. When kidney function fails, dialysis must be done to artificially rid the body of wastes. This is
a vital process to keep patients alive. In some cases, the patients undergo artificial dialysis until they are
eligible for a kidney transplant. In others who are not candidates for kidney transplants, dialysis is a life-long
necessity.
Dialysis technicians typically work in hospitals and clinics. While some roles in this field include equipment
development and maintenance, most dialysis technicians work in direct patient care. Their on-the-job
duties, which typically occur under the direct supervision of a registered nurse, focus on providing dialysis
treatments. This can include reviewing patient history and current condition, assessing and responding to
patient needs before and during treatment, and monitoring the dialysis process. Treatment may include
taking and reporting a patient’s vital signs and preparing solutions and equipment to ensure accurate and
sterile procedures.
41.2 | The Kidneys and Osmoregulatory Organs
By the end of this section, you will be able to do the following:
• Explain how the kidneys serve as the main osmoregulatory organs in mammalian systems
• Describe the structure of the kidneys and the functions of the parts of the kidney
• Describe how the nephron is the functional unit of the kidney and explain how it actively filters blood and
generates urine
• Detail the three steps in the formation of urine: glomerular filtration, tubular reabsorption, and tubular
secretion
Although the kidneys are the major osmoregulatory organ, the skin and lungs also play a role in the process.
Water and electrolytes are lost through sweat glands in the skin, which helps moisturize and cool the skin
surface, while the lungs expel a small amount of water in the form of mucous secretions and via evaporation of
water vapor.
Kidneys: The Main Osmoregulatory Organ
The kidneys, illustrated in Figure 41.4, are a pair of bean-shaped structures that are located just below and
posterior to the liver in the peritoneal cavity. The adrenal glands sit on top of each kidney and are also called the
suprarenal glands. Kidneys filter blood and purify it. All the blood in the human body is filtered many times a day
by the kidneys; these organs use up almost 25 percent of the oxygen absorbed through the lungs to perform this
function. Oxygen allows the kidney cells to efficiently manufacture chemical energy in the form of ATP through
aerobic respiration. The filtrate coming out of the kidneys is called urine.
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Renal vein
Ureter
Bladder
Urethra
Figure 41.4 Kidneys filter the blood, producing urine that is stored in the bladder prior to elimination through the
urethra, (credit: modification of work by NCI)
Kidney Structure
Externally, the kidneys are surrounded by three layers, illustrated in Figure 41.5. The outermost layer is a tough
connective tissue layer called the renal fascia. The second layer is called the perirenal fat capsule, which
helps anchor the kidneys in place. The third and innermost layer is the renal capsule. Internally, the kidney
has three regions—an outer cortex, a medulla in the middle, and the renal pelvis in the region called the
hilum of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter
and exit the kidney; it is also the point of exit for the ureters. The renal cortex is granular due to the presence of
nephrons— the functional unit of the kidney. The medulla consists of multiple pyramidal tissue masses, called
the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels
pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. There are, on average, eight
renal pyramids in each kidney. The renal pyramids along with the adjoining cortical region are called the lobes
of the kidney. The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the
renal pelvis branches out into two or three extensions called the major calyces, which further branch into the
minor calyces. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.
Renal artery
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Chapter 411 Osmotic Regulation and Excretion
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visual
CONNECTION
Figure 41.5 The internal structure of the kidney is shown, (credit: modification of work by NCI)
Which of the following statements about the kidney is false?
a. The renal pelvis drains into the ureter.
b. The renal pyramids are in the medulla.
c. The cortex covers the capsule.
d. Nephrons are in the renal cortex.
Because the kidney filters blood, its network of blood vessels is an important component of its structure and
function. The arteries, veins, and nerves that supply the kidney enter and exit at the renal hilum. Renal blood
supply starts with the branching of the aorta into the renal arteries (which are each named based on the
region of the kidney they pass through) and ends with the exiting of the renal veins to join the inferior vena
cava. The renal arteries split into several segmental arteries upon entering the kidneys. Each segmental artery
splits further into several interlobar arteries and enters the renal columns, which supply the renal lobes. The
interlobar arteries split at the junction of the renal cortex and medulla to form the arcuate arteries. The arcuate
“bow shaped” arteries form arcs along the base of the medullary pyramids. Cortical radiate arteries, as the
name suggests, radiate out from the arcuate arteries. The cortical radiate arteries branch into numerous afferent
arterioles, and then enter the capillaries supplying the nephrons. Veins trace the path of the arteries and have
similar names, except there are no segmental veins.
As mentioned previously, the functional unit of the kidney is the nephron, illustrated in Figure 41.6. Each kidney
is made up of over one million nephrons that dot the renal cortex, giving it a granular appearance when sectioned
sagittally. There are two types of nephrons— cortical nephrons (85 percent), which are deep in the renal
cortex, and juxtamedullary nephrons (15 percent), which lie in the renal cortex close to the renal medulla. A
nephron consists of three parts—a renal corpuscle, a renal tubule, and the associated capillary network, which
originates from the cortical radiate arteries.
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visual
CONNECTION
Proximal convoluted
Figure 41.6 The nephron is the functional unit of the kidney. The glomerulus and convoluted tubules are located
in the kidney cortex, while collecting ducts are located in the pyramids of the medulla, (credit: modification of work
by NIDDK)
Which of the following statements about the nephron is false?
a. The collecting duct empties into the distal convoluted tubule.
b. The Bowman’s capsule surrounds the glomerulus.
c. The loop of Henle is between the proximal and distal convoluted tubules.
d. The loop of Henle empties into the distal convoluted tubule.
Renal Corpuscle
The renal corpuscle, located in the renal cortex, is made up of a network of capillaries known as the glomerulus
and the capsule, a cup-shaped chamber that surrounds it, called the glomerular or Bowman's capsule.
Renal Tubule
The renal tubule is a long and convoluted structure that emerges from the glomerulus and can be divided into
three parts based on function. The first part is called the proximal convoluted tubule (PCT) due to its proximity
to the glomerulus; it stays in the renal cortex. The second part is called the loop of Henle, or nephritic loop,
because it forms a loop (with descending and ascending limbs) that goes through the renal medulla. The third
part of the renal tubule is called the distal convoluted tubule (DCT) and this part is also restricted to the renal
cortex. The DCT, which is the last part of the nephron, connects and empties its contents into collecting ducts
that line the medullary pyramids. The collecting ducts amass contents from multiple nephrons and fuse together
as they enter the papillae of the renal medulla.
Capillary Network within the Nephron
The capillary network that originates from the renal arteries supplies the nephron with blood that needs to
be filtered. The branch that enters the glomerulus is called the afferent arteriole. The branch that exits the
glomerulus is called the efferent arteriole. Within the glomerulus, the network of capillaries is called the
glomerular capillary bed. Once the efferent arteriole exits the glomerulus, it forms the peritubular capillary
network, which surrounds and interacts with parts of the renal tubule. In cortical nephrons, the peritubular
capillary network surrounds the PCT and DCT. In juxtamedullary nephrons, the peritubular capillary network
forms a network around the loop of Henle and is called the vasa recta.
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LINK TQ LEARNING
Go to this website (http:// 0 penstaxc 0 llege. 0 rg/l/kidney_secti 0 n) to see another coronal section of the
kidney and to explore an animation of the workings of nephrons.
Kidney Function and Physiology
Kidneys filter blood in a three-step process. First, the nephrons filter blood that runs through the capillary network
in the glomerulus. Almost all solutes, except for proteins, are filtered out into the glomerulus by a process called
glomerular filtration. Second, the filtrate is collected in the renal tubules. Most of the solutes get reabsorbed
in the PCT by a process called tubular reabsorption. In the loop of Henle, the filtrate continues to exchange
solutes and water with the renal medulla and the peritubular capillary network. Water is also reabsorbed during
this step. Then, additional solutes and wastes are secreted into the kidney tubules during tubular secretion,
which is, in essence, the opposite process to tubular reabsorption. The collecting ducts collect filtrate coming
from the nephrons and fuse in the medullary papillae. From here, the papillae deliver the filtrate, now called
urine, into the minor calyces that eventually connect to the ureters through the renal pelvis. This entire process
is illustrated in Figure 41.7.
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O Di-nvimol rrvm/nli itoH ti ■ hi i lo ■
Distal tubule:
selectively secretes
and absorbs different
ions to maintain blood
pH and electrolyte
balance
6. Collecting
duct:
reabsorbs
solutes and
water from
the filtrate
4. Ascending loop
of Henle:
3. Descending
loop of Henle:
aquaporins
allow water
to pass from
the filtrate
into the
interstitial fluid
reabsorbs Na + and
Cl - from the filtrate
into the interstitial
fluid
U
Figure 41.7 Each part of the nephron performs a different function in filtering waste and maintaining homeostatic
balance. (1) The glomerulus forces small solutes out of the blood by pressure. (2) The proximal convoluted tubule
reabsorbs ions, water, and nutrients from the filtrate into the interstitial fluid, and actively transports toxins and drugs
from the interstitial fluid into the filtrate. The proximal convoluted tubule also adjusts blood pH by selectively secreting
ammonia (NH3) into the filtrate, where it reacts with H + to form NH 4 + . The more acidic the filtrate, the more ammonia
is secreted. (3) The descending loop of Henle is lined with cells containing aquaporins that allow water to pass from
the filtrate into the interstitial fluid. (4) In the thin part of the ascending loop of Henle, Na + and Cl" ions diffuse into the
interstitial fluid. In the thick part, these same ions are actively transported into the interstitial fluid. Because salt but not
water is lost, the filtrate becomes more dilute as it travels up the limb. (5) In the distal convoluted tubule, K + and H + ions
are selectively secreted into the filtrate, while Na + , Cl", and HCO 3 " ions are reabsorbed to maintain pH and electrolyte
balance in the blood. (6) The collecting duct reabsorbs solutes and water from the filtrate, forming dilute urine, (credit:
modification of work by NIDDK)
Glomerular Filtration
Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in
the afferent arteriole. The blood pressure in the glomerulus is maintained independent of factors that affect
systemic blood pressure. The “leaky" connections between the endothelial cells of the glomerular capillary
network allow solutes to pass through easily. All solutes in the glomerular capillaries, except for macromolecules
like proteins, pass through by passive diffusion. There is no energy requirement at this stage of the filtration
process. Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys.
GFR is regulated by multiple mechanisms and is an important indicator of kidney function.
To learn more about the vascular system of kidneys, click through this review (http:// 0 penstaxc 0 llege. 0 rg/l/
kidneys) and the steps of blood flow.
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Chapter 411 Osmotic Regulation and Excretion
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Tubular Reabsorption and Secretion
Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed, and this
occurs either by passive or active transport. Reabsorption of water and some key electrolytes are regulated
and can be influenced by hormones. Sodium (Na + ) is the most abundant ion and most of it is reabsorbed by
active transport and then transported to the peritubular capillaries. Because Na + is actively transported out of
the tubule, water follows it to even out the osmotic pressure. Water is also independently reabsorbed into the
peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT. This occurs due to
the low blood pressure and high osmotic pressure in the peritubular capillaries. However, every solute has a
transport maximum and the excess is not reabsorbed.
in the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water,
not solutes; the opposite is true for the ascending limb. Additionally, the loop of Henle invades the renal medulla,
which is naturally high in salt concentration and tends to absorb water from the renal tubule and concentrate
the filtrate. The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop
of Henle perform opposing functions, as illustrated in Figure 41.8, it acts as a countercurrent multiplier. The
vasa recta around it acts as the countercurrent exchanger.
visual
CONNECTION
Filtrate enters the
descending limb.
Filtrate exits the
ascending limb.
Loop of
Henle
Interstitial
fluid
Figure 41.8 The loop of Henle acts as a countercurrent multiplier that uses energy to create concentration
gradients. The descending limb is water permeable. Water flows from the filtrate to the interstitial fluid, so
osmolality inside the limb increases as it descends into the renal medulla. At the bottom, the osmolality is higher
inside the loop than in the interstitial fluid. Thus, as filtrate enters the ascending limb, Na + and Cl" ions exit through
ion channels present in the cell membrane. Further up, Na + is actively transported out of the filtrate and Cl" follows.
Osmolarity is given in units of milliosmoles per liter (mOsm/L).
Loop diuretics are drugs sometimes used to treat hypertension. These drugs inhibit the reabsorption of Na +
and Cl" ions by the ascending limb of the loop of Henle. A side effect is that they increase urination. Why do
you think this is the case?
By the time the filtrate reaches the DCT, most of the urine and solutes have been reabsorbed. If the body
requires additional water, all of it can be reabsorbed at this point. Further reabsorption is controlled by hormones,
which will be discussed in a later section. Excretion of wastes occurs due to lack of reabsorption combined with
tubular secretion. Undesirable products like metabolic wastes, urea, uric acid, and certain drugs, are excreted
by tubular secretion. Most of the tubular secretion happens in the DCT, but some occurs in the early part of the
collecting duct. Kidneys also maintain an acid-base balance by secreting excess H + ions.
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Although parts of the renal tubules are named proximal and distal, in a cross-section of the kidney, the tubules
are placed close together and in contact with each other and the glomerulus. This allows for exchange of
chemical messengers between the different cell types. For example, the DCT ascending limb of the loop of
Henle has masses of cells called macula densa, which are in contact with cells of the afferent arterioles called
juxtaglomerular cells. Together, the macula densa and juxtaglomerular cells form the juxtaglomerular complex
(JGC). The JGC is an endocrine structure that secretes the enzyme renin and the hormone erythropoietin. When
hormones trigger the macula densa cells in the DCT due to variations in blood volume, blood pressure, or
electrolyte balance, these cells can immediately communicate the problem to the capillaries in the afferent and
efferent arterioles, which can constrict or relax to change the glomerular filtration rate of the kidneys.
career connection
Nephrologist
A nephrologist studies and deals with diseases of the kidneys—both those that cause kidney failure (such as
diabetes) and the conditions that are produced by kidney disease (such as hypertension). Blood pressure,
blood volume, and changes in electrolyte balance come under the purview of a nephrologist.
Nephrologists usually work with other physicians who refer patients to them or consult with them about
specific diagnoses and treatment plans. Patients are usually referred to a nephrologist for symptoms such
as blood or protein in the urine, very high blood pressure, kidney stones, or renal failure.
Nephrology is a subspecialty of internal medicine. To become a nephrologist, medical school is followed
by additional training to become certified in internal medicine. An additional two or more years is spent
specifically studying kidney disorders and their accompanying effects on the body.
41.3 | Excretion Systems
By the end of this section, you will be able to do the following:
• Explain how vacuoles, present in microorganisms, work to excrete waste
• Describe the way in which flame cells and nephridia in worms perform excretory functions and maintain
osmotic balance
• Explain how insects use Malpighian tubules to excrete wastes and maintain osmotic balance
Microorganisms and invertebrate animals use more primitive and simple mechanisms to get rid of their metabolic
wastes than the mammalian system of kidney and urinary function. Three excretory systems evolved in
organisms before complex kidneys: vacuoles, flame cells, and Malpighian tubules.
Contractile Vacuoles in Microorganisms
The most fundamental feature of life is the presence of a cell. In other words, a cell is the simplest functional
unit of a life. Bacteria are unicellular, prokaryotic organisms that have some of the least complex life processes
in place; however, prokaryotes such as bacteria do not contain membrane-bound vacuoles. The cells of
microorganisms like bacteria, protozoa, and fungi are bound by cell membranes and use them to interact with
the environment. Some cells, including some leucocytes in humans, are able to engulf food by endocytosis—the
formation of vesicles by involution of the cell membrane within the cells. The same vesicles are able to interact
and exchange metabolites with the intracellular environment. In some unicellular eukaryotic organisms such as
the amoeba, shown in Figure 41.9, cellular wastes and excess water are excreted by exocytosis, when the
contractile vacuoles merge with the cell membrane and expel wastes into the environment. Contractile vacuoles
(CV) should not be confused with vacuoles, which store food or water.
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Chapter 411 Osmotic Regulation and Excretion
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Figure 41.9 Some unicellular organisms, such as the amoeba, ingest food by endocytosis. The food vesicle fuses with
a lysosome, which digests the food. Waste is excreted by exocytosis.
Flame Cells of Planaria and Nephridia of Worms
As multicellular systems evolved to have organ systems that divided the metabolic needs of the body, individual
organs evolved to perform the excretory function. Planaria are flatworms that live in freshwater. Their excretory
system consists of two tubules connected to a highly branched duct system. The cells in the tubules are called
flame cells (or protonephridia) because they have a cluster of cilia that looks like a flickering flame when
viewed under the microscope, as illustrated in Figure 41.10a. The cilia propel waste matter down the tubules and
out of the body through excretory pores that open on the body surface; cilia also draw water from the interstitial
fluid, allowing for filtration. Any valuable metabolites are recovered by reabsorption. Flame cells are found in
flatworms, including parasitic tapeworms and free-living planaria. They also maintain the organism’s osmotic
balance.
Intestine
(a) Flame cell of a planarian (b) Nephridium of an earthworm
Figure 41.10 In the excretory system of the (a) planaria, cilia of flame cells propel waste through a tubule formed
by a tube cell. Tubules are connected into branched structures that lead to pores located all along the sides of the
body. The filtrate is secreted through these pores. In (b) annelids such as earthworms, nephridia filter fluid from the
coelom, or body cavity. Beating cilia at the opening of the nephridium draw water from the coelom into a tubule. As
the filtrate passes down the tubules, nutrients and other solutes are reabsorbed by capillaries. Filtered fluid containing
nitrogenous and other wastes is stored in a bladder and then secreted through a pore in the side of the body.
Earthworms (annelids) have slightly more evolved excretory structures called nephridia, illustrated in Figure
41.10b. A pair of nephridia is present on each segment of the earthworm. They are similar to flame cells in that
they have a tubule with cilia. Excretion occurs through a pore called the nephridiopore. They are more evolved
than the flame cells in that they have a system for tubular reabsorption by a capillary network before excretion.
Malpighian Tubules of Insects
Malpighian tubules are found lining the gut of some species of arthropods, such as the bee illustrated in Figure
41.11. They are usually found in pairs and the number of tubules varies with the species of insect. Malpighian
tubules are convoluted, which increases their surface area, and they are lined with microvilli for reabsorption
and maintenance of osmotic balance. Malpighian tubules work cooperatively with specialized glands in the wall
of the rectum. Body fluids are not filtered as in the case of nephridia; urine is produced by tubular secretion
mechanisms by the cells lining the Malpighian tubules that are bathed in hemolymph (a mixture of blood and
interstitial fluid that is found in insects and other arthropods as well as most mollusks). Metabolic wastes like
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uric acid freely diffuse into the tubules. There are exchange pumps lining the tubules, which actively transport
H + ions into the cell and K + or Na + ions out; water passively follows to form urine. The secretion of ions alters
the osmotic pressure which draws water, electrolytes, and nitrogenous waste (uric acid) into the tubules. Water
and electrolytes are reabsorbed when these organisms are faced with low-water environments, and uric acid is
excreted as a thick paste or powder. Not dissolving wastes in water helps these organisms to conserve water;
this is especially important for life in dry environments.
Figure 41.11 Malpighian tubules of insects and other terrestrial arthropods remove nitrogenous wastes and other
solutes from the hemolymph. Na + and/or K + ions are actively transported into the lumen of the tubules. Water then
enters the tubules via osmosis, forming urine. The urine passes through the intestine, and into the rectum. There,
nutrients diffuse back into the hemolymph. Na + and/or K + ions are pumped into the hemolymph, and water follows.
The concentrated waste is then excreted.
LINK TQ LEARNING
See a dissected cockroach, including a close-up look at its Malpighian tubules, in this video
(https:// 0 penstax. 0 rg/l/malpighian) .
41.4 | Nitrogenous Wastes
By the end of this section, you will be able to do the following:
• Compare and contrast the way in which aquatic animals and terrestrial animals can eliminate toxic
ammonia from their systems
• Compare the major byproduct of ammonia metabolism in vertebrate animals to that of birds, insects, and
reptiles
Of the four major macromolecules in biological systems, both proteins and nucleic acids contain nitrogen.
During the catabolism, or breakdown, of nitrogen-containing macromolecules, carbon, hydrogen, and oxygen
are extracted and stored in the form of carbohydrates and fats. Excess nitrogen is excreted from the body.
Nitrogenous wastes tend to form toxic ammonia, which raises the pH of body fluids. The formation of ammonia
itself requires energy in the form of ATP and large quantities of water to dilute it out of a biological system.
Animals that live in aquatic environments tend to release ammonia into the water. Animals that excrete ammonia
are said to be ammonotelic. Terrestrial organisms have evolved other mechanisms to excrete nitrogenous
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Chapter 411 Osmotic Regulation and Excretion
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wastes. The animals must detoxify ammonia by converting it into a relatively nontoxic form such as urea or uric
acid. Mammals, including humans, produce urea, whereas reptiles and many terrestrial invertebrates produce
uric acid. Animals that secrete urea as the primary nitrogenous waste material are called ureotelic animals.
Nitrogenous Waste in Terrestrial Animals: The Urea Cycle
The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver
and excreted in urine. The overall chemical reaction by which ammonia is converted to urea is 2 NH3 (ammonia)
+ CO2 + 3 ATP + H2O - H2N-CO-NH2 (urea) + 2 ADP + 4 Pi + AMP.
The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to
urea, as shown in Figure 41.12. The amino acid L-ornithine gets converted into different intermediates before
being regenerated at the end of the urea cycle. Hence, the urea cycle is also referred to as the ornithine cycle.
The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle and its deficiency can lead to
accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria and the
last three reactions occur in the cytosol. Urea concentration in the blood, called blood urea nitrogen or BUN, is
used as an indicator of kidney function.
Fumarate
Figure 41.12 The urea cycle converts ammonia to urea.
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Chapter 411 Osmotic Regulation and Excretion
e olution CONNECTION
Excretion of Nitrogenous Waste
The theory of evolution proposes that life started in an aquatic environment. It is not surprising to see
that biochemical pathways like the urea cycle evolved to adapt to a changing environment when terrestrial
life forms evolved. Arid conditions probably led to the evolution of the uric acid pathway as a means of
conserving water.
Nitrogenous Waste in Birds and Reptiles: Uric Acid
Birds, reptiles, and most terrestrial arthropods convert toxic ammonia to uric acid or the closely related
compound guanine (guano) instead of urea. Mammals also form some uric acid during breakdown of nucleic
acids. Uric acid is a compound similar to purines found in nucleic acids. It is water insoluble and tends to form a
white paste or powder; it is excreted by birds, insects, and reptiles. Conversion of ammonia to uric acid requires
more energy and is much more complex than conversion of ammonia to urea Figure 41.13.
(a) Many invertebrates and (b) Mammals, many adult (c) Insects, land snails, birds,
aquatic species excrete amphibians, and some marine and many reptiles excrete
ammonia. species excrete urea. uric acid.
Figure 41.13 Nitrogenous waste is excreted in different forms by different species. These include (a) ammonia, (b)
urea, and (c) uric acid, (credit a: modification of work by Eric Engbretson, USFWS; credit b: modification of work by B.
"Moose" Peterson, USFWS; credit c: modification of work by Dave Menke, USFWS)
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Chapter 411 Osmotic Regulation and Excretion
1293
everyday CONNECTION
Gout
Mammals use uric acid crystals as an antioxidant in their cells. However, too much uric acid tends to form
kidney stones and may also cause a painful condition called gout, where uric acid crystals accumulate in the
joints, as illustrated in Figure 41.14. Food choices that reduce the amount of nitrogenous bases in the diet
help reduce the risk of gout. For example, tea, coffee, and chocolate have purine-like compounds, called
xanthines, and should be avoided by people with gout and kidney stones.
Figure 41.14 Gout causes the inflammation visible in this person’s left big toe joint, (credit: "Gonzosft"/Wikimedia
Commons)
41.5 | Hormonal Control of Osmoregulatory Functions
By the end of this section, you will be able to do the following:
• Explain how hormonal cues help the kidneys synchronize the osmotic needs of the body
• Describe how hormones like epinephrine, norepinephrine, renin-angiotensin, aldosterone, anti-diuretic
hormone, and atrial natriuretic peptide help regulate waste elimination, maintain correct osmolarity, and
perform other osmoregulatory functions
While the kidneys operate to maintain osmotic balance and blood pressure in the body, they also act in concert
with hormones. Hormones are small molecules that act as messengers within the body. Hormones are typically
secreted from one cell and travel in the bloodstream to affect a target cell in another portion of the body. Different
regions of the nephron bear specialized cells that have receptors to respond to chemical messengers and
hormones. Table 41.1 summarizes the hormones that control the osmoregulatory functions.
Hormones That Affect Osmoregulation
Hormone
Where produced
Function
Epinephrine and
Norepinephrine
Adrenal medulla
Can decrease kidney function temporarily by vasoconstriction
Renin
Kidney nephrons
Increases blood pressure by acting on angiotensinogen
Table 41.1
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Chapter 411 Osmotic Regulation and Excretion
Hormones That Affect Osmoregulation
Hormone
Where produced
Function
Angiotensin
Liver
Angiotensin II affects multiple processes and increases blood
pressure
Aldosterone
Adrenal cortex
Prevents loss of sodium and water
Anti-diuretic
hormone
(vasopressin)
Hypothalamus (stored
in the posterior
pituitary)
Prevents water loss
Atrial natriuretic
peptide
Heart atrium
Decreases blood pressure by acting as a vasodilator and
increasing glomerular filtration rate; decreases sodium
reabsorption in kidneys
Table 41.1
Epinephrine and Norepinephrine
Epinephrine and norepinephrine are released by the adrenal medulla and nervous system respectively. They
are the flight/fight hormones that are released when the body is under extreme stress. During stress, much of
the body’s energy is used to combat imminent danger. Kidney function is halted temporarily by epinephrine and
norepinephrine. These hormones function by acting directly on the smooth muscles of blood vessels to constrict
them. Once the afferent arterioles are constricted, blood flow into the nephrons stops. These hormones go one
step further and trigger the renin-angiotensin-aldosterone system.
Renin-Angiotensin-Aldosterone
The renin-angiotensin-aldosterone system, illustrated in Figure 41.15 proceeds through several steps to
produce angiotensin II, which acts to stabilize blood pressure and volume. Renin (secreted by a part of
the juxtaglomerular complex) is produced by the granular cells of the afferent and efferent arterioles. Thus,
the kidneys control blood pressure and volume directly. Renin acts on angiotensinogen, which is made in
the liver and converts it to angiotensin I. Angiotensin converting enzyme (ACE) converts angiotensin I to
angiotensin II. Angiotensin II raises blood pressure by constricting blood vessels. It also triggers the release of
the mineralocorticoid aldosterone from the adrenal cortex, which in turn stimulates the renal tubules to reabsorb
more sodium. Angiotensin II also triggers the release of anti-diuretic hormone (ADH) from the hypothalamus,
leading to water retention in the kidneys. It acts directly on the nephrons and decreases glomerular filtration rate.
Medically, blood pressure can be controlled by drugs that inhibit ACE (called ACE inhibitors).
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The renin-angiotensin-aldosterone system increases blood volume and pressure
Renin
ACE
Angiotensin
c=
=>
Angiotensin 1
1=
=>
Angiotensin II
Angiotensin is
made by the liver.
Aldosterone is
produced by the
adrenal glands,
located on top
of the kidneys.
Triggers release of
other hormones
/ V
■ ADH is made in the
hypothalamus and
released by the
posterior pituitary.
ANP is made
by atrial cells
in the heart.
Aldosterone
ADH
Direct effects:
• Causes arteries to constrict and
increases cardiac output resulting
in an increase in blood pressure
and volume
• Decreases glomerular filtration rate
resulting in water retention
• Increases thirst
Causes nephron distal
tubules to reabsorb more
Na + and water, which
increases blood volume
■ Mediates insertion of aquaporins
into nephron collecting duct cells;
as a result, more water is reabsorbed
into the blood
■ Increases sodium reabsorption in the
medulla of the kidney
Renin is
produced
by the kidney.
ANP is a hormone antagonistic to the angiotensin pathway.
ANP decreases blood voluume and pressure by:
• Increasing the glomular filtration rate
• Decreasing of reabsorption of Na + by nephrons
• Inhibiting the release of renin, aldosterone, and ADH
Figure 41.15 The renin-angiotensin-aldosterone system increases blood pressure and volume. The hormone ANP has
antagonistic effects, (credit: modification of work by Mikael Haggstrom)
Mineralocorticoids
Mineralocorticoids are hormones synthesized by the adrenal cortex that affect osmotic balance. Aldosterone is a
mineralocorticoid that regulates sodium levels in the blood. Almost all of the sodium in the blood is reclaimed by
the renal tubules under the influence of aldosterone. Because sodium is always reabsorbed by active transport
and water follows sodium to maintain osmotic balance, aldosterone manages not only sodium levels but also
the water levels in body fluids. In contrast, the aldosterone also stimulates potassium secretion concurrently
with sodium reabsorption. In contrast, absence of aldosterone means that no sodium gets reabsorbed in the
renal tubules and all of it gets excreted in the urine. In addition, the daily dietary potassium load is not secreted
and the retention of K + can cause a dangerous increase in plasma K + concentration. Patients who have
Addison's disease have a failing adrenal cortex and cannot produce aldosterone. They lose sodium in their urine
constantly, and if the supply is not replenished, the consequences can be fatal.
Antidiurectic Hormone
As previously discussed, antidiuretic hormone or ADH (also called vasopressin), as the name suggests,
helps the body conserve water when body fluid volume, especially that of blood, is low. It is formed by the
hypothalamus and is stored and released from the posterior pituitary. It acts by inserting aquaporins in the
collecting ducts and promotes reabsorption of water. ADH also acts as a vasoconstrictor and increases blood
pressure during hemorrhaging.
Atrial Natriuretic Peptide Hormone
The atrial natriuretic peptide (ANP) lowers blood pressure by acting as a vasodilator. It is released by cells in
the atrium of the heart in response to high blood pressure and in patients with sleep apnea. ANP affects salt
release, and because water passively follows salt to maintain osmotic balance, it also has a diuretic effect. ANP
also prevents sodium reabsorption by the renal tubules, decreasing water reabsorption (thus acting as a diuretic)
and lowering blood pressure. Its actions suppress the actions of aldosterone, ADH, and renin.
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Chapter 411 Osmotic Regulation and Excretion
KEY TERMS
afferent arteriole arteriole that branches from the cortical radiate artery and enters the glomerulus
ammonia compound made of one nitrogen atom and three hydrogen atoms
ammonotelic describes an animal that excretes ammonia as the primary waste material
angiotensin converting enzyme (ACE) enzyme that converts angiotensin I to angiotensin II
angiotensin I product in the renin-angiotensin-aldosterone pathway
angiotensin II molecule that affects different organs to increase blood pressure
anti-diuretic hormone (ADH) hormone that prevents the loss of water
antioxidant agent that prevents cell destruction by reactive oxygen species
arcuate artery artery that branches from the interlobar artery and arches over the base of the renal pyramids
ascending limb part of the loop of Henle that ascends from the renal medulla to the renal cortex
blood urea nitrogen (BUN) estimate of urea in the blood and an indicator of kidney function
Bowman's capsule structure that encloses the glomerulus
calyx structure that connects the renal pelvis to the renal medulla
cortex (animal) outer layer of an organ like the kidney or adrenal gland
cortical nephron nephron that lies in the renal cortex
cortical radiate artery artery that radiates from the arcuate arteries into the renal cortex
countercurrent exchanger peritubular capillary network that allows exchange of solutes and water from the
renal tubules
countercurrent multiplier osmotic gradient in the renal medulla that is responsible for concentration of urine
descending limb part of the loop of Henle that descends from the renal cortex into the renal medulla
distal convoluted tubule (DCT) part of the renal tubule that is the most distant from the glomerulus
efferent arteriole arteriole that exits from the glomerulus
electrolyte solute that breaks down into ions when dissolved in water
flame cell (also, protonephridia) excretory cell found in flatworms
glomerular filtration filtration of blood in the glomerular capillary network into the glomerulus
glomerular filtration rate (GFR) amount of filtrate formed by the glomerulus per minute
glomerulus (renal) part of the renal corpuscle that contains the capillary network
hilum region in the renal pelvis where blood vessels, nerves, and ureters bunch before entering or exiting the
kidney
inferior vena cava one of the main veins in the human body
interlobar artery artery that branches from the segmental artery and travels in between the renal lobes
juxtaglomerular cell cell in the afferent and efferent arterioles that responds to stimuli from the macula densa
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juxtamedullary nephron nephron that lies in the cortex but close to the renal medulla
kidney organ that performs excretory and osmoregulatory functions
lobes of the kidney renal pyramid along with the adjoining cortical region
loop of Henle part of the renal tubule that loops into the renal medulla
macula densa group of cells that senses changes in sodium ion concentration; present in parts of the renal
tubule and collecting ducts
Malpighian tubule excretory tubules found in arthropods
medulla middle layer of an organ like the kidney or adrenal gland
microvilli cellular processes that increase the surface area of cells
molality number of moles of solute per kilogram of solvent
molarity number of moles of solute per liter of solution
mole gram equivalent of the molecular weight of a substance
nephridia excretory structures found in annelids
nephridiopore pore found at the end of nephridia
nephron functional unit of the kidney
non-electrolyte solute that does not break down into ions when dissolved in water
osmoconformer organism that changes its tonicity based on its environment
osmoregulation mechanism by which water and solute concentrations are maintained at desired levels
osmoregulator organism that maintains its tonicity irrespective of its environment
osmotic balance balance of the amount of water and salt input and output to and from a biological system
without disturbing the desired osmotic pressure and solute concentration in every compartment
osmotic pressure pressure exerted on a membrane to equalize solute concentration on either side
perirenal fat capsule fat layer that suspends the kidneys
peritubular capillary network capillary network that surrounds the renal tubule after the efferent artery exits
the glomerulus
proximal convoluted tubule (PCT) part of the renal tubule that lies close to the glomerulus
renal artery branch of the artery that enters the kidney
renal capsule layer that encapsulates the kidneys
renal column area of the kidney through which the interlobar arteries travel in the process of supplying blood to
the renal lobes
renal corpuscle glomerulus and the Bowman's capsule together
renal fascia connective tissue that supports the kidneys
renal pelvis region in the kidney where the calyces join the ureters
renal pyramid conical structure in the renal medulla
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Chapter 411 Osmotic Regulation and Excretion
renal tubule tubule of the nephron that arises from the glomerulus
renal vein branch of a vein that exits the kidney and joins the inferior vena cava
renin-angiotensin-aldosterone biochemical pathway that activates angiotensin II, which increases blood
pressure
segmental artery artery that branches from the renal artery
semi-permeable membrane membrane that allows only certain solutes to pass through
transport maximum maximum amount of solute that can be transported out of the renal tubules during
reabsorption
tubular reabsorption reclamation of water and solutes that got filtered out in the glomerulus
tubular secretion process of secretion of wastes that do not get reabsorbed
urea cycle pathway by which ammonia is converted to urea
ureotelic describes animals that secrete urea as the primary nitrogenous waste material
ureter urine-bearing tube coming out of the kidney; carries urine to the bladder
uric acid byproduct of ammonia metabolism in birds, insects, and reptiles
urinary bladder structure that the ureters empty the urine into; stores urine
urine filtrate produced by kidneys that gets excreted out of the body
vasa recta peritubular network that surrounds the loop of Henle of the juxtamedullary nephrons
vasodilator compound that increases the diameter of blood vessels
vasopressin another name for anti-diuretic hormone
CHAPTER SUMMARY
41.1 Osmoregulation and Osmotic Balance
Solute concentrations across semi-permeable membranes influence the movement of water and solutes across
the membrane. It is the number of solute molecules and not the molecular size that is important in osmosis.
Osmoregulation and osmotic balance are important bodily functions, resulting in water and salt balance. Not all
solutes can pass through a semi-permeable membrane. Osmosis is the movement of water across the
membrane. Osmosis occurs to equalize the number of solute molecules across a semi-permeable membrane
by the movement of water to the side of higher solute concentration. Facilitated diffusion utilizes protein
channels to move solute molecules from areas of higher to lower concentration while active transport
mechanisms are required to move solutes against concentration gradients. Osmolarity is measured in units of
milliequivalents or milliosmoles, both of which take into consideration the number of solute particles and the
charge on them. Fish that live in freshwater or saltwater adapt by being osmoregulators or osmoconformers.
41.2 The Kidneys and Osmoregulatory Organs
The kidneys are the main osmoregulatory organs in mammalian systems; they function to filter blood and
maintain the osmolarity of body fluids at 300 mOsm. They are surrounded by three layers and are made up
internally of three distinct regions—the cortex, medulla, and pelvis.
The blood vessels that transport blood into and out of the kidneys arise from and merge with the aorta and
inferior vena cava, respectively. The renal arteries branch out from the aorta and enter the kidney where they
further divide into segmental, interlobar, arcuate, and cortical radiate arteries.
The nephron is the functional unit of the kidney, which actively filters blood and generates urine. The nephron is
made up of the renal corpuscle and renal tubule. Cortical nephrons are found in the renal cortex, while
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Chapter 411 Osmotic Regulation and Excretion
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juxtamedullary nephrons are found in the renal cortex close to the renal medulla. The nephron filters and
exchanges water and solutes with two sets of blood vessels and the tissue fluid in the kidneys.
There are three steps in the formation of urine: glomerular filtration, which occurs in the glomerulus; tubular
reabsorption, which occurs in the renal tubules; and tubular secretion, which also occurs in the renal tubules.
41.3 Excretion Systems
Many systems have evolved for excreting wastes that are simpler than the kidney and urinary systems of
vertebrate animals. The simplest system is that of contractile vacuoles present in microorganisms. Flame cells
and nephridia in worms perform excretory functions and maintain osmotic balance. Some insects have evolved
Malpighian tubules to excrete wastes and maintain osmotic balance.
41.4 Nitrogenous Wastes
Ammonia is the waste produced by metabolism of nitrogen-containing compounds like proteins and nucleic
acids. While aquatic animals can easily excrete ammonia into their watery surroundings, terrestrial animals
have evolved special mechanisms to eliminate the toxic ammonia from their systems. Urea is the major
byproduct of ammonia metabolism in vertebrate animals. Uric acid is the major byproduct of ammonia
metabolism in birds, terrestrial arthropods, and reptiles.
41.5 Hormonal Control of Osmoregulatory Functions
Hormonal cues help the kidneys synchronize the osmotic needs of the body. Hormones like epinephrine,
norepinephrine, renin-angiotensin, aldosterone, anti-diuretic hormone, and atrial natriuretic peptide help
regulate the needs of the body as well as the communication between the different organ systems.
VISUAL CONNECTION QUESTIONS
1. Figure 41.5 Which of the following statements
about the kidney is false?
a. The renal pelvis drains into the ureter.
b. The renal pyramids are in the medulla.
c. The cortex covers the capsule.
d. Nephrons are in the renal cortex.
2. Figure 41.6 Which of the following statements
about the nephron is false?
REVIEW QUESTIONS
4. When a dehydrated human patient needs to be
given fluids intravenously, he or she is given:
a. water, which is hypotonic with respect to
body fluids
b. saline at a concentration that is isotonic with
respect to body fluids
c. glucose because it is a non-electrolyte
d. blood
5. The sodium ion is at the highest concentration in:
a. The collecting duct empties into the distal
convoluted tubule.
b. The Bowman’s capsule surrounds the
glomerulus.
c. The loop of Henle is between the proximal
and distal convoluted tubules.
d. The loop of Henle empties into the distal
convoluted tubule.
3. Figure 41.8 Loop diuretics are drugs sometimes
used to treat hypertension. These drugs inhibit the
reabsorption of Na + and Cl" ions by the ascending
limb of the loop of Henle. A side effect is that they
increase urination. Why do you think this is the case?
a. intracellular fluid
b. extracellular fluid
c. blood plasma
d. none of the above
6 . Cells in a hypertonic solution tend to:
a. shrink due to water loss
b. swell due to water gain
c. stay the same size due to water moving into
and out of the cell at the same rate
d. none of the above
7. The macula densa is/are:
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Chapter 411 Osmotic Regulation and Excretion
a. present in the renal medulla.
b. dense tissue present in the outer layer of
the kidney.
c. cells present in the DCT and collecting
tubules.
d. present in blood capillaries.
8 . The osmolarity of body fluids is maintained at
a. 100 mOsm
b. 300 mOsm
c. 1000 mOsm
d. it is not constantly maintained
9. The gland located at the top of the kidney is the
_gland.
a. adrenal
b. pituitary
c. thyroid
d. thymus
10. Active transport of K + in Malpighian tubules
ensures that:
a. water follows K + to make urine
b. osmotic balance is maintained between
waste matter and bodily fluids
c. both a and b
d. neither a nor b
11. Contractile vacuoles in microorganisms:
a. exclusively perform an excretory function
b. can perform many functions, one of which is
excretion of metabolic wastes
c. originate from the cell membrane
d. both b and c
12. Flame cells are primitive excretory organs found
in_.
CRITICAL THINKING QUESTIONS
18. Why is excretion important in order to achieve
osmotic balance?
19. Why do electrolyte ions move across membranes
by active transport?
20. Why are the loop of Henle and vasa recta
important for the formation of concentrated urine?
21. Describe the structure of the kidney.
22. Why might specialized organs have evolved for
excretion of wastes?
23. Explain two different excretory systems other
a. arthropods
b. annelids
c. mammals
d. flatworms
13. BUN is_.
a. blood urea nitrogen
b. blood uric acid nitrogen
c. an indicator of blood volume
d. an indicator of blood pressure
14. Human beings accumulate_before
excreting nitrogenous waste.
a. nitrogen
b. ammonia
c. urea
d. uric acid
15. Renin is made by_.
a. granular cells of the juxtaglomerular
apparatus
b. the kidneys
c. the nephrons
d. all of the above
16. Patients with Addison's disease_.
a. retain water
b. retain salts
c. lose salts and water
d. have too much aldosterone
17. Which hormone elicits the “fight or flight"
response?
a. epinephrine
b. mineralcorticoids
c. anti-diuretic hormone
d. thyroxine
than the kidneys.
24. In terms of evolution, why might the urea cycle
have evolved in organisms?
25. Compare and contrast the formation of urea and
uric acid.
26. Describe how hormones regulate blood pressure,
blood volume, and kidney function.
27. How does the renin-angiotensin-aldosterone
mechanism function? Why is it controlled by the
kidneys?
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Chapter 42 | The Immune System
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42 | THE IMMUNE SYSTEM
Figure 42.1 In this compound light micrograph purple-stained neutrophil (upper left) and eosinophil (lower right)
are white blood cells that float among red blood cells in this blood smear. Neutrophils provide an early, rapid, and
nonspecific defense against invading pathogens. Eosinophils play a variety of roles in the immune response. Red
blood cells are about 7-8 pm in diameter, and a neutrophil is about 10-12pm. (credit: modification of work by Dr. David
Csaba)
42.1: Innate Immune Response
42.2: Adaptive Immune Response
42.3: Antibodies
42.4: Disruptions in the Immune System
Introduction
The environment consists of numerous pathogens, which are agents, usually microorganisms, that cause
diseases in their hosts. A host is the organism that is invaded and often harmed by a pathogen. Pathogens
include bacteria, protists, fungi and other infectious organisms. We are constantly exposed to pathogens in food
and water, on surfaces, and in the air. Mammalian immune systems evolved for protection from such pathogens;
they are composed of an extremely diverse array of specialized cells and soluble molecules that coordinate a
rapid and flexible defense system capable of providing protection from a majority of these disease agents.
Components of the immune system constantly search the body for signs of pathogens. When pathogens are
found, immune factors are mobilized to the site of an infection. The immune factors identify the nature of
the pathogen, strengthen the corresponding cells and molecules to combat it efficiently, and then halt the
immune response after the infection is cleared to avoid unnecessary host cell damage. The immune system can
remember pathogens to which it has been exposed to create a more efficient response upon reexposure. This
memory can last several decades. Features of the immune system, such as pathogen identification, specific
response, amplification, retreat, and remembrance are essential for survival against pathogens. The immune
response can be classified as either innate or active. The innate immune response is always present and
attempts to defend against all pathogens rather than focusing on specific ones. Conversely, the adaptive immune
response stores information about past infections and mounts pathogen-specific defenses.
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Chapter 42 | The Immune System
42.1 1 Innate Immune Response
By the end of this section, you will be able to do the following:
• Describe physical and chemical immune barriers
• Explain immediate and induced innate immune responses
• Discuss natural killer cells
• Describe major histocompatibility class I molecules
• Summarize how the proteins in a complement system function to destroy extracellular pathogens
The immune system comprises both innate and adaptive immune responses. Innate immunity occurs naturally
because of genetic factors or physiology; it is not induced by infection or vaccination but works to reduce the
workload for the adaptive immune response. Both the innate and adaptive levels of the immune response involve
secreted proteins, receptor-mediated signaling, and intricate cell-to-cell communication. The innate immune
system developed early in animal evolution, roughly a billion years ago, as an essential response to infection.
Innate immunity has a limited number of specific targets: any pathogenic threat triggers a consistent sequence
of events that can identify the type of pathogen and either clear the infection independently or mobilize a highly
specialized adaptive immune response. For example, tears and mucus secretions contain microbicidal factors.
Physical and Chemical Barriers
Before any immune factors are triggered, the skin functions as a continuous, impassable barrier to potentially
infectious pathogens. Pathogens are killed or inactivated on the skin by desiccation (drying out) and by the
skin’s acidity. In addition, beneficial microorganisms that coexist on the skin compete with invading pathogens,
preventing infection. Regions of the body that are not protected by skin (such as the eyes and mucus
membranes) have alternative methods of defense, such as tears and mucus secretions that trap and rinse away
pathogens, and cilia in the nasal passages and respiratory tract that push the mucus with the pathogens out of
the body. Throughout the body are other defenses, such as the low pH of the stomach (which inhibits the growth
of pathogens), blood proteins that bind and disrupt bacterial cell membranes, and the process of urination (which
flushes pathogens from the urinary tract).
Despite these barriers, pathogens may enter the body through skin abrasions or punctures, or by collecting on
mucosal surfaces in large numbers that overcome the mucus or cilia. Some pathogens have evolved specific
mechanisms that allow them to overcome physical and chemical barriers. When pathogens do enter the body,
the innate immune system responds with inflammation, pathogen engulfment, and secretion of immune factors
and proteins.
Pathogen Recognition
An infection may be intracellular or extracellular, depending on the pathogen. All viruses infect cells and
replicate within those cells (intracellularly), whereas bacteria and other parasites may replicate intracellularly or
extracellularly, depending on the species. The innate immune system must respond accordingly: by identifying
the extracellular pathogen and/or by identifying host cells that have already been infected. When a pathogen
enters the body, cells in the blood and lymph detect the specific pathogen-associated molecular patterns
(PAMPs) on the pathogen’s surface. PAMPs are carbohydrate, polypeptide, and nucleic acid “signatures”
that are expressed by viruses, bacteria, and parasites but which differ from molecules on host cells. The
immune system has specific cells, described in Figure 42.2 and shown in Figure 42.3, with receptors that
recognize these PAMPs. A macrophage is a large phagocytic cell that engulfs foreign particles and pathogens.
Macrophages recognize PAMPs via complementary pattern recognition receptors (PRRs). PRRs are
molecules on macrophages and dendritic cells which are in contact with the external environment. A monocyte
is a type of white blood cell that circulates in the blood and lymph and differentiates into macrophages after
it moves into infected tissue. Dendritic cells bind molecular signatures of pathogens and promote pathogen
engulfment and destruction. Toll-like receptors (TLRs) are a type of PRR that recognizes molecules that are
shared by pathogens but distinguishable from host molecules. TLRs are present in invertebrates as well as
vertebrates, and appear to be one of the most ancient components of the immune system. TLRs have also been
identified in the mammalian nervous system.
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Chapter 42 | The Immune System
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Cell type
Characteristics
Location
Image
Mast cell
Dilates blood vessels and induces
inflammation through release of histamines
and heparin. Recruits macrophages and
neutrophils. Involved in wound healing and
defense against pathogens but can also be
responsible for allergic reactions.
Connective tissues,
mucous membranes
§•)
\ 0 _ 0 /
Macrophage
Phagocytic cell that consumes foreign
pathogens and cancer cells. Stimulates
response of other immune cells.
Migrates from blood
vessels into tissues.
Natural
killer cell
Kills tumor cells and virus-infected cells.
Circulates in blood and
migrates into tissues.
l°^lp ° )
U)
Dendritic cell
Presents antigens on its surface, thereby
triggering adaptive immunity.
Present in epithelial tissue,
including skin, lung and
tissues of the digestive tract.
Migrates to lymph nodes
upon activation.
Monocyte
Differentiates into macrophages and
dendritic cells in response to inflammation.
Stored in spleen, moves
through blood vessels to
infected tissues.
<§>
Neutrophil
First responders at the site of infection or
trauma, this abundant phagocytic cell
represents 50-60 percent of all leukocytes.
Releases toxins that kill or inhibit bacteria
and fungi and recruits other immune cells
to the site of infection.
Migrates from blood
vessels into tissues.
%
Basophil
Responsible for defense against parasites.
Releases histamines that cause
inflammation and may be responsible for
allergic reactions.
Circulates in blood and
migrates to tissues.
#
Eosinophil
Releases toxins that kill bacteria and
parasites but also causes tissue damage.
Circulates in blood and
migrates to tissues.
#
Figure 42.2 The characteristics and location of cells involved in the innate immune system are described, (credit:
modification of work by NIH)
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Chapter 42 | The Immune System
Figure 42.3 Cells of the blood include (1) monocytes, (2) lymphocytes, (3) neutrophils, (4) red blood cells, and (5)
platelets. Note the very similar morphologies of the leukocytes (1, 2, 3). (credit: modification of work by Bruce Wetzel,
Harry Schaefer, NCI; scale-bar data from Matt Russell)
Cytokine Release Effect
The binding of PRRs with PAMPs triggers the release of cytokines, which signal that a pathogen is present and
needs to be destroyed along with any infected cells. A cytokine is a chemical messenger that regulates cell
differentiation (form and function), proliferation (production), and gene expression to affect immune responses.
At least 40 types of cytokines exist in humans that differ in terms of the cell type that produces them, the cell type
that responds to them, and the changes they produce. One type of cytokine, interferon, is illustrated in Figure
42.4.
One subclass of cytokines is the interleukin (IL), so named because they mediate interactions between
leukocytes (white blood cells). Interleukins are involved in bridging the innate and adaptive immune responses.
In addition to being released from cells after PAMP recognition, cytokines are released by the infected cells
which bind to nearby uninfected cells and induce those cells to release cytokines, which results in a cytokine
burst.
A second class of early-acting cytokines is interferons, which are released by infected cells as a warning to
nearby uninfected cells. One of the functions of an interferon is to inhibit viral replication. They also have
other important functions, such as tumor surveillance. Interferons work by signaling neighboring uninfected
cells to destroy RNA and reduce protein synthesis, signaling neighboring infected cells to undergo apoptosis
(programmed cell death), and activating immune cells.
In response to interferons, uninfected cells alter their gene expression, which increases the cells’ resistance to
infection. One effect of interferon-induced gene expression is a sharply reduced cellular protein synthesis. Virally
infected cells produce more viruses by synthesizing large quantities of viral proteins. Thus, by reducing protein
synthesis, a cell becomes resistant to viral infection.
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Chapter 42 | The Immune System
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Interferon
Virus
Activates
immune cells.
Signals neighboring
uninfected cells to
destroy RNAand
reduce protein
synthesis.
Signals neighboring
infected cells to
undergo apoptosis.
Figure 42.4 Interferons are cytokines that are released by a cell infected with a virus. Response of neighboring cells
to interferon helps stem the infection.
Phagocytosis and Inflammation
The first cytokines to be produced are pro-inflammatory; that is, they encourage inflammation, the localized
redness, swelling, heat, and pain that result from the movement of leukocytes and fluid through increasingly
permeable capillaries to a site of infection. The population of leukocytes that arrives at an infection site depends
on the nature of the infecting pathogen. Both macrophages and dendritic cells engulf pathogens and cellular
debris through phagocytosis. A neutrophil is also a phagocytic leukocyte that engulfs and digests pathogens.
Neutrophils, shown in Figure 42.3, are the most abundant leukocytes of the immune system. Neutrophils have
a nucleus with two to five lobes, and they contain organelles, called lysosomes, that digest engulfed pathogens.
An eosinophil is a leukocyte that works with other eosinophils to surround a parasite; it is involved in the allergic
response and in protection against helminthes (parasitic worms).
Neutrophils and eosinophils are particularly important leukocytes that engulf large pathogens, such as bacteria
and fungi. A mast cell is a leukocyte that produces inflammatory molecules, such as histamine, in response
to large pathogens. A basophil is a leukocyte that, like a neutrophil, releases chemicals to stimulate the
inflammatory response as illustrated in Figure 42.5. Basophils are also involved in allergy and hypersensitivity
responses and induce specific types of inflammatory responses. Eosinophils and basophils produce additional
inflammatory mediators to recruit more leukocytes. A hypersensitive immune response to harmless antigens,
such as in pollen, often involves the release of histamine by basophils and mast cells.
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Chapter 42 | The Immune System
Figure 42.5 In response to a cut, mast cells secrete histamines that cause nearby capillaries to dilate. Neutrophils and
monocytes leave the capillaries. Monocytes mature into macrophages. Neutrophils, dendritic cells, and macrophages
release chemicals to stimulate the inflammatory response. Neutrophils and macrophages also consume invading
bacteria by phagocytosis.
Cytokines also send feedback to cells of the nervous system to bring about the overall symptoms of feeling
sick, which include lethargy, muscle pain, and nausea. These effects may have evolved because the symptoms
encourage the individual to rest and prevent the spreading of the infection to others. Cytokines also increase
the core body temperature, causing a fever, which causes the liver to withhold iron from the blood. Without iron,
certain pathogens, such as some bacteria, are unable to replicate; this is called nutritional immunity.
LINK
T a
LEARNING
Watch this 23-second stop-motion video (http:// 0 penstaxc 0 llege. 0 rg/l/c 0 nidia) showing a neutrophil that
searches for and engulfs fungus spores during an elapsed time of about 79 minutes.
Natural Killer Cells
Lymphocytes are leukocytes that are histologically identifiable by their large, darkly staining nuclei; they are
small cells with very little cytoplasm, as shown in Figure 42.6. Infected cells are identified and destroyed by
natural killer (NK) cells, lymphocytes that can kill cells infected with viruses or tumor cells (abnormal cells
that uncontrollably divide and invade other tissue). T cells and B cells of the adaptive immune system also
are classified as lymphocytes. T cells are lymphocytes that mature in the thymus gland, and B cells are
lymphocytes that mature in the bone marrow. NK cells identify intracellular infections, especially from viruses, by
the altered expression of major histocompatibility class (MHC) I molecules on the surface of infected cells.
MHC I molecules are proteins on the surfaces of all nucleated cells, thus they are scarce on red blood cells and
platelets which are non-nucleated. The function of MHC I molecules is to display fragments of proteins from the
infectious agents within the cell to T cells; healthy cells will be ignored, while “non-self” or foreign proteins will
be attacked by the immune system. MHC II molecules are found mainly on cells containing antigens (“non-self
proteins") and on lymphocytes. MHC II molecules interact with helper T cells to trigger the appropriate immune
response, which may include the inflammatory response.
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Chapter 42 | The Immune System
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5 pm
Figure 42.6 Lymphocytes, such as NK cells, are characterized by their large nuclei that actively absorb Wright stain
and therefore appear dark colored under a microscope.
An infected cell (or a tumor cell) is usually incapable of synthesizing and displaying MHC 1 molecules
appropriately. The metabolic resources of cells infected by some viruses produce proteins that interfere with
MHC i processing and/or trafficking to the cell surface. The reduced MHC I on host cells varies from virus to virus
and results from active inhibitors being produced by the viruses. This process can deplete host MHC I molecules
on the cell surface, which NK cells detect as “unhealthy” or “abnormal” while searching for cellular MHC I
molecules. Similarly, the dramatically altered gene expression of tumor cells leads to expression of extremely
deformed or absent MHC I molecules that also signal “unhealthy" or “abnormal."
NK cells are always active; an interaction with normal, intact MHC I molecules on a healthy cell disables the
killing sequence, and the NK cell moves on. After the NK cell detects an infected or tumor cell, its cytoplasm
secretes granules comprised of perforin, a destructive protein that creates a pore in the target cell. Granzymes
are released along with the perforin in the immunological synapse. A granzyme is a protease that digests
cellular proteins and induces the target cell to undergo programmed cell death, or apoptosis. Phagocytic cells
then digest the cell debris left behind. NK cells are constantly patrolling the body and are an effective mechanism
for controlling potential infections and preventing cancer progression.
Complement
An array of approximately 20 types of soluble proteins, called a complement system, functions to destroy
extracellular pathogens. Cells of the liver and macrophages synthesize complement proteins continuously;
these proteins are abundant in the blood serum and are capable of responding immediately to infecting
microorganisms. The complement system is so named because it is complementary to the antibody response of
the adaptive immune system. Complement proteins bind to the surfaces of microorganisms and are particularly
attracted to pathogens that are already bound by antibodies. Binding of complement proteins occurs in a specific
and highly regulated sequence, with each successive protein being activated by cleavage and/or structural
changes induced upon binding of the preceding protein(s). After the first few complement proteins bind, a
cascade of sequential binding events follows in which the pathogen rapidly becomes coated in complement
proteins.
Complement proteins perform several functions. The proteins serve as a marker to indicate the presence of a
pathogen to phagocytic cells, such as macrophages and B cells, and enhance engulfment; this process is called
opsonization. Certain complement proteins can combine to form attack complexes that open pores in microbial
cell membranes. These structures destroy pathogens by causing their contents to leak, as illustrated in Figure
42.7.
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Chapter 42 ] The Immune System
Classic Pathway
Cl binds to an antigen-antibody
complex on an invading
pathogen, causing complement
components C2 and C4 to
split in two.
Alternate Pathway-► C3 convertase Fragments from C2 and C4 combine to
form an enzyme called C3 convertase.
/
Host cell
I
C3
C3 convertase splits C3 in two. One
the fragments from C3 joins C3
convertase to form C5 convertase.
Q
c>
of
/
& <
A
Endogenous
proteins protect
host cells from lysis.
I
o
A fragment from C5 joins C6, C7, C8, and C9
to form a complex that makes a hole in the
plasma membrane of the invading cell. The cell
swells and bursts.
Invading pathogen
Figure 42.7 The classic pathway for the complement cascade involves the attachment of several initial complement
proteins to an antibody-bound pathogen followed by rapid activation and binding of many more complement proteins
and the creation of destructive pores in the microbial cell envelope and cell wall. The alternate pathway does not
involve antibody activation. Rather, C3 convertase spontaneously breaks down C3. Endogenous regulatory proteins
prevent the complement complex from binding to host cells. Pathogens lacking these regulatory proteins are lysed,
(credit: modification of work by NIH)
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Chapter 42 | The Immune System
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42.2 | Adaptive Immune Response
By the end of this section, you will be able to do the following:
• Explain adaptive immunity
• Compare and contrast adaptive and innate immunity
• Describe cell-mediated immune response and humoral immune response
• Describe immune tolerance
The adaptive, or acquired, immune response takes days or even weeks to become established—much longer
than the innate response; however, adaptive immunity is more specific to pathogens and has memory. Adaptive
immunity is an immunity that occurs after exposure to an antigen either from a pathogen or a vaccination. This
part of the immune system is activated when the innate immune response is insufficient to control an infection.
In fact, without information from the innate immune system, the adaptive response could not be mobilized. There
are two types of adaptive responses: the cell-mediated immune response, which is carried out by T cells, and
the humoral immune response, which is controlled by activated B cells and antibodies. Activated T cells and
B cells that are specific to molecular structures on the pathogen proliferate and attack the invading pathogen.
Their attack can kill pathogens directly or secrete antibodies that enhance the phagocytosis of pathogens and
disrupt the infection. Adaptive immunity also involves a memory to provide the host with long-term protection
from reinfection with the same type of pathogen; on reexposure, this memory will facilitate an efficient and quick
response.
Antigen-presenting Cells
Unlike NK cells of the innate immune system, B cells (B lymphocytes) are a type of white blood cell that gives rise
to antibodies, whereas T cells (T lymphocytes) are a type of white blood cell that plays an important role in the
immune response. T cells are a key component in the cell-mediated response—the specific immune response
that utilizes T cells to neutralize cells that have been infected with viruses and certain bacteria. There are three
types of T cells: cytotoxic, helper, and suppressor T cells. Cytotoxic T cells destroy virus-infected cells in the
cell-mediated immune response, and helper T cells play a part in activating both the antibody and the cell-
mediated immune responses. Suppressor T cells deactivate T cells and B cells when needed, and thus prevent
the immune response from becoming too intense.
An antigen is a foreign or “non-self” macromolecule that reacts with cells of the immune system. Not all antigens
will provoke a response. For instance, individuals produce innumerable “self” antigens and are constantly
exposed to harmless foreign antigens, such as food proteins, pollen, or dust components. The suppression of
immune responses to harmless macromolecules is highly regulated and typically prevents processes that could
be damaging to the host, known as tolerance.
The innate immune system contains cells that detect potentially harmful antigens, and then inform the adaptive
immune response about the presence of these antigens. An antigen-presenting cell (APC) is an immune
cell that detects, engulfs, and informs the adaptive immune response about an infection. When a pathogen
is detected, these APCs will phagocytose the pathogen and digest it to form many different fragments of the
antigen. Antigen fragments will then be transported to the surface of the APC, where they will serve as an
indicator to other immune cells. Dendritic cells are immune cells that process antigen material; they are present
in the skin (Langerhans cells) and the lining of the nose, lungs, stomach, and intestines. Sometimes a dendritic
cell presents on the surface of other cells to induce an immune response, thus functioning as an antigen-
presenting cell. Macrophages also function as APCs. Before activation and differentiation, B cells can also
function as APCs.
After phagocytosis by APCs, the phagocytic vesicle fuses with an intracellular lysosome forming
phagolysosome. Within the phagolysosome, the components are broken down into fragments; the fragments
are then loaded onto MHC class I or MHC class II molecules and are transported to the cell surface for antigen
presentation, as illustrated in Figure 42.8. Note that T lymphocytes cannot properly respond to the antigen
unless it is processed and embedded in an MHC II molecule. APCs express MHC on their surfaces, and when
combined with a foreign antigen, these complexes signal a “non-self” invader. Once the fragment of antigen is
embedded in the MHC II molecule, the immune cell can respond. Helper T cells are one of the main lymphocytes
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Chapter 42 | The Immune System
that respond to antigen-presenting cells. Recall that all other nucleated cells of the body expressed MHC I
molecules, which signal “healthy" or “normal."
(l) A bacterium
(3) Antigens from digested
bacterium are presented with
MHC II on the cell surface.
Figure 42.8 An APC, such as a macrophage, engulfs and digests a foreign bacterium. An antigen from the bacterium is
presented on the cell surface in conjunction with an MHC II molecule. Lymphocytes of the adaptive immune response
interact with antigen-embedded MHC II molecules to mature into functional immune cells.
This animation (http:// 0 penstaxc 0 llege. 0 rg/l/immune_system) from Rockefeller University shows how
dendritic cells act as sentinels in the body's immune system.
T and B Lymphocytes
Lymphocytes in human circulating blood are approximately 80 to 90 percent T cells, shown in Figure 42.9, and
10 to 20 percent B cells. Recall that the T cells are involved in the cell-mediated immune response, whereas B
cells are part of the humoral immune response.
T cells encompass a heterogeneous population of cells with extremely diverse functions. Some T cells respond
to APCs of the innate immune system, and indirectly induce immune responses by releasing cytokines. Other
T cells stimulate B cells to prepare their own response. Another population of T cells detects APC signals and
directly kills the infected cells. Other T cells are involved in suppressing inappropriate immune reactions to
harmless or “self" antigens.
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Chapter 42 | The Immune System
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Figure 42.9 This scanning electron micrograph shows a T lymphocyte, which is responsible for the cell-mediated
immune response. T cells are able to recognize antigens, (credit: modification of work by NCI; scale-bar data from Matt
Russell)
T and B cells exhibit a common theme of recognition/binding of specific antigens via a complementary receptor,
followed by activation and self-amplification/maturation to specifically bind to the particular antigen of the
infecting pathogen. T and B lymphocytes are also similar in that each cell only expresses one type of antigen
receptor. Any individual may possess a population of T and B cells that together express a near limitless variety
of antigen receptors that are capable of recognizing virtually any infecting pathogen. T and B cells are activated
when they recognize small components of antigens, called epitopes, presented by APCs, illustrated in Figure
42.10. Note that recognition occurs at a specific epitope rather than on the entire antigen; for this reason,
epitopes are known as “antigenic determinants.” In the absence of information from APCs, T and B cells remain
inactive, or naive, and are unable to prepare an immune response. The requirement for information from the
APCs of innate immunity to trigger B cell or T cell activation illustrates the essential nature of the innate immune
response to the functioning of the entire immune system.
Figure 42.10 An antigen is a macromolecule that reacts with components of the immune system. A given antigen may
contain several motifs that are recognized by immune cells. Each motif is an epitope. In this figure, the entire structure
is an antigen, and the orange, salmon and green components projecting from it represent potential epitopes.
Naive T cells can express one of two different molecules, CD4 or CD8, on their surface, as shown in Figure
42.11, and are accordingly classified as CD4 + or CD8 + cells. These molecules are important because they
regulate how a T cell will interact with and respond to an APC. Naive CD4 + cells bind APCs via their antigen-
embedded MHC II molecules and are stimulated to become helper T (Th) lymphocytes, cells that go on to
stimulate B cells (or cytotoxic T cells) directly or secrete cytokines to inform more and various target cells about
the pathogenic threat. In contrast, CD8 + cells engage antigen-embedded MHC I molecules on APCs and are
stimulated to become cytotoxic T lymphocytes (CTLs), which directly kill infected cells by apoptosis and emit
cytokines to amplify the immune response. The two populations of T cells have different mechanisms of immune
protection, but both bind MHC molecules via their antigen receptors called T cell receptors (TCRs). The CD4 or
CD8 surface molecules differentiate whether the TCR will engage an MHC II or an MHC I molecule. Because
they assist in binding specificity, the CD4 and CD8 molecules are described as coreceptors.
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visual
CONNECTION
Figure 42.11 Naive CD4 + T cells engage MHC II molecules on antigen-presenting cells (APCs) and become
activated. Clones of the activated helper T cell, in turn, activate B cells and CD8 + T cells, which become cytotoxic
T cells. Cytotoxic T cells kill infected cells.
Which of the following statements about T cells is false?
a. Helper T cells release cytokines while cytotoxic T cells kill the infected cell.
b. Helper T cells are CD4 + , while cytotoxic T cells are CD8 + ,
c. MHC II is a receptor found on most body cells, while MHC I is a receptor found on immune cells only.
d. The T cell receptor is found on both CD4 + and CD8 + T cells.
Consider the innumerable possible antigens that an individual will be exposed to during a lifetime. The
mammalian adaptive immune system is adept in responding appropriately to each antigen. Mammals have
an enormous diversity of T cell populations, resulting from the diversity of TCRs. Each TCR consists of two
polypeptide chains that span the T cell membrane, as illustrated in Figure 42.12; the chains are linked by a
disulfide bridge. Each polypeptide chain is comprised of a constant domain and a variable domain: a domain,
in this sense, is a specific region of a protein that may be regulatory or structural. The intracellular domain is
involved in intracellular signaling. A single T cell will express thousands of identical copies of one specific TCR
variant on its cell surface. The specificity of the adaptive immune system occurs because it synthesizes millions
of different T cell populations, each expressing a TCR that differs in its variable domain. This TCR diversity is
achieved by the mutation and recombination of genes that encode these receptors in stem cell precursors of T
cells. The binding between an antigen-displaying MHC molecule and a complementary TCR “match” indicates
that the adaptive immune system needs to activate and produce that specific T cell because its structure is
appropriate to recognize and destroy the invading pathogen.
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Antigen binding site
Figure 42.12 A T cell receptor spans the membrane and projects variable binding regions into the extracellular space
to bind processed antigens via MHC molecules on APCs.
Helper T Lymphocytes
The Th lymphocytes function indirectly to identify potential pathogens for other cells of the immune system.
These cells are important for extracellular infections, such as those caused by certain bacteria, helminths,
and protozoa. Th lymphocytes recognize specific antigens displayed in the MHC II complexes of APCs. There
are two major populations of Th cells: ThI and Th2. ThI cells secrete cytokines to enhance the activities of
macrophages and other T cells. ThI cells activate the action of cyotoxic T cells, as well as macrophages. Th 2
cells stimulate naive B cells to destroy foreign invaders via antibody secretion. Whether a ThI or a Th 2 immune
response develops depends on the specific types of cytokines secreted by cells of the innate immune system,
which in turn depends on the nature of the invading pathogen.
The THl-mediated response involves macrophages and is associated with inflammation. Recall the frontline
defenses of macrophages involved in the innate immune response. Some intracellular bacteria, such as
Mycobacterium tuberculosis, have evolved to multiply in macrophages after they have been engulfed. These
pathogens evade attempts by macrophages to destroy and digest the pathogen. When M. tuberculosis infection
occurs, macrophages can stimulate naive T cells to become ThI cells. These stimulated T cells secrete specific
cytokines that send feedback to the macrophage to stimulate its digestive capabilities and allow it to destroy the
colonizing M. tuberculosis. In the same manner, THl-activated macrophages also become better suited to ingest
and kill tumor cells. In summary; ThI responses are directed toward intracellular invaders while Th 2 responses
are aimed at those that are extracellular.
B Lymphocytes
When stimulated by the Th2 pathway, naive B cells differentiate into antibody-secreting plasma cells. A plasma
cell is an immune cell that secrets antibodies; these cells arise from B cells that were stimulated by antigens.
Similar to T cells, naive B cells initially are coated in thousands of B cell receptors (BCRs), which are membrane-
bound forms of Ig (immunoglobulin, or an antibody). The B cell receptor has two heavy chains and two light
chains connected by disulfide linkages. Each chain has a constant and a variable region; the latter is involved in
antigen binding. Two other membrane proteins, Ig alpha and Ig beta, are involved in signaling. The receptors of
any particular B cell, as shown in Figure 42.13 are all the same, but the hundreds of millions of different B cells
in an individual have distinct recognition domains that contribute to extensive diversity in the types of molecular
structures to which they can bind. In this state, B cells function as APCs. They bind and engulf foreign antigens
via their BCRs and then display processed antigens in the context of MHC II molecules to Th2 cells. When a
T h2 cell detects that a B cell is bound to a relevant antigen, it secretes specific cytokines that induce the B cell to
proliferate rapidly, which makes thousands of identical (clonal) copies of it, and then it synthesizes and secretes
antibodies with the same antigen recognition pattern as the BCRs. The activation of B cells corresponding
to one specific BCR variant and the dramatic proliferation of that variant is known as clonal selection. This
phenomenon drastically, but briefly, changes the proportions of BCR variants expressed by the immune system,
and shifts the balance toward BCRs specific to the infecting pathogen.
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Antigen binding site
Figure 42.13 B cell receptors are embedded in the membranes of B cells and bind a variety of antigens through their
variable regions. The signal transduction region transfers the signal into the cell.
T and B cells differ in one fundamental way: whereas T cells bind antigens that have been digested and
embedded in MHC molecules by APCs, B cells function as APCs that bind intact antigens that have not been
processed. Although T and B cells both react with molecules that are termed “antigens,” these lymphocytes
actually respond to very different types of molecules. B cells must be able to bind intact antigens because they
secrete antibodies that must recognize the pathogen directly, rather than digested remnants of the pathogen.
Bacterial carbohydrate and lipid molecules can activate B cells independently from the T cells.
Cytotoxic T Lymphocytes
CTLs, a subclass of T cells, function to clear infections directly. The cell-mediated part of the adaptive immune
system consists of CTLs that attack and destroy infected cells. CTLs are particularly important in protecting
against viral infections; this is because viruses replicate within cells where they are shielded from extracellular
contact with circulating antibodies. When APCs phagocytize pathogens and present MHC l-embedded antigens
to naive CD8 + T cells that express complementary TCRs, the CD8 + T cells become activated to proliferate
according to clonal selection. These resulting CTLs then identify non-APCs displaying the same MHC I-
embedded antigens (for example, viral proteins)—for example, the CTLs identify infected host cells.
intracellularly, infected cells typically die after the infecting pathogen replicates to a sufficient concentration and
lyses the cell, as many viruses do. CTLs attempt to identify and destroy infected cells before the pathogen
can replicate and escape, thereby halting the progression of intracellular infections. CTLs also support NK
lymphocytes to destroy early cancers. Cytokines secreted by the ThI response that stimulates macrophages
also stimulate CTLs and enhance their ability to identify and destroy infected cells and tumors.
CTLs sense MHC l-embedded antigens by directly interacting with infected cells via their TCRs. Binding of TCRs
with antigens activates CTLs to release perforin and granzyme, degradative enzymes that will induce apoptosis
of the infected cell. Recall that this is a similar destruction mechanism to that used by NK cells. In this process,
the CTL does not become infected and is not harmed by the secretion of perforin and granzymes. in fact, the
functions of NK cells and CTLs are complementary and maximize the removal of infected cells, as illustrated
in Figure 42.14. If the NK cell cannot identify the “missing self” pattern of down-regulated MHC I molecules,
then the CTL can identify it by the complex of MHC I with foreign antigens, which signals “altered self.” Similarly,
if the CTL cannot detect antigen-embedded MHC i because the receptors are depleted from the cell surface,
NK cells will destroy the cell instead. CTLs also emit cytokines, such as interferons, that alter surface protein
expression in other infected cells, such that the infected cells can be easily identified and destroyed. Moreover,
these interferons can also prevent virally infected cells from releasing virus particles.
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Chapter 42 | The Immune System
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visual
CONNECTION
A natural killer (NK) cell recognizes MHC I
on a healthy cell and does not kill it.
NK cell
Healthy cell
An infected cell that does not
present MHC I is killed.
NK cell
Infected cell
Figure 42.14 Natural killer (NK) cells recognize the MHC I receptor on healthy cells. If MHC I is absent, the cell is
lysed.
Based on what you know about MHC receptors, why do you think an organ transplanted from an
incompatible donor to a recipient will be rejected?
Plasma cells and CTLs are collectively called effector cells: they represent differentiated versions of their naive
counterparts, and they are involved in bringing about the immune defense of killing pathogens and infected host
cells.
Mucosal Surfaces and Immune Tolerance
The innate and adaptive immune responses discussed thus far comprise the systemic immune system (affecting
the whole body), which is distinct from the mucosal immune system. Mucosal immunity is formed by mucosa-
associated lymphoid tissue, which functions independently of the systemic immune system, and which has
its own innate and adaptive components. Mucosa-associated lymphoid tissue (MALT), illustrated in Figure
42.15, is a collection of lymphatic tissue that combines with epithelial tissue lining the mucosa throughout the
body. This tissue functions as the immune barrier and response in areas of the body with direct contact to the
external environment. The systemic and mucosal immune systems use many of the same cell types. Foreign
particles that make their way to MALT are taken up by absorptive epithelial cells called M cells and delivered to
APCs located directly below the mucosal tissue. M cells function in the transport described, and are located in
the Peyer’s patch, a lymphoid nodule. APCs of the mucosal immune system are primarily dendritic cells, with
B cells and macrophages having minor roles. Processed antigens displayed on APCs are detected by T cells
in the MALT and at various mucosal induction sites, such as the tonsils, adenoids, appendix, or the mesenteric
lymph nodes of the intestine. Activated T cells then migrate through the lymphatic system and into the circulatory
system to mucosal sites of infection.
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Chapter 42 ] The Immune System
Antigen
Mucous
Epithilial cell
Organized
lymphoid
follicles
Lymphatic system
T cell
B cell
MHC II
38* < 3 %;
B cell
! cell receptor
Dendritic cell
B cell
Figure 42.15 The topology and function of intestinal MALT is shown. Pathogens are taken up by M cells in the intestinal
epithelium and excreted into a pocket formed by the inner surface of the cell. The pocket contains antigen-presenting
cells such as dendritic cells, which engulf the antigens, then present them with MHC II molecules on the cell surface.
The dendritic cells migrate to an underlying tissue called a Peyer’s patch. Antigen-presenting cells, T cells, and B cells
aggregate within the Peyer’s patch, forming organized lymphoid follicles. There, some T cells and B cells are activated.
Other antigen-loaded dendritic cells migrate through the lymphatic system where they activate B cells, T cells, and
plasma cells in the lymph nodes. The activated cells then return to MALT tissue effector sites. IgA and other antibodies
are secreted into the intestinal lumen.
MALT is a crucial component of a functional immune system because mucosal surfaces, such as the nasal
passages, are the first tissues onto which inhaled or ingested pathogens are deposited. The mucosal tissue
includes the mouth, pharynx, and esophagus, and the gastrointestinal, respiratory, and urogenital tracts.
The immune system has to be regulated to prevent wasteful, unnecessary responses to harmless substances,
and more importantly so that it does not attack “self.” The acquired ability to prevent an unnecessary or
harmful immune response to a detected foreign substance known not to cause disease is described as immune
tolerance. Immune tolerance is crucial for maintaining mucosal homeostasis given the tremendous number
of foreign substances (such as food proteins) that APCs of the oral cavity, pharynx, and gastrointestinal
mucosa encounter. Immune tolerance is brought about by specialized APCs in the liver, lymph nodes, small
intestine, and lung that present harmless antigens to an exceptionally diverse population of regulatory T
(Treg) cells, specialized lymphocytes that suppress local inflammation and inhibit the secretion of stimulatory
immune factors. The combined result of Treg cells is to prevent immunologic activation and inflammation in
undesired tissue compartments and to allow the immune system to focus on pathogens instead. In addition to
promoting immune tolerance of harmless antigens, other subsets of Treg cells are involved in the prevention of
the autoimmune response, which is an inappropriate immune response to host cells or self-antigens. Another
Treg class suppresses immune responses to harmful pathogens after the infection has cleared to minimize host
cell damage induced by inflammation and cell lysis.
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Chapter 42 | The Immune System
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Immunological Memory
The adaptive immune system possesses a memory component that allows for an efficient and dramatic
response upon reinvasion of the same pathogen. Memory is handled by the adaptive immune system with little
reliance on cues from the innate response. During the adaptive immune response to a pathogen that has not
been encountered before, called a primary response, plasma cells secreting antibodies and differentiated T cells
increase, then plateau over time. As B and T cells mature into effector cells, a subset of the naive populations
differentiates into B and T memory cells with the same antigen specificities, as illustrated in Figure 42.16.
A memory cell is an antigen-specific B or T lymphocyte that does not differentiate into effector cells during
the primary immune response, but that can immediately become effector cells upon reexposure to the same
pathogen. During the primary immune response, memory cells do not respond to antigens and do not contribute
to host defenses. As the infection is cleared and pathogenic stimuli subside, the effectors are no longer needed,
and they undergo apoptosis. In contrast, the memory cells persist in the circulation.
visual
CONNECTION
Figure 42.16 After initially binding an antigen to the B cell receptor (BCR), a B cell internalizes the antigen and
presents it on MHC II. A helper T cell recognizes the MHC 11—antigen complex and activates the B cell. As a result,
memory B cells and plasma cells are made.
The Rh antigen is found on Rh-positive red blood cells. An Rh-negative female can usually carry an Rh-
positive fetus to term without difficulty. However, if she has a second Rh-positive fetus, her body may launch
an immune attack that causes hemolytic disease of the newborn. Why do you think hemolytic disease is
only a problem during the second or subsequent pregnancies?
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Chapter 42 | The Immune System
If the pathogen is never encountered again during the individual’s lifetime, B and T memory cells will circulate
for a few years or even several decades and will gradually die off, having never functioned as effector cells.
However, if the host is reexposed to the same pathogen type, circulating memory cells will immediately
differentiate into plasma cells and CTLs without input from APCs or Th cells. One reason the adaptive immune
response is delayed is because it takes time for naive B and T cells with the appropriate antigen specificities
to be identified and activated. Upon reinfection, this step is skipped, and the result is a more rapid production
of immune defenses. Memory B cells that differentiate into plasma cells output tens to hundreds-fold greater
antibody amounts than were secreted during the primary response, as the graph in Figure 42.17 illustrates. This
rapid and dramatic antibody response may stop the infection before it can even become established, and the
individual may not realize he or she had been exposed.
Figure 42.17 In the primary response to infection, antibodies are secreted first from plasma cells. Upon reexposure
to the same pathogen, memory cells differentiate into antibody-secreting plasma cells that output a greater amount of
antibody for a longer period of time.
Vaccination is based on the knowledge that exposure to noninfectious antigens, derived from known pathogens,
generates a mild primary immune response. The immune response to vaccination may not be perceived by the
host as illness but still confers immune memory. When exposed to the corresponding pathogen to which an
individual was vaccinated, the reaction is similar to a secondary exposure. Because each reinfection generates
more memory cells and increased resistance to the pathogen, and because some memory cells die, certain
vaccine courses involve one or more booster vaccinations to mimic repeat exposures: for instance, tetanus
boosters are necessary every ten years because the memory cells only live that long.
Mucosal Immune Memory
A subset of T and B cells of the mucosal immune system differentiates into memory cells just as in the
systemic immune system. Upon reinvasion of the same pathogen type, a pronounced immune response occurs
at the mucosal site where the original pathogen deposited, but a collective defense is also organized within
interconnected or adjacent mucosal tissue. For instance, the immune memory of an infection in the oral cavity
would also elicit a response in the pharynx if the oral cavity was exposed to the same pathogen.
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Chapter 42 | The Immune System
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ca eer connection
Vaccinologist
Vaccination (or immunization) involves the delivery, usually by injection as shown in Figure 42.18, of
noninfectious antigen(s) derived from known pathogens. Other components, called adjuvants, are delivered
in parallel to help stimulate the immune response. Immunological memory is the reason vaccines work.
Ideally, the effect of vaccination is to elicit immunological memory, and thus resistance to specific pathogens
without the individual having to experience an infection.
Figure 42.18 Vaccines are often delivered by injection into the arm. (credit: U.S. Navy Photographer's Mate
Airman Apprentice Christopher D. Blachly)
Vaccinologists are involved in the process of vaccine development from the initial idea to the availability
of the completed vaccine. This process can take decades, can cost millions of dollars, and can involve
many obstacles along the way. For instance, injected vaccines stimulate the systemic immune system,
eliciting humoral and cell-mediated immunity, but have little effect on the mucosal response, which presents
a challenge because many pathogens are deposited and replicate in mucosal compartments, and the
injection does not provide the most efficient immune memory for these disease agents. For this reason,
vaccinologists are actively involved in developing new vaccines that are applied via intranasal, aerosol,
oral, or transcutaneous (absorbed through the skin) delivery methods. Importantly, mucosal-administered
vaccines elicit both mucosal and systemic immunity and produce the same level of disease resistance as
injected vaccines.
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Chapter 42 | The Immune System
Figure 42.19 The polio vaccine can be administered orally, (credit: modification of work by UNICEF Sverige)
Currently, a version of intranasal influenza vaccine is available, and the polio and typhoid vaccines can
be administered orally, as shown in Figure 42.19. Similarly, the measles and rubella vaccines are being
adapted to aerosol delivery using inhalation devices. Eventually, transgenic plants may be engineered to
produce vaccine antigens that can be eaten to confer disease resistance. Other vaccines may be adapted
to rectal or vaginal application to elicit immune responses in rectal, genitourinary, or reproductive mucosa.
Finally, vaccine antigens may be adapted to transdermal application in which the skin is lightly scraped
and microneedles are used to pierce the outermost layer. In addition to mobilizing the mucosal immune
response, this new generation of vaccines may end the anxiety associated with injections and, in turn,
improve patient participation.
Primary Centers of the Immune System
Although the immune system is characterized by circulating cells throughout the body, the regulation, maturation,
and intercommunication of immune factors occur at specific sites. The blood circulates immune cells, proteins,
and other factors through the body. Approximately 0.1 percent of all cells in the blood are leukocytes, which
encompass monocytes (the precursor of macrophages) and lymphocytes. The majority of cells in the blood are
erythrocytes (red blood cells). Lymph is a watery fluid that bathes tissues and organs with protective white blood
cells and does not contain erythrocytes. Cells of the immune system can travel between the distinct lymphatic
and blood circulatory systems, which are separated by interstitial space, by a process called extravasation
(passing through to surrounding tissue).
The cells of the immune system originate from hematopoietic stem cells in the bone marrow. Cytokines stimulate
these stem cells to differentiate into immune cells. B cell maturation occurs in the bone marrow, whereas naive
T cells transit from the bone marrow to the thymus for maturation. In the thymus, immature T cells that express
TCRs complementary to self-antigens are destroyed. This process helps prevent autoimmune responses.
On maturation, T and B lymphocytes circulate to various destinations. Lymph nodes scattered throughout the
body, as illustrated in Figure 42.20, house large populations of T and B cells, dendritic cells, and macrophages.
Lymph gathers antigens as it drains from tissues. These antigens then are filtered through lymph nodes before
the lymph is returned to circulation. APCs in the lymph nodes capture and process antigens and inform nearby
lymphocytes about potential pathogens.
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Afferent lymphatic
vessel
Efferent lymphatic
vessel
(b)
Figure 42.20 (a) Lymphatic vessels carry a clear fluid called lymph throughout the body. The liquid enters (b) lymph
nodes through afferent vessels. Lymph nodes are filled with lymphocytes that purge infecting cells. The lymph then
exits through efferent vessels, (credit: modification of work by NIH, NCI)
The spleen houses B and T cells, macrophages, dendritic cells, and NK cells. The spleen, shown in Figure
42.21, is the site where APCs that have trapped foreign particles in the blood can communicate with
lymphocytes. Antibodies are synthesized and secreted by activated plasma cells in the spleen, and the spleen
filters foreign substances and antibody-complexed pathogens from the blood. Functionally, the spleen is to the
blood as lymph nodes are to the lymph.
Figure 42.21 The spleen is similar to a lymph node but is much larger and filters blood instead of lymph. Blood enters
the spleen through arteries and exits through veins. The spleen contains two types of tissue: red pulp and white
pulp. Red pulp consists of cavities that store blood. Within the red pulp, damaged red blood cells are removed and
replaced by new ones. White pulp is rich in lymphocytes that remove antigen-coated bacteria from the blood, (credit:
modification of work by NCI)
42.3 | Antibodies
By the end of this section, you will be able to do the following:
• Explain cross-reactivity
• Describe the structure and function of antibodies
• Discuss antibody production
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An antibody, also known as an immunoglobulin (Ig), is a protein that is produced by plasma cells after
stimulation by an antigen. Antibodies are the functional basis of humoral immunity. Antibodies occur in the blood,
in gastric and mucus secretions, and in breast milk. Antibodies in these bodily fluids can bind pathogens and
mark them for destruction by phagocytes before they can infect cells.
Antibody Structure
An antibody molecule is comprised of four polypeptides: two identical heavy chains (large peptide units) that are
partially bound to each other in a “Y” formation, which are flanked by two identical light chains (small peptide
units), as illustrated in Figure 42.22. Bonds between the cysteine amino acids in the antibody molecule attach
the polypeptides to each other. The areas where the antigen is recognized on the antibody are variable domains
and the antibody base is composed of constant domains.
In germ-line B cells, the variable region of the light chain gene has 40 variable (V) and five joining (J) segments.
An enzyme called DNA recombinase randomly excises most of these segments out of the gene, and splices
one V segment to one J segment. During RNA processing, all but one V and J segment are spliced out.
Recombination and splicing may result in over 10 6 possible VJ combinations. As a result, each differentiated B
cell in the human body typically has a unique variable chain. The constant domain, which does not bind antibody,
is the same for all antibodies.
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Germ-line
V 2
V38 V39
V40
DNA of differentiated B cell
J2 J4 Intron
Transcription
pre-mRNA
V2 J2 J3 J4 Intron
J3
l
J A
J1 J2 J3 J4 J5 Intron
DNA rearrangement by
recombinase
mRNA
V2 J2
RNA processing
protein
Translation
Variable Constant
region region
(a)
C
Antigen Antigen
binding site binding site
Figure 42.22 (a) As a germ-line B cell matures, an enzyme called DNA recombinase randomly excises V and J
segments from the light chain gene. Splicing at the mRNA level results in further gene rearrangement. As a result, (b)
each antibody has a unique variable region capable of binding a different antigen.
Similar to TCRs and BCRs, antibody diversity is produced by the mutation and recombination of approximately
300 different gene segments encoding the light and heavy chain variable domains in precursor cells that are
destined to become B cells. The variable domains from the heavy and light chains interact to form the binding
site through which an antibody can bind a specific epitope on an antigen. The numbers of repeated constant
domains in Ig classes are the same for all antibodies corresponding to a specific class. Antibodies are structurally
similar to the extracellular component of the BCRs, and B cell maturation to plasma cells can be visualized in
simple terms as the cell acquires the ability to secrete the extracellular portion of its BCR in large quantities.
Antibody Classes
Antibodies can be divided into five classes—IgM, IgG, IgA, IgD, IgE—based on their physiochemical, structural,
and immunological properties. IgGs, which make up about 80 percent of all antibodies, have heavy chains that
consist of one variable domain and three identical constant domains. IgA and IgD also have three constant
domains per heavy chain, whereas IgM and IgE each have four constant domains per heavy chain. The variable
domain determines binding specificity and the constant domain of the heavy chain determines the immunological
mechanism of action of the corresponding antibody class. It is possible for two antibodies to have the same
binding specificities but be in different classes and, therefore, to be involved in different functions.
After an adaptive defense is produced against a pathogen, typically plasma cells first secrete IgM into the blood.
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BCRs on naive B cells are of the IgM class and occasionally IgD class. IgM molecules make up approximately
ten percent of all antibodies. Prior to antibody secretion, plasma cells assemble IgM molecules into pentamers
(five individual antibodies) linked by a joining (J) chain, as shown in Figure 42.23. The pentamer arrangement
means that these macromolecules can bind ten identical antigens. However, IgM molecules released early in
the adaptive immune response do not bind to antigens as stably as IgGs, which are one of the possible types
of antibodies secreted in large quantities upon reexposure to the same pathogen. Figure 42.23 summarizes the
properties of immunoglobulins and illustrates their basic structures.
Name
Properties
Structure
IgA
Found in mucous, saliva, tears, and breast milk. Protects against
pathogens.
Y
A
IgD
Part of the B cell receptor. Activates basophils and mast cells.
Y
igE
Protects against parasitic worms. Responsible for allergic reactions.
Y
igG
Secreted by plasma cells in the blood. Able to cross the placenta into
the fetus.
Y
igM
May be attached to the surface of a B cell or secreted into the blood.
Responsible for early stages of immunity.
Figure 42.23 Immunoglobulins have different functions, but all are composed of light and heavy chains that form a
Y-shaped structure.
IgAs populate the saliva, tears, breast milk, and mucus secretions of the gastrointestinal, respiratory, and
genitourinary tracts. Collectively, these bodily fluids coat and protect the extensive mucosa (4000 square feet
in humans). The total number of IgA molecules in these bodily secretions is greater than the number of IgG
molecules in the blood serum. A small amount of IgA is also secreted into the serum in monomeric form.
Conversely, some IgM is secreted into bodily fluids of the mucosa. Similar to IgM, IgA molecules are secreted
as polymeric structures linked with a J chain. However, IgAs are secreted mostly as dimeric molecules, not
pentamers.
IgE is present in the serum in small quantities and is best characterized in its role as an allergy mediator. IgD is
also present in small quantities. Similar to IgM, BCRs of the IgD class are found on the surface of naive B cells.
This class supports antigen recognition and maturation of B cells to plasma cells.
Antibody Functions
Differentiated plasma cells are crucial players in the humoral response, and the antibodies they secrete
are particularly significant against extracellular pathogens and toxins. Antibodies circulate freely and act
independently of plasma cells. Antibodies can be transferred from one individual to another to temporarily protect
against infectious disease. For instance, a person who has recently produced a successful immune response
against a particular disease agent can donate blood to a nonimmune recipient and confer temporary immunity
through antibodies in the donor’s blood serum. This phenomenon is called passive immunity; it also occurs
naturally during breastfeeding, which makes breastfed infants highly resistant to infections during the first few
months of life.
Antibodies coat extracellular pathogens and neutralize them, as illustrated in Figure 42.24, by blocking key
sites on the pathogen that enhance their infectivity (such as receptors that “dock” pathogens on host cells).
Antibody neutralization can prevent pathogens from entering and infecting host cells, as opposed to the CTL-
mediated approach of killing cells that are already infected to prevent progression of an established infection.
The neutralized antibody-coated pathogens can then be filtered by the spleen and eliminated in urine or feces.
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(a) Neutralization Antibodies prevent a virus or toxic protein
from binding their target.
(b) Opsonization A pathogen tagged by antibodies is consumed
by a macrophage or neutrophil.
Pathogen
Macrophage
(c) Complement activation Antibodies attached to the surface
of a pathogen cell activate the complement system.
Pores formed
by complement
Figure 42.24 Antibodies may inhibit infection by (a) preventing the antigen from binding its target, (b) tagging a
pathogen for destruction by macrophages or neutrophils, or (c) activating the complement cascade.
Antibodies also mark pathogens for destruction by phagocytic cells, such as macrophages or neutrophils,
because phagocytic cells are highly attracted to macromolecules complexed with antibodies. Phagocytic
enhancement by antibodies is called opsonization. In a process called complement fixation, IgM and IgG in
serum bind to antigens and provide docking sites onto which sequential complement proteins can bind. The
combination of antibodies and complement enhances opsonization even further and promotes rapid clearing of
pathogens.
Affinity, Avidity, and Cross Reactivity
Not all antibodies bind with the same strength, specificity, and stability. In fact, antibodies exhibit different
affinities (attraction) depending on the molecular complementarity between antigen and antibody molecules, as
illustrated in Figure 42.25. An antibody with a higher affinity for a particular antigen would bind more strongly and
stably, and thus would be expected to present a more challenging defense against the pathogen corresponding
to the specific antigen.
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(a) Affinity versus avidity
Affinity refers to the strength
of a single antibody-antigen
interaction. Each IgG antigen
binding site typically has high
affinity for its target.
Avidity refers to the strength
of all interactions combined.
IgM typically has low affinity
antigen binding sites, but
there are ten of them, so
avidity is high.
(b) Cross reactivity
Figure 42.25 (a) Affinity refers to the strength of single interaction between antigen and antibody, while avidity refers
to the strength of all interactions combined, (b) An antibody may cross react with different epitopes.
The term avidity describes binding by antibody classes that are secreted as joined, multivalent structures (such
as IgM and IgA). Although avidity measures the strength of binding, just as affinity does, the avidity is not simply
the sum of the affinities of the antibodies in a multimeric structure. The avidity depends on the number of identical
binding sites on the antigen being detected, as well as other physical and chemical factors. Typically, multimeric
antibodies, such as pentameric IgM, are classified as having lower affinity than monomeric antibodies, but high
avidity. Essentially, the fact that multimeric antibodies can bind many antigens simultaneously balances their
slightly lower binding strength for each antibody/antigen interaction.
Antibodies secreted after binding to one epitope on an antigen may exhibit cross reactivity for the same or similar
epitopes on different antigens. Because an epitope corresponds to such a small region (the surface area of
about four to six amino acids), it is possible for different macromolecules to exhibit the same molecular identities
and orientations over short regions. Cross reactivity describes when an antibody binds not to the antigen that
elicited its synthesis and secretion, but to a different antigen.
Cross reactivity can be beneficial if an individual develops immunity to several related pathogens despite having
only been exposed to or vaccinated against one of them. For instance, antibody cross reactivity may occur
against the similar surface structures of various Gram-negative bacteria. Conversely, antibodies raised against
pathogenic molecular components that resemble self molecules may incorrectly mark host cells for destruction
and cause autoimmune damage. Patients who develop systemic lupus erythematosus (SLE) commonly exhibit
antibodies that react with their own DNA. These antibodies may have been initially raised against the nucleic
acid of microorganisms but later cross-reacted with self-antigens. This phenomenon is also called molecular
mimicry.
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Antibodies of the Mucosal Immune System
Antibodies synthesized by the mucosal immune system include IgA and IgM. Activated B cells differentiate into
mucosal plasma cells that synthesize and secrete dimeric IgA, and to a lesser extent, pentameric IgM. Secreted
IgA is abundant in tears, saliva, breast milk, and in secretions of the gastrointestinal and respiratory tracts.
Antibody secretion results in a local humoral response at epithelial surfaces and prevents infection of the mucosa
by binding and neutralizing pathogens.
42.4 | Disruptions in the Immune System
By the end of this section, you will be able to do the following:
• Describe hypersensitivity
• Define autoimmunity
A functioning immune system is essential for survival, but even the sophisticated cellular and molecular defenses
of the mammalian immune response can be defeated by pathogens at virtually every step. In the competition
between immune protection and pathogen evasion, pathogens have the advantage of more rapid evolution
because of their shorter generation time and other characteristics. For instance, Streptococcus pneumoniae
(bacterium that cause pneumonia and meningitis) surrounds itself with a capsule that inhibits phagocytes from
engulfing it and displaying antigens to the adaptive immune system. Staphylococcus aureus (bacterium that can
cause skin infections, abscesses, and meningitis) synthesizes a toxin called leukocidin that kills phagocytes after
they engulf the bacterium. Other pathogens can also hinder the adaptive immune system. HIV infects Th cells
via their CD4 surface molecules, gradually depleting the number of Th cells in the body; this inhibits the adaptive
immune system’s capacity to generate sufficient responses to infection or tumors. As a result, HIV-infected
individuals often suffer from infections that would not cause illness in people with healthy immune systems but
which can cause devastating illness to immune-compromised individuals. Maladaptive responses of immune
cells and molecules themselves can also disrupt the proper functioning of the entire system, leading to host cell
damage that could become fatal.
Immunodeficiency
Failures, insufficiencies, or delays at any level of the immune response can allow pathogens or tumor cells to
gain a foothold and replicate or proliferate to high enough levels that the immune system becomes overwhelmed.
Immunodeficiency is the failure, insufficiency, or delay in the response of the immune system, which may
be acquired or inherited. Immunodeficiency can be acquired as a result of infection with certain pathogens
(such as HIV), chemical exposure (including certain medical treatments), malnutrition, or possibly by extreme
stress. For instance, radiation exposure can destroy populations of lymphocytes and elevate an individual’s
susceptibility to infections and cancer. Dozens of genetic disorders result in immunodeficiencies, including
Severe Combined Immunodeficiency (SCID), Bare lymphocyte syndrome, and MHC II deficiencies. Rarely,
primary immunodeficiencies that are present from birth may occur. Neutropenia is one form in which the immune
system produces a below-average number of neutrophils, the body’s most abundant phagocytes. As a result,
bacterial infections may go unrestricted in the blood, causing serious complications.
Hypersensitivities
Maladaptive immune responses toward harmless foreign substances or self antigens that occur after tissue
sensitization are termed hypersensitivities. The types of hypersensitivities include immediate, delayed, and
autoimmunity. A large proportion of the population is affected by one or more types of hypersensitivity.
Allergies
The immune reaction that results from immediate hypersensitivities in which an antibody-mediated immune
response occurs within minutes of exposure to a harmless antigen is called an allergy. In the United States,
20 percent of the population exhibits symptoms of allergy or asthma, whereas 55 percent test positive against
one or more allergens. Upon initial exposure to a potential allergen, an allergic individual synthesizes antibodies
of the IgE class via the typical process of APCs presenting processed antigen to Th cells that stimulate B
cells to produce IgE. This class of antibodies also mediates the immune response to parasitic worms. The
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constant domain of the IgE molecules interact with mast cells embedded in connective tissues. This process
primes, or sensitizes, the tissue. Upon subsequent exposure to the same allergen, IgE molecules on mast cells
bind the antigen via their variable domains and stimulate the mast cell to release the modified amino acids
histamine and serotonin; these chemical mediators then recruit eosinophils which mediate allergic responses.
Figure 42.26 shows an example of an allergic response to ragweed pollen. The effects of an allergic reaction
range from mild symptoms like sneezing and itchy, watery eyes to more severe or even life-threatening reactions
involving intensely itchy welts or hives, airway contraction with severe respiratory distress, and plummeting blood
pressure. This extreme reaction is known as anaphylactic shock. If not treated with epinephrine to counter the
blood pressure and breathing effects, this condition can be fatal.
Upon initial exposure to the
antigen. IgE antibody is
produced and attached to
mast cells.
Chemicals
I
Upon a second exposure,
binding of the antigen to the
IgE-primed mast cells causes
the release of chemical
mediators that elicit an allergic
reaction.
Figure 42.26 On first exposure to an allergen, an IgE antibody is synthesized by plasma cells in response to a
harmless antigen. The IgE molecules bind to mast cells, and on secondary exposure, the mast cells release histamines
and other modulators that affect the symptoms of allergy, (credit: modification of work by NIH)
Delayed hypersensitivity is a cell-mediated immune response that takes approximately one to two days after
secondary exposure for a maximal reaction to be observed. This type of hypersensitivity involves the ThI
cytokine-mediated inflammatory response and may manifest as local tissue lesions or contact dermatitis (rash
or skin irritation). Delayed hypersensitivity occurs in some individuals in response to contact with certain types
of jewelry or cosmetics. Delayed hypersensitivity facilitates the immune response to poison ivy and is also the
reason why the skin test for tuberculosis results in a small region of inflammation on individuals who were
previously exposed to Mycobacterium tuberculosis. That is also why cortisone is used to treat such responses:
it will inhibit cytokine production.
Autoimmunity
Autoimmunity is a type of hypersensitivity to self antigens that affects approximately five percent of the
population. Most types of autoimmunity involve the humoral immune response. Antibodies that inappropriately
mark self components as foreign are termed autoantibodies. In patients with the autoimmune disease
myasthenia gravis, muscle cell receptors that induce contraction in response to acetylcholine are targeted by
antibodies. The result is muscle weakness that may include marked difficultly with fine and/or gross motor
functions. In systemic lupus erythematosus, a diffuse autoantibody response to the individual’s own DNA and
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proteins results in various systemic diseases. As illustrated in Figure 42.27, systemic lupus erythematosus may
affect the heart, joints, lungs, skin, kidneys, central nervous system, or other tissues, causing tissue damage via
antibody binding, complement recruitment, lysis, and inflammation.
■ Face rash
Ulcers of the mouth-
and nose
Muscle aches -
If
Inflammation of
' the pericardium
Poor circulation
in the fingers
and toes
Figure 42.27 Systemic lupus erythematosus is characterized by autoimmunity to the individual’s own DNA and/or
proteins, which leads to varied dysfunction of the organs, (credit: modification of work by Mikael Haggstrom)
Autoimmunity can develop with time, and its causes may be rooted in molecular mimicry. Antibodies and TCRs
may bind self antigens that are structurally similar to pathogen antigens, which the immune receptors first raised.
As an example, infection with Streptococcus pyogenes (bacterium that causes strep throat) may generate
antibodies or T cells that react with heart muscle, which has a similar structure to the surface of S. pyogenes.
These antibodies can damage heart muscle with autoimmune attacks, leading to rheumatic fever. Insulin-
dependent (Type 1) diabetes mellitus arises from a destructive inflammatory ThI response against insulin-
producing cells of the pancreas. Patients with this autoimmunity must be injected with insulin that originates from
other sources.
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KEY TERMS
adaptive immunity immunity that has memory and occurs after exposure to an antigen either from a pathogen
or a vaccination
affinity attraction of molecular complementarity between antigen and antibody molecules
allergy immune reaction that results from immediate hypersensitivities in which an antibody-mediated immune
response occurs within minutes of exposure to a harmless antigen
antibody protein that is produced by plasma cells after stimulation by an antigen; also known as an
immunoglobulin
antigen foreign or “non-self” protein that triggers the immune response
antigen-presenting cell (APC) immune cell that detects, engulfs, and informs the adaptive immune response
about an infection by presenting the processed antigen on the cell surface
autoantibody antibody that incorrectly marks “self” components as foreign and stimulates the immune
response
autoimmune response inappropriate immune response to host cells or self-antigens
autoimmunity type of hypersensitivity to self antigens
avidity total binding strength of a multivalent antibody with antigen
B cell lymphocyte that matures in the bone marrow and differentiates into antibody-secreting plasma cells
basophil leukocyte that releases chemicals usually involved in the inflammatory response
cell-mediated immune response adaptive immune response that is carried out by T cells
clonal selection activation of B cells corresponding to one specific BCR variant and the dramatic proliferation of
that variant
complement system array of approximately 20 soluble proteins of the innate immune system that enhance
phagocytosis, bore holes in pathogens, and recruit lymphocytes; enhances the adaptive response when
antibodies are produced
cross reactivity binding of an antibody to an epitope corresponding to an antigen that is different from the one
the antibody was raised against
cytokine chemical messenger that regulates cell differentiation, proliferation, gene expression, and cell
trafficking to effect immune responses
cytotoxic T lymphocyte (CTL) adaptive immune cell that directly kills infected cells via perforin and
granzymes, and releases cytokines to enhance the immune response
dendritic cell immune cell that processes antigen material and presents it on the surface of other cells to induce
an immune response
effector cell lymphocyte that has differentiated, such as a B cell, plasma cell, or cytotoxic T lymphocyte
eosinophil leukocyte that responds to parasites and is involved in the allergic response
epitope small component of an antigen that is specifically recognized by antibodies, B cells, and T cells; the
antigenic determinant
granzyme protease that enters target cells through perforin and induces apoptosis in the target cells; used by
NK cells and killer T cells
helper T lymphocyte (Th) cell of the adaptive immune system that binds APCs via MHC II molecules and
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stimulates B cells or secretes cytokines to initiate the immune response
host an organism that is invaded by a pathogen or parasite
humoral immune response adaptive immune response that is controlled by activated B cells and antibodies
hypersensitivities spectrum of maladaptive immune responses toward harmless foreign particles or self
antigens; occurs after tissue sensitization and includes immediate-type (allergy), delayed-type, and
autoimmunity
immune tolerance acquired ability to prevent an unnecessary or harmful immune response to a detected
foreign body known not to cause disease or to self-antigens
immunodeficiency failure, insufficiency, or delay at any level of the immune system, which may be acquired or
inherited
inflammation localized redness, swelling, heat, and pain that results from the movement of leukocytes and fluid
through opened capillaries to a site of infection
innate immunity immunity that occurs naturally because of genetic factors or physiology, and is not induced by
infection or vaccination
interferon cytokine that inhibits viral replication and modulates the immune response
lymph watery fluid that bathes tissues and organs with protective white blood cells and does not contain
erythrocytes
lymphocyte leukocyte that is histologically identifiable by its large nuclei; it is a small cell with very little
cytoplasm
macrophage large phagocytic cell that engulfs foreign particles and pathogens
major histocompatibility class (MHC) I/ll molecule protein found on the surface of all nucleated cells (I) or
specifically on antigen-presenting cells (II) that signals to immune cells whether the cell is healthy/normal or
is infected/cancerous; it provides the appropriate template into which antigens can be loaded for
recognition by lymphocytes
mast cell leukocyte that produces inflammatory molecules, such as histamine, in response to large pathogens
and allergens
memory cell antigen-specific B or T lymphocyte that does not differentiate into effector cells during the primary
immune response but that can immediately become an effector cell upon reexposure to the same pathogen
monocyte type of white blood cell that circulates in the blood and lymph and differentiates into macrophages
after it moves into infected tissue
mucosa-associated lymphoid tissue (MALT) collection of lymphatic tissue that combines with epithelial
tissue lining the mucosa throughout the body
natural killer (NK) cell lymphocyte that can kill cells infected with viruses or tumor cells
neutrophil phagocytic leukocyte that engulfs and digests pathogens
opsonization process that enhances phagocytosis using proteins to indicate the presence of a pathogen to
phagocytic cells
passive immunity transfer of antibodies from one individual to another to provide temporary protection against
pathogens
pathogen an agent, usually a microorganism, that causes disease in the organisms that it invades
pathogen-associated molecular pattern (PAMP) carbohydrate, polypeptide, and nucleic acid “signature” that
is expressed by viruses, bacteria, and parasites but differs from molecules on host cells
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Chapter 42 | The Immune System
pattern recognition receptor (PRR) molecule on macrophages and dendritic cells that binds molecular
signatures of pathogens and promotes pathogen engulfment and destruction
perforin destructive protein that creates a pore in the target cell; used by NK cells and killer T cells
plasma cell immune cell that secrets antibodies; these cells arise from B cells that were stimulated by antigens
regulatory T (Treg) cell specialized lymphocyte that suppresses local inflammation and inhibits the secretion of
cytokines, antibodies, and other stimulatory immune factors; involved in immune
tolerance
T cell lymphocyte that matures in the thymus gland; one of the main cells involved in the adaptive immune
system
CHAPTER SUMMARY
42.1 Innate Immune Response
The innate immune system serves as a first responder to pathogenic threats that bypass natural physical and
chemical barriers of the body. Using a combination of cellular and molecular attacks, the innate immune system
identifies the nature of a pathogen and responds with inflammation, phagocytosis, cytokine release, destruction
by NK cells, and/or a complement system. When innate mechanisms are insufficient to clear an infection, the
adaptive immune response is informed and mobilized.
42.2 Adaptive Immune Response
The adaptive immune response is a slower-acting, longer-lasting, and more specific response than the innate
response. However, the adaptive response requires information from the innate immune system to function.
APCs display antigens via MHC molecules to complementary naive T cells. In response, the T cells
differentiate and proliferate, becoming Th cells or CTLs. Th cells stimulate B cells that have engulfed and
presented pathogen-derived antigens. B cells differentiate into plasma cells that secrete antibodies, whereas
CTLs induce apoptosis in intracellularly infected or cancerous cells. Memory cells persist after a primary
exposure to a pathogen. If reexposure occurs, memory cells differentiate into effector cells without input from
the innate immune system. The mucosal immune system is largely independent from the systemic immune
system but functions in a parallel fashion to protect the extensive mucosal surfaces of the body.
42.3 Antibodies
Antibodies (immunoglobulins) are the molecules secreted from plasma cells that mediate the humoral immune
response. There are five antibody classes; an antibody's class determines its mechanism of action and
production site but does not control its binding specificity. Antibodies bind antigens via variable domains and
can either neutralize pathogens or mark them for phagocytosis or activate the complement cascade.
42.4 Disruptions in the Immune System
immune disruptions may involve insufficient immune responses or inappropriate immune targets.
Immunodeficiency increases an individual's susceptibility to infections and cancers. Hypersensitivities are
misdirected responses either to harmless foreign particles, as in the case of allergies, or to host factors, as in
the case of autoimmunity. Reactions to self components may be the result of molecular mimicry.
VISUAL CONNECTION QUESTIONS
1. Figure 42.11 Which of the following statements
about T cells is false?
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a. Helper T cells release cytokines while
cytotoxic T cells kill the infected cell.
b. Helper T cells are CD4+, while cytotoxic T
cells are CD8 + .
c. MHC II is a receptor found on most body
cells, while MHC I is a receptor found on
immune cells only.
d. The T cell receptor is found on both CD4 +
and CD8 + T cells.
2. Figure 42.14 Based on what you know about MHC
receptors, why do you think an organ transplanted
REVIEW QUESTIONS
4. Which of the following is a barrier against
pathogens provided by the skin?
a. high pH
b. mucus
c. tears
d. desiccation
5. Although interferons have several effects, they are
particularly useful against infections with which type
of pathogen?
a. bacteria
b. viruses
c. fungi
d. helminths
6. Which organelle do phagocytes use to digest
engulfed particles?
a. lysosome
b. nucleus
c. endoplasmic reticulum
d. mitochondria
7. Which innate immune system component uses
MHC I molecules directly in its defense strategy?
a. macrophages
b. neutrophils
c. NK cells
d. interferon
8. Which of the following is both a phagocyte and an
antigen-presenting cell?
a. NK cell
b. eosinophil
c. neutrophil
d. macrophage
9. Which immune cells bind MHC molecules on
APCs via CD8 coreceptors on their cell surfaces?
a. Th cells
b. CTLs
c. mast cells
d. basophils
10. What “self” pattern is identified by NK cells?
from an incompatible donor to a recipient will be
rejected?
3. Figure 42.16 The Rh antigen is found on Rh-
positive red blood cells. An Rh-negative female can
usually carry an Rh-positive fetus to term without
difficulty. However, if she has a second Rh-positive
fetus, her body may launch an immune attack that
causes hemolytic disease of the newborn. Why do
you think hemolytic disease is only a problem during
the second or subsequent pregnancies?
a. altered self
b. missing self
c. normal self
d. non-self
11. The acquired ability to prevent an unnecessary or
destructive immune reaction to a harmless foreign
particle, such as a food protein, is called_.
a. the Th 2 response
b. allergy
c. immune tolerance
d. autoimmunity
12. A memory B cell can differentiate upon
reexposure to a pathogen of which cell type?
a. CTL
b. naive B cell
c. memory T cell
d. plasma cell
13. Foreign particles circulating in the blood are
filtered by the_.
a. spleen
b. lymph nodes
c. MALT
d. lymph
14. The structure of an antibody is similar to the
extracellular component of which receptor?
a. MHC I
b. MHC II
c. BCR
d. none of the above
15. The first antibody class to appear in the serum in
response to a newly encountered pathogen is
a. IgM
b. IgA
c. IgG
d. IgE
16. What is the most abundant antibody class
detected in the serum upon reexposure to a
pathogen or in reaction to a vaccine?
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a. IgM
b. IgA
c. IgG
d. IgE
17. Breastfed infants typically are resistant to disease
because of_.
a. active immunity
b. passive immunity
c. immune tolerance
d. immune memory
18. Allergy to pollen is classified as:
a. an autoimmune reaction
b. immunodeficiency
c. delayed hypersensitivity
d. immediate hypersensitivity
19. A potential cause of acquired autoimmunity is
CRITICAL THINKING QUESTIONS
22. Different MHC I molecules between donor and
recipient cells can lead to rejection of a transplanted
organ or tissue. Suggest a reason for this.
23. If a series of genetic mutations prevented some,
but not all, of the complement proteins from binding
antibodies or pathogens, would the entire
complement system be compromised?
24. Explain the difference between an epitope and an
antigen.
25. What is a naive B or T cell?
26. How does the ThI response differ from the Th 2
response?
a. tissue hypersensitivity
b. molecular mimicry
c. histamine release
d. radiation exposure
20. Autoantibodies are probably involved in:
a. reactions to poison ivy
b. pollen allergies
c. systemic lupus erythematosus
d. HIV/AIDS
21. Which of the following diseases is not due to
autoimmunity?
a. rheumatic fever
b. systemic lupus erythematosus
c. diabetes mellitus
d. HIV/AIDS
27. In mammalian adaptive immune systems, T cell
receptors are extraordinarily diverse. What function
of the immune system results from this diversity, and
how is this diversity achieved?
28. How do B and T cells differ with respect to
antigens that they bind?
29. Why is the immune response after reinfection
much faster than the adaptive immune response after
the initial infection?
30. What are the benefits and costs of antibody cross
reactivity?
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Chapter 43 | Animal Reproduction and Development
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43 | ANIMAL
REPRODUCTION AND
DEVELOPMENT
Figure 43.1 Female seahorses produce eggs for reproduction that are then fertilized by the male. Unlike almost all
other animals, the male seahorse then gestates the young until birth, (credit: modification of work by "cliffl066"/Flickr)
Chapter Outline
43.1: Reproduction Methods
43.2: Fertilization
43.3: Human Reproductive Anatomy and Gametogenesis
43.4: Hormonal Control of Human Reproduction
43.5: Human Pregnancy and Birth
43.6: Fertilization and Early Embryonic Development
43.7: Organogenesis and Vertebrate Formation
Introduction
Animal reproduction is necessary for the survival of a species, in the animal kingdom, there are innumerable
ways that species reproduce. Asexual reproduction produces genetically identical organisms (clones), whereas
in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically
different from their parents. During sexual reproduction the male gamete (sperm) may be placed inside the
female’s body for internal fertilization, or the sperm and eggs may be released into the environment for external
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fertilization. Seahorses, like the one shown in Figure 43.1, provide an example of the latter. Following a mating
dance, the female lays eggs in the male seahorse’s abdominal brood pouch where they are fertilized. The eggs
hatch and the offspring develop in the pouch for several weeks.
43.1 1 Reproduction Methods
By the end of this section, you will be able to do the following:
• Describe advantages and disadvantages of asexual and sexual reproduction
• Discuss asexual reproduction methods
• Discuss sexual reproduction methods
Animals produce offspring through asexual and/or sexual reproduction. Both methods have advantages and
disadvantages. Asexual reproduction produces offspring that are genetically identical to the parent because
the offspring are all clones of the original parent. A single individual can produce offspring asexually and large
numbers of offspring can be produced quickly. In a stable or predictable environment, asexual reproduction is
an effective means of reproduction because all the offspring will be adapted to that environment, in an unstable
or unpredictable environment asexually-reproducing species may be at a disadvantage because all the offspring
are genetically identical and may not have the genetic variation to survive in new or different conditions. On the
other hand, the rapid rates of asexual reproduction may allow for a speedy response to environmental changes if
individuals have mutations. An additional advantage of asexual reproduction is that colonization of new habitats
may be easier when an individual does not need to find a mate to reproduce.
During sexual reproduction the genetic material of two individuals is combined to produce genetically diverse
offspring that differ from their parents. The genetic diversity of sexually produced offspring is thought to give
species a better chance of surviving in an unpredictable or changing environment. Species that reproduce
sexually must maintain two different types of individuals, males and females, which can limit the ability to
colonize new habitats as both sexes must be present.
Asexual Reproduction
Asexual reproduction occurs in prokaryotic microorganisms (bacteria) and in some eukaryotic single-celled and
multi-celled organisms. There are a number of ways that animals reproduce asexually.
Fission
Fission, also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled
organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic
organisms undergo binary fission by mitosis. In other organisms, part of the individual separates and forms
a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the
central disk. Some sea anemones and some coral polyps (Figure 43.2) also reproduce through fission.
Figure 43.2 Coral polyps reproduce asexually by fission, (credit: G. P. Schmahl, NOAA FGBNMS Manager)
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Budding
Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region
leading to a separation from the original organism into two individuals. Budding occurs commonly in some
invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult and breaks
away from the main body, as illustrated in Figure 43.3, whereas in coral budding, the bud does not detach and
multiplies as part of a new colony.
Figure 43.3 Hydra reproduce asexually through budding.
LINK TQ LEARNING
Watch a video of a hydra budding. (This multimedia resource will open in a browser.) (http://cnx.org/
content/m66668/1.3/#eip-id!170503731944)
Fragmentation
Fragmentation is the breaking of the body into two parts with subsequent regeneration. If the animal is capable
of fragmentation, and the part is big enough, a separate individual will regrow.
For example, in many sea stars, asexual reproduction is accomplished by fragmentation. Figure 43.4 illustrates
a sea star for which an arm of the individual is broken off and regenerates a new sea star. Fisheries workers
have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing
them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting
in twice as many sea stars to prey upon the oysters and clams. Fragmentation also occurs in annelid worms,
turbellarians, and poriferans.
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Figure 43.4 Sea stars can reproduce through fragmentation. The large arm, a fragment from another sea star, is
developing into a new individual.
Note that in fragmentation, there is generally a noticeable difference in the size of the individuals, whereas in
fission, two individuals of approximate size are formed.
Parthenogenesis
Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without
being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the
species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, some ants,
wasps, and bees. Bees use parthenogenesis to produce haploid males (drones). If eggs are fertilized, diploid
females develop, and if the fertilized eggs are fed a special diet (so called royal jelly), a queen is produced.
Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through
parthenogenesis. Although more common in plants, parthenogenesis has been observed in animal species that
were segregated by sex in terrestrial or marine zoos. Two female Komodo dragons, a hammerhead shark, and
a blacktop shark have produced parthenogenic young when the females have been isolated from males.
Sexual Reproduction
Sexual reproduction is the combination of (usually haploid) reproductive cells from two individuals to form a third
(usually diploid) unique offspring. Sexual reproduction produces offspring with novel combinations of genes. This
can be an adaptive advantage in unstable or unpredictable environments. As humans, we are used to thinking
of animals as having two separate sexes—male and female—determined at conception. However, in the animal
kingdom, there are many variations on this theme.
Hermaphroditism
Hermaphroditism occurs in animals where one individual has both male and female reproductive parts.
Invertebrates such as earthworms, slugs, tapeworms and snails, shown in Figure 43.5, are often
hermaphroditic. Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other
and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile,
such as barnacles and clams.
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Chapter 43 | Animal Reproduction and Development
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Figure 43.5 Many snails are hermaphrodites. When two individuals mate, they can produce up to one hundred eggs
each, (credit: Assaf Shtilman)
Sex Determination
Mammalian sex determination is determined genetically by the presence of X and Y chromosomes. Individuals
homozygous for X (XX) are female and heterozygous individuals (XY) are male. The presence of a Y
chromosome causes the development of male characteristics and its absence results in female characteristics.
The XY system is also found in some insects and plants.
Avian sex determination is dependent on the presence of Z and W chromosomes. Homozygous for Z (ZZ) results
in a male and heterozygous (ZW) results in a female. The W appears to be essential in determining the sex of
the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies
and moths), and reptiles use this system.
The sex of some species is not determined by genetics but by some aspect of the environment. Sex
determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical
periods of egg development. This is referred to as environmental sex determination, or more specifically as
temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce
males and warm temperatures produce females. In some crocodiles, moderate temperatures produce males
and both warm and cool temperatures produce females. In some species, sex is both genetic- and temperature-
dependent.
Individuals of some species change their sex during their lives, alternating between male and female. If the
individual is female first, it is termed protogyny or “first female,” if it is male first, its termed protandry or “first
male.” Oysters, for example, are born male, grow, and become female and lay eggs; some oyster species
change sex multiple times.
43.2 | Fertilization
By the end of this section, you will be able to do the following:
• Discuss internal and external methods of fertilization
• Describe the methods used by animals for development of offspring during gestation
• Describe the anatomical adaptions that occurred in animals to facilitate reproduction
Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. This can
occur either inside ( internal fertilization) or outside ( external fertilization) the body of the female. Humans
provide an example of the former whereas seahorse reproduction is an example of the latter.
External Fertilization
External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the
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water. After the sperm reaches the egg, fertilization takes place. Most external fertilization happens during the
process of spawning where one or several females release their eggs and the male(s) release sperm in the
same area, at the same time. The release of the reproductive material may be triggered by water temperature
or the length of daylight. Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such
as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers). Figure 43.6 shows salmon
spawning in a shallow stream. Frogs, like those shown in Figure 43.7, corals, molluscs, and sea cucumbers also
spawn.
Figure 43.6 Salmon reproduce through spawning, (credit: Dan Bennett)
Figure 43.7 During sexual reproduction in toads, the male grasps the female from behind and externally fertilizes the
eggs as they are deposited, (credit: "OakleyOriginals'VFlickr)
Pairs of fish that are not broadcast spawners may exhibit courtship behavior. This allows the female to select a
particular male. The trigger for egg and sperm release (spawning) causes the egg and sperm to be placed in a
small area, enhancing the possibility of fertilization.
External fertilization in an aquatic environment protects the eggs from drying out. Broadcast spawning can result
in a greater mixture of the genes within a group, leading to higher genetic diversity and a greater chance of
species survival in a hostile environment. For sessile aquatic organisms like sponges, broadcast spawning is
the only mechanism for fertilization and colonization of new environments. The presence of the fertilized eggs
and developing young in the water provides opportunities for predation resulting in a loss of offspring. Therefore,
millions of eggs must be produced by individuals, and the offspring produced through this method must mature
rapidly. The survival rate of eggs produced through broadcast spawning is low.
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Chapter 43 | Animal Reproduction and Development
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Internal Fertilization
internal fertilization occurs most often in land-based animals, although some aquatic animals also use this
method. There are three ways that offspring are produced following internal fertilization. In oviparity, fertilized
eggs are laid outside the female’s body and develop there, receiving nourishment from the yolk that is a
part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two
mammals, and all birds. Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with
high concentrations of calcium carbonate in the shell, making them hard. Chicken eggs are an example of this
second type.
In ovoviparity, fertilized eggs are retained in the female, but the embryo obtains its nourishment from the
egg’s yolk and the young are fully developed when they are hatched. This occurs in some bony fish (like the
guppy Lebistes reticulatus), some sharks, some lizards, some snakes (such as the garter snake Thamnophis
sirtalis), some vipers, and some invertebrate animals (like the Madagascar hissing cockroach Gromphadorhina
portentosa).
In viviparity the young develop within the female, receiving nourishment from the mother’s blood through
a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some
cartilaginous fish, and a few reptiles.
Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is
isolated within the female, which limits predation on the young. Internal fertilization enhances the fertilization of
eggs by a specific male. Fewer offspring are produced through this method, but their survival rate is higher than
that for external fertilization.
The Evolution of Reproduction
Once multicellular organisms evolved and developed specialized cells, some also developed tissues and organs
with specialized functions. An early development in reproduction occurred in the Annelids. These organisms
produce sperm and eggs from undifferentiated cells in their coelom and store them in that cavity. When the
coelom becomes filled, the cells are released through an excretory opening or by the body splitting open.
Reproductive organs evolved with the development of gonads that produce sperm and eggs. These cells went
through meiosis, an adaption of mitosis, which reduced the number of chromosomes in each reproductive cell
by half, while increasing the number of cells through cell division.
Complete reproductive systems were developed in insects, with separate sexes. Sperm are made in testes
and then travel through coiled tubes to the epididymis for storage. Eggs mature in the ovary. When they are
released from the ovary, they travel to the uterine tubes for fertilization. Some insects have a specialized sac,
called a spermatheca, which stores sperm for later use, sometimes up to a year. Fertilization can be timed with
environmental or food conditions that are optimal for offspring survival.
Vertebrates have similar structures, with a few differences. Non-mammals, such as birds and reptiles, have
a common body opening, called a cloaca, for the digestive, excretory and reproductive systems. Coupling
between birds usually involves positioning the cloaca openings opposite each other for transfer of sperm.
Mammals have separate openings for the systems in the female and a uterus for support of developing offspring.
The uterus has two chambers in species that produce large numbers of offspring at a time, while species that
produce one offspring, such as primates, have a single uterus.
Sperm transfer from the male to the female during reproduction ranges from releasing the sperm into the watery
environment for external fertilization, to the joining of cloaca in birds, to the development of a penis for direct
delivery into the female’s vagina in mammals.
43.3 | Human Reproductive Anatomy and
Gametogenesis
By the end of this section, you will be able to do the following:
• Describe human male and female reproductive anatomies
• Discuss the human sexual response
• Describe spermatogenesis and oogenesis and discuss their differences and similarities
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As animals became more complex, specific organs and organ systems developed to support specific functions
for the organism. The reproductive structures that evolved in land animals allow males and females to mate,
fertilize internally, and support the growth and development of offspring.
Human Reproductive Anatomy
The reproductive tissues of male and female humans develop similarly in utero until a low level of the hormone
testosterone is released from male gonads. Testosterone causes the undeveloped tissues to differentiate into
male sexual organs. When testosterone is absent, the tissues develop into female sexual tissues. Primitive
gonads become testes or ovaries. Tissues that produce a penis in males produce a clitoris in females. The tissue
that will become the scrotum in a male becomes the labia in a female; that is, they are homologous structures.
Male Reproductive Anatomy
In the male reproductive system, the scrotum houses the testicles or testes (singular: testis), including providing
passage for blood vessels, nerves, and muscles related to testicular function. The testes are a pair of male
reproductive organs that produce sperm and some reproductive hormones. Each testis is approximately 2.5 by
3.8 cm (1.5 by 1 in.) in size and divided into wedge-shaped lobules by connective tissue called septa. Coiled in
each wedge are seminiferous tubules that produce sperm.
Sperm are immobile at body temperature; therefore, the scrotum and penis are external to the body, as illustrated
in Figure 43.8 so that a proper temperature is maintained for motility. In land mammals, the pair of testes must be
suspended outside the body at about 2 C lower than body temperature to produce viable sperm. Infertility can
occur in land mammals when the testes do not descend through the abdominal cavity during fetal development.
visual
ft CONNECTION
Pubic bone
Bladder
Seminal vesicle
Seminiferous tubules
Figure 43.8 The reproductive structures of the human male are shown.
Which of the following statements about the male reproductive system is false?
a. The vas deferens carries sperm from the testes to the penis.
b. Sperm mature in seminiferous tubules in the testes.
c. Both the prostate and the bulbourethral glands produce components of the semen.
d. The prostate gland is located in the testes.
Sperm mature in seminiferous tubules that are coiled inside the testes, as illustrated in Figure 43.8. The walls
of the seminiferous tubules are made up of the developing sperm cells, with the least developed sperm at the
periphery of the tubule and the fully developed sperm in the lumen. The sperm cells are mixed with “nursemaid”
cells called Sertoli cells which protect the germ cells and promote their development. Other cells mixed in the
wall of the tubules are the interstitial cells of Leydig. These cells produce high levels of testosterone once the
male reaches adolescence.
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Chapter 43 | Animal Reproduction and Development
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When the sperm have developed flagella and are nearly mature, they leave the testicles and enter the
epididymis, shown in Figure 43.8. This structure resembles a comma and lies along the top and posterior portion
of the testes; it is the site of sperm maturation. The sperm leave the epididymis and enter the vas deferens
(or ductus deferens), which carries the sperm, behind the bladder, and forms the ejaculatory duct with the duct
from the seminal vesicles. During a vasectomy, a section of the vas deferens is removed, preventing sperm from
being passed out of the body during ejaculation and preventing fertilization.
Semen is a mixture of sperm and spermatic duct secretions (about 10 percent of the total) and fluids from
accessory glands that contribute most of the semen’s volume. Sperm are haploid cells, consisting of a flagellum
as a tail, a neck that contains the cell’s energy-producing mitochondria, and a head that contains the genetic
material. Figure 43.9 shows a micrograph of human sperm as well as a diagram of the parts of the sperm. An
acrosome is found at the top of the head of the sperm. This structure contains lysosomal enzymes that can digest
the protective coverings that surround the egg to help the sperm penetrate and fertilize the egg. An ejaculate will
contain from two to five milliliters of fluid with from 50-120 million sperm per milliliter.
Figure 43.9 Human sperm, visualized using scanning electron microscopy, have a flagellum, neck, and head, (credit
b: modification of work by Mariana Ruiz Villareal; scale-bar data from Matt Russell)
The bulk of the semen comes from the accessory glands associated with the male reproductive system. These
are the seminal vesicles, the prostate gland, and the bulbourethral gland, all of which are illustrated in Figure
43.8. The seminal vesicles are a pair of glands that lie along the posterior border of the urinary bladder. The
glands make a solution that is thick, yellowish, and alkaline. As sperm are only motile in an alkaline environment,
a basic pH is important to reverse the acidity of the vaginal environment. The solution also contains mucus,
fructose (a sperm mitochondrial nutrient), a coagulating enzyme, ascorbic acid, and local-acting hormones called
prostaglandins. The seminal vesicle glands account for 60 percent of the bulk of semen.
The penis, illustrated in Figure 43.8, is an organ that drains urine from the renal bladder and functions as
a copulatory organ during intercourse. The penis contains three tubes of erectile tissue running through the
length of the organ. These consist of a pair of tubes on the dorsal side, called the corpus cavernosum, and a
single tube of tissue on the ventral side, called the corpus spongiosum. This tissue will become engorged with
blood, becoming erect and hard, in preparation for intercourse. The organ is inserted into the vagina culminating
with an ejaculation. During intercourse, the smooth muscle sphincters at the opening to the renal bladder close
and prevent urine from entering the penis. An orgasm is a two-stage process: first, glands and accessory
organs connected to the testes contract, then semen (containing sperm) is expelled through the urethra during
ejaculation. After intercourse, the blood drains from the erectile tissue and the penis becomes flaccid.
The walnut-shaped prostate gland surrounds the urethra, the connection to the urinary bladder. It has a series
of short ducts that directly connect to the urethra. The gland is a mixture of smooth muscle and glandular tissue.
The muscle provides much of the force needed for ejaculation to occur. The glandular tissue makes a thin, milky
fluid that contains citrate (a nutrient), enzymes, and prostate specific antigen (PSA). PSA is a proteolytic enzyme
that helps to liquefy the ejaculate several minutes after release from the male. Prostate gland secretions account
for about 30 percent of the bulk of semen.
The bulbourethral gland, or Cowper’s gland, releases its secretion prior to the release of the bulk of the semen.
It neutralizes any acid residue in the urethra left over from urine. This usually accounts for a couple of drops
of fluid in the total ejaculate and may contain a few sperm. Withdrawal of the penis from the vagina before
ejaculation to prevent pregnancy may not work if sperm are present in the bulbourethral gland secretions. The
location and functions of the male reproductive organs are summarized in Table 43.1.
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Male Reproductive Anatomy
Organ
Location
Function
Scrotum
External
Carry and support testes
Penis
External
Deliver urine, copulating organ
Testes
Internal
Produce sperm and male hormones
Seminal Vesicles
Internal
Contribute to semen production
Prostate Gland
Internal
Contribute to semen production
Bulbourethral Glands
Internal
Clean urethra at ejaculation
Table 43.1
Female Reproductive Anatomy
A number of reproductive structures are exterior to the female’s body. These include the breasts and the vulva,
which consists of the mons pubis, clitoris, labia majora, labia minora, and the vestibular glands, all illustrated in
Figure 43.10. The location and functions of the female reproductive organs are summarized in Table 43.2. The
vulva is an area associated with the vestibule which includes the structures found in the inguinal (groin) area
of women. The mons pubis is a round, fatty area that overlies the pubic symphysis. The clitoris is a structure
with erectile tissue that contains a large number of sensory nerves and serves as a source of stimulation during
intercourse. The labia majora are a pair of elongated folds of tissue that run posterior from the mons pubis
and enclose the other components of the vulva. The labia majora derive from the same tissue that produces
the scrotum in a male. The labia minora are thin folds of tissue centrally located within the labia majora. These
labia protect the openings to the vagina and urethra. The mons pubis and the anterior portion of the labia majora
become covered with hair during adolescence; the labia minora is hairless. The greater vestibular glands are
found at the sides of the vaginal opening and provide lubrication during intercourse.
Figure 43.10 The reproductive structures of the human female are shown, (credit a: modification of work by Gray's
Anatomy; credit b: modification of work by CDC)
Female Reproductive Anatomy
Organ
Location
Function
Clitoris
External
Sensory organ
Table 43.2
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Chapter 43 | Animal Reproduction and Development
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Female Reproductive Anatomy
Organ
Location
Function
Mons pubis
External
Fatty area overlying pubic bone
Labia majora
External
Covers labia minora
Labia minora
External
Covers vestibule
Greater vestibular glands
External
Secrete mucus; lubricate vagina
Breasts
External
Produce and deliver milk
Ovaries
Internal
Carry and develop eggs
Oviducts (Fallopian tubes)
Internal
Transport egg to uterus
Uterus
Internal
Support developing embryo
Vagina
Internal
Common tube for intercourse, birth canal, passing menstrual flow
Table 43.2
The breasts consist of mammary glands and fat. The size of the breast is determined by the amount of fat
deposited behind the gland. Each gland consists of 15 to 25 lobes that have ducts that empty at the nipple and
that supply the nursing child with nutrient- and antibody-rich milk to aid development and protect the child.
Internal female reproductive structures include ovaries, oviducts, the uterus, and the vagina, shown in Figure
43.10. The pair of ovaries is held in place in the abdominal cavity by a system of ligaments. Ovaries consist of
a medulla and cortex: the medulla contains nerves and blood vessels to supply the cortex with nutrients and
remove waste. The outer layers of cells of the cortex are the functional parts of the ovaries. The cortex is made
up of follicular cells that surround eggs that develop during fetal development in utero. During the menstrual
period, a batch of follicular cells develops and prepares the eggs for release. At ovulation, one follicle ruptures
and one egg is released, as illustrated in Figure 43.11a.
Maturing follicle
Medulla
(inner part
of ovary)
(a) (b)
Figure 43.11 Oocytes develop in (a) follicles, located in the ovary. At the beginning of the menstrual cycle, the follicle
matures. At ovulation, the follicle ruptures, releasing the egg. The follicle becomes a corpus luteum, which eventually
degenerates. The (b) follicle in this light micrograph has an oocyte at its center, (credit a: modification of work by NIH;
scale-bar data from Matt Russell)
The oviducts, or fallopian tubes, extend from the uterus in the lower abdominal cavity to the ovaries, but they
are not in contact with the ovaries. The lateral ends of the oviducts flare out into a trumpet-like structure and
have a fringe of finger-like projections called fimbriae, illustrated in Figure 43.10b. When an egg is released at
ovulation, the fimbrae help the nonmotile egg enter into the tube and passage to the uterus. The walls of the
oviducts are ciliated and are made up mostly of smooth muscle. The cilia beat toward the middle, and the smooth
muscle contracts in the same direction, moving the egg toward the uterus. Fertilization usually takes place within
the oviducts and the developing embryo is moved toward the uterus for development. It usually takes the egg or
embryo a week to travel through the oviduct. Sterilization in women is called a tubal ligation; it is analogous to a
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vasectomy in males in that the oviducts are severed and sealed.
The uterus is a structure about the size of a woman’s fist. This is lined with an endometrium rich in blood vessels
and mucus glands. The uterus supports the developing embryo and fetus during gestation. The thickest portion
of the wall of the uterus is made of smooth muscle. Contractions of the smooth muscle in the uterus aid in
passing the baby through the vagina during labor. A portion of the lining of the uterus sloughs off during each
menstrual period, and then builds up again in preparation for an implantation. Part of the uterus, called the cervix,
protrudes into the top of the vagina. The cervix functions as the birth canal.
The vagina is a muscular tube that serves several purposes. It allows menstrual flow to leave the body. It is
the receptacle for the penis during intercourse and the vessel for the delivery of offspring. It is lined by stratified
squamous epithelial cells to protect the underlying tissue.
Sexual Response during Intercourse
The sexual response in humans is both psychological and physiological. Both sexes experience sexual arousal
through psychological and physical stimulation. There are four phases of the sexual response. During phase
one, called excitement, vasodilation leads to vasocongestion in erectile tissues in both men and women. The
nipples, clitoris, labia, and penis engorge with blood and become enlarged. Vaginal secretions are released to
lubricate the vagina to facilitate intercourse. During the second phase, called the plateau, stimulation continues,
the outer third of the vaginal wall enlarges with blood, and breathing and heart rate increase.
During phase three, or orgasm, rhythmic, involuntary contractions of muscles occur in both sexes. In the male,
the reproductive accessory glands and tubules constrict placing semen in the urethra, then the urethra contracts
expelling the semen through the penis. In women, the uterus and vaginal muscles contract in waves that may
last slightly less than a second each. During phase four, or resolution, the processes described in the first three
phases reverse themselves and return to their normal state. Men experience a refractory period in which they
cannot maintain an erection or ejaculate for a period of time ranging from minutes to hours.
Gametogenesis (Spermatogenesis and Oogenesis)
Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis. During meiosis,
two cell divisions separate the paired chromosomes in the nucleus and then separate the chromatids that were
made during an earlier stage of the cell’s life cycle. Meiosis produces haploid cells with half of each pair of
chromosomes normally found in diploid cells. The production of sperm is called spermatogenesis and the
production of eggs is called oogenesis.
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Chapter 43 | Animal Reproduction and Development
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Spermatogenesis
2n Spermatogonium
Mitosis
I
2n 2 n
Meiosis I
I
in in
Secondary spermatocyte
I'
Meiosis II
1
In ) In Spermatid
Differentiation
t Sperm
'
Figure 43.12 During spermatogenesis, four sperm result from each primary spermatocyte.
Spermatogenesis, illustrated in Figure 43.12, occurs in the wall of the seminiferous tubules (Figure 43.8),
with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under
the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia (singular:
spermatagonium), go through mitosis with one offspring going on to differentiate into a sperm cell and the other
giving rise to the next generation of sperm.
Meiosis starts with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell
is produced called a secondary spermatocyte. This cell is haploid and must go through another meiotic cell
division. The cell produced at the end of meiosis is called a spermatid and when it reaches the lumen of the
tubule and grows a flagellum, it is called a sperm cell. Four sperm result from each primary spermatocyte that
goes through meiosis.
Stem cells are deposited during gestation and are present at birth through the beginning of adolescence, but in
an inactive state. During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of
these cells and the production of viable sperm. This continues into old age.
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Visit this site (http:// 0 penstaxc 0 llege. 0 rg/l/spermat 0 genesis) to see the process of spermatogenesis.
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Oogenesis
Oogenesis, illustrated in Figure 43.13, occurs in the outermost layers of the ovaries. As with sperm production,
oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to
increase in number, eventually resulting in up to about one to two million cells in the embryo.
2 n Oogonium
t
Before birth
Mitosis
Primary oocyte
"* / (arrests in prophase I)
After puberty
1
Polar body
J
© In
Meiosis continues
Secondary oocyte
(arrests in metaphase II)
Ovulation, sperm entry
—■<Z> In
Meiosis, fertilization
Polar body in 2n ) Fertilized Egg
Figure 43.13 The process of oogenesis occurs in the ovary’s outermost layer.
The cell starting meiosis is called a primary oocyte, as shown in Figure 43.13. This cell will start the first meiotic
division and be arrested in its progress in the first prophase stage. At the time of birth, all future eggs are in
the prophase stage. At adolescence, anterior pituitary hormones cause the development of a number of follicles
in an ovary. This results in the primary oocyte finishing the first meiotic division. The cell divides unequally,
with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set
of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar
body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this
secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is
fertilized, the cell continues through the meiosis II, producing a second polar body and a fertilized egg containing
all 46 chromosomes of a human being, half of them coming from the sperm.
Egg production begins before birth, is arrested during meiosis until puberty, and then individual cells continue
through at each menstrual cycle. One egg is produced from each meiotic process, with the extra chromosomes
and chromatids going into polar bodies that degenerate and are reabsorbed by the body.
43.4 | Hormonal Control of Human Reproduction
By the end of this chapter, you will be able to do the following:
• Describe the roles of male and female reproductive hormones
• Discuss the interplay of the ovarian and menstrual cycles
• Describe the process of menopause
The human male and female reproductive cycles are controlled by the interaction of hormones from the
hypothalamus and anterior pituitary with hormones from reproductive tissues and organs. In both sexes, the
hypothalamus monitors and causes the release of hormones from the pituitary gland. When the reproductive
hormone is required, the hypothalamus sends a gonadotropin-releasing hormone (GnRH) to the anterior
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Chapter 43 | Animal Reproduction and Development
1349
pituitary. This causes the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from
the anterior pituitary into the blood. Note that the body must reach puberty in order for the adrenals to release the
hormones that must be present for GnRH to be produced. Although FSH and LH are named after their functions
in female reproduction, they are produced in both sexes and play important roles in controlling reproduction.
Other hormones have specific functions in the male and female reproductive systems.
Male Hormones
At the onset of puberty, the hypothalamus causes the release of FSH and LH into the male system for the first
time. FSH enters the testes and stimulates the Sertoli cells to begin facilitating spermatogenesis using negative
feedback, as illustrated in Figure 43.14. LH also enters the testes and stimulates the interstitial cells of Leydig
to make and release testosterone into the testes and the blood.
Testosterone, the hormone responsible for the secondary sexual characteristics that develop in the male during
adolescence, stimulates spermatogenesis. These secondary sex characteristics include a deepening of the
voice, the growth of facial, axillary, and pubic hair, and the beginnings of the sex drive.
Figure 43.14 Hormones control sperm production in a negative feedback system.
A negative feedback system occurs in the male with rising levels of testosterone acting on the hypothalamus and
anterior pituitary to inhibit the release of GnRH, FSH, and LH. The Sertoli cells produce the hormone inhibin,
which is released into the blood when the sperm count is too high. This inhibits the release of GnRH and FSH,
which will cause spermatogenesis to slow down. If the sperm count reaches 20 million/ml, the Sertoli cells cease
the release of inhibin, and the sperm count increases.
Female Hormones
The control of reproduction in females is more complex. As with the male, the anterior pituitary hormones
cause the release of the hormones FSH and LH. in addition, estrogens and progesterone are released from
the developing follicles. Estrogen is the reproductive hormone in females that assists in endometrial regrowth,
ovulation, and calcium absorption; it is also responsible for the secondary sexual characteristics of females.
These include breast development, flaring of the hips, and a shorter period necessary for bone maturation.
Progesterone assists in endometrial regrowth and inhibition of FSH and LH release.
In females, FSH stimulates development of egg cells, called ova, which develop in structures called follicles.
Follicle cells produce the hormone inhibin, which inhibits FSH production. LH also plays a role in the
development of ova, induction of ovulation, and stimulation of estradiol and progesterone production by the
ovaries. Estradiol and progesterone are steroid hormones that prepare the body for pregnancy. Estradiol
produces secondary sex characteristics in females, while both estradiol and progesterone regulate the menstrual
cycle.
The Ovarian Cycle and the Menstrual Cycle
The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual
cycle governs the preparation and maintenance of the uterine lining. These cycles occur concurrently and are
coordinated over a 22-32 day cycle, with an average length of 28 days.
The first half of the ovarian cycle is the follicular phase shown in Figure 43.15. Slowly rising levels of FSH and
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LH cause the growth of follicles on the surface of the ovary. This process prepares the egg for ovulation. As
the follicles grow, they begin releasing estrogens and a low level of progesterone. Progesterone maintains the
endometrium to help ensure pregnancy. The trip through the fallopian tube takes about seven days. At this stage
of development, called the morula, there are 30-60 cells. If pregnancy implantation does not occur, the lining is
sloughed off. After about five days, estrogen levels rise and the menstrual cycle enters the proliferative phase.
The endometrium begins to regrow, replacing the blood vessels and glands that deteriorated during the end of
the last cycle.
visual
CONNECTION
II Ovulation
Pituitary hormone
effect: LH and FSH
stimulate maturation
of one of the
growing follicles.
Hypothalamus
Ovarian
hormone
effects:
Follicles produce
low levels of
estradiol that
• Inhibit GnRH
secretion by
the hypothalamus,
keeping LH and
FSH levels low.
• Cause endrometrial
arteries to constrict,
resulting in
menstruation.
| G
I FSH
Anterior
pituitary
Ovaries
Follicles
Ovarian
hormone
effects:
Growing follicles
begin to produce
high levels of
estradiol, which
• Stimulate GnRH
secretion by the
hypothalamus.
LH and FSH levels
rise, resulting in
ovulation about
a day later.
• Cause the
endometrium
to thicken.
Ovarian
hormone
effects:
The corpus
luteum secretes
estradiol and
progesterone that
• Block GnRH
production by the
hypothalamus
and LH and
FSH production
by the pituitary.
• Cause the
endometrium to
further develop.
Figure 43.15 The ovarian and menstrual cycles of female reproduction are regulated by hormones produced by
the hypothalamus, pituitary, and ovaries.
Which of the following statements about hormone regulation of the female reproductive cycle is false?
a. LH and FSH are produced in the pituitary, and estradiol and progesterone are produced in the ovaries.
b. Estradiol and progesterone secreted from the corpus luteum cause the endometrium to thicken.
c. Both progesterone and estradiol are produced by the follicles.
d. Secretion of GnRH by the hypothalamus is inhibited by low levels of estradiol but stimulated by high
levels of estradiol.
Just prior to the middle of the cycle (approximately day 14), the high level of estrogen causes FSH and especially
LH to rise rapidly, then fall. The spike in LH causes ovulation: the most mature follicle, like that shown in Figure
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Chapter 43 | Animal Reproduction and Development
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43.16, ruptures and releases its egg. The follicles that did not rupture degenerate and their eggs are lost. The
level of estrogen decreases when the extra follicles degenerate.
Figure 43.16 This mature egg follicle may rupture and release an egg. (credit: scale-bar data from Matt Russell)
Following ovulation, the ovarian cycle enters its luteal phase, illustrated in Figure 43.15 and the menstrual cycle
enters its secretory phase, both of which run from about day 15 to 28. The luteal and secretory phases refer
to changes in the ruptured follicle. The cells in the follicle undergo physical changes and produce a structure
called a corpus luteum. The corpus luteum produces estrogen and progesterone. The progesterone facilitates
the regrowth of the uterine lining and inhibits the release of further FSH and LH. The uterus is being prepared to
accept a fertilized egg, should it occur during this cycle. The inhibition of FSH and LH prevents any further eggs
and follicles from developing, while the progesterone is elevated. The level of estrogen produced by the corpus
luteum increases to a steady level for the next few days.
If no fertilized egg is implanted into the uterus, the corpus luteum degenerates and the levels of estrogen and
progesterone decrease. The endometrium begins to degenerate as the progesterone levels drop, initiating the
next menstrual cycle. The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior
pituitary, releasing FSH and LH and starting the cycles again. Figure 43.17 visually compares the ovarian and
uterine cycles as well as the commensurate hormone levels.
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Chapter 43 | Animal Reproduction and Development
visual
a CONNECTION
Ovarian cycle phases
Follicular phase Luteal phase
,^> ^>,
Follicle is
released
Follicle grows Corpus luteum forms then
degenerates
Uterine cycle phases
rual Proliferative
phase
Secretory phase
Pituitary
Ovulation
Figure 43.17 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles, (credit:
modification of work by Mikael Haggstrom)
Which of the following statements about the menstrual cycle is false?
a. Progesterone levels rise during the luteal phase of the ovarian cycle and the secretory phase of the
uterine cycle.
b. Menstruation occurs just after LH and FSH levels peak.
c. Menstruation occurs after progesterone levels drop.
d. Estrogen levels rise before ovulation, while progesterone levels rise after.
Menopause
As women approach their mid-40s to mid-50s, their ovaries begin to lose their sensitivity to FSH and LH.
Menstrual periods become less frequent and finally cease; this is menopause. There are still eggs and potential
follicles on the ovaries, but without the stimulation of FSH and LH, they will not produce a viable egg to be
released. The outcome of this is the inability to have children.
The side effects of menopause include hot flashes, heavy sweating (especially at night), headaches, some hair
loss, muscle pain, vaginal dryness, insomnia, depression, weight gain, and mood swings. Estrogen is involved
in calcium metabolism and, without it, blood levels of calcium decrease. To replenish the blood, calcium is lost
from bone which may decrease the bone density and lead to osteoporosis. Supplementation of estrogen in the
form of hormone replacement therapy (HRT) can prevent bone loss, but the therapy can have negative side
effects. While HRT is thought to give some protection from colon cancer, osteoporosis, heart disease, macular
degeneration, and possibly depression, its negative side effects include increased risk of: stroke or heart attack,
blood clots, breast cancer, ovarian cancer, endometrial cancer, gall bladder disease, and possibly dementia.
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Chapter 43 | Animal Reproduction and Development
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ca eer connection
Reproductive Endocrinologist
A reproductive endocrinologist is a physician who treats a variety of hormonal disorders related to
reproduction and infertility in both men and women. The disorders include menstrual problems, infertility,
pregnancy loss, sexual dysfunction, and menopause. Doctors may use fertility drugs, surgery, or assisted
reproductive techniques (ART) in their therapy. ART involves the use of procedures to manipulate the egg
or sperm to facilitate reproduction, such as in vitro fertilization.
Reproductive endocrinologists undergo extensive medical training, first in a four-year residency in obstetrics
and gynecology, then in a three-year fellowship in reproductive endocrinology. To be board certified in this
area, the physician must pass written and oral exams in both areas.
43.5 | Human Pregnancy and Birth
By the end of this section, you will be able to do the following:
• Explain fetal development during the three trimesters of gestation
• Describe labor and delivery
• Compare the efficacy and duration of various types of contraception
• Discuss causes of infertility and the therapeutic options available
Pregnancy begins with the fertilization of an egg and continues through to the birth of the individual. The length
of time of gestation varies among animals, but is very similar among the great apes: human gestation is 266
days, while chimpanzee gestation is 237 days, a gorilla’s is 257 days, and orangutan gestation is 260 days long.
The fox has a 57-day gestation. Dogs and cats have similar gestations averaging 60 days. The longest gestation
for a land mammal is an African elephant at 640 days. The longest gestations among marine mammals are the
beluga and sperm whales at 460 days.
Human Gestation
Twenty-four hours before fertilization, the egg has finished meiosis and becomes a mature oocyte. When
fertilized (at conception) the egg becomes known as a zygote. The zygote travels through the oviduct to the
uterus (Figure 43.18). The developing embryo must implant into the wall of the uterus within seven days, or
it will deteriorate and die. The outer layers of the zygote (blastocyst) grow into the endometrium by digesting
the endometrial cells, and wound healing of the endometrium closes up the blastocyst into the tissue. Another
layer of the blastocyst, the chorion, begins releasing a hormone called human beta chorionic gonadotropin
(/J-HCG) which makes its way to the corpus luteum and keeps that structure active. This ensures adequate
levels of progesterone that will maintain the endometrium of the uterus for the support of the developing embryo.
Pregnancy tests determine the level of /3-HCG in urine or serum. If the hormone is present, the test is positive.
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Chapter 43 | Animal Reproduction and Development
Figure 43.18 In humans, fertilization occurs soon after the oocyte leaves the ovary. Implantation occurs eight or nine
days later.(credit: Ed Uthman)
The gestation period is divided into three equal periods or trimesters. During the first two to four weeks of
the first trimester, nutrition and waste are handled by the endometrial lining through diffusion. As the trimester
progresses, the outer layer of the embryo begins to merge with the endometrium, and the placenta forms. This
organ takes over the nutrient and waste requirements of the embryo and fetus, with the mother’s blood passing
nutrients to the placenta and removing waste from it. Chemicals from the fetus, such as bilirubin, are processed
by the mother’s liver for elimination. Some of the mother’s immunoglobulins will pass through the placenta,
providing passive immunity against some potential infections.
Internal organs and body structures begin to develop during the first trimester. By five weeks, limb buds, eyes,
the heart, and liver have been basically formed. By eight weeks, the term fetus applies, and the body is
essentially formed, as shown in Figure 43.19. The individual is about five centimeters (two inches) in length
and many of the organs, such as the lungs and liver, are not yet functioning. Exposure to any toxins is
especially dangerous during the first trimester, as all of the body’s organs and structures are going through initial
development. Anything that affects that development can have a severe effect on the fetus’ survival.
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Figure 43.19 Fetal development is shown at nine weeks gestation, (credit: Ed Uthman)
During the second trimester, the fetus grows to about 30 cm (12 inches), as shown in Figure 43.20. It becomes
active and the mother usually feels the first movements. All organs and structures continue to develop. The
placenta has taken over the functions of nutrition and waste and the production of estrogen and progesterone
from the corpus luteum, which has degenerated. The placenta will continue functioning up through the delivery
of the baby.
Figure 43.20 This fetus is just entering the second trimester, when the placenta takes over more of the functions
performed as the baby develops, (credit: National Museum of Health and Medicine)
During the third trimester, the fetus grows to 3 to 4 kg (6 Vz -8 Vz lbs.) and about 50 cm (19-20 inches) long, as
illustrated in Figure 43.21. This is the period of the most rapid growth during the pregnancy. Organ development
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Chapter 43 | Animal Reproduction and Development
continues to birth (and some systems, such as the nervous system and liver, continue to develop after birth).
The mother will be at her most uncomfortable during this trimester. She may urinate frequently due to pressure
on the bladder from the fetus. There may also be intestinal blockage and circulatory problems, especially in her
legs. Clots may form in her legs due to pressure from the fetus on returning veins as they enter the abdominal
cavity.
Umbilical
Cervix
Figure 43.21 There is rapid fetal growth during the third trimester, (credit: modification of work by Gray’s Anatomy)
LINK TQ LEARNING
Visit this site (http:// 0 penstaxc 0 llege. 0 rg/l/embry 0 _fetus) to see the stages of human fetal development.
Labor and Birth
Labor is the physical efforts of expulsion of the fetus and the placenta from the uterus during birth (parturition).
Toward the end of the third trimester, estrogen causes receptors on the uterine wall to develop and bind the
hormone oxytocin. At this time, the baby reorients, facing forward and down with the back or crown of the head
engaging the cervix (uterine opening). This causes the cervix to stretch and nerve impulses are sent to the
hypothalamus, which signals for the release of oxytocin from the posterior pituitary. The oxytocin causes the
smooth muscle in the uterine wall to contract. At the same time, the placenta releases prostaglandins into the
uterus, increasing the contractions. A positive feedback relay occurs between the uterus, hypothalamus, and
the posterior pituitary to assure an adequate supply of oxytocin. As more smooth muscle cells are recruited, the
contractions increase in intensity and force.
There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby
and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm. During stage two, the
baby is expelled from the uterus. The uterus contracts and the mother pushes as she compresses her abdominal
muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and
the organ has completely disengaged from the uterine wall. If labor should stop before stage two is reached,
synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.
An alternative to labor and delivery is the surgical delivery of the baby through a procedure called a Caesarian
section. This is major abdominal surgery and can lead to post-surgical complications for the mother, but in some
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Chapter 43 | Animal Reproduction and Development
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cases it may be the only way to safely deliver the baby.
The mother’s mammary glands go through changes during the third trimester to prepare for lactation and
breastfeeding. When the baby begins suckling at the breast, signals are sent to the hypothalamus causing the
release of prolactin from the anterior pituitary. Prolactin causes the mammary glands to produce milk. Oxytocin
is also released, promoting the release of the milk. The milk contains nutrients for the baby’s development and
growth as well as immunoglobulins to protect the child from bacterial and viral infections.
Contraception and Birth Control
The prevention of a pregnancy comes under the terms contraception or birth control. Strictly speaking,
contraception refers to preventing the sperm and egg from joining. Both terms are, however, frequently used
interchangeably.
Contraceptive Methods
Method
Examples
Failure Rate in Typical Use
Over 12 Months
Barrier
male condom, female condom, sponge, cervical cap,
diaphragm, spermicides
15 to 24%
Hormonal
oral, patch, vaginal ring
8%
injection
3%
implant
less than 1%
Other
natural family planning
12 to 25%
withdrawal
27%
sterilization
less than 1%
Table 43.3
Table 43.3 lists common methods of contraception. The failure rates listed are not the ideal rates that could be
realized, but the typical rates that occur. A failure rate is the number of pregnancies resulting from the method’s
use over a twelve-month period. Barrier methods, such as condoms, cervical caps, and diaphragms, block
sperm from entering the uterus, preventing fertilization. Spermicides are chemicals that are placed in the vagina
that kill sperm. Sponges, which are saturated with spermicides, are placed in the vagina at the cervical opening.
Combinations of spermicidal chemicals and barrier methods achieve lower failure rates than do the methods
when used separately.
Nearly a quarter of the couples using barrier methods, natural family planning, or withdrawal can expect a failure
of the method. Natural family planning is based on the monitoring of the menstrual cycle and having intercourse
only during times when the egg is not available. A woman’s body temperature may rise a degree Celsius at
ovulation and the cervical mucus may increase in volume and become more pliable. These changes give a
general indication of when intercourse is more or less likely to result in fertilization. Withdrawal involves the
removal of the penis from the vagina during intercourse, before ejaculation occurs. This is a risky method with a
high failure rate due to the possible presence of sperm in the bulbourethral gland’s secretion, which may enter
the vagina prior to removing the penis.
Hormonal methods use synthetic progesterone (sometimes in combination with estrogen), to inhibit the
hypothalamus from releasing FSH or LH, and thus prevent an egg from being available for fertilization. The
method of administering the hormone affects failure rate. The most reliable method, with a failure rate of less
than 1 percent, is the implantation of the hormone under the skin. The same rate can be achieved through the
sterilization procedures of vasectomy in the man or of tubal ligation in the woman, or by using an intrauterine
device (IUD). lUDs are inserted into the uterus and establish an inflammatory condition that prevents fertilized
eggs from implanting into the uterine wall.
Compliance with the contraceptive method is a strong contributor to the success or failure rate of any particular
method. The only method that is completely effective at preventing conception is abstinence. The choice
of contraceptive method depends on the goals of the woman or couple. Tubal ligation and vasectomy are
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Chapter 43 | Animal Reproduction and Development
considered permanent prevention, while other methods are reversible and provide short-term contraception.
Termination of an existing pregnancy can be spontaneous or voluntary. Spontaneous termination is a
miscarriage and usually occurs very early in the pregnancy, usually within the first few weeks. This occurs when
the fetus cannot develop properly and the gestation is naturally terminated. Voluntary termination of a pregnancy
is an abortion. Laws regulating abortion vary between states and tend to view fetal viability as the criteria for
allowing or preventing the procedure.
Infertility
Infertility is the inability to conceive a child or carry a child to birth. About 75 percent of causes of infertility
can be identified; these include diseases, such as sexually transmitted diseases that can cause scarring of the
reproductive tubes in either men or women, or developmental problems frequently related to abnormal hormone
levels in one of the individuals. Inadequate nutrition, especially starvation, can delay menstruation. Stress can
also lead to infertility. Short-term stress can affect hormone levels, while long-term stress can delay puberty and
cause less frequent menstrual cycles. Other factors that affect fertility include toxins (such as cadmium), tobacco
smoking, marijuana use, gonadal injuries, and aging.
If infertility is identified, several assisted reproductive technologies (ART) are available to aid conception. A
common type of ART is in vitro fertilization (IVF) where an egg and sperm are combined outside the body and
then placed in the uterus. Eggs are obtained from the woman after extensive hormonal treatments that prepare
mature eggs for fertilization and prepare the uterus for implantation of the fertilized egg. Sperm are obtained from
the man and they are combined with the eggs and supported through several cell divisions to ensure viability of
the zygotes. When the embryos have reached the eight-cell stage, one or more is implanted into the woman’s
uterus. If fertilization is not accomplished by simple IVF, a procedure that injects the sperm into an egg can
be used. This is called intracytoplasmic sperm injection (ICSI) and is shown in Figure 43.22. IVF procedures
produce a surplus of fertilized eggs and embryos that can be frozen and stored for future use. The procedures
can also result in multiple births.
Figure 43.22 A sperm is inserted into an egg for fertilization during intracytoplasmic sperm injection (ICSI). (credit:
scale-bar data from Matt Russell)
43.6 | Fertilization and Early Embryonic Development
By the end of this section, you will be able to do the following:
• Discuss how fertilization occurs
• Explain how the embryo forms from the zygote
• Discuss the role of cleavage and gastrulation in animal development
The process in which an organism develops from a single-celled zygote to a multi-cellular organism is complex
and well-regulated. The early stages of embryonic development are also crucial for ensuring the fitness of the
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Chapter 43 | Animal Reproduction and Development
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organism.
Fertilization
Fertilization, pictured in Figure 43.23a is the process in which gametes (an egg and sperm) fuse to form a
zygote. The egg and sperm each contain one set of chromosomes. To ensure that the offspring has only one
complete diploid set of chromosomes, only one sperm must fuse with one egg. In mammals, the egg is protected
by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. When a sperm
binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. In
placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein
matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as
illustrated in Figure 43.23b. The fusion of these two membranes creates an opening through which the sperm
nucleus is transferred into the ovum. The nuclear membranes of the egg and sperm break down and the two
haploid genomes condense to form a diploid genome.
Figure 43.23 (a) Fertilization is the process in which sperm and egg fuse to form a zygote, (b) Acrosomal reactions
help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus, (credit:
(b) modification of work by Mariana Ruiz Villareal; scale-bar data from Matt Russell)
To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one
location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing
with the egg. If this mechanism fails, multiple sperm can fuse with the egg, resulting in polyspermy. The
resulting embryo is not genetically viable and dies within a few days.
Cleavage and Blastula Stage
The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell
division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage. Cleavage is
illustrated in (Figure 43.24a). After the cleavage has produced over 100 cells, the embryo is called a blastula.
The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the
blastocoel). Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass
that is distinct from the surrounding blastula, shown in Figure 43.24b. During cleavage, the cells divide without
an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells. Each cell within
the blastula is called a blastomere.
OQQ
(a) (b)
Figure 43.24 (a) During cleavage, the zygote rapidly divides into multiple cells without increasing in size, (b) The cells
rearrange themselves to form a hollow ball with a fluid-filled or yolk-filled cavity called the blastula.
Cleavage can take place in two ways: holoblastic (total) cleavage or meroblastic (partial) cleavage. The
type of cleavage depends on the amount of yolk in the eggs. In placental mammals (including humans) where
nourishment is provided by the mother’s body, the eggs have a very small amount of yolk and undergo
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Chapter 43 | Animal Reproduction and Development
holoblastic cleavage. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during
development, undergo meroblastic cleavage.
In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula
arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner
cell mass is also known as the embryoblast and this mass of cells will go on to form the embryo. At this
stage of development, illustrated in Figure 43.25 the inner cell mass consists of embryonic stem cells that will
differentiate into the different cell types needed by the organism. The trophoblast will contribute to the placenta
and nourish the embryo.
Figure 43.25 The rearrangement of the cells in the mammalian blastula to two layers—the inner cell mass and the
trophoblast—results in the formation of the blastocyst.
LINK TQ LEARNING
Visit the Virtual Human Embryo project (http:// 0 penstaxc 0 llege. 0 rg/l/human_embry 0 ) at the Endowment
for Human Development site to step through an interactive that shows the stages of embryo development,
including micrographs and rotating 3-D images.
Gastrulation
The typical blastula is a ball of cells. The next stage in embryonic development is the formation of the body
plan. The cells in the blastula rearrange themselves spatially to form three layers of cells. This process is called
gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these
layers is called a germ layer and each germ layer differentiates into different organ systems.
The three germ layers, shown in Figure 43.26, are the endoderm, the ectoderm, and the mesoderm. The
ectoderm gives rise to the nervous system and the epidermis. The mesoderm gives rise to the muscle cells and
connective tissue in the body. The endoderm gives rise to columnar cells found in the digestive system and many
internal organs.
Lung cells Thyroid Digestive
(alveolar cells cells
cell) (pancreatic
cell)
Cardiac Skeletal
muscle muscle
cells cells
Tubule Red blood Smooth
cells of cells muscle
the kidney cells
(in gut)
Figure 43.26 The three germ layers give rise to different cell types in the animal body, (credit: modification of work by
NIH, NCBI)
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Chapter 43 | Animal Reproduction and Development
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everyday CONNECTION
Are Designer Babies in Our Future?
«uc«mcs DRAWS ITS HATCRIAtS FROM MADV SOURCSS ADD 0RCADIZ6S
Th6M IDTO AD hARMODIOUS CDTITV.
Figure 43.27 This logo from the Second International Eugenics Conference in New York City in September of
1921 shows how eugenics attempted to merge several fields of study with the goal of producing a genetically
superior human race.
If you could prevent your child from getting a devastating genetic disease, would you do it? Would you select
the sex of your child or select for their attractiveness, strength, or intelligence? How far would you go to
maximize the possibility of resistance to disease? The genetic engineering of a human child, the production
of "designer babies" with desirable phenotypic characteristics, was once atopic restricted to science fiction.
This is the case no longer: science fiction is now overlapping into science fact. Many phenotypic choices
for offspring are already available, with many more likely to be possible in the not too distant future. Which
traits should be selected and how they should be selected are topics of much debate within the worldwide
medical community. The ethical and moral line is not always clear or agreed upon, and some fear that
modern reproductive technologies could lead to a new form of eugenics.
Eugenics is the use of information and technology from a variety of sources to improve the genetic
makeup of the human race. The goal of creating genetically superior humans was quite prevalent (although
controversial) in several countries during the early 20 th century, but fell into disrepute when Nazi Germany
developed an extensive eugenics program in the 1930s and 40s. As part of their program, the Nazis
forcibly sterilized hundreds of thousands of the so-called "unfit" and killed tens of thousands of institutionally
disabled people as part of a systematic program to develop a genetically superior race of Germans known
as Aryans. Ever since, eugenic ideas have not been as publicly expressed, but there are still those who
promote them.
Efforts have been made in the past to control traits in human children using donated sperm from men with
desired traits. In fact, eugenicist Robert Klark Graham established a sperm bank in 1980 that included
samples exclusively from donors with high IQs. The "genius" sperm bank failed to capture the public's
imagination and the operation closed in 1999.
In more recent times, the procedure known as prenatal genetic diagnosis (PGD) has been developed. PGD
involves the screening of human embryos as part of the process of in vitro fertilization, during which embryos
are conceived and grown outside the mother's body for some period of time before they are implanted. The
term PGD usually refers to both the diagnosis, selection, and the implantation of the selected embryos.
In the least controversial use of PGD, embryos are tested for the presence of alleles which cause genetic
diseases such as sickle cell disease, muscular dystrophy, and hemophilia, in which a single disease-causing
allele or pair of alleles has been identified. By excluding embryos containing these alleles from implantation
into the mother, the disease is prevented, and the unused embryos are either donated to science or
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Chapter 43 | Animal Reproduction and Development
discarded. There are relatively few in the worldwide medical community that question the ethics of this type
of procedure, which allows individuals scared to have children because of the alleles they carry to do so
successfully. The major limitation to this procedure is its expense. Not usually covered by medical insurance
and thus out of reach financially for most couples, only a very small percentage of all live births use such
complicated methodologies. Yet, even in cases like these where the ethical issues may seem to be clear-
cut, not everyone agrees with the morality of these types of procedures. For example, to those who take
the position that human life begins at conception, the discarding of unused embryos, a necessary result of
PGD, is unacceptable under any circumstances.
A murkier ethical situation is found in the selection of a child's sex, which is easily performed by PGD.
Currently, countries such as Great Britain have banned the selection of a child's sex for reasons other than
preventing sex-linked diseases. Other countries allow the procedure for "family balancing", based on the
desire of some parents to have at least one child of each sex. Still others, including the United States, have
taken a scattershot approach to regulating these practices, essentially leaving it to the individual practicing
physician to decide which practices are acceptable and which are not.
Even murkier are rare instances of disabled parents, such as those with deafness or dwarfism, who select
embryos via PGD to ensure that they share their disability. These parents usually cite many positive aspects
of their disabilities and associated culture as reasons for their choice, which they see as their moral right. To
others, to purposely cause a disability in a child violates the basic medical principle of Primum non nocere,
"first, do no harm." This procedure, although not illegal in most countries, demonstrates the complexity of
ethical issues associated with choosing genetic traits in offspring.
Where could this process lead? Will this technology become more affordable and how should it be used?
With the ability of technology to progress rapidly and unpredictably, a lack of definitive guidelines for the use
of reproductive technologies before they arise might make it difficult for legislators to keep pace once they
are in fact realized, assuming the process needs any government regulation at all. Other bioethicists argue
that we should only deal with technologies that exist now, and not in some uncertain future. They argue that
these types of procedures will always be expensive and rare, so the fears of eugenics and "master" races
are unfounded and overstated. The debate continues.
43.7 | Organogenesis and Vertebrate Formation
By the end of this section, you will be able to do the following:
• Describe the process of organogenesis
• Identify the anatomical axes formed in vertebrates
Gastrulation leads to the formation of the three germ layers that give rise, during further development, to the
different organs in the animal body. This process is called organogenesis. Organogenesis is characterized by
rapid and precise movements of the cells within the embryo.
Organogenesis
Organs form from the germ layers through the process of differentiation. During differentiation, the embryonic
stem cells express specific sets of genes which will determine their ultimate cell type. For example, some cells in
the ectoderm will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal
cells. The process of differentiation is regulated by cellular signaling cascades.
Scientists study organogenesis extensively in the lab in fruit flies ( Drosophila ) and the nematode Caenorhabditis
elegans. Drosophila have segments along their bodies, and the patterning associated with the segment
formation has allowed scientists to study which genes play important roles in organogenesis along the length of
the embryo at different time points. The nematode C.elegans has roughly 1000 somatic cells and scientists have
studied the fate of each of these cells during their development in the nematode life cycle. There is little variation
in patterns of cell lineage between individuals, unlike in mammals where cell development from the embryo is
dependent on cellular cues.
In vertebrates, one of the primary steps during organogenesis is the formation of the neural system. The
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ectoderm forms epithelial cells and tissues, and neuronal tissues. During the formation of the neural system,
special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become
epidermis cells. The remaining cells in the center form the neural plate. If the signaling by growth factors were
disrupted, then the entire ectoderm would differentiate into neural tissue.
The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube,
as illustrated in Figure 43.28. In further development, the neural tube will give rise to the brain and the spinal
cord.
Neural plate border Neural plate Epidermis
Figure 43.28 The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal
cord.
The mesoderm that lies on either side of the vertebrate neural tube will develop into the various connective
tissues of the animal body. A spatial pattern of gene expression reorganizes the mesoderm into groups of cells
called somites with spaces between them. The somites, illustrated in Figure 43.29 will further develop into the
ribs, lungs, and segmental (spine) muscle. The mesoderm also forms a structure called the notochord, which is
rod-shaped and forms the central axis of the animal body.
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Figure 43.29 In this five-week old human embryo, somites are segments along the length of the body, (credit:
modification of work by Ed Uthman)
Vertebrate Axis Formation
Even as the germ layers form, the ball of cells still retains its spherical shape. However, animal bodies have
lateral-medial (left-right), dorsal-ventral (back-belly), and anterior-posterior (head-feet) axes, illustrated in Figure
43.30.
Left-right
axis
side
(lateral)
side
Anteroposterior
axis
Anterior
end
Oorsoventral
axis
Dorsal
side
Right
(lateral)
Posterior
end
Figure 43.30 Animal bodies have three axes for symmetry, (credit: modification of work by NOAA)
How are these established? In one of the most seminal experiments ever to be carried out in developmental
biology, Spemann and Mangold took dorsal cells from one embryo and transplanted them into the belly region
of another embryo. They found that the transplanted embryo now had two notochords: one at the dorsal site
from the original cells and another at the transplanted site. This suggested that the dorsal cells were genetically
programmed to form the notochord and define the axis. Since then, researchers have identified many genes that
are responsible for axis formation. Mutations in these genes leads to the loss of symmetry required for organism
development.
Animal bodies have externally visible symmetry. However, the internal organs are not symmetric. For example,
the heart is on the left side and the liver on the right. The formation of the central left-right axis is an important
process during development. This internal asymmetry is established very early during development and involves
many genes. Research is still ongoing to fully understand the developmental implications of these genes.
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KEY TERMS
acrosomal reaction series of biochemical reactions that the sperm uses to break through the zona pellucida
asexual reproduction form of reproduction that produces offspring that are genetically identical to the parent
blastocyst structure formed when cells in the mammalian blastula separate into an inner and outer layer
budding form of asexual reproduction that results from the outgrowth of a part of a cell leading to a separation
from the original animal into two individuals
bulbourethral gland secretion that cleanses the urethra prior to ejaculation
clitoris sensory structure in females; stimulated during sexual arousal
cloaca common body opening for the digestive, excretory, and reproductive systems found in non-mammals,
such as birds
contraception (also, birth control) various means used to prevent pregnancy
estrogen reproductive hormone in females that assists in endometrial regrowth, ovulation, and calcium
absorption
external fertilization fertilization of egg by sperm outside animal body, often during spawning
fission (also, binary fission) method by which multicellular organisms increase in size or asexual reproduction in
which a unicellular organism splits into two separate organisms by mitosis
follicle stimulating hormone (FSH) reproductive hormone that causes sperm production in men and follicle
development in women
fragmentation cutting or fragmenting of the original animal into parts and the growth of a separate animal from
each part
gastrulation process in which the blastula folds over itself to form the three germ layers
gestation length of time for fetal development to birth
gonadotropin-releasing hormone (GnRH) hormone from the hypothalamus that causes the release of FSH
and LH from the anterior pituitary
hermaphroditism state of having both male and female reproductive parts within the same individual
holoblastic complete cleavage; takes place in cells with a small amount of yolk
human beta chorionic gonadotropin (/J-HCG) hormone produced by the chorion of the zygote that helps to
maintain the corpus luteum and elevated levels of progesterone
infertility inability to conceive, carry, and deliver children
inhibin hormone made by Sertoli cells; provides negative feedback to hypothalamus in control of FSH and
GnRH release
inner cell mass inner layer of cells in the blastocyst
internal fertilization fertilization of egg by sperm inside the body of the female
interstitial cell of Leydig cell in seminiferous tubules that makes testosterone
labia majora large folds of tissue covering the inguinal area
labia minora smaller folds of tissue within the labia majora
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luteinizing hormone (LH) reproductive hormone in both men and women, causes testosterone production in
men and ovulation and lactation in women
menopause loss of reproductive capacity in women due to decreased sensitivity of the ovaries to FSH and LH
menstrual cycle cycle of the degradation and regrowth of the endometrium
meroblastic partial cleavage; takes place in cells with a large amount of yolk
morning sickness condition in the mother during the first trimester; includes feelings of nausea
neural tube tube-like structure that forms from the ectoderm and gives rise to the brain and spinal cord
oogenesis process of producing haploid eggs
organogenesis process of organ formation
ovarian cycle cycle of preparation of egg for ovulation and the conversion of the follicle to the corpus luteum
oviduct (also, fallopian tube) muscular tube connecting the uterus with the ovary area
oviparity process by which fertilized eggs are laid outside the female’s body and develop there, receiving
nourishment from the yolk that is a part of the egg
ovoviparity process by which fertilized eggs are retained within the female; the embryo obtains its nourishment
from the egg’s yolk and the young are fully developed when they are hatched
ovulation release of the egg by the most mature follicle
parthenogenesis form of asexual reproduction where an egg develops into a complete individual without being
fertilized
penis male reproductive structure for urine elimination and copulation
placenta organ that supports the diffusion of nutrients and waste between the mother’s and fetus’ blood
polyspermy condition in which one egg is fertilized by multiple sperm
progesterone reproductive hormone in women; assists in endometrial regrowth and inhibition of FSH and LH
release
prostate gland structure that is a mixture of smooth muscle and glandular material and that contributes to
semen
scrotum sac containing testes; exterior to the body
semen fluid mixture of sperm and supporting materials
seminal vesicle secretory accessory gland in males; contributes to semen
seminiferous tubule site of sperm production in testes
Sertoli cell cell in seminiferous tubules that assists developing sperm and makes inhibin
sexual reproduction mixing of genetic material from two individuals to produce genetically unique offspring
somite group of cells separated by small spaces that form from the mesoderm and give rise to connective tissue
spermatheca specialized sac that stores sperm for later use
spermatogenesis process of producing haploid sperm
testes pair of reproductive organs in males
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testosterone reproductive hormone in men that assists in sperm production and promoting secondary sexual
characteristics
trophoblast outer layer of cells in the blastocyst
uterus environment for developing embryo and fetus
vagina muscular tube for the passage of menstrual flow, copulation, and birth of offspring
viviparity process in which the young develop within the female, receiving nourishment from the mother’s blood
through a placenta
zona pellucida protective layer of glycoproteins on the mammalian egg
CHAPTER SUMMARY
43.1 Reproduction Methods
Reproduction may be asexual when one individual produces genetically identical offspring, or sexual when the
genetic material from two individuals is combined to produce genetically diverse offspring. Asexual
reproduction occurs through fission, budding, and fragmentation. Sexual reproduction may mean the joining of
sperm and eggs within animals’ bodies or it may mean the release of sperm and eggs into the environment. An
individual may be one sex, or both; it may start out as one sex and switch during its life, or it may stay male or
female.
43.2 Fertilization
Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. This can
occur either outside the bodies or inside the female. Both methods have advantages and disadvantages. Once
fertilized, the eggs can develop inside the female or outside. If the egg develops outside the body, it usually has
a protective covering over it. Animal anatomy evolved various ways to fertilize, hold, or expel the egg. The
method of fertilization varies among animals. Some species release the egg and sperm into the environment,
some species retain the egg and receive the sperm into the female body and then expel the developing embryo
covered with shell, while still other species retain the developing offspring through the gestation period.
43.3 Human Reproductive Anatomy and Gametogenesis
As animals became more complex, specific organs and organ systems developed to support specific functions
for the organism. The reproductive structures that evolved in land animals allow males and females to mate,
fertilize internally, and support the growth and development of offspring. Processes developed to produce
reproductive cells that had exactly half the number of chromosomes of each parent so that new combinations
would have the appropriate amount of genetic material. Gametogenesis, the production of sperm
(spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis.
43.4 Hormonal Control of Human Reproduction
The male and female reproductive cycles are controlled by hormones released from the hypothalamus and
anterior pituitary as well as hormones from reproductive tissues and organs. The hypothalamus monitors the
need for the FSH and LH hormones made and released from the anterior pituitary. FSH and LH affect
reproductive structures to cause the formation of sperm and the preparation of eggs for release and possible
fertilization. In the male, FSH and LH stimulate Sertoli cells and interstitial cells of Leydig in the testes to
facilitate sperm production. The Leydig cells produce testosterone, which also is responsible for the secondary
sexual characteristics of males. In females, FSH and LH cause estrogen and progesterone to be produced.
They regulate the female reproductive system which is divided into the ovarian cycle and the menstrual cycle.
Menopause occurs when the ovaries lose their sensitivity to FSH and LH and the female reproductive cycles
slow to a stop.
43.5 Human Pregnancy and Birth
Human pregnancy begins with fertilization of an egg and proceeds through the three trimesters of gestation.
The labor process has three stages (contractions, delivery of the fetus, expulsion of the placenta), each
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propelled by hormones. The first trimester lays down the basic structures of the body, including the limb buds,
heart, eyes, and the liver. The second trimester continues the development of all of the organs and systems.
The third trimester exhibits the greatest growth of the fetus and culminates in labor and delivery. Prevention of
a pregnancy can be accomplished through a variety of methods including barriers, hormones, or other means.
Assisted reproductive technologies may help individuals who have infertility problems.
43.6 Fertilization and Early Embryonic Development
The early stages of embryonic development begin with fertilization. The process of fertilization is tightly
controlled to ensure that only one sperm fuses with one egg. After fertilization, the zygote undergoes cleavage
to form the blastula. The blastula, which in some species is a hollow ball of cells, undergoes a process called
gastrulation, in which the three germ layers form. The ectoderm gives rise to the nervous system and the
epidermal skin cells, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the
endoderm gives rise to columnar cells and internal organs.
43.7 Organogenesis and Vertebrate Formation
Organogenesis is the formation of organs from the germ layers. Each germ layer gives rise to specific tissue
types. The first stage is the formation of the neural system in the ectoderm. The mesoderm gives rise to
somites and the notochord. Formation of vertebrate axis is another important developmental stage.
VISUAL CONNECTION QUESTIONS
1. Figure 43.8 Which of the following statements
about the male reproductive system is false?
a. The vas deferens carries sperm from the
testes to the penis.
b. Sperm mature in seminiferous tubules in the
testes.
c. Both the prostate and the bulbourethral
glands produce components of the semen.
d. The prostate gland is located in the testes.
2. Figure 43.15 Which of the following statements
about hormone regulation of the female reproductive
cycle is false?
REVIEW QUESTIONS
4. Which form of reproduction is thought to be best in
a stable environment?
a. asexual
b. sexual
c. budding
d. parthenogenesis
5. Which form of reproduction can result from
damage to the original animal?
a. LH and FSH are produced in the pituitary,
and estradiol and progesterone are
produced in the ovaries.
b. Estradiol and progesterone secreted from
the corpus luteum cause the endometrium
to thicken.
c. Both progesterone and estradiol are
produced by the follicles.
d. Secretion of GnRH by the hypothalamus is
inhibited by low levels of estradiol but
stimulated by high levels of estradiol.
3. Figure 43.17 Which of the following statements
about the menstrual cycle is false?
a. Progesterone levels rise during the luteal
phase of the ovarian cycle and the secretory
phase of the uterine cycle.
b. Menstruation occurs just after LH and FSH
levels peak.
c. Menstruation occurs after progesterone
levels drop.
d. Estrogen levels rise before ovulation, while
progesterone levels rise after.
a. asexual
b. fragmentation
c. budding
d. parthenogenesis
6. Which form of reproduction is useful to an animal
with little mobility that reproduces sexually?
a. fission
b. budding
c. parthenogenesis
d. hermaphroditism
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Chapter 43 | Animal Reproduction and Development
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7. Genetically unique individuals are produced
through_.
a. sexual reproduction
b. parthenogenesis
c. budding
d. fragmentation
8. External fertilization occurs in which type of
environment?
a. aquatic
b. forested
c. savanna
d. steppe
9. Which term applies to egg development within the
female with nourishment derived from a yolk?
a. oviparity
b. viviparity
c. ovoviparity
d. ovovoparity
10. Which term applies to egg development outside
the female with nourishment derived from a yolk?
a. oviparity
b. viviparity
c. ovoviparity
d. ovovoparity
11. Sperm are produced in the_.
a. scrotum
b. seminal vesicles
c. seminiferous tubules
d. prostate gland
12. Most of the bulk of semen is made by the
a. scrotum
b. seminal vesicles
c. seminiferous tubules
d. prostate gland
13. Which of the following cells in spermatogenesis is
diploid?
a. primary spermatocyte
b. secondary spermatocyte
c. spermatid
d. sperm
14. Which female organ has the same embryonic
origin as the penis?
a. clitoris
b. labia majora
c. greater vestibular glands
d. vagina
15. Which female organ has an endometrial lining
that will support a developing baby?
a. labia minora
b. breast
c. ovaries
d. uterus
16. How many eggs are produced as a result of one
meiotic series of cell divisions?
a. one
b. two
c. three
d. four
17. Which hormone causes Leydig cells to make
testosterone?
a. FSH
b. LH
c. inhibin
d. estrogen
18. Which hormone causes FSH and LH to be
released?
a. testosterone
b. estrogen
c. GnRH
d. progesterone
19. Which hormone signals ovulation?
a. FSH
b. LH
c. inhibin
d. estrogen
20. Which hormone causes the regrowth of the
endometrial lining of the uterus?
a. testosterone
b. estrogen
c. GnRH
d. progesterone
21. Nutrient and waste requirements for the
developing fetus are handled during the first few
weeks by:
a. the placenta
b. diffusion through the endometrium
c. the chorion
d. the blastocyst
22. Progesterone is made during the third trimester
by the:
a. placenta
b. endometrial lining
c. chorion
d. corpus luteum
23. Which contraceptive method is 100 percent
effective at preventing pregnancy?
a. condom
b. oral hormonal methods
c. sterilization
d. abstinence
24. Which type of short term contraceptive method is
generally more effective than others?
a. barrier
b. hormonal
c. natural family planning
d. withdrawal
25. Which hormone is primarily responsible for the
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Chapter 43 | Animal Reproduction and Development
contractions during labor?
a.
oxytocin
b.
estrogen
c.
/3-HCG
d.
progesterone
26. Major organs begin to develop during which part
of human gestation?
a. fertilization
b. first trimester
c. second trimester
d. third trimester
27. Which of the following is false?
a. The endoderm, mesoderm, ectoderm are
germ layers.
b. The trophoblast is a germ layer.
c. The inner cell mass is a source of
embryonic stem cells.
d. The blastula is often a hollow ball of cells.
CRITICAL THINKING QUESTIONS
31. Why is sexual reproduction useful if only half the
animals can produce offspring and two separate cells
must be combined to form a third?
32. What determines which sex will result in offspring
of birds and mammals?
33. What are the advantages and disadvantages of
external and internal forms of fertilization?
34. Why would paired external fertilization be
preferable to group spawning?
35. Describe the phases of the human sexual
response.
36. Compare spermatogenesis and oogenesis as to
timing of the processes and the number and type of
cells finally produced.
37. If male reproductive pathways are not cyclical,
28. During cleavage, the mass of cells:
a. increases
b. decreases
c. doubles with every cell division
d. does not change significantly
29. Which of the following gives rise to the skin cells?
a. ectoderm
b. endoderm
c. mesoderm
d. none of the above
30. The ribs form from the_.
a. notochord
b. neural plate
c. neural tube
d. somites
how are they controlled?
38. Describe the events in the ovarian cycle leading
up to ovulation.
39. Describe the major developments during each
trimester of human gestation.
40. Describe the stages of labor.
41. What do you think would happen if multiple
sperm fused with one egg?
42. Why do mammalian eggs have a small
concentration of yolk, while bird and reptile eggs
have a large concentration of yolk?
43. Explain how the different germ layers give rise to
different tissue types.
44. Explain the role of axis formation in development.
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Chapter 44 | Ecology and the Biosphere
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44 | ECOLOGY AND THE
BIOSPHERE
(a) (b) (c)
Figure 44.1 The (a) deer tick carries the bacterium that produces Lyme disease in humans, often evident in (b) a
symptomatic bull’s eye rash. The (c) white-footed mouse is one well-known host to deer ticks carrying the Lyme
disease bacterium, (credit a: modification of work by Scott Bauer, USDA ARS; credit b: modification of work by James
Gathany, CDC; credit c: modification of work by Rob Ireton)
Chapter Outline
44.1: The Scope of Ecology
44.2: Biogeography
44.3: Terrestrial Biomes
44.4: Aquatic Biomes
44.5: Climate and the Effects of Global Climate Change
Introduction
Why study ecology? Perhaps you are interested in learning about the natural world and how living things
have adapted to the physical conditions of their environment. Or, perhaps you’re a future physician seeking to
understand the connection between your patients' health and their environment.
Humans are a part of the ecological landscape, and human health is one important part of human interaction
with our physical and living environment. Lyme disease, for instance, serves as one modern-day example of the
connection between our health and the natural world (Figure 44.1). More formally known as Lyme borreliosis,
Lyme disease is a bacterial infection that can be transmitted to humans when they are bitten by the deer tick
(Ixodes scapularis in the eastern U.S., and Ixodes pacificus along the Pacific coast). Deer ticks are the primary
vectors (a vector is an organism that transmits a pathogen) for this disease. However, not all ticks carry the
pathogen, and not all deer ticks carry the bacteria that will cause Lyme disease in humans. Also, the ticks /.
scapularis and pacificus can have other hosts besides deer. In fact, it turns out that the probability of infection
depends on the type of host upon which the tick develops: a higher proportion of ticks that live on white-footed
mice carry the bacterium than do ticks that live on deer. Knowledge about the environments and population
densities in which the host species is abundant would help a physician or an epidemiologist better understand
how Lyme disease is transmitted and how its incidence could be reduced.
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44.1 1 The Scope of Ecology
By the end of this section, you will be able to do the following:
• Define ecology and the four basic levels of ecological research
• Describe examples of the ways in which ecology requires the integration of different scientific disciplines
• Distinguish between abiotic and biotic components of the environment
• Recognize the relationship between abiotic and biotic components of the environment
Ecology is the study of the interactions of living organisms with their environment. One core goal of ecology is
to understand the distribution and abundance of living things in the physical environment. Attainment of this goal
requires the integration of scientific disciplines inside and outside of biology, such as mathematics, statistics,
biochemistry, molecular biology, physiology, evolution, biodiversity, geology, and climatology.
LINK TQ LEARNING
Climate change can alter where organisms live, which can sometimes directly affect human health. Watch
the PBS video “Feeling the Effects of Climate Change” (http:// 0 penstaxc 0 llege. 0 rg/l/climate_health) in
which researchers discover a pathogenic organism living far outside of its normal range.
Levels of Ecological Study
When a discipline such as biology is studied, it is often helpful to subdivide it into smaller, related areas. For
instance, cell biologists interested in cell signaling need to understand the chemistry of the signal molecules
(which are usually proteins) as well as the result of cell signaling. Ecologists interested in the factors that
influence the survival of an endangered species might use mathematical models to predict how current
conservation efforts affect endangered organisms.
To produce a sound set of management options, a conservation biologist needs to collect accurate data,
including current population size, factors affecting reproduction (like physiology and behavior), habitat
requirements (such as plants and soils), and potential human influences on the endangered population and its
habitat (which might be derived through studies in sociology and urban ecology). Within the discipline of ecology,
researchers work at four general levels, which sometimes overlap. These levels are organism, population,
community, and ecosystem (Figure 44.2).
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Organisms: In a
forest, each pine
tree is an organism.
Populations: Together,
all the pine trees make
up a population.
Communities: All
the plant and animal
species comprise a
community.
Ecosystems: This
coastal ecosystem in
the southeastern United
States includes living
organisms and the
environment in which
they live.
Figure 44.2 Ecologists study within several biological levels of organization, (credit “organisms”: modification of work
by yeowatzup’VFlickr; credit “populations”: modification of work by "Crystl'VFlickr; credit “communities”: modification of
work by US Fish and Wildlife Service; credit “ecosystems”: modification of work by Tom Carlisle, US Fish and Wildlife
Service Headquarters; credit “biosphere": NASA)
Organismal Ecology
Researchers studying ecology at the organismal level are interested in the adaptations that enable individuals
to live in specific habitats. These adaptations can be morphological, physiological, and behavioral. For instance,
the Karner blue butterfly (Lycaeides melissa samuelis) (Figure 44.3) is considered a specialist because the
females only oviposit (that is, lay eggs) on wild lupine (Lupinus perennis). This specific requirement and
adaptation means that the Karner blue butterfly is completely dependent on the presence of wild lupine plants
for its survival.
Figure 44.3 The Karner blue butterfly (Lycaeides melissa samuelis) is a rare butterfly that lives only in open areas
with few trees or shrubs, such as pine barrens and oak savannas. It can only lay its eggs on lupine plants, (credit:
modification of work by J & K Hollingsworth, USFWS)
After hatching, the (first instar) caterpillars emerge and spend four to six weeks feeding solely on wild lupine
(Figure 44.4). The caterpillars pupate as a chrysalis to undergo the final stage of metamorphosis and emerge
as butterflies after about four weeks. The adult butterflies feed on the nectar of flowers of wild lupine and other
plant species, such as milkweeds. Generally there are two broods of the Karner blue each year.
A researcher interested in studying Karner blue butterflies at the organismal level might, in addition to asking
questions about egg laying requirements, ask questions about the butterflies’ preferred thoracic flight
temperature (a physiological question), or the behavior of the caterpillars when they are at different larval stages
(a behavioral question).
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Figure 44.4 The wild lupine (Lupinus perennis ) is the only known host plant for the Karner blue butterfly.
Population Ecology
A population is a group of interbreeding organisms that are members of the same species living in the same
area at the same time. (Organisms that are all members of the same species are called conspecifics.) A
population is identified, in part, by where it lives, and its area of population may have natural or artificial
boundaries. Natural boundaries might be rivers, mountains, or deserts, while artificial boundaries may be mowed
grass, manmade structures, or roads. The study of population ecology focuses on the number of individuals in
an area and how and why population size changes over time.
For example, population ecologists are particularly interested in counting the Karner blue butterfly because
it is classified as a federally endangered species. However, the distribution and density of this species is
highly influenced by the distribution and abundance of wild lupine, and the biophysical environment around it.
Researchers might ask questions about the factors leading to the decline of wild lupine and how these affect
Karner blue butterflies. For example, ecologists know that wild lupine thrives in open areas where trees and
shrubs are largely absent. In natural settings, intermittent wildfires regularly remove trees and shrubs, helping to
maintain the open areas that wild lupine requires. Mathematical models can be used to understand how wildfire
suppression by humans has led to the decline of this important plant for the Karner blue butterfly.
Community Ecology
A biological community consists of the different species within an area, typically a three-dimensional space,
and the interactions within and among these species. Community ecologists are interested in the processes
driving these interactions and their consequences. Questions about conspecific interactions often focus on
competition among members of the same species for a limited resource. Ecologists also study interactions
between various species; members of different species are called heterospecifics. Examples of heterospecific
interactions include predation, parasitism, herbivory, competition, and pollination. These interactions can have
regulating effects on population sizes and can impact ecological and evolutionary processes affecting diversity.
For example, Karner blue butterfly larvae form mutualistic relationships with ants (especially Formica spp).
Mutualism is a form of long-term relationship that has coevolved between two species and from which each
species benefits. For mutualism to exist between individual organisms, each species must receive some benefit
from the other as a consequence of the relationship. Researchers have shown that there is an increase in
survival when ants protect Karner blue butterfly larvae (caterpillars) from predaceous insects and spiders, an
act known as “tending.” This might be because the larvae spend less time in each life stage when tended by
ants, which provides an advantage for the larvae. Meanwhile, to attract the ants, the Karner blue butterfly larvae
secrete ant-like pheromones and a carbohydrate-rich substance that is an important energy source for the ants.
Both the Karner blue larvae and the ants benefit from their interaction, although the species of attendant ants
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Chapter 44 | Ecology and the Biosphere
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may be partially opportunistic and vary over the range of the butterfly.
Ecosystem Ecology
Ecosystem ecology is an extension of organismal, population, and community ecology. The ecosystem is
composed of all the biotic components (living things) in an area along with the abiotic components (nonliving
things) of that area. Some of the abiotic components include air, water, and soil. Ecosystem biologists ask
questions about how nutrients and energy are stored and how they move among organisms and through the
surrounding atmosphere, soil, and water.
The Karner blue butterflies and the wild lupine live in an oak-pine barren habitat. This habitat is characterized by
natural disturbance and nutrient-poor soils that are low in nitrogen. The availability of nutrients is an important
factor in the distribution of the plants that live in this habitat. Researchers interested in ecosystem ecology could
ask questions about the importance of limited resources and the movement of resources, such as nutrients,
though the biotic and abiotic portions of the ecosystem.
_ _
ca eer connection
Ecologist
A career in ecology contributes to many facets of human society. Understanding ecological issues can help
society meet the basic human needs of food, shelter, and health care. Ecologists can conduct their research
in the laboratory and outside in natural environments (Figure 44.5). These natural environments can be as
close to home as the stream running through your campus or as far away as the hydrothermal vents at the
bottom of the Pacific Ocean. Ecologists manage natural resources such as white-tailed deer populations
(■Odocoileus virginianus) for hunting or aspen (Populus spp.) timber stands for paper production. Ecologists
also work as educators who teach children and adults at various institutions including universities, high
schools, museums, and nature centers. Ecologists may also work in advisory positions assisting local, state,
and federal policymakers to develop laws that are ecologically sound, or they may develop those policies
and legislation themselves. To become an ecologist requires at least an undergraduate degree, usually in a
natural science. The undergraduate degree is often followed by specialized training or an advanced degree,
depending on the area of ecology selected. Ecologists should also have a broad background in the physical
sciences, as well as a solid foundation in mathematics and statistics.
Figure 44.5 This landscape ecologist is releasing a black-footed ferret into its native habitat as part of a study,
(credit: USFWS Mountain Prairie Region, NPS)
LINK
T &
LEARNING
Visit this site (http:// 0 penstaxc 0 llege. 0 rg/l/ec 0 l 0 gist_mle) to see Stephen Wing, a marine ecologist from
the University of Otago, discuss the role of an ecologist and the types of issues ecologists explore.
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44.2 | Biogeography
By the end of this section, you will be able to do the following:
• Define biogeography
• List and describe abiotic factors that affect the global distribution of plant and animal species
• Compare the impact of abiotic forces on aquatic and terrestrial environments
• Summarize the effects of abiotic factors on net primary productivity
Many forces influence the communities of living organisms present in different parts of the biosphere (all of the
parts of Earth inhabited by life). The biosphere extends into the atmosphere (several kilometers above Earth)
and into the depths of the oceans. Despite its apparent vastness to an individual human, the biosphere occupies
only a minute space when compared to the known universe. Many abiotic forces influence where life can exist
and the types of organisms found in different parts of the biosphere. The abiotic factors influence the distribution
of biomes: large areas of land with similar climate, flora, and fauna.
Biogeography
Biogeography is the study of the geographic distribution of living things and the abiotic factors that affect their
distribution. Abiotic factors such as temperature and rainfall vary based mainly on latitude and elevation. As
these abiotic factors change, the composition of plant and animal communities also changes. For example, if you
were to begin a journey at the equator and walk north, you would notice gradual changes in plant communities.
At the beginning of your journey, you would see tropical wet forests with broad-leaved evergreen trees, which are
characteristic of plant communities found near the equator. As you continued to travel north, you would see these
broad-leaved evergreen plants eventually give rise to seasonally dry forests with scattered trees. You would also
begin to notice changes in temperature and moisture. At about 30 degrees north, these forests would give way
to deserts, which are characterized by low precipitation and high insolation (sunlight).
Moving farther north, you would see that deserts are replaced by grasslands or prairies. Eventually, grasslands
are replaced by deciduous temperate forests. These deciduous forests give way to the boreal forests and taiga
found in the subarctic, the area south of the Arctic Circle. Finally, you would reach the Arctic tundra, which is
found at the most northern latitudes. This trek north reveals gradual changes in both climate and the types of
organisms that have adapted to environmental factors associated with ecosystems found at different latitudes.
However, different ecosystems exist at the same latitude due in part to abiotic factors such as jet streams, the
Gulf Stream, and ocean currents. If you were to hike up a mountain, the changes you would see in the vegetation
would parallel in many ways those as you move to higher latitudes.
Ecologists who study biogeography examine patterns of species distribution. No species exists everywhere; for
example, the Venus flytrap ( Dionaea muscipula) is endemic to a small area in North and South Carolina. An
endemic species is one which is naturally found only in a specific geographic area that is usually restricted
in size. Other species are generalists: species which live in a wide variety of geographic areas; the raccoon
(Procyon spp) for example, is native to most of North and Central America.
Species distribution patterns are based on biotic and abiotic factors and their influences during the very long
periods of time required for species evolution; therefore, early studies of biogeography were closely linked to
the emergence of evolutionary thinking in the eighteenth century. Some of the most distinctive assemblages of
plants and animals occur in regions that have been physically separated for millions of years by geographic
barriers. Biologists estimate that Australia, for example, has between 600,000 and 700,000 species of plants and
animals. Approximately 3/4 of living plant and mammal species are endemic species found solely in Australia
(Figure 44.6ab).
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Figure 44.6 Australia is home to many endemic species. The (a) wallaby (Wallabia bicolor), a medium-sized member
of the kangaroo family, is a pouched mammal, or marsupial. The (b) echidna (Tachyglossus aculeatus) is an egg-laying
mammal, (credit a: modification of work by Derrick Coetzee; credit b: modification of work by Allan Whittome)
Sometimes ecologists discover unique patterns of species distribution by determining where species are not
found. Despite being tropical, Hawaii, for example, has no native land species of reptiles or amphibians, only a
few native species of butterflies, and only one native terrestrial mammal, the hoary bat. Most of New Guinea, as
another example, lacks placental mammals.
LINK
T &
LEARNING
Check out this video (http:// 0 penstaxc 0 llege. 0 rg/l/platypus) to observe a platypus swimming in its natural
habitat in New South Wales, Australia.
Like animals, plants can be endemic or generalists: endemic plants are found only on specific regions of the
Earth, while generalists are found on many regions. Isolated land masses—such as Australia, Hawaii, and
Madagascar—often have large numbers of endemic plant species. Some of these plants are endangered due to
human activity. The forest gardenia (Gardenia brighamii), for instance, is endemic to Hawaii; only an estimated
15-20 trees are thought to exist (Figure 44.7).
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Figure 44.7 Listed as federally endangered, the forest gardenia is a small tree with distinctive flowers. It is found only
in five of the Hawaiian Islands in small populations consisting of a few individual specimens, (credit: Forest & Kim
Starr)
Energy Sources
Energy from the sun is captured by green plants, algae, cyanobacteria, and photosynthetic protists. These
organisms convert solar energy into the chemical energy needed by all living things. Light availability can be an
important force directly affecting the evolution of adaptations in photosynthesizers. For instance, plants in the
understory of a temperate forest are shaded when the trees above them in the canopy completely leaf out in
the late spring. Not surprisingly, understory plants have adaptations to successfully capture available light that
passes through the canopy. One such adaptation is the rapid growth of spring ephemeral plants such as the
spring beauty (Claytonia virginica) (Figure 44.8). These spring flowers achieve much of their growth and finish
their life cycle (reproduce) early in the season before the trees in the canopy develop leaves.
Figure 44.8 The spring beauty is an ephemeral spring plant that flowers early in the spring to avoid competing with
larger forest trees for sunlight, (credit: John Beetham)
In aquatic ecosystems, the availability of light may be limited because sunlight is absorbed by water, plants,
suspended particles, and resident microorganisms. Toward the bottom of a lake, pond, or ocean, there is a zone
that light cannot reach (because most wavelengths except for the shortest blues are absorbed by the water
column). Photosynthesis cannot take place there and, as a result, a number of adaptations have evolved that
enable living things to survive without light. For instance, aquatic plants have photosynthetic tissue near the
surface of the water. You can think of the broad, floating leaves of a water lily—water lilies cannot survive without
light. In environments such as hydrothermal vents, some bacteria extract energy from inorganic chemicals
because there is no light for photosynthesis.
The availability of nutrients in aquatic systems such as oceans is also an important aspect of energy or
photosynthesis. Many organisms sink to the bottom of the ocean when they die in the open water; when this
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occurs, the energy found in that living organism is sequestered for some time unless ocean upwelling occurs.
Ocean upwelling is the rising of deep ocean waters that occurs when prevailing winds blow along surface
waters near a coastline (Figure 44.9). As the wind pushes ocean waters offshore, water from the bottom of
the ocean moves up to replace this water. As a result, the nutrients once contained in dead organisms become
available for reuse by other living organisms.
Figure 44.9 Ocean upwelling is an important process that recycles nutrients and energy in the ocean. As wind (green
arrows) pushes offshore, it causes water from the ocean bottom (red arrows) to move to the surface, bringing up
nutrients from the ocean depths.
In freshwater systems, such as lakes, the recycling of nutrients occurs in response to air temperature and wind
changes. The nutrients at the bottom of lakes are recycled twice each year: in the spring and fall turnover.
The spring-and-fall turnover are seasonal processes that recycle nutrients and oxygen from the bottom of
a freshwater lake to the top of the lake (Figure 44.10). These turnovers are caused by the formation of a
thermocline: layers of water with temperatures that are significantly different from those above and below it.
In wintertime, the surface of lakes found in many northern regions is frozen. However, the water under the ice is
slightly warmer, and the water at the bottom of the lake is warmer yet at 4 °C to 5 °C (39.2 °F to 41 °F). Water
is densest at about 4 °C; therefore, the deepest water is also the densest. The deepest water is oxygen-poor
because the decomposition of organic material at the bottom of the lake uses up available oxygen that cannot
be replaced by means of oxygen diffusion into the surface of the water, due to the surface ice layer.
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CONNECTION
V 1 "f 21°
\ / 10 °
10 °
Summer stratification
Fall turnover
Figure 44.10 The spring and fall turnovers are important processes in freshwater lakes that act to move the
nutrients and oxygen at the bottom of deep lakes to the top. Turnover occurs because water has a maximum
density at 4 °C. Surface water temperature changes as the seasons progress, and denser water sinks.
How might turnover in tropical lakes differ from turnover in lakes that exist in temperate regions? Think of
the variation, or lack of variation, in seasonal temperature change.
In springtime, air temperatures increase and surface ice melts. When the temperature of the surface water
begins to approach 4 °C, the water becomes heavier and sinks to the bottom. The water at the bottom of the
lake is then displaced by the heavier and denser surface water and, thus, rises to the top. As that water rises
to the top, the sediments and nutrients from the lake bottom are brought along with it. This is called the spring
turnover. During the summer months, the lake water stratifies, or forms layers, with the warmest water at the
lake surface.
As air temperatures drop in the fall, the temperature of the lake water cools to 4 °C; therefore, this causes fait
turnover as the heavy cold water sinks and displaces the water at the bottom. The oxygen-rich water at the
surface of the lake then moves to the bottom of the lake, while the nutrients at the bottom of the lake rise to
the surface (Figure 44.10). During the winter, the oxygen at the bottom of the lake is used by decomposers and
other organisms requiring oxygen, such as fish. It is important to note, however, that the relative transparency
of ice also allows the penetration of the shorter wavelengths of visible light so that photosynthesis, especially by
algae can continue.
Temperature
Temperature affects the physiology of organisms as well as the density and state of water. Temperature exerts
an important influence on living things because few living things can survive at temperatures below 0 °C
(32 °F) due to metabolic constraints. It is also rare for living things to survive at temperatures exceeding 45
°C (113 °F); this is a reflection of evolutionary response to typical temperatures near the Earth’s surface.
Enzymes are most efficient within a narrow and specific range of temperatures; enzyme degradation can occur
at higher temperatures. Therefore, organisms either must maintain an internal temperature or they must inhabit
an environment that will keep the body within a temperature range that supports metabolism. Some animals
have adapted to enable their bodies to survive significant temperature fluctuations, such as seen in hibernation
or reptilian torpor. Similarly, some Archaea bacteria have evolved to tolerate extremely hot temperatures such
as those found in the geysers within Yellowstone National Park. Such bacteria are examples of extremophiles'.
organisms that thrive in extreme environments.
The temperature (of both water and air) can limit the distribution of living things. Animals faced with temperature
fluctuations may respond with adaptations, such as migration, in order to survive. Migration, the regular
movement from one place to another, is an adaptation found in many animals, including many that inhabit
seasonally cold climates. Migration solves problems related to temperature, locating food, and finding a mate.
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For example, the Arctic Tern ( Sterna paradisaea) makes a 40,000 km (24,000 mi) round-trip flight each year
between its feeding grounds in the southern hemisphere and its breeding grounds in the Arctic Ocean. Monarch
butterflies ( Danaus plexippus) live in the eastern and western United States in the warmer months, where they
build up enormous populations, and migrate to areas around Michoacan, Mexico as well as areas along the
Pacific Coast, and the southern United States in the wintertime. Some species of mammals also make migratory
forays. Reindeer ( Rangifer tarandus) travel about 5,000 km (3,100 mi) each year to find food. Amphibians and
reptiles are more limited in their distribution because they generally lack migratory ability. Not all animals that
could migrate do so: migration carries risk and comes at a high-energy cost.
Some animals hibernate or estivate to survive hostile temperatures. Hibernation enables animals to survive
cold conditions, and estivation allows animals to survive the hostile conditions of a hot, dry climate. Animals
that hibernate or estivate enter a state known as torpor: a condition in which their metabolic rate is significantly
lowered. This enables the animal to wait until its environment better supports its survival. Some amphibians,
such as the wood frog ( Rana syivatica), have an antifreeze-like chemical in their cells, which retains the cells’
integrity and prevents them from freezing and bursting.
Water
Water is required by all living things because it is critical for cellular processes. Since terrestrial organisms lose
water to the environment, they have evolved many adaptations to retain water.
• Plants have a number of interesting features on their leaves, such as leaf hairs and a waxy cuticle, that
serve to decrease the rate of water loss via transpiration and convection.
• Freshwater organisms are surrounded by water and are constantly in danger of having water rush into their
cells because of osmosis. Many adaptations of organisms living in freshwater environments have evolved
to ensure that solute concentrations in their bodies remain within appropriate levels. One such adaptation
is the excretion of dilute urine.
• Marine organisms are surrounded by water with a higher solute concentration than the organism and, thus,
are in danger of losing water to the environment because of osmosis. These organisms have morphological
and physiological adaptations to retain water and release solutes into the environment. For example, Marine
iguanas ( Amblyrhynchus cristatus), sneeze out water vapor that is high in salt in order to maintain solute
concentrations within an acceptable range while swimming in the ocean and eating marine plants.
Inorganic Nutrients and Soil
Inorganic nutrients, such as nitrogen and phosphorus, are important in determining the distribution and the
abundance of living things. Plants obtain these inorganic nutrients from the soil when water moves into the plant
through the roots. Therefore, soil structure (particle size of soil components), soil pH, and soil nutrient content
together all play an important role in the distribution of plants. Animals obtain inorganic nutrients from the food
they consume. Therefore, animal distributions are related to the distribution of what they eat. In some cases,
animals will follow their food resource as it moves through the environment.
Other Aquatic Factors
Some abiotic factors, such as oxygen, are important in aquatic ecosystems as well as terrestrial environments.
Terrestrial animals obtain oxygen from the air they breathe. Oxygen availability can be an issue for organisms
living at very high elevations, however, where there are fewer molecules of oxygen in the air. In aquatic systems,
the concentration of dissolved oxygen is related to water temperature and the speed at which the water moves.
Cold water has more dissolved oxygen than warmer water. In addition, salinity, currents, and tidal changes can
be important abiotic factors in aquatic ecosystems.
Other Terrestrial Factors
Wind can be an important abiotic factor because it influences the rate of evaporation, transpiration, and
convective heat loss from the surface of all organisms. The physical force of wind is also important because it
can move soil, water, or other abiotic factors, as well as an ecosystem’s organisms.
Fire is another terrestrial factor that can be an important agent of disturbance in terrestrial ecosystems. Some
organisms are adapted to fire and, thus, require the high heat associated with fire to complete a part of their life
cycle. For example, the jack pine ( Pinus banksiana) requires heat from fire for its seed cones to open (Figure
44.11). Through the burning of pine needles, fire adds nitrogen to the soil and limits competition by destroying
undergrowth.
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Figure 44.11 The mature cones of the jack pine (Pinus banksiana) open only when exposed to high temperatures,
such as during a forest fire. A fire is likely to kill most vegetation, so a seedling that germinates after a fire is more likely
to receive ample sunlight than one that germinates under normal conditions, (credit: USDA)
Abiotic Factors Influencing Plant Growth
Temperature and moisture are important influences on plant production (primary productivity) and the amount
of organic matter available as food (net primary productivity). Net primary productivity is an estimation of
all of the organic matter available as food; it is calculated as the total amount of carbon fixed per year minus
the amount that is oxidized during cellular respiration. In terrestrial environments, net primary productivity is
estimated by measuring the above-ground biomass per unit area, which is the total mass of living plants,
excluding roots (whose mass is very difficult to measure). This means that a large percentage of plant biomass
which exists underground is not included in this measurement. Net primary productivity is an important variable
when considering differences in biomes. Very productive biomes have a high level of aboveground biomass.
Annual biomass production is directly related to the abiotic components of the environment. Environments with
the greatest amount of biomass produce conditions in which photosynthesis, plant growth, and the resulting net
primary productivity are optimized. The climate of these areas is warm and wet. Photosynthesis can proceed
at a high rate, enzymes can work most efficiently, and stomata can remain open without the risk of excessive
transpiration; together, these factors lead to the maximal amount of carbon dioxide (CO 2 ) moving into the plant,
resulting in high biomass production. The above-ground biomass produces several important resources for
other living things, including habitat and food. Conversely, dry and cold environments have lower photosynthetic
rates and therefore less biomass. The animal communities living there will also be affected by the decrease in
available food.
44.3 | Terrestrial Biomes
By the end of this section, you will be able to do the following:
• Identify the two major abiotic factors that determine terrestrial biomes
• Recognize distinguishing characteristics of each of the eight major terrestrial biomes
The Earth’s biomes are categorized into two major groups: terrestrial and aquatic. Terrestrial biomes are based
on land, while aquatic biomes include both ocean and freshwater biomes. The eight major terrestrial biomes
on Earth are each distinguished by characteristic temperatures and amount of precipitation. Comparing the
annual totals of precipitation and fluctuations in precipitation from one biome to another provides clues as to the
importance of abiotic factors in the distribution of biomes. Temperature variation on a daily and seasonal basis is
also important for predicting the geographic distribution of the biome and the vegetation type in the biome. The
distribution of these biomes shows that the same biome can occur in geographically distinct areas with similar
climates (Figure 44.12).
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Chapter 44 | Ecology and the Biosphere
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visual
CONNECTION
■ Tropical forest
□ Boreal forest
l i Savanna
■ Tundra
8 Desert
I Mountains
■ Chaparral
□ Polar ice
Temperate forest
Temperate grassland
Figure 44.12 Each of the world’s major biomes is distinguished by characteristic temperatures and amounts of
precipitation. Polar ice and mountains are also shown.
Which of the following statements about biomes is false?
a. Chaparral is dominated by shrubs.
b. Savannas and temperate grasslands are dominated by grasses.
c. Boreal forests are dominated by deciduous trees.
d. Lichens are common in the arctic tundra.
Tropical Wet Forest
Tropical wet forests are also referred to as tropical rainforests. This biome is found in equatorial regions
(Figure 44.12). The vegetation is characterized by plants with broad leaves that fall and are replaced throughout
the year. Unlike the trees of deciduous forests, the trees in this biome do not have a seasonal loss of leaves
associated with variations in temperature and sunlight; these forests are “evergreen” year-round.
The temperature and sunlight profiles of tropical wet forests are very stable in comparison to that of other
terrestrial biomes, with the temperatures ranging from 20 °C to 34 °C (68 °F to 93 °F). When one compares
the annual temperature variation of tropical wet forests with that of other forest biomes, the lack of seasonal
temperature variation in the tropical wet forest becomes apparent. This lack of seasonality leads to year-round
plant growth, rather than the seasonal (spring, summer, and fall) growth seen in other more temperate biomes.
In contrast to other ecosystems, tropical ecosystems do not have long days and short days during the yearly
cycle. Instead, a constant daily amount of sunlight (11-12 hrs per day) provides more solar radiation, thereby, a
longer period of time for plant growth.
The annual rainfall in tropical wet forests ranges from 125 cm to 660 cm (50-200 in) with some monthly variation.
While sunlight and temperature remain fairly consistent, annual rainfall is highly variable. Tropical wet forests
typically have wet months in which there can be more than 30 cm (11-12 in) of precipitation, as well as dry
months in which there are fewer than 10 cm (3.5 in) of rainfall. However, the driest month of a tropical wet forest
still exceeds the annual rainfall of some other biomes, such as deserts.
Tropical wet forests have high net primary productivity because the annual temperatures and precipitation values
in these areas are ideal for plant growth. Therefore, the extensive biomass present in the tropical wet forest
leads to plant communities with very high species diversities (Figure 44.13). Tropical wet forests have more
species of trees than any other biome; on average between 100 and 300 species of trees are present in a
single hectare (2.5 acres) of South American Amazonian rain forest. One way to visualize this is to compare the
distinctive horizontal layers within the tropical wet forest biome. On the forest floor is a sparse layer of plants
and decaying plant matter. Above that is an understory of short shrubby foliage. A layer of trees rises above this
understory and is topped by a closed upper canopy —the uppermost overhead layer of branches and leaves.
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Some additional trees emerge through this closed upper canopy. These layers provide diverse and complex
habitats for the variety of plants, fungi, animals, and other organisms within the tropical wet forests.
For example, epiphytes are plants that grow on other plants, which typically are not harmed. Epiphytes are
found throughout tropical wet forest biomes. Many species of animals use the variety of plants and the complex
structure of the tropical wet forests for food and shelter. Some organisms live several meters above ground and
have adapted to this arboreal lifestyle.
Figure 44.13 Tropical wet forests, such as these forests along the Madre de Dios river, Peru, near the Amazon River,
have high species diversity, (credit: Roosevelt Garcia)
Savannas
Savannas are grasslands with scattered trees, and they are located in Africa, South America, and northern
Australia (Figure 44.12). Savannas are usually hot, tropical areas with temperatures averaging from 24 °C to 29
°C (75 °F to 84 °F) and an annual rainfall of 10-40 cm (3.9-15.7 in). Savannas have an extensive dry season;
for this reason, forest trees do not grow as well as they do in the tropical wet forest (or other forest biomes).
As a result, within the grasses and forbs (herbaceous flowering plants) that dominate the savanna, there are
relatively few trees (Figure 44.14). Since fire is an important source of disturbance in this biome, plants have
evolved well-developed root systems that allow them to quickly resprout after a fire.
Figure 44.14 Savannas, like this one in Taita Hills Wildlife Sanctuary in Kenya, are dominated by grasses, (credit:
Christopher T. Cooper)
Subtropical Deserts
Subtropical deserts exist between 15° and 30° north and south latitude and are centered on the Tropics of
Cancer and Capricorn (Figure 44.12). This biome is very dry; in some years, evaporation exceeds precipitation.
Subtropical hot deserts can have daytime soil surface temperatures above 60 °C (140 °F) and nighttime
temperatures approaching 0 °C (32 °F). This is largely due to the lack of atmospheric water. In cold deserts,
temperatures can be as high as 25 °C and can drop below -30 °C (-22 °F). Subtropical deserts are characterized
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by low annual precipitation of fewer than 30 cm (12 in) with little monthly variation and lack of predictability in
rainfall. In some cases, the annual rainfall can be as low as 2 cm (0.8 in) in subtropical deserts located in central
Australia (“the Outback") and northern Africa.
The vegetation and low animal diversity of this biome is closely related to low and unpredictable precipitation.
Very dry deserts lack perennial vegetation that lives from one year to the next; instead, many plants are annuals
that grow quickly and reproduce when rainfall does occur, and then die. Many other plants in these areas are
characterized by having a number of adaptations that conserve water, such as deep roots, reduced foliage,
and water-storing stems (Figure 44.15). Seed plants in the desert produce seeds that can be in dormancy for
extended periods between rains. Adaptations in desert animals include nocturnal behavior and burrowing.
Figure 44.15 To reduce water loss, many desert plants have tiny leaves or no leaves at all. The leaves of ocotillo
(Fouquieria splendens), shown here in the Sonora Desert near Gila Bend, Arizona, appear only after rainfall, and then
are shed.
Chaparral
The chaparral is also called the scrub forest and is found in California, along the Mediterranean Sea, and along
the southern coast of Australia (Figure 44.12). The annual rainfall in this biome ranges from 65 cm to 75 cm
(25.6-29.5 in), and the majority of the rain falls in the winter. Summers are very dry and many chaparral plants
are dormant during the summertime. The chaparral vegetation, shown in Figure 44.16, is dominated by shrubs
adapted to periodic fires, with some plants producing seeds that only germinate after a hot fire. The ashes left
behind after a fire are rich in nutrients like nitrogen that fertilize the soil and promote plant regrowth.
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Figure 44.16 The chaparral is dominated by shrubs, (credit: Miguel Vieira)
Temperate Grasslands
Temperate grasslands are found throughout central North America, where they are also known as prairies',
they are also in Eurasia, where they are known as steppes (Figure 44.12). Temperate grasslands have
pronounced annual fluctuations in temperature with hot summers and cold winters. The annual temperature
variation produces specific growing seasons for plants. Plant growth is possible when temperatures are warm
enough to sustain plant growth and when ample water is available, which occurs in the spring, summer, and fall.
During much of the winter, temperatures are low, and water, which is stored in the form of ice, is not available for
plant growth.
Annual precipitation ranges from 25 cm to 75 cm (9.8-29.5 in). Because of relatively lower annual precipitation in
temperate grasslands, there are few trees except for those found growing along rivers or streams. The dominant
vegetation tends to consist of grasses dense enough to sustain populations of grazing animals Figure 44.17.
The vegetation is very dense and the soils are fertile because the subsurface of the soil is packed with the
roots and rhizomes (underground stems) of these grasses. The roots and rhizomes act to anchor plants into the
ground and replenish the organic material (humus) in the soil when they die and decay.
Figure 44.17 The American bison (Bison bison), more commonly called the buffalo, is a grazing mammal that once
populated American prairies in huge numbers, (credit: Jack Dykinga, USDA Agricultural Research Service)
Fires, mainly caused by lightning, are a natural disturbance in temperate grasslands. When fire is suppressed in
temperate grasslands, the vegetation eventually converts to scrub and sometimes dense forests with drought-
tolerant tree species. Often, the restoration or management of temperate grasslands requires the use of
controlled burns to suppress the growth of trees and maintain the grasses.
Temperate Forests
Temperate forests are the most common biome in eastern North America, Western Europe, Eastern Asia,
Chile, and New Zealand (Figure 44.12). This biome is found throughout mid-latitude regions. Temperatures
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range between -30 °C and 30 °C (-22 °F to 86 °F) and drop to below freezing periodically during cold winters.
These temperatures mean that temperate forests have defined growing seasons during the spring, summer,
and early fall. Precipitation is relatively constant throughout the year and ranges between 75 cm and 150 cm
(29.5-59 in).
Because of the moderate annual rainfall and temperatures, deciduous trees are the dominant plant in this
biome (Figure 44.18). Deciduous trees lose their leaves each fall and remain leafless in the winter. Thus, no
photosynthesis occurs in the deciduous trees during the dormant winter period. Each spring, new leaves appear
as the temperature increases. Because of the dormant period, the net primary productivity of temperate forests
is less than that of tropical wet forests, in addition, temperate forests show less diversity of tree species than
tropical wet forest biomes.
Figure 44.18 Deciduous trees are the dominant plant in the temperate forest, (credit: Oliver Herold)
The trees of the temperate forests leaf out and shade much of the ground; however, this biome is more open
than tropical wet forests because most trees in the temperate forests do not grow as tall as the trees in tropical
wet forests. The soils of the temperate forests are rich in inorganic and organic nutrients. This is due to the
thick layer of leaf litter on forest floors, which does not develop in tropical rainforests. As this leaf litter decays,
nutrients are returned to the soil. The leaf litter also protects soil from erosion, insulates the ground, and provides
habitats for invertebrates (such as the pill bug or roly-poly, Armadillidium vulgare) and their predators, such as
the red-backed salamander (Plethodon cinereus).
Boreal Forests
The boreal forest, also known as taiga or coniferous forest, is found south of the Arctic Circle and across most
of Canada, Alaska, Russia, and northern Europe (Figure 44.12). This biome has cold, dry winters and short,
cool, wet summers. The annual precipitation is from 40 cm to 100 cm (15.7-39 in) and usually takes the form of
snow. Little evaporation occurs because of the cold temperatures.
The long and cold winters in the boreal forest have led to the predominance of cold-tolerant cone-bearing
(coniferous) plants. These are evergreen coniferous trees like pines, spruce, and fir, which retain their needle-
shaped leaves year-round. Evergreen trees can photosynthesize earlier in the spring than deciduous trees
because less energy from the sun is required to warm a needle-like leaf than a broad leaf. This benefits
evergreen trees, which grow faster than deciduous trees in the boreal forest, in addition, soils in boreal forest
regions tend to be acidic with little available nitrogen. Leaves are a nitrogen-rich structure and deciduous trees
must produce a new set of these nitrogen-rich structures each year. Therefore, coniferous trees that retain
nitrogen-rich needles may have a competitive advantage over the broad-leafed deciduous trees.
The net primary productivity of boreal forests is lower than that of temperate forests and tropical wet forests.
The above-ground biomass of boreal forests is high because these slow-growing tree species are long-lived
and accumulate a large standing biomass over time. Plant species diversity is less than that seen in temperate
forests and tropical wet forests. Boreal forests lack the pronounced elements of the layered forest structure seen
in tropical wet forests. The structure of a boreal forest is often only a tree layer and a ground layer (Figure
44.19). When conifer needles are dropped, they decompose more slowly than broad leaves; therefore, fewer
nutrients are returned to the soil to fuel plant growth.
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Figure 44.19 The boreal forest (taiga) has low lying plants and conifer trees, (credit: L.B. Brubaker)
Arctic Tundra
The Arctic tundra lies north of the subarctic boreal forest and is located throughout the Arctic regions of the
northern hemisphere (Figure 44.12). The average winter temperature is -34 °C (29.2 °F) and the average
summer temperature is from 3 °C to 12 °C (37 °F-52 °F). Plants in the arctic tundra have a very short growing
season of approximately 10-12 weeks.
However, during this time, there are almost 24 hours of daylight and plant growth is rapid. The annual
precipitation of the Arctic tundra is very low with little annual variation in precipitation. And, as in the boreal
forests, there is little evaporation due to the cold temperatures.
Plants in the Arctic tundra are generally low to the ground (Figure 44.20). There is little species diversity, low net
primary productivity, and low above-ground biomass. The soils of the Arctic tundra may remain in a perennially
frozen state referred to as permafrost. The permafrost makes it impossible for roots to penetrate deep into the
soil and slows the decay of organic matter, which inhibits the release of nutrients from organic matter. During the
growing season, the ground of the Arctic tundra can be completely covered with plants or lichens.
Figure 44.20 Low-growing plants such as shrub willow dominate the tundra landscape, shown here in the Arctic
National Wildlife Refuge, (credit: USFWS Arctic National Wildlife Refuge)
LINK TQ LEARNING
Watch this Assignment Discovery: Biomes video (http:// 0 penstaxc 0 llege. 0 rg/l/bi 0 mes) for an overview
of biomes. To explore further, select one of the biomes on the extended playlist: desert, savanna, temperate
forest, temperate grassland, tropic, tundra.
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44.4 | Aquatic Biomes
By the end of this section, you will be able to do the following:
• Describe the effects of abiotic factors on the composition of plant and animal communities in aquatic
biomes
• Compare and contrast the characteristics of the ocean zones
• Summarize the characteristics of standing water and flowing water freshwater biomes
Abiotic Factors Influencing Aquatic Biomes
Like terrestrial biomes, aquatic biomes are influenced by a series of abiotic factors. The aquatic
medium—water— has different physical and chemical properties than air, however. Even if the water in a pond
or other body of water is perfectly clear (there are no suspended particles), water, on its own, absorbs light.
As one descends into a deep body of water, there will eventually be a depth which the sunlight cannot reach.
While there are some abiotic and biotic factors in a terrestrial ecosystem that might obscure light (like fog, dust,
or insect swarms), usually these are not permanent features of the environment. The importance of light in
aquatic biomes is central to the communities of organisms found in both freshwater and marine ecosystems.
In freshwater systems, stratification due to differences in density is perhaps the most critical abiotic factor and
is related to the energy aspects of light. The thermal properties of water (rates of heating and cooling and the
ability to store much larger amounts of energy than the air) are significant to the function of marine systems and
have major impacts on global climate and weather patterns. Marine systems are also influenced by large-scale
physical water movements, such as currents; these are less important in most freshwater lakes.
The ocean is categorized by several areas or zones (Figure 44.21). All of the ocean’s open water is referred to
as the pelagic realm (or zone). The benthic realm (or zone) extends along the ocean bottom from the shoreline
to the deepest parts of the ocean floor. Within the pelagic realm is the photic zone, which is the portion of
the ocean that light can penetrate (approximately 200 m or 650 ft). At depths greater than 200 m, light cannot
penetrate; thus, this is referred to as the aphotic zone. The majority of the ocean is aphotic and lacks sufficient
light for photosynthesis. The deepest part of the ocean, the Challenger Deep (in the Mariana Trench, located
in the western Pacific Ocean), is about 11,000 m (about 6.8 mi) deep. To give some perspective on the depth
of this trench, the ocean is, on average, 4267 m or 14,000 ft deep. These realms and zones are relevant to
freshwater lakes as well.
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CONNECTION
Oceanic zone
Neritic Intertidal
zone 1 zone
Pelagic realm —
Photic zone
10,000 m
Benthic realm
Continental shelf
Figure 44.21 The ocean is divided into different zones based on water depth and distance from the shoreline.
In which of the following regions would you expect to find photosynthetic organisms?
a. the aphotic zone, the neritic zone, the oceanic zone, and the benthic realm
b. the photic zone, the intertidal zone, the neritic zone, and the oceanic zone
c. the photic zone, the abyssal zone, the neritic zone, and the oceanic zone
d. the pelagic realm, the aphotic zone, the neritic zone, and the oceanic zone
Marine Biomes
The ocean is the largest marine biome. It is a continuous body of salt water that is relatively uniform in chemical
composition; in fact, it is a weak solution of mineral salts and decayed biological matter. Within the ocean, coral
reefs are a second kind of marine biome. Estuaries, coastal areas where salt water and fresh water mix, form a
third unique marine biome.
Ocean
The physical diversity of the ocean is a significant influence on plants, animals, and other organisms. The ocean
is categorized into different zones based on how far light reaches into the water. Each zone has a distinct group
of species adapted to the biotic and abiotic conditions particular to that zone.
The intertidal zone, which is the zone between high and low tide, is the oceanic region that is closest to land
(Figure 44.21). Generally, most people think of this portion of the ocean as a sandy beach. In some cases, the
intertidal zone is indeed a sandy beach, but it can also be rocky or muddy. The intertidal zone is an extremely
variable environment because of action of tidal ebb and flow. Organisms are exposed to air and sunlight at
low tide and are underwater most of the time, especially during high tide. Therefore, living things that thrive in
the intertidal zone are adapted to being dry for long periods of time. The shore of the intertidal zone may also
be repeatedly struck by waves, and the organisms found there are adapted to withstand damage from their
pounding action (Figure 44.22). The exoskeletons of shoreline crustaceans (such as the shore crab, Carcinus
maenas) are tough and protect them from desiccation (drying out) and wave damage. Another consequence of
the pounding waves is that few algae and plants establish themselves in the constantly moving rocks, sand, or
mud.
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Figure 44.22 Sea urchins, mussel shells, and starfish are often found in the intertidal zone, shown here in Kachemak
Bay, Alaska, (credit: NOAA)
The neritic zone (Figure 44.21) extends from the intertidal zone to depths of about 200 m (or 650 ft) at the edge
of the continental shelf (the underwater landmass that extends from a continent). Since light can penetrate this
depth, photosynthesis can still occur in the neritic zone. The water here contains silt and is well-oxygenated, low
in pressure, and stable in temperature. Phytoplankton and floating Sargassum (a type of free-floating marine
seaweed) provide a habitat for some sea life found in the neritic zone. Zooplankton, protists, small fishes, and
shrimp are found in the neritic zone and are the base of the food chain for most of the world’s fisheries.
Beyond the neritic zone is the open ocean area known as the pelagic or open oceanic zone (Figure 44.21).
Within the oceanic zone there is thermal stratification where warm and cold waters mix because of ocean
currents. Abundant plankton serve as the base of the food chain for larger animals such as whales and dolphins.
Nutrients are scarce and this is a relatively less productive part of the marine biome. When photosynthetic
organisms and the protists and animals that feed on them die, their bodies fall to the bottom of the ocean, where
they remain. Unlike freshwater lakes, most of the open ocean lacks a process for bringing the organic nutrients
back up to the surface. (Exceptions include major oceanic upwellings within the Humboldt Current along the
western coast of South America.) The majority of organisms in the aphotic zone include sea cucumbers (phylum
Echinodermata) and other organisms that survive on the nutrients contained in the dead bodies of organisms in
the photic zone.
Beneath the pelagic zone is the benthic realm, the deep-water region beyond the continental shelf (Figure
44.21). The bottom of the benthic realm is composed of sand, silt, and dead organisms. Temperature decreases,
remaining above freezing, as water depth increases. This is a nutrient-rich portion of the ocean because of the
dead organisms that fall from the upper layers of the ocean. Because of this high level of nutrients, a diversity of
fungi, sponges, sea anemones, marine worms, sea stars, fishes, and bacteria exist.
The deepest part of the ocean is the abyssal zone, which is at depths of 4000 m or greater. The abyssal zone
(Figure 44.21) is very cold and has very high pressure, high oxygen content, and low nutrient content. There are
a variety of invertebrates and fishes found in this zone, but the abyssal zone does not have plants because of
the lack of light. Hydrothermal vents are found primarily in the abyssal zone; chemosynthetic bacteria utilize the
hydrogen sulfide and other minerals emitted from the vents. These chemosynthetic bacteria use the hydrogen
sulfide as an energy source and serve as the base of the food chain found in the abyssal zone.
Coral Reefs
Coral reefs are ocean ridges formed by marine invertebrates, comprising mostly cnidarians and molluscs, living
in warm shallow waters within the photic zone of the ocean. They are found within 30° north and south of the
equator. The Great Barrier Reef is perhaps the best-known and largest reef system in the world—visible from
the International Space Station! This massive and ancient reef is located several miles off the northeastern coast
of Australia. Other coral reef systems are fringing islands, which are directly adjacent to land, or atolls, which are
circular reef systems surrounding a former landmass that is now underwater. The coral organisms (members of
phylum Cnidaria) are colonies of saltwater polyps that secrete a calcium carbonate skeleton. These calcium-rich
skeletons slowly accumulate, forming the underwater reef (Figure 44.23). Corals found in shallower waters (at
a depth of approximately 60 m or about 200 ft) have a mutualistic relationship with photosynthetic unicellular
algae. The relationship provides corals with the majority of the nutrition and the energy they require. The waters
in which these corals live are nutritionally poor and, without this mutualism, it would not be possible for large
corals to grow. Some corals living in deeper and colder water do not have a mutualistic relationship with algae;
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these corals attain energy and nutrients using stinging cells called cnidocytes on their tentacles to capture prey.
LINK TQ LEARNING
Watch this National Oceanic and Atmospheric Administration (NOAA) video
(http:// 0 penstaxc 0 llege. 0 rg/l/marine_bi 0 l 0 gy) to see marine ecologist Dr. Peter Etnoyer discuss his
research on coral organisms.
It is estimated that more than 4,000 fish species inhabit coral reefs. These fishes can feed on coral, the
cryptofauna (invertebrates found within the calcium carbonate substrate of the coral reefs), or the seaweed and
algae that are associated with the coral. In addition, some fish species inhabit the boundaries of a coral reef;
these species include predators, herbivores, and planktivores, which consume planktonic organisms such as
bacteria, archaea, algae, and protists floating in the pelagic zone.
Figure 44.23 Coral reefs are formed by the calcium carbonate skeletons of coral organisms, which are marine
invertebrates in the phylum Cnidaria. (credit: Terry Hughes)
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Chapter 44 | Ecology and the Biosphere
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V /
e olution CONNECTION
Global Decline of Coral Reefs
It takes many thousands of years to build a coral reef. The animals that create coral reefs have evolved
over millions of years, continuing to slowly deposit the calcium carbonate that forms their characteristic
ocean homes. Bathed in warm tropical waters, the coral animals and their symbiotic algal partners evolved
to survive at the upper limit of ocean water temperature.
Together, climate change and human activity pose dual threats to the long-term survival of the world’s
coral reefs. As global warming due to fossil fuel emissions raises ocean temperatures, coral reefs are
suffering. The excessive warmth causes the reefs to lose their symbiotic, food-producing algae, resulting
in a phenomenon known as bleaching. When bleaching occurs, the reefs lose much of their characteristic
color as the algae and the coral animals die if loss of the symbiotic zooxanthellae is prolonged.
Rising levels of atmospheric carbon dioxide further threaten the corals in other ways; as CO 2 dissolves
in ocean waters, it lowers the pH and increases ocean acidity. As acidity increases, it interferes with the
calcification that normally occurs when coral animals build their calcium carbonate shelters.
When a coral reef begins to die, species diversity plummets as animals lose food and shelter. Coral reefs
are also economically important tourist destinations, so the decline of coral reefs poses a serious threat to
coastal economies.
Human population growth has damaged corals in other ways, too. As human coastal populations increase,
the runoff of sediment and agricultural chemicals has increased, as well, causing some of the once-clear
tropical waters to become cloudy. At the same time, overfishing of popular fish species has allowed the
predator species that eat corals to go unchecked.
Although a rise in global temperatures of 1-2 °C (a conservative scientific projection) in the coming decades
may not seem large, it is very significant to this biome. When change occurs rapidly, species can become
extinct before evolution can offer new adaptations. Many scientists believe that global warming, with its rapid
(in terms of evolutionary time) and inexorable increases in temperature, is tipping the balance beyond the
point at which many of the world’s coral reefs can recover.
Estuaries: Where the Ocean Meets Fresh Water
Estuaries are biomes that occur where a source of fresh water, such as a river, meets the ocean. Therefore,
both fresh water and salt water are found in the same vicinity; mixing results in a diluted ( brackish ) saltwater.
Estuaries form protected areas where many of the young offspring of crustaceans, molluscs, and fish begin their
lives, which also creates important breeding grounds for other animals. Salinity is a very important factor that
influences the organisms and the adaptations of the organisms found in estuaries. The salinity of estuaries varies
considerably and is based on the rate of flow of its freshwater sources, which may depend on the seasonal
rainfall. Once or twice a day, high tides bring salt water into the estuary. Low tides occurring at the same
frequency reverse the current of salt water.
The short-term and rapid variation in salinity due to the mixing of fresh water and salt water is a difficult
physiological challenge for the plants and animals that inhabit estuaries. Many estuarine plant species are
halophytes: plants that can tolerate salty conditions. Halophytic plants are adapted to deal with the salinity
resulting from saltwater on their roots or from sea spray. In some halophytes, filters in the roots remove the salt
from the water that the plant absorbs. Other plants are able to pump oxygen into their roots. Animals, such as
mussels and clams (phylum Mollusca), have developed behavioral adaptations that expend a lot of energy to
function in this rapidly changing environment. When these animals are exposed to low salinity, they stop feeding,
close their shells, and switch from aerobic respiration (in which they use gills to remove oxygen from the water)
to anaerobic respiration (a process that does not require oxygen and takes place in the cytoplasm of the animal’s
cells). When high tide returns to the estuary, the salinity and oxygen content of the water increases, and these
animals open their shells, begin feeding, and return to aerobic respiration.
Freshwater Biomes
Freshwater biomes include lakes and ponds (standing water) as well as rivers and streams (flowing water). They
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also include wetlands, which will be discussed later. Humans rely on freshwater biomes to provide ecosystem
benefits, which are aquatic resources for drinking water, crop irrigation, sanitation, and industry. Lakes and ponds
are connected with abiotic and biotic factors influencing their terrestrial biomes.
Lakes and Ponds
Lakes and ponds can range in area from a few square meters to thousands of square kilometers. Temperature
is an important abiotic factor affecting living things found in lakes and ponds. In the summer, as we have
seen, thermal stratification of lakes and ponds occurs when the upper layer of water is warmed by the sun
and does not mix with deeper, cooler water. Light can penetrate within the photic zone of the lake or pond.
Phytoplankton (algae and cyanobacteria) are found here and carry out photosynthesis, providing the base of
the food web of lakes and ponds. Zooplankton, such as rotifers and larvae and adult crustaceans, consume
these phytoplankton. At the bottom of lakes and ponds, bacteria in the aphotic zone break down dead organisms
that sink to the bottom.
Nitrogen and phosphorus are important limiting nutrients in lakes and ponds. Because of this, they are the
determining factors in the amount of phytoplankton growth that takes place in lakes and ponds. When there is a
large input of nitrogen and phosphorus (from sewage and runoff from fertilized lawns and farms, for example),
the growth of algae skyrockets, resulting in a large accumulation of algae called an algal bloom. Algal blooms
(Figure 44.24) can become so extensive that they reduce light penetration in water. They may also release
toxic byproducts into the water, contaminating any drinking water taken from that source. In addition, the lake or
pond becomes aphotic, and photosynthetic plants cannot survive. When the algae die and decompose, severe
oxygen depletion of the water occurs. Fishes and other organisms that require oxygen are then more likely to
die, resulting in a dead zone. Lake Erie and the Gulf of Mexico represent freshwater and marine habitats where
phosphorus control and storm water runoff pose significant environmental challenges.
Figure 44.24 The uncontrolled growth of algae in this lake has resulted in an algal bloom, (credit: Jeremy Nettleton)
Rivers and Streams
Rivers and streams are continuously moving bodies of water that carry large amounts of water from the source,
or headwater, to a lake or ocean. The largest rivers include the Nile River in Africa, the Amazon River in South
America, and the Mississippi River in North America.
Abiotic features of rivers and streams vary along the length of the river or stream. Streams begin at a point of
origin referred to as source water. The source water is usually cold, low in nutrients, and clear. The channel
(the width of the river or stream) is narrower than at any other place along the length of the river or stream.
Because of this, the current is often faster here than at any other point of the river or stream.
The fast-moving water results in minimal silt accumulation at the bottom of the river or stream; therefore, the
water is usually clear and free of debris. Photosynthesis here is mostly attributed to algae that are growing on
rocks; the swift current inhibits the growth of phytoplankton. An additional input of energy can come from leaves
and other organic material that fall downstream into the river or stream, as well as from trees and other plants
that border the water. When the leaves decompose, the organic material and nutrients in the leaves are returned
to the water. Plants and animals have adapted to this fast-moving water. For instance, leeches (phylum Annelida)
have elongated bodies and suckers on the anterior and ventral areas of the body. These suckers attach to
the substrate, keeping the leech anchored in place, and are also used to attach to their prey. Freshwater trout
species (phylum Chordata) are an important predator in these fast-moving rivers and streams.
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As the river or stream flows away from the source, the width of the channel gradually widens and the current
slows. This slow-moving water, caused by the gradient decrease and the volume increase as tributaries unite,
has more sedimentation. Phytoplankton can also be suspended in slow-moving water. Therefore, the water will
not be as clear as it is near the source. The water is also warmer. Worms (phylum Annelida) and insects (phylum
Arthropoda) can be found burrowing into the mud. The higher order predator vertebrates (phylum Chordata)
include waterfowl, frogs, and fishes. These predators must find food in these slow moving, sometimes murky,
waters and, unlike the trout in the waters at the source, these vertebrates may not be able to use vision as their
primary sense to find food. Instead, they are more likely to use taste or chemical cues to find prey.
Wetlands
Wetlands are environments in which the soil is either permanently or periodically saturated with water. Wetlands
are different from lakes because wetlands are shallow bodies of water whereas lakes vary in depth. Emergent
vegetation consists of wetland plants that are rooted in the soil but have portions of leaves, stems, and flowers
extending above the water’s surface. There are several types of wetlands including marshes, swamps, bogs,
mudflats, and salt marshes (Figure 44.25). The three shared characteristics among these types—what makes
them wetlands—are their hydrology, hydrophytic vegetation, and hydric soils.
Figure 44.25 Located in southern Florida, Everglades National Park is vast array of wetland environments, including
sawgrass marshes, cypress swamps, and estuarine mangrove forests. Here, a great egret walks among cypress trees,
(credit: NPS)
Freshwater marshes and swamps are characterized by slow and steady water flow. Bogs, however, develop in
depressions where water flow is low or nonexistent. Bogs usually occur in areas where there is a clay bottom
with poor percolation of water. (Percolation is the movement of water through the pores in the soil or rocks.) The
water found in a bog is stagnant and oxygen-depleted because the oxygen used during the decomposition of
organic matter is not readily replaced. As the oxygen in the water is depleted, decomposition slows. This leads
to a buildup of acids and a lower water pH. The lower pH creates challenges for plants because it limits the
available nitrogen. As a result, some bog plants (such as sundews, pitcher plants, and Venus flytraps) capture
insects in order to extract the nitrogen from their bodies. Bogs have low net primary productivity because the
water found in bogs has low levels of nitrogen and oxygen.
44.5 | Climate and the Effects of Global Climate
Change
By the end of this section, you will be able to do the following:
• Define global climate change
• Summarize the effects of the Industrial Revolution on global atmospheric carbon dioxide concentration
• Describe three natural factors affecting long-term global climate
• List two or more greenhouse gases and describe their role in the greenhouse effect
All biomes are universally affected by global conditions, such as climate, that ultimately shape each biome’s
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environment. Scientists who study climate have noted a series of marked changes that have gradually become
increasingly evident during the last sixty years. Global climate change is the term used to describe altered
global weather patterns, especially a worldwide increase in temperature and resulting changes in the climate,
due largely to rising levels of atmospheric carbon dioxide.
Climate and Weather
A common misconception about global climate change is that a specific weather event occurring in a particular
region (for example, a very cool week in June in central Indiana) provides evidence of global climate change.
However, a cold week in June is a weather-related event and not a climate-related one. These misconceptions
often arise because of confusion over the terms climate and weather.
Climate refers to the long-term, predictable atmospheric conditions of a specific area. The climate of a biome
is characterized by having consistent seasonal temperature and rainfall ranges. Climate does not address the
amount of rain that fell on one particular day in a biome or the colder-than-average temperatures that occurred
on one day. In contrast, weather refers to the conditions of the atmosphere during a short period of time.
Weather forecasts are usually made for 48-hour cycles. Long-range weather forecasts are available but can be
unreliable.
To better understand the difference between climate and weather, imagine that you are planning an outdoor
event in northern Wisconsin. You would be thinking about climate when you plan the event in the summer rather
than the winter because you have long-term knowledge that any given Saturday in the months of May to August
would be a better choice for an outdoor event in Wisconsin than any given Saturday in January. However, you
cannot determine the specific day that the event should be held on because it is difficult to accurately predict the
weather on a specific day. Climate can be considered “average" weather that takes place over many years.
Global Climate Change
Climate change can be understood by approaching three areas of study:
• evidence of current and past global climate change
• drivers of global climate change
• documented results of climate change
It is helpful to keep these three different aspects of climate change clearly separated when consuming media
reports about global climate change. We should note that it is common for reports and discussions about global
climate change to confuse the data showing that Earth’s climate is changing with the factors that drive this
climate change.
Evidence for Global Climate Change
Since scientists cannot go back in time to directly measure climatic variables, such as average temperature and
precipitation, they must instead indirectly measure temperature. To do this, scientists rely on historical evidence
of Earth’s past climate.
Antarctic ice cores are a key example of such evidence for climate change. These ice cores are samples of polar
ice obtained by means of drills that reach thousands of meters into ice sheets or high mountain glaciers. Viewing
the ice cores is like traveling backwards through time; the deeper the sample, the earlier the time period. Trapped
within the ice are air bubbles and other biological evidence that can reveal temperature and carbon dioxide data.
Antarctic ice cores have been collected and analyzed to indirectly estimate the temperature of the Earth over the
past 400,000 years (Figure 44.26a). The 0 °C on this graph refers to the long-term average. Temperatures that
are greater than 0 °C exceed Earth’s long-term average temperature. Conversely, temperatures that are less
than 0 °C are less than Earth’s average temperature. This figure shows that there have been periodic cycles of
increasing and decreasing temperature.
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(a) (b)
Figure 44.26 Scientists drill for ice cores in polar regions. The ice contains air bubbles and biological substances that
provide important information for researchers, (credit: a: Helle Astrid Kjaer; b: National Ice Core Laboratory, USGS)
Before the late 1800s, the Earth has been as much as 9 °C cooler and about 3 °C warmer. Note that the graph in
Figure 44.27b shows that the atmospheric concentration of carbon dioxide has also risen and fallen in periodic
cycles. Also note the relationship between carbon dioxide concentration and temperature. Figure 44.27b shows
that carbon dioxide levels in the atmosphere have historically cycled between 180 and 300 parts per million
(ppm) by volume.
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Figure 44.27 Ice at the Russian Vostok station in East Antarctica was laid down over the course of 420,000 years and
reached a depth of over 3,000 m. By measuring the amount of CO 2 trapped in the ice, scientists have determined past
atmospheric CO 2 concentrations. Temperatures relative to modern day were determined from the amount of deuterium
(a nonradioactive isotope of hydrogen) present.
Figure 44.27a does not show the last 2,000 years with enough detail to compare the changes of Earth’s
temperature during the last 400,000 years with the temperature change that has occurred in the more recent
past. Two significant temperature anomalies, or irregularities , have occurred in the last 2,000 years. These
are the Medieval Climate Anomaly (or the Medieval Warm Period) and the Little Ice Age. A third temperature
anomaly aligns with the Industrial Era. The Medieval Climate Anomaly occurred between 900 and 1300 AD.
During this time period, many climate scientists think that slightly warmer weather conditions prevailed in many
parts of the world; the higher-than-average temperature changes varied between 0.10 °C and 0.20 °C above the
norm. Although 0.10 °C does not seem large enough to produce any noticeable change, it did free seas of ice.
Because of this warming, the Vikings were able to colonize Greenland.
The Little Ice Age was a cold period that occurred between 1550 AD and 1850 AD. During this time, a slight
cooling of a little less than 1 °C was observed in North America, Europe, and possibly other areas of the Earth.
This 1 °C change in global temperature is a seemingly small deviation in temperature (as was observed during
the Medieval Climate Anomaly); however, it also resulted in noticeable climatic changes. Historical accounts
reveal a time of exceptionally harsh winters with much snow and frost.
The Industrial Revolution, which began around 1750, was characterized by changes in much of human society.
Advances in agriculture increased the food supply, which improved the standard of living for people in Europe
and the United States. New technologies were invented that provided jobs and cheaper goods. These new
technologies were powered using fossil fuels, especially coal. The industrial Revolution starting in the early
nineteenth century ushered in the beginning of the Industrial Era. When a fossil fuel is burned, carbon dioxide is
released. With the beginning of the Industrial Era, atmospheric carbon dioxide began to rise (Figure 44.28).
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Figure 44.28 The atmospheric concentration of CO 2 has risen steadily since the beginning of industrialization.
Current and Past Drivers of Global Climate Change
Because it is not possible to go back in time to directly observe and measure climate, scientists must use indirect
evidence to determine the drivers, or factors, that may be responsible for climate change. The indirect evidence
includes data collected using ice cores, boreholes (a narrow shaft bored into the ground), tree rings, glacier
lengths, pollen remains, and ocean sediments. The data shows a correlation between the timing of temperature
changes and drivers of climate change. Before the Industrial Era (pre-1780), there were three drivers of climate
change that were not related to human activity or atmospheric gases. The first of these is the Milankovitch cycles.
The Milankovitch cycles describe the effects of slight changes in the Earth’s orbit on Earth’s climate. The length
of the Milankovitch cycles ranges between 19,000 and 100,000 years. In other words, one could expect to see
some predictable changes in the Earth’s climate associated with changes in the Earth’s orbit at a minimum of
every 19,000 years.
The variation in the sun’s intensity is the second natural factor responsible for climate change. Solar intensity
is the amount of solar power or energy the sun emits in a given amount of time. There is a direct relationship
between solar intensity and temperature. As solar intensity increases (or decreases), the Earth’s temperature
correspondingly increases (or decreases). Changes in solar intensity have been proposed as one of several
possible explanations for the Little Ice Age.
Finally, volcanic eruptions are a third natural driver of climate change. Volcanic eruptions can last a few days, but
the solids and gases released during an eruption can influence the climate over a period of a few years, causing
short-term climate changes. The gases and solids released by volcanic eruptions can include carbon dioxide,
water vapor, sulfur dioxide, hydrogen sulfide, hydrogen, and carbon monoxide. Generally, volcanic eruptions cool
the climate. This occurred in 1783 when volcanos in Iceland erupted and caused the release of large volumes
of sulfuric oxide. This led to haze-effect cooling, a global phenomenon that occurs when dust, ash, or other
suspended particles block out sunlight and trigger lower global temperatures as a result; haze-effect cooling
usually extends for one or more years before dissipating in intensity. In Europe and North America, haze-effect
cooling produced some of the lowest average winter temperatures on record in 1783 and 1784.
Greenhouse gases are probably the most significant drivers of the climate. When heat energy from the sun
strikes the Earth, gases known as greenhouse gases trap the heat in the atmosphere, in a similar manner
as do the glass panes of a greenhouse keep heat from escaping. The greenhouse gases that affect Earth
include carbon dioxide, methane, water vapor, nitrous oxide, and ozone. Approximately half of the radiation from
the sun passes through these gases in the atmosphere and strikes the Earth. This radiation is converted into
thermal (infrared) radiation on the Earth’s surface, and then a portion of that energy is re-radiated back into the
atmosphere. Greenhouse gases, however, reflect much of the thermal energy back to the Earth’s surface. The
more greenhouse gases there are in the atmosphere, the more thermal energy is reflected back to the Earth’s
surface, heating it up and the atmosphere immediately above it. Greenhouse gases absorb and emit radiation
and are an important factor in the greenhouse effect: the warming of Earth due to carbon dioxide and other
greenhouse gases in the atmosphere.
Direct evidence supports the relationship between atmospheric concentrations of carbon dioxide and
temperature: as carbon dioxide rises, global temperature rises. Since 1950, the concentration of atmospheric
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carbon dioxide has increased from about 280 ppm to 382 ppm in 2006. in 2011, the atmospheric carbon dioxide
concentration was 392 ppm. However, the planet would not be inhabitable by current life forms if water vapor did
not produce its drastic greenhouse warming effect.
Scientists look at patterns in data and try to explain differences or deviations from these patterns. The
atmospheric carbon dioxide data reveal a historical pattern of carbon dioxide increasing and decreasing, cycling
between a low of 180 ppm and a high of 300 ppm. Scientists have concluded that it took around 50,000 years
for the atmospheric carbon dioxide level to increase from its low minimum concentration to its higher maximum
concentration. However, beginning only a few centuries ago, atmospheric carbon dioxide concentrations have
increased beyond the historical maximum of 300 ppm. The current increases in atmospheric carbon dioxide have
happened very quickly—in a matter of hundreds of years rather than thousands of years. What is the reason for
this difference in the rate of change and the amount of increase in carbon dioxide? A key factor that must be
recognized when comparing the historical data and the current data is the presence and industrial activities of
modern human society; no other driver of climate change has yielded changes in atmospheric carbon dioxide
levels at this rate or to this magnitude.
Human activity releases carbon dioxide and methane, two of the most important greenhouse gases, into the
atmosphere in several ways. The primary mechanism that releases carbon dioxide is the burning of fossil
fuels, such as gasoline, coal, and natural gas (Figure 44.29). Deforestation, cement manufacture, animal
agriculture, the clearing of land, and the burning of forests are other human activities that release carbon dioxide.
Methane (CH 4 ) is produced when bacteria break down organic matter under anaerobic conditions. Anaerobic
conditions can happen when organic matter is trapped underwater (such as in rice paddies) or in the intestines
of herbivores. Methane can also be released from natural gas fields and the decomposition of animal and plant
material that occurs in landfills. Another source of methane is the melting of clathrates. Clathrates are frozen
chunks of ice and methane found at the bottom of the ocean. When water warms, these chunks of ice melt
and methane is released. As the ocean’s water temperature increases, the rate at which clathrates melt is
increasing, releasing even more methane. This leads to increased levels of methane in the atmosphere, which
further accelerates the rate of global warming. This is an example of the positive feedback loop that is leading to
the rapid rate of increase of global temperatures.
Figure 44.29 The burning of fossil fuels in industry and by vehicles releases carbon dioxide and other greenhouse
gases into the atmosphere, (credit: “Pbllo'VWikimedia Commons)
Documented Results of Climate Change: Past and Present
Scientists have geological evidence of the consequences of long-ago climate change. Modern-day phenomena
such as retreating glaciers and melting polar ice cause a continual rise in sea level. Meanwhile, changes in
climate can negatively affect organisms.
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Geological Climate Change
Global warming has been associated with at least one planet-wide extinction event during the geological past.
The Permian extinction event occurred about 251 million years ago toward the end of the roughly 50-million-
year-long geological time span known as the Permian period. This geologic time period was one of the three
warmest periods in Earth’s geologic history. Scientists estimate that approximately 70 percent of the terrestrial
plant and animal species and 84 percent of marine species became extinct, vanishing forever near the end of
the Permian period.
Organisms that had adapted to wet and warm climatic conditions, such as annual rainfall of 300-400 cm
(118-157 in) and 20 °C-30 °C (68 °F-86 °F) in the tropical wet forest, may not have been able to survive the
Permian climate change.
LINK
T a
LEARNING
Watch this NASA video (http:// 0 penstaxc 0 llege. 0 rg/l/climate_plants) to discover the mixed effects of global
warming on plant growth. While scientists found that warmer temperatures in the 1980s and 1990s caused an
increase in plant productivity, this advantage has since been counteracted by more frequent droughts.
Present Climate Change
A number of global events have occurred that may be attributed to climate change during our lifetimes. Glacier
National Park in Montana is undergoing the retreat of many of its glaciers, a phenomenon known as glacier
recession. In 1850, the area contained approximately 150 glaciers. By 2010, however, the park contained only
about 24 glaciers greater than 25 acres in size. One of these glaciers is the Grinnell Glacier (Figure 44.30) at
Mount Gould. Between 1966 and 2005, the size of Grinnell Glacier shrank by 40 percent. Similarly, the mass
of the ice sheets in Greenland and the Antarctic is decreasing: Greenland lost 150-250 km 3 of ice per year
between 2002 and 2006. in addition, the size and thickness of the Arctic sea ice is decreasing.
Figure 44.30 The effect of global warming can be seen in the continuing retreat of Grinnel Glacier. The mean annual
temperature in the park has increased 1.33 °C since 1900. The loss of a glacier results in the loss of summer
meltwaters, sharply reducing seasonal water supplies and severely affecting local ecosystems, (credit: modification of
work by USGS)
This loss of ice is leading to increases in the global sea level. On average, the sea is rising at a rate of 1.8 mm
per year. However, between 1993 and 2010 the rate of sea level increase ranged between 2.9 and 3.4 mm per
year. A variety of factors affect the volume of water in the ocean, especially the temperature of the water (the
density of water is related to its temperature: water volume expands as it warms, thus raising sea levels), as well
as the amount of water found in rivers, lakes, glaciers, polar ice caps, and sea ice. As glaciers and polar ice caps
melt, there is a significant contribution of liquid water that was previously frozen.
In addition to some abiotic conditions changing in response to climate change, many organisms are also
being affected by the changes in temperature. Temperature and precipitation play key roles in determining
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the geographic distribution and phenology of plants and animals. ( Phenology is the study of the effects of
climatic conditions on the timing of periodic life cycle events, such as flowering in plants or migration in birds.)
Researchers have shown that 385 plant species in Great Britain are flowering 4.5 days sooner than was
recorded earlier during the previous 40 years. In addition, insect-pollinated species were more likely to flower
earlier than wind-pollinated species. The impact of changes in flowering date would be mitigated if the insect
pollinators emerged earlier. This mismatched timing of plants and pollinators could result in injurious ecosystem
effects because, for continued survival, insect-pollinated plants must flower when their pollinators are present.
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Chapter 44 | Ecology and the Biosphere
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KEY TERMS
abiotic nonliving components of the environment
above-ground biomass total mass of aboveground living plants per area
abyssal zone deepest part of the ocean at depths of 4000 m or greater
algal bloom rapid increase of algae in an aquatic system
aphotic zone part of the ocean where no light penetrates
benthic realm (also, benthic zone) part of the ocean that extends along the ocean bottom from the shoreline to
the deepest parts of the ocean floor
biogeography study of the geographic distribution of living things and the abiotic factors that affect their
distribution
biome ecological community of plants, animals, and other organisms that is adapted to a characteristic set of
environmental conditions
biotic living components of the environment
canopy branches and foliage of trees that form a layer of overhead coverage in a forest
channel width of a river or stream from one bank to the other bank
clathrates frozen chunks of ice and methane found at the bottom of the ocean
climate long-term, predictable atmospheric conditions present in a specific area
conspecifics individuals that are members of the same species
coral reef ocean ridges formed by marine invertebrates living in warm, shallow waters within the photic zone
cryptofauna invertebrates found within the calcium carbonate substrate of coral reefs
ecology study of interaction between living things and their environment
ecosystem services human benefits and services provided by natural ecosystems
emergent vegetation wetland plants that are rooted in the soil but have portions of leaves, stems, and flowers
extending above the water’s surface
endemic species found only in a specific geographic area that is usually restricted in size
estuary biomes where a source of fresh water, such as a river, meets the ocean
fall and spring turnover seasonal process that recycles nutrients and oxygen from the bottom of a freshwater
ecosystem to the top
global climate change altered global weather patterns, including a worldwide increase in temperature, due
largely to rising levels of atmospheric carbon dioxide
greenhouse effect warming of Earth due to carbon dioxide and other greenhouse gases in the atmosphere
greenhouse gases atmospheric gases such as carbon dioxide and methane that absorb and emit radiation,
thus trapping heat in Earth’s atmosphere
haze-effect cooling effect of the gases and solids from a volcanic eruption on global climate
heterospecifics individuals that are members of different species
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intertidal zone part of the ocean that is closest to land; parts extend above the water at low tide
Milankovitch cycles cyclic changes in the Earth's orbit that may affect climate
neritic zone part of the ocean that extends from low tide to the edge of the continental shelf
net primary productivity measurement of the energy accumulation within an ecosystem, calculated as the
total amount of carbon fixed per year minus the amount that is oxidized during cellular respiration
ocean upwelling rising of deep ocean waters that occurs when prevailing winds blow along surface waters near
a coastline
oceanic zone part of the ocean that begins offshore where the water measures 200 m deep or deeper
pelagic realm (also, pelagic zone) open ocean waters that are not close to the bottom or near the shore
permafrost perennially frozen portion of the Arctic tundra soil
photic zone portion of the ocean that light can penetrate
planktivore animal species that eats plankton
predator organism that kills and consumes another organism
Sargassum type of free-floating marine seaweed
solar intensity amount of solar power energy the sun emits in a given amount of time
source water point of origin of a river or stream
thermocline layer of water with a temperature that is significantly different from that of the surrounding layers
weather conditions of the atmosphere during a short period of time
CHAPTER SUMMARY
44.1 The Scope of Ecology
Ecology is the study of the interactions of living things with their environment. Ecologists ask questions that
comprise four levels of general biological organization—organismal, population, community, and ecosystem. At
the organismal level, ecologists study individual organisms and how they interact with their environments. At
the population and community levels, ecologists explore, respectively, how a population of organisms changes
over time and the ways in which that population interacts with other species in the community. Ecologists
studying an ecosystem examine the living species (the biotic components) of the ecosystem as well as the
nonliving portions (the abiotic components), such as air, water, and soil, of the environment.
44.2 Biogeography
Biogeography is the study of the geographic distribution of living things as well as the abiotic factors that affect
their distribution. Endemic species are species that are naturally found only in a specific geographic area. The
distribution of living things is influenced by several environmental factors that are, in part, controlled by the
latitude or elevation at which a species is found. Ocean upwellings, and spring and fall turnovers are important
processes regulating the distribution of nutrients and other abiotic factors important in aquatic ecosystems.
Energy sources, temperature, water, inorganic nutrients, and soil are factors limiting the distribution of living
things in terrestrial systems. Net primary productivity is a measure of the amount of biomass produced by a
biome.
44.3 Terrestrial Biomes
The Earth has terrestrial biomes and aquatic biomes. Aquatic biomes include both freshwater and marine
environments. There are eight major terrestrial biomes: tropical wet forests, savannas, subtropical deserts,
chaparral, temperate grasslands, temperate forests, boreal forests, and Arctic tundra. The same biome can
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Chapter 44 | Ecology and the Biosphere
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occur in different geographic locations with similar climates. Temperature and precipitation, and variations in
both, are key abiotic factors that shape the composition of animal and plant communities in terrestrial biomes.
Some biomes, such as temperate grasslands and temperate forests, have distinct seasons, with cold weather
and hot weather alternating throughout the year. In warm, moist biomes, such as the tropical wet forest, net
primary productivity is high, as warm temperatures, abundant water, and a year-round growing season fuel
plant growth and supply energy for high diversity throughout the food web. Other biomes, such as deserts and
tundras, have low primary productivity due to extreme temperatures and a shortage of available water.
44.4 Aquatic Biomes
Aquatic ecosystems include both saltwater and freshwater biomes. The abiotic factors important for the
structuring of aquatic ecosystems can be different than those seen in terrestrial systems. Sunlight is a driving
force behind the structure of forests and also is an important factor in bodies of water, especially those that are
very deep, because of the role of photosynthesis in sustaining certain organisms.
Density and temperature shape the structure of aquatic systems. Oceans may be thought of as consisting of
different zones based on water depth and distance from the shoreline and light penetrance. Different kinds of
organisms are adapted to the conditions found in each zone. Coral reefs are unique marine ecosystems that
are home to a wide variety of species. Estuaries are found where rivers meet the ocean; their shallow waters
provide nourishment and shelter for young crustaceans, mollusks, fishes, and many other species. Freshwater
biomes include lakes, ponds, rivers, streams, and wetlands. Bogs are an interesting type of wetland
characterized by standing water, lower pH, and a lack of nitrogen.
44.5 Climate and the Effects of Global Climate Change
The Earth has gone through periodic cycles of increases and decreases in temperature. During the past 2,000
years, the Medieval Climate Anomaly was a warmer period, while the Little Ice Age was unusually cool. Both of
these irregularities can be explained by natural causes of changes in climate, and, although the temperature
changes were small, they had significant effects. Natural drivers of climate change include Milankovitch cycles,
changes in solar activity, and volcanic eruptions. None of these factors, however, leads to rapid increases in
global temperature or sustained increases in carbon dioxide.
The burning of fossil fuels is an important source of greenhouse gases, which play a major role in the
greenhouse effect. Two hundred and fifty million years ago, global warming resulted in the Permian extinction:
a large-scale extinction event that is documented in the fossil record. Currently, modern-day climate change is
associated with the increased melting of glaciers and polar ice sheets, resulting in a gradual increase in sea
level. Plants and animals can also be affected by global climate change when the timing of seasonal events,
such as flowering or pollination, is affected by global warming.
VISUAL CONNECTION QUESTIONS
1. Figure 44.10 How might turnover in tropical lakes
differ from turnover in lakes that exist in temperate
regions?
2. Figure 44.12 Which of the following statements
about biomes is false?
a. Chaparral is dominated by shrubs.
b. Savannas and temperate grasslands are
dominated by grasses.
c. Boreal forests are dominated by deciduous
trees.
d. Lichens are common in the arctic tundra.
REVIEW QUESTIONS
4. Which of the following is a biotic factor? 5. The study of nutrient cycling though the
a. wind environment is an example of which of the following?
b. disease-causing microbe
c. temperature
d. soil particle size
3. Figure 44.21 In which of the following regions
would you expect to find photosynthetic organisms?
a. the aphotic zone, the neritic zone, the
oceanic zone, and the benthic realm
b. the photic zone, the intertidal zone, the
neritic zone, and the oceanic zone
c. the photic zone, the abyssal zone, the
neritic zone, and the oceanic zone
d. the pelagic realm, the aphotic zone, the
neritic zone, and the oceanic zone
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a. organismal ecology
b. population ecology
c. community ecology
d. ecosystem ecology
6. Understory plants in a temperate forest have
adaptations to capture limited_.
a. water
b. nutrients
c. heat
d. sunlight
7. An ecologist hiking up a mountain may notice
different biomes along the way due to changes in all
of the following except:
a. elevation
b. rainfall
c. latitude
d. temperature
8 . Which of the following biomes is characterized by
abundant water resources?
a. deserts
b. boreal forests
c. savannas
d. tropical wet forests
9. Which of the following biomes is characterized by
short growing seasons?
a. deserts
b. tropical wet forests
c. Arctic tundras
d. savannas
CRITICAL THINKING QUESTIONS
14. Ecologists often collaborate with other
researchers interested in ecological questions.
Describe the levels of ecology that would be easier
for collaboration because of the similarities of
questions asked. What levels of ecology might be
more difficult for collaboration?
15. The population is an important unit in ecology as
well as other biological sciences. How is a population
defined, and what are the strengths and weaknesses
of this definition? Are there some species that at
certain times or places are not in populations?
16. Compare and contrast ocean upwelling and
spring and fall turnovers.
17. Many endemic species are found in areas that
are geographically isolated. Suggest a plausible
scientific explanation for why this is so.
18. The extremely low precipitation of subtropical
desert biomes might lead one to expect fire to be a
10. Where would you expect to find the most
photosynthesis in an ocean biome?
a. aphotic zone
b. abyssal zone
c. benthic realm
d. intertidal zone
11. A key feature of estuaries is:
a. low light conditions and high productivity
b. salt water and fresh water
c. frequent algal blooms
d. little or no vegetation
12. Which of the following is an example of a weather
event?
a. The hurricane season lasts from June 1
through November 30.
b. The amount of atmospheric CO 2 has
steadily increased during the last century.
c. A windstorm blew down trees in the
Boundary Waters Canoe Area in Minnesota
on July 4, 1999.
d. Deserts are generally dry ecosystems
having very little rainfall.
13. Which of the following natural forces is
responsible for the release of carbon dioxide and
other atmospheric gases?
a. the Milankovitch cycles
b. volcanoes
c. solar intensity
d. burning of fossil fuels
major disturbance factor; however, fire is more
common in the temperate grassland biome than in
the subtropical desert biome. Why is this?
19. In what ways are the subtropical desert and the
arctic tundra similar?
20. Scientists have discovered the bodies of humans
and other living things buried in bogs for hundreds of
years, but not yet decomposed. Suggest a possible
biological explanation for why such bodies are so
well-preserved.
21. Describe the conditions and challenges facing
organisms living in the intertidal zone.
22. Compare and contrast how natural- and human-
induced processes have influenced global climate
change.
23. Predict possible consequences if carbon
emissions from fossil fuels continue to rise.
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Chapter 45 | Population and Community Ecology
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45 | POPULATION AND
COMMUNITY ECOLOGY
Figure 45.1 Asian carp jump out of the water in response to electrofishing. The Asian carp in the inset photograph were
harvested from the Little Calumet River in Illinois in May, 2010, using rotenone, a toxin often used as an insecticide, in
an effort to learn more about the population of the species, (credit main image: modification of work by USGS; credit
inset: modification of work by Lt. David French, USCG)
Chapter Outline
45.1: Population Demography
45.2: Life Histories and Natural Selection
45.3: Environmental Limits to Population Growth
45.4: Population Dynamics and Regulation
45.5: Human Population Growth
45.6: Community Ecology
45.7: Behavioral Biology: Proximate and Ultimate Causes of Behavior
Introduction
Imagine sailing down a river in a small motorboat on a weekend afternoon; the water is smooth and you are
enjoying the warm sunshine and cool breeze when suddenly you are hit in the head by a 20-pound silver carp.
This is now a risk on many rivers and canal systems in Illinois and Missouri because of the presence of Asian
carp.
This fish—actually a group of species including the silver, black, grass, and big head carp—has been farmed
and eaten in China for over 1000 years. It is one of the most important aquaculture food resources worldwide.
In the United States, however, Asian carp is considered a dangerous invasive species that disrupts community
structure and composition to the point of threatening native species.
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Chapter 45 | Population and Community Ecology
45.1 1 Population Demography
By the end of this section, you will be able to do the following:
• Describe how ecologists measure population size and density
• Describe three different patterns of population distribution
• Use life tables to calculate mortality rates
• Describe the three types of survivorship curves and relate them to specific populations
Populations are dynamic entities. Populations consist all of the species living within a specific area, and
populations fluctuate based on a number of factors: seasonal and yearly changes in the environment, natural
disasters such as forest fires and volcanic eruptions, and competition for resources between and within species.
The statistical study of population dynamics, demography, uses a series of mathematical tools to investigate
how populations respond to changes in their biotic and abiotic environments. Many of these tools were originally
designed to study human populations. For example, life tables, which detail the life expectancy of individuals
within a population, were initially developed by life insurance companies to set insurance rates. In fact, while the
term “demographics” is commonly used when discussing humans, all living populations can be studied using
this approach.
Population Size and Density
The study of any population usually begins by determining how many individuals of a particular species
exist, and how closely associated they are with each other. Within a particular habitat, a population can be
characterized by its population size ( N ), the total number of individuals, and its population density, the number
of individuals within a specific area or volume. Population size and density are the two main characteristics used
to describe and understand populations. For example, populations with more individuals may be more stable
than smaller populations based on their genetic variability, and thus their potential to adapt to the environment.
Alternatively, a member of a population with low population density (more spread out in the habitat), might have
more difficulty finding a mate to reproduce compared to a population of higher density. As is shown in Figure
45.2, smaller organisms tend to be more densely distributed than larger organisms.
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Chapter 45 | Population and Community Ecology
1409
CONNECTION
Relationship between Population and Body Mass in Australian Mammals
i, 2.0-
V)
c
0 )
■c
o
3 . 0 -
1 . 0 -
A
0.0
0.0
1.0
2.0
3.0
4.0
5.0
Log mass (grams)
Figure 45.2 Australian mammals show a typical inverse relationship between population density and body size.
As this graph shows, population density typically decreases with increasing body size. Why do you think
this is the case?
Population Research Methods
The most accurate way to determine population size is to simply count all of the individuals within the habitat.
However, this method is often not logistically or economically feasible, especially when studying large habitats.
Thus, scientists usually study populations by sampling a representative portion of each habitat and using this
data to make inferences about the habitat as a whole. A variety of methods can be used to sample populations
to determine their size and density. For immobile organisms such as plants, or for very small and slow-moving
organisms, a quadrat may be used (Figure 45.3). A quadrat is a way of marking off square areas within a
habitat, either by staking out an area with sticks and string, or by the use of a wood, plastic, or metal square
placed on the ground. After setting the quadrats, researchers then count the number of individuals that lie within
their boundaries. Multiple quadrat samples are performed throughout the habitat at several random locations
to estimate the population size and density within the entire habitat. The number and size of quadrat samples
depends on the type of organisms under study and other factors, including the density of the organism. For
example, if sampling daffodils, aim 2 quadrat might be used. With giant redwoods, on the other hand, a larger
quadrat of 100 m 2 might be employed. This ensures that enough individuals of the species are counted to get
an accurate sample that correlates with the habitat, including areas not sampled.
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Chapter 45 | Population and Community Ecology
Figure 45.3 A scientist uses a quadrat to measure population size and density, (credit: NPS Sonoran Desert Network)
For mobile organisms, such as mammals, birds, or fish, scientists use a technique called mark and recapture.
This method involves marking a sample of captured animals in some way (such as tags, bands, paint, or other
body markings), and then releasing them back into the environment to allow them to mix with the rest of the
population. Later, researchers collect a new sample, including some individuals that are marked (recaptures)
and some individuals that are unmarked (Figure 45.4).
(a) (b) (c)
Figure 45.4 Mark and recapture is used to measure the population size of mobile animals such as (a) bighorn sheep,
(b) the California condor, and (c) salmon, (credit a: modification of work by Neal Herbert, NPS; credit b: modification of
work by Pacific Southwest Region USFWS; credit c: modification of work by Ingrid Taylar)
Using the ratio of marked and unmarked individuals, scientists determine how many individuals are in the
sample. From this, calculations are used to estimate the total population size. This method assumes that the
larger the population, the lower the percentage of tagged organisms that will be recaptured since they will have
mixed with more untagged individuals. For example, if 80 deer are captured, tagged, and released into the forest,
and later 100 deer are captured and 20 of them are already marked, we can estimate the population size (/V)
using the following equation:
(number marked fir t catch x total number of second catch) _ ^
number marked second catch
Using our example, the population size would be estimated at 400.
(80x 100)
20
Therefore, there are an estimated 400 total individuals in the original population.
There are some limitations to the mark and recapture method. Some animals from the first catch may learn to
avoid capture in the second round, thus inflating population estimates. Alternatively, some animals may prefer to
be retrapped (especially if a food reward is offered), resulting in an underestimate of population size. Also, some
species may be harmed by the marking technique, reducing their survival. A variety of other techniques have
been developed, including the electronic tracking of animals tagged with radio transmitters and the use of data
from commercial fishing and trapping operations to estimate the size and health of populations and communities.
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Chapter 45 | Population and Community Ecology
1411
Species Distribution
in addition to measuring simple density, further information about a population can be obtained by looking
at the distribution of the individuals. Species dispersion patterns (or distribution patterns) show the spatial
relationship between members of a population within a habitat at a particular point in time. In other words, they
show whether members of the species live close together or far apart, and what patterns are evident when they
are spaced apart.
Individuals in a population can be equally spaced apart, dispersed randomly with no predictable pattern, or
clustered in groups. These are known as uniform, random, and clumped dispersion patterns, respectively
(Figure 45.5). Uniform dispersion is observed in plants that secrete substances inhibiting the growth of nearby
individuals (such as the release of toxic chemicals by the sage plant Salvia leucophylla, a phenomenon called
allelopathy) and in animals like the penguin that maintain a defined territory. An example of random dispersion
occurs with dandelion and other plants that have wind-dispersed seeds that germinate wherever they happen
to fall in a favorable environment. A clumped dispersion may be seen in plants that drop their seeds straight
to the ground, such as oak trees, or in animals that live in groups (schools of fish or herds of elephants).
Clumped dispersions may also be a function of habitat heterogeneity. Thus, the dispersion of the individuals
within a population provides more information about how they interact with each other than does a simple density
measurement. Just as lower density species might have more difficulty finding a mate, solitary species with a
random distribution might have a similar difficulty when compared to social species clumped together in groups.
Uniform Random Clumped
Figure 45.5 Species may have uniform, random, or clumped distribution. Territorial birds such as penguins tend
to have uniform distribution. Plants such as dandelions with wind-dispersed seeds tend to be randomly distributed.
Animals such as elephants that travel in groups exhibit clumped distribution, (credit a: modification of work by Ben
Tubby; credit b: modification of work by Rosendahl; credit c: modification of work by Rebecca Wood)
Demography
While population size and density describe a population at one particular point in time, scientists must use
demography to study the dynamics of a population. Demography is the statistical study of population changes
over time: birth rates, death rates, and life expectancies. Each of these measures, especially birth rates, may
be affected by the population characteristics described above. For example, a large population size results in a
higher birth rate because more potentially reproductive individuals are present. In contrast, a large population
size can also result in a higher death rate because of competition, disease, and the accumulation of waste.
Similarly, a higher population density or a clumped dispersion pattern results in more potential reproductive
encounters between individuals, which can increase birth rate. Lastly, a female-biased sex ratio (the ratio of
males to females) or age structure (the proportion of population members at specific age ranges) composed of
many individuals of reproductive age can increase birth rates.
In addition, the demographic characteristics of a population can influence how the population grows or declines
over time. If birth and death rates are equal, the population remains stable. However, the population size will
increase if birth rates exceed death rates; the population will decrease if birth rates are less than death rates.
Life expectancy is another important factor; the length of time individuals remain in the population impacts local
resources, reproduction, and the overall health of the population. These demographic characteristics are often
displayed in the form of a life table.
Life Tables
Life tables provide important information about the life history of an organism. Life tables divide the population
into age groups and often sexes, and show how long a member of that group is likely to live. They are modeled
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after actuarial tables used by the insurance industry for estimating human life expectancy. Life tables may
include the probability of individuals dying before their next birthday (i.e., their mortality rate), the percentage
of surviving individuals dying at a particular age interval, and their life expectancy at each interval. An example
of a life table is shown in Table 45.1 from a study of Dali mountain sheep, a species native to northwestern
North America. Notice that the population is divided into age intervals (column A). The mortality rate (per 1000),
shown in column D, is based on the number of individuals dying during the age interval (column B) divided by
the number of individuals surviving at the beginning of the interval (Column C), multiplied by 1000.
.. number of individuals dying
mortality rate =---——7-—-— -A—?—
number ol individuals surviving
x 1000
For example, between ages three and four, 12 individuals die out of the 776 that were remaining from the original
1000 sheep. This number is then multiplied by 1000 to get the mortality rate per thousand.
mortality rate = x 1000 « 15.5
As can be seen from the mortality rate data (column D), a high death rate occurred when the sheep were
between 6 and 12 months old, and then increased even more from 8 to 12 years old, after which there were few
survivors. The data indicate that if a sheep in this population were to survive to age one, it could be expected to
live another 7.7 years on average, as shown by the life expectancy numbers in column E.
[i]
Life Table of Dali Mountain Sheep
Age
interval
(years)
Number
dying in age
interval out
of 1000 born
Number surviving
at beginning of
age interval out of
1000 born
Mortality rate
per 1000 alive
at beginning of
age interval
Life expectancy or
mean lifetime
remaining to those
attaining age interval
0-0.5
54
1000
54.0
7.06
0.5-1
145
946
153.3
-
1-2
12
801
15.0
7.7
2-3
13
789
16.5
6.8
3-4
12
776
15.5
5.9
4-5
30
764
39.3
5.0
5-6
46
734
62.7
4.2
6-7
48
688
69.8
3.4
7-8
69
640
107.8
2.6
8-9
132
571
231.2
1.9
9-10
187
439
426.0
1.3
10-11
156
252
619.0
0.9
11-12
90
96
937.5
0.6
12-13
3
6
500.0
1.2
13-14
3
3
1000
0.7
Table 45.1 This life table of Ovis dalli shows the number of deaths, number of survivors, mortality rate,
and life expectancy at each age interval for the Dali mountain sheep.
Survivorship Curves
Another tool used by population ecologists is a survivorship curve, which is a graph of the number of
1. Data Adapted from Edward S. Deevey, Jr., “Life Tables for Natural Populations of Animals,” The Quarterly Review of Biology 22, no. 4
(December 1947): 283-314.
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Chapter 45 | Population and Community Ecology
1413
individuals surviving at each age interval plotted versus time (usually with data compiled from a life table).
These curves allow us to compare the life histories of different populations (Figure 45.6). Humans and most
primates exhibit a Type I survivorship curve because a high percentage of offspring survive their early and middle
years—death occurs predominantly in older individuals. These types of species usually have small numbers of
offspring at one time, and they give a high amount of parental care to them to ensure their survival. Birds are
an example of an intermediate or Type II survivorship curve because birds die more or less equally at each age
interval. These organisms also may have relatively few offspring and provide significant parental care. Trees,
marine invertebrates, and most fishes exhibit a Type III survivorship curve because very few of these organisms
survive their younger years; however, those that make it to an old age are more likely to survive for a relatively
long period of time. Organisms in this category usually have a very large number of offspring, but once they are
born, little parental care is provided. Thus these offspring are “on their own" and vulnerable to predation, but
their sheer numbers assure the survival of enough individuals to perpetuate the species.
Figure 45.6 Survivorship curves show the distribution of individuals in a population according to age. Humans and
most mammals have a Type I survivorship curve because death primarily occurs in the older years. Birds have a Type
II survivorship curve, as death at any age is equally probable. Trees have a Type III survivorship curve because very
few survive the younger years, but after a certain age, individuals are much more likely to survive.
45.2 | Life Histories and Natural Selection
By the end of this section, you will be able to do the following:
• Describe how life history patterns are influenced by natural selection
• Explain different life history patterns and how different reproductive strategies affect species’ survival
A species’ life history describes the series of events over its lifetime, such as how resources are allocated
for growth, maintenance, and reproduction. Life history traits affect the life table of an organism. A species’ life
history is genetically determined and shaped by the environment and natural selection.
Life History Patterns and Energy Budgets
Energy is required by all living organisms for their growth, maintenance, and reproduction; at the same time,
energy is often a major limiting factor in determining an organism’s survival. Plants, for example, acquire energy
from the sun via photosynthesis, but must expend this energy to grow, maintain health, and produce energy-rich
seeds to produce the next generation. Animals have the additional burden of using some of their energy reserves
to acquire food. Furthermore, some animals must expend energy caring for their offspring. Thus, all species
have an energy budget: they must balance energy intake with their use of energy for metabolism, reproduction,
parental care, and energy storage (such as bears building up body fat for winter hibernation).
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Chapter 45 | Population and Community Ecology
Parental Care and Fecundity
Fecundity is the potential reproductive capacity of an individual within a population. In other words, fecundity
describes how many offspring could ideally be produced if an individual has as many offspring as possible,
repeating the reproductive cycle as soon as possible after the birth of the offspring. In animals, fecundity is
inversely related to the amount of parental care given to an individual offspring. Species, such as many marine
invertebrates, that produce many offspring usually provide little if any care for the offspring (they would not have
the energy or the ability to do so anyway). Most of their energy budget is used to produce many tiny offspring.
Animals with this strategy are often self-sufficient at a very early age. This is because of the energy tradeoff
these organisms have made to maximize their evolutionary fitness. Because their energy is used for producing
offspring instead of parental care, it makes sense that these offspring have some ability to be able to move
within their environment and find food and perhaps shelter. Even with these abilities, their small size makes
them extremely vulnerable to predation, so the production of many offspring allows enough of them to survive to
maintain the species.
Animal species that have few offspring during a reproductive event usually give extensive parental care, devoting
much of their energy budget to these activities, sometimes at the expense of their own health. This is the case
with many mammals, such as humans, kangaroos, and pandas. The offspring of these species are relatively
helpless at birth and need to develop before they achieve self-sufficiency.
Plants with low fecundity produce few energy-rich seeds (such as coconuts and chestnuts) with each having a
good chance to germinate into a new organism; plants with high fecundity usually have many small, energy-poor
seeds (like orchids) that have a relatively poor chance of surviving. Although it may seem that coconuts and
chestnuts have a better chance of surviving, the energy tradeoff of the orchid is also very effective. It is a matter
of where the energy is used, for large numbers of seeds or for fewer seeds with more energy.
Early versus Late Reproduction
The timing of reproduction in a life history also affects species survival. Organisms that reproduce at an early
age have a greater chance of producing offspring, but this is usually at the expense of their growth and the
maintenance of their health. Conversely, organisms that start reproducing later in life often have greater fecundity
or are better able to provide parental care, but they risk that they will not survive to reproductive age. Examples
of this can be seen in fishes. Small fish, like guppies, use their energy to reproduce rapidly, but never attain the
size that would give them defense against some predators. Larger fish, like the bluegill or shark, use their energy
to attain a large size, but do so with the risk that they will die before they can reproduce or at least reproduce
to their maximum. These different energy strategies and tradeoffs are key to understanding the evolution of
each species as it maximizes its fitness and fills its niche. In terms of energy budgeting, some species “blow it
all" and use up most of their energy reserves to reproduce early before they die. Other species delay having
reproduction to become stronger, more experienced individuals and to make sure that they are strong enough to
provide parental care if necessary.
Single versus Multiple Reproductive Events
Some life history traits, such as fecundity, timing of reproduction, and parental care, can be grouped together into
general strategies that are used by multiple species. Semelparity occurs when a species reproduces only once
during its lifetime and then dies. Such species use most of their resource budget during a single reproductive
event, sacrificing their health to the point that they do not survive. Examples of semelparity are bamboo, which
flowers once and then dies, and the Chinook salmon (Figure 45.7a), which uses most of its energy reserves to
migrate from the ocean to its freshwater nesting area, where it reproduces and then dies. Scientists have posited
alternate explanations for the evolutionary advantage of the Chinook’s post-reproduction death: a programmed
suicide caused by a massive release of corticosteroid hormones, presumably so the parents can become food
for the offspring, or simple exhaustion caused by the energy demands of reproduction; these are still being
debated.
Iteroparity describes species that reproduce repeatedly during their lives. Some animals are able to mate only
once per year, but survive multiple mating seasons. The pronghorn antelope is an example of an animal that
goes into a seasonal estrus cycle (“heat”): a hormonally induced physiological condition preparing the body for
successful mating (Figure 45.7b). Females of these species mate only during the estrus phase of the cycle. A
different pattern is observed in primates, including humans and chimpanzees, which may attempt reproduction
at any time during their reproductive years, even though their menstrual cycles make pregnancy likely only a few
days per month during ovulation (Figure 45.7c).
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Chapter 45 | Population and Community Ecology
1415
(a) (b) (c)
Figure 45.7 The (a) Chinook salmon mates once and dies. The (b) pronghorn antelope mates during specific times
of the year during its reproductive life. Primates, such as humans and (c) chimpanzees, may mate on any day,
independent of ovulation, (credit a: modification of work by Roger Tabor, USFWS; credit b: modification of work by
Mark Gocke, USDA; credit c: modification of work by “Shiny Things7Flickr)
LINK
T &
LEARNING
Play this interactive PBS evolution-based mating game (http://0penstaxc0llege.0rg/l/mating_game) to
learn more about reproductive strategies.
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Chapter 45 | Population and Community Ecology
V /
e olution CONNECTION
Energy Budgets, Reproductive Costs, and Sexual Selection in
Drosophila
Research into how animals allocate their energy resources for growth, maintenance, and reproduction has
used a variety of experimental animal models. Some of this work has been done using the common fruit
fly, Drosophila melanogaster. Studies have shown that not only does reproduction have a cost as far as
how long male fruit flies live, but also fruit flies that have already mated several times have limited sperm
remaining for reproduction. Fruit flies maximize their last chances at reproduction by selecting optimal
mates.
In a 1981 study, male fruit flies were placed in enclosures with either virgin or inseminated females. The
males that mated with virgin females had shorter life spans than those in contact with the same number
of inseminated females with which they were unable to mate. This effect occurred regardless of how large
(indicative of their age) the males were. Thus, males that did not mate lived longer, allowing them more
opportunities to find mates in the future.
More recent studies, performed in 2006, show how males select the female with which they will mate and
[ 2 ]
how this is affected by previous matings (Figure 45.8). Males were allowed to select between smaller
and larger females. Findings showed that larger females had greater fecundity, producing twice as many
offspring per mating as the smaller females did. Males that had previously mated, and thus had lower
supplies of sperm, were termed “resource-depleted,” while males that had not mated were termed “non-
resource-depleted.” The study showed that although non-resource-depleted males preferentially mated with
larger females, this selection of partners was more pronounced in the resource-depleted males. Thus,
males with depleted sperm supplies, which were limited in the number of times that they could mate before
they replenished their sperm supply, selected larger, more fecund females, thus maximizing their chances
for offspring. This study was one of the first to show that the physiological state of the male affected its
mating behavior in away that clearly maximizes its use of limited reproductive resources.
Ratio large/small females mated
Non sperm-depleted
8 ± 5
Sperm-depleted
15 ±5
Figure 45.8 Male fruit flies that had previously mated (sperm-depleted) picked larger, more fecund females more
often than those that had not mated (non-sperm-depleted). This change in behavior causes an increase in the
efficiency of a limited reproductive resource: sperm.
These studies demonstrate two ways in which the energy budget is a factor in reproduction. First, energy
expended on mating may reduce an animal’s lifespan, but by this time they have already reproduced, so
in the context of natural selection this early death is not of much evolutionary importance. Second, when
resources such as sperm (and the energy needed to replenish it) are low, an organism’s behavior can
change to give them the best chance of passing their genes on to the next generation. These changes in
behavior, so important to evolution, are studied in a discipline known as behavioral biology, or ethology, at
the interface between population biology and psychology.
2. Adapted from Phillip G. Byrne and William R. Rice, “Evidence for adaptive male mate choice in the fruit fly Drosophila melanogaster,"
Proc Biol Sci. 273, no. 1589 (2006): 917-922, doi: 10.1098/rspb.2005.3372.
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Chapter 45 | Population and Community Ecology
1417
45.3 | Environmental Limits to Population Growth
By the end of this section, you will be able to do the following:
• Explain the characteristics of and differences between exponential and logistic growth patterns
• Give examples of exponential and logistic growth in natural populations
• Describe how natural selection and environmental adaptation led to the evolution of particular life history
patterns
Although life histories describe the way many characteristics of a population (such as their age structure) change
over time in a general way, population ecologists make use of a variety of methods to model population dynamics
mathematically. These more precise models can then be used to accurately describe changes occurring in a
population and better predict future changes. Certain long-accepted models are now being modified or even
abandoned due to their lack of predictive ability, and scholars strive to create effective new models.
Exponential Growth
Charles Darwin, in his theory of natural selection, was greatly influenced by the English clergyman Thomas
Malthus. Malthus published a book in 1798 stating that populations with unlimited natural resources grow very
rapidly, and then population growth decreases as resources become depleted. This accelerating pattern of
increasing population size is called exponential growth.
The best example of exponential growth is seen in bacteria. Bacteria reproduce by prokaryotic fission. This
division takes about an hour for many bacterial species. If 1000 bacteria are placed in a large flask with an
unlimited supply of nutrients (so the nutrients will not become depleted), after an hour, there is one round of
division and each organism divides, resulting in 2000 organisms—an increase of 1000. In another hour, each
of the 2000 organisms will double, producing 4000, an increase of 2000 organisms. After the third hour, there
should be 8000 bacteria in the flask, an increase of 4000 organisms. The important concept of exponential
growth is the accelerating population growth rate —the number of organisms added in each reproductive
generation—that is, it is increasing at a greater and greater rate. After 1 day and 24 of these cycles, the
population would have increased from 1000 to more than 16 billion. When the population size, N, is plotted over
time, a J-shaped growth curve is produced (Figure 45.9).
The bacteria example is not representative of the real world where resources are limited. Furthermore, some
bacteria will die during the experiment and thus not reproduce, lowering the growth rate. Therefore, when
calculating the growth rate of a population, the death rate (D) (number organisms that die during a particular
time interval) is subtracted from the birth rate (6) (number organisms that are born during that interval). This is
shown in the following formula:
AN (change in number)
AT (change in time)
= B (birth rate) - D (death rate)
The birth rate is usually expressed on a per capita (for each individual) basis. Thus, B (birth rate) = bN (the per
capita birth rate “b” multiplied by the number of individuals “/V”) and D (death rate) = dN (the per capita death
rate “d” multiplied by the number of individuals “/V"). Additionally, ecologists are interested in the population at a
particular point in time, an infinitely small time interval. For this reason, the terminology of differential calculus is
used to obtain the “instantaneous” growth rate, replacing the change in number and time with an instant-specific
measurement of number and time.
^ = bN - dN=(b - d)N
Notice that the “d" associated with the first term refers to the derivative (as the term is used in calculus) and is
different from the death rate, also called “d.” The difference between birth and death rates is further simplified by
substituting the term “r” (intrinsic rate of increase) for the relationship between birth and death rates:
The value “r” can be positive, meaning the population is increasing in size; or negative, meaning the population
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Chapter 45 | Population and Community Ecology
is decreasing in size; or zero, where the population’s size is unchanging, a condition known as zero population
growth. A further refinement of the formula recognizes that different species have inherent differences in
their intrinsic rate of increase (often thought of as the potential for reproduction), even under ideal conditions.
Obviously, a bacterium can reproduce more rapidly and have a higher intrinsic rate of growth than a human. The
maximal growth rate for a species is its biotic potential, or r m ax, thus changing the equation to:
dN =
dT
r maxN
Exponential Growth
Logistic Growth
Figure 45.9 When resources are unlimited, populations exhibit exponential growth, resulting in a J-shaped curve.
When resources are limited, populations exhibit logistic growth. In logistic growth, population expansion decreases as
resources become scarce, and it levels off when the carrying capacity of the environment is reached, resulting in an
S-shaped curve.
Logistic Growth
Exponential growth is possible only when infinite natural resources are available; this is not the case in the real
world. Charles Darwin recognized this fact in his description of the “struggle for existence," which states that
individuals will compete (with members of their own or other species) for limited resources. The successful ones
will survive to pass on their own characteristics and traits (which we know now are transferred by genes) to
the next generation at a greater rate (natural selection). To model the reality of limited resources, population
ecologists developed the logistic growth model.
Carrying Capacity and the Logistic Model
In the real world, with its limited resources, exponential growth cannot continue indefinitely. Exponential growth
may occur in environments where there are few individuals and plentiful resources, but when the number of
individuals gets large enough, resources will be depleted, slowing the growth rate. Eventually, the growth rate
will plateau or level off (Figure 45.9). This population size, which represents the maximum population size that
a particular environment can support, is called the carrying capacity, or K.
The formula we use to calculate logistic growth adds the carrying capacity as a moderating force in the growth
rate. The expression “K - N ” indicates how many individuals may be added to a population at a given stage, and
“K - N” divided by “K" is the fraction of the carrying capacity available for further growth. Thus, the exponential
growth model is restricted by this factor to generate the logistic growth equation:
dN_ _ dN _
dT ~ ' raax dT ~
<N-
C K - AQ
K
Notice that when N is very small, ( K-N)IK becomes close to KJK or 1, and the right side of the equation reduces
to rmaxN, which means the population is growing exponentially and is not influenced by carrying capacity. On the
other hand, when N is large, ( K-N)IK comes close to zero, which means that population growth will be slowed
greatly or even stopped. Thus, population growth is greatly slowed in large populations by the carrying capacity
K. This model also allows for the population of a negative population growth, or a population decline. This occurs
when the number of individuals in the population exceeds the carrying capacity (because the value of (K-N)/K is
negative).
A graph of this equation yields an S-shaped curve (Figure 45.9), and it is a more realistic model of population
growth than exponential growth. There are three different sections to an S-shaped curve. Initially, growth is
exponential because there are few individuals and ample resources available. Then, as resources begin to
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Chapter 45 | Population and Community Ecology
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become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment,
with little change in population size over time.
Role of Intraspecific Competition
The logistic model assumes that every individual within a population will have equal access to resources and,
thus, an equal chance for survival. For plants, the amount of water, sunlight, nutrients, and the space to grow are
the important resources, whereas in animals, important resources include food, water, shelter, nesting space,
and mates.
In the real world, phenotypic variation among individuals within a population means that some individuals will
be better adapted to their environment than others. The resulting competition between population members of
the same species for resources is termed intraspecific competition (intra- = “within”; -specific = “species").
Intraspecific competition for resources may not affect populations that are well below their carrying
capacity—resources are plentiful and all individuals can obtain what they need. However, as population size
increases, this competition intensifies. In addition, the accumulation of waste products can reduce an
environment’s carrying capacity.
Examples of Logistic Growth
Yeast, a microscopic fungus used to make bread and alcoholic beverages, exhibits the classical S-shaped curve
when grown in a test tube (Figure 45.10a). Its growth levels off as the population depletes the nutrients. In the
real world, however, there are variations to this idealized curve. Examples in wild populations include sheep
and harbor seals (Figure 45.10b). In both examples, the population size exceeds the carrying capacity for short
periods of time and then falls below the carrying capacity afterwards. This fluctuation in population size continues
to occur as the population oscillates around its carrying capacity. Still, even with this oscillation, the logistic model
is confirmed.
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(a)
(b)
Figure 45.10 (a) Yeast grown in ideal conditions in a test tube show a classical S-shaped logistic growth curve,
whereas (b) a natural population of seals shows real-world fluctuation.
If the major food source of the seals declines due to pollution or overfishing, which of the following would
likely occur?
a. The carrying capacity of seals would decrease, as would the seal population.
b. The carrying capacity of seals would decrease, but the seal population would remain the same.
c. The number of seal deaths would increase but the number of births would also increase, so the
population size would remain the same.
d. The carrying capacity of seals would remain the same, but the population of seals would decrease.
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Chapter 45 | Population and Community Ecology
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45.4 | Population Dynamics and Regulation
By the end of this section, you will be able to do the following:
• Give examples of how the carrying capacity of a habitat may change
• Compare and contrast density-dependent growth regulation and density-independent growth regulation,
giving examples
• Give examples of exponential and logistic growth in wild animal populations
• Describe how natural selection and environmental adaptation leads to the evolution of particular life-
history patterns
The logistic model of population growth, while valid in many natural populations and a useful model, is a
simplification of real-world population dynamics. Implicit in the model is that the carrying capacity of the
environment does not change, which is not the case. The carrying capacity varies annually: for example, some
summers are hot and dry whereas others are cold and wet. In many areas, the carrying capacity during the winter
is much lower than it is during the summer. Also, natural events such as earthquakes, volcanoes, and fires can
alter an environment and hence its carrying capacity. Additionally, populations do not usually exist in isolation.
They engage in interspecific competition: that is, they share the environment with other species competing for
the same resources. These factors are also important to understanding how a specific population will grow.
Nature regulates population growth in a variety of ways. These are grouped into density-dependent factors, in
which the density of the population at a given time affects growth rate and mortality, and density-independent
factors, which influence mortality in a population regardless of population density. Note that in the former,
the effect of the factor on the population depends on the density of the population at onset. Conservation
biologists want to understand both types because this helps them manage populations and prevent extinction or
overpopulation.
Density-Dependent Regulation
Most density-dependent factors are biological in nature (biotic), and include predation, inter- and intraspecific
competition, accumulation of waste, and diseases such as those caused by parasites. Usually, the denser
a population is, the greater its mortality rate. For example, during intra- and interspecific competition, the
reproductive rates of the individuals will usually be lower, reducing their population’s rate of growth. In addition,
low prey density increases the mortality of its predator because it has more difficulty locating its food source.
An example of density-dependent regulation is shown in Figure 45.11 with results from a study focusing on
the giant intestinal roundworm (Ascaris lumbricoides), a parasite of humans and other mammals. Denser
populations of the parasite exhibited lower fecundity: they contained fewer eggs. One possible explanation for
this is that females would be smaller in more dense populations (due to limited resources) and that smaller
females would have fewer eggs. This hypothesis was tested and disproved in a 2009 study which showed that
female weight had no influence. The actual cause of the density-dependence of fecundity in this organism is
still unclear and awaiting further investigation.
3. N.A. Croll et al., “The Population Biology and Control of Ascaris lumbricoides in a Rural Community in Iran." Transactions of the Royal
Society of Tropical Medicine and Hygiene 76, no. 2 (1982): 187-197, doi:10.1016/0035-9203(82)90272-3.
4. Martin Walker et al., “Density-Dependent Effects on the Weight of Female Ascaris lumbricoides Infections of Humans and its Impact on
Patterns of Egg Production." Parasites & Vectors 2, no. 11 (February 2009), doi:10.1186/1756-3305-2-11.
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Chapter 45 | Population and Community Ecology
[5]
Figure 45.11 In this population of roundworms, fecundity (number of eggs) decreases with population density.
Density-Independent Regulation and Interaction with Density-
Dependent Factors
Many factors, typically physical or chemical in nature (abiotic), influence the mortality of a population regardless
of its density, including weather, natural disasters, and pollution. An individual deer may be killed in a forest
fire regardless of how many deer happen to be in that area. Its chances of survival are the same whether the
population density is high or low. The same holds true for cold winter weather.
In real-life situations, population regulation is very complicated and density-dependent and independent factors
can interact. A dense population that is reduced in a density-independent manner by some environmental
factor(s) will be able to recover differently than a sparse population. For example, a population of deer affected
by a harsh winter will recover faster if there are more deer remaining to reproduce.
5. N.A. Croll et at, “The Population Biology and Control of Ascaris lumbricoides in a Rural Community in Iran." Transactions of the Royal
Society of Tropical Medicine and Hygiene 76, no. 2 (1982): 187-197, doi:10.1016/0035-9203(82)90272-3.
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Chapter 45 | Population and Community Ecology
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V /
e olution CONNECTION
Why Did the Woolly Mammoth Go Extinct?
Figure 45.12 The three photos include: (a) 1916 mural of a mammoth herd from the American Museum of Natural
History, (b) the only stuffed mammoth in the world, from the Museum of Zoology located in St. Petersburg, Russia,
and (c) a one-month-old baby mammoth, named Lyuba, discovered in Siberia in 2007. (credit a: modification of
work by Charles R. Knight; credit b: modification of work by “Tanapon'VFlickr; credit c: modification of work by Matt
Howry)
It's easy to get lost in the discussion about why dinosaurs went extinct 65 million years ago. Was it due to a
meteor slamming into Earth near the coast of modern-day Mexico, or was it from some long-term weather
cycle that is not yet understood? Scientists are continually exploring these and other theories.
Woolly mammoths began to go extinct much more recently, when they shared the Earth with humans who
were no different anatomically than humans today (Figure 45.12). Mammoths survived in isolated island
populations as recently as 1700 BC. We know a lot about these animals from carcasses found frozen in the
ice of Siberia and other regions of the north. Scientists have sequenced at least 50 percent of its genome
and believe mammoths are between 98 and 99 percent identical to modern elephants.
It is commonly thought that climate change and human hunting led to their extinction. A 2008 study
estimated that climate change reduced the mammoth’s range from 3,000,000 square miles 42,000 years
[ 6 ]
ago to 310,000 square miles 6,000 years ago. It is also well documented that humans hunted these
animals. A 2012 study showed that no single factor was exclusively responsible for the extinction of these
magnificent creatures. In addition to human hunting, climate change, and reduction of habitat, these
scientists demonstrated another important factor in the mammoth’s extinction was the migration of humans
across the Bering Strait to North America during the last ice age 20,000 years ago.
The maintenance of stable populations was and is very complex, with many interacting factors determining
the outcome. It is important to remember that humans are also part of nature. We once contributed to a
species’ decline using only primitive hunting technology.
Life Histories of /(-selected and r-selected Species
While reproductive strategies play a key role in life histories, they do not account for important factors like limited
resources and competition. The regulation of population growth by these factors can be used to introduce a
classical concept in population biology, that of K-selected versus r-selected species.
The concept relates to a species’ reproductive strategies, habitat, and behavior, especially in the way that
they obtain resources and care for their young. It includes length of life and survivorship factors as well.
Population biologists have grouped species into the two large categories—K-selected and r-selected—although
the categories are really two ends of a continuum.
K-selected species are species selected by stable, predictable environments. Populations of K-selected
species tend to exist close to their carrying capacity (hence the term K-selected) where intraspecific competition
is high. These species have few, large offspring, a long gestation period, and often give long-term care to their
offspring (Table 45.2). While larger in size when born, the offspring are relatively helpless and immature at birth.
6. David Nogues-Bravo et al., "Climate Change, Humans, and the Extinction of the Woolly Mammoth." PLoS Biol 6 (April 2008): e79,
doi: 10.1371/journal, pbio.0060079.
7. G.M. MacDonald et al., “Pattern of Extinction of the Woolly Mammoth in Beringia." Nature Communications 3, no. 893 (June 2012),
doi:10.1038/ncommsl881.
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By the time they reach adulthood, they must develop skills to compete for natural resources. In plants, scientists
think of parental care more broadly: how long fruit takes to develop or how long it remains on the plant are
determining factors in the time to the next reproductive event. Examples of K-selected species are primates
(including humans), elephants, and plants such as oak trees (Figure 45.13a).
Oak trees grow very slowly and take, on average, 20 years to produce their first seeds, known as acorns. As
many as 50,000 acorns can be produced by an individual tree, but the germination rate is low as many of these
rot or are eaten by animals such as squirrels. In some years, oaks may produce an exceptionally large number
of acorns, and these years may be on a two- or three-year cycle depending on the species of oak (/"-selection).
As oak trees grow to a large size and for many years before they begin to produce acorns, they devote a
large percentage of their energy budget to growth and maintenance. The tree’s height and size allow it to
dominate other plants in the competition for sunlight, the oak’s primary energy resource. Furthermore, when it
does reproduce, the oak produces large, energy-rich seeds that use their energy reserve to become quickly
established (K-selection).
In contrast, /"-selected species have a large number of small offspring (hence their r designation (Table 45.2)).
This strategy is often employed in unpredictable or changing environments. Animals that are r-selected do
not give long-term parental care and the offspring are relatively mature and self-sufficient at birth. Examples
of r-selected species are marine invertebrates, such as jellyfish, and plants, such as the dandelion (Figure
45.13b). Dandelions have small seeds that are wind dispersed long distances. Many seeds are produced
simultaneously to ensure that at least some of them reach a hospitable environment. Seeds that land in
inhospitable environments have little chance for survival since their seeds are low in energy content. Note that
survival is not necessarily a function of energy stored in the seed itself.
Characteristics of /(-selected and r-selected species
Characteristics of /C-selected species
Characteristics of r-selected species
Mature late
Mature early
Greater longevity
Lower longevity
Increased parental care
Decreased parental care
Increased competition
Decreased competition
Fewer offspring
More offspring
Larger offspring
Smaller offspring
Table 45.2
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Chapter 45 | Population and Community Ecology
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(a) K-selected species
(b) r-selected species
Figure 45.13 (a) Elephants are considered K-selected species as they live long, mature late, and provide long-term
parental care to few offspring. Oak trees produce many offspring that do not receive parental care, but are considered
K-selected species based on longevity and late maturation, (b) Dandelions and jellyfish are both considered r-selected
species as they mature early, have short lifespans, and produce many offspring that receive no parental care.
Modern Theories of Life History
By the second half of the twentieth century, the concept of K- and r-selected species was used extensively and
successfully to study populations. The r- and K-selection theory, although accepted for decades and used for
much groundbreaking research, has now been reconsidered, and many population biologists have abandoned
or modified it. Over the years, several studies attempted to confirm the theory, but these attempts have largely
failed. Many species were identified that did not follow the theory’s predictions. Furthermore, the theory ignored
the age-specific mortality of the populations which scientists now know is very important. New demographic-
based models of life history evolution have been developed which incorporate many ecological concepts
included in r- and K-selection theory as well as population age structure and mortality factors.
45.5 | Human Population Growth
By the end of this section, you will be able to do the following:
• Discuss exponential human population growth
• Explain how humans have expanded the carrying capacity of their habitat
• Relate population growth and age structure to the level of economic development in different countries
• Discuss the long-term implications of unchecked human population growth
Population dynamics can be applied to human population growth. Earth’s human population is growing rapidly,
to the extent that some worry about the ability of the earth’s environment to sustain this population. Long-term
exponential growth carries the potential risks of famine, disease, and large-scale death.
Although humans have increased the carrying capacity of their environment, the technologies used to achieve
this transformation have caused unprecedented changes to Earth’s environment, altering ecosystems to the
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point where some may be in danger of collapse. The depletion of the ozone layer, erosion due to acid rain,
and damage from global climate change are caused by human activities. The ultimate effect of these changes
on our carrying capacity is unknown. As some point out, it is likely that the negative effects of increasing
carrying capacity will outweigh the positive ones—the world’s carrying capacity for human beings might actually
decrease.
The human population is currently experiencing exponential growth even though human reproduction is far
below its biotic potential (Figure 45.14). To reach its biotic potential, all females would have to become pregnant
every nine months or so during their reproductive years. Also, resources would have to be such that the
environment would support such growth. Neither of these two conditions exists. In spite of this fact, human
population is still growing exponentially.
Figure 45.14 Human population growth since 1000 AD is exponential (dark blue line). Notice that while the population
in Asia (yellow line), which has many economically underdeveloped countries, is increasing exponentially, the
population in Europe (light blue line), where most of the countries are economically developed, is growing much more
slowly.
A consequence of exponential human population growth is a reduction in time that it takes to add a particular
number of humans to the Earth. Figure 45.15 shows that 123 years were necessary to add 1 billion humans
in 1930, but it only took 24 years to add two billion people between 1975 and 1999. As already discussed,
our ability to increase our carrying capacity indefinitely my be limited. Without new technological advances, the
human growth rate has been predicted to slow in the coming decades. However, the population will still be
increasing and the threat of overpopulation remains.
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Chapter 45 | Population and Community Ecology
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Time between Billions in World Population Growth
1 billion: 1800
2 billion: 1930
3 billion: 1960
4 billion: 1975
5 billion: 1987
6 billion: 1999
7 billion: 2012
8 billion: 2028
9 billion: 2054
Figure 45.15 The time between the addition of each billion human beings to Earth decreases over time, (credit:
modification of work by Ryan T. Cragun)
0 20 40 60 80 100 120 140
Years
Source: Population Reference Bureau
LINK TQ LEARNING
Click through this interactive view (http:// 0 penstaxc 0 llege. 0 rg/l/human growth) of how human populations
have changed over time.
Overcoming Density-Dependent Regulation
Humans are unique in their ability to alter their environment with the conscious purpose of increasing carrying
capacity. This ability is a major factor responsible for human population growth and a way of overcoming density-
dependent growth regulation. Much of this ability is related to human intelligence, society, and communication.
Humans can construct shelter to protect them from the elements and have developed agriculture and
domesticated animals to increase their food supplies. In addition, humans use language to communicate this
technology to new generations, allowing them to improve upon previous accomplishments.
Other factors in human population growth are migration and public health. Humans originated in Africa, but have
since migrated to nearly all inhabitable land on the Earth. Public health, sanitation, and the use of antibiotics and
vaccines have decreased the ability of infectious disease to limit human population growth. In the past, diseases
such as the bubonic plaque of the fourteenth century killed between 30 and 60 percent of Europe’s population
and reduced the overall world population by as many as 100 million people. Today, the threat of infectious
disease, while not gone, is certainly less severe. According to the World Health Organization, global death
from infectious disease declined from 16.4 million in 1993 to 14.7 million in 1992. To compare to some of the
epidemics of the past, the percentage of the world's population killed between 1993 and 2002 decreased from
0.30 percent of the world's population to 0.24 percent. Thus, infectious disease influence on human population
growth is becoming less significant.
Age Structure, Population Growth, and Economic Development
The age structure of a population is an important factor in population dynamics. Age structure is the proportion
of a population at different age ranges. Age structure allows better prediction of population growth, plus the
ability to associate this growth with the level of economic development in the region. Countries with rapid growth
have a pyramidal shape in their age structure diagrams, showing a preponderance of younger individuals,
many of whom are of reproductive age or will be soon (Figure 45.16). This pattern is most often observed in
underdeveloped countries where individuals do not live to old age because of less-than-optimal living conditions.
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Age structures of areas with slow growth, including developed countries such as the United States, still have a
pyramidal structure, but with many fewer young and reproductive-aged individuals and a greater proportion of
older individuals. Other developed countries, such as Italy, have zero population growth. The age structure of
these populations is more conical, with an even greater percentage of middle-aged and older individuals. The
actual growth rates in different countries are shown in Figure 45.17, with the highest rates tending to be in the
less economically developed countries of Africa and Asia.
visual
CONNECTION
Age Structure Diagrams
<u
Male
<
Female
Stage 1: Rapid growth Stage 2: Slow growth
Stage 3
: Stable
Stage 4: ?
Figure 45.16 Typical age structure diagrams are shown. The rapid growth diagram narrows to a point, indicating
that the number of individuals decreases rapidly with age. In the slow growth model, the number of individuals
decreases steadily with age. Stable population diagrams are rounded on the top, showing that the number of
individuals per age group decreases gradually, and then increases for the older part of the population.
Age structure diagrams for rapidly growing, slow growing, and stable populations are shown in stages 1
through 3. What type of population change do you think stage 4 represents?
Percent Growth in Population
■ -0% ■ 0%-l% ■ 1% □ 2% □ 3%+
Figure 45.17 The percent growth rate of population in different countries is shown. Notice that the highest growth is
occurring in less economically developed countries in Africa and Asia.
Long-Term Consequences of Exponential Human Population Growth
Many dire predictions have been made about the world’s population leading to a major crisis called the
“population explosion.” In the 1968 book The Population Bomb , biologist Dr. Paul R. Ehrlich wrote, “The battle
to feed all of humanity is over. In the 1970s hundreds of millions of people will starve to death in spite of any
crash programs embarked upon now. At this late date nothing can prevent a substantial increase in the world
death rate.” While many experts view this statement as incorrect based on evidence, the laws of exponential
population growth are still in effect, and unchecked human population growth cannot continue indefinitely.
Several nations have instituted policies aimed at influencing population. Efforts to control population growth led
8. Paul R. Erlich, prologue to The Population Bomb, (1968; repr., New York: Ballantine, 1970).
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Chapter 45 | Population and Community Ecology
1429
to the one-child policy in China, which is now being phased out. India also implements national and regional
populations to encourage family planning. On the other hand, Japan, Spain, Russia, Iran, and other countries
have made efforts to increase population growth after birth rates dipped. Such policies are controversial, and the
human population continues to grow. At some point the food supply may run out, but the outcomes are difficult
to predict. The United Nations estimates that future world population growth may vary from 6 billion (a decrease)
to 16 billion people by the year 2100.
Another result of population growth is the endangerment of the natural environment. Many countries have
attempted to reduce the human impact on climate change by reducing their emission of the greenhouse gas
carbon dioxide. However, these treaties have not been ratified by every country. The role of human activity in
causing climate change has become a hotly debated socio-political issue in some countries, including the United
States. Thus, we enter the future with considerable uncertainty about our ability to curb human population growth
and protect our environment.
LINK TQ LEARNING
Visit this website (http://openstaxcollege.Org/l/populations) and select “Launch movie” for an animation
discussing the global impacts of human population growth.
45.6 | Community Ecology
By the end of this section, you will be able to do the following:
• Discuss the predator-prey cycle
• Give examples of defenses against predation and herbivory
• Describe the competitive exclusion principle
• Give examples of symbiotic relationships between species
• Describe community structure and succession
Populations rarely, if ever, live in isolation from populations of other species. In most cases, numerous species
share a habitat. The interactions between these populations play a major role in regulating population growth
and abundance. All populations occupying the same habitat form a community: populations inhabiting a specific
area at the same time. The number of species occupying the same habitat and their relative abundance is known
as species diversity. Areas with low diversity, such as the glaciers of Antarctica, still contain a wide variety of
living things, whereas the diversity of tropical rainforests is so great that it cannot be counted. Ecology is studied
at the community level to understand how species interact with each other and compete for the same resources.
Predation and Herbivory
Perhaps the classical example of species interaction is predation: the consumption of prey by its predator. Nature
shows on television highlight the drama of one living organism killing another. Populations of predators and prey
in a community are not constant over time: in most cases, they vary in cycles that appear to be related. The most
often cited example of predator-prey dynamics is seen in the cycling of the lynx (predator) and the snowshoe
hare (prey), using nearly 200 year-old trapping data from North American forests (Figure 45.18). This cycle of
predator and prey lasts approximately 10 years, with the predator population lagging 1-2 years behind that of
the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx
population to increase as well. When the lynx population grows to a threshold level, however, they kill so many
hares that hare population begins to decline, followed by a decline in the lynx population because of scarcity of
food. When the lynx population is low, the hare population size begins to increase due, at least in part, to low
predation pressure, starting the cycle anew.
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Predator-prey Dynamics
Time (years)
Figure 45.18 The cycling of lynx and snowshoe hare populations in Northern Ontario is an example of predator-prey
dynamics.
Some researchers question the idea that predation models entirely control the population cycling of the two
species. More recent studies have pointed to undefined density-dependent factors as being important in the
cycling, in addition to predation. One possibility is that the cycling is inherent in the hare population due
to density-dependent effects such as lower fecundity (maternal stress) caused by crowding when the hare
population gets too dense. The hare cycling would then induce the cycling of the lynx because it is the lynxes’
major food source. The more we study communities, the more complexities we find, allowing ecologists to derive
more accurate and sophisticated models of population dynamics.
Herbivory describes the consumption of plants by insects and other animals, and it is another interspecific
relationship that affects populations. Unlike animals, most plants cannot outrun predators or use mimicry to hide
from hungry animals. Some plants have developed mechanisms to defend against herbivory. Other species have
developed mutualistic relationships; for example, herbivory provides a mechanism of seed distribution that aids
in plant reproduction.
Defense Mechanisms against Predation and Herbivory
The study of communities must consider evolutionary forces that act on the members of the various populations
contained within it. Species are not static, but slowly changing and adapting to their environment by natural
selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and
herbivory. These defenses may be mechanical, chemical, physical, or behavioral.
Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal
predation and herbivory by causing physical pain to the predator or by physically preventing the predator from
being able to eat the prey. Chemical defenses are produced by many animals as well as plants, such as the
foxglove which is extremely toxic when eaten. Figure 45.19 shows some organisms’ defenses against predation
and herbivory.
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Chapter 45 | Population and Community Ecology
1431
Figure 45.19 The (a) honey locust tree (Gteditsia triacanthos) uses thorns, a mechanical defense, against herbivores,
while the (b) Florida red-bellied turtle (Pseudemys nelsoni) uses its shell as a mechanical defense against predators,
(c) Foxglove (Digitalis sp.) uses a chemical defense: toxins produced by the plant can cause nausea, vomiting,
hallucinations, convulsions, or death when consumed, (d) The North American millipede (Narceus americanus) uses
both mechanical and chemical defenses: when threatened, the millipede curls into a defensive ball and produces a
noxious substance that irritates eyes and skin, (credit a: modification of work by Huw Williams; credit b: modification
of work by “JamieS937Flickr; credit c: modification of work by Philip Jagenstedt; credit d: modification of work by Cory
Zanker)
Many species use their body shape and coloration to avoid being detected by predators. The tropical walking
stick is an insect with the coloration and body shape of a twig which makes it very hard to see when stationary
against a background of real twigs (Figure 45.20a). In another example, the chameleon can change its color to
match its surroundings (Figure 45.20b). Both of these are examples of camouflage, or avoiding detection by
blending in with the background.
(a) (b)
Figure 45.20 (a) The tropical walking stick and (b) the chameleon use body shape and/or coloration to prevent
detection by predators, (credit a: modification of work by Linda Tanner; credit b: modification of work by Frank Vassen)
Some species use coloration as a way of warning predators that they are not good to eat. For example, the
cinnabar moth caterpillar, the fire-bellied toad, and many species of beetle have bright colors that warn of a foul
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taste, the presence of toxic chemicals, and/or the ability to sting or bite, respectively. Predators that ignore this
coloration and eat the organisms will experience their unpleasant taste or presence of toxic chemicals and learn
not to eat them in the future. This type of defensive mechanism is called aposematic coloration, or warning
coloration (Figure 45.21).
Figure 45.21 (a) The strawberry poison dart frog (Oophaga pumilio) uses aposematic coloration to warn predators
that it is toxic, while the (b) striped skunk (Mephitis mephitis) uses aposematic coloration to warn predators of the
unpleasant odor it produces, (credit a: modification of work by Jay Iwasaki; credit b: modification of work by Dan
Dzurisin)
While some predators learn to avoid eating certain potential prey because of their coloration, other species have
evolved mechanisms to mimic this coloration to avoid being eaten, even though they themselves may not be
unpleasant to eat or contain toxic chemicals. In Batesian mimicry, a harmless species imitates the warning
coloration of a harmful one. Assuming they share the same predators, this coloration then protects the harmless
ones, even though they do not have the same level of physical or chemical defenses against predation as the
organism they mimic. Many insect species mimic the coloration of wasps or bees, which are stinging, venomous
insects, thereby discouraging predation (Figure 45.22).
Figure 45.22 Batesian mimicry occurs when a harmless species mimics the coloration of a harmful species, as is seen
with the (a) bumblebee and (b) bee-like robber fly. (credit a, b: modification of work by Cory Zanker)
In Mullerian mimicry, multiple species share the same warning coloration, but all of them actually have
defenses. Figure 45.23 shows a variety of foul-tasting butterflies with similar coloration. In Emsleyan/
Mertensian mimicry, a deadly prey mimics a less dangerous one, such as the venomous coral snake mimicking
the nonvenomous milk snake. This type of mimicry is extremely rare and more difficult to understand than the
previous two types. For this type of mimicry to work, it is essential that eating the milk snake has unpleasant but
not fatal consequences. Then, these predators learn not to eat snakes with this coloration, protecting the coral
snake as well. If the snake were fatal to the predator, there would be no opportunity for the predator to learn not
to eat it, and the benefit for the less toxic species would disappear.
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Figure 45.23 Several unpleasant-tasting Heliconius butterfly species share a similar color pattern with better-tasting
varieties, an example of Mullerian mimicry, (credit: Joron M, Papa R, Beltran M, Chamberlain N, Mavarez J, et al.)
LINK TQ LEARNING
Go to this website (http:// 0 penstaxc 0 llege. 0 rg/l/find_the_mimic) to view stunning examples of mimicry.
Competitive Exclusion Principle
Resources are often limited within a habitat and multiple species may compete to obtain them. All species have
an ecological niche in the ecosystem, which describes how they acquire the resources they need and how they
interact with other species in the community. The competitive exclusion principle states that two species
cannot occupy the same niche in a habitat. In other words, different species cannot coexist in a community if
they are competing for all the same resources. An example of this principle is shown in Figure 45.24, with two
protozoan species, Paramecium aurelia and Paramecium caudatum. When grown individually in the laboratory,
they both thrive. But when they are placed together in the same test tube (habitat), P. aurelia outcompetes P.
caudatum for food, leading to the latter’s eventual extinction.
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P. caudatum alone
(b)
0 5 10 15 20
Time (days)
(C)
Figure 45.24 Paramecium aurelia and Paramecium caudatum grow well individually, but when they compete for the
same resources, the P. aurelia outcompetes the P. caudatum.
This exclusion may be avoided if a population evolves to make use of a different resource, a different area of the
habitat, or feeds during a different time of day, called resource partitioning. The two organisms are then said to
occupy different microniches. These organisms coexist by minimizing direct competition.
Symbiosis
Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species
over an extended period of time which impact the abundance and distribution of the associating populations.
Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where
both individuals benefit from the interaction. In this discussion, the broader definition will be used.
Commensalism
A commensal relationship occurs when one species benefits from the close, prolonged interaction, while the
other neither benefits nor is harmed. Birds nesting in trees provide an example of a commensal relationship
(Figure 45.25). The tree is not harmed by the presence of the nest among its branches. The nests are light
and produce little strain on the structural integrity of the branch, and most of the leaves, which the tree uses to
get energy by photosynthesis, are above the nest so they are unaffected. The bird, on the other hand, benefits
greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators. Another example
of a commensal relationship is the clown fish and the sea anemone. The sea anemone is not harmed by the fish,
and the fish benefits with protection from predators who would be stung upon nearing the sea anemone.
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Figure 45.25 The southern masked-weaver bird is starting to make a nest in a tree in Zambezi Valley, Zambia. This
is an example of a commensal relationship, in which one species (the bird) benefits, while the other (the tree) neither
benefits nor is harmed, (credit: “Hanay’VWikimedia Commons)
Mutualism
A second type of symbiotic relationship is called mutualism, where two species benefit from their interaction.
Some scientists believe that these are the only true examples of symbiosis. For example, termites have a
mutualistic relationship with protozoa that live in the insect’s gut (Figure 45.26a). The termite benefits from the
ability of bacterial symbionts within the protozoa to digest cellulose. The termite itself cannot do this, and without
the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats).
The protozoa and the bacterial symbionts benefit by having a protective environment and a constant supply of
food from the wood chewing actions of the termite. Lichens have a mutualistic relationship between fungus and
photosynthetic algae or bacteria (Figure 45.26b). As these symbionts grow together, the glucose produced by
the algae provides nourishment for both organisms, whereas the physical structure of the lichen protects the
algae from the elements and makes certain nutrients in the atmosphere more available to the algae.
(a) (b)
Figure 45.26 (a) Termites form a mutualistic relationship with symbiotic protozoa in their guts, which allow both
organisms to obtain energy from the cellulose the termite consumes, (b) Lichen is a fungus that has symbiotic
photosynthetic algae living inside its cells, (credit a: modification of work by Scott Bauer, USDA; credit b: modification
of work by Cory Zanker)
Parasitism
A parasite is an organism that lives in or on another living organism and derives nutrients from it. In this
relationship, the parasite benefits, but the host is harmed. The host is usually weakened by the parasite as it
siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the
host, especially not quickly, because this would allow no time for the organism to complete its reproductive cycle
by spreading to another host.
The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species.
A tapeworm is a parasite that causes disease in humans when contaminated, undercooked meat is consumed
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(Figure 45.27). The tapeworm can live inside the intestine of the host for several years, benefiting from the food
the host is eating, and may grow to be over 50 ft long by adding segments. The parasite moves from species to
species in a cycle, making two hosts necessary to complete its life cycle.
Another common parasite is Plasmodium falciparum, the protozoan cause of malaria, a significant disease in
many parts of the world. Living in human liver and red blood cells, the organism reproduces asexually in the
gut of blood-feeding mosquitoes to complete its life cycle. Thus malaria is spread from human to human by
mosquitoes, one of many arthropod-borne infectious diseases.
Tapeworm (Taenia) Infection
Q Tapeworm embryos
hatch, penetrate the
intestinal wall,
and circulate
to musculature
in pigs or humans.
Embryos develop into
larvae in muscles of
pigs or humans.
Q Eggs or segments
are ingested by pigs
or humans.
Figure 45.27 This diagram shows the life cycle of a pork tapeworm (Taenia solium), a human worm parasite, (credit:
modification of work by CDC)
Characteristics of Communities
Communities are complex entities that can be characterized by their structure (the types and numbers of species
present) and dynamics (how communities change over time). Understanding community structure and dynamics
enables community ecologists to manage ecosystems more effectively.
Foundation Species
Foundation species are considered the “base" or “bedrock” of a community, having the greatest influence on
its overall structure. They are usually the primary producers: organisms that bring most of the energy into the
community. Kelp, or brown algae, is a foundation species, forming the basis of the kelp forests off the coast of
California.
Foundation species may physically modify the environment to produce and maintain habitats that benefit the
other organisms that use them. An example is the photosynthetic corals of the coral reef (Figure 45.28).
Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called
zooxanthellae) that perform photosynthesis; this is another example of a mutualism. The exoskeletons of living
and dead coral make up most of the reef structure, which protects many other species from waves and ocean
currents.
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Figure 45.28 Coral is the foundation species of coral reef ecosystems, (credit: Jim E. Maragos, USFWS)
Biodiversity, Species Richness, and Relative Species Abundance
Biodiversity describes a community’s biological complexity: it is measured by the number of different species
(species richness) in a particular area and their relative abundance (species evenness). The area in question
could be a habitat, a biome, or the entire biosphere. Species richness is the term that is used to describe the
number of species living in a habitat or biome. Species richness varies across the globe (Figure 45.29). One
factor in determining species richness is latitude, with the greatest species richness occurring in ecosystems
near the equator, which often have warmer temperatures, large amounts of rainfall, and low seasonality. The
lowest species richness occurs near the poles, which are much colder, drier, and thus less conducive to life in
Geologic time (time since glaciations). The predictability of climate or productivity is also an important factor.
Other factors influence species richness as well. For example, the study of island biogeography attempts to
explain the relatively high species richness found in certain isolated island chains, including the Galapagos
Islands that inspired the young Darwin. Relative species abundance is the number of individuals in a species
relative to the total number of individuals in all species within a habitat, ecosystem, or biome. Foundation species
often have the highest relative abundance of species.
Figure 45.29 The greatest species richness for mammals in North and South America is associated with the equatorial
latitudes, (credit: modification of work by NASA, CIESIN, Columbia University)
Keystone Species
A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to
upholding an ecological community’s structure. The intertidal sea star, Pisaster ochraceus, of the northwestern
United States is a keystone species (Figure 45.30). Studies have shown that when this organism is removed
from communities, populations of their natural prey (mussels) increase, completely altering the species
composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams,
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which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. If these
fish were to become extinct, the community would be greatly affected.
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everyday CONNECTION
Invasive Species
Invasive species are nonnative organisms that, when introduced to an area out of their native range,
threaten the ecosystem balance of that habitat. Many such species exist in the United States, as shown in
Figure 45.31. Whether enjoying a forest hike, taking a summer boat trip, or simply walking down an urban
street, you have likely encountered an invasive species.
(d) (e) (f)
Figure 45.31 In the United States, invasive species like (a) purple loosestrife ( Lythrum salicaria) and the (b) zebra
mussel ( Dreissena polymorpha) threaten certain aquatic ecosystems. Some forests are threatened by the spread
of (c) common buckthorn ( Rhamnus cathartica), (d) garlic mustard ( Alliaria petiolata), and (e) the emerald ash
borer ( Agrilus planipennis). The (f) European starling ( Sturnus vulgaris) may compete with native bird species for
nest holes, (credit a: modification of work by Liz West; credit b: modification of work by M. McCormick, NOAA;
credit c: modification of work by E. Dronkert; credit d: modification of work by Dan Davison; credit e: modification
of work by USDA; credit f: modification of work by Don DeBold)
One of the many recent proliferations of an invasive species concerns the growth of Asian carp populations.
Asian carp were introduced to the United States in the 1970s by fisheries and sewage treatment facilities
that used the fish’s excellent filter feeding capabilities to clean their ponds of excess plankton. Some of
the fish escaped, however, and by the 1980s they had colonized many waterways of the Mississippi River
basin, including the Illinois and Missouri Rivers.
Voracious eaters and rapid reproducers, Asian carp may outcompete native species for food, potentially
leading to their extinction. For example, black carp are voracious eaters of native mussels and snails,
limiting this food source for native fish species. Silver carp eat plankton that native mussels and snails
feed on, reducing this food source by a different alteration of the food web. In some areas of the
Mississippi River, Asian carp species have become the most predominant, effectively outcompeting native
fishes for habitat. In some parts of the Illinois River, Asian carp constitute 95 percent of the community's
biomass. Although edible, the fish is bony and not a desired food in the United States. Moreover, their
presence threatens the native fish and fisheries of the Great Lakes, which are important to local economies
and recreational anglers. Asian carp have even injured humans. The fish, frightened by the sound of
approaching motorboats, thrust themselves into the air, often landing in the boat or directly hitting the
boaters.
The Great Lakes and their prized salmon and lake trout fisheries are also being threatened by these invasive
fish. Asian carp have already colonized rivers and canals that lead into Lake Michigan. One infested
waterway of particular importance is the Chicago Sanitary and Ship Channel, the major supply waterway
linking the Great Lakes to the Mississippi River. To prevent the Asian carp from leaving the canal, a series of
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electric barriers have been successfully used to discourage their migration; however, the threat is significant
enough that several states and Canada have sued to have the Chicago channel permanently cut off from
Lake Michigan. Local and national politicians have weighed in on how to solve the problem, but no one
knows whether the Asian carp will ultimately be considered a nuisance, like other invasive species such as
the water hyacinth and zebra mussel, or whether it will be the destroyer of the largest freshwater fishery of
the world.
The issues associated with Asian carp show how population and community ecology, fisheries
management, and politics intersect on issues of vital importance to the human food supply and economy.
Socio-political issues like this make extensive use of the sciences of population ecology (the study of
members of a particular species occupying a particular area known as a habitat) and community ecology
(the study of the interaction of all species within a habitat).
Community Dynamics
Community dynamics are the changes in community structure and composition over time. Sometimes these
changes are induced by environmental disturbances such as volcanoes, earthquakes, storms, fires, and
climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the
community may or may not return to the equilibrium state.
Succession describes the sequential appearance and disappearance of species in a community over time.
In primary succession, newly exposed or newly formed land is colonized by living things; in secondary
succession, part of an ecosystem is disturbed and remnants of the previous community remain.
Primary Succession and Pioneer Species
Primary succession occurs when new land is formed or rock is exposed: for example, following the eruption
of volcanoes, such as those on the Big Island of Hawaii. As lava flows into the ocean, new land is continually
being formed. On the Big Island, approximately 32 acres of land is added each year. First, weathering and other
natural forces break down the substrate enough for the establishment of certain hearty plants and lichens with
few soil requirements, known as pioneer species (Figure 45.32). These species help to further break down
the mineral rich lava into soil where other, less hardy species will grow and eventually replace the pioneer
species. In addition, as these early species grow and die, they add to an ever-growing layer of decomposing
organic material and contribute to soil formation. Over time the area will reach an equilibrium state, with a set of
organisms quite different from the pioneer species.
Figure 45.32 During primary succession in lava on Maui, Hawaii, succulent plants are the pioneer species, (credit:
Forest and Kim Starr)
Secondary succession
A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire (Figure 45.33).
Wildfires will burn most vegetation and kill those animals unable to flee the area. Their nutrients, however, are
returned to the ground in the form of ash. Thus, even when areas are devoid of life due to severe fires, the area
will soon be ready for new life to take hold.
Before the fire, the vegetation was dominated by tall trees with access to the major plant energy resource:
sunlight. Their height gave them access to sunlight while also shading the ground and other low-lying species.
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After the fire, though, these trees are no longer dominant. Thus, the first plants to grow back are usually annual
plants followed within a few years by quickly growing and spreading grasses and other pioneer species. Due
to, at least in part, changes in the environment brought on by the growth of the grasses and other species,
over many years, shrubs will emerge along with small pine, oak, and hickory trees. These organisms are
called intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point where species
composition is no longer changing and resembles the community before the fire. This equilibrium state is referred
to as the climax community, which will remain stable until the next disturbance.
Secondary Succession of an Oak and Hickory Forest
Pioneer species Intermediate species Climax community
Annual plants grow and are succeeded Shrubs, then pines, and young oak The mature oak and hickory forest
by grasses and perennials. and hickory begin to grow. remains stable until the next disturbance.
Figure 45.33 Secondary succession is shown in an oak and hickory forest after a forest fire.
45.7 | Behavioral Biology: Proximate and Ultimate
Causes of Behavior
By the end of this section, you will be able to do the following:
• Compare innate and learned behavior
• Discuss how movement and migration behaviors are a result of natural selection
• Discuss the different ways members of a population communicate with each other
• Give examples of how species use energy for mating displays and other courtship behaviors
• Differentiate between various mating systems
• Describe different ways that species learn
Behavior is the change in activity of an organism in response to a stimulus. Behavioral biology is the study
of the biological and evolutionary bases for such changes. The idea that behaviors evolved as a result of the
pressures of natural selection is not new. For decades, several types of scientists have studied animal behavior.
Biologists do so in the science of ethology; psychologists in the science of comparative psychology; and other
scientists in the science of neurobiology. The first two, ethology and comparative psychology, are the most
consequential for the study of behavioral biology.
One goal of behavioral biology is to the innate behaviors, which have a strong genetic component and are
largely independent of environmental influences, from the learned behaviors, which result from environmental
conditioning. Innate behavior, or instinct, is important because there is no risk of an incorrect behavior being
learned. They are “hard wired" into the system. On the other hand, learned behaviors, although riskier, are
flexible, dynamic, and can be altered according to changes in the environment.
Innate Behaviors: Movement and Migration
Innate or instinctual behaviors rely on response to stimuli. The simplest example of this is a reflex action, an
involuntary and rapid response to stimulus. To test the “knee-jerk” reflex, a doctor taps the patellar tendon below
the kneecap with a rubber hammer. The stimulation of the nerves leads to the reflex of extending the leg at the
knee. This is similar to the reaction of someone who touches a hot stove and instinctually pulls his or her hand
away. Even humans, with our great capacity to learn, still exhibit a variety of innate behaviors.
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Kinesis and Taxis
Another activity or movement of innate behavior is kinesis, or the undirected movement in response to a
stimulus. Orthokinesis is the increased or decreased speed of movement of an organism in response to a
stimulus. Woodlice, for example, increase their speed of movement when exposed to high or low temperatures.
This movement, although random, increases the probability that the insect spends less time in the unfavorable
environment. Another example is klinokinesis, an increase in turning behaviors. It is exhibited by bacteria
such as E. coli which, in association with orthokinesis, helps the organisms randomly find a more hospitable
environment.
A similar, but more directed version of kinesis is taxis: the directed movement towards or away from a stimulus.
This movement can be in response to light (phototaxis), chemical signals (chemotaxis), or gravity (geotaxis)
and can be directed toward (positive) or away (negative) from the source of the stimulus. An example of a
positive chemotaxis is exhibited by the unicellular protozoan Tetrahymena thermophila. This organism swims
using its cilia, at times moving in a straight line, and at other times making turns. The attracting chemotactic
agent alters the frequency of turning as the organism moves directly toward the source, following the increasing
concentration gradient.
Fixed Action Patterns
A fixed action pattern is a series of movements elicited by a stimulus such that even when the stimulus
is removed, the pattern goes on to completion. An example of such a behavior occurs in the three-spined
stickleback, a small freshwater fish (Figure 45.34). Males of this species develop a red belly during breeding
season and show instinctual aggressiveness to other males during this time. In laboratory experiments,
researchers exposed such fish to objects that in no way resemble a fish in their shape, but which were painted
red on their lower halves. The male sticklebacks responded aggressively to the objects just as if they were real
male sticklebacks.
Figure 45.34 Male three-spined stickleback fish exhibit a fixed action pattern. During mating season, the males, which
develop a bright red belly, react strongly to red-bottomed objects that in no way resemble fish.
Migration
Migration is the long-range seasonal movement of animals. It is an evolved, adapted response to variation in
resource availability, and it is a common phenomenon found in all major groups of animals. Birds fly south for
the winter to get to warmer climates with sufficient food, and salmon migrate to their spawning grounds. The
popular 2005 documentary March of the Penguins followed the 62-mile migration of emperor penguins through
Antarctica to bring food back to their breeding site and to their young. Wildebeests (Figure 45.35) migrate over
1800 miles each year in search of new grasslands.
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Figure 45.35 Wildebeests migrate in a clockwise fashion over 1800 miles each year in search of rain-ripened grass,
(credit: Eric Inafuku)
Although migration is thought of as innate behavior, only some migrating species always migrate (obligate
migration). Animals that exhibit facultative migration can choose to migrate or not. Additionally, in some animals,
only a portion of the population migrates, whereas the rest does not migrate (incomplete migration). For
example, owls that live in the tundra may migrate in years when their food source, small rodents, is relatively
scarce, but not migrate during the years when rodents are plentiful.
Foraging
Foraging is the act of searching for and exploiting food resources. Feeding behaviors that maximize energy
gain and minimize energy expenditure are called optimal foraging behaviors, and these are favored by natural
section. The painted stork, for example, uses its long beak to search the bottom of a freshwater marshland for
crabs and other food (Figure 45.36).
Figure 45.36 The painted stork uses its long beak to forage, (credit: J.M. Garg)
Innate Behaviors: Living in Groups
Not all animals live in groups, but even those that live relatively solitary lives, with the exception of those that can
reproduce asexually, must mate. Mating usually involves one animal signaling another so as to communicate the
desire to mate. There are several types of energy-intensive behaviors or displays associated with mating, called
mating rituals. Other behaviors found in populations that live in groups are described in terms of which animal
benefits from the behavior. In selfish behavior, only the animal in question benefits; in altruistic behavior, one
animal’s actions benefit another animal; cooperative behavior describes when both animals benefit. All of these
behaviors involve some sort of communication between population members.
Communication within a Species
Animals communicate with each other using stimuli known as signals. An example of this is seen in the
three-spined stickleback, where the visual signal of a red region in the lower half of a fish signals males to
become aggressive and signals females to mate. Other signals are chemical (pheromones), aural (sound), visual
(courtship and aggressive displays), or tactile (touch). These types of communication may be instinctual or
learned or a combination of both. These are not the same as the communication we associate with language,
which has been observed only in humans and perhaps in some species of primates and cetaceans.
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A pheromone is a secreted chemical signal used to obtain a response from another individual of the same
species. The purpose of pheromones is to elicit a specific behavior from the receiving individual. Pheromones
are especially common among social insects, but they are used by many species to attract the opposite sex,
to sound alarms, to mark food trails, and to elicit other, more complex behaviors. Even humans are thought to
respond to certain pheromones called axillary steroids. These chemicals influence human perception of other
people, and in one study were responsible for a group of women synchronizing their menstrual cycles. The role
of pheromones in human-to-human communication is not fully understood and continues to be researched.
Songs are an example of an aural signal, one that needs to be heard by the recipient. Perhaps the best known
of these are songs of birds, which identify the species and are used to attract mates. Other well-known songs
are those of whales, which are of such low frequency that they can travel long distances underwater. Dolphin
species communicate with each other (and occasionally even with other species of dolphins) using a wide variety
of vocalizations. Male crickets make chirping sounds using a specialized organ to attract a mate, repel other
males, and to announce a successful mating.
Courtship displays are a series of ritualized visual behaviors (signals) designed to attract and convince a
member of the opposite sex to mate. These displays are ubiquitous in the animal kingdom. Often these displays
involve a series of steps, including an initial display by one member followed by a response from the other. If at
any point, the display is performed incorrectly or a proper response is not given, the mating ritual is abandoned
and the mating attempt will be unsuccessful. The mating display of the common stork is shown in Figure 45.37.
Aggressive displays are also common in the animal kingdom. For example, a dog bares its teeth when it
wants another dog to back down. Presumably, these displays communicate not only the willingness of the animal
to fight, but also its fighting ability. Although these displays do signal aggression on the part of the sender, it
is thought that these displays are actually a mechanism to reduce the amount of actual fighting that occurs
between members of the same species: they allow individuals to assess the fighting ability of their opponent and
thus decide whether it is “worth the fight.” The testing of certain hypotheses using game theory has led to the
conclusion that some of these displays may overstate an animal’s actual fighting ability and are used to “bluff"
the opponent. This type of interaction, even if “dishonest," would be favored by natural selection if it is successful
more times than not.
Figure 45.37 This stork’s courtship display is designed to attract potential mates, (credit: Linda “jinterwas’VFlickr)
Distraction displays are seen in birds and some fish. They are designed to attract a predator away from the
nest. This is an example of an altruistic behavior: it benefits the young more than the individual performing the
display, which is putting itself at risk by doing so.
Many animals, especially primates, communicate with other members in the group through touch. Activities such
as grooming, touching the shoulder or root of the tail, embracing, lip contact, and greeting ceremonies have
all been observed in the Indian langur, an Old World monkey. Similar behaviors are found in other primates,
especially in the great apes.
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LINK TQ LEARNING
The killdeer bird distracts predators from its eggs by faking a broken wing display in this video taken in Boise,
Idaho. (This multimedia resource will open in a browser.) (http://cnx.org/content/m66699/1.3/#eip-
id!171734275996)
Altruistic Behaviors
Behaviors that lower the fitness of the individual but increase the fitness of another individual are termed
altruistic. Examples of such behaviors are seen widely across the animal kingdom. Social insects such as worker
bees have no ability to reproduce, yet they maintain the queen so she can populate the hive with her offspring.
Meerkats keep a sentry standing guard to warn the rest of the colony about intruders, even though the sentry is
putting itself at risk. Wolves and wild dogs bring meat to pack members not present during a hunt. Lemurs take
care of infants unrelated to them. Although on the surface, these behaviors appear to be altruistic, the truth may
not be so simple.
There has been much discussion over why altruistic behaviors exist. Do these behaviors lead to overall
evolutionary advantages for their species? Do they help the altruistic individual pass on its own genes? And what
about such activities between unrelated individuals? One explanation for altruistic-type behaviors is found in the
genetics of natural selection. In the 1976 book, The Selfish Gene, scientist Richard Dawkins attempted to explain
many seemingly altruistic behaviors from the viewpoint of the gene itself. Although a gene obviously cannot be
selfish in the human sense, it may appear that way if the sacrifice of an individual benefits related individuals that
share genes that are identical by descent (present in relatives because of common lineage). Mammal parents
make this sacrifice to take care of their offspring. Emperor penguins migrate miles in harsh conditions to bring
food back for their young. Selfish gene theory has been controversial over the years and is still discussed among
scientists in related fields.
Even less-related individuals, those with less genetic identity than that shared by parent and offspring, benefit
from seemingly altruistic behavior. The activities of social insects such as bees, wasps, ants, and termites are
good examples. Sterile workers in these societies take care of the queen because they are closely related to it,
and as the queen has offspring, she is passing on genes from the workers indirectly. Thus, it is of fitness benefit
for the worker to maintain the queen without having any direct chance of passing on its genes due to its sterility.
The lowering of individual fitness to enhance the reproductive fitness of a relative and thus one’s inclusive fitness
evolves through kin selection. This phenomenon can explain many superficially altruistic behaviors seen in
animals. However, these behaviors may not be truly defined as altruism in these cases because the actor is
actually increasing its own fitness either directly (through its own offspring) or indirectly (through the inclusive
fitness it gains through relatives that share genes with it).
Unrelated individuals may also act altruistically to each other, and this seems to defy the “selfish gene”
explanation. An example of this observed in many monkey species where a monkey will present its back to an
unrelated monkey to have that individual pick the parasites from its fur. After a certain amount of time, the roles
are reversed and the first monkey now grooms the second monkey. Thus, there is reciprocity in the behavior.
Both benefit from the interaction and their fitness is raised more than if neither cooperated nor if one cooperated
and the other did not cooperate. This behavior is still not necessarily altruism, as the “giving" behavior of the
actor is based on the expectation that it will be the “receiver” of the behavior in the future, termed reciprocal
altruism. Reciprocal altruism requires that individuals repeatedly encounter each other, often the result of living
in the same social group, and that cheaters (those that never “give back”) are punished.
Evolutionary game theory, a modification of classical game theory in mathematics, has shown that many of
these so-called “altruistic behaviors” are not altruistic at all. The definition of “pure” altruism, based on human
behavior, is an action that benefits another without any direct benefit to oneself. Most of the behaviors previously
described do not seem to satisfy this definition, and game theorists are good at finding “selfish” components in
them. Others have argued that the terms “selfish” and “altruistic” should be dropped completely when discussing
animal behavior, as they describe human behavior and may not be directly applicable to instinctual animal
activity. What is clear, though, is that heritable behaviors that improve the chances of passing on one’s genes
or a portion of one’s genes are favored by natural selection and will be retained in future generations as long
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as those behaviors convey a fitness advantage. These instinctual behaviors may then be applied, in special
circumstances, to other species, as long as it doesn’t lower the animal’s fitness.
Finding Sex Partners
Not all animals reproduce sexually, but many that do have the same challenge: they need to find a suitable
mate and often have to compete with other individuals to obtain one. Significant energy is spent in the process
of locating, attracting, and mating with the sex partner. Two types of selection occur during this process:
intersexual selection, where individuals of one sex choose mates of the other sex, and intrasexual selection,
the competition for mates between species members of the same sex. Intersexual selection is often complex
because choosing a mate may be based on a variety of visual, aural, tactile, and chemical cues. An example of
intersexual selection is when female peacocks choose to mate with the male with the brightest plumage. This
type of selection often leads to traits in the chosen sex that do not enhance survival, but are those traits most
attractive to the opposite sex (often at the expense of survival). Intrasexual selection involves mating displays
and aggressive mating rituals such as rams butting heads—the winner of these battles is the one that is able to
mate. Many of these rituals use up considerable energy but result in the selection of the healthiest, strongest,
and/or most dominant individuals for mating.
Three general mating systems, all involving innate as opposed to learned behaviors, are seen in animal
populations: monogamous, polygynous, and polyandrous.
LINK TQ LEARNING
Visit this website (http:// 0 penstaxc 0 llege. 0 rg/l/sex_selecti 0 n) for informative videos on sexual selection.
In monogamous systems, one male and one female are paired for at least one breeding season. In some
animals, such as the gray wolf, these associations can last much longer, even a lifetime. Several theories may
explain this type of mating system. The “mate-guarding hypothesis” states that males stay with the female to
prevent other males from mating with her. This behavior is advantageous in such situations where mates are
scarce and difficult to find. Another explanation is the “male-assistance hypothesis,” where males that help guard
and rear their young will have more and healthier offspring. Monogamy is observed in many bird populations
where, in addition to the parental care from the female, the male is also a major provider of parental care
for the chicks. A third explanation for the evolutionary advantages of monogamy is the “female-enforcement
hypothesis.” In this scenario, the female ensures that the male does not have other offspring that might compete
with her own, so she actively interferes with the male’s signaling to attract other mates.
Polygynous mating refers to one male mating with multiple females. In these situations, the female must be
responsible for most of the parental care as the single male is not capable of providing care to that many
offspring. In resourced-based polygyny, males compete for territories with the best resources, and then mate with
females that enter the territory, drawn to its resource richness. The female benefits by mating with a dominant,
genetically fit male; however, it is at the cost of having no male help in caring for the offspring. An example is
seen in the yellow-rumped honeyguide, a bird whose males defend beehives because the females feed on their
wax. As the females approach, the male defending the nest will mate with them. Harem mating structures are
a type of polygynous system where certain males dominate mating while controlling a territory with resources.
Harem mating occurs in elephant seals, where the alpha male dominates the mating within the group. A third
type of polygyny is a lek system. Here there is a communal courting area where several males perform elaborate
displays for females, and the females choose their mate from this group. This behavior is observed in several
bird species including the sage grouse and the prairie chicken.
In polyandrous mating systems, one female mates with many males. These types of systems are much rarer
than monogamous and polygynous mating systems. In pipefishes and seahorses, males receive the eggs from
the female, fertilize them, protect them within a pouch, and give birth to the offspring (Figure 45.38). Therefore,
the female is able to provide eggs to several males without the burden of carrying the fertilized eggs.
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(a) (b)
Figure 45.38 Polyandrous mating, in which one female mates with many males, occurs in the (a) seahorse and the (b)
pipefish, (credit a: modification of work by Brian Gratwicke; credit b: modification of work by Stephen Childs)
Simple Learned Behaviors
The majority of the behaviors previously discussed were innate or at least have an innate component (variations
on the innate behaviors may be learned). They are inherited and the behaviors do not change in response
to signals from the environment. Conversely, learned behaviors, even though they may have instinctive
components, allow an organism to adapt to changes in the environment and are modified by previous
experiences. Simple learned behaviors include habituation and imprinting—both are important to the maturation
process of young animals.
Habituation
Habituation is a simple form of learning in which an animal stops responding to a stimulus after a period
of repeated exposure. This is a form of non-associative learning, as the stimulus is not associated with any
punishment or reward. Prairie dogs typically sound an alarm call when threatened by a predator, but they
become habituated to the sound of human footsteps when no harm is associated with this sound, therefore, they
no longer respond to them with an alarm call. In this example, habituation is specific to the sound of human
footsteps, as the animals still respond to the sounds of potential predators.
Imprinting
Imprinting is a type of learning that occurs at a particular age or a life stage that is rapid and independent of the
species involved. Hatchling ducks recognize the first adult they see, their mother, and make a bond with her. A
familiar sight is ducklings walking or swimming after their mothers (Figure 45.39). This is another type of non-
associative learning, but is very important in the maturation process of these animals as it encourages them to
stay near their mother so they will be protected, greatly increasing their chances of survival. However, if newborn
ducks see a human before they see their mother, they will imprint on the human and follow it in just the same
manner as they would follow their real mother.
Figure 45.39 The attachment of ducklings to their mother is an example of imprinting, (credit: modification of work by
Mark Harkin)
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LINK TQ LEARNING
The International Crane Foundation has helped raise the world’s population of whooping cranes from 21
individuals to about 600. Imprinting hatchlings has been a key to success: biologists wear full crane costumes
so the birds never “see” humans. Watch this video to learn more. (This multimedia resource will open in a
browser.) (http://cnx.Org/content/m66699/l.3/#eip-id5650108)
Conditioned Behavior
Conditioned behaviors are types of associative learning, where a stimulus becomes associated with a
consequence. During operant conditioning, the behavioral response is modified by its consequences, with
regards to its form, strength, or frequency.
Classical Conditioning
In classical conditioning, a response called the conditioned response is associated with a stimulus that it
had previously not been associated with, the conditioned stimulus. The response to the original, unconditioned
stimulus is called the unconditioned response. The most cited example of classical conditioning is Ivan Pavlov’s
experiments with dogs (Figure 45.40). In Pavlov’s experiments, the unconditioned response was the salivation
of dogs in response to the unconditioned stimulus of seeing or smelling their food. The conditioning stimulus
that researchers associated with the unconditioned response was the ringing of a bell. During conditioning,
every time the animal was given food, the bell was rung. This was repeated during several trials. After some
time, the dog learned to associate the ringing of the bell with food and to respond by salivating. After the
conditioning period was finished, the dog would respond by salivating when the bell was rung, even when the
unconditioned stimulus, the food, was absent. Thus, the ringing of the bell became the conditioned stimulus and
the salivation became the conditioned response. Although it is thought by some scientists that the unconditioned
and conditioned responses are identical, even Pavlov discovered that the saliva in the conditioned dogs had
characteristic differences when compared to the unconditioned dog.
Figure 45.40 In the classic Pavlovian response, the dog becomes conditioned to associate the ringing of the bell with
food.
It had been thought by some scientists that this type of conditioning required multiple exposures to the paired
stimulus and response, but it is now known that this is not necessary in all cases, and that some conditioning
can be learned in a single pairing experiment. Classical conditioning is a major tenet of behaviorism, a branch
of psychological philosophy that proposes that all actions, thoughts, and emotions of living things are behaviors
that can be treated by behavior modification and changes in the environment.
Operant Conditioning
in operant conditioning, the conditioned behavior is gradually modified by its consequences as the animal
responds to the stimulus. A major proponent of such conditioning was psychologist B.F. Skinner, the inventor of
the Skinner box. Skinner put rats in his boxes that contained a lever that would dispense food to the rat when
depressed. While initially the rat would push the lever a few times by accident, it eventually associated pushing
the lever with getting the food. This type of learning is an example of operant conditioning. Operant learning
is the basis of most animal training. The conditioned behavior is continually modified by positive or negative
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reinforcement, often a reward such as food or some type of punishment, respectively, in this way, the animal
is conditioned to associate a type of behavior with the punishment or reward, and, over time, can be induced
to perform behaviors that they would not have done in the wild, such as the “tricks" dolphins perform at marine
amusement park shows (Figure 45.41).
Figure 45.41 The training of dolphins by rewarding them with food is an example of positive reinforcement operant
conditioning, (credit: Roland Tanglao)
Cognitive Learning
Classical and operant conditioning are inefficient ways for humans and other intelligent animals to learn. Some
primates, including humans, are able to learn by imitating the behavior of others and by taking instructions. The
development of complex language by humans has made cognitive learning, the manipulation of information
using the mind, the most prominent method of human learning. In fact, that is how students are learning
right now by reading this book. As students read, they can make mental images of objects or organisms
and imagine changes to them, or behaviors by them, and anticipate the consequences. In addition to visual
processing, cognitive learning is also enhanced by remembering past experiences, touching physical objects,
hearing sounds, tasting food, and a variety of other sensory-based inputs. Cognitive learning is so powerful that
it can be used to understand conditioning in detail. In the reverse scenario, conditioning cannot help someone
learn about cognition.
Classic work on cognitive learning was done by Wolfgang Kohler with chimpanzees. He demonstrated that these
animals were capable of abstract thought by showing that they could learn how to solve a puzzle. When a
banana was hung in their cage too high for them to reach, and several boxes were placed randomly on the floor,
some of the chimps were able to stack the boxes one on top of the other, climb on top of them, and get the
banana. This implies that they could visualize the result of stacking the boxes even before they had performed
the action. This type of learning is much more powerful and versatile than conditioning.
Cognitive learning is not limited to primates, although they are the most efficient in using it. Maze running
experiments done with rats by H.C. Blodgett in the 1920s were the first to show cognitive skills in a simple
mammal. The motivation for the animals to work their way through the maze was a piece of food at its end. In
these studies, the animals in Group I were run in one trial per day and had food available to them each day
on completion of the run (Figure 45.42). Group II rats were not fed in the maze for the first six days and then
subsequent runs were done with food for several days after. Group III rats had food available on the third day
and every day thereafter. The results were that the control rats, Group I, learned quickly, and figured out how to
run the maze in seven days. Group III did not learn much during the three days without food, but rapidly caught
up to the control group when given the food reward. Group II learned very slowly for the six days with no reward
to motivate them, and they did not begin to catch up to the control group until the day food was given, and then
it took two days longer to learn the maze.
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Redrawn after H. C. Blodgett, The effect of the introduction of reward upon the maze performance of rats. Univ. Calif. Publ.
Psychol., 1929, 4, No. 8, pages 117 and 120.
Figure 45.42 Group I (the green solid line) found food at the end of each trial, group II (the blue dashed line) did not
find food for the first 6 days, and group III (the red dotted line) did not find food during runs on the first three days.
Notice that rats given food earlier learned faster and eventually caught up to the control group. The orange dots on the
group II and III lines show the days when food rewards were added to the mazes.
It may not be immediately obvious that this type of learning is different than conditioning. Although one might be
tempted to believe that the rats simply learned how to find their way through a conditioned series of right and left
turns, E.C. Tolman proved a decade later that the rats were making a representation of the maze in their minds,
which he called a “cognitive map.” This was an early demonstration of the power of cognitive learning and how
these abilities were not just limited to humans.
Sociobiology
Sociobiology is an interdisciplinary science originally popularized by social insect researcher E.O. Wilson in the
1970s. Wilson defined the science as “the extension of population biology and evolutionary theory to social
organization.” The main thrust of sociobiology is that animal and human behavior, including aggressiveness
and other social interactions, can be explained almost solely in terms of genetics and natural selection. This
science is controversial; noted scientists such as the late Stephen Jay Gould criticized the approach for ignoring
the environmental effects on behavior. This is another example of the “nature versus nurture" debate of the role
of genetics versus the role of environment in determining an organism’s characteristics.
Sociobiology also links genes with behaviors and has been associated with “biological determinism,” the belief
that all behaviors are hardwired into our genes. No one disputes that certain behaviors can be inherited and that
natural selection plays a role retaining them. It is the application of such principles to human behavior that sparks
this controversy, which remains active today.
9. Edward O. Wilson. On Human Nature (1978; repr., Cambridge: Harvard University Press, 2004), xx.
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KEY TERMS
age structure proportion of population members at specific age ranges
aggressive display visual display by a species member to discourage other members of the same species or
different species
aposematic coloration warning coloration used as a defensive mechanism against predation
Batesian mimicry type of mimicry where a non-harmful species takes on the warning colorations of a harmful
one
behavior change in an organism’s activities in response to a stimulus
behavioral biology study of the biology and evolution of behavior
biotic potential (r m ax) maximal potential growth rate of a species
birth rate (B) number of births within a population at a specific point in time
camouflage avoid detection by blending in with the background
carrying capacity ( K) number of individuals of a species that can be supported by the limited resources of a
habitat
classical conditioning association of a specific stimulus and response through conditioning
climax community final stage of succession, where a stable community is formed by a characteristic
assortment of plant and animal species
cognitive learning knowledge and skills acquired by the manipulation of information in the mind
commensalism relationship between species wherein one species benefits from the close, prolonged
interaction, while the other species neither benefits nor is harmed
competitive exclusion principle no two species within a habitat can coexist when they compete for the same
resources at the same place and time
conditioned behavior behavior that becomes associated with a specific stimulus through conditioning
courtship display visual display used to attract a mate
death rate (D) number of deaths within a population at a specific point in time
demographic-based population model modern model of population dynamics incorporating many features of
the r- and K-selection theory
demography statistical study of changes in populations over time
density-dependent regulation regulation of population that is influenced by population density, such as
crowding effects; usually involves biotic factors
density-independent regulation regulation of populations by factors that operate independent of population
density, such as forest fires and volcanic eruptions; usually involves abiotic factors
distraction display visual display used to distract predators away from a nesting site
Emsleyan/Mertensian mimicry type of mimicry where a harmful species resembles a less harmful one
energy budget allocation of energy resources for body maintenance, reproduction, and parental care
environmental disturbance change in the environment caused by natural disasters or human activities
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ethology biological study of animal behavior
exponential growth accelerating growth pattern seen in species under conditions where resources are not
limiting
fecundity potential reproductive capacity of an individual
fixed action pattern series of instinctual behaviors that, once initiated, always goes to completion regardless of
changes in the environment
foraging behaviors species use to find food
foundation species species which often forms the major structural portion of the habitat
habituation ability of a species to ignore repeated stimuli that have no consequence
host organism a parasite lives on
imprinting identification of parents by newborns as the first organism they see after birth
innate behavior instinctual behavior that is not altered by changes in the environment
intersexual selection selection of a desirable mate of the opposite sex
interspecific competition competition between species for resources in a shared habitat or environment
intrasexual selection competition between members of the same sex for a mate
intraspecific competition competition between members of the same species
island biogeography study of life on island chains and how their geography interacts with the diversity of
species found there
iteroparity life history strategy characterized by multiple reproductive events during the lifetime of a species
J-shaped growth curve shape of an exponential growth curve
/(-selected species species suited to stable environments that produce a few, relatively large offspring and
provide parental care
keystone species species whose presence is key to maintaining biodiversity in an ecosystem and to upholding
an ecological community’s structure
kin selection sacrificing one’s own life so that one’s genes will be passed on to future generations by relatives
kinesis undirected movement of an organism in response to a stimulus
learned behavior behavior that responds to changes in the environment
life history inherited pattern of resource allocation under the influence of natural selection and other
evolutionary forces
life table table showing the life expectancy of a population member based on its age
logistic growth leveling off of exponential growth due to limiting resources
mark and recapture technique used to determine population size in mobile organisms
migration long-range seasonal movement of animal species
monogamy mating system whereby one male and one female remain coupled for at least one mating season
mortality rate proportion of population surviving to the beginning of an age interval that die during the age
interval
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mutualism symbiotic relationship between two species where both species benefit
Mullerian mimicry type of mimicry where species share warning coloration and all are harmful to predators
one-child policy China’s policy to limit population growth by limiting urban couples to have only one child or
face the penalty of a fine
operant conditioning learned behaviors in response to positive and/or negative reinforcement
parasite organism that uses resources from another species, the host
pioneer species first species to appear in primary and secondary succession
polyandry mating system where one female mates with many males
polygyny mating system where one male mates with many females
population density number of population members divided by the area or volume being measured
population growth rate number of organisms added in each reproductive generation
population size ( N) number of population members in a habitat at the same time
primary succession succession on land that previously has had no life
quadrat square made of various materials used to determine population size and density in slow moving or
stationary organisms
r-selected species species suited to changing environments that produce many offspring and provide little or
no parental care
reflex action action in response to direct physical stimulation of a nerve
relative species abundance absolute population size of a particular species relative to the population sizes of
other species within the community
S-shaped growth curve shape of a logistic growth curve
secondary succession succession in response to environmental disturbances that move a community away
from its equilibrium
semelparity life history strategy characterized by a single reproductive event followed by death
signal method of communication between animals including those obtained by the senses of smell, hearing,
sight, or touch
species dispersion pattern (also, species distribution pattern) spatial location of individuals of a given species
within a habitat at a particular point in time
species richness number of different species in a community
survivorship curve graph of the number of surviving population members versus the relative age of the
member
symbiosis close interaction between individuals of different species over an extended period of time that
impacts the abundance and distribution of the associating populations
taxis directed movement in response to a stimulus
zero population growth steady population size where birth rates and death rates are equal
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CHAPTER SUMMARY
45.1 Population Demography
Populations are individuals of a species that live in a particular habitat. Ecologists measure characteristics of
populations: size, density, dispersion pattern, age structure, and sex ratio. Life tables are useful to calculate life
expectancies of individual population members. Survivorship curves show the number of individuals surviving
at each age interval plotted versus time.
45.2 Life Histories and Natural Selection
All species have evolved a pattern of living, called a life history strategy, in which they partition energy for
growth, maintenance, and reproduction. These patterns evolve through natural selection; they allow species to
adapt to their environment to obtain the resources they need to successfully reproduce. There is an inverse
relationship between fecundity and parental care. A species may reproduce early in life to ensure surviving to a
reproductive age or reproduce later in life to become larger and healthier and better able to give parental care.
A species may reproduce once (semelparity) or many times (iteroparity) in its life.
45.3 Environmental Limits to Population Growth
Populations with unlimited resources grow exponentially, with an accelerating growth rate. When resources
become limiting, populations follow a logistic growth curve. The population of a species will level off at the
carrying capacity of its environment.
45.4 Population Dynamics and Regulation
Populations are regulated by a variety of density-dependent and density-independent factors. Species are
divided into two categories based on a variety of features of their life history patterns: r-selected species, which
have large numbers of offspring, and K-selected species, which have few offspring. The r- and K-selection
theory has fallen out of use; however, many of its key features are still used in newer, demographically-based
models of population dynamics.
45.5 Human Population Growth
The world’s human population is growing at an exponential rate. Humans have increased the world’s carrying
capacity through migration, agriculture, medical advances, and communication. The age structure of a
population allows us to predict population growth. Unchecked human population growth could have dire long¬
term effects on our environment.
45.6 Community Ecology
Communities include all the different species living in a given area. The variety of these species is called
species richness. Many organisms have developed defenses against predation and herbivory, including
mechanical defenses, warning coloration, and mimicry, as a result of evolution and the interaction with other
members of the community. Two species cannot exist in the same habitat competing directly for the same
resources. Species may form symbiotic relationships such as commensalism or mutualism. Community
structure is described by its foundation and keystone species. Communities respond to environmental
disturbances by succession (the predictable appearance of different types of plant species) until a stable
community structure is established.
45.7 Behavioral Biology: Proximate and Ultimate Causes of Behavior
Behaviors are responses to stimuli. They can either be instinctual/innate behaviors, which are not influenced by
the environment, or learned behaviors, which are influenced by environmental changes. Instinctual behaviors
include mating systems and methods of communication. Learned behaviors include imprinting and habituation,
conditioning, and, most powerfully, cognitive learning.
VISUAL CONNECTION QUESTIONS
1. Figure 45.2 As this graph shows, population density typically decreases with increasing body size.
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Why do you think this is the case?
2. Figure 45.10b If the major food source of the
seals declines due to pollution or overfishing, which
of the following would likely occur?
a. The carrying capacity of seals would
decrease, as would the seal population.
b. The carrying capacity of seals would
decrease, but the seal population would
remain the same.
c. The number of seal deaths would increase
but the number of births would also
increase, so the population size would
remain the same.
d. The carrying capacity of seals would remain
the same, but the population of seals would
decrease.
REVIEW QUESTIONS
4. Which of the following methods will tell an
ecologist about both the size and density of a
population?
a. mark and recapture
b. mark and release
c. quadrat
d. life table
5. Which of the following is best at showing the life
expectancy of an individual within a population?
a. quadrat
b. mark and recapture
c. survivorship curve
d. life table
6. Humans have which type of survivorship curve?
a. Type i
b. Type II
c. Type III
d. Type IV
7. How is a clumped population distribution beneficial
for prey animals?
a. Being a member of a larger group provides
protection for each individual from
predators.
b. Prey animals rely on each other to acquire
food.
c. Prey animals live in small family groups to
raise young.
d. Clumped population distributions ensure
that at least one member of the population
knows how to identify the seasonal
migration route.
8. Which of the following is associated with long-term
parental care?
3. Figure 45.16 Age structure diagrams for rapidly
growing, slow growing, and stable populations are
shown in stages 1 through 3. What type of population
change do you think stage 4 represents?
a. few offspring
b. many offspring
c. semelparity
d. fecundity
9. Which of the following is associated with multiple
reproductive episodes during a species’ lifetime?
a. semiparity
b. iteroparity
c. semelparity
d. fecundity
10. Which of the following is associated with the
reproductive potential of a species?
a. few offspring
b. many offspring
c. semelparity
d. fecundity
11. Species with limited resources usually exhibit
a(n)_growth curve.
a. logistic
b. logical
c. experimental
d. exponential
12. The maximum rate of increased characteristic of
a species is called its_.
a. limit
b. carrying capacity
c. biotic potential
d. exponential growth pattern
13. The population size of a species capable of being
supported by the environment is called its_.
a. limit
b. carrying capacity
c. biotic potential
d. logistic growth pattern
14. Species that have many offspring at one time are
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usually:
a. r-selected
b. K-selected
c. both r- and K-selected
d. not selected
15. A forest fire is an example of_
regulation.
a. density-dependent
b. density-independent
c. r-selected
d. K-selected
16. Primates are examples of:
a. density-dependent species
b. density-independent species
c. r-selected species
d. K-selected species
17. Which of the following statements does not
support the conclusion that giraffes are /c-selected
species?
a. Giraffes are approximately 6’ tall and weigh
150 lbs at birth.
b. Wild giraffes begin mating at 6-7 years of
age.
c. Newborn giraffes are capable of coordinated
walking within an hour of birth, and running
within 24 hours of birth.
d. Giraffes rarely give birth to twins.
18. Which of the following events would not
negatively impact Yellowstone’s grey wolf carrying
capacity?
a. snow in winter
b. a beaver damming a river upstream
c. a forest fire
d. chronic wasting disease in the deer
population
19. A country with zero population growth is likely to
be_.
a. in Africa
b. in Asia
c. economically developed
d. economically underdeveloped
20. Which type of country has the greatest proportion
of young individuals?
a. economically developed
b. economically underdeveloped
c. countries with zero population growth
d. countries in Europe
21. Which of the following is not a way that humans
have increased the carrying capacity of the
environment?
a. agriculture
b. using large amounts of natural resources
c. domestication of animals
d. use of language
22. The first species to live on new land, such as that
formed from volcanic lava, are called_.
a. climax community
b. keystone species
c. foundation species
d. pioneer species
23. Which type of mimicry involves multiple species
with similar warning coloration that are all toxic to
predators?
a. Batesian mimicry
b. Mullerian mimicry
c. Emsleyan/Mertensian mimicry
d. Mertensian mimicry
24. A symbiotic relationship where both of the
coexisting species benefit from the interaction is
called_.
a. commensalism
b. parasitism
c. mutualism
d. communism
25. Which of the following is not a mutualistic
relationship?
a. a shark using an aquatic cleaning station
b. a helminth feeding from its host
c. a bumblebee collecting pollen from a flower
d. bacteria living in the gut of humans
26. The ability of rats to learn how to run a maze is
an example of_.
a. imprinting
b. classical conditioning
c. operant conditioning
d. cognitive learning
27. The training of animals usually involves
a. imprinting
b. classical conditioning
c. operant conditioning
d. cognitive learning
28. The sacrifice of the life of an individual so that the
genes of relatives may be passed on is called
a. operant learning
b. kin selection
c. kinesis
d. imprinting
29. Why are polyandrous mating systems more rare
than polygynous matings?
a. Only males are capable of multiple rounds
of reproduction within a single breeding
season.
b. Only females care for the young.
c. Females usually experience more
intrasexual selection pressure than males.
d. Females usually devote more energy to
offspring production and development.
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CRITICAL THINKING QUESTIONS
30. Describe how a researcher would determine the
size of a penguin population in Antarctica using the
mark and release method.
31. The CDC released the following data in its 2013
Vital Statistics report.
Age
interval
Number dying
in age interval
Number surviving at
beginning of age interval
0-10
756
100,000
11-20
292
99,244
21-30
890
98,953
31-40
1,234
98,164
41-50
2,457
96,811
51-60
5,564
94,352
61-70
10,479
88,788
Table 45.3
Calculate the mortality rate for each age interval, and
describe the trends in adult and childhood mortality
per 100,000 births in the United States in 2013.
32. Why is long-term parental care not associated
with having many offspring during a reproductive
episode?
33. Describe the difference in evolutionary pressures
experienced by an animal that begins reproducing
early and an animal that reproduces late in its
lifecycle.
34. Describe the rate of population growth that would
be expected at various parts of the S-shaped curve
of logistic growth.
35. Describe how the population of a species that
survives a mass extinction event would change in
size and growth pattern over time beginning
immediately after the extinction event.
36. Give an example of how density-dependent and
density-independent factors might interact.
37. Describe the age structures in rapidly growing
countries, slowly growing countries, and countries
with zero population growth.
38. Since the introduction of the Endangered Species
Act the number of species on the protected list has
more than doubled. Describe how the human
population’s growth pattern contributes to the rise in
endangered species.
39. Describe the competitive exclusion principle and
its effects on competing species.
40. Jaguars are a keystone species in the Amazon.
Describe how they can be so essential to the
ecosystem despite being significantly less abundant
than many other species.
41. Describe Pavlov’s dog experiments as an
example of classical conditioning.
42. Describe the advantage of using an aural or
pheromone signal to attract a mate as opposed to a
visual signal. How might the population density
contribute to the evolution of aural or visual mating
rituals?
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Chapter 46 | Ecosystems
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46 | ECOSYSTEMS
Figure 46.1 In the southwestern United States, rainy weather causes an increase in production of pinyon nuts, causing
the deer mouse population to explode. Deer mice may carry a virus called Sin Nombre (a hantavirus) that causes
respiratory disease in humans and has a high fatality rate. In 1992-1993, wet El Nino weather caused a Sin Nombre
epidemic. Navajo healers, who were aware of the link between this disease and weather, predicted the outbreak,
(credit "highway": modification of work by Phillip Capper; credit "mouse": modification of work by USFWS)
Chapter Outline
46.1: Ecology of Ecosystems
46.2: Energy Flow through Ecosystems
46.3: Biogeochemical Cycles
Introduction
in 1993, an interesting example of ecosystem dynamics occurred when a rare lung disease struck inhabitants
of the southwestern United States. This disease had an alarming rate of fatalities, killing more than half of
early patients, many of whom were Native Americans. These formerly healthy young adults died from complete
respiratory failure. The disease was unknown, and the Centers for Disease Control (CDC), the United States
government agency responsible for managing potential epidemics, was brought in to investigate. The scientists
could have learned about the disease had they known to talk with the Navajo healers who lived in the area and
who had observed the connection between rainfall and mice populations, thereby predicting the 1993 outbreak.
The cause of the disease, determined within a few weeks by the CDC investigators, was the hantavirus known
as Sin Nombre, the virus with “no name." With insights from traditional Navajo medicine, scientists were able
to characterize the disease rapidly and institute effective health measures to prevent its spread. This example
illustrates the importance of understanding the complexities of ecosystems and how they respond to changes in
the environment.
46.1 1 Ecology of Ecosystems
By the end of this section, you will be able to do the following:
• Describe the basic ecosystem types
• Explain the methods that ecologists use to study ecosystem structure and dynamics
• Identify the different methods of ecosystem modeling
• Differentiate between food chains and food webs and recognize the importance of each
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Life in an ecosystem is often about competition for limited resources, a characteristic of the theory of natural
selection. Competition in communities (all living things within specific habitats) is observed both within species
and among different species. The resources for which organisms compete include organic material, sunlight,
and mineral nutrients, which provide the energy for living processes and the matter to make up organisms’
physical structures. Other critical factors influencing community dynamics are the components of its physical and
geographic environment: a habitat’s latitude, amount of rainfall, topography (elevation), and available species.
These are all important environmental variables that determine which organisms can exist within a particular
area.
An ecosystem is a community of living organisms and their interactions with their abiotic (nonliving)
environment. Ecosystems can be small, such as the tide pools found near the rocky shores of many oceans, or
large, such as the Amazon Rainforest in Brazil (Figure 46.2).
Figure 46.2 A (a) tidal pool ecosystem in Matinicus Island in Maine is a small ecosystem, while the (b) Amazon
Rainforest in Brazil is a large ecosystem, (credit a: modification of work by “takomabibelot’VFlickr; credit b: modification
of work by Ivan Mlinaric)
There are three broad categories of ecosystems based on their general environment: freshwater, ocean water,
and terrestrial. Within these broad categories are individual ecosystem types based on the organisms present
and the type of environmental habitat.
Ocean ecosystems are the most common, comprising over 70 percent of the Earth's surface and consisting
of three basic types: shallow ocean, deep ocean water, and deep ocean surfaces (the low depth areas of the
deep oceans). The shallow ocean ecosystems include extremely biodiverse coral reef ecosystems, and the deep
ocean surface is known for its large numbers of plankton and krill (small crustaceans) that support it. These two
environments are especially important to aerobic respirators worldwide as the phytoplankton perform 40 percent
of all photosynthesis on Earth. Although not as diverse as the other two, deep ocean ecosystems contain a wide
variety of marine organisms. Such ecosystems exist even at the bottom of the ocean where light is unable to
penetrate through the water.
Freshwater ecosystems are the rarest, occurring on only 1.8 percent of the Earth's surface. Lakes, rivers,
streams, and springs comprise these systems. They are quite diverse, and they support a variety of fish,
amphibians, reptiles, insects, phytoplankton, fungi, and bacteria.
Terrestrial ecosystems, also known for their diversity, are grouped into large categories called biomes, such
as tropical rain forests, savannas, deserts, coniferous forests, deciduous forests, and tundra. Grouping these
ecosystems into just a few biome categories obscures the great diversity of the individual ecosystems within
them. For example, there is great variation in desert vegetation: the saguaro cacti and other plant life in the
Sonoran Desert, in the United States, are relatively abundant compared to the desolate rocky desert of Boa
Vista, an island off the coast of Western Africa (Figure 46.3).
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Chapter 46 | Ecosystems
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(a) (b)
Figure 46.3 Desert ecosystems, like all ecosystems, can vary greatly. The desert in (a) Saguaro National Park,
Arizona, has abundant plant life, while the rocky desert of (b) Boa Vista island, Cape Verde, Africa, is devoid of plant
life, (credit a: modification of work by Jay Galvin; credit b: modification of work by Ingo Wolbern)
Ecosystems are complex with many interacting parts. They are routinely exposed to various disturbances, or
changes in the environment that effect their compositions: yearly variations in rainfall and temperature and
the slower processes of plant growth, which may take several years. Many of these disturbances result from
natural processes. For example, when lightning causes a forest fire and destroys part of a forest ecosystem, the
ground is eventually populated by grasses, then by bushes and shrubs, and later by mature trees, restoring the
forest to its former state. The impact of environmental disturbances caused by human activities is as important
as the changes wrought by natural processes. Human agricultural practices, air pollution, acid rain, global
deforestation, overfishing, eutrophication, oil spills, and waste dumping on land and into the ocean are all issues
of concern to conservationists.
Equilibrium is the steady state of an ecosystem where all organisms are in balance with their environment
and with each other. In ecology, two parameters are used to measure changes in ecosystems: resistance and
resilience. Resistance is the ability of an ecosystem to remain at equilibrium in spite of disturbances. Resilience
is the speed at which an ecosystem recovers equilibrium after being disturbed. Ecosystem resistance and
resilience are especially important when considering human impact. The nature of an ecosystem may change
to such a degree that it can lose its resilience entirely. This process can lead to the complete destruction or
irreversible altering of the ecosystem.
Food Chains and Food Webs
The term “food chain” is sometimes used metaphorically to describe human social situations. Individuals who
are considered successful are seen as being at the top of the food chain, consuming all others for their benefit,
whereas the less successful are seen as being at the bottom.
The scientific understanding of a food chain is more precise than in its everyday usage. In ecology, a food
chain is a linear sequence of organisms through which nutrients and energy pass: primary producers, primary
consumers, and higher-level consumers are used to describe ecosystem structure and dynamics. There is a
single path through the chain. Each organism in a food chain occupies what is called a trophic level. Depending
on their role as producers or consumers, species or groups of species can be assigned to various trophic levels.
In many ecosystems, the bottom of the food chain consists of photosynthetic organisms (plants and/or
phytoplankton), which are called primary producers. The organisms that consume the primary producers
are herbivores: the primary consumers. Secondary consumers are usually carnivores that eat the primary
consumers. Tertiary consumers are carnivores that eat other carnivores. Higher-level consumers feed on the
next lower tropic levels, and so on, up to the organisms at the top of the food chain: the apex consumers. In the
Lake Ontario food chain shown in Figure 46.4, the Chinook salmon is the apex consumer at the top of this food
chain.
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Chapter 46 | Ecosystems
Figure 46.4 These are the trophic levels of a food chain in Lake Ontario at the United States-Canada border. Energy
and nutrients flow from photosynthetic green algae at the bottom to the top of the food chain: the Chinook salmon.
One major factor that limits the length of food chains is energy. Energy is lost as heat between each trophic level
due to the second law of thermodynamics. Thus, after a limited number of trophic energy transfers, the amount
of energy remaining in the food chain may not be great enough to support viable populations at yet a higher
trophic level.
The loss of energy between trophic levels is illustrated by the pioneering studies of Howard T. Odum in the
Silver Springs, Florida, ecosystem in the 1940s (Figure 46.5). The primary producers generated 20,819 kcal/
m 2 /yr (kilocalories per square meter per year), the primary consumers generated 3368 kcal/m 2 /yr, the secondary
consumers generated 383 kcal/m 2 /yr, and the tertiary consumers only generated 21 kcal/m 2 /yr. Thus, there is
little energy remaining for another level of consumers in this ecosystem.
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Chapter 46 | Ecosystems
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Relative Energy Content in Trophic Levels
Tertiary (apex)
consumers
Secondary consumers
Primary consumers
Primary producers
0 5,000 10,000 15,000 20,000
Energy content (kcal/m 2 /yr)
Figure 46.5 The relative energy in trophic levels in a Silver Springs, Florida, ecosystem is shown. Each trophic level
has less energy available and supports fewer organisms at the next level.
There is a one problem when using food chains to accurately describe most ecosystems. Even when all
organisms are grouped into appropriate trophic levels, some of these organisms can feed on species from more
than one trophic level; likewise, some of these organisms can be eaten by species from multiple trophic levels.
In other words, the linear model of ecosystems, the food chain, is not completely descriptive of ecosystem
structure. A holistic model—which accounts for all the interactions between different species and their complex
interconnected relationships with each other and with the environment—is a more accurate and descriptive
model for ecosystems. A food web is a graphic representation of a holistic, nonlinear web of primary producers,
primary consumers, and higher-level consumers used to describe ecosystem structure and dynamics (Figure
46.6).
Figure 46.6 This food web shows the interactions between organisms across trophic levels in the Lake Ontario
ecosystem. Primary producers are outlined in green, primary consumers in orange, secondary consumers in blue, and
tertiary (apex) consumers in purple. Arrows point from an organism that is consumed to the organism that consumes
it. Notice how some lines point to more than one trophic level. For example, the opossum shrimp eats both primary
producers and primary consumers, (credit: NOAA, GLERL)
A comparison of the two types of structural ecosystem models shows strength in both. Food chains are more
flexible for analytical modeling, are easier to follow, and are easier to experiment with, whereas food web
models more accurately represent ecosystem structure and dynamics, and data can be directly used as input for
simulation modeling.
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Chapter 46 | Ecosystems
LINK TQ LEARNING
Head to this online interactive simulator (http:// 0 penstaxc 0 llege. 0 rg/l/f 00 d_web) to investigate food web
function. In the Interactive Labs box, under Food Web , click Step 1. Read the instructions first, and then click
Step 2 for additional instructions. When you are ready to create a simulation, in the upper-right corner of the
Interactive Labs box, click OPEN SIMULATOR.
Two general types of food webs are often shown interacting within a single ecosystem. A grazing food web
(such as the Lake Ontario food web in Figure 46.6) has plants or other photosynthetic organisms at its base,
followed by herbivores and various carnivores. A detrital food web consists of a base of organisms that feed
on decaying organic matter (dead organisms), called decomposers or detritivores. These organisms are usually
bacteria or fungi that recycle organic material back into the biotic part of the ecosystem as they themselves are
consumed by other organisms. As all ecosystems require a method to recycle material from dead organisms,
most grazing food webs have an associated detrital food web. For example, in a meadow ecosystem, plants may
support a grazing food web of different organisms, primary and other levels of consumers, while at the same
time supporting a detrital food web of bacteria, fungi, and detrivorous invertebrates feeding off dead plants and
animals.
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Chapter 46 | Ecosystems
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V /
e olution CONNECTION
Three-spined Stickleback
It is well established by the theory of natural selection that changes in the environment play a major role in
the evolution of species within an ecosystem. However, little is known about how the evolution of species
within an ecosystem can alter the ecosystem environment. In 2009, Dr. Luke Harmon, from the University of
Idaho, published a paper that for the first time showed that the evolution of organisms into subspecies can
have direct effects on their ecosystem environment.
The three-spined stickleback (Gasterosteus aculeatus) is a freshwater fish that evolved from a saltwater fish
to live in freshwater lakes about 10,000 years ago, which is considered a recent development in evolutionary
time (Figure 46.7). Over the last 10,000 years, these freshwater fish then became isolated from each other
in different lakes. Depending on which lake population was studied, findings showed that these sticklebacks
then either remained as one species or evolved into two species. The divergence of species was made
possible by their use of different areas of the pond for feeding called micro niches.
Dr. Harmon and his team created artificial pond microcosms in 250-gallon tanks and added muck from
freshwater ponds as a source of zooplankton and other invertebrates to sustain the fish. In different
experimental tanks they introduced one species of stickleback from either a single-species or double¬
species lake.
Over time, the team observed that some of the tanks bloomed with algae while others did not. This puzzled
the scientists, and they decided to measure the water's dissolved organic carbon (DOC), which consists of
mostly large molecules of decaying organic matter that give pond-water its slightly brownish color. It turned
out that the water from the tanks with two-species fish contained larger particles of DOC (and hence darker
water) than water with single-species fish. This increase in DOC blocked the sunlight and prevented algal
blooming. Conversely, the water from the single-species tank contained smaller DOC particles, allowing
more sunlight penetration to fuel the algal blooms.
This change in the environment, which is due to the different feeding habits of the stickleback species in
each lake type, probably has a great impact on the survival of other species in these ecosystems, especially
other photosynthetic organisms. Thus, the study shows that, at least in these ecosystems, the environment
and the evolution of populations have reciprocal effects that may now be factored into simulation models.
Figure 46.7 The three-spined stickleback evolved from a saltwater fish to freshwater fish, (credit: Barrett Paul,
USFWS)
Research into Ecosystem Dynamics: Ecosystem Experimentation and
Modeling
The study of the changes in ecosystem structure caused by changes in the environment (disturbances) or
by internal forces is called ecosystem dynamics. Ecosystems are characterized using a variety of research
1. Nature (Vol. 458, April 1, 2009)
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Chapter 46 | Ecosystems
methodologies. Some ecologists study ecosystems using controlled experimental systems, while some study
entire ecosystems in their natural state, and others use both approaches.
A holistic ecosystem model attempts to quantify the composition, interaction, and dynamics of entire
ecosystems; it is the most representative of the ecosystem in its natural state. A food web is an example of a
holistic ecosystem model. However, this type of study is limited by time and expense, as well as the fact that it
is neither feasible nor ethical to do experiments on large natural ecosystems. It is difficult to quantify all different
species in an ecosystem and the dynamics in their habitat, especially when studying large habitats such as the
Amazon Rainforest.
For these reasons, scientists study ecosystems under more controlled conditions. Experimental systems usually
involve either partitioning a part of a natural ecosystem that can be used for experiments, termed a mesocosm,
or by recreating an ecosystem entirely in an indoor or outdoor laboratory environment, which is referred
to as a microcosm. A major limitation to these approaches is that removing individual organisms from
their natural ecosystem or altering a natural ecosystem through partitioning may change the dynamics of
the ecosystem. These changes are often due to differences in species numbers and diversity and also to
environment alterations caused by partitioning (mesocosm) or recreating (microcosm) the natural habitat. Thus,
these types of experiments are not totally predictive of changes that would occur in the ecosystem from which
they were gathered.
As both of these approaches have their limitations, some ecologists suggest that results from these experimental
systems should be used only in conjunction with holistic ecosystem studies to obtain the most representative
data about ecosystem structure, function, and dynamics.
Scientists use the data generated by these experimental studies to develop ecosystem models that demonstrate
the structure and dynamics of ecosystems. They use three basic types of ecosystem modeling in research
and ecosystem management: a conceptual model, an analytical model, and a simulation model. A conceptual
model is an ecosystem model that consists of flow charts to show interactions of different compartments of
the living and nonliving components of the ecosystem. A conceptual model describes ecosystem structure
and dynamics and shows how environmental disturbances affect the ecosystem; however, its ability to predict
the effects of these disturbances is limited. Analytical and simulation models, in contrast, are mathematical
methods of describing ecosystems that are indeed capable of predicting the effects of potential environmental
changes without direct experimentation, although with some limitations as to accuracy. An analytical model is
an ecosystem model that is created using simple mathematical formulas to predict the effects of environmental
disturbances on ecosystem structure and dynamics. A simulation model is an ecosystem model that is created
using complex computer algorithms to holistically model ecosystems and to predict the effects of environmental
disturbances on ecosystem structure and dynamics. Ideally, these models are accurate enough to determine
which components of the ecosystem are particularly sensitive to disturbances, and they can serve as a guide
to ecosystem managers (such as conservation ecologists or fisheries biologists) in the practical maintenance of
ecosystem health.
Conceptual Models
Conceptual models are useful for describing ecosystem structure and dynamics and for demonstrating the
relationships between different organisms in a community and their environment. Conceptual models are usually
depicted graphically as flow charts. The organisms and their resources are grouped into specific compartments
with arrows showing the relationship and transfer of energy or nutrients between them. Thus, these diagrams
are sometimes called compartment models.
To model the cycling of mineral nutrients, organic and inorganic nutrients are subdivided into those that are
bioavailable (ready to be incorporated into biological macromolecules) and those that are not. For example,
in a terrestrial ecosystem near a deposit of coal, carbon will be available to the plants of this ecosystem as
carbon dioxide gas in a short-term period, not from the carbon-rich coal itself. However, over a longer period,
microorganisms capable of digesting coal will incorporate its carbon or release it as natural gas (methane, CH 4 ),
changing this unavailable organic source into an available one. This conversion is greatly accelerated by the
combustion of fossil fuels by humans, which releases large amounts of carbon dioxide into the atmosphere. This
is thought to be a major factor in the rise of the atmospheric carbon dioxide levels in the industrial age. The
carbon dioxide released from burning fossil fuels is produced faster than photosynthetic organisms can use it.
This process is intensified by the reduction of photosynthetic trees because of worldwide deforestation. Most
scientists agree that high atmospheric carbon dioxide is a major cause of global climate change.
Conceptual models are also used to show the flow of energy through particular ecosystems. Figure 46.8 is
based on Howard T. Odum’s classical study of the Silver Springs, Florida, holistic ecosystem in the mid-twentieth
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Chapter 46 | Ecosystems
1467
[2]
century.
This study shows the energy content and transfer between various ecosystem compartments.
visual
CONNECTION
Sunlight
1,700,000
kcal/m 2 /yr
(U
(SI
o
o.
E
o
u
0)
•o
o
Gross productivity
Net productivity
Figure 46.8 This conceptual model shows the flow of energy through a spring ecosystem in Silver Springs,
Florida. Notice that the energy decreases with each increase in trophic level.
Why do you think the value for gross productivity of the primary producers is the same as the value for total
heat and respiration (20,810 kcal/m 2 /yr)?
Analytical and Simulation Models
The major limitation of conceptual models is their inability to predict the consequences of changes in ecosystem
species and/or environment. Ecosystems are dynamic entities and subject to a variety of abiotic and biotic
disturbances caused by natural forces and/or human activity. Ecosystems altered from their initial equilibrium
state can often recover from such disturbances and return to a state of equilibrium. As most ecosystems are
subject to periodic disturbances and are often in a state of change, they are usually either moving toward or away
from their equilibrium state. There are many of these equilibrium states among the various components of an
2. Howard T. Odum, “Trophic Structure and Productivity of Silver Springs, Florida," Ecological Monographs 27, no. 1 (1957): 47-112.
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Chapter 46 | Ecosystems
ecosystem, which affects the ecosystem overall. Furthermore, as humans have the ability to greatly and rapidly
alter the species content and habitat of an ecosystem, the need for predictive models that enable understanding
of how ecosystems respond to these changes becomes more crucial.
Analytical models often use simple, linear components of ecosystems, such as food chains, and are known to be
complex mathematically; therefore, they require a significant amount of mathematical knowledge and expertise.
Although analytical models have great potential, their simplification of complex ecosystems is thought to limit
their accuracy. Simulation models that use computer programs are better able to deal with the complexities of
ecosystem structure.
A recent development in simulation modeling uses supercomputers to create and run individual-based
simulations, which accounts for the behavior of individual organisms and their effects on the ecosystem as a
whole. These simulations are considered to be the most accurate and predictive of the complex responses of
ecosystems to disturbances.
LINK TQ LEARNING
Visit The Darwin Project (http:// 0 penstaxc 0 llege. 0 rg/l/Darwin_pr 0 ject) to view a variety of ecosystem
models.
46.2 | Energy Flow through Ecosystems
By the end of this section, you will be able to do the following:
• Describe how organisms acquire energy in a food web and in associated food chains
• Explain how the efficiency of energy transfers between trophic levels affects ecosystem structure and
dynamics
• Discuss trophic levels and how ecological pyramids are used to model them
All living things require energy in one form or another. Energy is required by most complex metabolic pathways
(often in the form of adenosine triphosphate, ATP), especially those responsible for building large molecules
from smaller compounds, and life itself is an energy-driven process. Living organisms would not be able to
assemble macromolecules (proteins, lipids, nucleic acids, and complex carbohydrates) from their monomeric
subunits without a constant energy input.
It is important to understand how organisms acquire energy and how that energy is passed from one organism to
another through food webs and their constituent food chains. Food webs illustrate how energy flows directionally
through ecosystems, including how efficiently organisms acquire it, use it, and how much remains for use by
other organisms of the food web.
How Organisms Acquire Energy in a Food Web
Energy is acquired by living things in three ways: photosynthesis, chemosynthesis, and the consumption and
digestion of other living or previously living organisms by heterotrophs.
Photosynthetic and chemosynthetic organisms are both grouped into a category known as autotrophs:
organisms capable of synthesizing their own food (more specifically, capable of using inorganic carbon as
a carbon source). Photosynthetic autotrophs (photoautotrophs) use sunlight as an energy source, whereas
chemosynthetic autotrophs (chemoautotrophs) use inorganic molecules as an energy source. Autotrophs are
critical for all ecosystems. Without these organisms, energy would not be available to other living organisms and
life itself would not be possible.
Photoautotrophs, such as plants, algae, and photosynthetic bacteria, serve as the energy source for a majority of
the world’s ecosystems. These ecosystems are often described by grazing food webs. Photoautotrophs harness
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Chapter 46 | Ecosystems
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the solar energy of the sun by converting it to chemical energy in the form of ATP (and NADP). The energy
stored in ATP is used to synthesize complex organic molecules, such as glucose.
Chemoautotrophs are primarily bacteria that are found in rare ecosystems where sunlight is not available,
such as in those associated with dark caves or hydrothermal vents at the bottom of the ocean (Figure 46.9).
Many chemoautotrophs in hydrothermal vents use hydrogen sulfide (H 2 S), which is released from the vents as
a source of chemical energy. This allows chemoautotrophs to synthesize complex organic molecules, such as
glucose, for their own energy and in turn supplies energy to the rest of the ecosystem.
Figure 46.9 Swimming shrimp, a few squat lobsters, and hundreds of vent mussels are seen at a hydrothermal vent
at the bottom of the ocean. As no sunlight penetrates to this depth, the ecosystem is supported by chemoautotrophic
bacteria and organic material that sinks from the ocean’s surface. This picture was taken in 2006 at the submerged NW
Eifuku volcano off the coast of Japan by the National Oceanic and Atmospheric Administration (NOAA). The summit of
this highly active volcano lies 1535 m below the surface.
Productivity within Trophic Levels
Productivity within an ecosystem can be defined as the percentage of energy entering the ecosystem
incorporated into biomass in a particular trophic level. Biomass is the total mass, in a unit area at the time
of measurement, of living or previously living organisms within a trophic level. Ecosystems have characteristic
amounts of biomass at each trophic level. For example, in the English Channel ecosystem the primary producers
account for a biomass of 4 g/m 2 (grams per square meter), while the primary consumers exhibit a biomass of 21
g/m 2 .
The productivity of the primary producers is especially important in any ecosystem because these organisms
bring energy to other living organisms by photoautotrophy or chemoautotrophy. The rate at which photosynthetic
primary producers incorporate energy from the sun is called gross primary productivity. An example of gross
primary productivity is shown in the compartment diagram of energy flow within the Silver Springs aquatic
ecosystem as shown (Figure 46.8). In this ecosystem, the total energy accumulated by the primary producers
(gross primary productivity) was shown to be 20,810 kcal/m 2 /yr.
Because all organisms need to use some of this energy for their own functions (like respiration and resulting
metabolic heat loss) scientists often refer to the net primary productivity of an ecosystem. Net primary
productivity is the energy that remains in the primary producers after accounting for the organisms’ respiration
and heat loss. The net productivity is then available to the primary consumers at the next trophic level. In our
Silver Springs example, 13,187 of the 20,810 kcal/m 2 /yr were used for respiration or were lost as heat, leaving
7,633 kcal/m 2 /yr of energy for use by the primary consumers.
Ecological Efficiency: The Transfer of Energy between Trophic Levels
As illustrated in (Figure 46.8), as energy flows from primary producers through the various trophic levels, the
ecosystem loses large amounts of energy. The main reason for this loss is the second law of thermodynamics,
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Chapter 46 | Ecosystems
which states that whenever energy is converted from one form to another, there is a tendency toward disorder
(entropy) in the system. In biologic systems, this energy takes the form of metabolic heat, which is lost when
the organisms consume other organisms, in the Silver Springs ecosystem example (Figure 46.8), we see that
the primary consumers produced 1103 kcal/m 2 /yr from the 7618 kcal/m 2 /yr of energy available to them from
the primary producers. The measurement of energy transfer efficiency between two successive trophic levels is
termed the trophic level transfer efficiency (TLTE) and is defined by the formula:
TLTE — P roc * uct i on at present trophic level
production at previous trophic level
In Silver Springs, the TLTE between the first two trophic levels was approximately 14.8 percent. The low
efficiency of energy transfer between trophic levels is usually the major factor that limits the length of food chains
observed in a food web. The fact is, after four to six energy transfers, there is not enough energy left to support
another trophic level. In the Lake Ontario example shown in (Figure 46.6), only three energy transfers occurred
between the primary producer, (green algae), and the apex consumer (Chinook salmon).
Ecologists have many different methods of measuring energy transfers within ecosystems. Measurement
difficulty depends on the complexity of the ecosystem and how much access scientists have to observe the
ecosystem, in other words, some ecosystems are more difficult to study than others, and sometimes the
quantification of energy transfers has to be estimated.
Other parameters are important in characterizing energy flow within an ecosystem. Net production efficiency
(NPE) allows ecologists to quantify how efficiently organisms of a particular trophic level incorporate the energy
they receive into biomass; it is calculated using the following formula:
. TT ,_ net consumer productivity ,,,,,
NPE = - : — 7T-. - -X 100
assimilation
Net consumer productivity is the energy content available to the organisms of the next trophic level.
Assimilation is the biomass (energy content generated per unit area) of the present trophic level after
accounting for the energy lost due to incomplete ingestion of food, energy used for respiration, and energy lost
as waste. Incomplete ingestion refers to the fact that some consumers eat only a part of their food. For example,
when a lion kills an antelope, it will eat everything except the hide and bones. The lion is missing the energy-rich
bone marrow inside the bone, so the lion does not make use of all the calories its prey could provide.
Thus, NPE measures how efficiently each trophic level uses and incorporates the energy from its food into
biomass to fuel the next trophic level. In general, cold-blooded animals (ectotherms), such as invertebrates, fish,
amphibians, and reptiles, use less of the energy they obtain for respiration and heat than warm-blooded animals
(endotherms), such as birds and mammals. The extra heat generated in endotherms, although an advantage
in terms of the activity of these organisms in colder environments, is a major disadvantage in terms of NPE.
Therefore, many endotherms have to eat more often than ectotherms to get the energy they need for survival. In
general, NPE for ectotherms is an order of magnitude (lOx) higher than for endotherms. For example, the NPE
for a caterpillar eating leaves has been measured at 18 percent, whereas the NPE for a squirrel eating acorns
may be as low as 1.6 percent.
The inefficiency of energy use by warm-blooded animals has broad implications for the world's food supply. It
is widely accepted that the meat industry uses large amounts of crops to feed livestock, and because the NPE
is low, much of the energy from animal feed is lost. For example, it costs about $0.01 to produce 1000 dietary
calories (kcal) of corn or soybeans, but approximately $0.19 to produce a similar number of calories growing
cattle for beef consumption. The same energy content of milk from cattle is also costly, at approximately $0.16
per 1000 kcal. Much of this difference is due to the low NPE of cattle. Thus, there has been a growing movement
worldwide to promote the consumption of nonmeat and nondairy foods so that less energy is wasted feeding
animals for the meat industry.
Modeling Ecosystems Energy Flow: Ecological Pyramids
The structure of ecosystems can be visualized with ecological pyramids, which were first described by the
pioneering studies of Charles Elton in the 1920s. Ecological pyramids show the relative amounts of various
parameters (such as number of organisms, energy, and biomass) across trophic levels.
Pyramids of numbers can be either upright or inverted, depending on the ecosystem. As shown in Figure 46.10,
typical grassland during the summer has a base of many plants, and the numbers of organisms decrease at
each trophic level. However, during the summer in a temperate forest, the base of the pyramid consists of few
trees compared with the number of primary consumers, mostly insects. Because trees are large, they have
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Chapter 46 | Ecosystems
1471
great photosynthetic capability, and dominate other plants in this ecosystem to obtain sunlight. Even in smaller
numbers, primary producers in forests are still capable of supporting other trophic levels.
Another way to visualize ecosystem structure is with pyramids of biomass. This pyramid measures the amount
of energy converted into living tissue at the different trophic levels. Using the Silver Springs ecosystem example,
this data exhibits an upright biomass pyramid (Figure 46.10), whereas the pyramid from the English Channel
example is inverted. The plants (primary producers) of the Silver Springs ecosystem make up a large percentage
of the biomass found there. However, the phytoplankton in the English Channel example make up less biomass
than the primary consumers, the zooplankton. As with inverted pyramids of numbers, this inverted pyramid
is not due to a lack of productivity from the primary producers, but results from the high turnover rate of
the phytoplankton. The phytoplankton are consumed rapidly by the primary consumers, thus, minimizing their
biomass at any particular point in time. However, phytoplankton reproduce quickly, thus they are able to support
the rest of the ecosystem.
Pyramid ecosystem modeling can also be used to show energy flow through the trophic levels. Notice that
these numbers are the same as those used in the energy flow compartment diagram in (Figure 46.8). Pyramids
of energy are always upright, and an ecosystem without sufficient primary productivity cannot be supported.
All types of ecological pyramids are useful for characterizing ecosystem structure. However, in the study of
energy flow through the ecosystem, pyramids of energy are the most consistent and representative models of
ecosystem structure (Figure 46.10).
visual
CONNECTION
A. Biomass (dry mass, g/m 2 )
Silver Springs, Florida
English Channel
Fishes 5
Decomposers
Fishes 11
21 Zooplankton
(fungi, bacteria) 5
Herbivorous insects,
snails 37
4 Phytoplankton
Plants 809
B. Number of individuals per 0.1 hectare
Grassland (summer)
Temperate forest (summer)
1 Bird
5 Birds
1 90,000 Predatory insects
120,000 Predatory insects
200,000 Herbivorous
150,000 Herbivorous insects
insects
1,500,000 Grass plants
200 trees
C. Energy (kcal/m 2 /yr)
Silver Springs, Florida
Tertiary (apex) consumer
Decomposers Fishes 21
(fungi, bacteria) _
5060 ■ Fishes 383
Insects, snails
3368
Secondary consumer
Primary consumer
Primary producer
Plants 20,810
Figure 46.10 Ecological pyramids depict the (a) biomass, (b) number of organisms, and (c) energy in each trophic
level.
Pyramids depicting the number of organisms or biomass may be inverted, upright, or even diamond-shaped.
Energy pyramids, however, are always upright. Why?
Consequences of Food Webs: Biological Magnification
One of the most important environmental consequences of ecosystem dynamics is biomagnification.
Biomagnification is the increasing concentration of persistent, toxic substances in organisms at each trophic
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Chapter 46 | Ecosystems
level, from the primary producers to the apex consumers. Many substances have been shown to bioaccumulate,
including the pesticide dichlorodiphenyltrichloroethane (DDT), which was described in the 1960s bestseller,
Silent Spring, by Rachel Carson. DDT was a commonly used pesticide before its dangers became known. In
some aquatic ecosystems, organisms from each trophic level consumed many organisms of the lower level,
which caused DDT to increase in birds (apex consumers) that ate fish. Thus, the birds accumulated sufficient
amounts of DDT to cause fragility in their eggshells. This effect increased egg breakage during nesting and was
shown to have adverse effects on these bird populations. The use of DDT was banned in the United States in
the 1970s.
Other substances that biomagnify are polychlorinated biphenyls (PCBs), which were used in coolant liquids in
the United States until their use was banned in 1979, and heavy metals, such as mercury, lead, and cadmium.
These substances were best studied in aquatic ecosystems, where fish species at different trophic levels
accumulate toxic substances brought through the ecosystem by the primary producers. As illustrated in a study
performed by the National Oceanic and Atmospheric Administration (NOAA) in the Saginaw Bay of Lake Huron
(Figure 46.11), PCB concentrations increased from the ecosystem’s primary producers (phytoplankton) through
the different trophic levels of fish species. The apex consumer (walleye) has more than four times the amount of
PCBs compared to phytoplankton. Also, based on results from other studies, birds that eat these fish may have
PCB levels at least one order of magnitude higher than those found in the lake fish.
Figure 46.11 This chart shows the PCB concentrations found at the various trophic levels in the Saginaw Bay
ecosystem of Lake Huron. Numbers on the x-axis reflect enrichment with heavy isotopes of nitrogen ( 15 N), which is a
marker for increasing trophic level. Notice that the fish in the higher trophic levels accumulate more PCBs than those
in lower trophic levels, (credit: Patricia Van Hoof, NOAA, GLERL)
Other concerns have been raised by the accumulation of heavy metals, such as mercury and cadmium, in certain
types of seafood. The United States Environmental Protection Agency (EPA) recommends that pregnant women
and young children should not consume any swordfish, shark, king mackerel, or tilefish because of their high
mercury content. These individuals are advised to eat fish low in mercury: salmon, tilapia, shrimp, pollock, and
catfish. Biomagnification is a good example of how ecosystem dynamics can affect our everyday lives, even
influencing the food we eat.
46.3 | Biogeochemical Cycles
By the end of this section, you will be able to do the following:
• Discuss the biogeochemical cycles of water, carbon, nitrogen, phosphorus, and sulfur
• Explain how human activities have impacted these cycles and the potential consequences for Earth
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Chapter 46 | Ecosystems
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Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for
chemoautotrophs) and leaving as heat during the many transfers between trophic levels. However, the matter
that makes up living organisms is conserved and recycled. The six most common elements associated with
organic molecules—carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur—take a variety of chemical
forms and may exist for long periods in the atmosphere, on land, in water, or beneath the Earth’s surface.
Geologic processes, such as weathering, erosion, water drainage, and the subduction of the continental plates,
all play a role in this recycling of materials. Because geology and chemistry have major roles in the study
of this process, the recycling of inorganic matter between living organisms and their environment is called a
biogeochemical cycle.
Water contains hydrogen and oxygen, which is essential to all living processes. The hydrosphere is the area
of the Earth where water movement and storage occurs. On or beneath the surface, water occurs in liquid or
solid form in rivers, lakes, oceans, groundwater, polar ice caps, and glaciers. And it occurs as water vapor in
the atmosphere. Carbon is found in all organic macromolecules and is an important constituent of fossil fuels.
Nitrogen is a major component of our nucleic acids and proteins and is critical to human agriculture. Phosphorus,
a major component of nucleic acid (along with nitrogen), is one of the main ingredients in artificial fertilizers used
in agriculture and their associated environmental impacts on our surface water. Sulfur is critical to the 3-D folding
of proteins, such as in disulfide binding.
The cycling of these elements is interconnected. For example, the movement of water is critical for the leaching
of nitrogen and phosphate into rivers, lakes, and oceans. Furthermore, the ocean itself is a major reservoir for
carbon. Thus, mineral nutrients are cycled, either rapidly or slowly, through the entire biosphere, from one living
organism to another, and between the biotic and abiotic world.
LINK TQ LEARNING
Head to this website (httpd/openstaxcollege.org/l/biogeochemical) to learn more about biogeochemical
cycles.
The Water (Hydrologic) Cycle
Water is the basis of all living processes on Earth. When examining the stores of water on Earth, 97.5 percent of
it is non-potable salt water (Figure 46.12). Of the remaining water, 99 percent is locked underground as water
or as ice. Thus, less than 1 percent of fresh water is easily accessible from lakes and rivers. Many living things,
such as plants, animals, and fungi, are dependent on that small amount of fresh surface water, a lack of which
can have massive effects on ecosystem dynamics. To be successful, organisms must adapt to fluctuating water
supplies. Humans, of course, have developed technologies to increase water availability, such as digging wells
to harvest groundwater, storing rainwater, and using desalination to obtain drinkable water from the ocean.
Lakes and rivers 0.3%
Groundwater (soil moisture, swamp
water, permafrost) 30.8%
Freshwater 2.5%
(35,000,000 km 3 )
Glaciers and permanent snow cover
68.9%
Figure 46.12 Only 2.5 percent of water on Earth is fresh water, and less than 1 percent of fresh water is easily
accessible to living things.
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Chapter 46 | Ecosystems
Water cycling is extremely important to ecosystem dynamics. Water has a major influence on climate and,
thus, on the environments of ecosystems. Most of the water on Earth is stored for long periods in the oceans,
underground, and as ice. Figure 46.13 illustrates the average time that an individual water molecule may spend
in the Earth’s major water reservoirs. Residence time is a measure of the average time an individual water
molecule stays in a particular reservoir.
Average Residence Time for Water Molecules
Biospheric (in living organisms) 1 week
Atmospheric 1.5 weeks
Rivers 2 weeks
Soil moisture 2 weeks-1 year
Swamps 1-10 years
Lakes & reservoirs 10 years
Oceans & seas 4,000 years
Groundwater 2 weeks to 10,000 years
Glaciers and permafrost 1,000-10,000 years
Figure 46.13 This graph shows the average residence time for water molecules in the Earth’s water reservoirs.
There are various processes that occur during the cycling of water, shown in Figure 46.14. These processes
include the following:
• evaporation/sublimation
• condensation/precipitation
• subsurface water flow
• surface runoff/snowmelt
• streamflow
The water cycle is driven by the sun’s energy as it warms the oceans and other surface waters. This leads to
the evaporation (water to water vapor) of liquid surface water and the sublimation (ice to water vapor) of frozen
water, which deposits large amounts of water vapor into the atmosphere. Over time, this water vapor condenses
into clouds as liquid or frozen droplets and is eventually followed by precipitation (rain or snow), which returns
water to the Earth’s surface. Rain eventually permeates into the ground, where it may evaporate again if it is
near the surface, flow beneath the surface, or be stored for long periods. More easily observed is surface runoff:
the flow of fresh water either from rain or melting ice. Runoff can then make its way through streams and lakes
to the oceans or flow directly to the oceans themselves.
LINK TQ LEARNING
Head to this website (http:// 0 penstaxc 0 llege. 0 rg/l/freshwater) to learn more about the world’s fresh water
supply.
Rain and surface runoff are major ways in which minerals, including carbon, nitrogen, phosphorus, and sulfur,
are cycled from land to water. The environmental effects of runoff will be discussed later as these cycles are
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Chapter 46 | Ecosystems
1475
described.
Figure 46.14 Water from the land and oceans enters the atmosphere by evaporation or sublimation, where it
condenses into clouds and falls as rain or snow. Precipitated water may enter freshwater bodies or infiltrate the soil.
The cycle is complete when surface or groundwater reenters the ocean, (credit: modification of work by John M. Evans
and Howard Perlman, USGS)
The Carbon Cycle
Carbon is the second most abundant element in living organisms. Carbon is present in all organic molecules,
and its role in the structure of macromolecules is of primary importance to living organisms.
The carbon cycle is most easily studied as two interconnected sub-cycles: one dealing with rapid carbon
exchange among living organisms and the other dealing with the long-term cycling of carbon through geologic
processes. The entire carbon cycle is shown in Figure 46.15.
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Chapter 46 | Ecosystems
Figure 46.15 Carbon dioxide gas exists in the atmosphere and is dissolved in water. Photosynthesis converts carbon
dioxide gas to organic carbon, and respiration cycles the organic carbon back into carbon dioxide gas. Long-term
storage of organic carbon occurs when matter from living organisms is buried deep underground and becomes
fossilized. Volcanic activity and, more recently, human emissions, bring this stored carbon back into the carbon cycle,
(credit: modification of work by John M. Evans and Howard Perlman, USGS)
LINK
T a
LEARNING
Click this link (http:// 0 penstaxc 0 llege. 0 rg/l/carb 0 n cycle) to read information about the United States
Carbon Cycle Science Program.
The Biological Carbon Cycle
Living organisms are connected in many ways, even between ecosystems. A good example of this connection
is the exchange of carbon between autotrophs and heterotrophs within and between ecosystems by way of
atmospheric carbon dioxide. Carbon dioxide is the basic building block that most autotrophs use to build
multicarbon, high energy compounds, such as glucose. The energy harnessed from the sun is used by these
organisms to form the covalent bonds that link carbon atoms together. These chemical bonds thereby store this
energy for later use in the process of respiration. Most terrestrial autotrophs obtain their carbon dioxide directly
from the atmosphere, while marine autotrophs acquire it in the dissolved form (carbonic acid, H 2 CO 3 ). However
carbon dioxide is acquired, a by-product of the process is oxygen. The photosynthetic organisms are responsible
for depositing approximately 21 percent oxygen content of the atmosphere that we observe today.
Heterotrophs and autotrophs are partners in biological carbon exchange (especially the primary consumers,
largely herbivores). Heterotrophs acquire the high-energy carbon compounds from the autotrophs by consuming
them, and breaking them down by respiration to obtain cellular energy, such as ATP. The most efficient type of
respiration, aerobic respiration, requires oxygen obtained from the atmosphere or dissolved in water. Thus, there
is a constant exchange of oxygen and carbon dioxide between the autotrophs (which need the carbon) and the
heterotrophs (which need the oxygen). Gas exchange through the atmosphere and water is one way that the
carbon cycle connects all living organisms on Earth.
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Chapter 46 | Ecosystems
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The Biogeochemical Carbon Cycle
The movement of carbon through the land, water, and air is complex, and in many cases, it occurs much more
slowly geologically than as seen between living organisms. Carbon is stored for long periods in what are known
as carbon reservoirs, which include the atmosphere, bodies of liquid water (mostly oceans), ocean sediment,
soil, land sediments (including fossil fuels), and the Earth’s interior.
As stated, the atmosphere is a major reservoir of carbon in the form of carbon dioxide and is essential to the
process of photosynthesis. The level of carbon dioxide in the atmosphere is greatly influenced by the reservoir
of carbon in the oceans. The exchange of carbon between the atmosphere and water reservoirs influences how
much carbon is found in each location, and each one affects the other reciprocally. Carbon dioxide (CO 2 ) from
the atmosphere dissolves in water and combines with water molecules to form carbonic acid, and then it ionizes
to carbonate and bicarbonate ions (Figure 46.16)
Step 1: C0 2 (atmospheric) - C0 2 (dissolved)
Step 2: C0 2 (dissolved) + H 2 0 - H 2 C0 3 (carbonic acid)
Step 3: H 2 C0 3 ^ + H + + HC0 3 ~ (biocarbonate ion)
Step 4: HC0 3 _ - H + + C0 3 2 ~ (carbonate ion)
Figure 46.16 Carbon dioxide reacts with water to form bicarbonate and carbonate ions.
The equilibrium coefficients are such that more than 90 percent of the carbon in the ocean is found as
bicarbonate ions. Some of these ions combine with seawater calcium to form calcium carbonate (CaCOs), a
major component of marine organism shells. These organisms eventually form sediments on the ocean floor.
Over geologic time, the calcium carbonate forms limestone, which comprises the largest carbon reservoir on
Earth.
On land, carbon is stored in soil as a result of the decomposition of living organisms (by decomposers) or from
weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirs by surface
runoff. Deeper underground, on land and at sea, are fossil fuels: the anaerobically decomposed remains of
plants that take millions of years to form. Fossil fuels are considered a nonrenewable resource because their
use far exceeds their rate of formation. A nonrenewable resource, such as fossil fuel, is either regenerated
very slowly or not at all. Another way for carbon to enter the atmosphere is from land (including land beneath the
surface of the ocean) by the eruption of volcanoes and other geothermal systems. Carbon sediments from the
ocean floor are taken deep within the Earth by the process of subduction: the movement of one tectonic plate
beneath another. Carbon is released as carbon dioxide when a volcano erupts or from volcanic hydrothermal
vents.
Humans contribute to atmospheric carbon by the burning of fossil fuels and other materials. Since the industrial
Revolution, humans have significantly increased the release of carbon and carbon compounds, which has in
turn affected the climate and overall environment.
Animal husbandry by humans also increases atmospheric carbon. The large numbers of land animals raised to
feed the Earth’s growing population results in increased carbon dioxide levels in the atmosphere due to farming
practices and respiration and methane production. This is another example of how human activity indirectly
affects biogeochemical cycles in a significant way. Although much of the debate about the future effects of
increasing atmospheric carbon on climate change focuses on fossils fuels, scientists take natural processes,
such as volcanoes and respiration, into account as they model and predict the future impact of this increase.
The Nitrogen Cycle
Getting nitrogen into the living world is difficult. Plants and phytoplankton are not equipped to incorporate
nitrogen from the atmosphere (which exists as tightly bonded, triple covalent N 2 ) even though this molecule
comprises approximately 78 percent of the atmosphere. Nitrogen enters the living world via free-living and
symbiotic bacteria, which incorporate nitrogen into their macromolecules through nitrogen fixation (conversion of
N 2 ). Cyanobacteria live in most aquatic ecosystems where sunlight is present; they play a key role in nitrogen
fixation. Cyanobacteria are able to use inorganic sources of nitrogen to “fix” nitrogen. Rhizobium bacteria live
symbiotically in the root nodules of legumes (such as peas, beans, and peanuts) and provide them with the
organic nitrogen they need. (For example, gardeners often grow peas both for their produce and to naturally
add nitrogen to the soil. This practice goes back to ancient times, even if the science has only been recently
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Chapter 46 | Ecosystems
understood.) Free-living bacteria, such as Azotobacter, are also important nitrogen fixers.
Organic nitrogen is especially important to the study of ecosystem dynamics since many ecosystem processes,
such as primary production and decomposition, are limited by the available supply of nitrogen. As shown
in Figure 46.17, the nitrogen that enters living systems by nitrogen fixation is successively converted from
organic nitrogen back into nitrogen gas by bacteria. This process occurs in three steps in terrestrial systems:
ammonification, nitrification, and denitrification. First, the ammonification process converts nitrogenous waste
from living animals or from the remains of dead animals into ammonium (NH 4 1 ") by certain bacteria and fungi.
Second, the ammonium is converted to nitrites (NO 2 ) by nitrifying bacteria, such as Nitrosomonas, through
nitrification. Subsequently, nitrites are converted to nitrates (NO 3 - ) by similar organisms. Third, the process
of denitrification occurs, whereby bacteria, such as Pseudomonas and Clostridium, convert the nitrates into
nitrogen gas, allowing it to reenter the atmosphere.
visual
CONNECTION
I Denitr ifi ca tipn]
JbyJjactRi 1a
Freshwater
[Nitrificatioriib vi
Ib'a'cterla'toiNO,
iNitroqenousl
[wasteslinfsoil]
' Ammonification '
[bvibacteria^ancj]
INitroq engs
(fixation
lb vi bacterial
The Nitrogen Cycle
Nitrogen gas in atmosphere (NJ
Fertilizers
Marine food webs
Runoff
Terrestnal
food webs
u k
Denitrification by
bacteria to N,
\
%
Nitrification by *
bacteria to NO,", NO,'
Oceans
Nitrogenous sediments
fall to ocean floor
Figure 46.17 Nitrogen enters the living world from the atmosphere via nitrogen-fixing bacteria. This nitrogen and
nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria, which also supply
terrestrial food webs with the organic nitrogen they need, (credit: modification of work by John M. Evans and
Howard Perlman, USGS)
Which of the following statements about the nitrogen cycle is false?
a. Ammonification converts organic nitrogenous matter from living organisms into ammonium (NH 4 + ).
b. Denitrification by bacteria converts nitrates (NO3 ) to nitrogen gas (N2).
c. Nitrification by bacteria converts nitrates (N 03 - ) to nitrites (N 02 - ).
d. Nitrogen fixing bacteria convert nitrogen gas (N 2 ) into organic compounds.
Human activity can release nitrogen into the environment by two primary means: the combustion of fossil fuels,
which releases different nitrogen oxides, and by the use of artificial fertilizers in agriculture, which are then
washed into lakes, streams, and rivers by surface runoff. Atmospheric nitrogen is associated with several effects
on Earth’s ecosystems including the production of acid rain (as nitric acid, HNO3) and greenhouse gas (as
nitrous oxide, N2O) potentially causing climate change. A major effect from fertilizer runoff is saltwater and
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Chapter 46 | Ecosystems
1479
freshwater eutrophication, a process whereby nutrient runoff causes the excess growth of microorganisms,
depleting dissolved oxygen levels and killing ecosystem fauna.
A similar process occurs in the marine nitrogen cycle, where the ammonification, nitrification, and denitrification
processes are performed by marine bacteria. Some of this nitrogen falls to the ocean floor as sediment, which
can then be moved to land in geologic time by uplift of the Earth’s surface and thereby incorporated into
terrestrial rock. Although the movement of nitrogen from rock directly into living systems has been traditionally
seen as insignificant compared with nitrogen fixed from the atmosphere, a recent study showed that this process
may indeed be significant and should be included in any study of the global nitrogen cycle.
The Phosphorus Cycle
Phosphorus is an essential nutrient for living processes; it is a major component of nucleic acid and
phospholipids, and, as calcium phosphate, makes up the supportive components of our bones. Phosphorus is
often the limiting nutrient (necessary for growth) in aquatic ecosystems (Figure 46.18).
Phosphorus occurs in nature as the phosphate ion (PO 4 3 ). In addition to phosphate runoff as a result of
human activity, natural surface runoff occurs when it is leached from phosphate-containing rock by weathering,
thus sending phosphates into rivers, lakes, and the ocean. This rock has its origins in the ocean. Phosphate-
containing ocean sediments form primarily from the bodies of ocean organisms and from their excretions.
However, in remote regions, volcanic ash, aerosols, and mineral dust may also be significant phosphate sources.
This sediment then is moved to land over geologic time by the uplifting of areas of the Earth’s surface.
Phosphorus is also reciprocally exchanged between phosphate dissolved in the ocean and marine ecosystems.
The movement of phosphate from the ocean to the land and through the soil is extremely slow, with the average
phosphate ion having an oceanic residence time between 20,000 and 100,000 years.
Figure 46.18 In nature, phosphorus exists as the phosphate ion (P 04 3- ). Weathering of rocks and volcanic activity
releases phosphate into the soil, water, and air, where it becomes available to terrestrial food webs. Phosphate enters
the oceans via surface runoff, groundwater flow, and river flow. Phosphate dissolved in ocean water cycles into marine
food webs. Some phosphate from the marine food webs falls to the ocean floor, where it forms sediment, (credit:
modification of work by John M. Evans and Howard Perlman, USGS)
As discussed in Chapter 44, excess phosphorus and nitrogen that enters these ecosystems from fertilizer runoff
3. Scott L. Morford, Benjamin Z. Houlton, and Randy A. Dahlgren, “Increased Forest Ecosystem Carbon and Nitrogen Storage from
Nitrogen Rich Bedrock," Nature 477, no. 7362 (2011): 78-81.
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Chapter 46 | Ecosystems
and from sewage causes excessive growth of microorganisms and depletes the dissolved oxygen, which leads
to the death of many ecosystem fauna, such as shellfish and finfish. This process is responsible for dead zones
in lakes and at the mouths of many major rivers (Figure 46.19).
Particulate Organic C arbon (mg/m*) Population Oentit y (pertonsAwv'l Dead Zone Sixe (km')
■■■■■■■■■■■■■■■■ *■■■■■■■■■■■■■■■ unkown. • • f A
10 20 50 100 200 500 1,000 1 10 100 1.000 10k 100k o.l 1 10 100 lk 10k
Figure 46.19 Dead zones occur when phosphorus and nitrogen from fertilizers cause excessive growth of
microorganisms, which depletes oxygen and kills fauna. Worldwide, large dead zones are found in coastal areas of
high population density, (credit: NASA Earth Observatory)
As discussed earlier, a dead zone is an area within a freshwater or marine ecosystem where large areas are
depleted of their normal flora and fauna; these zones can be caused by eutrophication, oil spills, dumping of
toxic chemicals, and other human activities. The number of dead zones has been increasing for several years,
and more than 400 of these zones were present as of 2008. One of the worst dead zones is off the coast of the
United States in the Gulf of Mexico, where fertilizer runoff from the Mississippi River basin has created a dead
zone of over 8463 square miles. Phosphate and nitrate runoff from fertilizers also negatively affect several lake
and bay ecosystems including the Chesapeake Bay in the eastern United States.
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Chapter 46 | Ecosystems
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everyday CONNECTION
Chesapeake Bay
Figure 46.20 This (a) satellite image shows the Chesapeake Bay, an ecosystem affected by phosphate and nitrate
runoff. A (b) member of the Army Corps of Engineers holds a clump of oysters being used as a part of the oyster
restoration effort in the bay. (credit a: modification of work by NASA/MODIS; credit b: modification of work by U.S.
Army)
The Chesapeake Bay has long been valued as one of the most scenic areas on Earth; it is now in distress
and is recognized as a declining ecosystem. In the 1970s, the Chesapeake Bay was one of the first
ecosystems to have identified dead zones, which continue to kill many fish and bottom-dwelling species,
such as clams, oysters, and worms. Several species have declined in the Chesapeake Bay due to surface
water runoff containing excess nutrients from artificial fertilizer used on land. The source of the fertilizers
(with high nitrogen and phosphate content) is not limited to agricultural practices. There are many nearby
urban areas and more than 150 rivers and streams empty into the bay that are carrying fertilizer runoff
from lawns and gardens. Thus, the decline of the Chesapeake Bay is a complex issue and requires the
cooperation of industry, agriculture, and everyday homeowners.
Of particular interest to conservationists is the oyster population; it is estimated that more than 200,000
acres of oyster reefs existed in the bay in the 1700s, but that number has now declined to only 36,000
acres. Oyster harvesting was once a major industry for Chesapeake Bay, but it declined 88 percent between
1982 and 2007. This decline was due not only to fertilizer runoff and dead zones but also to overharvesting.
Oysters require a certain minimum population density because they must be in close proximity to reproduce.
Human activity has altered the oyster population and locations, greatly disrupting the ecosystem.
The restoration of the oyster population in the Chesapeake Bay has been ongoing for several years with
mixed success. Not only do many people find oysters good to eat, but they also clean up the bay. Oysters
are filter feeders, and as they eat, they clean the water around them. In the 1700s, it was estimated that
it took only a few days for the oyster population to filter the entire volume of the bay. Today, with changed
water conditions, it is estimated that the present population would take nearly a year to do the same job.
Restoration efforts have been ongoing for several years by nonprofit organizations, such as the Chesapeake
Bay Foundation. The restoration goal is to find a way to increase population density so the oysters can
reproduce more efficiently. Many disease-resistant varieties (developed at the Virginia Institute of Marine
Science for the College of William and Mary) are now available and have been used in the construction
of experimental oyster reefs. Efforts to clean and restore the bay by Virginia and Delaware have been
hampered because much of the pollution entering the bay comes from other states, which stresses the need
for interstate cooperation to gain successful restoration.
The new, hearty oyster strains have also spawned a new and economically viable industry—oyster
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Chapter 46 | Ecosystems
aquaculture—which not only supplies oysters for food and profit, but also has the added benefit of cleaning
the bay.
The Sulfur Cycle
Sulfur is an essential element for the macromolecules of living things. As a part of the amino acid cysteine, it is
involved in the formation of disulfide bonds within proteins, which help to determine their 3-D folding patterns,
and hence their functions. As shown in Figure 46.21, sulfur cycles between the oceans, land, and atmosphere.
Atmospheric sulfur is found in the form of sulfur dioxide (SO 2 ) and enters the atmosphere in three ways: from
the decomposition of organic molecules, from volcanic activity and geothermal vents, and from the burning of
fossil fuels by humans.
decomposition]
The Sulfur Cycle
, Atmospheric sulfur (SO z )
Fallout
Volcano
eruption
(H 2 S)
Precipitation
Terrestrial
ecosystems
Human
emissions
(H 2 S)
Marine
sulfate
(SCV)
Runoff
oceans
Marine
ecosystems
Soillsulfates
Pyrite
(so;
Figure 46.21 Sulfur dioxide from the atmosphere becomes available to terrestrial and marine ecosystems when it is
dissolved in precipitation as weak sulfuric acid or when it falls directly to the Earth as fallout. Weathering of rocks also
makes sulfates available to terrestrial ecosystems. Decomposition of living organisms returns sulfates to the ocean,
soil, and atmosphere, (credit: modification of work by John M. Evans and Howard Perlman, USGS)
On land, sulfur is deposited in four major ways: precipitation, direct fallout from the atmosphere, rock weathering,
and geothermal vents (Figure 46.21). Atmospheric sulfur is found in the form of sulfur dioxide (SO 2 ), and as rain
falls through the atmosphere, sulfur is dissolved in the form of weak sulfuric acid (H 2 SO 4 ). Sulfur can also fall
directly from the atmosphere in a process called fallout. Also, the weathering of sulfur-containing rocks releases
sulfur into the soil. These rocks originate from ocean sediments that are moved to land by the geologic uplifting
of ocean sediments. Terrestrial ecosystems can then make use of these soil sulfates (S0 4 - ), and upon the
death and decomposition of these organisms, release the sulfur back into the atmosphere as hydrogen sulfide
(H 2 S) gas.
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Chapter 46 | Ecosystems
1483
Figure 46.22 At this sulfur vent in Lassen Volcanic National Park in northeastern California, the yellowish sulfur
deposits are visible near the mouth of the vent.
Sulfur enters the ocean via runoff from land, from atmospheric fallout, and from underwater geothermal vents.
Some ecosystems (Figure 46.9) rely on chemoautotrophs using sulfur as a biological energy source. This sulfur
then supports marine ecosystems in the form of sulfates.
Human activities have played a major role in altering the balance of the global sulfur cycle. The burning of
large quantities of fossil fuels, especially from coal, releases larger amounts of hydrogen sulfide gas into the
atmosphere. Acid rain is caused by rainwater falling to the ground through this sulfur dioxide gas, turning it into
weak sulfuric acid. Acid rain damages the natural environment by lowering the pH of lakes, which kills many of
the resident fauna; it also affects the man-made environment through the chemical degradation of buildings. For
example, many marble monuments, such as the Lincoln Memorial in Washington, DC, have suffered significant
damage from acid rain over the years.
LINK TQ LEARNING
Click this link (http:// 0 penstaxc 0 llege. 0 rg/l/climate_change) to learn more about global climate change.
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Chapter 46 | Ecosystems
KEY TERMS
acid rain corrosive rain caused by rainwater falling to the ground through sulfur dioxide gas, turning it into weak
sulfuric acid; can damage structures and ecosystems
analytical model ecosystem model that is created with mathematical formulas to predict the effects of
environmental disturbances on ecosystem structure and dynamics
apex consumer organism at the top of the food chain
assimilation biomass consumed and assimilated from the previous trophic level after accounting for the energy
lost due to incomplete ingestion of food, energy used for respiration, and energy lost as waste
biogeochemical cycle cycling of mineral nutrients through ecosystems and through the nonliving world
biomagnification increasing concentrations of persistent, toxic substances in organisms at each trophic level,
from the primary producers to the apex consumers
biomass total weight, at the time of measurement, of living or previously living organisms in a unit area within a
trophic level
chemoautotroph organism capable of synthesizing its own food using energy from inorganic molecules
conceptual model (also, compartment model) ecosystem model that consists of flow charts that show the
interactions of different compartments of the living and nonliving components of the ecosystem
dead zone area within an ecosystem in lakes and near the mouths of rivers where large areas of ecosystems
are depleted of their normal flora and fauna; these zones can be caused by eutrophication, oil spills,
dumping of toxic chemicals, and other human activities
detrital food web type of food web in which the primary consumers consist of decomposers; these are often
associated with grazing food webs within the same ecosystem
ecological pyramid (also, Eltonian pyramid) graphical representation of different trophic levels in an ecosystem
based of organism numbers, biomass, or energy content
ecosystem community of living organisms and their interactions with their abiotic environment
ecosystem dynamics study of the changes in ecosystem structure caused by changes in the environment or
internal forces
equilibrium steady state of an ecosystem where all organisms are in balance with their environment and each
other
eutrophication process whereby nutrient runoff causes the excess growth of microorganisms, depleting
dissolved oxygen levels and killing ecosystem fauna
fallout direct deposit of solid minerals on land or in the ocean from the atmosphere
food chain linear representation of a chain of primary producers, primary consumers, and higher-level
consumers used to describe ecosystem structure and dynamics
food web graphic representation of a holistic, nonlinear web of primary producers, primary consumers, and
higher-level consumers used to describe ecosystem structure and dynamics
grazing food web type of food web in which the primary producers are either plants on land or phytoplankton in
the water; often associated with a detrital food web within the same ecosystem
gross primary productivity rate at which photosynthetic primary producers incorporate energy from the sun
holistic ecosystem model study that attempts to quantify the composition, interactions, and dynamics of entire
ecosystems; often limited by economic and logistical difficulties, depending on the ecosystem
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Chapter 46 | Ecosystems
1485
hydrosphere area of the Earth where water movement and storage occurs
mesocosm portion of a natural ecosystem to be used for experiments
microcosm re-creation of natural ecosystems entirely in a laboratory environment to be used for experiments
net consumer productivity energy content available to the organisms of the next trophic level
net primary productivity energy that remains in the primary producers after accounting for the organisms’
respiration and heat loss
net production efficiency (NPE) measure of the ability of a trophic level to convert the energy it receives from
the previous trophic level into biomass
nonrenewable resource resource, such as fossil fuel, that is either regenerated very slowly or not at all
primary consumer trophic level that obtains its energy from the primary producers of an ecosystem
primary producer trophic level that obtains its energy from sunlight, inorganic chemicals, or dead and/or
decaying organic material
residence time measure of the average time an individual water molecule stays in a particular reservoir
resilience (ecological) speed at which an ecosystem recovers equilibrium after being disturbed
resistance (ecological) ability of an ecosystem to remain at equilibrium in spite of disturbances
secondary consumer usually a carnivore that eats primary consumers
simulation model ecosystem model that is created with computer programs to holistically model ecosystems
and to predict the effects of environmental disturbances on ecosystem structure and dynamics
subduction movement of one tectonic plate beneath another
tertiary consumer carnivore that eats other carnivores
trophic level position of a species or group of species in a food chain or a food web
trophic level transfer efficiency (TLTE) energy transfer efficiency between two successive trophic levels
CHAPTER SUMMARY
46.1 Ecology of Ecosystems
Ecosystems exist on land, at sea, in the air, and underground. Different ways of modeling ecosystems are
necessary to understand how environmental disturbances will affect ecosystem structure and dynamics.
Conceptual models are useful to show the general relationships between organisms and the flow of materials
or energy between them. Analytical models are used to describe linear food chains, and simulation models
work best with holistic food webs.
46.2 Energy Flow through Ecosystems
Organisms in an ecosystem acquire energy in a variety of ways, which is transferred between trophic levels as
the energy flows from the bottom to the top of the food web, with energy being lost at each transfer. The
efficiency of these transfers is important for understanding the different behaviors and eating habits of warm¬
blooded versus cold-blooded animals. Modeling of ecosystem energy is best done with ecological pyramids of
energy, although other ecological pyramids provide other vital information about ecosystem structure.
46.3 Biogeochemical Cycles
Mineral nutrients are cycled through ecosystems and their environment. Of particular importance are water,
carbon, nitrogen, phosphorus, and sulfur. All of these cycles have major impacts on ecosystem structure and
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Chapter 46 | Ecosystems
function. A variety of human activities, such as pollution, oil spills, and other events have damaged
ecosystems, potentially causing global climate change. The health of Earth depends on understanding these
cycles and how to protect the environment from irreversible damage.
VISUAL CONNECTION QUESTIONS
1. Figure 46.8 Why do you think the value for gross
productivity of the primary producers is the same as
the value for total heat and respiration (20,810 kcal/
m 2 /yr)?
2. Figure 46.10 Pyramids depicting the number of
organisms or biomass may be inverted, upright, or
even diamond-shaped. Energy pyramids, however,
are always upright. Why?
3. Figure 46.17 Which of the following statements
about the nitrogen cycle is false?
REVIEW QUESTIONS
4. The ability of an ecosystem to return to its
equilibrium state after an environmental disturbance
is called_.
a. resistance
b. restoration
c. reformation
d. resilience
5. A re-created ecosystem in a laboratory
environment is known as a_.
a. mesocosm
b. simulation
c. microcosm
d. reproduction
6. Decomposers are associated with which class of
food web?
a. grazing
b. detrital
c. inverted
d. aquatic
7. The primary producers in an ocean grazing food
web are usually_.
a. plants
b. animals
c. fungi
d. phytoplankton
8. What term describes the use of mathematical
equations in the modeling of linear aspects of
ecosystems?
a. analytical modeling
b. simulation modeling
c. conceptual modeling
d. individual-based modeling
9. The position of an organism along a food chain is
known as its_.
a. Ammonification converts organic
nitrogenous matter from living organisms
into ammonium (NH 4 + ).
b. Denitrification by bacteria converts nitrates
(NO 3 ) to nitrogen gas (N 2 ).
c. Nitrification by bacteria converts nitrates
(NO 3 ) to nitrites (NO 2 ).
d. Nitrogen fixing bacteria convert nitrogen gas
(N 2 ) into organic compounds.
a. locus
b. location
c. trophic level
d. microcosm
10. The loss of an apex consumer would impact
which trophic level of a food web?
a. primary producers
b. primary consumers
c. secondary consumers
d. all of the above
11. A food chain would be a better resource than a
food web to answer which question?
a. How does energy move from an organism in
one trophic level to an organism on the next
trophic level?
b. How does energy move within a trophic
level?
c. What preys on grasses?
d. How is organic matter recycled in a forest?
12. The weight of living organisms in an ecosystem
at a particular point in time is called:
a. energy
b. production
c. entropy
d. biomass
13. Which term describes the process whereby toxic
substances increase along trophic levels of an
ecosystem?
a. biomassification
b. biomagnification
c. bioentropy
d. heterotrophy
14. Organisms that can make their own food using
inorganic molecules are called:
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Chapter 46 | Ecosystems
1487
a. autotrophs
b. heterotrophs
c. photoautotrophs
d. chemoautotrophs
15. In the English Channel ecosystem, the number of
primary producers is smaller than the number of
primary consumers because_.
a. the apex consumers have a low turnover
rate
b. the primary producers have a low turnover
rate
c. the primary producers have a high turnover
rate
d. the primary consumers have a high turnover
rate
16. What law of chemistry determines how much
energy can be transferred when it is converted from
one form to another?
a. the first law of thermodynamics
b. the second law of thermodynamics
c. the conservation of matter
d. the conservation of energy
17. The mussels that live at the NW Eifuku volcano
are examples of_.
a. chemoautotrophs
b. photoautotrophs
c. apex predators
d. primary consumers
18. The movement of mineral nutrients through
organisms and their environment is called a
_cycle.
a. biological
b. bioaccumulation
c. biogeochemical
d. biochemical
19. Carbon is present in the atmosphere as
a. carbon dioxide
b. carbonate ion
c. carbon dust
d. carbon monoxide
20. The majority of water found on Earth is:
CRITICAL THINKING QUESTIONS
26. Compare and contrast food chains and food
webs. What are the strengths of each concept in
describing ecosystems?
27. Describe freshwater, ocean, and terrestrial
ecosystems.
28. Compare grazing and detrital food webs. Why
would they both be present in the same ecosystem?
29. How does the microcosm modeling approach
differ from utilizing a holistic model for ecological
research?
a. ice
b. water vapor
c. fresh water
d. saltwater
21. The average time a molecule spends in its
reservoir is known as_.
a. residence time
b. restriction time
c. resilience time
d. storage time
22. The process whereby oxygen is depleted by the
growth of microorganisms due to excess nutrients in
aquatic systems is called_.
a. dead zoning
b. eutrophication
c. retrofication
d. depletion
23. The process whereby nitrogen is brought into
organic molecules is called_.
a. nitrification
b. denitrification
c. nitrogen fixation
d. nitrogen cycling
24. Which of the following approaches would be the
most effective way to reduce greenhouse carbon
dioxide?
a. Increase waste deposition into the deep
ocean.
b. Plant more environmentally-suitable plants.
c. increase use of fuel sources that do not
produce carbon dioxide as a by-product.
d. Decrease livestock agriculture.
25. How would loss of fungi in a forest effect
biogeochemical cycles in the area?
a. Nitrogen could no longer be fixed into
organic molecules.
b. Phosphorus stores would be released for
use by other organisms.
c. Sulfur release from eroding rocks would
cease.
d. Carbon would accumulate in dead organic
matter and waste.
30. How do conceptual and analytical models of
ecosystems compliment each other?
31. Compare the three types of ecological pyramids
and how well they describe ecosystem structure.
Identify which ones can be inverted and give an
example of an inverted pyramid for each.
32. How does the amount of food a warm-blooded
animal (endotherm) eats relate to its net production
efficiency (NPE)?
33. A study uses an inverted pyramid to demonstrate
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Chapter 46 | Ecosystems
the relationship between sharks, their aquatic prey,
and phytoplankton in an ocean region. What type of
pyramid must be used? What does this convey to
readers about predation in the area?
34. Describe what a pyramid of numbers would like if
an ecologist models the relationship between bird
parasites, blue jays, and oak trees in a hectare. Does
this match the energy flow pyramid?
35. Describe nitrogen fixation and why it is important
to agriculture.
36. What are the factors that cause dead zones?
Describe eutrophication, in particular, as a cause.
37. Why are drinking water supplies still a major
concern for many countries?
38. Discuss how the human disruption of the carbon
cycle has caused ocean acidification.
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Chapter 47 | Conservation Biology and Biodiversity
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47 | CONSERVATION
BIOLOGY AND
BIODIVERSITY
Figure 47.1 Lake Victoria in Africa, shown in this satellite image, was the site of one of the most extraordinary
evolutionary findings on the planet, as well as a casualty of devastating biodiversity loss, (credit: modification of work
by Rishabh Tatiraju, using NASA World Wind software)
Chapter Outline
47.1: The Biodiversity Crisis
47.2: The Importance of Biodiversity to Human Life
47.3: Threats to Biodiversity
47.4: Preserving Biodiversity
Introduction
In the 1980s, biologists working in Lake Victoria in Africa discovered one of the most extraordinary products
of evolution on the planet. Located in the Great Rift Valley, Lake Victoria is an enormous and deep lake about
68,900 km 2 in area (larger than Lake Huron, the second largest of North America’s Great Lakes). Biologists were
studying species of a family of fish called cichlids. When they sampled for fish in different locations of the lake,
the researchers identified over 500 evolved species in total. However, the scientists soon discovered that the
invasive Nile Perch was destroying the lake’s cichlid population, bringing hundreds of cichlid species to extinction
with devastating rapidity.
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Chapter 47 | Conservation Biology and Biodiversity
47.1 1 The Biodiversity Crisis
By the end of this section, you will be able to do the following:
• Define biodiversity in terms of species diversity and abundance
• Describe biodiversity as the equilibrium of naturally fluctuating rates of extinction and speciation
• Identify historical causes of high extinction rates in Earth’s history
Traditionally, ecologists have measured biodiversity, a general term for the number of species present in the
biosphere, by taking into account both the number of species and their relative abundance to each other.
Biodiversity can be estimated at a number of levels of organization of living organisms. These estimation indices,
which came from information theory, are most useful as a first step in quantifying biodiversity between and within
ecosystems; they are less useful when the main concern among conservation biologists is simply the loss of
biodiversity. However, biologists recognize that measures of biodiversity, in terms of species diversity, may help
focus efforts to preserve the biologically or technologically important elements of biodiversity.
The Lake Victoria cichlids provide an example with which we can begin to understand biodiversity. The biologists
studying cichlids in the 1980s discovered hundreds of cichlid species representing a variety of specializations to
specialized habitat types and specific feeding strategies: such as eating plankton floating in the water, scraping/
eating algae from rocks, eating insect larvae from the lake bottom, and eating the eggs of other species of
cichlid. The cichlids of Lake Victoria are the product of an complex adaptive radiation. An adaptive radiation
is a rapid (less than three million years in the case of the Lake Victoria cichlids) branching through speciation
of a phylogenetic clade into many closely related species. Typically, the species “radiate” into different habitats
and niches. The Galapagos Island finches are an example of a modest adaptive radiation with 15 species. The
cichlids of Lake Victoria are an example of a spectacular adaptive radiation that formerly included about 500
species.
At the time biologists were making this discovery, some species began to quickly disappear. A culprit in these
declines was the Nile perch, a species of large predatory fish that was introduced to Lake Victoria by fisheries to
feed the people living around the lake. The Nile perch was introduced in 1963, but its populations did not begin
to surge until the 1980s. The perch population grew by consuming cichlids, driving species after species to the
point of extinction (the disappearance of a species), in fact, there were several factors that played a role in
the extinction of perhaps 200 cichlid species in Lake Victoria: the Nile perch, declining lake water quality due to
agriculture and land clearing on the shores of Lake Victoria, and increased fishing pressure. Scientists had not
even catalogued all of the species present—so many were lost that were never named. The diversity is now a
shadow of what it once was.
The cichlids of Lake Victoria are a thumbnail sketch of contemporary rapid species loss that occurs all over Earth
that is caused primarily by human activity. Extinction is a natural process of macroevolution that occurs at the
rate of about one out of 1 million species becoming extinct per year. The fossil record reveals that there have
been five periods of mass extinction in history with much higher rates of species loss, and the rate of species
loss today is comparable to those periods of mass extinction. However, there is a major difference between
the previous mass extinctions and the current extinction we are experiencing: human activity. Specifically, three
human activities have a major impact: 1) destruction of habitat, 2) introduction of exotic species, and 3) over¬
harvesting. Predictions of species loss within the next century, a tiny amount of time on geological timescales,
range from 10 percent to 50 percent. Extinctions on this scale have only happened five other times in the history
of the planet, and these extinctions were caused by cataclysmic events that changed the course of the history of
life in each instance.
Types of Biodiversity
Scientists generally accept that the term biodiversity describes the number and kinds of species and their
abundance in a given location or on the planet. Species can be difficult to define, but most biologists still feel
comfortable with the concept and are able to identify and count eukaryotic species in most contexts. Biologists
have also identified alternate measures of biodiversity, some of which are important for planning how to preserve
biodiversity.
Genetic diversity is one of those alternate concepts. Genetic diversity, or genetic variation defines the raw
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Chapter 47 | Conservation Biology and Biodiversity
1491
material for evolution and adaptation in a species. A species’ future potential for adaptation depends on the
genetic diversity held in the genomes of the individuals in populations that make up the species. The same is
true for higher taxonomic categories. A genus with very different types of species will have more genetic diversity
than a genus with species that are genetically similar and have similar ecologies. If there were a choice between
one of these genera of species being preserved, the one with the greatest potential for subsequent evolution is
the most genetically diverse one.
Many genes code for proteins, which in turn carry out the metabolic processes that keep organisms alive and
reproducing. Genetic diversity can be measured as chemical diversity in that different species produce a
variety of chemicals in their cells, both the proteins as well as the products and byproducts of metabolism. This
chemical diversity has potential benefit for humans as a source of pharmaceuticals, so it provides one way to
measure diversity that is important to human health and welfare.
Humans have generated diversity in domestic animals, plants, and fungi, among many other organisms. This
diversity is also suffering losses because of migration, market forces, and increasing globalism in agriculture,
especially in densely populated regions such as China, India, and Japan. The human population directly
depends on this diversity as a stable food source, and its decline is troubling biologists and agricultural scientists.
It is also useful to define ecosystem diversity, meaning the number of different ecosystems on the planet or
within a given geographic area (Figure 47.2). Whole ecosystems can disappear even if some of the species
might survive by adapting to other ecosystems. The loss of an ecosystem means the loss of interactions between
species, the loss of unique features of coadaptation, and the loss of biological productivity that an ecosystem
is able to create. An example of a largely extinct ecosystem in North America is the prairie ecosystem.
Prairies once spanned central North America from the boreal forest in northern Canada down into Mexico.
They are now all but gone, replaced by crop fields, pasture lands, and suburban sprawl. Many of the species
survive elsewhere, but the hugely productive ecosystem that was responsible for creating the most productive
agricultural soils in the United States is now gone. As a consequence, native soils are disappearing or must be
maintained and enhanced at great expense.
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Chapter 47 | Conservation Biology and Biodiversity
(a)
(b)
Figure 47.2 The variety of ecosystems on Earth—from (a) coral reef to (b) prairie—enables a great diversity of species
to exist, (credit a: modification of work by Jim Maragos, USFWS; credit b: modification of work by Jim Minnerath,
USFWS)
Current Species Diversity
Despite considerable effort, knowledge of the species that inhabit the planet is limited and always will be
because of a continuing lack of financial resources and political willpower. A recent estimate suggests that the
eukaryote species for which science has names, about 1.5 million species, account for less than 20 percent of
the total number of eukaryote species present on the planet (8.7 million species, by one estimate). Estimates of
numbers of prokaryotic species are largely guesses, but biologists agree that science has only begun to catalog
their diversity. Even with what is known, there is no central repository of names or samples of the described
species; therefore, there is no way to be sure that the 1.5 million descriptions is an accurate accounting. It is a
best guess based on the opinions of experts in different taxonomic groups. Given that Earth is losing species at
an accelerating pace, science is very much in the place it was with the Lake Victoria cichlids: knowing little about
what is being lost. Table 47.1 presents recent estimates of biodiversity in different groups.
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Chapter 47 | Conservation Biology and Biodiversity
1493
Estimates of the Numbers of Described and Predicted Species by Taxonomic
Group
[i] p] Groombridge & Jenkins
Mora et al. 2011 Chapman 2009 2002 PI
Described
Predicted
Described
Predicted
Described
Predicted
Animalia
1,124,516
9,920,000
1,424,153
6,836,330
1,225,500
10,820,000
Chromista
17,892
34,900
25,044
200,500
—
—
Fungi
44,368
616,320
98,998
1,500,000
72,000
1,500,000
Plantae
224,244
314,600
310,129
390,800
270,000
320,000
Protozoa
16,236
72,800
28,871
1,000,000
80,000
600,000
Prokaryotes
—
—
10,307
1,000,000
10,175
—
Total
1,438,769
10,960,000
1,897,502
10,897,630
1,657,675
13,240,000
Table 47.1
There are various initiatives to catalog described species in accessible ways, and the internet is facilitating that
effort. Nevertheless, it has been pointed out that at the current rate of new species descriptions (which according
to the State of Observed Species Report is 17,000 to 20,000 new species per year), it will take close to 500 years
[4]
to finish describing life on this planet. Over time, the task becomes both increasingly difficult and increasingly
easier as extinction removes species from the planet.
Naming and counting species may seem like an unimportant pursuit given the other needs of humanity, but
determining biodiversity it is not simply an accounting of species. Describing a species is a complex process
through which biologists determine an organism’s unique characteristics and whether or not that organism
belongs to any other described species or genus. It allows biologists to find and recognize the species after
the initial discovery, and allows them to follow up on questions about its biology. In addition, the unique
characteristics of each species make it potentially valuable to humans or other species on which humans
depend.
Patterns of Biodiversity
Biodiversity is not evenly distributed on Earth. Lake Victoria contained almost 500 species of cichlids alone,
ignoring the other fish families present in the lake. All of these species were found only in Lake Victoria;
therefore, the 500 species of cichlids were endemic. Endemic species are found in only one location. Endemics
with highly restricted distributions are particularly vulnerable to extinction. Higher taxonomic levels, such as
genera and families, can also be endemic. Lake Michigan contains about 79 species of fish, many of which are
found in other lakes in North America. What accounts for the difference in fish diversity in these two lakes? Lake
Victoria is an ancient tropical lake, while Lake Michigan is a recently formed temperate lake. Lake Michigan in
its present form is only about 7,000 years old, while Lake Victoria in its present form is about 15,000 years old,
although its basin is about 400,000 years in age. Biogeographers have suggested these two factors, latitude and
age, are two of several hypotheses to explain biodiversity patterns on the planet.
1. Mora Camilo et al., “How Many Species Are There on Earth and in the Ocean?” PLoS Biology (2011), doi:10.1371/journal.pbio.1001127.
2. Arthur D. Chapman, Numbers of Living Species in Australia and the World, 2nd ed. (Canberra, AU: Australian Biological Resources
Study, 2009). https://www.environment.gov.au/system/files/pages/2ee3f4al-fl30-465b-9c7a-79373680a067/files/nlsaw-2nd-complete.pdf/
(http:// 0 penstax. 0 rg/l/Aus_diversity) .
3. Brian Groombridge and Martin D. Jenkins. World Atlas of Biodiversity: Earth's Living Resources in the 21 st Century. Berkeley: University
of California Press, 2002.
4. International Institute for Species Exploration (USE), 2011 State of Observed Species (SOS). Tempe, AZ: USE, 2011. Accessed May, 20,
2012. http://www.esf.edu/species/ (http:// 0 penstax. 0 rg/l/ 0 bserved_species).
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Chapter 47 | Conservation Biology and Biodiversity
ca eer connection
Biogeographer
Biogeography is the study of the distribution of the world’s species—both in the past and in the present.
The work of biogeographers is critical to understanding our physical environment, how the environment
affects species, and how environmental changes impact the distribution of a species; it has also been critical
to developing modern evolutionary theory. Biogeographers need to understand both biology and ecology.
They also need to be well-versed in evolutionary studies, soil science, and climatology.
There are three main fields of study under the heading of biogeography: ecological biogeography, historical
biogeography (called paleobiogeography), and conservation biogeography. Ecological biogeography
studies the current factors affecting the distribution of plants and animals. Historical biogeography, as the
name implies, studies the past distribution of species. Conservation biogeography, on the other hand, is
focused on the protection and restoration of species based upon known historical and current ecological
information. Each of these fields considers both zoogeography and phytogeography—the past and present
distribution of animals and plants.
One of the oldest observed patterns in ecology is that species biodiversity in almost every taxonomic group
increases as latitude declines. In other words, biodiversity increases closer to the equator (Figure 47.3).
Number of species
□ 16-20 □ 21-30 m 31-40 ■ 41-60 ■ 61-144
Figure 47.3 This map illustrates the number of amphibian species across the globe and shows the trend toward higher
biodiversity at lower latitudes. A similar pattern is observed for most taxonomic groups. The white areas indicate a lack
of data in this particular study.
It is not yet clear why biodiversity increases closer to the equator, but scientists have several hypotheses.
One factor may be the greater age of the ecosystems in the tropics versus those in temperate regions; the
temperate regions were largely devoid of life or were drastically reduced during the last glaciation. The idea
is that greater age provides more time for speciation. Another possible explanation is the increased direct
energy the tropics receive from the sun versus the decreased intensity of the solar energy that temperate and
polar regions receive. Tropical ecosystem complexity may promote speciation by increasing the heterogeneity,
or number of ecological niches , in the tropics relative to higher latitudes. The greater heterogeneity provides
more opportunities for coevolution, specialization, and perhaps greater selection pressures leading to population
differentiation. However, this hypothesis suffers from some circularity—ecosystems with more species
encourage speciation, but how did they get more species to begin with?
The tropics have been perceived as being more stable than temperate regions, which have a pronounced
climate and day-length seasonality. The tropics have their own forms of seasonality, such as rainfall, but they
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are generally assumed to be more stable environments and this stability might promote speciation into highly
specialized niches.
Regardless of the mechanisms, it is certainly true that all levels of biodiversity are greatest in the tropics.
Additionally, the rate of endemism is highest, and there are more biodiversity “hotspots.” However, this richness
of diversity also means that knowledge of species is unfortunately very low, and there is a high potential for
biodiversity loss.
Conservation of Biodiversity
In 1988, British environmentalist Norman Myers developed a conservation concept to identify areas rich in
species and at significant risk for species loss: biodiversity hotspots. Biodiversity hotspots are geographical
areas that contain high numbers of endemic species. The purpose of the concept was to identify important
locations on the planet for conservation efforts, a kind of conservation triage. By protecting hotspots,
governments are able to protect a larger number of species. The original criteria for a hotspot included the
presence of 1500 or more endemic plant species and 70 percent of the area disturbed by human activity. There
are now 34 biodiversity hotspots (Figure 47.4) containing large numbers of endemic species, which include half
of Earth’s endemic plants.
Figure 47.4 Conservation International has identified 34 biodiversity hotspots, which cover only 2.3 percent of the
Earth’s surface but have endemic to them 42 percent of the terrestrial vertebrate species and 50 percent of the world’s
plants.
Biodiversity Change through Geological Time
The number of species on the planet, or in any geographical area, is the result of an equilibrium of two
evolutionary processes that are continuously ongoing: speciation and extinction. Both are natural “birth" and
“death” processes of macroevolution. When speciation rates begin to outstrip extinction rates, the number of
species will increase; likewise, the number of species will decrease when extinction rates begin to overtake
speciation rates. Throughout Earth’s history, these two processes have fluctuated—sometimes leading to
dramatic changes in the number of species on Earth as reflected in the fossil record (Figure 47.5).
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Figure 47.5 Percent extinction occurrences as reflected in the fossil record have fluctuated throughout Earth’s history.
Sudden and dramatic losses of biodiversity, called mass extinctions, have occurred five times.
Paleontologists have identified five strata in the fossil record that appear to show sudden and dramatic (greater
than half of all extant species disappearing from the fossil record) losses in biodiversity. These are called mass
extinctions. There are many lesser, yet still dramatic, extinction events, but the five mass extinctions have
attracted the most research. An argument can be made that the five mass extinctions are only the five most
extreme events in a continuous series of large extinction events throughout the Phanerozoic (since 542 million
years ago). In most cases, the hypothesized causes are still controversial; however, the most recent mass
extinction event seems clear.
The Five Mass Extinctions
The fossil record of the mass extinctions was the basis for defining periods of geological history, so they typically
occur at the transition point between geological periods. The transition in fossils from one period to another
reflects the dramatic loss of species and the gradual origin of new species. These transitions can be seen in the
rock strata. Table 47.2 provides data on the five mass extinctions.
Mass Extinctions
Geological Period
Mass Extinction Name
Time (millions of years ago)
Ordovician-Silurian
end-Ordovician O-S
450-440
Late Devonian
end-Devonian
375-360
Permian-Triassic
end-Permian
251
Triassic-Jurassic
end-Triassic
205
Cretaceous-Paleogene
end-Cretaceous K-Pg (K-T)
65.5
Table 47.2 This table shows the names and dates for the five mass extinctions in
Earth’s history.
The Ordovician-Silurian extinction event is the first recorded mass extinction and the second largest. During
this period, about 85 percent of marine species (few species lived outside the oceans) became extinct. The main
hypothesis for its cause is a period of glaciation and then warming. The extinction event actually consists of two
extinction events separated by about 1 million years. The first event was caused by cooling, and the second
event was due to the subsequent warming. The climate changes affected temperatures and sea levels. Some
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researchers have suggested that a gamma-ray burst, caused by a nearby supernova, was a possible cause of
the Ordovician-Silurian extinction. The gamma-ray burst would have stripped away the Earth’s protective ozone
layer, allowing intense ultraviolet radiation from the sun to reach the surface of the earth—and may account
for climate changes observed at the time. The hypothesis is very speculative, and extraterrestrial influences on
Earth’s history are an active line of research. Recovery of biodiversity after the mass extinction took from 5 to 20
million years, depending on the location.
The late Devonian extinction may have occurred over a relatively long period of time. It appears to have mostly
affected marine species and not so much the plants or animals inhabiting terrestrial habitats. The causes of this
extinction are poorly understood.
The end-Permian extinction was the largest in the history of life. Indeed, an argument could be made that
Earth became nearly devoid of life during this extinction event. Estimates are that 96 percent of all marine
species and 70 percent of all terrestrial species were lost. It was at this time, for example, that the trilobites, a
group that survived the Ordovician-Silurian extinction, became extinct. The causes for this mass extinction are
not clear, but the leading suspect is extended and widespread volcanic activity that led to a runaway global-
warming event. The oceans became largely anoxic, suffocating marine life. Terrestrial tetrapod diversity took 30
million years to recover after the end-Permian extinction. The Permian extinction dramatically altered Earth’s
biodiversity makeup and the course of evolution.
The causes of the Triassic-Jurassic extinction event are not clear, and researchers argue hypotheses including
climate change, asteroid impact, and volcanic eruptions. The extinction event occurred just before the breakup
of the supercontinent Pangaea, although recent scholarship suggests that the extinctions may have occurred
more gradually throughout the Triassic.
The causes of the end-Cretaceous extinction event are the ones that are best understood. It was during this
extinction event about 65 million years ago that the majority of the dinosaurs, the dominant vertebrate group for
millions of years, disappeared from the planet (with the exception of a theropod clade that gave rise to birds).
The cause of this extinction is now understood to be the result of a cataclysmic impact of a large meteorite,
or asteroid, off the coast of what is now the Yucatan Peninsula. This hypothesis, proposed first in 1980, was a
radical explanation based on a sharp spike in the levels of iridium (which enters our atmosphere from meteors
at a fairly constant rate but is otherwise absent on Earth’s surface) in the rock stratum that marks the boundary
between the Cretaceous and Paleogene periods (Figure 47.6). This boundary marked the disappearance of the
dinosaurs in fossils as well as many other taxa. The researchers who discovered the iridium spike interpreted it
as a rapid influx of iridium from space to the atmosphere (in the form of a large asteroid) rather than a slowing
in the deposition of sediments during that period. It was a radical explanation, but the report of an appropriately
aged and sized impact crater in 1991 made the hypothesis more believable. Now an abundance of geological
evidence supports the theory. Recovery times for biodiversity after the end-Cretaceous extinction are shorter, in
geological time, than for the end-Permian extinction, on the order of 10 million years.
Another possibility, perhaps coincidental with the impact of the Yucatan asteroid, was extensive volcanism that
began forming about 66 million years ago, about the same time as the Yucatan asteroid impact, at the end of
the Cretaceous. The lava flows covered over 50 percent of what is now India. The release of volcanic gases,
particularly sulphur dioxide, during the formation of the traps contributed to climate change, which may have
induced the mass extinction.
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visual
CONNECTION
Figure 47.6 In 1980, Luis and Walter Alvarez, Frank Asaro, and Helen Michels discovered, across the world,
a spike in the concentration of iridium within the sedimentary layer at the K-Pg boundary. These researchers
hypothesized that this iridium spike was caused by an asteroid impact that resulted in the K-Pg mass extinction.
In the photo, the iridium layer is the light band, (credit: USGS)
Scientists measured the relative abundance of fern spores above and below the K-Pg boundary in this rock
sample. Which of the following statements most likely represents their findings?
a. An abundance of fern spores from several species was found below the K-Pg boundary, but none was
found above.
b. An abundance of fern spores from several species was found above the K-Pg boundary, but none was
found below.
c. An abundance of fern spores was found both above and below the K-Pg boundary, but only one
species was found below the boundary, and many species were found above the boundary.
d. Many species of fern spores were found both above and below the boundary, but the total number of
spores was greater below the boundary.
LINK
T a
LEARNING
Explore this interactive website (http:// 0 penstaxc 0 llege. 0 rg/l/extincti 0 ns) about mass extinctions.
The Pleistocene Extinction
The Pleistocene Extinction is one of the lesser extinctions, and a recent one. It is well known that the
North American, and to some degree Eurasian, megafauna —large vertebrate animals—disappeared toward
the end of the last glaciation period. The extinction appears to have happened in a relatively restricted time
period of 10,000-12,000 years ago. In North America, the losses were quite dramatic and included the woolly
mammoths (with an extant population existing until about 4,000 years ago in isolation on Wrangel Island,
Canada), mastodon, giant beavers, giant ground sloths, saber-toothed cats, and the North American camel, just
to name a few. In the early 1900s, scientists first suggested the possibility that over-hunting caused the rapid
extinction of these large animals. Research into this hypothesis continues today.
In general, the timing of the Pleistocene extinctions correlated with the arrival of paleo-humans, perhaps as long
as 40,000 years ago, and not with climate-change events, which is the main competing hypothesis for these
extinctions. The extinctions began in Australia about 40,000 to 50,000 years ago, just after the arrival of humans
in the area: a marsupial lion, a giant one-ton wombat, and several giant kangaroo species disappeared. In North
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America, the extinctions of almost all of the large mammals occurred 10,000-12,000 years ago. All that are left
are the smaller mammals such as bears, elk, moose, and cougars. Finally, on many remote oceanic islands, the
extinctions of many species occurred coincidentally with human arrivals. Not all of the islands had large animals,
but when there were large animals, they were often forced into extinction. Madagascar was colonized about
2,000 years ago and the large mammals that lived there became extinct. Eurasia and Africa do not show this
pattern, but they also did not experience a recent arrival of hunter-gatherer humans. Rather, humans arrived in
Eurasia hundreds of thousands of years ago. This topic remains an area of active research and hypothesizing.
It seems clear that even if climate played a role, in most cases human hunting precipitated the extinctions.
Recent Extinctions
The sixth, or Holocene, mass extinction appears to have begun earlier than previously believed and is largely
due to the disruptive activities of modern Homo sapiens. Since the beginning of the Holocene period, there
are numerous recent extinctions of individual species that are recorded in human writings. Most of these are
coincident with the expansion of the European colonies since the 1500s.
One of the earlier and popularly known examples is the dodo bird. The odd pigeon-like bird lived in the forests of
Mauritius (an island in the Indian Ocean) and became extinct around 1662. The dodo was hunted for its meat by
sailors and was easy prey because it approached people without fear (the dodo had not evolved with humans).
Pigs, rats, and dogs brought to the island by European ships also killed dodo young and eggs.
Steller's sea cow became extinct in 1768; it was related to the manatee and probably once lived along the
northwest coast of North America. Steller's sea cow was first discovered by Europeans in 1741 and was
overhunted for meat and oil. The last sea cow was killed in 1768. That amounts to just 27 years between the sea
cow’s first contact with Europeans and extinction of the species!
Since 1900, a variety of species have gone extinct, including the following:
• In 1914, the last living passenger pigeon died in a zoo in Cincinnati, Ohio. This species had once darkened
the skies of North America during its migrations, but it was overhunted and suffered from habitat loss that
resulted from the clearing of forests for farmland.
• The Carolina parakeet, once common in the eastern United States, died out in 1918. It suffered habitat loss
and was hunted to prevent it from eating orchard fruit. (The parakeet ate orchard fruit because its native
foods were destroyed to make way for farmland.)
• The Japanese sea lion, which inhabited a broad area around Japan and the coast of Korea, became extinct
in the 1950s due to fishermen.
• The Caribbean monk seal was distributed throughout the Caribbean Sea but was driven to extinction via
hunting by 1952.
These are only a few of the recorded extinctions in the past 500 years. The International Union for Conservation
of Nature (IUCN) keeps a list of extinct and endangered species called the Red List. The list is not complete,
but it describes 380 extinct species of vertebrates after 1500 AD, 86 of which were driven extinct by overhunting
or overfishing.
Estimates of Present-Time Extinction Rates
Estimates of extinction rates are hampered by the fact that most extinctions are probably happening without
observation. The extinction of a bird or mammal is likely to be noticed by humans, especially if it has been hunted
or used in some other way. But there are many organisms that are of less interest to humans (not necessarily of
less value) and many that are undescribed.
The background extinction rate is estimated to be about one per million species per year (E/MSY). For
example, assuming there are about ten million species in existence, the expectation is that ten species would
become extinct each year (each year represents ten million species per year).
One contemporary extinction rate estimate uses the extinctions in the written record since the year 1500. For
birds alone this method yields an estimate of 26 E/MSY. However, this value may be an underestimate for three
reasons. First, many species would not have been described until much later in the time period, so their loss
would have gone unnoticed. Second, the number of recently extinct vertebrate species is increasing because
extinct species now are being described from skeletal remains. And third, some species are probably already
extinct even though conservationists are reluctant to name them as such. Taking these factors into account
raises the estimated extinction rate closer to 100 E/MSY. The predicted rate by the end of the century is 1500 El
MSY.
A second approach to estimating present-time extinction rates is to correlate species loss with habitat loss by
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measuring forest-area loss and understanding species-area relationships. The species-area relationship is
the rate at which new species are seen when the area surveyed is increased. Studies have shown that the
number of species present increases as the size of the island increases. This phenomenon has also been
shown to hold true in other island-like habitats as well, such as the mountain-top tepuis of Venezuela, which
are surrounded by tropical forest. Turning this relationship around, if the habitat area is reduced, the number
of species living there will also decline. Estimates of extinction rates based on habitat loss and species-area
relationships have suggested that with about 90 percent habitat loss an expected 50 percent of species would
become extinct. Species-area estimates have led to species extinction rate calculations of about 1000 E/MSY
and higher. In general, actual observations do not show this amount of loss and suggestions have been made
that there is a delay in extinction. Recent work has also called into question the applicability of the species-area
relationship when estimating the loss of species. This work argues that the species-area relationship leads to an
overestimate of extinction rates. A better relationship to use may be the endemics-area relationship. Using this
method would bring estimates down to around 500 E/MSY in the coming century. Note that this value is stiii 500
times the background rate.
Figure 47.7 Studies have shown that the number of species present increases with the size of the habitat, (credit:
modification of work by Adam B. Smith)
LINK TQ LEARNING
Check out this interactive exploration (http:// 0 penstaxc 0 llege. 0 rg/l/what_is_missing) of endangered and
extinct species, their ecosystems, and the causes of the endangerment or extinction.
47.2 | The Importance of Biodiversity to Human Life
By the end of this section, you will be able to do the following:
• identify chemical diversity benefits to humans
• identify biodiversity components that support human agriculture
• Describe ecosystem services
It may not be clear why biologists are concerned about biodiversity loss. When biodiversity loss is thought of
as the extinction of the passenger pigeon, the dodo bird, and even the woolly mammoth, the loss may appear
to be an emotional one. But is the loss practically important for the welfare of the human species? From the
perspective of evolution and ecology, the loss of a particular individual species is unimportant (however, we
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should note that the loss of a keystone species can lead to ecological disaster). Extinction is a normal part of
macroevolution. But the accelerated extinction rate translates into the loss of tens of thousands of species within
our lifetimes, and it is likely to have dramatic effects on human welfare through the collapse of ecosystems and
in added costs to maintain food production, clean air and water, and human health.
Agriculture began after early hunter-gatherer societies first settled in one place and heavily modified their
immediate environment. This cultural transition has made it difficult for humans to recognize their dependence on
undomesticated living things on the planet. Biologists recognize the human species is embedded in ecosystems
and is dependent on them, just as every other species on the planet is dependent. Technology smooths out the
extremes of existence, but ultimately the human species cannot exist without a supportive ecosystem.
Human Health
Archeological evidence indicates that humans have been using plants for medicinal uses for thousands of years.
A Chinese document from approximately 2800 BC is believed to be the the first written account of herbal
remedies, and such references occur throughout the global historical record. Contemporary indigenous societies
that live close to the land often retain broad knowledge of the medicinal uses of plants growing in their area.
Most plants produce secondary plant compounds, which are toxins used to protect the plant from insects and
other animals that eat them, but some of which also work as medication.
Modern pharmaceutical science also recognizes the importance of these plant compounds. Examples of
significant medicines derived from plant compounds include aspirin, codeine, digoxin, atropine, and vincristine
(Figure 47.8). Many medicines were once derived from plant extracts but are now synthesized. It is estimated
that, at one time, 25 percent of modern drugs contained at least one plant extract. That number has probably
decreased to about 10 percent as natural plant ingredients are replaced by synthetic versions. Antibiotics, which
are responsible for extraordinary improvements in health and lifespans in developed countries, are compounds
largely derived from fungi and bacteria.
Figure 47.8 Catharanthus roseus, the Madagascar periwinkle, has various medicinal properties. Among other uses, it
is a source of vincristine, a drug used in the treatment of lymphomas, (credit: Forest and Kim Starr)
in recent years, animal venoms and poisons have excited intense research for their medicinal potential. By
2007, the FDA had approved five drugs based on animal toxins to treat diseases such as hypertension, chronic
pain, and diabetes. Another five drugs are undergoing clinical trials, and at least six drugs are being used in
other countries. Other toxins under investigation come from mammals, snakes, lizards, various amphibians, fish,
snails, octopuses, and scorpions.
Aside from representing billions of dollars in profits, these medicines improve people’s lives. Pharmaceutical
companies are always looking for new compounds synthesized by living organisms that can function as
medicines. It is estimated that 1/3 of pharmaceutical research and development is spent on natural compounds
and that about 35 percent of new drugs brought to market between 1981 and 2002 were derived from natural
compounds. The opportunities for new medications will be reduced in direct proportion to the disappearance of
species.
Agricultural Diversity
Since the beginning of human agriculture more than 10,000 years ago, human groups have been breeding and
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selecting crop varieties. This crop diversity matched the cultural diversity of highly subdivided populations of
humans. For example, potatoes were domesticated beginning around 7,000 years ago in the central Andes of
Peru and Bolivia. The potatoes grown in that region belong to seven species and the number of varieties likely is
in the thousands. Even the Inca capital of Machu Picchu had numerous gardens growing varieties of potatoes.
Each variety has been bred to thrive at particular elevations and soil and climate conditions. The diversity is
driven by the diverse demands of the topography, the limited movement of people, and the demands created by
crop rotation for different varieties that will do well in different fields.
Potatoes are only one example of human-generated diversity. Every plant, animal, and fungus that has been
cultivated by humans has been bred from original wild ancestor species into diverse varieties arising from the
demands for food value, adaptation to growing conditions, and resistance to pests.
The potato also demonstrates risks of low crop diversity. The tragic Irish potato famine occurred when the single
variety grown in Ireland became susceptible to a potato blight, wiping out the entire crop. The loss of the potato
crop led to mass famine and the related deaths of over one million people, as well as mass emigration of nearly
two million people.
Disease resistance is a chief benefit of crop biodiversity, and lack of diversity in contemporary crop species
carries similar risks. Seed companies, which are the source of most crop varieties in developed countries, must
continually breed new varieties to keep up with evolving pest organisms. These same seed companies, however,
have participated in the decline of the number of varieties available as they focus on selling fewer varieties in
more areas of the world.
The ability to create new crop varieties relies on the diversity of varieties available and the accessibility of wild
forms related to the crop plant. These wild forms are often the source of new gene variants that can be bred
with existing varieties to create varieties with new attributes. Loss of wild species related to a crop will mean the
loss of potential in crop improvement. Maintaining the genetic diversity of wild species related to domesticated
species ensures our continued food supply.
Since the 1920s, government agriculture departments have maintained seed banks of crop varieties as a way
of maintaining crop diversity. This system has flaws because, over time, seed banks are lost through accidents,
and there is no way to replace them, in 2008, the Svalbard Global Seed Vault (Figure 47.9) began storing
seeds from around the world as a backup system to the regional seed banks. If a regional seed bank stores
varieties in Svalbard, losses can be replaced from Svalbard. Conditions within the vault are maintained at ideal
temperature and humidity for seed survival, but the deep underground location of the vault in the arctic means
that failure of the vault’s systems will not compromise the climatic conditions inside the vault.
visual
CONNECTION
Figure 47.9 The Svalbard Global Seed Vault is a storage facility for seeds of Earth’s diverse crops, (credit: Mari
Tefre, Svalbard Global Seed Vault)
The Svalbard Global Seed Vault is located on Spitsbergen island in Norway, which has an arctic climate.
Why might an arctic climate be good for seed storage?
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Crop success is largely dependent on the quality of the soil. Although some agricultural soils are rendered
sterile using controversial cultivation and chemical treatments, most contain a huge diversity of organisms that
maintain nutrient cycles—breaking down organic matter into nutrient compounds that crops need for growth.
These organisms also maintain soil texture that affects water and oxygen dynamics in the soil that are necessary
for plant growth. If farmers had to maintain arable soil using alternate means, the cost of food would be much
higher than it is now. These kinds of processes are called ecosystem services. They occur within ecosystems,
such as soil ecosystems, as a result of the diverse metabolic activities of the organisms living there, but they
provide benefits to human food production, drinking water availability, and breathable air.
Plant pollination is another key ecosystem service, provided by various species of bees, other insects, and birds.
One estimate indicates that honey bee pollination provides the United States a $1.6 billion annual benefit.
Honey bee populations in North America have been suffering large losses caused by a syndrome known as
colony collapse disorder, whose cause is unclear. (Evidence suggests the possible culprits may be the invasive
varroa mite coupled with the Nosema gut parasite and acute paralysis virus.) Loss of these species would render
it very difficult, if not impossible, to grow any of the 150 United States crops requiring pollination, including
grapes, oranges, lemons, peppers, most brassica (broccoli and cauliflower), and many berries, melons, and
nuts.
Finally, humans compete for their food with crop pests, most of which are insects. Pesticides control these
competitors; however, pesticides are costly and lose their effectiveness over time as pest populations adapt and
evolve. They also lead to collateral damage by killing non-pest species and risking the health of consumers and
agricultural workers. Ecologists believe that the bulk of the work in removing pests is actually done by predators
and parasites of those pests, but the impact has not been well studied. A review found that in 74 percent of
studies that looked for an effect of landscape complexity on natural enemies of pests, the greater the complexity,
the greater the effect of pest-suppressing organisms. An experimental study found that introducing multiple
enemies of pea aphids (an important alfalfa pest) increased the yield of alfalfa significantly. This study shows the
importance of landscape diversity via the question of whether a diversity of pests is more effective at control than
one single pest; the results showed this to be the case. Loss of diversity in pest enemies will inevitably make it
more difficult and costly to grow food.
Wild Food Sources
In addition to growing crops and raising animals for food, humans obtain food resources from wild populations,
primarily fish populations. In fact, for approximately 1 billion people worldwide, aquatic resources provide the
main source of animal protein. But since 1990, global fish production has declined, sometimes dramatically.
Unfortunately, and despite considerable effort, few fisheries on the planet are managed for sustainability.
Fishery extinctions rarely lead to complete extinction of the harvested species, but rather to a radical
restructuring of the marine ecosystem in which a dominant species is so over-harvested that it becomes a minor
player, ecologically. In addition to humans losing the food source, these alterations affect many other species in
ways that are difficult or impossible to predict. The collapse of fisheries has dramatic and long-lasting effects on
local populations that work in the fishery. In addition, the loss of an inexpensive protein source to populations
that cannot afford to replace it will increase the cost of living and limit societies in other ways. In general, the
fish taken from fisheries have shifted to smaller species as larger species are fished to extinction. The ultimate
outcome could clearly be the loss of aquatic systems as food sources.
LINK TQ LEARNING
View a brief video (http://openstaxcollege.Org/l/declining_fish) discussing declining fish stocks.
Psychological and Moral Value
Finally, it has been clearly shown that humans benefit psychologically from living in a biodiverse world. A chief
proponent of this idea is Harvard entomologist E. O. Wilson. He argues that human evolutionary history has
adapted us to live in a natural environment and that city environments generate psychological stressors that
affect human health and well-being. There is considerable research into the psychological regenerative benefits
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of natural landscapes that suggests the hypothesis may hold some truth. In addition, there is a moral argument
that humans have a responsibility to inflict as little harm as possible on other species.
47.3 | Threats to Biodiversity
By the end of this section, you will be able to do the following:
• Identify significant threats to biodiversity
• Explain the effects of habitat loss, the introduction of exotic species, and hunting on biodiversity
• identify the early and predicted effects of climate change on biodiversity
The core threat to biodiversity on the planet, and therefore a threat to human welfare, is the combination of
human population growth and resource exploitation. The human population requires resources to survive and
grow, and those resources are being removed unsustainably from the environment. The three greatest proximate
threats to biodiversity are habitat loss, overharvesting, and the introduction of exotic species. The first two of
these are a direct result of human population growth and resource use. The third results from increased mobility
and trade. A fourth major cause of extinction, anthropogenic climate change, has not yet had a large impact, but
it is predicted to become significant during this century. Global climate change is also a consequence of human
population needs for energy and the use of fossil fuels to meet those needs (Figure 47.10). Environmental
issues, such as toxic pollution, have specific targeted effects on species, but they are not generally seen as
threats at the magnitude of the others.
Figure 47.10 Atmospheric carbon dioxide levels fluctuate in a cyclical manner. However, the burning of fossil fuels in
recent history has caused a dramatic increase in the levels of carbon dioxide in the Earth’s atmosphere, which have
now reached levels never before seen in human history. Scientists predict that the addition of this “greenhouse gas” to
the atmosphere is resulting in climate change that will significantly impact biodiversity in the coming century.
Habitat Loss
Humans rely on technology to modify their environment and replace certain functions that were once performed
by the natural ecosystem. Other species cannot do this. Elimination of their ecosystem—whether it is a forest,
a desert, a grassland, a freshwater estuarine, or a marine environment—will kill the individuals belonging to
the species. The species will become extinct if we remove the entire habitat within the range of a species.
Human destruction of habitats accelerated in the latter half of the twentieth century. Consider the exceptional
biodiversity of Sumatra: it is home to one species of orangutan, a species of critically endangered elephant, and
the Sumatran tiger, but half of Sumatra’s forest is now gone. The neighboring island of Borneo, home to the other
species of orangutan, has lost a similar area of forest. Forest loss continues in protected areas of Borneo. All
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three species of orangutan are now listed as endangered by the International Union for Conservation of Nature
(IUCN), but they are simply the most visible of thousands of species that will not survive the disappearance of
the forests in Sumatra and Borneo. The forests are removed for timber and to plant palm oil plantations (Figure
47.11). Palm oil is used in many products including food products, cosmetics, and biodiesel in Europe. A five-
year estimate of global forest cover loss for the years 2000-2005 was 3.1 percent. In the humid tropics where
forest loss is primarily from timber extraction, 272,000 km 2 was lost out of a global total of 11,564,000 km 2 (or
2.4 percent). In the tropics, these losses certainly also represent the extinction of species because of high levels
of endemism —species unique to a defined geographic location, and found nowhere else.
(a) (b)
(d) (e)
Figure 47.11 (a) One of three species of orangutan, Pongo pygmaeus, is found only in the rainforests of Borneo,
and an other species of orangutan ( Pongo abelii) is found only in the rainforests of Sumatra. These animals are
examples of the exceptional biodiversity of (c) the islands of Sumatra and Borneo. Other species include the (b)
Sumatran tiger ( Panthera tigris sumatrae ) and the (d) Sumatran elephant ( Elephas maximus sumatranus), both
critically endangered species. Rainforest habitat is being removed to make way for (e) oil palm plantations such as this
one in Borneo’s Sabah Province, (credit a: modification of work by Thorsten Bachner; credit b: modification of work
by Dick Mudde; credit c: modification of work by U.S. CIA World Factbook; credit d: modification of work by “Nonprofit
Organizations’VFlickr; credit e: modification of work by Dr. Lian Pin Koh)
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everyday CONNECTION
Preventing Habitat Destruction with Wise Wood Choices
Most consumers are not aware that the home improvement products they buy might be contributing to
habitat loss and species extinctions. Yet the market for illegally harvested tropical timber is huge, and the
wood products often find themselves in building supply stores in the United States. One estimate is that 10
percent of the imported timber stream in the United States, which is the world’s largest consumer of wood
products, is potentially illegally logged. In 2006, this amounted to $3.6 billion in wood products. Most of the
illegal products are imported from countries that act as intermediaries and are not the originators of the
wood.
How is it possible to determine if a wood product, such as flooring, was harvested sustainably or even
legally? The Forest Stewardship Council (FSC) certifies sustainably harvested forest products, therefore,
looking for their certification on flooring and other hardwood products is one way to ensure that the wood
has not been taken illegally from a tropical forest. Certification applies to specific products, not to a producer;
some producers’ products may not have certification while other products are certified. While there are other
industry-backed certifications other than the FSC, these are unreliable due to lack of independence from
the industry. Another approach is to buy domestic wood species. While it would be great if there was a
list of legal versus illegal wood products, it is not that simple. Logging and forest management laws vary
from country to country; what is illegal in one country may be legal in another. Where and how a product is
harvested and whether the forest from which it comes is being maintained sustainably all factor into whether
a wood product will be certified by the FSC. If you are in doubt, it is always a good idea to ask questions
about where a wood product came from and how the supplier knows that it was harvested legally.
Habitat destruction can affect ecosystems other than forests. Rivers and streams are important ecosystems
that are frequently modified through land development, damming, channelizing, or water removal. Damming
affects the water flow to all parts of a river, which can reduce or eliminate populations that had adapted to the
natural flow of the river. For example, an estimated 91 percent of United States rivers have been altered in some
way. Modifications include dams, to create energy or store water; levees, to prevent flooding; and dredging or
rerouting, to create land that is more suitable for human development. Many fish and amphibian species and
numerous freshwater clams in the United States have seen declines caused by river damming and habitat loss.
Overharvesting
Overharvesting is a serious threat to many species, but particularly to aquatic (both marine and freshwater)
species. Despite regulation and monitoring, there are recent examples of fishery collapse. The western Atlantic
cod fishery is the among the most significant. While it was a hugely productive fishery for 400 years, the
introduction of modern factory trawlers in the 1980s caused it to become unsustainable. Fisheries collapse as
a result of both economic and political factors. Fisheries are managed as a shared international resource even
when the fishing territory lies within an individual country’s territorial waters. Common resources are subject to
an economic pressure known as the tragedy of the commons, in which essentially no fisher has a motivation
to exercise restraint in harvesting a fishery when it is not owned by that fisher. Overexploitation is a common
outcome. This overexploitation is exacerbated when access to the fishery is open and unregulated and when
technology gives fishers the ability to overfish. In a few fisheries, the biological growth of the resource is less than
the potential growth of the profits made from fishing if that time and money were invested elsewhere. In these
cases—whales are an example—economic forces will always drive toward fishing the population to extinction.
LINK
%
LEARNING
Explore a U S. Fish & Wildlife Service interactive map (http:// 0 penstaxc 0 llege. 0 rg/l/habitat_map) of critical
habitat for endangered and threatened species in the United States. To begin, select “Visit the online mapper.”
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For the most part, fishery extinction is not equivalent to biological extinction—the last fish of a species is rarely
fished out of the ocean. At the same time, fishery extinction is still harmful to fish species and their ecosystems.
There are some instances in which true extinction is a possibility. Whales have slow-growing populations due
to low reproductive rates, and therefore are at risk of complete extinction through hunting. There are some
species of sharks with restricted distributions that are at risk of extinction. The groupers are another population
of generally slow-growing fishes that, in the Caribbean, includes a number of species that are at risk of extinction
from overfishing.
Coral reefs are extremely diverse marine ecosystems that face immediate peril from several processes. Reefs
are home to 1/3 of the world’s marine fish species—about 4,000 species—despite making up only 1 percent
of marine habitat. Most home marine aquaria are stocked with wild-caught organisms, not cultured organisms.
Although no species is known to have been driven extinct by the pet trade in marine species, there are studies
showing that populations of some species have declined in response to harvesting, indicating that the harvest is
not sustainable at those levels. There are concerns about the effect of the pet trade on some terrestrial species
such as turtles, amphibians, birds, plants, and even the orangutan.
View a brief video (http:// 0 penstaxc 0 llege. 0 rg/l/ 0 cean_matters) discussing the role of marine ecosystems
in supporting human welfare and the decline of ocean ecosystems.
Bush meat is the generic term used for wild animals killed for food. Hunting is practiced throughout the world,
but hunting practices, particularly in equatorial Africa and parts of Asia, are believed to threaten a number of
species with extinction. Traditionally, bush meat in Africa was hunted to feed families directly; however, recent
commercialization of the practice now has bush meat available in grocery stores, which has increased harvest
rates to the level of unsustainability. Additionally, human population growth has increased the need for protein
foods that are not being met from agriculture. Species threatened by the bush meat trade are mostly mammals
including many primates living in the Congo basin.
Exotic Species
Exotic species are species that have been intentionally or unintentionally introduced into an ecosystem in which
they did not evolve. For example, Kudzu (Pueraria lobata), which is native to Japan, was introduced in the United
States in 1876. It was later planted for soil conservation. Problematically, it grows too well in the southeastern
United States—up to a foot a day. It is now an invasive pest species and covers over 7 million acres in the
southeastern United States. If an introduced species is able to survive in its new habitat, that introduction is
now reflected in the observed range of the species. Human transportation of people and goods, including the
intentional transport of organisms for trade, has dramatically increased the introduction of species into new
ecosystems, sometimes at distances that are well beyond the capacity of the species to ever travel itself and
outside the range of the species’ natural predators.
Most exotic species introductions probably fail because of the low number of individuals introduced or poor
adaptation to the ecosystem they enter. Some species, however, possess pre-adaptations that can make them
especially successful in a new ecosystem. These exotic species often undergo dramatic population increases
in their new habitat and reset the ecological conditions in the new environment, threatening the species that
exist there. For this reason, exotic species are also called invasive species. Exotic species can threaten other
species through competition for resources, predation, or disease. For example, the Eurasian star thistle, also
called spotted knapweed, has invaded and rendered useless some of the open prairies of the western states.
However, it is a great nectar-bearing flower for the production of honey and supports numerous pollinating
insects, including migrating monarch butterflies in the north-central states such as Michigan.
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LINK
T &
LEARNING
Explore an interactive global database (http:// 0 penstaxc 0 llege. 0 rg/l/ex 0 tic_invasive) of exotic or invasive
species.
Lakes and islands are particularly vulnerable to extinction threats from introduced species. In Lake Victoria, as
mentioned earlier, the intentional introduction of the Nile perch was largely responsible for the extinction of about
200 species of endemic cichlids. The accidental introduction of the brown tree snake via aircraft (Figure 47.12)
from the Solomon Islands to Guam in 1950 has led to the extinction of three species of birds and three to five
species of reptiles endemic to the island. Several other species are still threatened. The brown tree snake is
adept at exploiting human transportation as a means to migrate; one was even found on an aircraft arriving in
Corpus Christi, Texas. Constant vigilance on the part of airport, military, and commercial aircraft personnel is
required to prevent the snake from moving from Guam to other islands in the Pacific, especially Hawaii. Islands
do not make up a large area of land on the globe, but they do contain a disproportionate number of endemic
species because of their isolation from mainland ancestors.
Figure 47.12 The brown tree snake, Boiga irregularis, is an exotic species that has caused numerous extinctions on
the island of Guam since its accidental introduction in 1950. (credit: NPS)
It now appears that the global decline in amphibian species recognized in the 1990s is, in some part, caused by
the fungus Batrachochytrium dendrobatidis, which causes the disease chytridiomycosis (Figure 47.13). There
is evidence that the fungus is native to Africa and may have been spread throughout the world by transport
of a commonly used laboratory and pet species: the African clawed toad ( Xenopus laevis). It may well be
that biologists themselves are responsible for spreading this disease worldwide. The North American bullfrog,
Rana catesbeiana, which has also been widely introduced as a food animal but which easily escapes captivity,
survives most infections of Batrachochytrium dendrobatidis, and can act as a reservoir for the disease. It also is
a voracious predator in freshwater lakes.
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Figure 47.13 This Limosa Harlequin Frog (Atelopus limosus), an endangered species from Panama, died from a fungal
disease called chytridiomycosis. The red lesions are symptomatic of the disease, (credit: Brian Gratwicke)
Early evidence suggests that another fungal pathogen, Ceomyces destructans, introduced from Europe is
responsible for white-nose syndrome, which infects cave-hibernating bats in eastern North America and has
spread from a point of origin in western New York State (Figure 47.14). The disease has decimated bat
populations and threatens extinction of species already listed as endangered: the Indiana bat, Myotis sodalis,
and potentially the Virginia big-eared bat, Corynorhinus townsendii virginianus. How the fungus was introduced
is unclear, but one logical presumption would be that recreational cavers unintentionally brought the fungus on
clothes or equipment from Europe.
Figure 47.14 This little brown bat in Greeley Mine, Vermont, March 26, 2009, was found to have white-nose syndrome,
(credit: Marvin Moriarty, USFWS)
Climate Change
Climate change, and specifically the anthropogenic (meaning, caused by humans) warming trend presently
escalating, is recognized as a major extinction threat, particularly when combined with other threats such as
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habitat loss and the expansion of disease organisms. Scientists disagree about the likely magnitude of the
effects, with extinction rate estimates ranging from 15 percent to 40 percent of species destined for extinction
by 2050. Scientists do agree, however, that climate change will alter regional climates, including rainfall and
snowfall patterns, making habitats less hospitable to the species living in them, in particular, the endemic
species. The warming trend will shift colder climates toward the north and south poles, forcing species to move
with their adapted climate norms while facing habitat gaps along the way. The shifting ranges will impose new
competitive regimes on species as they find themselves in contact with other species not present in their historic
range. One such unexpected species contact is between polar bears and grizzly bears. Previously, these two
distinct species had separate ranges. Now, their ranges are overlapping and there are documented cases of
these two species mating and producing viable offspring, which may or may not be viable crossing back to
either parental species. Changing climates also throw off species’ delicate timed adaptations to seasonal food
resources and breeding times. Many contemporary mismatches to shifts in resource availability and timing have
already been documented.
■ Historic grizzly Extended range of y/y Polar bear
bear habitat grizzly bear habitat '//. habitat
Figure 47.15 Since 2008, grizzly bears (Ursus arctos horribilis) have been spotted farther north than their historic
range, a possible consequence of climate change. As a result, grizzly bear habitat now overlaps polar bear
(Ursus maritimus) habitat. The two species of bears, which are capable of mating and producing viable offspring, are
considered separate “ecological” species because historically they lived in different habitats and never met. However,
in 2006 a hunter shot a wild grizzly-polar bear hybrid known as a grolar bear, the first wild hybrid ever found.
Range shifts are already being observed: for example, some European bird species ranges have moved 91 km
northward. The same study suggested that the optimal shift based on warming trends was double that distance,
suggesting that the populations are not moving quickly enough. Range shifts have also been observed in plants,
butterflies, other insects, freshwater fishes, reptiles, and mammals.
Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating
the habitat for those species adapted to the highest elevations. Some climates will completely disappear. The
accelerating rate of warming in the arctic significantly reduces snowfall and the formation of sea ice. Without the
ice, species like polar bears cannot successfully hunt seals, which are their only reliable source of food. Sea ice
coverage has been decreasing since observations began in the mid-twentieth century, and the rate of decline
observed in recent years is far greater than previously predicted.
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Finally, global warming will raise ocean levels due to meltwater from glaciers and the greater volume of warmer
water. Shorelines will be inundated, reducing island size, which will have an effect on some species, and a
number of islands will disappear entirely. Additionally, the gradual melting and subsequent refreezing of the
poles, glaciers, and higher elevation mountains—a cycle that has provided freshwater to environments for
centuries—will also be jeopardized. This could result in an overabundance of salt water and a shortage of fresh
water.
47.4 | Preserving Biodiversity
By the end of this section, you will be able to do the following:
• Identify new technologies and methods for describing biodiversity
• Explain the legislative framework for conservation
• Describe principles and challenges of conservation preserve design
• Identify examples of the effects of habitat restoration
• Discuss the role of zoos in biodiversity conservation
Preserving biodiversity is an extraordinary challenge that must be met by greater understanding of biodiversity
itself, changes in human behavior and beliefs, and various preservation strategies.
Measuring Biodiversity
The technology of molecular genetics and data processing and storage are maturing to the point where
cataloguing the planet’s species in an accessible way is now feasible. DNA barcoding is one molecular genetic
method, which takes advantage of rapid evolution in a mitochondrial gene (cytochrome c oxidase 1) present
in eukaryotes, except for plants, to identify species using the sequence of portions of the gene. However,
plants may be barcoded using a combination of chloroplast genes. Rapid mass sequencing machines make
the molecular genetics portion of the work relatively inexpensive and quick. Computer resources store and
make available the large volumes of data. Projects are currently underway to use DNA barcoding to catalog
museum specimens, which have already been named and studied, as well as testing the method on less-studied
groups. As of mid 2017, close to 200,000 named species had been barcoded. Early studies suggest there are
significant numbers of undescribed species that looked too much like sibling species to previously be recognized
as different. These now can be identified with DNA barcoding.
Numerous computer databases now provide information about named species and a framework for adding new
species. However, as already noted, at the present rate of description of new species, it will take close to 500
years before the complete catalog of life is known. Many, perhaps most, species on the planet do not have that
much time.
There is also the problem of understanding which species known to science are threatened and to what degree
they are threatened. This task is carried out by the non-profit IUCN which, as previously mentioned, maintains
the Red List—an online listing of endangered species categorized by taxonomy, type of threat, and other criteria
(Figure 47.16). The Red List is supported by scientific research. In 2011, the list contained 61,000 species, all
with supporting documentation.
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CONNECTION
Amphibians
Mammals
Fishes
Insects
Mollusks
Birds
Reptiles
Plants
I I I I ] I » t I 1 I I I \ | » I I 1 | I I \ I | > I » I
0% 5% 10% 15% 20% 25% 30%
The percentage of species in several
groups that are listed as: ■ critically
endangered, a endangered, or
vulnerable on the 2007 IUCN Red List
Figure 47.16 This chart shows the percentage of various animal species, by group, on the IUCN Red List as of
2007.
Which of the following statements is not supported by this graph?
a. There are more vulnerable fishes than critically endangered and endangered fishes combined.
b. There are more critically endangered amphibians than vulnerable, endangered and critically
endangered reptiles combined.
c. Within each group, there are more critically endangered species than vulnerable species.
d. A greater percentage of bird species are critically endangered than mollusk species.
Changing Human Behavior
Legislation throughout the world has been enacted to protect species. The legislation includes international
treaties as well as national and state laws. The Convention on International Trade in Endangered Species of Wild
Fauna and Flora (CITES) treaty came into force in 1975. The treaty, and the national legislation that supports
it, provides a legal framework for preventing approximately 33,000 listed species from being transported across
nations’ borders, thus protecting them from being caught or killed when international trade is involved. The treaty
is limited in its reach because it only deals with international movement of organisms or their parts. It is also
limited by various countries’ ability or willingness to enforce the treaty and supporting legislation. The illegal trade
in organisms and their parts is probably a market in the hundreds of millions of dollars. Illegal wildlife trade is
monitored by another non-profit: Trade Records Analysis of Flora and Fauna in Commerce (TRAFFIC).
Within many countries there are laws that protect endangered species and regulate hunting and fishing. In the
United States, the Endangered Species Act (ESA) was enacted in 1973. Species at risk are listed by the Act;
the U.S. Fish & Wildlife Service is required by law to develop plans that protect the listed species and bring
them back to sustainable numbers. The Act, and others like it in other countries, is a useful tool, but it suffers
because it is often difficult to get a species listed, or to get an effective management plan in place once it is listed.
Additionally, species may be controversially taken off the list without necessarily having had a change in their
situation. More fundamentally, the approach to protecting individual species rather than entire ecosystems is
both inefficient and focuses efforts on a few highly visible and often charismatic species, perhaps at the expense
of other species that go unprotected. At the same time, the Act has a critical habitat provision outlined in the
recovery mechanism that may benefit species other than the one targeted for management.
The Migratory Bird Treaty Act (MBTA) is an agreement between the United States and Canada that was signed
into law in 1918 in response to declines in North American bird species caused by hunting. The Act now lists
over 800 protected species. It makes it illegal to disturb or kill the protected species or distribute their parts (much
of the hunting of birds in the past was for their feathers).
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As already mentioned, the private non-profit sector plays a large role in the conservation effort both in North
America and around the world. The approaches range from species-specific organizations to the broadly
focused IUCN and TRAFFIC. The Nature Conservancy takes a novel approach. It purchases land and protects
it in an attempt to set up preserves for ecosystems.
Although it is focused largely on reducing carbon and related emissions, the Paris Climate Agreement is a
significant step toward altering human behavior in a way that should affect biodiversity. If the agreement is
successful in its goal of halting global temperature rise, many species negatively affected by climate change
may benefit. Assessments of the accord’s implementation will not take place until 2023, and measurement of
its effects will not be feasible for some time. However, the agreement, signed by over 194 countries, represents
the world’s most concerted and unified effort to reduce greenhouse gas emissions, embrace alternative energy
sources, and ease climate pressure on ecosystems.
Conservation in Preserves
Establishment of wildlife and ecosystem preserves is one of the key tools in conservation efforts. A preserve is
an area of land set aside with varying degrees of protection for the organisms that exist within the boundaries
of the preserve. Preserves can be effective in the short term for protecting both species and ecosystems, but
they face challenges that scientists are still exploring to strengthen their viability as long-term solutions to the
preservation of biodiversity and the prevention of extinction.
How Much Area to Preserve?
Due to the way protected lands are allocated and the way biodiversity is distributed, it is challenging to determine
how much land or marine habitat should be protected. The IUCN World Parks Congress estimated that 11.5
percent of Earth’s land surface was covered by preserves of various kinds in 2003. We should note that this
area is greater than previous goals; however, it only includes 9 out of 14 recognized major biomes. Similarly,
individual animals or types of animals are not equally represented on preserves. For example, high quality
preserves include only about 50 percent of threatened amphibian species. To guarantee that all threatened
species will be properly protected, either the protected areas must increase in size, or the percentage of high
quality preserves must increase, or preserves must be targeted with greater attention to biodiversity protection.
Researchers indicate that more attention to the latter solution is required.
Preserve Design
There has been extensive research into optimal preserve designs for maintaining biodiversity. The fundamental
principle behind much of the research has been the seminal theoretical work of Robert H. MacArthur and Edward
[5]
O. Wilson published in 1967 on island biogeography. This work sought to understand the factors affecting
biodiversity on islands. The fundamental conclusion was that biodiversity on an island was a function of the
origin of species through migration, speciation, and extinction on that island. Islands farther from a mainland are
harder to get to, so migration is lower and the equilibrium number of species is lower. Within island populations,
evidence suggests that the number of species gradually increases to a level similar to the numbers on the
mainland from which the species is suspected to have migrated. In addition, smaller islands are harder to find, so
their immigration rates for new species are typically lower. Smaller islands are also less geographically diverse
so all things being equal, there are fewer niches to promote speciation. And finally, smaller islands support
smaller populations, so the probability of extinction is higher.
As islands get larger, the number of species able to colonize the island and find suitable niches on the island
increases, although the effect of island area on species numbers is not a direct correlation. Conservation
preserves can be seen as “islands” of habitat within “an ocean" of non-habitat. For a species to persist in a
preserve, the preserve must be large enough to support it. The critical size depends, in part, on the home
range that is characteristic of the species. A preserve for wolves, which range hundreds of kilometers, must be
much larger than a preserve for butterflies, which might range within ten kilometers during its lifetime. But larger
preserves have more core area of optimal habitat for individual species, they have more niches to support more
species, and they attract more species because they can be found and reached more easily.
Preserves perform better when there are buffer zones around them of suboptimal habitat. The buffer allows
organisms to exit the boundaries of the preserve without immediate negative consequences from predation or
lack of resources. One large preserve is better than the same area of several smaller preserves because there
is more core habitat unaffected by edges. For this same reason, preserves in the shape of a square or circle will
be better than a preserve with many thin “arms.” If preserves must be smaller, then providing wildlife corridors
5. Robert H. MacArthur and Edward O. Wilson, E. O., The Theory of Island Biogeography (Princeton, N.J.: Princeton University Press,
1967).
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between them so that individuals (and their genes) can move between the preserves, for example along rivers
and streams, will make the smaller preserves behave more like a large one. All of these factors are taken into
consideration when planning the nature of a preserve before the land is set aside.
In addition to the physical, biological, and ecological specifications of a preserve, there are a variety of policy,
legislative, and enforcement specifications related to uses of the preserve for functions other than protection
of species. These can include anything from timber extraction, mineral extraction, regulated hunting, human
habitation, and nondestructive human recreation. Many of these policy decisions are made based on political
pressures rather than conservation considerations. In some cases, wildlife protection policies have been so
strict that subsistence-living indigenous populations have been forced from ancestral lands that fell within a
preserve. In other cases, even if a preserve is designed to protect wildlife, if the protections are not or cannot be
enforced, the preserve status will have little meaning in the face of illegal poaching and timber extraction. This is
a widespread problem with preserves in areas of the tropics.
Limitations on Preserves
Some of the limitations on preserves as conservation tools are evident from the discussion of preserve design.
Political and economic pressures typically make preserves smaller, rather than larger, so setting aside areas that
are large enough is difficult. If the area set aside is sufficiently large, there may not be sufficient area to create
a buffer around the preserve. In this case, an area on the outer edges of the preserve inevitably becomes a
riskier suboptimal habitat for the species in the preserve. Enforcement of protections is also a significant issue
in countries without the resources or political will to prevent poaching and illegal resource extraction.
Climate change will create inevitable problems with the location of preserves. The species within them may
migrate to higher latitudes as the habitat of the preserve becomes less favorable. Scientists are planning for the
effects of global warming on future preserves and striving to predict the need for new preserves to accommodate
anticipated changes to habitats; however, the end effectiveness is tenuous since these efforts are prediction
based.
Finally, an argument can be made that conservation preserves indicate that humans are growing more separate
from nature, and that humans only operate in ways that do damage to biodiversity. Creating preserves may
reduce the pressure on humans outside the preserve to be sustainable and non-damaging to biodiversity. On
the other hand, properly managed, high quality preserves present opportunities for humans to witness nature
in a less damaging way, and preserves may present some financial benefits to local economies. Ultimately, the
economic and demographic pressures on biodiversity are unlikely to be mitigated by preserves alone. In order
to fully benefit from biodiversity, humans will need to alter activities that damage it.
LINK TQ LEARNING
An interactive global data system (http:// 0 penstaxc 0 llege. 0 rg/l/pr 0 tected_areas) of protected areas can
be found at this website. Review data about individual protected areas by location or study statistics on
protected areas by country or region.
Habitat Restoration
Habitat restoration holds considerable promise as a mechanism for restoring and maintaining biodiversity. Of
course, once a species has become extinct, its restoration is impossible. However, restoration can improve
the biodiversity of degraded ecosystems. Reintroducing wolves, a top predator, to Yellowstone National Park
in 1995 led to dramatic changes in the ecosystem that increased biodiversity. The wolves (Figure 47.17)
function to suppress elk and coyote populations and provide more abundant resources to the guild of carrion
eaters. Reducing elk populations has allowed revegetation of riparian areas, which has increased the diversity
of species in that habitat. Decreasing the coyote population has increased the populations of species that
were previously suppressed by this predator. The number of species of carrion eaters has increased because
of the predatory activities of the wolves. In this habitat, the wolf is a keystone species, meaning a species
that is instrumental in maintaining diversity in an ecosystem. Removing a keystone species from an ecological
community may cause a collapse in diversity. The results from the Yellowstone experiment suggest that restoring
a keystone species can have the effect of restoring biodiversity in the community. Ecologists have argued for the
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identification of keystone species where possible and for focusing protection efforts on those species; likewise,
it also makes sense to attempt to return them to their ecosystem if they have been removed.
(c) (d)
Figure 47.17 (a) The Gibbon wolf pack in Yellowstone National Park, March 1, 2007, represents a keystone species.
The reintroduction of wolves into Yellowstone National Park in 1995 led to a change in the grazing behavior of (b)
elk. To avoid predation, the elk no longer grazed exposed stream and riverbeds, such as (c) the Lamar Riverbed
in Yellowstone. This allowed willow and cottonwood seedlings to grow and recolonized large areas. The seedlings
decreased erosion and provided shading to the creek, which improved fish habitat. A new colony of (d) beaver may
also have benefited from the habitat change, (credit a: modification of work by Doug Smith, NPS; credit c: modification
of work by Jim Peaco, NPS; credit d: modification of work by “Shiny Things’VFlickr)
Other large-scale restoration experiments underway involve dam removal, which is a national movement that is
accelerating in importance. In the United States, since the mid-1980s, many aging dams are being considered
for removal rather than replacement because of shifting beliefs about the ecological value of free-flowing rivers
and because many dams no longer provide the benefit and functions that they did when they were first built.
The measured benefits of dam removal include restoration of naturally fluctuating water levels (the purpose
of dams is frequently to reduce variation in river flows), which leads to increased fish diversity and improved
water quality. In the Pacific Northwest, dam removal projects are expected to increase populations of salmon,
which is considered a keystone species because it transports key nutrients to inland ecosystems during its
annual spawning migrations. In other regions such as the Atlantic coast, dam removal has allowed the return of
spawning anadromous fish species (species that are born in fresh water, live most of their lives in salt water, and
return to fresh water to spawn). Some of the largest dam removal projects have yet to occur or have happened
too recently for the consequences to be measured. The large-scale ecological experiments that these removal
projects constitute will provide valuable data for other dam projects slated either for removal or construction.
The Role of Captive Breeding
Zoos have sought to play a role in conservation efforts both through captive breeding programs and education.
The transformation of the missions of zoos from collection and exhibition facilities to organizations that are
dedicated to conservation is ongoing and gaining strength. In general, it has been recognized that, except in
some specific targeted cases, captive breeding programs for endangered species are inefficient and often prone
to failure when the species are reintroduced to the wild. However, captive breeding programs have yielded some
success stories, such as the American condor reintroduction to the Grand Canyon and the reestablishment of
the Whooping Crane along the Midwest flyway.
Unfortunately, zoo facilities are far too limited to contemplate captive breeding programs for the numbers of
species that are now at risk. Education is another potential positive impact of zoos on conservation efforts,
particularly given the global trend to urbanization and the consequent reduction in contacts between people and
wildlife. A number of studies have been performed to look at the effectiveness of zoos on people’s attitudes and
actions regarding conservation; at present, the results tend to be mixed.
1516
Chapter 47 | Conservation Biology and Biodiversity
KEY TERMS
adaptive radiation rapid branching through speciation of a phylogenetic tree into many closely related species
biodiversity variety of a biological system, typically conceived as the number of species, but also applying to
genes, biochemistry, and ecosystems
biodiversity hotspot concept originated by Norman Myers to describe a geographical region with a large
number of endemic species and a large percentage of degraded habitat
bush meat wild-caught animal used as food (typically mammals, birds, and reptiles); usually referring to hunting
in the tropics of sub-Saharan Africa, Asia, and the Americas
chemical diversity variety of metabolic compounds in an ecosystem
chytridiomycosis disease of amphibians caused by the fungus Batrachochytrium dendrobatidis; thought to be
a major cause of the global amphibian decline
DNA barcoding molecular genetic method for identifying a unique genetic sequence to associate with a species
ecosystem diversity variety of ecosystems
endemic species species native to one place
exotic species (also, invasive species) species that has been introduced to an ecosystem in which it did not
evolve
extinction disappearance of a species from Earth; local extinction is the disappearance of a species from a
region
extinction rate number of species becoming extinct over time, sometimes defined as extinctions per million
species-years to make numbers manageable (E/MSY)
genetic diversity variety of genes in a species or other taxonomic group or ecosystem, the term can refer to
allelic diversity or genome-wide diversity
heterogeneity number of ecological niches
megafauna large animals
secondary plant compound compound produced as byproducts of plant metabolic processes that is usually
toxic, but is sequestered by the plant to defend against herbivores
species-area relationship relationship between area surveyed and number of species encountered; typically
measured by incrementally increasing the area of a survey and determining the cumulative numbers of
species
tragedy of the commons economic principle that resources held in common will inevitably be overexploited
white-nose syndrome disease of cave-hibernating bats in the eastern United States and Canada associated
with the fungus Geomyces destructans
CHAPTER SUMMARY
47.1 The Biodiversity Crisis
Biodiversity exists at multiple levels of organization and is measured in different ways depending on the
scientific goals of those taking the measurements. These measurements include numbers of species, genetic
diversity, chemical diversity, and ecosystem diversity. The number of described species is estimated to be 1.5
million with about 17,000 new species being described each year. Estimates for the total number of species on
Earth vary but are on the order of 10 million. Biodiversity is negatively correlated with latitude for most taxa,
meaning that biodiversity is higher in the tropics. The mechanism for this pattern is not known with certainty, but
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Chapter 47 | Conservation Biology and Biodiversity
1517
several plausible hypotheses have been advanced.
Five mass extinctions with losses of more than 50 percent of extant species are observable in the fossil record.
Biodiversity recovery times after mass extinctions vary, but may be as long as 30 million years. Recent
extinctions are recorded in written history and are the basis for one method of estimating contemporary
extinction rates. The other method uses measures of habitat loss and species-area relationships. Estimates of
contemporary extinction rates vary, but some rates are as high as 500 times the background rate, as
determined from the fossil record, and are predicted to rise.
47.2 The Importance of Biodiversity to Human Life
Humans use many compounds that were first discovered or derived from living organisms as medicines:
secondary plant compounds, animal toxins, and antibiotics produced by bacteria and fungi. More medicines will
undoubtedly be discovered in nature. Loss of biodiversity will impact the number of pharmaceuticals available
to humans.
Crop diversity is a requirement for food security, and it is being lost. The loss of wild relatives to crops also
threatens breeders’ abilities to create new varieties. Ecosystems provide ecosystem services that support
human agriculture: pollination, nutrient cycling, pest control, and soil development and maintenance. Loss of
biodiversity threatens these ecosystem services and risks making food production more expensive or
impossible. Wild food sources are mainly aquatic, but few of these resources are being managed for
sustainability. Fisheries’ ability to provide protein to human populations is threatened when extinction occurs.
Biodiversity may provide important psychological benefits to humans. Additionally, there are moral arguments
for the maintenance of biodiversity.
47.3 Threats to Biodiversity
The core threats to biodiversity are human population growth and unsustainable resource use. To date, the
most significant causes of extinctions are habitat loss, introduction of exotic species, and overharvesting.
Climate change is predicted to be a significant cause of extinctions in the coming century. Habitat loss occurs
through deforestation, damming of rivers, and other disruptive human activities. Overharvesting is a threat
particularly to aquatic species, while the taking of bush meat in the humid tropics threatens many species in
Asia, Africa, and the Americas. Exotic species have been the cause of a number of extinctions and are
especially damaging to islands and lakes. Exotic species’ introductions are increasing damaging native
ecosystems around the world because of the increased mobility of human populations and growing global trade
and transportation. Climate change is forcing range changes that may lead to extinction. It is also affecting
adaptations to the timing of resource availability that negatively affects species in seasonal environments. The
impacts of climate change are greatest in the arctic. Global warming will also raise sea levels, eliminating some
islands and reducing the area of all others.
47.4 Preserving Biodiversity
New technological methods such as DNA barcoding and information processing and accessibility are
facilitating the cataloging of the planet’s biodiversity. There is also a legislative framework for biodiversity
protection. International treaties such as CITES regulate the transportation of endangered species across
international borders. Legislation within individual countries protecting species and agreements on global
warming have had limited success; the Paris Climate accord is currently being implemented as a means to
reduce global climate change.
In the United States, the Endangered Species Act protects listed species but is hampered by procedural
difficulties and a focus on individual species. The Migratory Bird Act is an agreement between Canada and the
United States to protect migratory birds. The non-profit sector is also very active in conservation efforts in a
variety of ways.
Conservation preserves are a major tool in biodiversity protection. Presently, 11 percent of Earth’s land surface
is protected in some way. The science of island biogeography has informed the optimal design of preserves;
however, preserves have limitations imposed by political and economic forces. In addition, climate change will
limit the effectiveness of preserves in the future. A downside of preserves is that they may lessen the pressure
on human societies to function more sustainably outside the preserves.
Habitat restoration has the potential to restore ecosystems to previous biodiversity levels before species
become extinct. Examples of restoration include reintroduction of keystone species and removal of dams on
1518
Chapter 47 | Conservation Biology and Biodiversity
rivers. Zoos have attempted to take a more active role in conservation and can have a limited role in captive
breeding programs. Zoos also have a useful role in education.
VISUAL CONNECTION QUESTIONS
1. Figure 47.6 Scientists measured the relative
abundance of fern spores above and below the K-Pg
boundary in this rock sample. Which of the following
statements most likely represents their findings?
a. An abundance of fern spores from several
species was found below the K-Pg
boundary, but none was found above.
b. An abundance of fern spores from several
species was found above the K-Pg
boundary, but none was found below.
c. An abundance of fern spores was found
both above and below the K-Pg boundary,
but only one species was found below the
boundary , and many species were found
above the boundary.
d. Many species of fern spores were found
both above and below the boundary, but the
total number of spores was greater below
the boundary.
2. Figure 47.9 The Svalbard Global Seed Vault is
located on Spitsbergen island in Norway, which has
REVIEW QUESTIONS
5. With an extinction rate of 100 E/MSY and an
estimated 10 million species, how many extinctions
are expected to occur in a century?
a. 100
b. 10,000
c. 100,000
d. 1,000,000
6. An adaptive radiation is_.
a. a burst of speciation
b. a healthy level of UV radiation
c. a hypothesized cause of a mass extinction
d. evidence of an asteroid impact
7. The number of currently described species on the
planet is about_.
a. 17,000
b. 150,000
c. 1.5 million
d. 10 million
8. A mass extinction is defined as_.
a. a loss of 95 percent of species
b. an asteroid impact
c. a boundary between geological periods
d. a loss of 50 percent of species
9. A secondary plant compound might be used for
which of the following?
an arctic climate. Why might an arctic climate be
good for seed storage?
3. Converting a prairie to a farm field is an example
of_.
a. overharvesting
b. habitat loss
c. exotic species
d. climate change
4. Figure 47.16 Which of the following statements is
not supported by this graph?
a. There are more vulnerable fishes than
critically endangered and endangered fishes
combined.
b. There are more critically endangered
amphibians than vulnerable, endangered
and critically endangered reptiles combined.
c. Within each group, there are more critically
endangered species than vulnerable
species.
d. A greater percentage of bird species are
critically endangered than mollusk species.
a. a new crop variety
b. a new drug
c. a soil nutrient
d. a pest of a crop pest
10. Pollination is an example of_.
a. a possible source of new drugs
b. chemical diversity
c. an ecosystem service
d. crop pest control
11. What is an ecosystem service that performs the
same function as a pesticide?
a. pollination
b. secondary plant compounds
c. crop diversity
d. predators of pests
12. Which two extinction risks may be a direct result
of the pet trade?
a. climate change and exotic species
introduction
b. habitat loss and overharvesting
c. overharvesting and exotic species
introduction
d. habitat loss and climate change
13. Exotic species are especially threatening to what
kind of ecosystem?
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Chapter 47 | Conservation Biology and Biodiversity
1519
a. deserts
b. marine ecosystems
c. islands
d. tropical forests
14. Certain parrot species cannot be brought to the
United States to be sold as pets. What is the name of
the legislation that makes this illegal?
a. Red List
b. Migratory Bird Act
c. CITES
d. Endangered Species Act (ESA)
15. What was the name of the first international
CRITICAL THINKING QUESTIONS
17. Describe the evidence for the cause of the
Cretaceous-Paleogene (K-Pg) mass extinction.
18. Describe the two methods used to calculate
contemporary extinction rates.
19. Explain how biodiversity loss can impact crop
diversity.
20. Describe two types of compounds from living
things that are used as medications.
21. Describe the mechanisms by which human
agreement on climate change?
a. Red List
b. Montreal Protocol
c. International Union for the Conservation of
Nature (IUCN)
d. Kyoto Protocol
16. About what percentage of land on the planet is
set aside as a preserve of some type?
a. 1 percent
b. 6 percent
c. 11 percent
d. 15 percent
population growth and resource use causes
increased extinction rates.
22. Explain what extinction threats a frog living on a
mountainside in Costa Rica might face.
23. Describe two considerations in conservation
preserve design.
24. Describe what happens to an ecosystem when a
keystone species is removed.
1520
Chapter 47 | Conservation Biology and Biodiversity
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Appendix A
1521
APPENDIX A | THE PERIODIC TABLE
OF ELEMENTS
CD
a
Group
1
Periodic Table of the Elements
18
1
H
1.008
hydrogen
2
13
14
15
16
17
2
He
4.003
3
Li
6.94
4
Be
9.012
beryllium
5
B
10.81
6
c
12.01
7
N
14.01
8
o
16.00
9
F
19.00
fluorine
10
Ne
20.18
11
Na
22.99
12
Mg
24.31
magnesium
3
4
5
6
7
8
9
10
11
12
13 .
Al
26.98
aluminum
14
Si
28.09
15
P
30.97
phosphorus
16 s
32.06
17 ci
35.45
18
Ar
39.95
19
K
39.10
potassium
20
Ca
40.08
calcium
21
Sc
44.96
scandium
22
Ti
47.87
titanium
23
V
50.94
vanadium
24
Cr
52.00
chromium
25
Mn
54.94
manganese
26
Fe
55.85
27
Co
58.93
28
Ni
58.69
nickel
29
Cu
63.55
copper
30
Zn
65.38
zinc
31
Ga
69.72
32
Ge
72.63
germanium
33
As
74.92
34
Se
78.97
selenium
35
Br
79.90
bromine
36
Kr
83.80
37
Rb
85.47
rubidium
38
Sr
87.62
strontium
39
Y
88.91
yttnum
40
Zr
91.22
zirconium
41
Nb
92.91
niobium
42
Mo
95.95
molybdenum
43
Tc
[97]
technetium
44
Ru
101.1
ruthenium
45
Rh
102.9
rhodium
46
Pd
106.4
palladium
47
Ag
10779
silver
48
Cd
112.4
49
In
114.8
50
Sn
118.7
51
Sb
121.8
antimony
52
Te
127.6
teilunum
53
1
126.9
54
Xe
131.3
55
Cs
132.9
56
Ba
137.3
57-71
La-
Lu *
72
Hf
178.5
hafnium
73
Ta
180.9
tantalum
74
w
183.8
tungsten
75
Re
186.2
rhenium
76
Os
190.2
77
lr
192.2
iridium
78
Pt
195.1
platinum
79
Au
197.0
gold
80
Hg
200:6
mercury
81
TI
204.4
thallium
82
Pb
207.2
lead
83
Bi
209.0
84
Po
[209]
polonium
85
At
pig
86
Rn
[222]
87
Fr
[2231
francium
88
Ra
[226]
89-103
Ac-
Lr **
104
Rf
. SSL
105
Db
[270]
dubruum
106
£f|
seaborgium
107
Bh
[270]
bohrium
108
Hs
[277]
hassium
109
Mt
meitnerium
no
Ds
[281]
darmaadiium
111
[282]
roentgenium
112
Cn
[285]
copernicium
113
Nh
jsa,
114
FI
[289]
flerovlum
115
Me
[288]
moscovium
116
Lv
[293]
livermonum
117
Ts
tennesslne
118
oganesson
V
57
La
138.9
lanthanum
58
Ce
140.1
59
Pr
140.9
iraseocv n' t
60
Nd
144.2
61
Pm
[145]
promethium
62
Sm
150.4
samarium
63
Eu
152.0
europium
64
Gd
157.3
65
Tb
158.9
66
Dy
1675
dysprosium
67
Ho
164.9
68
Er
167.3
69
Tm
168.9
thulium
70
Yb
173.1
ytterbium
71
Lu
175.0
lutetium
**
89
Ac
[227]
actinium
90
Th
232.0
thorium
91
Pa
231.0
protactinium
92
u
238.0
uranium
93
Np
[237]
neptunium
94
Pu
[244]
plutonium
95
Am
[243]
americium
96
Cm
[247]
97
Bk
[247]
berkelium
98
Cf
[251]
californium
99
Es
[252]
einsteinium
100
Fm
[257]
lermium
101
Md
[258]
mendelevium
102
No
[259]
nobelium
103
Lr
[262]
lawrencium
Symbol
Atomic mass
Name
Color Code
Metal
Solid
1 1 Metalloid
Liquid
Nonmetal
Gas
Figure A1
1522
Appendix A
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Appendix B
1523
APPENDIX B | GEOLOGICAL TIME
2 Ma:
ca. 2300 Ma:
Atmosphere becomes oxygen-rich;
ca. 3500 Ma:
Photosynthesis starts
ca. 380 Ma:
First vertebrate land animals
ca. 4000 Ma: End of the
Late Heavy Bombardment;
first life
ca. 530 Ma:
Cambrian explosion
750-635 Ma:
Two Snowball Earths
First Hominids
230-65 Ma:
Dinosaurs
4550 Ma:
Formation of the Earth
Hominids
Mammals
Land plants
Animals
Multicellular life
Eukaryotes
r-*—>-— Formation of the Moon
4527 Ma:
first Snowball Earth
Figure B1 Geological Time Clock
Phanerozoic
Present
Proterozoic
542 mva
Archaean
m,a
Hadean
3800 mva
Cenozoic Present
Mesozoic 65.5 mva
Paleozoic 251 mva
Pliocene
Miocene 5.33 mya
Oligocene 23.03 mva
Eocene
33.9 mva
Pleistocene 11500 >
Pliocene i B 06 mva
Recent
History
Today
Iron Age
500 AD
Bronze Age iioobc
Chalcolithic
3000 BC
Neolithic
4000 BC
Mesolithic 65_QO_BC
Figure B2 Geological Time Chart
(credit: Richard S. Murphy, Jr.)
1524
Appendix B
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Appendix C
1525
APPENDIX C | MEASUREMENTS AND
THE METRIC SYSTEM
Measurements and the Metric System
Measurement
Unit
Abbreviation
Metric
Equivalent
Approximate Standard
Equivalent
Length
nanometer
nm
1 nm = 10 9 m
1 mm - 0.039 inch
1 cm = 0.394 inch
1 m = 39.37 inches
1 m = 3.28 feet
1 m = 1.093 yards
1 km = 0.621 miles
micrometer
pm
1 pm = 10“ 6 m
millimeter
mm
1 mm = 0.001 m
centimeter
cm
1 cm = 0.01 m
meter
m
1 m = 100 cm
1 m = 1000 mm
kilometer
km
1 km = 1000 m
Mass
microgram
MS
1 pg = 10“ 6 g
1 g = 0.035 ounce
1 kg = 2.205 pounds
milligram
mg
1 mg = 10~ 3 g
gram
g
1 g = 1000 mg
kilogram
kg
1 kg = 1000 g
Volume
microliter
Ml
1 pi = 10“ 6 1
I ml - 0.034 fluid ounce
II = 1.057 quarts
1 kl = 264.172 gallons
milliliter
ml
1 ml = 10“ 3 1
liter
1
11 = 1000 ml
kiloliter
kl
1 kl = 1000 1
Area
square
centimeter
cm 2
1 cm 2 = 100 mm 2
1 cm 2 = 0.155 square inch
1 m 2 = 10.764 square feet
1 m 2 = 1.196 square yards
1 ha = 2.471 acres
square meter
m 2
1 m 2 = 10,000
2
cm
hectare
ha
1 ha = 10,000 m 2
Temperature
Celsius
°C
—
1 °C = 5/9 x (°F - 32)
Table Cl
1526
Appendix C
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Answer Key
1527
ANSWER KEY
Chapter 1
1 Figure 1.6 1: C; 2: F; 3: A; 4: B; 5: D; 6: E. The original hypothesis is incorrect, as the coffeemaker works when
plugged into the outlet. Alternative hypotheses include that the toaster might be broken or that the toaster wasn't turned
on. 3 Figure 1.16 Communities exist within populations which exist within ecosystems. 4B 6D 8C IOC 12 B 14
D 16 Answers will vary, but should apply the steps of the scientific method. One possibility could be a car which
doesn't start. The hypothesis could be that the car doesn’t start because the battery is dead. The experiment would be
to change the battery or to charge the battery and then check whether the car starts or not. If it starts, the problem was
due to the battery, and the hypothesis is accepted. 18 Answers will vary. Topics that fall inside the area of biological
study include how diseases affect human bodies, how pollution impacts a species’ habitat, and how plants respond
to their environments. Topics that fall outside of biology (the “study of life”) include how metamorphic rock is formed
and how planetary orbits function. 20 Answers will vary. Layers of sedimentary rock have order but are not alive.
Technology is capable of regulation but is not, of itself, alive. 22 During your walk, you may begin to perspire, which
cools your body and helps your body to maintain a constant internal temperature. You might also become thirsty and
pause long enough for a cool drink, which will help to restore the water lost during perspiration.
Chapter 2
1 Figure 2.3 Carbon-12 has six neutrons. Carbon-13 has seven neutrons. 3 Figure 2.24 C4A6C8D 10
C 12 D 14 Ionic bonds are created between ions. The electrons are not shared between the atoms, but rather are
associated more with one ion than the other. Ionic bonds are strong bonds, but are weaker than covalent bonds,
meaning it takes less energy to break an ionic bond compared with a covalent one. 16 Buffers absorb the free
hydrogen ions and hydroxide ions that result from chemical reactions. Because they can bond these ions, they
prevent increases or decreases in pH. An example of a buffer system is the bicarbonate system in the human body.
This system is able to absorb hydrogen and hydroxide ions to prevent changes in pH and keep cells functioning
properly. 18 Carbon is unique and found in all living things because it can form up to four covalent bonds between
atoms or molecules. These can be nonpolar or polar covalent bonds, and they allow for the formation of long chains of
carbon molecules that combine to form proteins and DNA.
Chapter 3
1 Figure 3.5 Glucose and galactose are aldoses. Fructose is a ketose. 3 Figure 3.33 Adenine is larger than cytosine
and will not be able to base pair properly with the guanine on the opposing strand. This will cause the DNA to bulge.
DNA repair enzymes may recognize the bulge and replace the incorrect nucleotide. 4 C 6 A 8 D 10 B 12 D 14
C 16 B 18 C 20 C 21 Biological macromolecules are organic because they contain carbon. 23 Amino acids can
be linked into long chains through condensation reactions. One of the hydrogen atoms bonded to the nitrogen atom
of an amino acid reacts with the -OH group attached to the terminal carbon on another amino acid. Since both ends
of the molecule can participate in condensation reactions, peptide bonds can be made in both directions to create
a long amino acid chain. 25 The (3 1-4 glycosidic linkage in cellulose cannot be broken down by human digestive
enzymes. Herbivores such as cows, koalas, and buffalos are able to digest grass that is rich in cellulose and use it as
a food source because bacteria and protists in their digestive systems, especially in the rumen, secrete the enzyme
cellulase. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by
the animal. 27 Fat serves as a valuable way for animals to store energy. It can also provide insulation. Waxes can
protect plant leaves and mammalian fur from getting wet. Phospholipids and steroids are important components of
animal cell membranes, as well as plant, fungal, and bacterial membranes. 29 Fats have a higher energy density
than carbohydrates (averaging 9kcal/gram versus 4.3kcal/gram respectively). Thus, on a per gram basis, more energy
can be stored in fats than can be stored in carbohydrates. Additionally, fats are packaged into spherical globules
to minimize interactions with the water-based plasma membrane, while glycogen is a large branched carbohydrate
that cannot be compacted for storage. 31 A change in gene sequence can lead to a different amino acid being
added to a polypeptide chain instead of the normal one. This causes a change in protein structure and function.
For example, in sickle cell anemia, the hemoglobin (3 chain has a single amino acid substitution—the amino acid
glutamic acid in position six is substituted by valine. Because of this change, hemoglobin molecules form aggregates,
and the disc-shaped red blood cells assume a crescent shape, which results in serious health problems. 33 The
protein must form a channel in the plasma membrane that allows water into the cell since water cannot cross the
plasma membrane by itself. Since aquaporins are embedded in the plasma membrane and connect with both the
intracellular and extracellular spaces, it must be amphipathic like the plasma membrane. The top and bottom of the
protein must contain charged or polar amino acids (hydrophilic) to interact with the aqueous environments. The exterior
transmembrane region must contain non-polar amino acids (hydrophobic) that can interact with the phospholipid tails.
1528
Answer Key
However, the inside of this channel must contain hydrophilic amino acids since they will interact with the traveling water
molecules. 35 The four types of RNA are messenger RNA, ribosomal RNA, transfer RNA, and microRNA. Messenger
RNA carries the information from the DNA that controls all cellular activities. The mRNA binds to the ribosomes that are
constructed of proteins and rRNA, and tRNA transfers the correct amino acid to the site of protein synthesis. microRNA
regulates the availability of mRNA for translation.
Chapter 4
1 Figure 4.7 Substances can diffuse more quickly through small cells. Small cells have no need for organelles and
therefore do not need to expend energy getting substances across organelle membranes. Large cells have organelles
that can separate cellular processes, enabling them to build molecules that are more complex. 3 Figure 4.18 It would
end up on the outside. After the vesicle passes through the Golgi apparatus and fuses with the plasma membrane, it
turns inside out. 4C 6D 8D 10 B 12 D 14 A 16 C 18 D 20 D 22 C 24 D 25 A light microscope would be ideal
when viewing a small living organism, especially when the cell has been stained to reveal details. 27 A transmission
electron microscope would be ideal for viewing the cell’s internal structures, because many of the internal structures
have membranes that are not visible by the light microscope. 29 The cell theory states: All living things are
made of cells.;Cells are the most basic unit of life.;New cells arise from existing cells. All humans are multicellular
organisms whose smallest building blocks are cells. Adult humans begin with the fusion of a male gamete cell with
a female gamete cell to form a fertilized egg (single cell). That cell then divides into two cells, which each divides
into two more cells, and so forth until all the cells of a human embryo are made. As the embryo passes through all
the developmental stages to make an adult human, the cells that are added arise from division of existing cells. 31
Some microbes are beneficial. For instance, E. coli bacteria populate the human gut and help break down fiber in
the diet. Some foods such as yogurt are formed by bacteria. 33 Both are similar in that they are enveloped in a
double membrane, both have an intermembrane space, and both make ATP. Both mitochondria and chloroplasts have
DNA, and mitochondria have inner folds called cristae and a matrix, while chloroplasts have chlorophyll and accessory
pigments in the thylakoids that form stacks (grana) and a stroma. 35 “Form follows function” refers to the idea that the
function of a body part dictates the form of that body part. As an example, compare your arm to a bat's wing. While
the bones of the two correspond, the parts serve different functions in each organism and their forms have adapted
to follow that function. 37 Centrioles and flagella are alike in that they are made up of microtubules. In centrioles, two
rings of nine microtubule “triplets” are arranged at right angles to one another. This arrangement does not occur in
flagella. 39 A macrophage engulfs a pathogen by rearranging its actin microfilaments to bend the plasma membrane
around the pathogen. Once the pathogen is sealed in an endosome inside the macrophage, the vesicle is walked
along microtubules until it combines with a lysosome to digest the pathogen. 41 They differ because plant cell walls
are rigid. Plasmodesmata, which a plant cell needs for transportation and communication, are able to allow movement
of really large molecules. Gap junctions are necessary in animal cells for transportation and communication. 43 E. coli
infections generally cause food poisoning, meaning that the invading bacteria cross from the lumen of the gut into the
rest of the body. Tight junctions hold the epithelial layer that lines the digestive tract together so that the material that
crosses into the body is tightly regulated. One way E. coli can avoid this regulation is to destroy the tight junctions so
that it can enter the body between the epithelial cells, rather than having to go through the cells.
Chapter 5
1 Figure 5.12 No, it must have been hypotonic as a hypotonic solution would cause water to enter the cells, thereby
making them burst. 3 Figure 5.19 A decrease in pH means an increase in positively charged H + ions, and an increase
in the electrical gradient across the membrane. The transport of amino acids into the cell will increase. 4 A 6 A 8
C 10 A 12 D 14 D 16 B 18 C 20 B 21 The fluid characteristic of the cell membrane allows greater flexibility to
the cell than it would if the membrane were rigid. It also allows the motion of membrane components, required for
some types of membrane transport. 23 Peripheral proteins can bind to other molecules in the extracellular space.
However, they cannot directly transmit a signal to the inside of the cell since they do not have a transmembrane domain
(region that goes through the plasma membrane to the inside of the cell). They must associate with integral membrane
proteins in order to pass the signal to the inside of the cell. 25 Water moves through a membrane in osmosis because
there is a concentration gradient across the membrane of solute and solvent. The solute cannot effectively move to
balance the concentration on both sides of the membrane, so water moves to achieve this balance. 27 Decreasing
temperature will decrease the kinetic energy in the system. A lower temperature means less energy in the molecules,
so they will move at a slower speed. Lowering temperature also decreases the kinetic energy of the molecules in
the plasma membrane, compressing them together. This increases the density of the plasma membrane, which slows
diffusion into the cell. 29 The cell harvests energy from ATP produced by its own metabolism to power active transport
processes, such as the activity of pumps. 31 Intestinal epithelial cells use active transport to fulfill their specific role as
the cells that transfer glucose from the digested food to the bloodstream. Intestinal cells are exposed to an environment
with fluctuating glucose levels. Immediately after eating, glucose in the gut lumen will be high, and could accumulate
in intestinal cells by diffusion. However, when the gut lumen is empty, glucose levels are higher in the intestinal cells. If
glucose moved by facilitated diffusion, this would cause glucose to flow back out of the intestinal cells and into the gut.
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Active transport proteins ensure that glucose moves into the intestinal cells, and cannot move back into the gut. It also
ensures that glucose transport continues to occur even if high levels of glucose are already present in the intestinal
cells. This maximizes the amount of energy the body can harvest from food. 33 The proteins allow a cell to select
what compound will be transported, meeting the needs of the cell and not bringing in anything else.
Chapter 6
1 Figure 6.8 A compost pile decomposing is an exergonic process; enthalpy increases (energy is released) and
entropy increases (large molecules are broken down into smaller ones). A baby developing from a fertilized egg is
an endergonic process; enthalpy decreases (energy is absorbed) and entropy decreases. Sand art being destroyed
is an exergonic process; there is no change in enthalpy, but entropy increases. A ball rolling downhill is an exergonic
process; enthalpy decreases (energy is released), but there is no change in entropy. 3 Figure 6.14 Three sodium ions
could be moved by the hydrolysis of one ATP molecule. The AG of the coupled reaction must be negative. Movement
of three sodium ions across the membrane will take 6.3 kcal of energy (2.1 kcal x 3 Na + ions = 6.3 kcal). Hydrolysis of
ATP provides 7.3 kcal of energy, more than enough to power this reaction. Movement of four sodium ions across the
membrane, however, would require 8.4 kcal of energy, more than one ATP molecule can provide. 4 C 6 C 8 B 10
A 12 A 14 C 16 Physical exercise involves both anabolic and catabolic processes. Body cells break down sugars
to provide ATP to do the work necessary for exercise, such as muscle contractions. This is catabolism. Muscle cells
also must repair muscle tissue damaged by exercise by building new muscle. This is anabolism. 18 A spontaneous
reaction is one that has a negative AG and thus releases energy. However, a spontaneous reaction need not occur
quickly or suddenly like an instantaneous reaction. It may occur over long periods due to a large energy of activation,
which prevents the reaction from occurring quickly. 20 The ant farm had lower entropy before the earthquake because
it was a highly ordered system. After the earthquake, the system became much more disordered and had higher
entropy. 22 The activation energy for hydrolysis is very low. Not only is ATP hydrolysis an exergonic process with a
large -AG, but ATP is also a very unstable molecule that rapidly breaks down into ADP + P; if not utilized quickly. This
suggests a very low Ea since it hydrolyzes so quickly. 24 Feedback inhibition allows cells to control the amounts of
metabolic products produced. If there is too much of a particular product relative to the cell’s needs, feedback inhibition
effectively causes the cell to decrease production of that particular product. In general, this reduces the production of
superfluous products and conserves energy, maximizing energy efficiency.
Chapter 7
1 Figure 7.11 After DNP poisoning, the electron transport chain can no longer form a proton gradient, and ATP
synthase can no longer make ATP. DNP is an effective diet drug because it uncouples ATP synthesis; in other words,
after taking it, a person obtains less energy out of the food he or she eats. Interestingly, one of the worst side effects of
this drug is hyperthermia, or overheating of the body. Since ATP cannot be formed, the energy from electron transport
is lost as heat. 3 Figure 7.14 The illness is caused by lactate accumulation. Lactate levels rise after exercise, making
the symptoms worse. Milk sickness is rare today but was common in the midwestern United States in the early
1800s. 4 A 6 C 8 B 10 C 12 C 14 A 16 A 18 ATP provides the cell with a way to handle energy in an efficient
manner. The molecule can be charged, stored, and used as needed. Moreover, the energy from hydrolyzing ATP
is delivered as a consistent amount. Harvesting energy from the bonds of several different compounds would result
in energy deliveries of different quantities. 20 All cells must consume energy to carry out basic functions, such as
pumping ions across membranes. A red blood cell would lose its membrane potential if glycolysis were blocked, and
it would eventually die. 22 Q and cytochrome c are transport molecules. Their function does not result directly in ATP
synthesis in that they are not pumps. Moreover, Q is the only component of the electron transport chain that is not
a protein. Ubiquinone and cytochrome c are small, mobile electron carriers, whereas the other components of the
electron transport chain are large complexes anchored in the inner mitochondrial membrane. 24 Fermentation uses
glycolysis only. Anaerobic respiration uses all three parts of cellular respiration, including the parts in the mitochondria
like the citric acid cycle and electron transport; it also uses a different final electron acceptor instead of oxygen gas. 26
Citrate can inhibit phosphofructokinase by feedback regulation.
Chapter 8
1 Figure 8.6 Levels of carbon dioxide (a necessary photosynthetic substrate) will immediately fall. As a result, the rate
of photosynthesis will be inhibited. 3 Figure 8.18 D 4 A 6 B 8 B 10 A 12 C 14 A 16 D 18 C 20 C 21 The
outcome of light reactions in photosynthesis is the conversion of solar energy into chemical energy that the chloroplasts
can use to do work (mostly anabolic production of carbohydrates from carbon dioxide). 23 The energy carriers that
move from the light-dependent reaction to the light-independent one are “full” because they bring energy. After the
energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy. There
is not much actual movement involved. Both ATP and NADPH are produced in the stroma where they are also used
and reconverted into ADP, Pi, and NADP + . 25 The stomata regulate the exchange of gases and water vapor between
a leaf and its surrounding environment. When the stomata are closed, the water molecules cannot escape the leaf,
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but the leaf also cannot acquire new carbon dioxide molecules from the environment. This limits the light-independent
reactions to only continuing until the carbon dioxide stores in the leaf are depleted. 27 Both of these molecules carry
energy; in the case of NADPH, it has reducing power that is used to fuel the process of making carbohydrate molecules
in light-independent reactions. 29 Because RuBP, the molecule needed at the start of the cycle, is regenerated from
G3P. 31 Because G3P has three carbon atoms, and each turn of the cycle takes in one carbon atom in the form of
carbon dioxide. 33 In the defined ecosystem, energy would radiate from the Sun, and be absorbed by the chlorophyll
in the leaves of the tree. Photosynthesis would occur in the leaves, transforming the light energy into stored chemical
energy in the covalent bonds of carbon molecules. The giraffe would eat the leaves of the tree, and digest the carbon
molecules to release energy. In the same ecosystem, nutrients would cycle between the tree and the giraffe. The
giraffe would consume oxygen and release carbon dioxide as its cells perform aerobic respiration to create chemical
energy. The tree will consume the released carbon dioxide during photosynthesis to create its own stored chemical
energy, and release oxygen as a by-product.
Chapter 9
1 Figure 9.8 C. The downstream cellular response would be inhibited. 3 Figure 9.17 C. 5 B 7 B 9 C 11 C 13
B 15 B 17 C 19 D 21 D 23 Intracellular signaling occurs within a cell, and intercellular signaling occurs between
cells. 25 Internal receptors are located inside the cell, and their ligands enter the cell to bind the receptor. The complex
formed by the internal receptor and the ligand then enters the nucleus and directly affects protein production by binding
to the chromosomal DNA and initiating the making of mRNA that codes for proteins. Cell-surface receptors, however,
are embedded in the plasma membrane, and their ligands do not enter the cell. Binding of the ligand to the cell-surface
receptor initiates a cell signaling cascade and does not directly influence the making of proteins; however, it may
involve the activation of intracellular proteins. 27 Insulin’s receptor is an enzyme-linked transmembrane receptor, as
can be determined from the “tyrosine kinase” in its name. This receptor is embedded in the plasma membrane, and
insulin binds to its extracellular (outer) surface to initiate intracellular signaling cascades. Normally, steroid hormones
cross the plasma membrane to bind with intracellular receptors. These intracellular hormone-receptor complexes then
interact directly with DNA to regulate transcription. This limits steroid hormones to be small, non-polar molecules so
they can cross the plasma membrane. However, since insulin does not have to cross into the cell it could be large
or polar (it is a small, polar molecule). 29 The binding of the ligand to the extracellular domain would activate the
pathway normally activated by the receptor donating the intracellular domain. 31 If a kinase is mutated so that it is
always activated, it will continuously signal through the pathway and lead to uncontrolled growth and possibly cancer.
If a kinase is mutated so that it cannot function, the cell will not respond to ligand binding. 33 Possible explanations:
EGFR dimer cannot separate.An upstream mutation (in Ras, Raf, MEK) constitutively activates the signaling
cascade.ERK has a mutation that prevents it from binding to its phosphatase.The cell has a mutation preventing the
expression or function of the ERK-specific phosphatase. 35 Multicellular organisms must coordinate many different
events in different cell types that may be very distant from each other. Single-celled organisms are only concerned with
their immediate environment and the presence of other cells in the area.
Chapter 10
1 Figure 10.6 D. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase
plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the cell divides. 3
Figure 10.14 D. E6 binding marks p53 for degradation. 4 C 6 D 8 B 10 B 12 D 14 A 16 A 18 D 20 C 22 C 24
D 26 C 28 Human somatic cells have 46 chromosomes: 22 pairs and 2 sex chromosomes that may or may not form a
pair. This is the 2 n or diploid condition. Human gametes have 23 chromosomes, one each of 23 unique chromosomes,
one of which is a sex chromosome. This is the n or haploid condition. 30 The DNA double helix is wrapped around
histone proteins to form structures called nucleosomes. Nucleosomes and the linker DNA in between them are coiled
into a 30-nm fiber. During cell division, chromatin is further condensed by packing proteins. 32 The mitotic spindle
is formed of microtubules. Microtubules are polymers of the protein tubulin; therefore, it is the mitotic spindle that is
disrupted by these drugs. Without a functional mitotic spindle, the chromosomes will not be sorted or separated during
mitosis. The cell will arrest in mitosis and die. 34 Many cells temporarily enter Go until they reach maturity. Some
cells are only triggered to enter Gi when the organism needs to increase that particular cell type. Some cells only
reproduce following an injury to the tissue. Some cells never divide once they reach maturity. 36 The Gi checkpoint
monitors adequate cell growth, the state of the genomic DNA, adequate stores of energy, and materials for S phase.
At the Q >2 checkpoint, DNA is checked to ensure that all chromosomes were duplicated and that there are no mistakes
in newly synthesized DNA. Additionally, cell size and energy reserves are evaluated. The M checkpoint confirms the
correct attachment of the mitotic spindle fibers to the kinetochores. 38 Cdk must bind to a cyclin, and it must be
phosphorylated in the correct position to become fully active. 40 If one of the genes that produces regulator proteins
becomes mutated, it produces a malformed, possibly non-functional, cell-cycle regulator, increasing the chance that
more mutations will be left unrepaired in the cell. Each subsequent generation of cells sustains more damage. The cell
cycle can speed up as a result of the loss of functional checkpoint proteins. The cells can lose the ability to self-destruct
and eventually become “immortalized.” 42 Regulatory mechanisms that might be lost include monitoring of the quality
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of the genomic DNA, recruiting of repair enzymes, and the triggering of apoptosis. 44 The common components of
eukaryotic cell division and binary fission are DNA duplication, segregation of duplicated chromosomes, and division
of the cytoplasmic contents.
Chapter 11
1 Figure 11.9 Yes, it will be able to reproduce asexually. 2C 4D 6C 8C 10 B 12 D 14 D 16 A 18 C 19 During
the meiotic interphase, each chromosome is duplicated. The sister chromatids that are formed during synthesis are
held together at the centromere region by cohesin proteins. All chromosomes are attached to the nuclear envelope
by their tips. As the cell enters prophase I, the nuclear envelope begins to fragment and the proteins holding
homologous chromosomes locate each other. The four sister chromatids align lengthwise, and a protein lattice called
the synaptonemal complex is formed between them to bind them together. The synaptonemal complex facilitates
crossover between nonsister chromatids, which is observed as chiasmata along the length of the chromosome. As
prophase I progresses, the synaptonemal complex breaks down and the sister chromatids become free, except where
they are attached by chiasmata. At this stage, the four chromatids are visible in each homologous pairing and are
called a tetrad. 21 In metaphase I, the homologous chromosomes line up at the metaphase plate. In anaphase I, the
homologous chromosomes are pulled apart and move to opposite poles. Sister chromatids are not separated until
meiosis II. The fused kinetochore formed during meiosis I ensures that each spindle microtubule that binds to the
tetrad will attach to both sister chromatids. 23 The chromosomes of the individual cannot cross over during meiosis if
the individual cannot make recombination nodules. This limits the genetic diversity of the individual’s gametes to what
occurs during independent assortment, with all daughter cells receiving complete maternal or paternal chromatids.
An individual who cannot produce diverse offspring is considered less fit than individuals who do produce diverse
offspring. 25 a. Crossover occurs in prophase I between nonsister homologous chromosomes. Segments of DNA
are exchanged between maternally derived and paternally derived chromosomes, and new gene combinations are
formed, b. Random alignment during metaphase I leads to gametes that have a mixture of maternal and paternal
chromosomes, c. Fertilization is random, in that any two gametes can fuse. 27 Sexual reproduction increases the
genetic variation within the population, because new individuals are made by randomly combining genetic material
from two parents. Because only fit individuals reach sexual maturity and reproduce, the overall population tends toward
increasing fitness in its environment. However, there is always a possibility that the random combination creating the
offspring’s genome will actually produce an organism less fit for the environment than its parents were. 29 Haploid-
dominant organisms undergo sexual reproduction by making a diploid zygote. The cells that make the gametes are
derived from haploid cells, but the + and - mating types that produce the zygote are randomly combined. The zygote
also undergoes meiosis to return to the haploid stage, so multiple steps add genetic diversity to haploid-dominant
organisms.
Chapter 12
1 Figure 12.5 You cannot be sure if the plant is homozygous or heterozygous as the data set is too small: by random
chance, all three plants might have acquired only the dominant gene even if the recessive one is present. If the round
pea parent is heterozygous, there is a one-eighth probability that a random sample of three progeny peas will all be
round. 3 Figure 12.12 Half of the female offspring would be heterozygous (X W X W ) with red eyes, and half would be
homozygous recessive (X^X 1 ^) with white eyes. Half of the male offspring would be hemizygous dominant (X W Y) withe
red yes, and half would be hemizygous recessive (X W Y) with white eyes. 5 A 7B 9C 11 C 13 D 15 D 17 A 19
D 21 D 22 The garden pea is sessile and has flowers that close tightly during self-pollination. These features help to
prevent accidental or unintentional fertilizations that could have diminished the accuracy of Mendel's data. 24 Since
we are calculating the probability of two independent events occurring simultaneously, we use the product rule. Fi
generation: Since green seed color is recessive, there is a 0% probability that any plants in the Fi generation will have
green, round seeds. F 2 generation: The probability of growing an F 2 generation plant with green seeds is %, while the
probability of growing an F 2 generation plant with round seeds is %. We can use the product rule to then calculate the
probability of a plant with green, round seeds: 1/ 4x3/ 4 = 3/ 16 26 Because axial is dominant, the gene(12.3)
would be designated as A. Fi would be all heterozygous Aa with axial phenotype. F 2 would have possible genotypes of
AA, Aa, and aa; these would correspond to axial, axial, and terminal phenotypes, respectively. 28 No, males can only
express color blindness. They cannot carry it because an individual needs two X chromosomes to be a carrier. 30
Considering each gene separately, the cross at A will produce offspring of which half are AA and half are Aa; B will
produce all Bb ; C will produce half Cc and half cc. Proportions then are (1/2) x (l) x (1/2), or 1/4 AABbCc; continuing for
the other possibilities yields 1/4 AABbcc, 1/4 AaBbCc, and 1/4 AaBbcc. The proportions therefore are 1:1:1:1. 32 The
cross can be represented as a 4 x 4 Punnett square, with the following gametes for each parent: WY, Wy, wY, and wy.
For all 12 of the offspring that express a dominant W gene, the offspring will be white. The three offspring that are
homozygous recessive for w but express a dominant Y gene will be yellow. The remaining wwyy offspring will be
green. 34 Mendelian inheritance would predict that all three genes are inherited independently. There are therefore 8
different gamete genotype possibilities: VYR, VYr, VyR, Vyr, vYR, vYr, vyR, vyr. If all three genes are found on the
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same chromosome arm, independent assortment is unlikely to occur because the genes are close together (linked).
Chapter 13
1 Figure 13.3 No. The predicted frequency of recombinant offspring ranges from 0% (for linked traits) to 50% (for
unlinked traits). 3 Figure 13.6 B. 4 A 6 C 8 B 10 C 12 D 14 The Chromosomal Theory of Inheritance proposed
that genes reside on chromosomes. The understanding that chromosomes are linear arrays of genes explained
linkage, and crossing over explained recombination.
Chapter 14
1 Figure 14.10 Compartmentalization enables a eukaryotic cell to divide processes into discrete steps so it can build
more complex protein and RNA products. But there is an advantage to having a single compartment as well: RNA
and protein synthesis occurs much more quickly in a prokaryotic cell. 3 Figure 14.21 If three nucleotides are added,
one additional amino acid will be incorporated into the protein chain, but the reading frame wont shift. 4 C 6 D 8
D 10 C 12 D 14 A 16 D 18 C 20 B 21 Live R cells acquired genetic information from the heat-killed S cells that
“transformed” the R cells into S cells. 23 If the tetranucleotide hypothesis were true, then DNA would have to contain
equal amounts of all four nucleotides (A=T=G=C). However, Chargaff demonstrated that A=T and G=C, but that the
four nucleotides are not present in equal amounts. 25 DNA has two strands in anti-parallel orientation. The sugar-
phosphate linkages form a backbone on the outside, and the bases are paired on the inside: A with T, and G with
C, like rungs on a spiral ladder. 27 Meselson’s experiments with E. coli grown in 15 N deduced this finding. 29 At
an origin of replication, two replication forks are formed that are extended in two directions. On the lagging strand,
Okazaki fragments are formed in a discontinuous manner. 31 1333 seconds or 22.2 minutes. 33 Primer provides a
3'-OH group for DNA pol to start adding nucleotides. There would be no reaction in the tube without a primer, and no
bands would be visible on the electrophoresis. 35 Telomerase has an inbuilt RNA template that extends the 3' end, so
primer is synthesized and extended. Thus, the ends are protected. 37 This is a frameshift mutation with a deletion of
an “A” in the 12 th position of the coding region. Patient: ATGGGGATATGGCATNormal: ATGGGGATATGAGCAT
Chapter 15
1 Figure 15.11 No. Prokaryotes use different promoters than eukaryotes. 3 Figure 15.16 Tetracycline: a;
Chloramphenicol: c. 4 D 6 C 8 B 10 B 12 B 14 A 16 C 18 A 19 For 200 commonly occurring amino acids,
codons consisting of four types of nucleotides would have to be at least four nucleotides long, because 4 4 = 256. There
would be much less degeneracy in this case. 21 Met Cys Arg Asn Ser Arg The first step to writing the amino acid
sequence is to find the start codon AUG. Then, the nucleotide sequence is separated into triplets: CU AUG UGU CGU
AAC AGC CGA UGA. We stop the translation at UGA because that triplet encodes a stop codon. When we convert
these codons to amino acids, the sequence becomes Met Cys Arg Asn Ser Arg. 23 Rho-dependent termination is
controlled by the rho protein, which tracks along behind the polymerase on the growing mRNA chain. Near the end
of the gene, the polymerase stalls at a run of G nucleotides on the DNA template. The rho protein collides with the
polymerase and releases mRNA from the transcription bubble. Rho-independent termination is controlled by specific
sequences in the DNA template strand. As the polymerase nears the end of the gene being transcribed, it encounters
a region rich in C-G nucleotides. This creates an mRNA hairpin that causes the polymerase to stall right as it begins
to transcribe a region rich in A-T nucleotides. Because A-U bonds are less thermostable, the core enzyme falls
away. 25 To determine that a RNA polymerase I mutation or deficiency is causing the defect in protein production,
the scientist would need to make observations that provide evidence that RNA polymerases II and III are working in
the cell. The observations eliminating RNA polymerase II as the defect could include: Transcription of mRNAs
in the nucleus;Presence of processed mRNAs in the cytoplasm The observations eliminating RNA polymerase III
could include: Isolation of small nuclear RNAs from the cell;lsolation of microRNAs from the cell;Transcription of
5S rRNA in the nucleus;Presence of tRNAs in the cytoplasm The observations implicating RNA polymerase I could
include: A lack of functional ribosomes in the cytoplasm (RNA polymerase I or lll);A lack of RNA polymerase
I protein;RNA polymerase I protein is non-functional 27 The mRNA would be: 5'-AUGGCCGGUUAUUAAGCA-3'.
The protein would be: MAGY. Even though there are six codons, the fifth codon corresponds to a stop, so the sixth
codon would not be translated. 29 Original mRNA: 5’ -UGCC AUG GUA AUA ACA CAU GAG GCC UGA AC- 3’;
Translation: Met - Val - lie - Thr - His - Glu - Ala; Mutated mRNA: 5’ -UGCC AUG GUU AAU AAC ACA UGA
GGCCUGAAC- 3’; Translation: Met - Val - Asn - Asn - Thr; Insertion mutations can have dramatic effects on proteins
because they shift the reading frame for the codons. This changes the amino acids encoded by the mRNA, and can
introduce premature start or stop sites.
Chapter 16
1 Figure 16.5 Tryptophan is an amino acid essential for making proteins, so the cell always needs to have some
on hand. However, if plenty of tryptophan is present, it is wasteful to make more, and the expression of the trp
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1533
receptor is repressed. Lactose, a sugar found in milk, is not always available. It makes no sense to make the
enzymes necessary to digest an energy source that is not available, so the lac operon is only turned on when
lactose is present. 3 Figure 16.13 Protein synthesis would be inhibited. 4 D 6 D 8 D 10 A 12 C 14 B 16
D 18 C 20 B 22 C 23 Eukaryotic cells have a nucleus, whereas prokaryotic cells do not. In eukaryotic cells,
DNA is confined within the nuclear region. Because of this, transcription and translation are physically separated.
This creates a more complex mechanism for the control of gene expression that benefits multicellular organisms
because it compartmentalizes gene regulation. Gene expression occurs at many stages in eukaryotic cells, whereas
in prokaryotic cells, control of gene expression only occurs at the transcriptional level. This allows for greater control
of gene expression in eukaryotes and more complex systems to be developed. Because of this, different cell types
can arise in an individual organism. 25 Environmental stimuli can increase or induce transcription in prokaryotic
cells. In this example, lactose in the environment will induce the transcription of the lac operon, but only if glucose
is not available in the environment. 27 You can create medications that reverse the epigenetic processes (to add
histone acetylation marks or to remove DNA methylation) and create an open chromosomal configuration. 29 Histone
acetylation reduces the positive charge of histone proteins, loosening the DNA wrapped around the histones. This
looser DNA can then interact with transcription factors to express genes found in that region. Normally, once the
gene is no longer needed, histone deacetylase enzymes remove the acetyl groups from histones so that the DNA
becomes tightly wound and inaccessible again. However, when there is a defect in HDAC9, the deacetylation may
not occur. In an immune cell, this would mean that inflammatory genes that were made accessible during an infection
are not tightly rewound around the histones. 31 If too much of an activating transcription factor were present, then
transcription would be increased in the cell. This could lead to dramatic alterations in cell function. 33 RNA binding
proteins (RBP) bind to the RNA and can either increase or decrease the stability of the RNA. If they increase the
stability of the RNA molecule, the RNA will remain intact in the cell for a longer period of time than normal. Since both
RBPs and miRNAs bind to the RNA molecule, RBP can potentially bind first to the RNA and prevent the binding of
the miRNA that will degrade it. 35 Because proteins are involved in every stage of gene regulation, phosphorylation
of a protein (depending on the protein that is modified) can alter accessibility to the chromosome, can alter translation
(by altering the transcription factor binding or function), can change nuclear shuttling (by influencing modifications to
the nuclear pore complex), can alter RNA stability (by binding or not binding to the RNA to regulate its stability), can
modify translation (increase or decrease), or can change post-translational modifications (add or remove phosphates
or other chemical modifications). 37 Environmental stimuli, like ultraviolet light exposure, can alter the modifications to
the histone proteins or DNA. Such stimuli may change an actively transcribed gene into a silenced gene by removing
acetyl groups from histone proteins or by adding methyl groups to DNA. 39 These drugs will keep the histone proteins
and the DNA methylation patterns in the open chromosomal configuration so that transcription is feasible. If a gene is
silenced, these drugs could reverse the epigenetic configuration to re-express the gene.
Chapter 17
1 Figure 17.7 B. The experiment would result in blue colonies only. 3 Figure 17.15 There are no right or wrong
answers to these questions. While it is true that prostate cancer treatment itself can be harmful, many men would rather
be aware that they have cancer so they can monitor the disease and begin treatment if it progresses. And while genetic
screening may be useful, it is expensive and may cause needless worry. People with certain risk factors may never
develop the disease, and preventative treatments may do more harm than good. 4 B 6 B 8 D 10 B 12 A 14 A 16
D 18 D 20 B 22 Southern blotting is the transfer of DNA that has been enzymatically cut into fragments and run on
an agarose gel onto a nylon membrane. The DNA fragments that are on the nylon membrane can be denatured to
make them single-stranded, and then probed with small DNA fragments that are radioactively or fluorescently labeled,
to detect the presence of specific sequences. An example of the use of Southern blotting would be in analyzing
the presence, absence, or variation of a disease gene in genomic DNA from a group of patients. 24 By identifying
an herbicide resistance gene and cloning it into a plant expression vector system, like the Ti plasmid system from
Agrobacterium tumefaclens. The scientist would then introduce it into the plant cells by transformation, and select
cells that have taken up and integrated the herbicide-resistance gene into the genome. 26 Genome mapping has
many different applications and provides comprehensive information that can be used for predictive purposes. 28
Metagenomics is revolutionary because it replaced the practice of using pure cultures. Pure cultures were used
to study individual species in the laboratory, but did not accurately represent what happens in the environment.
Metagenomics studies the genomes of bacterial populations in their environmental niche. 30 Proteomics has provided
a way to detect biomarkers and protein signatures, which have been used to screen for the early detection of cancer.
Chapter 18
1 Figure 18.14 Loss of genetic material is almost always lethal, so offspring with 2n+l chromosomes are more
likely to survive. 3 Figure 18.23 Answer B 4 B 6 D 8 A 10 B 12 C 14 D 16 C 17 The plants that can best use
the resources of the area, including competing with other individuals for those resources will produce more seeds
themselves and those traits that allowed them to better use the resources will increase in the population of the next
generation. 19 In science, a theory is a thoroughly tested and verified set of explanations for a body of observations of
nature. It is the strongest form of knowledge in science. In contrast, a theory in common vernacular can mean a guess
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Answer Key
or speculation about something, meaning that the knowledge implied by the theory is very weak. 21 Organisms of
one species can arrive to an island together and then disperse throughout the chain, each settling into different niches
and exploiting different food resources to reduce competition. 23 The formation of gametes with new n numbers can
occur in one generation. After a couple of generations, enough of these new hybrids can form to reproduce together
as a new species. 25 If the hybrid offspring are as fit or more fit than the parents, reproduction would likely continue
between both species and the hybrids, eventually bringing all organisms under the umbrella of one species.
Chapter 19
1 Figure 19.2 The expected distribution is 320 VV, 160Vv, and 20 vv plants. Plants with VV or Vv genotypes would
have violet flowers, and plants with the vv genotype would have white flowers, so a total of 480 plants would be
expected to have violet flowers, and 20 plants would have white flowers. 3 Figure 19.8 Moths have shifted to a lighter
color. 4 C 6 D 8C 10 A 12 D 14 A 16 p = (8*2 + 4)/48 = .42; q = (12*2 + 4)/48 = .58; p 2 = .17; 2pq = .48; q 2
= .34 18 Red is recessive so q2 = 200/800 = 0.25; q = 0.5; p = 1 - q = 0.5; p2 = 0.25; 2pq = 0.5. You would expect
200 homozygous blue flowers, 400 heterozygous blue flowers, and 200 red flowers. 20 The theory of natural selection
stems from the observation that some individuals in a population survive longer and have more offspring than others:
thus, more of their genes are passed to the next generation. For example, a big, powerful male gorilla is much more
likely than a smaller, weaker one to become the population’s silverback: the pack’s leader who mates far more than
the other males of the group. Therefore, the pack leader will father more offspring who share half of his genes and are
likely to grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in
the population, and the average body size, as a result, will grow larger on average. 22 The peacock’s tail is a good
example of the handicap principle. The tail, which makes the males more visible to predators and less able to escape,
is clearly a disadvantage to the bird’s survival. But because it is a disadvantage, only the most fit males should be able
to survive with it. Thus, the tail serves as an honest signal of quality to the females of the population; therefore, the
male will earn more matings and greater reproductive success.
Chapter 20
1 Figure 20.6 Cats and dogs are part of the same group at five levels: both are in the domain Eukarya, the kingdom
Animalia, the phylum Chordata, the class Mammalia, and the order Carnivora. 3 Figure 20.11 The largest clade
encompasses the entire tree. 4 C 6 D 8 C 10 B 12 C 14 A 16 The phylogenetic tree shows the order in which
evolutionary events took place and in what order certain characteristics and organisms evolved in relation to others.
It does not relate to time. 18 domain, kingdom, phylum, class, order, family, genus, species 20 Phylogenetic trees
are based on evolutionary connections. If an analogous similarity were used on a tree, this would be erroneous and,
furthermore, would cause the subsequent branches to be inaccurate. 22 Some hypotheses propose that mitochondria
were acquired first, followed by the development of the nucleus. Others propose that the nucleus evolved first and
that this new eukaryotic cell later acquired the mitochondria. Still others hypothesize that prokaryotes descended from
eukaryotes by the loss of genes and complexity.
Chapter 21
1 Figure 21.5 D 3 Figure 21.10 C 4 B 6 D 8 A 10 B 12 D 14 D 16 A 18 D 20 C 22 Viruses pass through
filters that eliminated all bacteria which were visible in the light microscopes at the time. As the bacteria-free filtrate
could still cause infections when given to a healthy organism, this observation demonstrated the existence of very
small infectious agents. These agents were later shown to be unrelated to bacteria and were classified as viruses. 24
Rabies virus is a (-) strand RNA virus that transcribes mRNAs from its genome (Group V). HIV-1 is a single-stranded
RNA retrovirus that uses reverse transcriptase to create a double-stranded DNA copy of its genome which is integrated
into the host human’s genome prior to making mRNAs (Group VI). The genome structure system classifies both viruses
as single-stranded RNA viruses with linear genomes. Baltimore classification sorts Rabies virus and HIV-1 into two
different groups, indicating that the two viruses have very different life cycles. However, genome structure classification
does not distinguish between the two viruses. This leaves out important information regarding virus function and
survival. 26 Reverse transcriptase is needed to make more HIV-1 viruses, so targeting the reverse transcriptase
enzyme may be a way to inhibit the replication of the virus. Importantly, by targeting reverse transcriptase, we do little
harm to the host cell, since host cells do not make reverse transcriptase. Thus, we can specifically attack the virus and
not the host cell when we use reverse transcriptase inhibitors. 28 Plant viruses infect crops, causing crop damage
and failure, and considerable economic losses. 30 Rabies vaccine works after a bite because it takes a week for the
virus to travel from the site of the bite to the central nervous system, where the most severe symptoms of the disease
occur. Adults are not routinely vaccinated for rabies for two reasons: first, because the routine vaccination of domestic
animals makes it unlikely that humans will contract rabies from an animal bite; second, if one is bitten by a wild animal
or a domestic animal that one cannot confirm has been immunized, there is still time to give the vaccine and avoid the
often fatal consequences of the disease. 32 This prion-based disease is transmitted through human consumption of
infected meat. 34 The botanist would need to isolate any foreign nucleic acids from infected plant cells, and confirm
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Answer Key
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that an RNA molecule is the etiological agent of disease. The botanist would then need to demonstrate that the RNA
can infect plant cells without a capsid, and that the RNA replicates, but is not translated to produce proteins.
Chapter 22
1 Figure 22.8 The extracellular matrix and outer layer of cells protects the inner bacteria. The close proximity of cells
also facilitates lateral gene transfer, a process by which genes such as antibiotic-resistance genes are transferred
from one bacterium to another. And even if lateral gene transfer does not occur, one bacterium that produces an
exo-enzyme that destroys antibiotic may save neighboring bacteria. 3 Figure 22.19 D4A6A8C 10 D 12
B 14 C 16 B 18 A 20 A 22 D 24 D 26 D 28 B 30 As the organisms are non-culturable, the presence could
be detected through molecular techniques, such as PCR. 32 Possible answers include: PsychrophileHypolith -
survival in low humidity/water environment 34 Both bacteria and archaea have cell membranes and they both contain
a hydrophobic portion. In the case of bacteria, it is a fatty acid; in the case of archaea, it is a hydrocarbon (phytanyl).
Both bacteria and archaea have a cell wall that protects them. In the case of bacteria, it is composed of peptidoglycan,
whereas in the case of archaea, it is pseudopeptidoglycan, polysaccharides, glycoproteins, or pure protein. Bacterial
and archaeal flagella also differ in their chemical structure. 36 Responses will vary. In a deep-sea hydrothermal vent,
there is no light, so prokaryotes would be chemotrophs instead of phototrophs. The source of carbon would be carbon
dioxide dissolved in the ocean, so they would be autotrophs. There is not a lot of organic material in the ocean,
so prokaryotes would probably use inorganic sources, thus they would be chemolitotrophs. The temperatures are
very high in the hydrothermal vent, so the prokaryotes would be thermophilic. 38 Losing the bacteria that serve as
decomposers in the ecosystem would disrupt the carbon cycle, but not stop it completely since fungi can also serve
as decomposers. Without bacterial decomposers functioning, organic waste would accumulate in the area, and less
carbon dioxide would be released back into the atmosphere. 40 £. coli colonizes the surface of the leaf, forming a
biofilm that is more difficult to remove than free (planktonic) cells. Additionally, bacteria can be taken up in the water
that plants are grown in, thereby entering the plant tissues rather than simply residing on the leaf surface. 42 Soap
indiscriminately kills bacteria on skin. This kills harmful bacteria, but can also eliminate “good” bacteria from the skin.
When the non-pathogenic bacteria are eliminated, pathogenic bacteria can colonize the empty surface.
Chapter 23
1 Figure 23.5 All eukaryotic cells have mitochondria, but not all eukaryotic cells have chloroplasts. 3 Figure 23.28
C 4 D 6 C 8 C 10 D 12 B 14 A 16 C 18 D 20 A 22 B 23 Eukaryotic cells arose through endosymbiotic events
that gave rise to the energy-producing organelles within the eukaryotic cells such as mitochondria and chloroplasts.
The nuclear genome of eukaryotes is related most closely to the Archaea, so it may have been an early archaean
that engulfed a bacterial cell that evolved into a mitochondrion. Mitochondria appear to have originated from an
alpha-proteobacterium, whereas chloroplasts originated as a cyanobacterium. There is also evidence of secondary
endosymbiotic events. Other cell components may also have resulted from endosymbiotic events. 25 The ability to
perform sexual reproduction allows protists to recombine their genes and produce new variations of progeny that may
be better suited to the new environment. In contrast, asexual reproduction generates progeny that are clones of the
parent. 27 Protists are defined as any eukaryotes that do not fall into the Plantae, Fungi, or Animal Kingdoms. Since
the unifying characteristics describe what they are NOT, rather than what they are, Protista can include almost any
cellular/organism organization. Possible examples of structure variety: Barrier to exterior world: cell wall, plasma
membrane, pellicleLocomotion: flagella, cilia, pseudopodia 29 By definition, an obligate saprobe lacks the ability to
perform photosynthesis, so it cannot directly obtain nutrition by searching for light. Instead, a chemotactic mechanism
that senses the odors released during decay might be a more effective sensing organ for a saprobe. 31 Possible
answers include: Two nuclei (a macronucleus and a micronucleus) instead of one nucleusAmitotic division/binary
fission during asexual reproduction instead of mitotic cell divisionMitosis of the micronucleus after meiosis instead
of direct meiotic production of gametes for sexual reproduction 33 The trypanosomes that cause this disease are
capable of expressing a glycoprotein coat with a different molecular structure with each generation. Because the
immune system must respond to specific antigens to raise a meaningful defense, the changing nature of trypanosome
antigens prevents the immune system from ever clearing this infection. Massive trypanosome infection eventually
leads to host organ failure and death.
Chapter 24
1 Figure 24.14 A 3 Figure 24.21 Without mycorrhiza, plants cannot absorb adequate nutrients, which stunts their
growth. Addition of fungal spores to sterile soil can alleviate this problem. 4 C 6 D 8 C 10 B 12 B 14 C 16 C
18 C 20 B 22 A 23 Asexual reproduction is fast and best under favorable conditions. Sexual reproduction allows
the recombination of genetic traits and increases the odds of developing new adaptations better suited to a changed
environment. 25 Fungi break down decaying matter in their environment to serve as their food source. Since the
digestion occurs externally, the large mycelium can secrete exoenzymes over a large area. The fungi must be able to
absorb the small molecules released by digestion, so having a large surface area increases the amount of digested
molecules that are captured by the fungi. 27 Chytridiomycota (Chytrids) may have a unicellular or multicellular body
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Answer Key
structure; some are aquatic with motile spores with flagella; an example is the Allomyces. Zygomycota (conjugated
fungi) have a multicellular body structure; features include zygospores and presence in soil; examples are bread and
fruit molds. Ascomycota (sac fungi) may have unicellular or multicellular body structure; a feature is sexual spores
in sacs (asci); examples include the yeasts used in bread, wine, and beer production. Basidiomycota (club fungi)
have multicellular bodies; features includes sexual spores in the basidiocarp (mushroom) and that they are mostly
decomposers; mushroom-producing fungi are an example. 29 The bark beetles and the fungus have a mutualistic
relationship since each partner benefits from interacting with the other. The beetle can provide food for its offspring,
while the fungus can spread to new trees. 31 Dermatophytes that colonize skin break down the keratinized layer of
dead cells that protects tissues from bacterial invasion. Once the integrity of the skin is breached, bacteria can enter the
deeper layers of tissues and cause infections. 33 The dough is often contaminated by toxic spores that float in the air.
It was one of Louis Pasteur's achievements to purify reliable strains of baker's yeast to produce bread consistently.
Chapter 25
1 Figure 25.6 B. 3 Figure 25.24 D. 4 A 6 C 8 C 10 C 12 D 14 A 16 B 18 D 20 D 22 D 24 Sunlight is
not filtered by water or other algae on land; therefore, there is no need to collect light at additional wavelengths
made available by other pigment coloration. 26 Possible challenges include: Climate: Deserts are more arid than
swamps, so there is less humidity in the air and less water in the soil.Reproduction: Cactuses are often not densely
populated, whereas cattails occur in groups.Temperature: During the day, deserts are usually hot, which increases
the risk of desiccation. The desert climate will also have broader temperature ranges (extremes). 28 It allows for
survival through periodic droughts and colonization of environments where the supply of water fluctuates. 30 The
bryophytes are divided into three phyla: the liverworts or Hepaticophyta, the hornworts or Anthocerotophyta, and the
mosses or true Bryophyta. 32 Similarities include: Sexual reproduction is dependent upon water in which the
male gamete swims.The haploid organism is the dominant part of the life cycle. Differences include: Bryophyte
gametotangia protect the gametes and the growing embryo.Bryophytes make sporangium to produce spores. 34
Ferns are considered the most advanced seedless vascular plants, because they display characteristics commonly
observed in seed plants—they form large leaves and branching roots.
Chapter 26
1 Figure 26.8 B. The diploid zygote forms after the pollen tube has finished forming, so that the male generative nuclei
can fuse with the female gametophyte. 3D 5 C 7 A 9 B 11 C 13 B 15 C 17 D 19 Both pollination and herbivory
contributed to diversity, with plants needing to attract some insects and repel others. 21 The trees are adapted to
arid weather, and do not lose as much water due to transpiration as non-conifers. 23 The resemblance between
cycads and palm trees is only superficial. Cycads are gymnosperms and do not bear flowers or fruit. Cycads produce
cones: large, female cones that produce naked seeds, and smaller male cones on separate plants. Palms do not. 25
Using animal pollinators promotes cross-pollination and increases genetic diversity. The odds that the pollen will reach
another flower are greatly increased compared with the randomness of wind pollination.
Chapter 27
1 Figure 27.5 The animal might develop two heads and no tail. 3 Figure 27.9 D 4 B 6 D 8 B 10 C 12 D 14 B 16
C 18 D 19 The development of specialized tissues affords more complex animal anatomy and physiology because
differentiated tissue types can perform unique functions and work together in tandem to allow the animal to perform
more functions. For example, specialized muscle tissue allows directed and efficient movement, and specialized
nervous tissue allows for multiple sensory modalities as well as the ability to respond to various sensory information;
these functions are not necessarily available to other nonanimal organisms. 21 Altered expression of homeotic genes
can lead to major changes in the morphology of the individual. Hox genes can affect the spatial arrangements of organs
and body parts. If a Hox gene was mutated or duplicated, it could affect where a leg might be on a fruit fly or how far
apart a person’s fingers are. 23 The evolution of bilateral symmetry led to designated head and tail body regions, and
promoted more efficient mobility for animals. This improved mobility allowed for more skillful seeking of resources and
prey escaping from predators. The appearance of the coelom in coelomates provides many internal organs with shock
absorption, making them less prone to physical damage from bodily assault. A coelom also gives the body greater
flexibility, which promotes more efficient movement. The relatively loose placement of organs within the coelom allows
them to develop and grow with some spatial freedom, which promoted the evolution of optimal organ arrangement.
The coelom also provides space for a circulatory system, which is an advantageous way to distribute body fluids and
gases. 25 In many cases, morphological similarities between animals may be only superficial similarities and may
not indicate a true evolutionary relationship. One of the reasons for this is that certain morphological traits can evolve
along very different evolutionary branches of animals for similar ecological reasons. 27 It is true that multiple mass
extinction events have taken place since the Cambrian period, when most currently existing animal phyla appeared,
and the majority of animal species were commonly wiped out during these events. However, a small number of animal
species representing each phylum were usually able to survive each extinction event, allowing the phylum to continue
to evolve rather than become altogether extinct.
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Answer Key
1537
Chapter 28
1 Figure 28.3 B 3 Figure 28.45 C 4 B 6 B 8 C 10 A 12 B 14 D 16 C 18 B 20 D 22 A 24 Pinacocytes
are epithelial-like cells, form the outermost layer of sponges, and enclose a jelly-like substance called mesohyl. In
some sponges, porocytes form ostia, single tube-shaped cells that act as valves to regulate the flow of water into
the spongocoel. Choanocytes (“collar cells”) are present at various locations, depending on the type of sponge, but
they always line some space through which water flows and are used in feeding. 26 Nematocysts are “stinging cells”
designed to paralyze prey. The nematocysts contain a neurotoxin that renders prey immobile. 28 There are two key
differences between Porifera (sponges) and Cubozoans (box jellyfish) - gamete production and fertilization strategy.
Box jellyfish have separate sexes, while a single sponge can produce both types of gametes. Box jellyfish also undergo
internal fertilization, while sponges reproduce by external fertilization. Internal fertilization allows box jellyfish to control
which sperm is used for fertilization and increases the likelihood of ova and spermatozoa meeting. 30 Mollusks have
a large muscular foot that may be modified in various ways, such as into tentacles, but it functions in locomotion.
They have a mantle, a structure of tissue that covers and encloses the dorsal portion of the animal, and secrete the
shell when it is present. The mantle encloses the mantle cavity, which houses the gills (when present), excretory
pores, anus, and gonadopores. The coelom of mollusks is restricted to the region around the systemic heart. The
main body cavity is a hemocoel. Many mollusks have a radula near the mouth that is used for scraping food. 32
Cephalopods have a closed circulatory system, while other members of the Mollusca phylum have open circulatory
systems. Having a closed system allows blood to be moved more efficiently and rapidly through the animal, since the
circulation is not limited by diffusion. For example, this allows the octopus to have a much more complex body plan,
with branching tentacles, compared to a snail. In many cases, a closed circulatory system also allows the development
of larger organisms. 34 There are nematodes with separate sexes and hermaphrodites in addition to species that
reproduce parthenogentically. The nematode Caenorhabditis elegans has a self-fertilizing hermaphrodite sex and a
pure male sex. 36 The Arthropoda include the Hexapoda, which are mandibulates with six legs; the Myriapoda,
which are mandibulates with many legs and include the centipedes and millipedes; the Crustacea, which are mostly
marine mandibulates; and the Chelicerata, which include the spiders and scorpions and their kin. 38 Insects are
the predominant members of the subphylum Hexapoda. Advantages: Pollination;Eliminate pests;Cheap food
source;Produce food products (ex. honey) Disadvantages: Damage to food crops;Transmit disease to agricultural
workers;Contaminate/spoil food;Destroy buildings storing food crops
Chapter 29
1 Figure 29.3 A 3 Figure 29.24 The ancestor of modern Testudines may at one time have had a second opening in
the skull, but over time this might have been lost. 4 B 6 A 8 B 10 D 12 C 14 D 16 D 18 D 20 A 22 B 23 The
characteristic features of the phylum Chordata are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a
post-anal tail. 25 Comparison of hagfishes with lampreys shows that the cranium evolved first in early vertebrates, as
it is seen in hagfishes, which evolved earlier than lampreys. This was followed by evolution of the vertebral column, a
primitive form of which is seen in lampreys and not in hagfishes. 27 A moist environment is required, as frog eggs lack
a shell and dehydrate quickly in dry environments. 29 Frogs (Anura) begin their lives as tadpoles, organisms restricted
to an aquatic environment that use gills to breathe. After metamorphosis, most frogs develop lungs and lose their gills,
although they will also continue to perform gas exchange through their skin. The lungs of an adult frog allow the animal
to move out of the water, and become terrestrial. This limits competition between adults and tadpoles by opening
new living space and food sources to the adult. 31 Lizards differ from snakes by having eyelids, external ears, and
less kinematic skulls. 33 This is suggested by similarities observed between theropod fossils and birds, specifically
in the design of the hip and wrist bones, as well as the presence of a furcula, or wishbone, formed by the fusing of
the clavicles. 35 Ostriches and penguins are flightless birds, but ostriches are entirely terrestrial, while penguins dive
and swim in the ocean to find food. Therefore, penguins and flight birds like terns have similar chest structures with a
keel sternum and relatively large pectoral muscles (penguins use their wings to “fly” through water). Conversely, since
ostriches move by running, they do not have a keel to their sternum. They also have smaller pectoral muscles than
would be predicted for a flying bird their size, but have larger thigh muscles. 37 In some mammals, the cerebral cortex
is highly folded, allowing for greater surface area than a smooth cortex. The optic lobes are divided into two parts in
mammals. Eutherian mammals also possess a specialized structure that links the two cerebral hemispheres, called
the corpus callosum. 39 Archaic Homo sapiens differed from modern humans by having a thick skull and a prominent
brow ridge, and lacking a prominent chin.
Chapter 30
1 Figure 30.7 A and B. The cortex, pith, and epidermis are made of parenchyma cells. 3 Figure 30.34 B. 4C 6C 8
A 10 B 12 A 14 B 16 C 18 B 20 D 22 C 24 C 26 C 27 Lawn grasses and other monocots have an intercalary
meristem, which is a region of meristematic tissue at the base of the leaf blade. This is beneficial to the plant because
it can continue to grow even when the tip of the plant is removed by grazing or mowing. 29 Stomata allow gases
to enter and exit the plant. Guard cells regulate the opening and closing of stomata. If these cells did not function
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Answer Key
correctly, a plant could not get the carbon dioxide needed for photosynthesis, nor could it release the oxygen produced
by photosynthesis. 31 In woody plants, the cork cambium is the outermost lateral meristem; it produces new cells
towards the interior, which enables the plant to increase in girth. The cork cambium also produces cork cells towards
the exterior, which protect the plant from physical damage while reducing water loss. 33 Annual rings can also indicate
the climate conditions that prevailed during each growing season. 35 A tap root system has a single main root that
grows down. A fibrous root system forms a dense network of roots that is closer to the soil surface. An example of a
tap root system is a carrot. Grasses such as wheat, rice, and corn are examples of fibrous root systems. Fibrous root
systems are found in monocots; tap root systems are found in dicots. 37 Monocots have leaves with parallel venation,
and dicots have leaves with reticulate, net-like venation. 39 The process of bulk flow moves water up the xylem and
moves photosynthates (solutes) up and down the phloem. 41 Gravitropism will allow roots to dig deep into the soil to
find water and minerals, whereas the seedling will grow towards light to enable photosynthesis. 43 To prevent further
entry of pathogens, stomata close, even if they restrict entry of CO 2 . Some pathogens secrete virulence factors that
inhibit the closing of stomata. Abscisic acid is the stress hormone responsible for inducing closing of stomata.
Chapter 31
1 Figure 31.6 The air content of the soil decreases. 3 Figure 31.10 Soybeans are able to fix nitrogen in their roots,
which are not harvested at the end of the growing season. The belowground nitrogen can be used in the next season
by the corn. 4 C 6 A 8 D 10 B 12 B 14 A 16 Deficiencies in these nutrients could result in stunted growth, slow
growth, and chlorosis. 18 Answers may vary. Essential macronutrients include carbon, hydrogen, oxygen, nitrogen,
phosphorus, potassium, calcium, magnesium, and sulfur. Essential micronutrients include iron, manganese, boron,
molybdenum, copper, zinc, chlorine, nickel, cobalt, sodium, and silicon. 20 Parent material, climate, topography,
biological factors, and time affect soil formation. Parent material is the material in which soils form. Climate describes
how temperature, moisture, and wind cause different patterns of weathering, influencing the characteristics of the soil.
Topography affects the characteristics and fertility of a soil. Biological factors include the presence of living organisms
that greatly affect soil formation. Processes such as freezing and thawing may produce cracks in rocks; plant roots
can penetrate these crevices and produce more fragmentation. Time affects soil because soil develops over long
periods. 22 Because it is natural and does not require use of a nonrenewable resource, such as natural gas. 24 A
nodule results from the symbiosis between a plant and bacterium. Within nodules, the process of nitrogen fixation
allows the plant to obtain nitrogen from the air.
Chapter 32
1 Figure 32.3 Pollen (or sperm); carpellate; staminate. 3 Figure 32.20 B 4 B 6 A 8 B 10 D 12 A 14 D 16 Inside
the flower are the reproductive organs of the plant. The stamen is the male reproductive organ. Pollen is produced in
the stamen. The carpel is the female reproductive organ. The ovary is the swollen base of the carpel where ovules
are found. Not all flowers have every one of the four parts. 18 A typical flower has four main parts, or whorls: the
calyx, corolla, androecium, and gynoecium. The outermost whorl of the flower has green, leafy structures known as
sepals, which are collectively called the calyx. It helps to protect the unopened bud. The second whorl is made up
of brightly colored petals that are known collectively as the corolla. The third whorl is the male reproductive structure
known as the androecium. The androecium has stamens, which have anthers on a stalk or filament. Pollen grains are
borne on the anthers. The gynoecium is the female reproductive structure. The carpel is the individual structure of the
gynoecium and has a stigma, the stalk or style, and the ovary. 20 Many seeds enter a period of inactivity or extremely
low metabolic activity, a process known as dormancy. Dormancy allows seeds to tide over unfavorable conditions and
germinate on return to favorable conditions. Favorable conditions could be as diverse as moisture, light, cold, fire, or
chemical treatments. After heavy rains, many new seedlings emerge. Forest fires also lead to the emergence of new
seedlings. 22 Asexual reproduction does not require the expenditure of the plant’s resources and energy that would
be involved in producing a flower, attracting pollinators, or dispersing seeds. Asexual reproduction results in plants that
are genetically identical to the parent plant, since there is no mixing of male and female gametes, resulting in better
survival. The cuttings or buds taken from an adult plant produce progeny that mature faster and are sturdier than a
seedling grown from a seed. 24 Plant species that complete their life cycle in one season are known as annuals.
Biennials complete their life cycle in two seasons. In the first season, the plant has a vegetative phase, whereas in
the next season, it completes its reproductive phase. Perennials, such as the magnolia, complete their life cycle in two
years or more.
Chapter 33
1 Figure 33.11 A 3 Figure 33.23 Pyrogens increase body temperature by causing the blood vessels to constrict,
inducing shivering, and stopping sweat glands from secreting fluid. 4A 6C 8D 10 D 12 C 14 D 16 D 18 A 20
B 22 C 24 A 26 B 28 A 29 Diffusion is effective over a very short distance. If a cell exceeds this distance in its size,
the center of the cell cannot get adequate nutrients nor can it expel enough waste to survive. To compensate for this,
cells can loosely adhere to each other in a liquid medium, or develop into multi-celled organisms that use circulatory
and respiratory systems to deliver nutrients and remove wastes. 31 In an open circulatory system, the heart(s) pump
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blood into an open cavity, bathing the tissues. As the blood diffuses through the tissue space, it delivers nutrients in
exchange for receiving metabolic wastes. The blood then diffuses back to the heart to be pumped again. However,
since this system relies on diffusion, the size of animals that use an open circulatory system is limited to fairly small
volumes so that the blood can diffuse rapidly enough to efficiently exchange molecules with the tissues. 33 Squamous
epithelia can be either simple or stratified. As a single layer of cells, it presents a very thin epithelia that minimally
inhibits diffusion. As a stratified epithelia, the surface cells can be sloughed off and the cells in deeper layers protect
the underlying tissues from damage. 35 In multiple sclerosis, the immune system attacks the oligodendrocytes. The
death of oligodendrocytes results in the loss of the insulating sheath around the axon of the neurons. When the sheath
is gone, the electrical impulses travel much more slowly down the length of the axon. 37 An adjustment to a change
in the internal or external environment requires a change in the direction of the stimulus. A negative feedback loop
accomplishes this, while a positive feedback loop would continue the stimulus and result in harm to the animal. 39
Diabetes is often associated with a lack in production of insulin. Without insulin, blood glucose levels go up after a
meal, but never go back down to normal levels.
Chapter 34
1 Figure 34.11 B 3 Figure 34.19 C 4 D 6 C 8 B 10 D 12 C 14 A 16 B 18 B 20 Animals with a polygastric
digestive system have a multi-chambered stomach. The four compartments of the stomach are called the rumen,
reticulum, omasum, and abomasum. These chambers contain many microbes that breakdown the cellulose and
ferment the ingested food. The abomasum is the “true” stomach and is the equivalent of a monogastric stomach
chamber where gastric juices are secreted. The four-compartment gastric chamber provides larger space and the
microbial support necessary for ruminants to digest plant material. 22 Accessory organs play an important role in
producing and delivering digestive juices to the intestine during digestion and absorption. Specifically, the salivary
glands, liver, pancreas, and gallbladder play important roles. Malfunction of any of these organs can lead to disease
states. 24 The stomach and the teeth both perform mechanical digestion, which is physically (as opposed to
chemically) breaking the food into smaller components. This exposes a larger surface area for chemical digestion
and release of nutrients. The teeth are vital to mastication, which breaks large bites of food down into smaller pieces
that are easily swallowed. The stomach's muscle contractions churn the food to expose all particles to the acid and
digestive enzymes. 26 Minerals—such as potassium, sodium, and calcium—are required for the functioning of many
cellular processes, including muscle contraction and nerve conduction. While minerals are required in trace amounts,
not having minerals in the diet can be potentially harmful. 28 Malnutrition, often in the form of not getting enough
calories or not enough of the essential nutrients, can have severe consequences. Many malnourished children have
vision and dental problems, and over the years may develop many serious health problems. 30 Fats are an essential
component of a healthy diet, and needed by the body to function. Fats are essential for many processes, including
the absorption of fat-soluble vitamins and production of some hormones. Fats also send a satiation signal to the brain
that regulates hunger. Without fats in their diets many people may have actually consumed more calories, which
would have resulted in weight gain. 32 The gut microbiome includes all the bacteria that aid in chemical digestion
in the intestines. Changing its composition can change the way that food is digested since not all bacteria have
the same macromolecule-digesting enzymes. Additionally, changes in gut microbiome can lead to the establishment
of pathogenic bacteria populations that cause inflammation in the gut or other disease. 34 Hormones control the
different digestive enzymes that are secreted in the stomach and the intestine during the process of digestion and
absorption. For example, the hormone gastrin stimulates stomach acid secretion in response to food intake. The
hormone somatostatin stops the release of stomach acid. 36 Somatostatin is the hormone that inhibits the release
of HCI into the stomach lumen after the chyme has moved to the intestine. If the receptor for somatostatin is
nonfunctional, somatostatin cannot signal to the stomach parietal cells to stop acid secretion. Thus, acid secretion
will continue when there is no food present, and can cause damage to the stomach tissue. However, as long as the
stomach remains intact the mutation should not slow digestion since acid will always be present in the stomach to
digest any new boluses of food.
Chapter 35
1 Figure 35.3 B 3 Figure 35.26 D 4 C 6 B 8 B 10 C 12 D 14 B 16 C 18 A 20 C 22 D 24 Neurons contain
organelles common to all cells, such as a nucleus and mitochondria. They are unique because they contain dendrites,
which can receive signals from other neurons, and axons that can send these signals to other cells. 26 A single
axon means that a neuron can only send one signal at a time (one electrical impulse down the length of the axon).
However, since the axon has multiple terminals it can send the signal to several other cells at once. This ensures
that the signal is rapidly propagated to the rest of the body. 28 An action potential travels along an axon until it
depolarizes the membrane at an axon terminal. Depolarization of the membrane causes voltage-gated Ca 2+ channels
to open and Ca 2+ to enter the cell. The intracellular calcium influx causes synaptic vesicles containing neurotransmitter
to fuse with the presynaptic membrane. The neurotransmitter diffuses across the synaptic cleft and binds to receptors
on the postsynaptic membrane. Depending on the specific neurotransmitter and postsynaptic receptor, this action
can cause positive (excitatory postsynaptic potential) or negative (inhibitory postsynaptic potential) ions to enter the
1540
Answer Key
cell. 30 To determine the function of a specific brain area, scientists can look at patients who have damage in that brain
area and see what symptoms they exhibit. Researchers can disable the brain structure temporarily using transcranial
magnetic stimulation. They can disable or remove the area in an animal model. fMRI can be used to correlate specific
functions with increased blood flow to brain regions. 32 Potential answers: Frontal lobe. Alzheimer’s patients
experience changes in personality, judgment, and behavior.;Parietal lobe. Alzheimer’s patients experience difficulties
with recalling and using language as disease progresses.;Temporal lobe. The hippocampus is one of the main areas of
the brain affected in Alzheimer’s disease. Patients lose the ability to make new memories and access memories. 34
The sensory-somatic nervous system transmits sensory information from the skin, muscles, and sensory organs
to the CNS. It also sends motor commands from the CNS to the muscles, causing them to contract. 36 Many
events in modern human life are not physical dangers; instead they are events we think of as “stress.” Finding the
money to pay your student loans or being nervous before a test still activate the sympathetic nervous system, but
these situations do not require the fight-or-flight response to survive. 38 Possible treatments for patients with major
depression include psychotherapy and prescription medications. MAO inhibitor drugs inhibit the breakdown of certain
neurotransmitters (including dopamine, serotonin, norepinephrine) in the synaptic cleft. SSRI medications inhibit the
reuptake of serotonin into the presynaptic neuron.
Chapter 36
1 Figure 36.5 D 3 Figure 36.18 A 4 B 6 A 8 B 10 B 12 A 14 B 16 D 18 B 20 B 22 D 24 Transmission of
sensory information from the receptor to the central nervous system will be impaired, and thus, perception of stimuli,
which occurs in the brain, will be halted. 26 General sensory receptors are located throughout the body in the skin
and internal organs. Conversely, special senses are all located in the head region, and require specialized organs.
28 Pain is a subjective sensation that relies on the brain interpreting the nociception signals received by the sensory
receptors (perception). Therefore, even though two people experience identical stimuli, their brains can perceive them
as very different sensory experiences. 30 The animal might not be able to recognize the differences in food sources
and thus might not be able to discriminate between spoiled food and safe food or between foods that contain necessary
nutrients, such as proteins, and foods that do not. 32 The sound would slow down, because it is transmitted through
the particles (gas) and there are fewer particles (lower density) at higher altitudes. 34 The first step in processing a
sound in humans is the collection of sound by the pinna. When a person encounters a sound, the pinna on both sides
of the head will collect the vibrations. Since the waves originate from a single site, the two pinnae will not collect the
sound at the exact same time. When the sound is processed by the auditory system, the brain is able to use this slight
difference in timing to determine the location of the sound. 36 The photoreceptors tonically inhibit the bipolar cells,
and stimulation of the receptors turns this inhibition off, activating the bipolar cells.
Chapter 37
1 Figure 37.5 Proteins unfold, or denature, at higher temperatures. 3 Figure 37.14 Patient A has symptoms
associated with decreased metabolism, and may be suffering from hypothyroidism. Patient B has symptoms
associated with increased metabolism, and may be suffering from hyperthyroidism. 4 C 6 D 8 D 10 A 12 C 14
A 16 B 18 C 20 C 21 Although there are many different hormones in the human body, they can be divided into
three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide hormones. One of the
key distinguishing features of the lipid-derived hormones is that they can diffuse across plasma membranes whereas
the amino acid-derived and peptide hormones cannot. 23 Glucagon acts in opposition to insulin, the peptide hormone
that stimulates cells to take up glucose from the bloodstream, to maintain blood glucose within healthy levels. When
glucagon is released into the blood in response to falling blood sugar levels, the liver catabolizes its glycogen stores
to release glucose. If glucagon does not function properly, the blood sugar will drop too low from insulin signaling
driving cellular uptake from the blood. 25 Depending on the location of the protein receptor on the target cell and the
chemical structure of the hormone, hormones can mediate changes directly by binding to intracellular receptors and
modulating gene transcription, or indirectly by binding to cell surface receptors and stimulating signaling pathways. 27
In addition to producing FSH and LH, the anterior pituitary also produces the hormone prolactin (PRL) in females.
Prolactin stimulates the production of milk by the mammary glands following childbirth. Prolactin levels are regulated
by the hypothalamic hormones prolactin-releasing hormone (PRH) and prolactin-inhibiting hormone (PIH) which is now
known to be dopamine. PRH stimulates the release of prolactin and PIH inhibits it. The posterior pituitary releases the
hormone oxytocin, which stimulates contractions during childbirth. The uterine smooth muscles are not very sensitive
to oxytocin until late in pregnancy when the number of oxytocin receptors in the uterus peaks. Stretching of tissues
in the uterus and vagina stimulates oxytocin release in childbirth. Contractions increase in intensity as blood levels of
oxytocin rise until the birth is complete. 29 The stressed patients that catch a cold must be chronically (long-term)
stressed. Long-term stress results in the production of glucocorticoids, such as cortisol. These hormones inhibit the
function of the immune system, which makes people more susceptible to infectious diseases. 31 The term humoral is
derived from the term humor, which refers to bodily fluids such as blood. Humoral stimuli refer to the control of hormone
release in response to changes in extracellular fluids such as blood or the ion concentration in the blood. For example,
a rise in blood glucose levels triggers the pancreatic release of insulin. Insulin causes blood glucose levels to drop,
which signals the pancreas to stop producing insulin in a negative feedback loop. Hormonal stimuli refer to the release
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Answer Key
1541
of a hormone in response to another hormone. A number of endocrine glands release hormones when stimulated by
hormones released by other endocrine organs. For example, the hypothalamus produces hormones that stimulate the
anterior pituitary. The anterior pituitary in turn releases hormones that regulate hormone production by other endocrine
glands. For example, the anterior pituitary releases thyroid-stimulating hormone, which stimulates the thyroid gland
to produce the hormones T 3 and T 4 . As blood concentrations of T 3 and T 4 rise they inhibit both the pituitary and
the hypothalamus in a negative feedback loop. 33 The main mineralocorticoid is aldosterone, which regulates the
concentration of ions in urine, sweat, and saliva. Aldosterone release from the adrenal cortex is stimulated by a
decrease in blood concentrations of sodium ions, blood volume, or blood pressure, or an increase in blood potassium
levels. 35 Damage to the posterior pituitary gland would prevent the release of ADH and oxytocin into the body.
However, the hypothalamus’s ability to produce ADH would not be affected. The hypothalamus would also still be able
to produce and release inhibiting hormones to regulate the anterior pituitary.
Chapter 38
1 Figure 38.19B 3 Figure 38.38 In the presence of Sarin, acetycholine is not removed from the synapse, resulting in
continuous stimulation of the muscle plasma membrane. At first, muscle activity is intense and uncontrolled, but the
ion gradients dissipate, so electrical signals in the T-tubules are no longer possible. The result is paralysis, leading to
death by asphyxiation. 4 A 6D 8D 10 C 12 C 14 D 16 D 18 A 20 D 22 D 24 D 25 The female pelvis is tilted
forward and is wider, lighter, and shallower than the male pelvis. It also has a pubic angle that is broader than the male
pelvis. 27 Hydrostatic skeletons protect internal organs from harm by cushioning them from external shock. However,
these skeletons do not provide protection from external trauma. Exoskeletons are hard structures that protect the
organs from damage caused by their environment. However, since they are rigid, they provide little shock absorption,
so the animal will need to have other ways of cushioning its internal organs. 29 Compact bone tissue forms the
hard external layer of all bones and consists of osteons. Compact bone tissue is prominent in areas of bone at which
stresses are applied in only a few directions. Spongy bone tissue forms the inner layer of all bones and consists of
trabeculae. Spongy bone is prominent in areas of bones that are not heavily stressed or at which stresses arrive from
many directions. 31 Thalidomide effected the development of the long bones of the arms, disrupting endochondral
ossification. The bones would have been able to develop into a template made of the calcified cartilage matrix, but
new blood vessels could not be created. Since no vessels invade the template, the structure is not converted into
trabecular bone. 33 Elevation is the movement of a bone upward, such as when the shoulders are shrugged, lifting
the scapulae. Depression is the downward movement of a bone, such as after the shoulders are shrugged and the
scapulae return to their normal position from an elevated position. 35 Because ATP is required for myosin to release
from actin, muscles would remain rigidly contracted until more ATP was available for the myosin cross-bridge release.
This is why dead vertebrates undergo rigor mortis. 37 Neurons will not be able to release neurotransmitter without
calcium. Skeletal muscles have calcium stored and don’t need any from the outside.
Chapter 39
1 Figure 39.7 B 3 Figure 39.20 The blood pH will drop and hemoglobin affinity for oxygen will decrease. 4 A 6
B 8 D 10 B 12 D 14 C 16 The main bronchus is the conduit in the lung that funnels air to the airways where gas
exchange occurs. The main bronchus attaches the lungs to the very end of the trachea where it bifurcates. The trachea
is the cartilaginous structure that extends from the pharynx to the primary bronchi. It serves to funnel air to the lungs.
The alveoli are the sites of gas exchange; they are located at the terminal regions of the lung and are attached to the
respiratory bronchioles. The acinus is the structure in the lung where gas exchange occurs. 18 FEV1/FVC measures
the forced expiratory volume in one second in relation to the total forced vital capacity (the total amount of air that is
exhaled from the lung from a maximal inhalation). This ratio changes with alterations in lung function that arise from
diseases such as fibrosis, asthma, and COPD. 20 Oxygen moves from the lung to the bloodstream to the tissues
according to the pressure gradient. This is measured as the partial pressure of oxygen. If the amount of oxygen drops
in the inspired air, there would be reduced partial pressure. This would decrease the driving force that moves the
oxygen into the blood and into the tissues. P 0o is also reduced at high elevations: P Qi at high elevations is lower
than at sea level because the total atmospheric pressure is less than atmospheric pressure at sea level. 22 Increased
airway resistance increases the volume and pressure in the lung; therefore, the intrapleural pressure would be less
negative and breathing would be more difficult. 24 The lung is particularly susceptible to changes in the magnitude
and direction of gravitational forces. When someone is standing or sitting upright, the pleural pressure gradient leads to
increased ventilation further down in the lung. 26 Carbon monoxide has a higher affinity for hemoglobin than oxygen.
This means that carbon monoxide will preferentially bind to hemoglobin over oxygen. Administration of 100 percent
oxygen is an effective therapy because at that concentration, oxygen will displace the carbon monoxide from the
hemoglobin.
Chapter 40
1 Figure 40.10 C 3 Figure 40.17 Blood in the legs is farthest away from the heart and has to flow up to reach it. 4
1542
Answer Key
A 6 C 8 C 10 C 12 A 14 A 16 A closed circulatory system is a closed-loop system, in which blood is not free in
a cavity. Blood is separate from the bodily interstitial fluid and contained within blood vessels. In this type of system,
blood circulates unidirectionally from the heart around the systemic circulatory route, and then returns to the heart. 18
Red blood cells are coated with proteins called antigens made of glycolipids and glycoproteins. When type A and type
B blood are mixed, the blood agglutinates because of antibodies in the plasma that bind with the opposing antigen.
Type O blood has no antigens. The Rh blood group has either the Rh antigen (Rh+) or no Rh antigen (Rh-). 20
Lymph capillaries take fluid from the blood to the lymph nodes. The lymph nodes filter the lymph by percolation through
connective tissue filled with white blood cells. The white blood cells remove infectious agents, such as bacteria and
viruses, to clean the lymph before it returns to the bloodstream. 22 The capillaries basically exchange materials with
their surroundings. Their walls are very thin and are made of one or two layers of cells, where gases, nutrients, and
waste are diffused. They are distributed as beds, complex networks that link arteries as well as veins.
Chapter 41
1 Figure 41.5 C 3 Figure 41.8 Loop diuretics decrease the excretion of salt into the renal medulla, thereby reducing
its osmolality. As a result, less water is excreted into the medulla by the descending limb, and more water is excreted
as urine. 4 B 6 A 8 B 10 C 12 D 14 C 16 C 18 Excretion allows an organism to rid itself of waste molecules that
could be toxic if allowed to accumulate. It also allows the organism to keep the amount of water and dissolved solutes
in balance. 20 The loop of Henle is part of the renal tubule that loops into the renal medulla. In the loop of Henle,
the filtrate exchanges solutes and water with the renal medulla and the vasa recta (the peritubular capillary network).
The vasa recta acts as the countercurrent exchanger. The kidneys maintain the osmolality of the rest of the body at a
constant 300 mOsm by concentrating the filtrate as it passes through the loop of Henle. 22 The removal of wastes,
which could otherwise be toxic to an organism, is extremely important for survival. Having organs that specialize in
this process and that operate separately from other organs provides a measure of safety for the organism. 24 It
is believed that the urea cycle evolved to adapt to a changing environment when terrestrial life forms evolved. Arid
conditions probably led to the evolution of the uric acid pathway as a means of conserving water. 26 Hormones
are small molecules that act as messengers within the body. Different regions of the nephron bear specialized cells,
which have receptors to respond to chemical messengers and hormones. The hormones carry messages to the
kidney. These hormonal cues help the kidneys synchronize the osmotic needs of the body. Hormones like epinephrine,
norepinephrine, renin-angiotensin, aldosterone, anti-diuretic hormone, and atrial natriuretic peptide help regulate the
needs of the body as well as the communication between the different organ systems.
Chapter 42
1 Figure 42.11 C 3 Figure 42.16 If the blood of the mother and fetus mixes, memory cells that recognize the Rh
antigen can form late in the first pregnancy. During subsequent pregnancies, these memory cells launch an immune
attack on the fetal blood cells. Injection of anti-Rh antibody during the first pregnancy prevents the immune response
from occurring. 4 D 6 A 8 D 10 B 12 D 14 C 16 C 18 D 20 C 22 If the MHC I molecules expressed on donor
cells differ from the MHC I molecules expressed on recipient cells, NK cells may identify the donor cells as “non¬
self” and produce perforin and granzymes to induce the donor cells to undergo apoptosis, which would destroy the
transplanted organ. 24 An antigen is a molecule that reacts with some component of the immune response (antibody,
B cell receptor, T cell receptor). An epitope is the region on the antigen through which binding with the immune
component actually occurs. 26 The ThI response involves the secretion of cytokines to stimulate macrophages and
CTLs and improve their destruction of intracellular pathogens and tumor cells. It is associated with inflammation. The
Th 2 response is involved in the stimulation of B cells into plasma cells that synthesize and secrete antibodies. 28 T
cells bind antigens that have been digested and embedded in MHC molecules by APCs. In contrast, B cells function
themselves as APCs to bind intact, unprocessed antigens. 30 Cross reactivity of antibodies can be beneficial when it
allows an individual's immune system to respond to an array of similar pathogens after being exposed to just one of
them. A potential cost of cross reactivity is an antibody response to parts of the body (self) in addition to the appropriate
antigen.
Chapter 43
1 Figure 43.8 D 3 Figure 43.17 B 4 A 6 D 8 A 10 A 12 C 14 A 16 A 18 C 20 D 22 A 24 B 26 B 28 D 30
D 31 Sexual reproduction produces a new combination of genes in the offspring that may better enable them to
survive changes in the environment and assist in the survival of the species. 33 External fertilization can create large
numbers of offspring without requiring specialized delivery or reproductive support organs. Offspring develop and
mature quickly compared to internally fertilizing species. A disadvantage is that the offspring are out in the environment
and predation can account for large loss of offspring. The embryos are susceptible to changes in the environment,
which further depletes their numbers. Internally fertilizing species control their environment and protect their offspring
from predators but must have specialized organs to complete these tasks and usually produce fewer embryos. 35 In
phase one (excitement), vasodilation leads to vasocongestion and enlargement of erectile tissues. Vaginal secretions
are released to lubricate the vagina during intercourse. In phase two (plateau), stimulation continues, the outer third
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Answer Key
1543
of the vaginal wall enlarges with blood, and breathing and heart rate increase. In phase three (orgasm), rhythmic,
involuntary contractions of muscles occur. In the male, reproductive accessory glands and tubules constrict, depositing
semen in the urethra; then, the urethra contracts, expelling the semen through the penis. In women, the uterus and
vaginal muscles contract in waves that may last slightly less than a second each. In phase four (resolution), the
processes listed in the first three phases reverse themselves and return to their normal state. Men experience a
refractory period in which they cannot maintain an erection or ejaculate for a period of time ranging from minutes to
hours. Women do not experience a refractory period. 37 Negative feedback in the male system is supplied through
two hormones: inhibin and testosterone. Inhibin is produced by Sertoli cells when the sperm count exceeds set limits.
The hormone inhibits GnRH and FSH, decreasing the activity of the Sertoli cells. Increased levels of testosterone affect
the release of both GnRH and LH, decreasing the activity of the Leydig cells, resulting in decreased testosterone and
sperm production. 39 The first trimester lays down the basic structures of the body, including the limb buds, heart,
eyes, and the liver. The second trimester continues the development of all of the organs and systems established
during the first trimester. The placenta takes over the production of estrogen and high levels of progesterone and
handles the nutrient and waste requirements of the fetus. The third trimester exhibits the greatest growth of the fetus,
culminating in labor and delivery. 41 Multiple sperm can fuse with the egg, resulting in polyspermy. The resulting
embryo is not genetically viable and dies within a few days. 43 Organs form from the germ layers through the process
of differentiation. During differentiation, the embryonic stem cells express a specific set of genes that will determine
their ultimate fate as a cell type. For example, some cells in the ectoderm will express the genes specific to skin
cells. As a result, these cells will differentiate into epidermal cells. The process of differentiation is regulated by cellular
signaling cascades.
Chapter 44
1 Figure 44.10 Tropical lakes don't freeze, so they don't undergo spring turnover in the same way temperate lakes do.
However, stratification does occur, as well as seasonal turnover. 3 Figure 44.21 B. the photic zone, the intertidal zone,
the neritic zone, and the oceanic zone 4 B 6 D 8 D 10 D 12 C 14 Ecologists working in organismal or population
ecology might ask similar questions about how the biotic and abiotic conditions affect particular organisms and,
thus, might find collaboration to be mutually beneficial. Levels of ecology such as community ecology or ecosystem
ecology might pose greater challenges for collaboration because these areas are very broad and may include many
different environmental components. 16 Ocean upwelling is a continual process that occurs year-round. Spring and
fall turnover in freshwater lakes and ponds, however, is a seasonal process that occurs due to temperature changes
in the water that take place during springtime warming and autumn cooling. Both ocean upwelling and spring and fall
turnover enable nutrients in the organic materials at the bottom of the body of water to be recycled and reused by
living things. 18 Fire is less common in desert biomes than in temperate grasslands because deserts have low net
primary productivity and, thus, very little plant biomass to fuel a fire. 20 Bogs are low in oxygen and high in organic
acids. The low oxygen content and the low pH both slow the rate of decomposition. 22 Natural processes such as
the Milankovitch cycles, variation in solar intensity, and volcanic eruptions can cause periodic, intermittent changes in
global climate. Human activity, in the form of emissions from the burning of fossil fuels, has caused a progressive rise
in the levels of atmospheric carbon dioxide.
Chapter 45
1 Figure 45.2 Smaller animals require less food and other resources, so the environment can support more of them. 3
Figure 45.16 Stage 4 represents a population that is decreasing. 4 C 6 A 8 A 10 D 12 C 14 A 16 D 18 A 20
B 22 D 24 C 26 D 28 B 30 The researcher would mark a certain number of penguins with a tag, release them
back into the population, and, at a later time, recapture penguins to see what percentage of the recaptured penguins
was tagged. This percentage would allow an estimation of the size of the penguin population. 32 Parental care is not
feasible for organisms having many offspring because they do not have the energy available to take care of offspring.
Most of their energy budget is used in the formation of seeds or offspring, so there is little left for parental care. Also,
the sheer number of offspring would make individual parental care impossible. 34 In the first part of the curve, when
few individuals of the species are present and resources are plentiful, growth is exponential, similar to a J-shaped
curve. Later, growth slows due to the species using up resources. Finally, the population levels off at the carrying
capacity of the environment, and it is relatively stable over time. 36 If a natural disaster such as a fire happened in
the winter, when populations are low, it would have a greater effect on the overall population and its recovery than if
the same disaster occurred during the summer, when population levels are high. 38 Continued exponential human
population growth results in the human population requiring more resources to sustain itself. These resources are
usually taken at the expense of the environment and the organisms that rely on the resources in that environment
(e.g., habitat destruction for human development, water rerouting for irrigation, etc.). The continued use of fossil fuels
to generate power for human activities also contributes to climate change, changing climates in some areas so that
certain species can no longer survive there. 40 Jaguars are an apex predator in the Amazon, eating a variety of prey
animals and not serving as prey to any other predators. Through predation, they control the population sizes of the
smaller herbivores and omnivores. If jaguars were to disappear from the ecosystem, the smaller herbivore populations
would dramatically increase, and could overconsume the plant populations. 42 Animals that use aural or pheromone
1544
Answer Key
signals to communicate with potential mates are able to signal over longer distances. Sound waves and chemicals
can diffuse out into an environment while visual cues require a direct line of sight between the sender and receiver.
Animals that use aural cues to acquire mates probably exhibit a lower population density than animals that use visual
cues.
Chapter 46
1 Figure 46.8 According to the first law of thermodynamics, energy can neither be created nor destroyed. Eventually,
all energy consumed by living systems is lost as heat or used for respiration, and the total energy output of the system
must equal the energy that went into it. 3 Figure 46.17 C: Nitrification by bacteria converts nitrates (NO 3 - ) to nitrites
(NO 2 - ). 4 D 6 B 8 A 10 D 12 D 14 D 16 B 18 C 20 D 22 B 24 B 26 Food webs show interacting groups of
different species and their many interconnections with each other and the environment. Food chains are linear aspects
of food webs that describe the succession of organisms consuming one another at defined trophic levels. Food webs
are a more accurate representation of the structure and dynamics of an ecosystem. Food chains are easier to model
and use for experimental studies. 28 Grazing food webs have a primary producer at their base, which is either a
plant for terrestrial ecosystems or a phytoplankton for aquatic ecosystems. The producers pass their energy to the
various trophic levels of consumers. At the base of detrital food webs are the decomposers, which pass this energy to
a variety of other consumers. Detrital food webs are important for the health of many grazing food webs because they
eliminate dead and decaying organic material, thus, clearing space for new organisms and removing potential causes
of disease. By breaking down dead organic matter, decomposers also make mineral nutrients available to primary
producers; this process is a vital link in nutrient cycling. 30 Conceptual models allow ecologists to see the “big picture”
of how different components of the ecosystem interact with each other, energy sources, and resources. However, this
approach is more descriptive than quantitative, so it is difficult to make conclusions about the resistance or resilience
of a system. Analytical modeling creates a model that can predict how the ecosystem’s relationships will change in
response to disturbances, but does not convey the complexity of the relationships seen with conceptual modeling.
32 NPE measures the rate at which one trophic level can use and make biomass from what it attained in the previous
level, taking into account respiration, defecation, and heat loss. Endotherms have high metabolism and generate a
lot of body heat. Although this gives them advantages in their activity level in colder temperatures, these organisms
are 10 times less efficient at harnessing the energy from the food they eat compared with cold-blooded animals, and
thus have to eat more and more often. 34 In this ecological model, the oak trees (producers) would be at the bottom,
the blue jays would be in the middle level (primary consumer of acorns), and the parasites would be at the top level
(secondary consumer). However, the pyramid would be inverted since each bird could support several parasites, and
each tree could support several birds. This pyramid would appear to be the opposite of the energy flow pyramid. 36
Many factors can kill life in a lake or ocean, such as eutrophication by nutrient-rich surface runoff, oil spills, toxic waste
spills, changes in climate, and the dumping of garbage into the ocean. Eutrophication is a result of nutrient-rich runoff
from land using artificial fertilizers high in nitrogen and phosphorus. These nutrients cause the rapid and excessive
growth of microorganisms, which deplete local dissolved oxygen and kill many fish and other aquatic organisms. 38
Human activity has greatly increased the amount of carbon dioxide gas in the Earth's atmosphere. The oceanic and
atmospheric levels of carbon dioxide are linked so that when atmospheric carbon dioxide levels increase, the amount
of dissolved carbon dioxide in the ocean also increases (partial pressure of oxygen). When carbon dioxide dissolves in
water it produces the weak acid bicarbonate. Since the Industrial Revolution the pH of the ocean has dropped 0.1 pH
units, a 30% increase in acidity.
Chapter 47
1 Figure 47.6 A. An abundance of fern spores from several species was found below the K-Pg boundary, but none
was found above. 3B 5C 7 C 9B 11 D 13 C 15 D 17 The hypothesized cause of the K-Pg extinction event is
an asteroid impact. The first piece of evidence of the impact is a spike in iridium (an element that is rare on Earth, but
common in meteors) in the geological layers that mark the K-Pg transition. The second piece of evidence is an impact
crater off the Yucatan Peninsula that is the right size and age to have caused the extinction event. 19 Crop plants
are derived from wild plants, and genes from wild relatives are frequently brought into crop varieties by plant breeders
to add valued characteristics to the crops. If the wild species are lost, then this genetic variation would no longer be
available. 21 Human population growth leads to unsustainable resource use, which causes habitat destruction to build
new human settlements, create agricultural fields, and so on. Larger human populations have also led to unsustainable
fishing and hunting of wild animal populations. Excessive use of fossil fuels also leads to global warming. 23 Larger
preserves will contain more species. Preserves should have a buffer around them to protect species from edge effects.
Preserves that are round or square are better than preserves with many thin arms.
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Index
1545
INDEX
Symbols
(macroevolution), 518
(microevolution), 518
3' UTR, 449, 455
5' cap, 449, 455
5' UTR, 449, 455
60S ribosomal subunit, 450
7-methylguanosine cap, 421,
430
cr-helix, 92
[ 3 -pleated sheet, 92
“degenerate.”, 410
A
A horizon, 957, 967
abdomen, 833
Abduction, 1200
abduction, 1215
abiotic, 1375, 1403
aboral madreporite, 838
above-ground biomass, 1382,
1403
abscisic acid, 940
abscisic acid (ABA), 943
abscission, 939, 943
absorption spectrum, 235, 245
abstract, 18, 29
abyssal zone, 1391, 1403
Acanthostega, 895
accessory fruit, 999
Accessory fruits, 991
acclimatization, 1025, 1028
acellular, 583
acetyl CoA, 206, 223
acetylcholine, 1093,1102
acetylcholinesterase, 1213,
1215
acid, 54, 63
Acid rain, 1483
acid rain, 1484
acidophile, 621
acoelomate, 785
acoelomates, 772
acromegaly, 1160, 1172
acrosomal reaction, 1365
acrosomal reactions, 1359
Actin, 1207
actin, 1215
Actinopterygii, 860, 895
action potential, 1076,1102
activation energy, 181, 194
activator, 455
activators, 438
active site, 187, 194
Active transport, 159
active transport, 168
acute disease, 572, 583
adaptation, 495, 512
adaptive evolution, 527, 533
Adaptive immunity, 1309
adaptive immunity, 1330
adaptive radiation, 503, 512,
1490, 1516
Addison’s disease, 1162, 1172
Adduction, 1201
adduction, 1215
adenosine triphosphate, 185
adenylate cyclase, 1172
adenylyl cyclase, 1150
adhesion, 53, 63
adrenal cortex, 1167, 1172
adrenal gland, 1172
adrenal glands, 1167
adrenal medulla, 1168, 1172
adrenocorticotropic hormone
(ACTH), 1161, 1172
Adventitious, 721
adventitious, 726
adventitious root, 943
adventitious roots, 915
aerobic, 605
aerobic respiration, 206, 223
afferent arteriole, 1284, 1296
affinities, 1325
affinity, 1330
age structure, 1411, 1451
Age structure, 1427
aggregate fruit, 991, 999
aggressive display, 1451
Aggressive displays, 1444
Agnatha, 855
air sacs, 878
aldosterone, 1151,1172
aleurone, 989, 999
algal bloom, 1394, 1403
alimentary canal, 1035, 1060
aliphatic hydrocarbon, 63
aliphatic hydrocarbons, 57
alkaliphile, 621
allantois, 867, 895
allele, 355
allele frequency, 519, 533
alleles, 334
allergy, 1327, 1330
Allopatric speciation, 502
allopatric speciation, 512
allopolyploid, 505, 512
allosteric inhibition, 190,194
alpha cell, 1172
alpha cells, 1168
alpha-helix structure (cr-helix),
101
alteration, 1025, 1028
alternation of generations, 322
alveolar duct, 1245
alveolar ducts, 1228
alveolar P 09 , 1245
alveolar P Qi , 1233
alveolar sac, 1245
alveolar sacs, 1228
alveolar ventilation, 1238, 1245
alveoli, 1228
alveolus, 1245
Alzheimer’s disease, 1096,
1102
ambulacral (water vascular)
system, 838
amino acid, 101
amino acid-derived hormone,
1172
amino acid-derived hormones,
1146
Amino acids, 87
aminoacyl tRNA synthetase,
430
aminoacyl tRNA synthetases,
426
aminopeptidase, 1054, 1060
ammonia, 1290, 1296
ammonification, 607, 621
ammonotelic, 1290, 1296
amnion, 868, 895
amniote, 895
amniote embryo, 867
amniotic egg, 867
amoebocyte, 841
amoebocytes, 791, 791
Amphiarthroses, 1200
amphiarthrosis, 1215
Amphibia, 895
amphibolic, 209
amphiphilic, 145, 168
ampulla of Lorenzini, 895
ampullae, 838
ampullae of Lorenzini, 858
amygdala, 1090,1102
Amyloplasts, 938
Anabolic, 176
anabolic, 194
anaerobic, 204, 223, 621
anaerobic cellular respiration,
214, 223
analogy, 543, 556
1546
Index
analytical model, 1466, 1484
Anaphase, 286
anaphase, 300
anapsid, 895
Anatomical dead space, 1240
anatomical dead space, 1245
androecium, 744, 972, 999
androgen, 1172
androgens, 1152
aneuploid, 369, 375
aneuploidy, 512
angina, 1264, 1272
Angiotensin converting enzyme
(ACE), 1294
angiotensin converting enzyme
(ACE), 1296
angiotensin I, 1294, 1296
angiotensin II, 1294, 1296
angular movement, 1215
Angular movements, 1200
anion, 63
anions, 45
Annelida, 841
anoxic, 591, 621
antenna protein, 245
antenna proteins, 237
anterior pituitary, 1165, 1172
anther, 744, 758
antheridium, 705, 726
Anthophyta, 748, 758
anthropoid, 895
anti-diuretic hormone (ADH),
1294, 1296
antibiotic, 614, 621
antibiotic resistance, 466, 484
antibody, 1322, 1330
anticodon, 424, 430
antidiuretic hormone (ADH),
1151, 1172
antigen, 1309, 1330
antigen-presenting cell (APC),
1309, 1330
antioxidant, 1293, 1296
antipodal, 977
antipodals, 999
antiporter, 160, 168
Anura, 863, 895
anus, 1045, 1060
aorta, 1262, 1272
apex consumer, 1484
apex consumers, 1461
aphotic zone, 1389, 1403
apical bud, 906, 943
apical meristem, 943
Apical meristems, 905
apocrine gland, 895
Apocrine glands, 882
Apoda, 863, 895
apodeme, 1028
apodemes, 1007
apomixis, 993, 999
apoptosis, 267, 273, 568
aposematic coloration, 1432,
1451
appendicular skeleton, 1186,
1215
applied science, 17, 29
Appositional growth, 1196
appositional growth, 1215
aquaporin, 168
Aquaporins, 154
aquatic biomes, 1382
arachnoid mater, 1085, 1102
arboreal hypothesis, 880
arbuscular mycorrhiza, 696
arbuscular mycorrhizae, 684
Arbuscular mycorrhizae, 696
Archaeopteryx, 895
archegonium, 705, 726
archenteron , 836, 841
archosaur, 895
archosaurs, 869, 879
arcuate arteries, 1283
arcuate artery, 1296
aromatic hydrocarbon, 63
aromatic hydrocarbons, 57
Arteries, 1266
arteriole, 1272
arterioles, 1266
artery, 1272
Arthropoda, 841
articulation, 1188, 1198, 1215
ascending limb, 1296
ascending limbs, 1284
ascocarp, 679, 696
Ascomycota, 678, 696
ascus, 679
Asexual reproduction, 1336
asexual reproduction, 1365
Assimilation, 1470
assimilation, 1484
assortative mating, 526, 533
astrocyte, 1102
Astrocytes, 1072
Asymmetrical, 1006
asymmetrical, 1028
asymptomatic disease, 583
asymptomatic infection, 572
Atherosclerosis, 1264
atherosclerosis, 1272
atom, 22, 29, 36, 63
atomic mass, 37, 63
atomic number, 37, 63
ATP, 185, 194
ATP synthase, 223
atria, 1255
atrial natriuretic peptide (ANP),
1170, 1172
atriopore, 852
atrioventricular valve, 1262,
1272
atrium, 852, 1272
attention deficit hyperactivity
disorder (ADHD), 1102
attention deficit/hyperactivity
disorder (ADHD), 1099
attenuating, 575
attenuation, 583
Audition, 1124
audition, 1138
auditory ossicle, 1215
auditory ossicles, 1183
Australopithecus, 895
Autism spectrum disorder
(ASD), 1098
autism spectrum disorder
(ASD), 1102
autoantibodies, 1328
autoantibody, 1330
autocrine signal, 273
Autocrine signals, 254
autoimmune response, 1316,
1330
Autoimmunity, 1328
autoimmunity, 1330
autoinducer, 273
Autoinducers, 269
autonomic nervous system,
1091, 1102
autopolyploid, 512
autopolyploidy, 505
autosome, 375
autosomes, 341, 355, 367
auxin, 943
Auxins, 939
avidity, 1326, 1330
axial skeleton, 1182, 1215
axillary bud, 906, 943
axon, 1068,1102
axon hillock, 1068, 1102
axon terminal, 1102
axon terminals, 1068
AZT, 583
B
B cell, 1330
B cells, 1306
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1547
B horizon, 967
back mutation, 583
back mutations, 575
background extinction, 1499
bacteriophage, 382, 583
bacteriophages, 561
balanced chemical equation, 44,
63
ball-and-socket joint, 1215
Ball-and-socket joints, 1205
Barcoding, 757
barcoding, 758
bark, 909, 943
basal angiosperms, 758
basal ganglia, 1089, 1102
basal metabolic rate (BMR),
1008, 1028
basal nuclei, 1089, 1102
basal taxon, 538, 556
base, 54, 63
Basic science, 17
basic science, 29
basidia, 681
basidiocarp, 682, 696
Basidiomycota, 681, 696
basidium, 681, 696
basilar membrane, 1126, 1138
basophil, 1305, 1330
Batesian mimicry, 1432, 1451
bedrock, 957, 967
Behavior, 1441
behavior, 1451
Behavioral biology, 1441
behavioral biology, 1451
Behavioral isolation, 507
behavioral isolation, 512
benthic realm, 1389, 1391, 1403
beta cell, 1172
beta cells, 1168
beta-pleated sheet (/3-pleated),
101
bicarbonate (HCOJ ) ion, 1245
bicarbonate buffer system,
1243, 1245
bicarbonate ions (HCOJ ),
1243
bicuspid valve, 1262, 1272
Bilateral symmetry, 770
bilateral symmetry, 785
Bile, 1044
bile, 1060
binary (prokaryotic) fission, 297
binary fission, 300
binomial nomenclature, 540,
556
biochemistry, 27, 29
biodiversity, 1490, 1516
biodiversity hotspot, 1516
Biodiversity hotspots, 1495
bioenergetics, 174, 194
biofilm, 594, 621
biogeochemical cycle, 1473,
1484
Biogeography, 1376, 1494
biogeography, 1403
biological carbon pump, 663
biological community, 1374
biological macromolecule, 101
biological macromolecules, 70
Biological nitrogen fixation, 617
biological nitrogen fixation, 621
biology, 10, 29
bioluminescence, 649, 663
Biomagnification, 1471
biomagnification, 1484
biomarker, 483, 484
Biomass, 1469
biomass, 1484
biome, 1403
biomes, 1376
bioremediation, 619, 621
biosphere, 24, 29, 1376
Biotechnology, 462
biotechnology, 484, 618, 621
biotic, 1375, 1403
biotic potential (rmax), 1451
biotic potential, or rmax , 1418
bipolar neuron, 1138
bipolar neurons, 1119
biramous, 841
birth rate (S), 1417, 1451
Black Death, 610, 621
blastocyst, 1360, 1365
blastomeres, 766
blastopore, 773, 785
blastula, 766, 785
bleaching, 1393
blending theory of inheritance,
355
Blood pressure (BP), 1268
blood pressure (BP), 1272
blood urea nitrogen, 1291
blood urea nitrogen (BUN),
1296
blotting, 466
body plan, 764, 785
bolus, 1040, 1060
Bone, 1191
bone, 1215
Bone remodeling, 1197
bone remodeling, 1215
boreal forest, 1387
botany, 28, 29
bottleneck effect, 524, 533
botulism, 621
Bowman's capsule, 1284, 1296
brachiation, 895
brainstem, 1090,1102
branch point, 538, 556
bronchi, 1228
bronchiole, 1245
bronchioles, 1228
bronchus, 1245
Brumation, 870
brumation, 895
Bryophytes, 710
budding , 568, 583, 1365
Budding, 1337
buffer, 63
buffer zones, 1513
Buffers, 55
bulb, 913, 943
bulbourethral gland, 1343,1365
Bush meat, 1507
bush meat, 1516
B horizon, 957
c
C horizon, 957, 967
CA-MRSA, 614, 621
CAAT box, 418, 430
caecilian, 895
caecilians, 865
Calcification, 1191
calcification, 1215
calcitonin, 1159, 1172
calorie, 51, 63
Calvin cycle, 240, 245
calyces, 1282
calyx, 744, 758, 1296
Cambrian explosion, 779, 785
camouflage, 1431, 1451
cAMP-dependent kinase (A-
kinase), 264
canaliculi, 1018
canaliculus, 1028
candela, 1131, 1138
canopy, 1383, 1403
capillaries, 1267
capillary, 1272
capillary action, 53, 63
capillary bed, 1272
Capillary beds, 1267
capsid, 561, 583
capsomere, 583
capsomeres, 561
1548
Index
capsule, 621, 716, 726
captacula, 841
carapace, 833, 875
carbaminohemoglobin, 1243,
1245
carbohydrate, 101
carbohydrates, 71
carbon, 241
carbon fixation, 245
Carbonic anhydrase (CA), 1243
carbonic anhydrase (CA), 1245
carboxypeptidase, 1053,1060
cardiac cycle, 1264, 1272
Cardiac muscle tissue, 1207
cardiac muscle tissue, 1215
cardiac output, 1272
cardiomyocyte, 1272
Cardiomyocytes, 1264
carnivore, 1060
Carnivores, 1034
carotenoid, 245
carotenoids, 235
carpel, 758
carpus, 1188, 1215
carrier protein, 155, 168
carrying capacity (K), 1451
carrying capacity, or K , 1418
Cartilage, 1017
cartilage, 1028
cartilaginous joint, 1215
Cartilaginous joints, 1199
Casineria, 895
Casparian strip, 917, 943
catabolic, 176, 194
catabolite activator protein
(CAP), 439, 455
Catarrhini, 895
cation, 63
Cations, 45
caveolin, 164, 168
cDNA library, 484
cell, 23, 29
cell cycle, 283, 300, 300
cell necrosis , 571, 583
cell plate, 286, 300
cell theory, 136
cell wall, 119, 136, 596
cell-cycle checkpoint, 300
cell-cycle checkpoints, 290
cell-mediated immune
response, 1309, 1330
cell-surface receptor, 273
Cell-surface receptors, 255
cellular cloning, 468, 484
Cellulose, 77
cellulose, 101
centimorgan (cM), 375
centimorgans (cM), 365
central dogma, 408, 430
central vacuole, 121, 136
centriole, 300
centrioles, 284
centromere, 282, 300
centrosome, 119, 136
cephalic phase, 1058, 1060
Cephalochordata, 852, 895
cephalophorax, 833
cephalothorax, 833, 841
cerebellum, 1090, 1102
cerebral cortex, 1086,1102
cerebrospinal fluid (CSF), 1085,
1102
chain termination method, 484
channel, 1394, 1403
channel protein, 168
Channel proteins, 154
chaperone, 101
chaperones, 95
Chargaff’s rules, 383
charophyte, 726
Charophytes, 702
chelicera, 841
chelicerae, 830
chemical bond, 63
chemical bonds, 44
chemical diversity, 1491,1516
chemical energy, 178, 194
chemical reaction, 63
Chemical reactions, 44
chemical reactivity, 40, 63
chemical synapse, 273
chemical synapses, 253
Chemiosmosis, 203
chemiosmosis, 223
chemoautotroph, 245, 1484
chemoautotrophs, 228
Chemoautotrophs, 1469
chemotroph, 621
Chemotrophs, 605
chiasmata, 322
chitin, 78, 101, 821
chloride shift, 1243, 1245
chlorophyll, 120, 136
Chlorophyll a, 235
chlorophyll a, 245
chlorophyll b, 235, 245
Chlorophytes, 702
chloroplast, 136, 230, 245
Chloroplasts, 120
choanocyte, 841
cholecystokinin, 1059, 1060
Chondrichthyes, 895
chondrocyte, 1028
chondrocytes, 1017
Chordata, 841, 850, 895
chorioallantoic placenta, 885
chorion, 867, 895
choroid plexus, 1085, 1102
chromatid, 300
chromatids, 282
chromatin, 116, 136
chromatophores, 816
chromophore, 935, 943
Chromosomal Theory of
Inheritance, 362, 375
chromosome, 136
chromosome inversion, 373,
375
chromosomes, 116, 280
chronic infection, 583
chronic infections, 572
chylomicron, 1060
chylomicrons, 1055
chyme, 1043, 1060
chymotrypsin, 1053, 1060
Chytridiomycetes, 676
chytridiomycosis, 1508, 1516
Chytridiomycota, 696
cilia, 129
cilium, 136
cingulate gyrus, 1090,1102
circadian, 1137, 1138
Circumduction, 1201
circumduction, 1215
cis-acting element, 446, 455
citric acid cycle, 207, 223
cladistics, 546, 556
class, 540, 556
classical conditioning, 1448,
1451
Clathrates, 1400
cl ath rates, 1403
clathrin, 163, 168
clavicle, 1215
clavicles, 1188
clay, 956, 967
cleavage, 766, 785
cleavage furrow, 286, 300
Climate, 1396
climate, 1403
Climate change, 1509
climax community, 1441, 1451
cline, 527, 533
clitellum, 818, 841
clitoris, 1344, 1365
cloaca, 878, 1341, 1365
clonal selection, 1313,1330
clone, 484
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1549
closed circulatory system, 1252,
1272
club mosses, 718, 726
Cnidaria, 794, 841
cnidocyte, 794, 841
cochlea, 1126, 1138
codominance, 339, 355
codon, 430
codons, 410
coelom, 772, 785, 803, 838
coelomic pouches, 836
coenocytic hypha, 696
coenzyme, 194
coenzymes, 191
cofactor, 194
cofactors, 191
cognitive learning, 1449, 1451
cohesin, 308, 322
cohesion, 52, 63
coleoptile, 990, 999
coleorhiza, 990, 999
colinear, 408, 430
collenchyma cell, 943
Collenchyma cells, 908
colloid, 1166, 1172
columella, 879
columnar epithelia, 1028
Columnar epithelial, 1013
commensal, 1434
Commensalism, 691
commensalism, 696,1451
community, 24, 29
Compact bone, 1193
compact bone, 1215
companion cell, 943
Companion cells, 911
competitive exclusion principle,
1433, 1451
competitive inhibition, 189, 194
complement system, 1307,
1330
complementary DNA (cDNA)
libraries, 474
compliance, 1239, 1245
compound, 63
compound leaf, 920, 943
compounds, 45
concentration gradient, 151, 168
conceptual model, 1466, 1484
conclusion, 19, 29
condensin, 300
condensin proteins, 285
conditioned behavior, 1451
Conditioned behaviors, 1448
condyloid joint, 1215
Condyloid joints, 1204
cone, 1138
cones, 1132
Conidiospores, 674
conifer, 758
Conifers, 741
conispiral, 841
conjugation, 603, 621
connective tissue, 1028
Connective tissues, 1015
consensus, 430
Conservation biogeography,
1494
conspecifics, 1374, 1403
contig, 477, 484
Continuous variation, 328
continuous variation, 355
contour feather, 895
Contour feathers, 876
contraception, 1357, 1365
contractile vacuole, 663
contractile vacuoles, 650
control, 29
control group, 13
convergent evolution, 496, 512
coral reef, 1403
Coral reefs, 1391
core enzyme, 414, 430
corm, 943
Corms, 913
cornea, 1131, 1138
corolla, 744, 758
corona, 808, 841
coronary arteries, 1264
coronary artery, 1272
coronary vein, 1272
coronary veins, 1264
corpus callosum, 1086, 1102
cortex, 911, 943, 1282
cortex (animal), 1296
Cortical, 1283
cortical nephron, 1296
cortical nephrons, 1283
cortical radiate artery, 1296
corticosteroid, 1172
corticosteroids, 1162
cortisol, 1162, 1172
cotyledon, 758, 999
cotyledons, 747, 987
countercurrent exchanger, 1287,
1296
countercurrent multiplier, 1287,
1296
courtship display, 1451
Courtship displays, 1444
covalent bond, 63
covalent bonds, 46
coxal bone, 1215
coxal bones, 1188
cranial bone, 1215
cranial bones, 1182
cranial nerve, 1102
cranial nerves, 1094
Craniata, 895
Craniata (or Vertebrata), 854
cranium, 854, 895
cristae, 632
Crocodilia, 872, 895
crop, 758
Cross reactivity, 1326
cross reactivity, 1330
Cross-pollination, 981
cross-pollination, 999
crossing over, 309
crossover, 309, 322
Cryogenian period, 779, 785
cryptobiosis, 826
cryptochrome, 943
Cryptochromes, 938
cryptofauna, 1392, 1403
ctenidium, 841
cuboidal epithelia, 1028
Cuboidal epithelial, 1013
Cushing’s disease, 1162, 1172
cutaneous respiration, 895
cuticle, 829, 921, 932, 943, 943
cuticle (animal), 841
cutting, 999
cuttings, 995
cyanobacteria, 591, 621
cycad, 758
Cycads, 741
cyclic AMP (cAMP), 264, 273
cyclic AMP-dependent kinase,
273
cyclin, 300
cyclin-dependent kinase (Cdk),
300
cyclin-dependent kinases, 292
cyclins, 292
cypris, 841
cysts, 639
cytochrome complex, 245
Cytogenetic mapping, 474
cytogenetic mapping, 484
cytokine, 1304,1330
cytokinesis, 283, 300
Cytokinesis, 286
cytokinin, 939, 943
cytopathic, 583
cytopathic effects, 568
cytoplasm, 115,136
cytoplasmic streaming, 663
1550
Index
cytoskeleton, 126, 136
cytosol, 115, 136
cytotoxic T lymphocyte (CTL),
1330
cytotoxic T lymphocytes (CTLs),
1311
D
dead space, 1240, 1245
dead zone, 1480, 1484
death rate (D), 1417, 1451
decomposer, 621
decomposers, 606
Deductive reasoning, 12
deductive reasoning, 29
degeneracy, 430
dehydration synthesis, 70, 101
demographic-based models,
1425
demographic-based population
model, 1451
demography, 1408,1451
denaturation, 87, 101
denature, 188, 194
dendrite, 1102
Dendrites, 1068
dendritic cell, 1330
Dendritic cells, 1309
denitrification, 621
density-dependent, 1421
density-dependent regulation,
1451
density-independent, 1421
density-independent regulation,
1451
dentary, 882, 896
deoxynucleotide, 476, 484
deoxyribonucleic acid (DNA),
96, 101
dephosphorylation, 202, 223
depolarization, 1077, 1102
Depression, 1201
depression, 1215
Dermal tissue, 905
dermal tissue, 943
descending, 1284
descending limb, 1296
Descriptive (or discovery)
science, 12
descriptive science, 29
desmosome, 136
desmosomes, 133
determinate cleavage, 774, 785
detrital food web, 1464, 1484
Deuteromycota, 684, 696
deuterostome, 785
Deuterostomes, 773
diabetes insipidus, 1151,1172
diabetes mellitus, 1155, 1172
diabetogenic effect, 1159, 1172
diacylglycerol (DAG), 264, 273
diaphragm, 1228, 1245
diaphysis, 1192, 1215
diapsid, 896
Diarthroses, 1200
diarthrosis, 1215
diastole, 1264, 1272
Dicer, 450, 455
dicot, 758
dicots, 750
dideoxynucleotide, 484
dideoxynucleotides, 477
Diffusion, 152
diffusion, 168
Digestion, 1052
digestion, 1060
dihybrid, 347, 355
dikaryon, 679
dimer, 261, 273
dimerization, 261, 273
dimorphic, 795
Dinosaurs, 871
dioecious, 738, 758
dipeptidase, 1054, 1060
diphyodont, 896
diphyodonts, 882
diploblast, 785
diploblasts, 771
diploid, 280, 300, 308
diplontic, 703, 726
directional selection, 528, 533
disaccharide, 101
Disaccharides, 74
discontinuous variation, 328,
355
discussion, 19, 29
Dispersal, 502
dispersal, 512
dissociation, 52, 63
distal convoluted tubule (DCT),
1284, 1296
distraction display, 1451
Distraction displays, 1444
divergent evolution, 496, 512
diversifying selection, 528, 533
DNA barcoding, 1511, 1516
DNA fingerprinting, 381
DNA methylation, 455
DNA microarray, 484
DNA microarrays, 479
dominant, 355
dominant lethal, 345, 355
Dominant traits, 331
dormancy, 990, 999
dorsal cavity, 1010, 1028
dorsal hollow nerve cord, 851,
896
Dorsiflexion, 1201
dorsiflexion, 1215
double circulation, 1255, 1272
double fertilization, 987, 999
down feather, 896
down feathers, 876
down-regulation, 1148, 1172
downstream, 430
duodenum, 1044, 1060
dura mater, 1085, 1102
E
eccrine gland, 896
Eccrine glands, 882
ecdysis, 827, 830
Ecdysozoa, 776, 785
Echinodermata, 841
Ecological biogeography, 1494
ecological pyramid, 1484
Ecological pyramids, 1470
Ecology, 1372
ecology, 1403
ecosystem, 24, 29, 1460, 1484
ecosystem diversity, 1491,1516
ecosystem dynamics, 1465,
1484
Ecosystem ecology, 1375
ecosystem services, 1403, 1503
ectomycorrhiza, 696
Ectomycorrhizae, 686, 696
ectotherm, 1028
ectothermic, 1008
ectotherms, 870
Ediacaran Period, 778
Ediacaran period, 785
effector cell, 1330
effector cells, 1315
efferent arteriole, 1284, 1296
elastase, 1053, 1060
elastic recoil, 1237, 1245
elastic work, 1238, 1245
electrocardiogram (ECG), 1265,
1272
electrochemical gradient, 159,
168
electrogenic pump, 162, 168
electrolyte, 63, 1278, 1296
electrolytes, 46
electromagnetic spectrum, 233,
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1551
245
electron, 63
electron configuration, 43, 63
electron microscope, 136
electron microscopes, 109
electron orbital, 63
electron orbitals, 42
electron transfer, 45, 63
electron transport chain, 210,
238, 245
electronegativity, 47, 63
Electrons, 37
electrophoresis, 402
element, 64
Elements, 36
Elevation, 1201
elevation, 1216
embryonic mesoderm, 836
embryophyte, 726
embryophytes, 704
Emergent vegetation, 1395
emergent vegetation, 1403
emerging disease, 611, 621
Emsleyan/Mertensian mimicry,
1432, 1451
Enantiomers, 60
enantiomers, 64
Enantiornithes, 896
end-Permian extinction, 1497
endemic, 1403
endemic disease, 609, 621
endemic species, 1376, 1516
Endemic species, 1493
endemism, 1505
endergonic, 194
endergonic reactions, 179
endocardium, 1263, 1272
endocarp, 991, 999
Endochondral ossification, 1196
endochondral ossification, 1216
endocrine, 1059
endocrine cell, 273
endocrine cells, 254
endocrine gland, 1172
endocrine glands, 1171
endocrine signal, 273
endocrine signals, 254
endocrine system, 1060
Endocytosis, 163
endocytosis, 168
endodermis, 916, 943
endomembrane system, 136
Endomycorrhizae, 686
endoplasmic reticulum (ER),
122,136
endoskeleton, 790, 837, 1181,
1216
endosperm, 746, 986, 999
endospermic dicot, 999
endospermic dicots, 989
endosymbiosis, 631, 663
endosymbiotic theory, 631, 663
endotherm, 1008,1028
endotherms, 870
energy budget, 1413, 1451
enhancer, 455
enhancers, 446
enterocoelom, 836, 841
enterocoely, 773, 785
enthalpy, 179, 194
entropy, 183
entropy (S), 194
envelope, 561, 583
environmental disturbance,
1451
environmental disturbances,
1440
enzyme, 101
enzyme-linked receptor, 273
Enzyme-linked receptors, 258
Enzymes, 87
eosinophil, 1305,1330
Ependymal, 1072
ependymal, 1102
epicardium, 1264, 1272
epicotyl, 989, 999
epidemic, 609, 621
epidermis, 841, 909, 943
epigenetic, 436, 455
epilepsy, 1101,1103
epinephrine, 1161,1172
epiphyseal plate, 1196, 1216
epiphyses, 1192
epiphysis, 1216
epiphyte, 965, 967
epiphytes, 1384
epistasis, 352, 355
epithelial tissue, 1028
Epithelial tissues, 1012
epitope, 1330
epitopes, 1311
equilibrium, 45, 64, 1484
Equilibrium, 1461
Erythropoietin (EPO), 1171
erythropoietin (EPO), 1172
esophagus, 1040, 1060
essential, 952
essential nutrient, 1060
essential nutrients, 1047
esthetes, 813
estivation, 1009, 1028, 1381
estrogen, 1146, 1365
Estrogen, 1349
estrogens, 1173
Estuaries, 1393
estuary, 1403
ethology, 1441, 1452
Ethylene, 940
ethylene, 943
eucoelomate, 785
eucoelomates, 772
eukaryote, 29
eukaryote-first, 552
eukaryote-first hypothesis, 556
eukaryotes, 23
eukaryotic cell, 136
eukaryotic cells, 113
eukaryotic initiation factor-2
(elF-2), 450, 455
Eumetazoa, 776, 785
euploid, 369, 375
eutherian mammal, 896
eutrophication, 1479, 1484
evaporation, 51, 64
Eversion, 1201
eversion, 1216
evolution, 24, 29
evolutionary (Darwinian) fitness,
527
evolutionary fitness, 533
excitatory postsynaptic potential
(EPSP), 1080, 1103
exergonic, 194
exergonic reactions, 179
exine, 977, 999
exocarp, 991, 999
Exocytosis, 166
exocytosis, 168
exon, 430
exons, 422, 447
exoskeleton, 1180, 1216
Exotic species, 1507
exotic species, 1516
expiratory reserve volume
(ERV), 1232, 1245
exponential growth, 1417, 1452
expressed sequence tag (EST),
474, 484
extant, 706, 726
Extension, 1200
extension, 1216
external fertilization, 1339, 1365
extinct, 706, 726
extinction, 1490, 1516
extinction rate, 1516
extinction rates, 1499
extracellular digestion, 841
extracellular domain, 255, 273
1552
Index
extracellular matrix, 131,136
extremophile, 621
extremophiles, 592
F
Fi, 329, 355
F2, 329, 355
facial bone, 1216
facial bones, 1183
facilitated transport, 154, 168
FACT, 419, 430
facultative anaerobes, 671, 696
fall and spring turnover, 1403
fallout, 1482, 1484
false negative, 483, 484
falsifiable, 13, 29
family, 540, 556
Fecundity, 1414
fecundity, 1452
Feedback inhibition, 192
feedback inhibition, 194
femur, 1189, 1216
fermentation, 214, 223
fern, 726
ferns, 721
fertilization, 322
FEV1/FVC ratio, 1232, 1245
fibrous connective tissue, 1028
Fibrous connective tissues,
1016
fibrous joint, 1216
fibrous joints, 1198
fibrous root system, 915, 943
fibula, 1189, 1216
field, 1110
filament, 744, 758
first messenger, 1150,1173
Fission, 1336
fission, 1365
fixation, 241
fixed action pattern, 1442, 1452
flagella, 129
flagellum, 136
flame cell, 1296
flame cells, 804, 1289
flat bone, 1216
Flat bones, 1192
Flexion, 1200
flexion, 1216
flight feather, 896
Flow-resistive, 1238
flow-resistive, 1245
flower, 758
fluid mosaic model, 144, 168
follicle stimulating hormone
(FSH), 1349, 1365
follicle-stimulating hormone
(FSH), 1152, 1173
food chain, 1461, 1484
food web, 1463, 1484
foodborne disease, 621
foot, 811, 812
Foraging, 1443
foraging, 1452
forced expiratory volume (FEV),
1232, 1245
forearm, 1188, 1216
foreign DNA, 466, 484
Forest Stewardship Council,
1506
fouling., 815
Foundation species, 1436
foundation species, 1452
founder effect, 519, 533
fovea, 1133, 1138
fragmentation, 839, 1365
Fragmentation, 1337
free energy, 179, 194
free nerve ending, 1115, 1138
frequency-dependent selection,
529,533
frog, 896
frontal (coronal) plane, 1028
frontal lobe, 1088, 1103
frontal plane, 1009
fruit, 758
FtsZ, 297, 300
functional group, 64
Functional groups, 60
functional residual capacity
(FRC), 1232, 1245
functional vital capacity (FVC),
1239, 1245
furcula, 877, 896
fusiform, 1006, 1028
fusion, 572, 583
G
G-protein, 1150, 1173
G-protein-linked receptor, 273
G-protein-linked receptors, 256
Go phase, 287, 300
Gi checkpoint, 291
Gi phase, 283, 300
G 2 phase, 284, 300
gall, 583
gallbladder, 1045, 1060
galls, 571
Gametangia, 705
gametangium, 726
gamete, 300
gametes, 280, 308
gametic barrier, 507, 512
gametophyte, 322, 971, 999
gametophytes, 320
gap junction, 136
Gap junctions, 134
gastric inhibitory peptide, 1059,
1060
gastric phase, 1059, 1060
gastrin, 1059, 1060
gastrodermis, 841
gastrovascular cavity, 841,
1035, 1060
gastrula, 785
Gastrulation, 766
gastrulation, 1360, 1365
GC-rich box, 430
GC-rich boxes, 418
Gel electrophoresis, 386, 463
gel electrophoresis, 484
gemma, 726
gemmae, 712, 712
gemmule, 841
Gemmules, 793
gene, 300
gene expression, 436, 455
gene flow, 525, 533
gene pool, 519, 533
Gene targeting, 469
gene targeting, 484
Gene therapy, 470
gene therapy, 484, 579, 583
gene transfer agent (GTA), 556
gene transfer agents (GTAs),
550
genes, 280
genetic diagnosis, 470, 484
Genetic diversity, 1490
genetic diversity, 1516
genetic drift, 522, 533
Genetic engineering, 469
genetic engineering, 484
genetic map, 472, 484
genetic marker, 472, 484
genetic recombination, 473, 484
genetic structure, 519, 533
genetic testing, 470, 484
genetic variance, 522, 533
genetically modified organism,
469
genetically modified organism
(GMO), 484
genome, 280, 300
genome annotation, 478, 484
genome fusion, 551, 556
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1553
Genome mapping, 472
genome mapping, 484
genomic libraries, 474
genomic library, 484
Genomics, 472
genomics, 484
genotype, 334, 355
genus, 540, 556
geographical variation, 527, 533
geometric isomer, 64
Geometric isomers, 58
germ cells, 319, 322
germ layer, 785
germ layers, 766
gestation, 1353, 1365
gibberellin (GA), 943
Gibberellins, 939
gigantism, 1160,1173
gill circulation, 1254, 1272
gills, 811
ginkgophyte, 758
ginkgophytes, 742
gizzard, 1037, 1060
glabrous, 1115, 1138
glia, 1068, 1103
gliding movement, 1216
Gliding movements, 1200
Global climate change, 1396
global climate change, 1403
Glomeromycota, 684, 696
glomerular filtration, 1285, 1296
Glomerular filtration rate (GFR),
1286
glomerular filtration rate (GFR),
1296
glomeruli, 1124
glomerulus, 1138, 1284
glomerulus (renal), 1296
glucagon, 1156, 1173
glucocorticoid, 1173
glucocorticoids, 1162
gluconeogenesis, 1156,1173
glucose-sparing effect, 1159,
1173
GLUT (glucose transporter)
proteins, 220
GLUT protein, 223
GLUT proteins, 204
Glycogen, 77, 217
glycogen, 101
glycogenolysis, 1156,1173
glycolipid, 168
glycolipids, 145
Glycolysis, 204
glycolysis, 223
glycoprotein, 168
glycoproteins, 145
glycosidic bond, 74, 101
gnathostome, 896
gnathostomes, 855
Gnathostomes, 857
gnetophyte, 758
gnetophytes, 742
goiter, 1158, 1173
Golgi apparatus, 124, 136
Golgi tendon organ, 1138
Golgi tendon organs, 1117
Gomphoses, 1199
gomphosis, 1216
gonadotropin, 1173
gonadotropin-releasing
hormone (GnRH), 1348, 1365
gonadotropins, 1152
Gondwana, 866
good genes hypothesis, 531,
533
Gorilla, 888, 896
gradual speciation model, 510,
512
grafting, 994, 999
Gram negative, 601, 621
Gram positive, 601, 621
granum, 230, 245
granzyme, 1307, 1330
gravitropism, 999
grazing food web, 1464,1484
Great Barrier Reef, 1391
greenhouse effect, 1399, 1403
greenhouse gases, 1399, 1403
gross primary productivity, 1469,
1484
Ground tissue, 905
ground tissue, 943
Group I, 566
group I virus, 583
Group II, 566
group II virus, 583
Group III, 566
group III virus, 583
Group IV, 566
group IV virus, 583
Group V, 566
group V virus, 583
Group VI, 566
group VI virus, 583
Group VII, 566
group VII virus, 583
growth factor, 273
growth factors, 266
Growth hormone (GH), 1159
growth hormone (GH), 1173
growth hormone-inhibiting
hormone (GHIH), 1160, 1173
growth hormone-releasing
hormone (GHRH), 1160, 1173
guanine diphosphate (GDP),
455
guanine triphosphate (GTP),
455
guanosine triphosphate (GTP),
450
guard cells, 909, 944
gustation, 1119, 1138
gymnosperm, 758
Gymnosperms, 738
gynoecium, 744, 758, 972, 999
gyri, 1086
gyrus, 1103
H
habitat isolation, 507, 512
Habituation, 1447
habituation, 1452
hagfish, 896
hair, 882
hairpin, 430
halophile, 621
handicap principle, 531, 533
haplodiplodontic, 726
haplodiplontic, 703
haploid, 280, 300, 308
haplontic, 703, 726
haustoria, 672, 696
Haversian canal, 1193, 1216
haze-effect cooling, 1399, 1403
heat, 194
Heat energy, 181
heat energy, 183, 194
heat of vaporization, 51
heat of vaporization of water, 64
heirloom seed, 758
Heirloom seeds, 757
helicase, 402
helper T (Th) lymphocytes,
1311
helper T lymphocyte (Th), 1330
hemal system, 838
heme group, 1241, 1245
hemizygous, 342, 355
hemocoel, 813, 829, 841, 1252,
1272
Hemoglobin, 1241
hemoglobin, 1245
hemolymph, 1252, 1272
herbaceous, 750, 758
herbivore, 1060
Herbivores, 1034
1554
Index
herbivory, 751, 758
Heritability, 522
heritability, 533
hermaphrodite, 841
Hermaphroditism, 1338
hermaphroditism, 1365
heterodont tooth, 896
heterogeneity, 1494, 1516
heterospecifics, 1374, 1403
heterosporous, 726
Heterothallic, 675
heterothallic, 696
heterotroph, 245
heterotrophs, 228
heterozygous, 334, 355
hibernation, 1009, 1028
Hibernation, 1381
hilum, 1282, 1296
hinge joint, 1216
hinge joints, 1203
hippocampus, 1088, 1103
histone, 300
histone acetylation, 452, 455
histone proteins, 281
Historical biogeography, 1494
HIV (human immunodeficiency
virus), 568
holistic ecosystem model, 1466,
1484
holoblastic, 1359, 1365
Holocene, mass extinction,
1499
holoenzyme, 414, 430
homeostasis, 22, 29, 1028
Homeostasis, 1023
homeotherms, 870
hominin, 896
hominoid, 896
Homo, 888, 896
Homo sapiens sapiens, 894,
896
homologous, 280
homologous chromosomes, 300
homologous recombination,
363, 375
homologous structures, 497,
512
homosporous, 726
homothallic, 675, 696
homozygous, 334, 355
honest signal, 531, 533
horizon, 957, 967
Horizontal gene transfer (HGT),
549
horizontal gene transfer (HGT),
556
horizontal transmission, 571,
583
Hormonal stimuli, 1163
hormonal stimuli, 1173
hormone, 101,1148
hormone receptor, 1173
Hormones, 87
hornworts, 726
horsetail, 726
horsetails, 719
host, 1301, 1331, 1435, 1452
host DNA, 466, 485
Hox gene, 785
Hox genes, 767
human beta chorionic
gonadotropin (/3-HCG), 1353,
1365
human growth hormone (HGH
or hGH), 290
humerus, 1188, 1216
humoral immune response,
1309, 1331
humoral stimuli, 1173
humoral stimulus, 1163
humus, 956, 967
hybrid, 501, 512
hybrid inviability, 508
hybrid zone, 509, 512
hybridization, 355
hybridizations, 329
hydrocarbon, 64
Hydrocarbons, 56
hydrogen bond, 48, 64
hydrogenosome, 663
hydrolysis, 101
hydrolysis reactions, 70
hydrophilic, 49, 64, 145, 168
hydrophobic, 49, 64, 168
Hydrophobic, 145
hydrosphere, 1473, 1485
hydrostatic skeleton, 1180, 1216
hydrothermal vent, 590, 622
Hylobatidae, 888, 896
Hylonomus, 896
hyoid bone, 1184, 1216
hyperextension, 1200, 1216
hyperglycemia, 1155, 1173
hyperopia, 1132, 1138
hyperplasia, 571, 583
hyperpolarization, 1103
hyperpolarizes, 1077
hypersensitivities, 1327,1331
hyperthermophile, 622
Hyperthyroidism, 1158
hyperthyroidism, 1173
hypertonic, 156, 168
hypha, 696
hypocotyl, 989, 999
hypoglycemia, 1155,1173
hypophyseal portal system,
1165, 1173
hypoplasia, 571, 583
hypothalamus, 1090, 1103,
1164
hypothesis, 10, 29
hypothesis-based science, 12,
29
Hypothyroidism, 1158
hypothyroidism, 1173
hypotonic, 156, 168
I
ileum, 1044, 1060
immune tolerance, 1316, 1331
Immunodeficiency, 1327
immunodeficiency, 1331
Imperfect fungi, 684
Imprinting, 1447
imprinting, 1452
inbreeding, 522, 533
inbreeding depression, 522, 533
incomplete dominance, 338,
355
incus, 1125,1138
indeterminate cleavage, 774,
785
induced fit, 188,194
induced mutation, 402
Induced mutations, 400
inducible operon, 455
Inductive reasoning, 12
inductive reasoning, 29
inert, 42
inert gas, 64
inferior vena cava, 1262, 1272,
1283, 1296
Infertility, 1358
infertility, 1365
inflammation, 1305, 1331
ingestion, 1052, 1061
inhibin, 1349, 1365
inhibitor, 266, 273
inhibitory postsynaptic potential
(IPSP), 1103
inhibitory postsynaptic potentials
(IPSPs), 1080
initiation complex, 426, 450, 455
initiation site, 413, 430
initiator tRNA, 426, 430
innate behavior, 1452
innate behaviors, 1441
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1555
Innate immunity, 1302
innate immunity, 1331
inner cell mass, 1360, 1365
inner ear, 1126, 1138
inorganic compound, 952, 967
Inorganic nutrients, 1381
inositol phospholipid, 273
inositol phospholipids, 264
inositol triphosphate (IP 3 ), 264,
273
insectivorous, 965
insectivorous plant, 967
inspiratory capacity (1C), 1232,
1245
inspiratory reserve volume
(IRV), 1232, 1245
Insulin, 1155
insulin, 1173
insulin-like growth factor (IGF),
1173
insulin-like growth factors
(IGFs), 1159
integral protein, 168
Integral proteins, 147
integument, 738, 758
intercalary meristem, 944
Intercalary meristems, 905
intercellular signaling, 252, 273
intercostal muscle, 1246
intercostal muscles, 1237
interferon, 1304, 1331
interkinesis, 313, 322
interlobar arteries, 1283
interlobar artery, 1296
intermediate filament, 136
Intermediate filaments, 129
intermittent, 572
intermittent symptom, 583
internal fertilization, 1339, 1365
internal receptor, 273
Internal receptors, 255
internode, 906, 944
interphase, 283, 301
intersexual selection, 1446,
1452
interspecific competition, 1421,
1452
interstitial cell of Leydig, 1365
interstitial cells of Leydig, 1349
interstitial fluid, 1252, 1273
intertidal zone, 1390, 1404
intervertebral disc, 1216
Intervertebral discs, 1185
intestinal phase, 1059, 1061
intine, 977, 999
intracellular hormone receptor,
1173
intracellular hormone receptors,
1148
intracellular mediator, 273
intracellular mediators, 254
intracellular signaling, 252, 273
Intramembranous ossification,
1195
intramembranous ossification,
1216
intrapleural space, 1237, 1246
intrasexual selection, 1446,
1452
intraspecific competition, 1419,
1452
introduction, 19, 29
intron, 430
introns, 422
Inversion, 1201
inversion, 1216
invertebrata, 842
ion, 64
ion channel-linked receptor, 273
Ion channel-linked receptors,
256
ionic bond, 64
Ionic bonds, 46
ions, 42
iridophores, 816
iris, 1131, 1138
irregular bone, 1216
Irregular bones, 1192
irreversible, 45
irreversible chemical reaction,
64
island biogeography, 1437,
1452
islets of Langerhans, 1168
islets of Langerhans (pancreatic
islets), 1174
isomerase, 204, 223
isomers, 58, 64
isotonic, 157, 168
isotope, 64
Isotopes, 38
isthmus, 1165, 1174
Iteroparity, 1414
iteroparity, 1452
J
J-shaped growth curve, 1417,
1452
Jacobson's organ, 875
Jasmonates, 941
jasmonates, 944
jejunum, 1044, 1061
joint, 1198, 1216
juxtaglomerular cell, 1296
juxtaglomerular cells, 1288
juxtamedullary nephron, 1297
juxtamedullary nephrons, 1283
K
/(-selected species, 1423, 1452
karyogamy, 675, 696
karyogram, 366, 375
karyokinesis, 284, 301
karyotype, 366, 375
keratin, 882
keystone species, 1437, 1452,
1514
kidney, 1297
kidneys, 1281
kin selection, 1445, 1452
kinase, 263, 273
kinesis, 1442, 1452
kinesthesia, 1110, 1138
kinetic energy, 177, 194
kinetochore, 285, 301
kinetoplast, 663
kingdom, 540, 556
Koch's postulates, 594
Kozak’s rules, 426, 430
Krebs cycle, 207, 223
L
labia majora, 1344, 1365
labia minora, 1344, 1365
labyrinth, 1126, 1138
lac operon, 440, 455
lactase, 1061
lactases, 1053
lacuna, 1028
lacunae, 1017
lagging strand, 393, 402
lamella, 1217
lamellae, 1193
lamina, 918, 944
lamprey, 896
lancelet, 896
large 60S ribosomal subunit,
455
large intestine, 1044, 1061
larynx, 1227, 1246
late Devonian extinction, 1497
latency, 570, 583
lateral line, 859, 896
lateral meristem, 944
Lateral meristems, 905
lateral rotation, 1201, 1217
1556
Index
Laurasia, 866
Laurentia, 866
aw of dominance, 346, 355
aw of independent assortment,
347, 355
aw of mass action, 45, 64
aw of segregation, 346, 355
Layering, 995
ayering, 999
eading strand, 393, 402
earned behavior, 1452
earned behaviors, 1441
ens, 1131, 1138
enticel, 944
enticels, 912
epidosaur, 896
epidosaurs, 869
eptin, 1171, 1174
eucophores, 816
ichen, 696
Lichens, 688
ife cycle, 322
ife cycles, 318
ife history, 1413, 1452
ife science, 29
ife sciences, 11
ife table, 1452
ife tables, 1408
igand, 252, 273
igase, 393, 402
ight harvesting complex, 245
ight microscope, 136
ight microscopes, 108
ight-dependent reaction, 245
ight-dependent reactions, 231
ight-harvesting complex, 237
ight-independent reaction, 245
ight-independent reactions, 231
ignin, 717, 726
imbic system, 1090, 1103
inkage, 349, 355
inkage analysis, 472, 485
ipase, 1040, 1061
ipid, 101
ipid hormones, 1146
ipid-derived hormone, 1174
Lipids, 80
itmus, 54
itmus paper, 64
Little Ice Age, 1398
iver, 1045, 1061
Liverworts, 711
iverworts, 726
oam, 967
oams, 956
obe, 1088, 1088, 1088
lobes of the kidney, 1282, 1297
locus, 280, 301
logistic growth, 1418, 1452
long bone, 1217
Long bones, 1192
Long-term depression (LTD),
1084
long-term depression (LTD),
1103
Long-term potentiation (LTP),
1084
long-term potentiation (LTP),
1103
loop of Henle, 1284, 1297
loose (areolar) connective
tissue, 1028
Loose connective tissue, 1016
Lophotrochozoa, 776, 785
lower limb, 1189, 1217
lung capacities, 1231
lung capacity, 1246
lung volume, 1246
lung volumes, 1231
luteinizing hormone (LH), 1349,
1366
lycophyte, 726
Lycopodiophyta, 718
Lymph, 1320
lymph, 1331
lymph node, 1273
Lymph nodes, 1269
lymphocyte, 1331
Lymphocytes, 1306
lysis, 568, 584
lysis buffer, 462, 485
lysogenic cycle , 570, 584
lysosome, 137
lysosomes, 119
lytic cycle, 570, 584
M
macroevolution, 533
macromolecule, 29
macromolecules, 22
macronutrient, 967
macronutrients, 953
macrophage, 1302, 1331
macula densa, 1288, 1297
madreporite, 838, 842
Major depression, 1101
major depression, 1103
major histocompatibility class
(MHC) I molecules, 1306
major histocompatibility class
(MHC) I/ll molecule, 1331
malleus, 1125, 1138
Malpighian tubule, 1297
Malpighian tubules, 1289
maltase, 1061
maltases, 1053
mammal, 896
Mammals, 881
mammary gland, 896
Mammary glands, 882
mantle, 811, 812, 812, 842
mark and recapture, 1410, 1452
marsupial, 896
mass extinction, 782, 785, 1490
mass extinctions, 1496
mass number, 37, 64
mast cell, 1305, 1331
mastax, 808, 842
materials and methods, 19, 29
mating factor, 268, 274
matrix, 1015, 1028
matrix protein, 584
matrix proteins, 563
Matter, 36
matter, 64
maximum parsimony, 548, 556
mechanoreceptor, 1110, 1138
medial rotation, 1201, 1217
medulla, 1282, 1297
medusa, 842
megafauna, 1498, 1516
megagametogenesis, 977, 999
megapascal (MPa), 944
megapascals, 926
megaphyll, 726
megaphylls, 718
megasporangium, 977, 999
megaspore, 726
megasporocyte, 739, 758
megasporogenesis, 977,1000
megasporophyll, 1000
megasporophylls, 979
meiosis, 307, 322
Meiosis, 308
meiosis I, 308, 322
Meiosis II, 308
meiosis II, 322
Meissner's corpuscle, 1138
Meissner's corpuscles, 1116
membrane potential, 1073,1103
memory cell, 1317,1331
meninge, 1103
meninges, 1085
menopause, 1352, 1366
menstrual cycle, 1349, 1366
meristem, 944
Meristematic tissue, 905
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1557
meristematic tissue, 944
meristems, 905
Merkel's disk, 1138
Merkel’s disks, 1115
meroblastic, 1359, 1366
mesocarp, 991, 1000
mesocosm, 1466, 1485
mesoglea, 842
mesohyl, 791, 842
mesophyll, 230, 245
messenger RNA (mRNA), 96,
101
metabolism, 174, 194
metabolome, 482, 485
Metabolomics, 482
metabolomics, 485
metacarpus, 1188, 1217
Metagenomics, 480
metagenomics, 485
metamerism, 818, 842
Metaphase, 286
metaphase, 301
metaphase plate, 286, 301
metatarsal, 1217
metatarsals, 1190
Metazoa, 776, 785
methicillin-resistant
Staphylococcus aureus
(MRSA), 614
MHC II molecules, 1306
microbial mat, 590, 622
Microbiology, 27
microbiology, 29
microcosm, 1466, 1485
microevolution, 533
microfilament, 137
microfilaments, 127
Microglia, 1072
microglia, 1103
micronutrient, 967
micronutrients, 954
microphyll, 726
microphylls, 718
Micropropagation, 996
micropropagation, 1000
micropyle, 978, 1000
microRNA (miRNA), 455
microRNAs, 450
microsatellite polymorphism,
485
microsatellite polymorphisms,
473
microscope, 108, 137
microsporangium, 975, 1000
microspore, 726
microsporocyte, 758
microsporocytes, 738
microsporophyll, 1000
microsporophylls, 979
microtubule, 137
microtubules, 129
microvilli, 1289, 1297
middle ear, 1125, 1139
midsagittal plane, 1009, 1028
Migration, 1380, 1442
migration, 1452
Milankovitch cycles, 1399, 1404
mineral, 1061
mineral soil, 967
mineral soils, 956
mineralocorticoid, 1151, 1174
Minerals, 1047
mismatch repair, 398, 402
mitochondria, 118, 137
mitochondria-first, 552
mitochondria-first hypothesis,
556
mitosis, 284, 301
mitosome, 663
mitotic phase, 283, 301
mitotic spindle, 284, 301
mixotroph, 663
mixotrophs, 638
model organism, 478, 485
model system, 328, 356
modern synthesis, 518, 533
molality, 1279, 1297
molarity, 1279, 1297
mold, 684, 696
mole, 1279, 1297
molecular biology, 27, 30
molecular cloning, 485
molecular systematics, 543, 556
molecule, 22, 30, 64
Molecules, 40
Mollusca, 842
molting, 827, 829
monocarpic, 997, 1000
monocot, 749, 759
monocyte, 1302, 1331
monoecious, 738, 759, 794
monogamous, 1446
monogamy, 1452
monogastric, 1036, 1061
monohybrid, 335, 356
monomer, 101
monomers, 70
monophyletic, 821
monophyletic group, 546, 556
monosaccharide, 101
Monosaccharides, 71
monosomy, 369, 375
monosulcate, 749
monotreme, 896
monotremes, 884
morganucodonts, 884
morning sickness, 1366
mortality rate, 1412, 1452
mosses, 714, 726
motor end plate, 1212, 1217
MRSA, 622
mucin, 1229, 1246
Mucosa-associated lymphoid
tissue (MALT), 1315
mucosa-associated lymphoid
tissue (MALT), 1331
mucus, 1229, 1246
Mullerian mimicry, 1432, 1453
multiple cloning site (MCS), 466,
485
Multiple fruit, 991
multiple fruit, 1000
muscle spindle, 1139
Muscle spindles, 1117
mutation, 402
Mutations, 400
Mutualism, 1374
mutualism, 1435, 1453
Myc, 453
myc, 455
mycelium, 670, 696
Mycetismus, 691
mycetismus, 696
Mycologists, 669
mycology, 668, 696
Mycorrhiza, 686
mycorrhiza, 696
mycorrhizae, 668, 696
mycosis, 691, 697
Mycotoxicosis, 691
mycotoxicosis, 697
myelin, 1068,1103
myocardial infarction, 1264,
1273
myocardium, 1263,1273
myofibril, 1217
myofibrils, 1207
myofilament, 1217
myofilaments, 1208
Myopia, 1132
myopia, 1139
myosin, 1207, 1217
Myxini, 856, 896
N
nacre, 842
nasal cavity, 1226, 1246
1558
Index
natural killer (NK) cell, 1331
natural killer (NK) cells, 1306
natural science, 30
natural sciences, 11
Natural selection, 493
natural selection, 512
Nature Conservancy, 1513
nauplius, 842
nectar, 759
nectar guide, 983, 1000
negative feedback loop, 1023,
1028
negative gravitropism, 938, 944
negative polarity, 566, 584
negative regulator, 455
negative regulators, 439
nematocyst, 842
nematocysts, 794
Nematoda, 842
Nemertea, 842
Neognathae, 896
Neornithes, 879, 897
Nephridia, 815
nephridia, 1289, 1297
nephridiopore, 1289, 1297
nephron, 1297
nephrons, 1282
neritic zone, 1391, 1404
nerve net, 797
Net consumer productivity, 1470
net consumer productivity, 1485
Net primary productivity, 1382,
1469
net primary productivity, 1404,
1485
Net production efficiency (NPE),
1470
net production efficiency (NPE),
1485
neural stimuli, 1164, 1174
neural tube, 1363, 1366
neurobiology, 27, 30
neurodegenerative disorder,
1103
Neurodegenerative disorders,
1096
neuron, 1103
neurons, 1067
neurotransmitter, 274
neurotransmitters, 253
neutron, 37, 64
neutrophil, 1305, 1331
next-generation sequencing,
478,485
nitrification, 622
nitrogen fixation, 607, 622
nitrogenase, 962, 967
noble gas, 64
noble gases, 42
nociception, 1118, 1139
node, 944
Nodes, 906
nodes of Ranvier, 1068, 1103
nodule, 622
nodules, 617, 962, 967
non-electrolyte, 1278, 1297
non-endospermic dicot, 1000
non-endospermic dicots, 989
non-vascular plant, 726
non-vascular plants, 707
noncellular, 561
nondisjunction, 368, 375
nonparental (recombinant) type,
375
nonparental types, 363
nonpolar covalent bond, 64
Nonpolar covalent bonds, 47
nonrandom mating, 526, 533
nonrenewable resource, 1477,
1485
nonsense codon, 430
nonsense codons, 410
nontemplate strand, 413, 430
norepinephrine, 1093, 1103,
1161, 1174
Northern blotting, 466, 485
notochord, 850, 897
nuclear envelope, 116, 137
nucleic acid, 101
Nucleic acids, 96
nucleoid, 111, 137, 596
nucleolus, 117, 137
nucleoplasm, 116, 137
nucleosome, 281, 301
nucleotide, 101
nucleotide excision repair, 399,
402
nucleotides, 96
nucleus, 36, 64, 116, 137
nucleus-first, 552
nucleus-first hypothesis, 556
nutrient, 622
nutrients, 590, 605
o
O horizon, 957, 967
obligate aerobes, 671, 697
obligate anaerobes, 671, 697
obstructive disease, 1246
Obstructive diseases, 1239
occipital, 1088
occipital lobe, 1103
Ocean upwelling, 1379
ocean upwelling, 1404
oceanic zone, 1404
Octamer box, 430
octamer boxes, 418
octet rule, 41, 65
odorant, 1139
Odorants, 1119
Okazaki fragment, 402
Okazaki fragments, 393
olfaction, 1119, 1139
olfactory bulb, 1124, 1139
olfactory epithelium, 1119, 1139
olfactory receptor, 1119, 1139
oligodendrocyte, 1103
Oligodendrocytes, 1072
oligosaccharin, 944
Oligosaccharins, 941
Omega, 83
omega fat, 101
omnivore, 1061
Omnivores, 1035
oncogene, 301
oncogenes, 295
oncogenic virus, 584
oncogenic viruses, 573
oncolytic virus, 584
Oncolytic viruses, 579
one-child policy, 1429, 1453
oogenesis, 1346, 1366
open circulatory system, 1252,
1273
operant conditioning, 1448,
1453
operator, 439, 455
operculum, 815
operon, 455
operons, 438
Opposition, 1201
opposition, 1217
opsonization, 1307, 1331
orbital, 65
orbitals, 40
order, 540, 556
Ordovician-Silurian extinction,
1496
organ, 30
organ of Corti, 1127,1139
organ system, 24, 30
organelle, 30, 137
organelles, 23, 113
organic compound, 952, 967
organic molecule, 65
organic molecules, 56
organic soil, 967
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1559
organic soils, 955
organism, 30
Organisms, 24
organogenesis, 766, 785, 1362,
1366
Organs, 23
origin, 297, 301
Ornithorhynchidae, 884, 897
Ornithurae, 880
osculum, 791, 842
osmoconformer, 1297
osmoconformers, 1280
Osmolarity, 156
osmolarity, 168
osmophile, 622
osmoreceptor, 1174
osmoreceptors, 1151
Osmoregulation, 1278
osmoregulation, 1297
osmoregulator, 1297
osmoregulatory, 1280
Osmosis, 155
osmosis, 169
osmotic balance, 1278, 1297
osmotic pressure, 1278, 1297
osseous tissue, 1191, 1217
ossicle, 1139
ossicles, 1125
Ossification, 1195
ossification, 1217
Osteichthyes, 860, 897
osteoblast, 1217
Osteoblasts, 1195
osteoclast, 1217
Osteoclasts, 1195
osteocyte, 1217
Osteocytes, 1195
osteon, 1028, 1217
osteons, 1018
Osteons, 1193
Osteoprogenitor cells, 1195
ostia, 1252
ostium, 842, 1273
ostracoderm, 897
outer ear, 1125, 1139
oval window, 1126, 1139
ovarian cycle, 1349, 1366
ovary, 744, 759
oviduct, 1366
oviducts, 1345
oviger, 842
ovigers,, 830
ovi parity, 1341, 1366
ovovi parity, 1341, 1366
ovulate cone, 759
ovulate cones, 739
ovulation, 1350, 1366
ovule, 759
oxidative phosphorylation, 203,
223
oxygen, 49
oxygen dissociation curve,
1242, 1246
oxygen-carrying capacity, 1242,
1246
oxytocin, 1155, 1174
P
P 0 , 329, 356
p21, 294, 301
p53, 294, 301
P680, 238, 245
P700, 239, 245
Pacinian corpuscle, 1139
Pacinian corpuscles, 1117
packing, 442
pairwise-end sequencing, 478
Paleognathae, 897
Paleontology, 28
paleontology, 30
Paleoptera, 835
palmately compound leaf, 920,
944
Pan, 888, 897
pancreas, 1045, 1061, 1168,
1174
pandemic, 609, 622
Pangaea, 866
papilla, 1139
papillae, 1121
papulae, 838
parabronchi, 878
paracentric, 373, 375
paracrine signal, 274
paracrine signals, 253
parafollicular cell, 1174
parafollicular cells, 1166
parapodia, 819
parapodium, 842
parasite, 1435, 1453
parasitic plant, 963, 967
Parasitism, 690
parasitism, 697
parasympathetic nervous
system, 1094, 1103
parathyroid gland, 1174
parathyroid glands, 1166
parathyroid hormone (PTH),
1158, 1174
Parazoa, 785
parenchyma cell, 944
Parenchyma cells, 907
parent material, 957, 967
Parental types, 363
parental types, 375
parietal, 1088
parietal lobe, 1103
Parkinson’s disease, 1097,
1103
Parthenogenesis, 1338
parthenogenesis, 1366
Partial pressure, 1230
partial pressure, 1246
particulate matter, 1229, 1246
passive immunity, 1324,1331
Passive transport, 151
passive transport, 169
patella, 1189, 1217
pathogen, 584, 1331
pathogen-associated molecular
pattern (PAMP), 1331
pathogen-associated molecular
patterns (PAMPs), 1302
pathogens, 579, 608, 1301
pattern recognition receptor
(PRR), 1332
pattern recognition receptors
(PRRs), 1302
peat moss, 724, 726
pectoral girdle, 1187, 1217
pedalia, 800
pedicellaria, 837
pedigree analysis, 337
pedipalp, 842
pedipalps, 830
peer-reviewed manuscript, 30
Peer-reviewed manuscripts, 18
pelagic realm, 1389, 1404
pellicle, 663
pellicles, 637
pelvic girdle, 1188, 1217
penis, 1343, 1366
Pepsin, 1042
pepsin, 1061
pepsinogen, 1042, 1061
peptide bond, 89,102
peptide hormone, 1174
peptide hormones, 1147
peptidoglycan, 601, 622
peptidyl transferase, 427, 431
Perception, 1112
perception, 1139
perfect flowers, 747
perforin, 1307, 1332
perianth, 744, 759, 972, 1000
pericardium, 1264, 1273
pericarp, 991, 1000
1560
Index
pericentric, 373, 375
pericycle, 917, 944
periderm, 912, 944
periodic table, 40, 65
peripheral protein, 169
Peripheral proteins, 148
peripheral resistance, 1270,
1273
perirenal fat capsule, 1282,
1297
peristalsis, 1040, 1061
peristome, 726
peritubular capillary network,
1284, 1297
permafrost, 1388, 1404
permanent tissue, 905, 944
Permian extinction, 1401
permissive, 567, 584
peroxisome, 137
Peroxisomes, 118
petal, 759
Petals, 744
petiole, 918, 944
Petromyzontida, 857
Petromyzontidae, 897
pH paper, 65
pH scale, 54, 65
phage therapy, 579, 584
phagolysosome, 637, 663
phalange, 1217
phalanges, 1188
Pharmacogenomics, 480
pharmacogenomics, 485
pharyngeal nerve ring, 822
pharyngeal slit, 897
Pharyngeal slits, 851
pharynx, 1227, 1246
Phenology, 1402
phenotype, 334, 356
pheromone, 1121,1139
pheromones, 835
Phloem, 717
phloem, 727
phosphatase, 274
phosphatases, 268
phosphoanhydride bond, 194
phosphoanhydride bonds, 185
phosphodiester, 97, 102
phosphodiesterase, 268, 274,
1150
phosphodiesterase (PDE), 1174
phospholipid, 102
Phospholipids, 84
Phosphorylation, 202
phosphorylation, 223
photic zone, 1389, 1404
photoact, 238, 245
photoautotroph, 245
photoautotrophs, 228
Photomorphogenesis, 935
photomorphogenesis, 944
photon, 237, 245
Photoperiodism, 935
photoperiodism, 944
photosystem, 237, 245
photosystem I, 237, 246
photosystem II, 237, 246
phototroph, 622
phototrophs, 591
Phototrophs, 605
phototropin, 944
phototropins, 937
Phototropism, 935
phototropism, 944
phyllotaxy, 919, 944
phylogenetic tree, 30, 538, 556
phylogeny, 537, 556
phylum, 540, 556
physical map, 472, 485
physical science, 30
physical sciences, 11
physiological dead space, 1240,
1246
phytochrome, 944
phytochromes, 936
Phytoplankton, 1394
pia mater, 1085, 1103
pigment, 230, 246
pilidium, 842
pilus, 622
pinacocyte, 842
pinna, 1125, 1139
pinnately compound leaf, 944
Pinnately compound leaves,
920
pinocytosis, 164, 169
pioneer species, 1440,1453
pistil, 744, 759
pith, 911, 944
pituitary dwarfism, 1160, 1174
pituitary gland, 1164,1174
pituitary stalk, 1164, 1174
pivot joint, 1217
Pivot joints, 1203
placenta, 1354, 1366
plagiarism, 19, 30
planar joint, 1217
Planar joints, 1202
planktivore, 1404
planktivores, 1392
plankton, 649, 663
planospiral, 842
Plantar flexion, 1201
plantar flexion, 1217
planuliform, 842
plasma, 1255, 1273
plasma cell, 1313, 1332
plasma membrane, 115, 137
plasma membrane hormone
receptor, 1174
plasma membrane hormone
receptors, 1149
plasmid, 431
plasmids, 413
plasmodesma, 137
plasmodesmata, 132
plasmogamy, 675, 697
plasmolysis, 158, 169
plastid, 633, 663
plastron, 875
platelet, 1273
platelets, 1255
Platyrrhini, 897
Pleistocene Extinction, 1498
Plesiadapis, 897
pleura, 1237, 1246
Pleurisy, 1238
pleurisy, 1246
plumule, 989, 1000
pneumatic bone, 897
Pneumatic bones, 877
point mutation, 402
polar covalent bond, 46, 65
polar microtubules, 285
polar nuclei, 977, 1000
pollen grain, 759
pollen tube, 759
pollination, 752, 759, 981, 1000
poly-A tail, 422, 431, 449, 455
polyandrous, 1446
polyandry, 1453
Polycarpic, 997
polycarpic, 1000
polygenic, 479, 485
Polygynous, 1446
polygyny, 1453
polymer, 102
polymerase chain reaction
(PCR), 485
polymers, 70
polymorphic, 842
polynucleotide, 96, 102
polyp, 842
polypeptide, 102
polyploid, 370, 375
polysaccharide, 76, 102
polysome, 425, 431
polyspermy, 1359, 1366
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1561
polytomy, 538, 556
Pongo, 888,897
population, 24, 30, 1374
population density, 1408, 1453
population genetics, 518, 533
population growth rate, 1417,
1453
population size ( N ), 1408, 1453
population variation, 521, 534
Porifera, 776, 790, 842
positive feedback loop, 1024,
1028
positive gravitropism, 938, 944
positive polarity, 584
positive regulator, 456
positive regulators, 439
post-anal tail, 851, 897
post-transcriptional, 436, 456
post-translational, 436, 456
posterior pituitary, 1165,1174
postzygotic barrier, 506, 512
potential energy, 177, 194
potocytosis, 164, 169
precapillary sphincter, 1273
predator, 1404
preening, 878
preinitiation complex, 431
presbyopia, 1132, 1139
prezygotic barrier, 506, 512
primary (main) bronchi, 1228
Primary active transport, 160
primary active transport, 169
primary bronchus, 1246
primary consumer, 1485
primary consumers, 1461
primary electron acceptor, 238,
246
primary feather, 897
Primary feathers, 877
primary growth, 911, 944
primary producer, 1485
primary producers, 1461
primary structure, 91,102
primary succession, 1440, 1453
primase, 392, 402
Primates, 897
primer, 392, 402
prion, 584
Prions, 579
probe, 485
probes, 465
product, 65
product rule, 332, 356
productive , 570, 584
products, 44
Progesterone, 1349
progesterone, 1366
prognathic jaw, 897
progymnosperm, 759
prokaryote, 30, 111, 137
Prokaryotes, 23
prolactin (PRL), 1155, 1174
prolactin-inhibiting hormone,
1174
prolactin-inhibiting hormone
(PIH), 1155
prolactin-releasing hormone,
1174
prolactin-releasing hormone
(PRH), 1155
Prometaphase, 285
prometaphase, 301
promoter, 414, 431, 445
Pronation, 1201
pronation, 1217
proofreading, 397, 402
prophage, 570, 584
Prophase, 285
prophase, 301
proprioception, 1088, 1103,
1110, 1139
prosimian, 897
prostate gland, 1343, 1366
prosthetic group, 210, 223
protease, 485
proteases, 462
proteasome, 451, 456
protein, 102
protein signature, 483, 485
Proteins, 87
proteome, 482, 485
Proteomics, 482
proteomics, 485
proto-oncogene, 301
proto-oncogenes, 295
proton, 37, 65
protonema, 714, 727
protonephridia, 811, 1289
protostome, 786
Protostomes, 773
Protraction, 1201
protraction, 1217
proventriculus, 1037,1061
proximal convoluted tubule
(PCT), 1284, 1297
PrP c , 580, 584
PrP sc , 580, 584
pseudocoelomate, 786
pseudocoelomates, 773
pseudopeptidoglycan, 602, 622
pseudostratified, 1014, 1028
psychrophile, 622
pterosaurs, 870
pterygotes, 834
pulmocutaneous circulation,
1255, 1273
pulmonary circulation, 1255,
1273
pump, 169
pumps, 160
punctuated equilibrium, 510,
512
Punnett square, 335, 356
pupil, 1139
pure culture, 480, 485
purine, 102
purines, 97
pygostyle, 880
pyloric caeca, 838
pyrimidine, 102
pyrimidines, 97, 383
pyruvate, 204, 223
Q
quadrat, 1409, 1453
quaternary structure, 94, 102
quiescent, 287, 301
quorum sensing, 269, 274
R
/•-selected species, 1424, 1453
radial cleavage, 774, 786
Radial glia, 1072
radial glia, 1103
Radial symmetry, 770
radial symmetry, 786
radiate arteries, 1283
Radiation hybrid mapping, 474
radiation hybrid mapping, 485
radicle, 989, 1000
radioisotope, 65
radioisotopes, 38
radioresistant, 592, 622
radius, 1188, 1217
radula, 811, 812, 842
raphe, 653, 663
reactant, 65
reactants, 44
reaction center, 237, 246
reading frame, 410, 410, 431
reception, 1110, 1139
receptive, 1110
receptive field, 1139
receptor, 274
receptor potential, 1110, 1139
receptor-mediated endocytosis,
1562
Index
165,169
receptors, 252, 1148
recessive, 356
recessive lethal, 344, 356
Recessive traits, 331
reciprocal cross, 331, 356
recombinant DNA, 466, 485
recombinant protein, 485
recombinant proteins, 466
recombination frequency, 365,
375
recombination nodules, 309,
322
recruitment, 1240, 1246
rectum, 1045, 1061
red blood cell, 1273
Red blood cells, 1256
Red List, 1499
redox reaction, 223
redox reactions, 200
reduction, 241, 246
reduction division, 322
reductional division, 316
reflex action, 1441, 1453
refractory period, 1077, 1104
regulatory T (Treg) cell, 1332
regulatory T (Treg) cells, 1316
reinforcement, 509, 512
relative fitness, 527, 534
Relative species abundance,
1437
relative species abundance,
1453
renal arteries, 1283
renal artery, 1297
renal capsule, 1282, 1297
renal column, 1297
renal columns, 1282
renal corpuscle, 1283, 1297
renal fascia, 1282, 1297
renal pelvis, 1282, 1297
renal pyramid, 1297
renal pyramids, 1282
renal tubule, 1283, 1298
renal vein, 1298
renal veins, 1283
renette cells, 822
renin, 1152, 1174
renin-angiotensin-aldosterone,
1294, 1298
replication fork, 402
replicative intermediate, 584
replicative intermediates , 566
repressor, 456
Repressors, 438
Reproductive cloning, 468
reproductive cloning, 485
reproductive isolation, 506, 512
Residence time, 1474
residence time, 1485
residual volume (RV), 1232,
1246
Resilience, 1461
resilience (ecological), 1485
resistance, 1239, 1246
Resistance, 1461
resistance (ecological), 1485
resorption, 1217
respiratory bronchiole, 1246
respiratory bronchioles, 1228
respiratory distress syndrome,
1239, 1246
respiratory quotient (RQ), 1233,
1246
respiratory rate, 1238, 1246
respiratory trees, 839
restriction endonuclease, 485
Restriction endonucleases, 466
restriction fragment length
polymorphism (RFLP), 485
restriction fragment length
polymorphisms, 473
restrictive disease, 1246
restrictive diseases, 1239
results, 19, 30
resuscitation, 594, 622
retina, 1131, 1139
retinoblastoma protein (Rb),
294, 301
Retraction, 1201
retraction, 1217
reverse genetics, 485
reverse transcriptase, 566, 584
reverse transcriptase PCR (RT-
PCR), 465, 486
reversible chemical reaction, 65
Reversible reactions, 45
review article, 30
Review articles, 19
Rhabdites, 803
rhizobia, 962, 967
rhizoids, 714, 727
rhizome, 913, 945
rhizosphere, 957, 967
Rho-dependent termination, 415
rho-dependent termination, 431
rho-independent, 431
Rho-independent termination,
415
rhodopsin, 1133, 1139
rhynchocoel, 810, 842
rib, 1217
ribonuclease, 486
ribonucleases, 462
ribonucleic acid (RNA), 96, 102
Ribosomal RNA (rRNA), 99
ribosomal RNA (rRNA), 102
ribosome, 137, 425
Ribosomes, 117
ribs, 1185
ring of life, 554, 556
RISC, 456
RNA editing, 421, 431
RNA stability, 456
RNA-binding protein (RBP), 456
RNA-binding proteins, 449
RNA-induced silencing complex
(RISC), 450
RNAs, 417
rod, 1139
rods, 1132
root cap, 915, 945
root hair, 945
Root hairs, 916
root system, 904, 945
rooted, 538, 556
Rotational movement, 1201
rotational movement, 1218
rough endoplasmic reticulum
(RER), 122, 137
roughage, 1038, 1061
Ruffini ending, 1139
Ruffini endings, 1116
ruminant, 1061
Ruminants, 1038
runner, 945
Runners, 913
s
S phase, 284, 301
S-layer, 622
S-shaped curve, 1418
S-shaped growth curve, 1453
saddle joint, 1218
Saddle joints, 1204
sagittal plane, 1009, 1029
salamander, 897
Salamanders, 863
salivary amylase, 1040, 1061
saltatory conduction, 1078,
1104
sand, 956, 967
saprobe, 697
saprobes, 671
saprophyte, 964, 967
sarcolemma, 1207,1218
sarcomere, 1218
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1563
sarcomeres, 1208
Sarcopterygii, 860, 897
Sargassum, 1391, 1404
Satellite glia, 1072
satellite glia, 1104
saturated fatty acid, 81, 102
sauropsid, 897
Sauropsids, 868
Savannas, 1384
scapula, 1218
scapulae, 1188
Scarification, 990
scarification, 1000
schizocoelom, 803, 842
schizocoely, 773, 786
Schizophrenia, 1100
schizophrenia, 1104
Schwann cell, 1072, 1104
science, 10, 30
scientific method, 10, 30
scion, 994, 1000
sclerenchyma cell, 945
Sclerenchyma cells, 908
sclerocyte, 842
scrotum, 1342, 1366
scutellum, 989, 1000
scutes, 875
sebaceous gland, 897
Sebaceous glands, 882
second messenger, 274
Second messengers, 263
Secondary active transport, 160
secondary active transport, 169
secondary consumer, 1485
Secondary consumers, 1461
secondary feather, 897
Secondary feathers, 877
Secondary growth, 911
secondary growth, 945
secondary plant compound,
1516
secondary plant compounds,
1501
secondary structure, 92, 102
secondary succession, 1440,
1453
secretin, 1059, 1061
seed, 744, 759
seedless vascular plant, 727
segmental arteries, 1283
segmental artery, 1298
selection pressure, 522
selective pressure, 534
selectively permeable, 151, 169
Self-pollination, 981
self-pollination, 1000
Semelparity, 1414
semelparity, 1453
Semen, 1343
semen, 1366
semi-permeable membrane,
1298
semi-permeable membranes,
1278
semicircular canal, 1140
semicircular canals, 1129
semilunar valve, 1262, 1273
seminal vesicle, 1366
seminal vesicles, 1343
seminiferous tubule, 1366
seminiferous tubules, 1342
senescence, 997,1000
sensory receptor, 1110, 1140
sensory transduction, 1110,
1140
sensory-somatic nervous
system, 1091, 1104
sepal, 759
sepals, 744
septa, 670, 697, 798
septum, 297, 301, 670
Sequence mapping , 474
sequence mapping, 486
serendipity, 18, 30
Serial hermaphroditism, 834
serotype, 622
Sertoli cell, 1366
Sertoli cells, 1349
serum, 1259, 1273
sesamoid bone, 1218
Sesamoid bones, 1192
sessile, 918, 945
set point, 1023, 1029
seta, 716, 727
seta/chaeta, 843
setae, 819
sex-linked, 356
sexual dimorphism, 534
sexual dimorphisms, 530
sexual reproduction, 1336, 1366
shared ancestral character, 547,
556
shared derived character, 547,
556
Shine-Dalgarno sequence, 431
shoot system, 904, 945
short bone, 1218
Short bones, 1192
shotgun sequencing, 477, 486
sickle cell anemia, 1242, 1246
sieve-tube cell, 945
sieve-tube cells, 911
signal, 1453
signal integration, 262, 274
signal sequence, 429, 431
signal transduction, 261, 274
signaling cell, 274
signaling cells, 252
signaling pathway, 261, 274
signals, 1443
silent mutation, 402
silt, 956, 967
simple epithelia, 1012, 1029
simple fruit, 991, 1000
simple leaf, 920, 945
simulation model, 1466, 1485
single nucleotide polymorphism
(SNP), 486
single nucleotide
polymorphisms, 473
single-strand binding protein,
402
Single-strand binding proteins,
392
sink, 945
sinks, 933
sinoatrial (SA) node, 1265, 1273
siphonophore, 843
siphonophores, 801
sister taxa, 538, 557
Skeletal muscle tissue, 1207
skeletal muscle tissue, 1218
skull, 1182, 1218
sliding clamp, 393, 402
small 40S ribosomal subunit,
456
small intestine, 1043, 1061
small nuclear, 417
small nuclear RNA, 431
smooth endoplasmic reticulum
(SER), 123, 137
Smooth muscle tissue, 1207
smooth muscle tissue, 1218
Soil, 955
soil, 967
soil profile, 957, 967
Solar intensity, 1399
solar intensity, 1404
solute, 155, 169
solutes, 154
solvent, 52, 65
somatic cell, 308, 322
somatosensation, 1088,1104
somatostatin, 1059, 1061
somite, 1366
somites, 1363
soredia, 697
source, 945
1564
Index
source water, 1394, 1404
sources, 933
Southern blotting, 466, 486
speciation, 501, 512
species, 500, 512, 540
species dispersion pattern, 1453
Species dispersion patterns,
1411
Species richness, 1437
species richness, 1453
species-area relationship, 1500,
1516
specific heat capacity, 51, 65
spectrophotometer, 236, 246
spermatheca, 1341, 1366
spermatogenesis, 1346, 1366
spermatophore, 863
spermatophyte, 759
Sphenodontia, 873, 897
sphere of hydration, 52, 65
sphincter, 1041,1061
spicule, 843
spinal cord, 1090, 1104
spinal nerve, 1104
Spinal nerves, 1095
spiracles. , 829
spiral cleavage, 773, 786
spirometry, 1232, 1247
splicing, 422, 431
spongocoel, 790, 843
spongy bone, 1194
spongy bone tissue, 1218
spontaneous mutation, 402
Spontaneous mutations, 400
sporangium, 673, 697, 711
spore, 322, 697
spores, 667
sporocyte, 727
sporocytes, 704
sporophyll, 727
sporophylls, 718, 738
sporophyte, 320, 322, 711, 971,
1000
sporopollenin, 705, 727
spring-and-fall turnover, 1379
Squamata, 873, 897
squamous epithelia, 1029
Squamous epithelial, 1012
stabilizing selection, 528, 534
stable hairpin, 415
stamen, 759
Stamens, 744
standard metabolic rate (SMR),
1008, 1029
stapes, 1125, 1140
starch, 71, 102
start codon, 426, 431
statolith, 945
statoliths, 938
stele, 916, 945
stereocilia, 1127,1140
stereoscopic vision, 886, 897
sternum, 1185, 1218
steroid, 102
steroids, 85
stigma, 744, 759
stipule, 945
stipules, 918
stolon, 945
Stolons, 913
stoma, 246
stomach, 1041, 1061
stomata, 230
stone canal, 838
stratified epithelia, 1012, 1029
streptophytes, 727
strigolactone, 945
Strigolactones, 941
Strobili, 718
strobili, 727
strobilus, 732, 738, 759
stroke volume, 1271, 1273
stroma, 230, 246
stromatolite, 591, 622
Structural isomers, 58
structural isomers, 65
style, 744, 759
subduction, 1477, 1485
substituted hydrocarbon, 65
substituted hydrocarbons, 60
substrate, 194
substrate-level phosphorylation,
203,223
substrates, 187
Subtropical deserts, 1384
sucrase, 1061
sucrases, 1053
sulci, 1086
sulcus, 1104
sum rule, 333, 356
summation, 1081,1104
superior colliculus, 1137, 1140
superior vena cava, 1262, 1273
Supination, 1201
supination, 1218
suprachiasmatic nucleus, 1137,
1140
surface tension, 52, 65
Surfactant, 1238
surfactant, 1247
survivorship curve, 1412, 1453
suspensor, 987, 1000
sutural bone, 1218
Sutural bones, 1193
suture, 1218
Sutures, 1198
Svalbard Global Seed Vault,
1502
swim bladder, 860, 897
symbiont, 964, 968
symbioses, 1434
Symbiosis, 686
symbiosis, 1453
Symbiotic nitrogen fixation, 617
sympathetic nervous system,
1093, 1104
Sympatric speciation, 502
sympatric speciation, 512
symphyses, 1199
symphysis, 1218
symporter, 160, 169
synapse, 1104
synapses, 1068
synapsid, 897
Synapsids, 868
synapsis, 309, 322
synaptic cleft, 1079, 1104
synaptic signal, 253, 274
synaptic vesicle, 1104
synaptic vesicles, 1079
synaptonemal complex, 308,
322
synarthrosis, 1200, 1218
synchondrosis, 1199, 1218
syncytium, 804, 821
Syndesmoses, 1199
syndesmosis, 1218
synergid, 977, 1000
synovial joint, 1218
Synovial joints, 1199
syrinx, 878
system, 1059
Systematics, 539
systematics, 557
systemic circulation, 1254, 1273
systems biology, 482, 486
systole, 1264, 1273
T
T cell, 1332
T cells, 1306
Tachyglossidae, 884, 897
tadpole, 897
taiga, 1387
tap root system, 915, 945
target cell, 274
target cells, 252
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1565
tarsal, 1218
tarsals, 1190
tastant, 1140
tastants, 1122
taste bud, 1121, 1140
TATA box, 431
taxis, 1442, 1453
taxon, 540, 557
Taxonomy, 540
taxonomy, 557
TCA cycle, 207, 223
tectorial membrane, 1126, 1140
tegmen, 989, 1000
teichoic acid, 622
teloblastic growth, 818
telomerase, 396, 402
telomere, 402
Telophase, 286
telophase, 301
Temperate forests, 1386
Temperate grasslands, 1386
template strand, 413, 431
temporal, 1088
temporal fenestra, 897
Temporal fenestrae, 868
temporal isolation, 506, 512
temporal lobe, 1104
tendril, 945
Tendrils, 914
terminal bronchiole, 1247
terminal bronchioles, 1228
Terrestrial biomes, 1382
terrestrial hypothesis, 880
tertiary consumer, 1485
Tertiary consumers, 1461
tertiary structure, 93,102
test, 663
test cross, 337, 356
testa, 989, 1000
testes, 1342, 1366
Testosterone, 1349
testosterone, 1367
tests, 647
Testudines, 875, 898
tetrad, 322
tetrads, 309
Tetrapod, 855
tetrapod, 898
thalamus, 1089, 1104
Thalassemia, 1242
thalassemia, 1247
thallus, 697
The chaparral, 1385
theory, 11, 30
thermocline, 1379, 1404
Thermodynamics, 182
thermodynamics, 194
thermophile, 622
thermoregulation, 1027, 1029
theropod, 898
thick filament, 1218
Thick filaments, 1208
Thigmomorphogenesis, 941
thigmomorphogenesis, 945
thigmonastic, 941, 945
thigmotropism, 941, 945
thin filament, 1218
Thin filaments, 1208
thoracic cage, 1185, 1218
thorn, 945
Thorns, 914
threshold of excitation, 1104
thylakoid, 246
thylakoid lumen, 230, 246
thylakoids, 230
thymus, 1171, 1174
thyroglobulin, 1157, 1174
thyroid gland, 1165, 1174
thyroid-stimulating hormone
(TSH), 1157, 1174
thyroxine, 1157
thyroxine (tetraiodothyronine,
T 4 ), 1175
Ti plasmid, 486
Ti plasmids, 472
tibia, 1189, 1218
Tidal volume (TV), 1232
tidal volume (TV), 1247
tight junction, 133, 137
tissue, 30
tissues, 23
tobacco mosaic disease, 560
tonic activity, 1136, 1140
Tonicity, 156
tonicity, 169
topoisomerase, 402
Torpor, 1009
torpor, 1029
torsion, 815
total lung capacity (TLC), 1232,
1247
trabecula, 1029
trabeculae, 1018, 1194, 1218
trachea, 1227, 1247
tracheid, 945
Tracheids, 910
tracheophyte, 727
tracheophytes, 716
tragedy of the commons, 1506,
1516
trait, 330, 356
trans, 122
trans fat, 82, 102
trans -acting element, 456
transcription, 100, 102
transcription bubble, 431
transcription bubble., 413
transcription factor, 456
transcription factor binding site,
456
transcription factors, 444
transcriptional, 436
transcriptional start site, 439,
456
transduction, 603, 622
Transfer RNA (tRNA), 100
transfer RNA (tRNA), 102
transformation, 380, 402, 603,
622
transgenic, 469, 486
transition state, 181, 195
Transition substitution, 400
transition substitution, 402
Transitional, 1014
transitional epithelia, 1029
translation, 100, 102
translational, 436
translocation, 374, 375, 933,
945
translocations, 368
transpiration, 909, 945
Transpiration, 929, 931
transport maximum, 1287, 1298
transport protein, 169
transport proteins, 154
transporter, 169
transporters, 160
transverse (horizontal) plane,
1029
transverse plane, 1009
Transversion substitution, 401
transversion substitution, 402
triacylglycerol (also,
triglyceride), 102
triacylglycerols, 81
Triassic-Jurassic, 1497
trichome, 945
Trichomes, 909
tricuspid valve, 1262, 1273
triglycerides, 81
triiodothyronine, 1157
triiodothyronine (T 3 ), 1175
triploblast, 786
triploblasts, 771
trisomy, 369, 375
trochophore, 843
trochophore larva, 812
trophic level, 1461, 1485
1566
Index
trophic level transfer efficiency
(TLTE), 1470, 1485
trophoblast, 1360, 1367
Tropical wet forests, 1383
Tropomyosin, 1212
tropomyosin, 1218
Troponin, 1212
troponin, 1218
trp operon, 456
trypsin, 1053, 1061
Tryptophan, 438
tryptophan, 456
tryptophan (trp) operon, 438
tube feet, 838
tuber, 946
Tubers, 913
tubular reabsorption, 1285,
1298
tubular secretion, 1285, 1298
tumor suppressor gene, 301
Tumor suppressor genes, 295
tunicate, 898
tunicates, 852
tympanum, 1125, 1140
u
ubiquinone, 210, 223
ulna, 1188, 1219
ultrasound, 1124,1140
umami, 1119, 1140
unidirectional circulation, 1273
unidirectionally, 1252
unified cell theory, 109, 137
uniporter, 160, 169
uniramous, 843
unsaturated, 82
unsaturated fatty acid, 102
untranslated region, 456
untranslated regions, 449
up-regulation, 1148, 1175
upstream, 431
urate salts, 878
urea cycle, 1291, 1298
ureotelic, 1291, 1298
ureter, 1282, 1298
uric acid, 1292, 1298
urinary bladder, 1282, 1298
urine, 1281, 1298
Urochordata, 852, 898
Urodela, 863, 898
uropygial gland, 878
uterus, 1345, 1367
V
vaccination, 575
vaccine, 584
Vaccines, 575
vacuole, 137
vacuoles, 118
vagina, 1346, 1367
valence shell, 41, 65
van der Waals interaction, 65
van der Waals interactions, 48
variable, 13, 30
variable number of tandem
repeats (VNTRs), 486
variants, 339
variation, 495, 513
vasa recta, 1284, 1298
vascular bundle, 905, 946
vascular cylinder, 905
vascular plant, 727
vascular stele, 905, 946
vascular tissue, 905, 946
vasoconstriction, 1267, 1273
vasodilation, 1267, 1273
vasodilator, 1295, 1298
vasopressin, 1295, 1298
vein, 718, 727, 1274
Veins, 1267
veliger, 812, 843
velum, 801
vena cava, 1274
venation, 919, 946
venous P cc , 2 , 1234, 1247
venous P 0o , 1247
venous P 0? , 1234
ventilation/perfusion (V/Q)
mismatch, 1240, 1247
ventral cavity, 1010, 1029
ventricle, 1104, 1255, 1274
ventricles, 1085
venule, 1274
venules, 1267
vernalization, 990, 1000
vertebrae, 854
vertebral column, 898, 1184,
1219
Vertebrata, 898
vertical transmission, 571, 584
vesicle, 137
Vesicles, 118
vessel element, 946
Vessel elements, 910
Vestibular sensation, 1110
vestibular sense, 1140
vestigial structure, 513
vestigial structures, 497
viable-but-non-culturable, 594
viable-but-non-culturable
(VBNC) state, 622
vicariance, 502, 513
villi, 1043, 1061
viral receptor, 584
viral receptors , 562
virion, 584
virions, 560
viroid, 584
Viroids, 580
virus core, 563, 584
visceral mass, 811, 812
Vision, 1130
vision, 1140
vital capacity (VC), 1232, 1247
vitamin, 1061
Vitamins, 1047
viviparity, 1341, 1367
vomerine teeth, 862
w
Water potential, 926
water potential (M J W ), 946
water vascular system, 843
wavelength, 233, 246
Wax, 84
wax, 102
weather, 1396, 1404
web of life, 553, 557
Wetlands, 1395
whisk fern, 727
whisk ferns, 720
white blood cell, 1274
white blood cells, 1255
white-nose syndrome, 1509,
1516
Whole-genome sequencing, 476
whole-genome sequencing, 486
whorled, 920, 946
wild type, 339
wildlife corridors, 1513
X
X inactivation, 371, 375
X-linked, 341, 356
Xylem, 717
xylem, 727
Y
yeast, 697
yolk sac, 867
z
zero population growth, 1418,
This OpenStax book is available for free at http://cnx.Org/content/col24361/l.8
Index
1567
1453
zoea, 843
zona pellucida, 1359, 1367
Zoology, 28
zoology, 30
Zoonoses, 611
zoonosis, 622
zoonotic diseases, 881
Zooplankton, 1394
Zygomycota, 677, 697
zygospore, 697
zygospores, 677
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